Extraterrestrial Life (Life Beyond Earth)

January 29, 2024 by Dr Rajiv Desai

Extraterrestrial Life (Life Beyond Earth):

Figure above shows Earth, as seen by NASA’s Voyager 1. In this image, taken from 4 billion miles away 33 years ago, Earth appears as a “pale blue dot” representing less than a pixel’s worth of light. Would this light reveal Earth as a habitable and inhabited world? Our search for life on exoplanets will depend on an ability to extract information about life from the faint light of faraway worlds.

“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”

-Arthur C. Clarke, science fiction author and former Planetary Society board member

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Section-1

Prologue:

Is mankind alone in the universe? Or are there somewhere other intelligent beings looking up into their night sky from very different worlds and asking the same kind of question? Are there civilizations more advanced than ours, civilizations that have achieved interstellar communication and have established a network of linked societies throughout our galaxy? Such questions, bearing on the deepest problems of the nature and destiny of mankind, were long the exclusive province of theology and speculative fiction. In less than 30 years, we’ve gone from not knowing whether planets existed outside our solar system (exoplanets) to being able to pinpoint potentially habitable planets and collect data that will enable us to look for the signatures of life. These advances offer unprecedented opportunities to answer the age-old question, are we alone? The mystique of imagined extraterrestrial civilizations living, dying, and conducting their affairs in far-distant kingdoms among the stars has likely stirred the minds of children for time immemorial. Their mothers have responded to the question “Do people live there?” by gently declaring that we do not know. And that answer stands today in the galactic realm, although it is now resolved negatively for our Moon and, arguably, for several of our planetary neighbors. NASA says that extraterrestrial life has not been discovered, but that does not rule out its existence. NASA is dedicated to exploring one of the most profound questions of all time: Could there be life beyond Earth? The answer will change us forever, whether it reveals a universe rich with life, one in which life is rare and fragile, or even a universe in which we can find no other life at all. The hunt for an answer is also revealing important details about our own place in the universe – where we came from, how life came about and, perhaps, where we’re headed.

Perceived prospects for life beyond Earth have waxed and waned over the centuries, but by the mid twentieth century the notion that we share the universe with other intelligent life made a pronounced shift from fantasy to science. This transition rested in part on a long accumulation of major discoveries including the Copernican model of the universe, which suggests that our Sun and planet are not privileged, and Darwin’s theory of evolution, which contends that life and intelligence are the result of entirely natural processes that could occur anywhere. There’s no scientific question more interesting than whether the life that carpets Earth is some sort of miracle, or merely an unremarkable example of a common, cosmic phenomenon. Philosophers and ordinary people have pondered the rise of life on our planet. Creation myths are fundamental to every civilization and culture, and reflect the profound resonance of the question of our origins. Today, astronomers have pushed back our understanding of the origins of the universe to within tiny fractions of microseconds of the big bang. However, science cannot, as yet, offer any complete definition of life, nor yet point to the exact time, conditions and mechanisms when organic matter first went from non-living to living.

Way back in 1960s, almost all scientists believed we are alone in the universe. The search for intelligent life beyond Earth was ridiculed; one might as well have professed an interest in looking for fairies. The focus of skepticism concerned the origin of life, which was widely assumed to have been a chemical fluke of such incredibly low probability it would never have happened twice. Today the pendulum has swung decisively the other way. During the last half of the twentieth century, rapid advancements in science and technology prompted many people to begin rethinking our place in the universe. These developments included ideas about cosmic evolution (which claims that the universe is evolving in the direction of greater complexity, consciousness, and culture), space exploration, recognition that asteroids and comets pose a threat to the survival of our species, and growing circumstantial evidence that we may share the universe with extraterrestrial civilizations. The idea that other thinking beings inhabit the cosmos is surely more popular today than ever before. “Aliens” (the routine shorthand for extraterrestrial sentients) infest books, movies, and television dramas.

Even if we find a true Earth-like planet, finding out whether it has life is far from easy. It needs a series of happy coincidences for life to evolve on a planet, apart from a stable climate over a few billion years. The Earth is a very special place. Earth is at the right distance from the Sun, it is protected from harmful solar radiation by its magnetic field, it has a moon that stabilises spin, it has giant planets outside that protects it from celestial impacts, it is kept warm by an insulating atmosphere, and it has the right chemical ingredients for life, including water and carbon. The processes that shape the Earth and its environment constantly cycle elements through the planet. This cycling sustains life and leads to the formation of the mineral and energy resources that are the foundation of modern technological society. Yet we know very little about the processes under which life forms because we have only one example so far. A second example, wherever it is, could change the nature of life science itself. Over the last decade, astronomers have expended great effort trying to find what traces of simple forms of life—known as “biosignatures”—might exist elsewhere in the universe. But what if an alien planet hosted intelligent life that built a technological civilization? Could there be “technosignatures” that civilization on another world would create that could be seen from Earth? And, could these technosignatures be even easier to detect than biosignatures?

In the mid-1990s, we didn’t know about planets that circle their stars in hours or others that take almost a million years. We didn’t know about planets that revolve around two stars, or rogue planets that don’t orbit any star but just wander about in space. In fact, we didn’t know for sure that any planets at all existed beyond our solar system, and a lot of the assumptions we made about planet-ness have turned out to be wrong. On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi. A giant planet crammed up against its star, winging around it in just four days. Since that finding—which won the scientists a portion of the 2019 Nobel Prize in Physics—researchers have discovered more than 5,000 exoplanets, including some Earth-like planets that may have the potential to harbour life. The majority were discovered by the Kepler space telescope, launched in 2009. Out of thousands of planets in our Milky Way galaxy, some of them are in Earth’s size range and orbiting in their stars’ “habitable zones” – the distance from the star at which liquid water could exist on the surface. Astronomers estimate there are billions of undiscovered planets just in our Galaxy.

Given the size of the universe it seems silly to think we’re all alone. The question is not really whether there is life out there. It’s whether there is life in close enough proximity to us that we could find it. Do you know that the fastest plane that was ever build would need to fly for a million years non-stop at full speed just to cover the distance to the very next star? Of course, spacecrafts might be faster, but physics prevent them from getting really fast. If we get a thousand times faster (which is rather unreasonable) then we would still need a thousand years. All of the known exoplanets i.e. planets outside of our solar system, are too far away to feasibly travel to. Even in the very optimistic case of a livable planet that is not too far, say a few dozen light years, which is not a lot, it’s in the neighbourhood, the time to go there is considerable. If we are talking about exoplanets, things should be clear: We will not migrate there. Humans are not the right lifeform to do interstellar flights.

In considering the design of present and future Search for Extraterrestrial Intelligence (SETI) searches and the interpretation of their (so far) negative results, an important factor is the estimated probability of detecting electromagnetic (EM) radiation from a civilization. This factor depends on the parameters of the search itself and on the probable number of civilizations emitting EM radiation in a manner and at a level that a search would detect it. In estimating the number of such civilizations, an important factor is the average length of time that any such civilization does emit such EM radiation. Since European explorers first reached all regions of the Earth, there have been no real “aliens” or “outsiders,” and all of humanity is currently linked into a single, global community. However, it is possible that in future contact will be made with a very different kind of alien—namely, non-human inhabitants of other worlds—and it is worth understanding our own degree of preparation for this event. My endeavour is to study whether extraterrestrial life exists or not. Extraterrestrial life could be similar to lifeforms on earth or alternate lifeforms, could be intelligent lifeforms (aliens) or simple lifeforms such as prokaryotes, could be organic lifeforms or inorganic lifeforms, could be far more advanced civilizations than humans or primitives.

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Abbreviations and synonyms:

ET = extraterrestrial = alien = intelligent extraterrestrial lifeform

ETL = extraterrestrial life

ETI = extraterrestrial intelligence

Gya = giga years ago, a unit of time equal to one billion years before present.

LUCA = last universal common ancestor

SETI = search for extraterrestrial intelligence

METI = messaging extraterrestrial intelligence

CETI = communication with extraterrestrial intelligence

SETA = search for extraterrestrial artifacts

C2H6 = ethane

CH3 = methyl

CH3SH = methanethiol

CH4 = methane

H2S = hydrogen sulfide

HNO3 = nitric acid

N2O = nitrous oxide

DMDS = dimethyl disulfide

DMS = dimethyl sulfide

OH = hydroxyl

SO2 = sulfur dioxide

ELTs = extremely large ground-based telescopes

HabEx = Habitable Exoplanet Imaging Mission

JWST = James Webb Space Telescope

HWO = Habitable Worlds Observatory

LUVOIR = Large UltraViolet Optical Infrared Surveyor

UV-VIS-NIR-MIR = ultraviolet, visible, near-infrared, and midinfrared

RV = radial velocity

S/N = signal-to-noise

WFIRST = Wide Field Infrared Survey Telescope

HZ = habitable zone

NDVI = Normalized Difference Vegetation Index

NExSS = Nexus for Exoplanet System Science

TPF = Terrestrial Planet Finder

VRE = vegetation red edge

FRB = fast radio burst

UAP = unidentified anomalous phenomena

UFO = unidentified flying objects

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Terminology:

Extraterrestrial:

The word itself is quite simple. “Extra” means outside of, and “terrestrial” means the Earth. Add that together, and it means something from outside of our Earth. Because it’s so general, “extraterrestrial” can describe other cosmic objects beyond our immediate backyard, such as space dust or meteorites. In the context of life, “extraterrestrial” would mean an organism originating and capable of surviving outside of Earth.

Habitable zone:

The habitable zone refers to the distance from a star at which water could exist on the surface of orbiting planets. These are called ‘Goldilocks’ zones’, as they are planets where conditions may be just right for life.

SETI:

The search for extraterrestrial intelligence (SETI) is a general term that refers to scientific searches for intelligent extraterrestrial life. One example is monitoring electromagnetic radiation for signs of transmissions from civilisations from other planets. Scientists have been actively searching for signs of extraterrestrial life through programs like the Search for Extraterrestrial Intelligence (SETI) Institute. The SETI Institute began with a NASA-funded project on the search for extraterrestrial intelligence and eventually grew into the nonprofit research organization it is now. The institute conducts research in a variety of fields related to astrobiology, astronomy, and space science, with a primary focus on detecting signals or signs of intelligent life in the universe.

Biosignature:

A biosignature is any characteristic, element, molecule, substance, or feature that can be used as evidence for past or present life

Technosignature:

A technosignature, or technomarker, is a property or effect that provides scientific proof of past or present technology.

Exoplanet:

All the planets in our solar system orbit the sun. Planets that orbit other stars are known as exoplanets. Exoplanets can vary in size and composition, and most are tethered to host stars.

Astronomer:

An astronomer is a scientist whose area of study focuses on questions outside the scope of Earth. They study astronomical objects like stars, planets and moons, and their topics of study can include things like the origins of stars or the formation of galaxies. Astronomy is study of stars, space and the universe as a whole.

Astrobiology:

Astrobiology is a multidisciplinary scientific field that explores the origin, evolution, distribution, and potential existence of life in the universe. It’s just as it sounds, a combination of astronomy and biology. Astrobiology is concerned with some of the deepest questions humanity can pose. Where do we come from? Where are we going? Are we alone?

Extremophile:

A microorganism that lives in conditions of extreme temperature, acidity, alkalinity or chemical concentration.

Hydrothermal vent:

An opening in the sea floor out of which heated, mineral-rich water flows.

Dwarf planet:

A planetary body which is large enough to be a planet, but has not become the dominant planetary body in its orbit.

Enceladus:

A moon of Saturn, and the brightest object in the Solar System due to its surface of ice.

Europa:

A moon of Jupiter, and the 6th-largest moon in the Solar System.

Iron Core:

A liquid iron core is important for protecting life on a planet’s surface. The movement of molten iron generates a magnetic field, which shields the atmosphere from stellar activity. Some planets with iron cores, like Earth, start with a completely liquid core which crystallizes over time. For planets with small cores, the core may completely solidify, turning off the magnetic field.

Magnetic Fields:

On Earth, magnetic fields are produced by a spinning molten iron core. The field protects the planet’s atmosphere from harmful activity from its star, which could impact the habitability for some forms of life.

Planet Size:

The size of a planet plays a large role in how much atmosphere it can hold. Planets that are too large hide their surfaces under atmospheres much thicker than Earth’s. Small planets can’t keep their stars’ stellar winds from blowing away their atmospheres.

Tidal locking:

Tidal locking is the phenomenon by which a body has the same rotational period as its orbital period around a partner. So, the Moon is tidally locked to the Earth because it rotates in exactly the same time as it takes to orbit the Earth. That is why we only see one side of the Moon.

Planet orbits:

How and where a planet orbits its star is very important for its habitability. Planets in eccentric orbits — or those experiencing dramatic changes in tilt — could have extreme seasons.

Face-on orbit means the plane of the exoplanet’s orbit is perpendicular to the line of sight with Earth.

Edge-on orbit means the plane of the exoplanet’s orbit is parallel to the line of sight with Earth.

Host star and planet:

Stars are huge celestial bodies made up of hydrogen and helium that can emit their light and energy from nuclear fusion reactions. Planets are celestial bodies that orbit around the host star. They don’t have the ability to emit light and energy from nuclear fusion reactions. Stars release UV light, X-rays, and energetic particles, all of which can be harmful to life and strip away a planet’s atmosphere. Some stars may be good for life, others may just be too extreme. These stellar factors determine where a habitable planet might be found and if life could survive there at all. Old stars expand quickly, engulfing nearby planets. Planets around large stars have to be far from their star and may not have enough time to develop life before the star dies. Planets around small stars must be very close to their volatile hosts. Any life could be fried by stellar activity. Habitable planets are likely found in the Goldilocks zone, meaning they’re just the right distance from their star for liquid water to exist on the surface.

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Units of cosmic distance measurement:

Astronomical unit:

Astronomical unit (AU, or au), a unit of length effectively equal to the average, or mean, distance between Earth and the Sun, defined as 149,597,870.7 km (92,955,807.3 miles). Alternately, it can be considered the length of the semimajor axis—i.e., the length of half of the maximum diameter—of Earth’s elliptical orbit around the Sun. The astronomical unit provides a convenient way to express and relate distances of objects in the solar system and to carry out various astronomical calculations. For example, stating that the planet Jupiter is 5.2 AU (5.2 Earth distances) from the Sun and that Pluto is nearly 40 AU gives ready comparisons of the distances of all three bodies.

Parsec:

The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9 trillion kilometres (19.2 trillion miles).

Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand parsecs, and the Andromeda galaxy at over 700 thousand parsecs.

Light-year (ly):

A light-year, alternatively spelled light year, (ly) is a unit of length used to express astronomical distances and is equal to exactly 9,460,730,472,580.8 km, which is approximately 5.88 trillion mi. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the word “year”, the term is sometimes misinterpreted as a unit of time. The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry.

1 light-year

≈ 9.461 trillion kilometres (5.879 trillion miles)

≈ 63241.077 astronomical units

≈ 0.306601 parsec

In the Milky Way Galaxy, wherein Earth is located, distances to remote stars are measured in terms of kiloparsecs (1 kiloparsec = 1,000 parsecs). The Sun is at a distance of 8.3 kiloparsecs from the centre of the Milky Way system. When dealing with other galaxies or clusters of galaxies, the convenient unit is the megaparsec (1 megaparsec = 1,000,000 parsecs). The distance to the Andromeda Galaxy (Messier 31) is about 0.76 megaparsec. The farthest galaxies and quasars have distances on the order of about 4,000 megaparsecs, or 13,000,000,000 light-years.

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Section-2

Basic astronomy vis-à-vis extraterrestrial life:

Earth is Humanity’s Home Base:

The image above shows the Western hemisphere as viewed from space 35,400 kilometers (about 22,000 miles) above Earth.

Our nearest astronomical neighbor is Earth’s satellite, commonly called the Moon. The Moon’s distance from Earth is about 30 times Earth’s diameter, or approximately 384,000 kilometers, and it takes about a month for the Moon to revolve around Earth. The Moon’s diameter is 3476 kilometers, about one fourth the size of Earth. Light (or radio waves) takes 1.3 seconds to travel between Earth and the Moon. If you’ve seen videos of the Apollo flights to the Moon, you may recall that there was a delay of about 3 seconds between the time Mission Control asked a question and the time the astronauts responded. This was not because the astronauts were thinking slowly, but rather because it took the radio waves almost 3 seconds to make the round trip.

Earth revolves around our host star, the Sun, which is about 150 million kilometers away—approximately 400 times as far away from us as the Moon. We call the average Earth–Sun distance an astronomical unit (AU) because, in the early days of astronomy, it was the most important measuring standard. Light takes slightly more than 8 minutes to travel 1 astronomical unit, which means the latest news we receive from the Sun is always 8 minutes old. The diameter of the Sun is about 1.5 million kilometers; Earth could fit comfortably inside one of the minor eruptions that occurs on the surface of our star. If the Sun were reduced to the size of a basketball, Earth would be a small apple seed about 30 meters from the ball.

Earth revolves in orbit around the Sun in 365 days, 6 hours, 9 minutes with reference to the stars, at a speed ranging from 29.29 to 30.29 km/s. The 6 hours, 9 minutes adds up to about an extra day every fourth year, which is designated a leap year, with the extra day added as February 29^th. Because gravity holds us firmly to Earth and there is no resistance to Earth’s motion in the vacuum of space, we participate in this extremely fast-moving trip without being aware of it day to day.

Earth is only one of eight planets that revolve around the Sun. These planets, along with their moons and swarms of smaller bodies such as dwarf planets, make up the solar system as seen in the figure below.

The Sun, the planets, and some dwarf planets are shown with their sizes drawn to scale. The orbits of the planets are much more widely separated than shown in this figure. Notice the size of Earth compared to the giant planets.

We are able to see the nearby planets in our skies only because they reflect the light of our local star, the Sun. If the planets were much farther away, the tiny amount of light they reflect would usually not be visible to us. The planets we have so far discovered orbiting other stars were found from the pull their gravity exerts on their parent stars, or from the light they block from their stars when they pass in front of them. We can’t see most of these planets directly, although a few are now being imaged directly. The Sun is our local star (host star), and all the other stars are also enormous balls of glowing gas that generate vast amounts of energy by nuclear reactions deep within. The other stars look faint only because they are very far away. If we continue our basketball analogy, Proxima Centauri, the nearest star beyond the Sun, which is 4.25 light-years away, would be almost 7000 kilometers from the basketball.

Galaxy:

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally ‘milky’, a reference to the Milky Way galaxy that contains our Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy’s center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many are thought to have supermassive black holes at their centers. The Milky Way’s central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.

It is estimated that there are between 200 billion (2×10^11) to 2 trillion galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy (with diameter of about 152,000 ly), its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)

There are an estimated 400 billion stars in the Milky Way and an estimated two trillion galaxies in the universe. According to findings by the Kepler Space Telescope and other ground-based and space-based observatories, virtually every star in the sky is circled by at least one planet, and many, like our sun, by a whole litter of them. That makes for trillions upon trillions of worlds on which life could, in theory at least, have taken hold.

When you look up at a star-filled sky on a clear night, all the stars visible to the unaided eye are part of a single collection of stars we call the Milky Way Galaxy, or simply the Galaxy. (When referring to the Milky Way, we capitalize Galaxy; when talking about other galaxies of stars, we use lowercase galaxy.) The Sun is one of hundreds of billions of stars that make up the Galaxy; its extent staggers the human imagination. Within a sphere 10 light-years in radius centered on the Sun, we find roughly ten stars. Within a sphere 100 light-years in radius, there are roughly 10,000 (10^4) stars—far too many to count or name—but we have still traversed only a tiny part of the Milky Way Galaxy. Within a 1000-light-year sphere, we find some ten million (10^7) stars; within a sphere of 100,000 light-years, we finally encompass the entire Milky Way Galaxy. Our Galaxy looks like a giant disk with a small ball in the middle. If we could move outside our Galaxy and look down on the disk of the Milky Way from above, it would probably resemble the galaxy as seen in the figure below, with its spiral structure outlined by the blue light of hot adolescent stars.

Figure above shows Spiral Galaxy. This galaxy of billions of stars, called by its catalog number NGC 1073, is thought to be similar to our own Milky Way Galaxy. Here we see the giant wheel-shaped system with a bar of stars across its middle.

The Sun is somewhat less than 30,000 light-years from the center of the Galaxy, in a location with nothing much to distinguish it. From our position inside the Milky Way Galaxy, we cannot see through to its far rim (at least not with ordinary light) because the space between the stars is not completely empty. It contains a sparse distribution of gas (mostly the simplest element, hydrogen) intermixed with tiny solid particles that we call interstellar dust. This gas and dust collect into enormous clouds in many places in the Galaxy, becoming the raw material for future generations of stars.

Figure above shows Milky Way Galaxy. The Milky Way is the galaxy that includes the Solar System. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

Typically, the interstellar material is so extremely sparse that the space between stars is a much better vacuum than anything we can produce in terrestrial laboratories. Yet, the dust in space, building up over thousands of light-years, can block the light of more distant stars. Like the distant buildings that disappear from our view on a smoggy day, the more distant regions of the Milky Way cannot be seen behind the layers of interstellar smog. Luckily, astronomers have found that stars and raw material shine with various forms of light, some of which do penetrate the smog, and so we have been able to develop a pretty good map of the Galaxy.

Recent observations, however, have also revealed a rather surprising and disturbing fact. There appears to be more—much more—to the Galaxy than meets the eye (or the telescope). From various investigations, we have evidence that much of our Galaxy is made of material we cannot currently observe directly with our instruments. We therefore call this component of the Galaxy dark matter. We know the dark matter is there by the pull its gravity exerts on the stars and raw material we can observe, but what this dark matter is made of and how much of it exists remain a mystery. Furthermore, this dark matter is not confined to our Galaxy; it appears to be an important part of other star groupings as well.

By the way, not all stars live by themselves, as the Sun does. Many are born in double or triple systems with two, three, or more stars revolving about each other. Because the stars influence each other in such close systems, multiple stars allow us to measure characteristics that we cannot discern from observing single stars. In a number of places, enough stars have formed together that we recognized them as star clusters as seen in the figure below. Some of the largest of the star clusters that astronomers have catalogued contain hundreds of thousands of stars and take up volumes of space hundreds of light-years across.

Figure above shows Star Cluster. This large star cluster is known by its catalogue number, M9. It contains some 250,000 stars and is seen more clearly from space using the Hubble Space Telescope. It is located roughly 25,000 light-years away.

You may hear stars referred to as “eternal,” but in fact no star can last forever. Since the “business” of stars is making energy, and energy production requires some sort of fuel to be used up, eventually all stars run out of fuel. This news should not cause you to panic, though, because our Sun still has at least 5 or 6 billion years to go. Ultimately, the Sun and all stars will die, and it is in their death throes that some of the most intriguing and important processes of the universe are revealed. For example, we now know that many of the atoms in our bodies were once inside stars. These stars exploded at the ends of their lives, recycling their material back into the reservoir of the Galaxy. In this sense, all of us are literally made of recycled “star dust.”

Star type:

One extremely important factor in the evaluation of life outside Earth is the star type around which the planets orbit. The sun worked for us because it has a balanced mass, not so big that it burns out quickly and not so small that it doesn’t produce enough energy. But what substitutes would be possible for the great ball of incandescent fire that warms us daily?

The first important concept here is that of dwarf stars. The sun, for example, is a yellow dwarf star and serves as a parameter to categorize others of its kind. Most of the known stars in the universe fall into this division, changing classifications solely due to color – it may sound silly, but the color indicates what stage of life the star is in. This information also concerns the energy classification and temperature of the star. The system used is the letters of the alphabet, with the following: O, B, A, F, G, K and M. Being a yellow dwarf, the sun is a dwarf of classification G. Generally speaking, those classified as O are the hottest, while those placed in the M category are the coldest. Following this order, we can also analyze the lifetime of stars: the hotter it is, the less time it will have – which means, here, that category O is the most ephemeral.

Figure below shows size comparison of main sequence stars in Morgan–Keenan classifications. Main sequence stars are those that fuse hydrogen into helium in their cores. The Morgan–Keenan system classifies stars based on their spectral characteristics. Our Sun is a G-type star. SISTINE-2’s target is Procyon A, an F-type star.

In determining the feasibility of extraterrestrial life, astronomers had long focused their attention on stars like the Sun. However, since planetary systems that resemble the Solar System are proving to be rare, they have begun to explore the possibility that life might form in systems very unlike the Sun’s. It is believed that F, G, K and M-type stars could host habitable exoplanets. About half of the stars similar in temperature to the Sun could have a rocky planet able to support liquid water on its surface, according to research using data from NASA’s Kepler Space Telescope. The analyses carried out by the researchers at the University of Washington point to dwarfs of type K as the most promising in maintaining life outside Earth. This is because they are not prone to radical changes and proton explosions, like the M-class ones, and they last longer than the G-class ones, contributing to the development of a broader biodiversity.

The most common class of star out there in the Universe — red dwarf (M-class) stars make up 75-80% of all stars — and there are all sorts of reasons why life is unlikely to exist there. Here are just a few:

M-class stars will tidally lock any Earth-sized (rocky) planets wherever liquid water is capable of forming on very short (~1 million years or less) timescales. All inner planets in a red dwarf system will be tidally locked, with one side always facing the star.
M-class stars flare ubiquitously, and would easily strip away an Earth-like atmosphere on short timescales.
X-rays emitted by these stars are too great and numerous, and would sufficiently irradiate the planet to make life as we know it untenable.
And that the lack of higher-energy (ultraviolet and yellow/green/blue/violet) light would make photosynthesis impossible, preventing primitive life from coming into existence.

If these are reasons for disfavouring life around the most common class of star in the Universe, where approximately 6% of these stars are thought to contain Earth-sized planets in what we call the habitable zone (at the right distance for a world with Earth-like conditions to have liquid water on its surface), you’re going to have to reconsider your assumptions. Tidal locking might not be necessarily as bad as we thought, as magnetic fields and substantial atmospheres with high winds could still provide changes in energy inputs. A planet (like Venus) that continuously generated new atmospheric particles could potentially survive solar wind/flare stripping events. Organisms could dive to deeper depths during X-ray events, shielding themselves from the radiation. And photosynthesis, like all life processes on Earth, is only based on the use of 20 amino acids, but more than 60 additional ones are known to occur naturally throughout the Universe.

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Since the Nobel-prize-winning discovery of a planet orbiting a sun-like star, the field of extrasolar planets is undergoing a true revolution. Thousands of planets have been found, of which some may be like Earth. Could there be biological activity on any of these, and how do we find out? Since the age of Copernicus and Galileo we know that Earth does not form the center of the Universe. We have found out that it orbits the Sun as one of multiple planets. The Sun is a normal star in the outskirts of the Milky Way, and the Milky Way is one of the billions of galaxies in the observable universe. It may be argued that our home planet is nothing special, and that the universe is teeming with life. In reality, however, the Earth may be quite unique. Circumstances may need to be just right for life to form and evolve, in particular for highly evolved species which may require a stable planetary climate for billions of years. We simply do not know. The search for extraterrestrial life is an important philosophical endeavour, which will shed light on the place of humanity in the cosmos. Are we alone?

Now, in an age of fast-paced discovery, we’ve learned we’re likely just one of trillions of planets in the Milky Way galaxy – and among the smaller ones at that. The universe is 13.8 billion years old. From what we can see, the universe is a sphere with a diameter of about 92 billion light-years. But it is still expanding to this day, and there is so much that we are not able to see.

How can the universe be 92 billion light-years but only 13.8 billion years old?

This seeming contradiction is due to the fact that the universe has been expanding since the Big Bang. As a result, the light from distant objects has had to travel a longer distance to reach us, which has caused the observable universe to appear larger than 13.8 billion light years.

Since the universe is only about 13.8 billion years old and light takes time to travel through space, then regardless of what direction we look, we see light that’s been traveling, at most, 13.8 billion years. So it’s logical to think that the observable universe must then be 2 times 13.8 equals 27.6 billion light years across, but it’s not. That’s because over time, space has been expanding, so the distant objects that gave off that light 13.8 billion years ago have since moved even farther away from us. Today, those distant objects are a bit more than 46 billion light years away. Multiply times 2, and you get 92 billion light years, the diameter of the observable universe.

As of January 2022, the Voyager 1 spacecraft was about 14.5 billion miles away from Earth. The spacecraft discovered many new things that we previously did not know about our solar system, including, but not limited to, two new moons orbiting around Jupiter. And because of the Hubble telescope orbiting around Earth, we can see other galaxies besides our own. Spacecrafts like these continue to relay back pictures of our universe to Earth, allowing us to obtain new information every day. However, these amazing discoveries that we have made over the years are minuscule when it comes to the vastness of the universe. The 14.5 billion miles that the Voyager 1 has travelled converts to not even one light-year. And when compared to the universe’s estimated 92 billion light-year diameter, it is almost sad to realize how little we have discovered. There are billions upon billions of light-years that are a complete mystery to us, so to assume that there is no other life in this universe besides on Earth is irrational.

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Cosmic evolution to origin of life:

Cosmic evolution begins with an enormous cosmic dust cloud, such as exists today between the stars. Such a cloud has a “cosmic” abundance of the elements, being composed primarily of hydrogen and helium, with only a small admixture of heavier elements. Here and there matter will be somewhat more dense than in nearby regions. The more diffuse regions will be gravitationally attracted to the denser region, which, in consequence, will grow in size and mass. As matter streams in towards the condensing central nucleus, conservation of angular momentum will cause the whole region, nucleus and streaming matter, to rotate faster and faster.

In addition, as large amounts of matter continue to collide with the nucleus, its temperature will steadily rise. After perhaps a hundred million years, the temperature at the center of the cloud will have risen to about fifteen million degrees. This is the ignition temperature for thermonuclear reactions, (such as the conversion of hydrogen to helium in the hydrogen bomb). At this time the nucleus of the cloud will become a star, “turning on” and radiating light and heat into nearby space. If the rotation is sufficiently fast, the forming star will separate under certain conditions into smaller parts, producing a double or multiple star system.

Now as the star forms, there still is a large dust cloud surrounding the star and rotating with it. In this cloud, the nebula of star, small denser regions begin attracting nearby matter, as in star formation. However, the protoplanets that grow from these regions, (in the gravitational field of the nearby star), never rise by collisional heating to the thermonuclear ignition temperature, and so become planets and not stars. In the forming protoplanets, there would be a tendency for the heavier elements to sink to the center, leaving the much more abundant hydrogen and helium as the principal constituents of the atmosphere surrounding the new planets. When the newly formed star “turns on,” radiation pressure will tend to blow away this atmosphere. However, if the protoplanet is very massive, or very far from the sun, the gravitational attraction of the protoplanet for a gas molecule may be greater than the force of radiation trying to blow it away, and the protoplanet may retain an atmosphere. This atmosphere can be residual from the proto-atmosphere, or may be due to gaseous exhalations from the planetary interior. For example, the earth’s present atmosphere is due to exhalations; Jupiter’s present atmosphere is residual.

In such a way, one can understand, generally, the atmospheres of the planets in this solar system:

-1. Mercury: Not massive, close to the sun, retains negligible atmosphere.

-2. Venus: More massive than Mercury, further from the sun, retains only the heavy gas, carbon dioxide.

-3. Earth: Retains the lighter gases, nitrogen, oxygen, and water vapor, but has lost almost all hydrogen and helium.

-4. Mars: Although further from the sun, is less massive than Earth or Venus, and so retains principally only the heavy gas, carbon dioxide.

-5. Jupiter, Saturn, Uranus, Neptune: Much further from the sun and very massive, they retain much hydrogen and helium, while the other planets have lost theirs.

One fact about our solar system that has rung the death knell of many cosmogonies is the fact that although over 99 per cent of the mass of the solar system is in the sun, over 98 per cent of the angular momentum of the system is in the planets. It is as if the rotational inertia has been transferred from the sun to the planets. H. Alfven has explained this as a magnetic braking of the sun’s rotation, due to the interaction of its magnetic field with the ionized solar nebula. On this basis, the existence of a solar nebula from which planetary systems form will cause the central star to rotate more and more slowly.

Now the origin of planets must be dependent on the temperature of the central star. If it is too cold, the atmosphere of the protoplanets will not be blown away, resulting perhaps in the formation of a system of planets similar to Jupiter, but even larger and more massive. On the other hand, if the star is too hot, radiation pressure will disperse the solar nebula rapidly, leaving, if anything, small atmosphereless planets, or a system of millions of tiny asteroids. For planets to be formed, the temperature of the star must be between these extremes.

There is another reason to believe that hot stars do not have planets. If the formation of planetary systems and the slowing down of stellar rotation both arise from the existence of nebulae, then we should expect the hot stars which dissipate their nebulae and do not form planets to rotate faster. This is exactly what is observed! The hotter the star, the faster the rotation. Cooler stars rotate more slowly than would otherwise be expected. At a temperature of about 7,000 degrees, characteristic of what are called F stars, there is a sudden large decrease in average rotational velocities, and it is possible, perhaps, that below this temperature all stars retain enough of their nebulae to form planets, (provided they have not used up their nebulae in forming double or multiple sun systems). The number of such stars is between one and ten per cent of the total number of stars, suggesting that there are as many as ten billion solar systems in our galaxy alone. Of these, perhaps one per cent, or 100 million have planets like the earth. What is the probability of life on these worlds?

The air we breathe has about 10^19 atoms in each cubic centimeter—and we usually think of air as empty space. In the interstellar gas of the Galaxy, there is about one atom in every cubic centimeter. Intergalactic space is filled so sparsely that to find one atom, on average, we must search through a cubic meter of space. Most of the universe is fantastically empty; places that are dense, such as the human body, are tremendously rare.

The most abundant elements in the cosmos are listed in Table below. Note that the list includes the four elements most common in life on Earth—hydrogen, carbon, nitrogen, and oxygen.

The Cosmically Abundant Elements:

Element	Symbol	Number of Atoms per Million Hydrogen Atoms
Hydrogen	H	1,000,000
Helium	He	80,000
Carbon	C	450
Nitrogen	N	92
Oxygen	O	740
Neon	Ne	130
Magnesium	Mg	40
Silicon	Si	37
Sulfur	S	19
Iron	Fe	32

Since the most abundant element, cosmically, is hydrogen, the atmosphere of the early protoplanets of any system must contain much hydrogen and hydrogen compounds. The hydrogen compounds of carbon, nitrogen, and oxygen are probably the most abundant hydrogen compounds in the proto-atmosphere. They are, respectively, methane, CH4, ammonia, NH3, and water vapor, H20.

In 1953, Stanley Miller, then a graduate student working under professor Harold C. Urey showed that when hydrogen, methane, ammonia, and water vapor are mixed together, and supplied with energy, some fundamental organic compounds are produced. (The energy source in protoatmospheres is probably ultraviolet light from the sun about which the protoplanet revolves.) These compounds are almost all amino acids, the biochemical building blocks from which protein is constructed. There is also some reason to believe that amino acids lead to the formation of purines and pyrimidines, which are in turn building blocks for nucleic acids. Proteins and nucleic acids are the two fundamental constituents of life as we know it on earth; hereditary materials such as genes and chromosomes are composed perhaps exclusively of nucleic acids and proteins. In addition, enzymes, which catalyse slow chemical reactions and thereby make complex life forms possible, are always proteins.

Experiments of comparable importance to those of Miller have been performed by S. W. Fox. Fox applied heat, in the range between 100 and 200 degrees Centigrade, to simple molecules, such as those synthesized by Miller. This simple procedure produced small amounts of complex organic molecules that happen to be widely distributed in all terrestrial organisms. In particular, Fox has produced ureidosuccinic acid, a key intermediary in the synthesis of nucleic acids. The temperatures required by Fox can easily be supplied by radioactive heating of the crust of the planet. There is evidence that such radioactive heating is a normal part of the early evolution of all planets.

Now it is really striking that the molecules produced by Miller and Fox are precisely the molecules necessary to form life as we know it. Almost no molecules were produced which are not fundamentally involved in modern terrestrial organisms. The processes described by Miller and Fox would probably occur on at least one planet of each star of moderate temperature. All that is required is a way of collecting the molecules produced by these processes into one place where they can interact. A liquid medium on the surface of the planet serves this purpose admirably. Molecules produced in the atmosphere would fall into these bodies of liquid, and molecules produced on land by the application of heat would also be washed into them. Although seas of liquid ammonia or hydrofluoric acid would serve, it can be shown that seas of water would be most efficient in collecting and preserving the bio-molecules. The one planet in each system that we are considering probably possessed liquid water seas early in its history, and therefore on such planets the production of proteins and nucleic acids may be expected.

Now proteins and nucleic acids have some unusual properties; so far as we know, ones not found in any other molecules. They can form a new molecule which not only can construct other identical molecules from the matter floating in the sea around it, but which if changed in some way can also construct copies of its changed structure. Such a mutating, self-reproducing molecule or collection of molecules must undergo natural selection. For these reasons, it must be identified as the first living being on the planet in question.

Thus, there may be 100 million planets in this galaxy alone on which flourish organisms at least biochemically akin to ourselves. On the other hand, due to natural selection, these organisms must be well adapted, each to its own environment. Since even slight differences in the environment eventually cause extreme differences in the structure of organisms, we should not accept extraterrestrial lifeforms to resemble anything familiar. But there is reason to believe they are out there.

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Drake to Mayor and Queloz:

It’s difficult to pin down when the search for life among the stars morphed from science fiction to science, but one key milestone was an astronomy meeting in November 1961. It was organized by Frank Drake, a young radio astronomer who was intrigued with the idea of searching for alien radio transmissions. When he called the meeting, the search for extraterrestrial intelligence, or SETI, “was essentially taboo in astronomy,” But with his lab director’s blessing, he brought in a handful of astronomers, chemists, biologists, and engineers, including a young planetary scientist named Carl Sagan, to discuss what is now called astrobiology, the science of life beyond Earth. In particular, Drake wanted some expert help in deciding how sensible it might be to devote significant radio telescope time to listening for alien broadcasts and what might be the most promising way to search. How many civilizations might reasonably be out there? he wondered. So before his guests arrived, he scribbled an equation on the blackboard.

That scribble, now famous as the Drake equation, lays out a process for answering his question. You start out with the formation rate of sunlike stars in the Milky Way, then multiply that by the fraction of such stars that have planetary systems. Take the resulting number and multiply that by the number of life-friendly planets on average in each such system—planets, that is, that are about the size of Earth and orbit at the right distance from their star to be hospitable to life. Multiply that by the fraction of those planets where life arises, then by the fraction of those where life evolves intelligence, and then by the fraction of those that might develop the technology to emit radio signals we could detect.

The final step: Multiply the number of radio-savvy civilizations by the average time they’re likely to keep broadcasting or even to survive. If such advanced societies typically blow themselves up in a nuclear holocaust just a few decades after developing radio technology, for example, there would probably be very few to listen for at any given time.

The equation made perfect sense, but there was one problem. Nobody had a clue what any of those fractions or numbers were, except for the very first variable in the equation: the formation rate of sunlike stars. The rest was pure guesswork. If SETI scientists managed to snag an extraterrestrial radio signal, of course, these uncertainties wouldn’t matter. But until that happened, experts on every item in the Drake equation would have to try to fill it in by nailing down the numbers—by finding the occurrence rate for planets around sunlike stars or by trying to solve the mystery of how life took root on Earth.

It would be a third of a century before scientists could even begin to put rough estimates into the equation. In 1995 Michel Mayor and Didier Queloz of the University of Geneva detected the first planet orbiting a sunlike star outside our solar system. That world, known as 51 Pegasi b, about 50 light-years from Earth, is a huge, gaseous blob about half the size of Jupiter, with an orbit so tight that its “year” is only four days long and its surface temperature close to 2000°F.

Nobody thought for a moment that life could ever take hold in such hellish conditions. But the discovery of even a single exoplanet was an enormous breakthrough. Early the next year Geoffrey Marcy, then at San Francisco State University would lead his own team in finding a second extrasolar planet, then a third. After that, the floodgates opened. To date, astronomers have confirmed nearly five thousand so-called exoplanets, ranging in size from smaller than Earth to bigger than Jupiter; most found by the exquisitely sensitive Kepler space telescope, which went into orbit in 2009.

None of these planets is an exact match for Earth, but scientists are confident they’ll find one that is before too long. Based on the discoveries of somewhat larger planets made to date, astronomers recently calculated that more than a fifth of stars like the sun harbor habitable, Earthlike planets. Statistically speaking, the nearest one could be a mere 12 light-years away, which is practically next door in cosmic terms.

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Section-3

Origin of life on earth:

Creationist vs Evolutionist:

Many people make a distinction between the origin of life and the evolution of life. In this view, biological evolution refers to the gradual development of the diversity of living things from a common ancestor, while the ultimate origin of life is a separate question.

This is a legitimate point, but evolution is about much more than just biology. The evolutionary worldview is that all of physical existence, both living and non-living, arose through purely natural processes. With this broad definition of evolution, abiogenesis–the spontaneous appearance of life from non-living matter–is a necessity. If life did arise on earth by itself, it would be inconceivable that this is the only planet upon which there is life. Otherwise, the earth would be a remarkably special place, and that could easily lead to theistic ideas. Consequently, most evolutionists believe that life must exist elsewhere in the universe.

The creation worldview is very different, because, as usual, we start with very different assumptions. We believe that life exists on earth because God created life here, but He first had to fashion the earth to be a suitable habitation for life. The evolutionist must believe that life is inevitable wherever conditions are suitable for life, but creationists understand that even if conditions on another planet could sustain life, life there is not possible–unless God created life there or permitted life somehow to travel to that planet from earth.

While we cannot prove biblically that God did not create life elsewhere, the strong implication of Scripture is that He did not. These very different predictions of the special creation and evolution models mean that the search for life elsewhere amounts to a powerful test between the two theories of origin.

What is life?

Most biologists would agree that self-replication, genetic continuity, is a fundamental trait of the life process. Systems that generally would be deemed nonbiological can exhibit a sort of self-replication, however. Examples would be the growth of a crystal lattice or a propagating clay structure. Crystals and clays propagate, unquestionably, but life they are not. There is no locus of genetic continuity, no organism. Such systems do not evolve, do not change in genetic ways to meet new challenges. Consequently, the definition of life should include the capacity for evolution as well as self-replication. Indeed, the mechanism of evolution—natural selection—is a consequence of the necessarily competing drives for self-replication that are manifest in all organisms. The definition based on those processes, then, would be that life is any self-replicating, evolving system.

Life on Earth is based on complex organic molecules, consisting of chains of carbon, hydrogen, nitrogen, and oxygen. However, organic molecules can be produced by simple chemical reactions as well as by biological activity. Thus, to determine if a process is truly biological, rather than simply a chemical reaction, it is necessary to define the criteria for life. The ability of an organism to reproduce itself is considered to be an essential feature of life. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the organic molecules that control heredity in terrestrial life forms. Thus, DNA and RNA are considered essential for reproduction of life on Earth. These two nucleic acids are produced only with the help of certain proteins. A major focus of exobiology is to understand how DNA, RNA, and the proteins essential in their production originated.

One of the most commonly accepted definitions of life comes from a discussion on astrobiology organized by NASA: “Life is a self-sustaining chemical system capable of undergoing Darwinian evolution” (Joyce et al., 1994; Benner, 2010). These scientists were interested in finding a definition that would apply not just to Earth life, but to life on another planet, if we ever find it. It is very significant that evolution was included as a defining feature of life.

In the last decade, astronomers have detected hundreds of planets in other star systems, and the increasing sensitivity of these observations means that it is getting progressively easier to detect smaller planets that might be similar to Earth. It now seems likely that quite a high fraction of stars have Earth-like planets (Petigura et al., 2013), i.e., rocky planets with a temperature that allows liquid water. One reason to be optimistic that life might be common elsewhere is that life occurs in a very wide range of environments on Earth, with micro-organisms (bacteria and archaea) being particularly versatile in their habitat. Organisms that live in conditions that appear ‘extreme’ to us are known as extremophiles (Rothschild and Mancinelli, 2001). For example, organisms are found in boiling hot springs at close to 100 °C, crevices in sea ice at close to 0 °C, and lakes of extremely high salt concentration. Although most organisms would die in these extreme conditions, the fact that some organisms can survive there suggests that evolution often finds a way to solve the challenges posed by unusual environments. Typical conditions on other planets might be very different from those on Earth, but the study of extremophiles on Earth tells us that we should not be too narrow in our expectations of what other kinds of planets might be able to support life.

The big unknown is how likely it is that life will evolve, given a suitable planet. It may still be very difficult for a living process to get started, even on a planet where physical and chemical conditions are appropriate for supporting life, if it arises. We still have no way of knowing how many other planets have life, and we cannot rule out the possibility that we are alone in our galaxy (or even the whole observable universe!). However, the timeline for life on Earth gives us another reason to be optimistic. It is known that the Earth and the rest of the solar system formed about 4.6 billion years ago. It is thought that there were many impacts of large meteorites and comets in the early period of the solar system that would have made Earth uninhabitable until about 4.0 or 3.9 billion years ago (Hartmann et al., 2007). There is also fairly strong evidence that life existed 3.5 billion years ago, and some indirect evidence that it was present as early as 3.8 billion years ago (Buick, 2007). Hence, the time window during which life evolved is at most 0.5 billion years, and could be as little as 0.1 billion years (100 million years). Although this may seem like a long time, it is much shorter than the age of the planet. There has been life on Earth for most of the time that Earth has been here. If the time for life to evolve on Earth is in any way typical, then we expect that there are many other planets that are old enough for life to have had a good chance of originating.

Of course, this does not mean that the origin of life is ‘easy.’ We cannot just throw together a few chemicals in a test tube and expect that a new kind of life will spontaneously appear (otherwise someone would have done it by now!). The origin of life is probably a very unusual event that takes millions of years to occur once on a whole planet. But once is all it takes, given that life can multiply exponentially once it gets started, and can adapt to areas of a planet that are much different to the place where it originated. In summary, the origin of life is not a miracle; it is an unusual event that can be studied from a scientific point of view, with an aim of understanding how it was most likely to have occurred on Earth and how it could occur elsewhere.

The existence of life on Earth rests on five main pillars: the distance from the Sun, neither too close nor too far away, just enough for liquid water; the magnetic core, which protects the atmosphere from the drag of the solar wind and life from cosmic radiation; the atmosphere itself, whose greenhouse effect prevents water from freezing; water, naturally, the universal solvent of life; and finally oxygen, which allows us to breathe. But unlike a recipe, these ingredients are not entirely independent of each other. And, above all, as in the preparation of any dish, only if they are combined in the right order and cooked in the correct way do we obtain the result that makes it possible for us to be here today.

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Astrobiology research is taking place because its time has come. Scientists across America and around the world are diving into origin-of-life and life-beyond-Earth issues and developing exciting and cutting-edge work. But NASA also has an astrobiology “strategy” describing where the agency sees promising lines of research – from the highly specific to the wide and broad — that the agency might support. A sampling of examples:

What were the steps that led inanimate materials – rocks, sediments, organic compounds, water – to come together and build living organisms, with replicating genes, cell walls, and the ability to reproduce?
What led to the proliferation of new life forms on Earth?
How do water and essential organic compounds arrive on planets and moons, and how do they interact with the planets and moons they land on?
Is it possible to learn from chemicals and minerals on the surface of planets whether microbes might live there, including beneath the planet’s surface?
Is it possible, likely even, that life exists elsewhere based on elements other than carbon and a system different than DNA? Could such life even exist here on Earth, but is as yet undetected?

These and so many other lines of research shed light on how to identify and find extraterrestrial life, as well as how to understand the origin of life on Earth. The two issues are inseparable.

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Requirements for life:

The search for life beyond Earth is one of the most monumental and consequential endeavours on which humanity has ever embarked. It is also a search that is fraught with intricacies and complexities. Our definition of life is necessarily limited by our understanding of life on Earth; however, we are aided by the universality of the laws of physics and chemistry. Through this notion of universality, a consensus has emerged that life requires three essential components: (1) an energy source to drive metabolic reactions, (2) a liquid solvent to mediate these reactions, and (3) a suite of nutrients both to build biomass and to produce enzymes that catalyze metabolic reactions (Cockell et al., 2016). The so-called “Weird Life Report” identified four fundamental requirements for life (in order of decreasing certainty): thermodynamic disequilibrium (Gibbs free energy), an environment capable of maintaining covalent bonds (especially between C, H, and other atoms), a liquid environment, and a molecular system that can support Darwinian evolution.

It is useful to categorize the requirements for life on Earth as four items: energy, carbon, liquid water, and various other elements. These are listed in Table below along with the occurrence of these factors in the Solar System. In our Solar System it is the occurrence of liquid water that appears to limit the occurrence of habitable environments and this appears to be the case for exoplanetary systems as well.

Ecological requirements for life:

Requirement	Occurrence in the Solar System
Energy	Common
Predominately light	Photosynthesis at 100 AU light levels
Chemical energy	e.g., H₂ + CO₂ → CH₄ + H₂O
Carbon	Common as CO₂ and CH₄
Liquid water	Rare, only on Earth for certain
N,P, S, Na, and other elements	Likely to be common

From basic thermodynamic considerations it is clear that life requires a source of energy. To power metabolism and growth, life on Earth uses only one energy source: that associated with the transfer of electrons by chemical reactions of reduction and oxidation. For example, methane-producing microbes use the reaction of CO2 with H2 to produce CH4. Photosynthetic organisms use a light-absorbing protein, such as chlorophyll, bacteriochlorophylls, and bacteriorhodopsin, to convert photon energy to the energy of an electron which then completes a redox reaction. The electrons from the redox reaction are used to create an electrochemical gradient across cell membranes. This occurs in the mitochondria in of most eukaryotes and in the cell membrane of prokaryotic cells. It has recently been shown that electrons provided directly as electrical current can also drive microbial metabolism. Although life can detect and generate other energy sources including magnetic, kinetic, gravitational, thermal gradient, and electrostatic, none of these is used for metabolic energy.

Darwinism is rarely considered when talking about habitability or biosignatures, it is nevertheless a fundamental aspect of life as we understand it. The requirement for a molecular system capable of such evolution significantly constrains life’s requirements. For example, if Darwinian evolution is fundamental to biology, then information processing is a core attribute of life, and this would require molecular recognition with a very high level of fidelity—a requirement that may limit the range of chemistries, solvents, and environments that are suitable for life.

The basic elements that life on Earth needs can be described by acronym “CHNOPS” (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur) or “SPONCH” (sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen). Carbon is used as the scaffolding element that allows for a large diversity of molecular structures. That diversity is greatest when carbon is in the intermediate oxidation state (between CO2 and CH4). Heteroatoms (nitrogen, oxygen, phosphorus, and sulfur) support a diverse range of covalent chemistry and also have polar bonds that allow for a variety of non-covalent interactions. Hydrogen provides hydrogen bonding (obviously), which is part of what allows for high-fidelity molecular recognition in our biochemistry. Watson-Crick base-pairing in DNA, for example, is based on hydrogen bonding between complementary nucleobases. Potential alternatives to these elements, would have to fill these same roles and do so as part of molecules that are stable over meaningful time scales.

The study of life on Earth and the general principles of chemistry and physics further suggests that the liquid solvent for life is likely to be water, both because of its cosmic abundance (it is one of the most cosmically abundance molecules, consisting of the first [H] and third [O] most abundant elements) and its distinct physicochemical properties that make it highly suitable for mediating macromolecular interactions. While one of water’s essential properties is its oft-cited ability to act as a solvent for polar molecules, promoted by its unique ability to engage in hydrogen bonding, water has much more expansive, active and, at times, subtle, roles within known living processes. For example, water plays an essential role in protein folding, protein substrate binding, enzyme actions, the rapid transport of protons in aqueous solution, maintaining the structural stability of proteins and DNA/RNA, and the inhomogeneous segregation of salt ions at cellular interfaces. Carbon chemistry is likewise favored as a basis for biomass because carbon has a high cosmic abundance and carries the ability to form an inordinate number of complex molecules. While acknowledging that alternative biochemistries may exist, their plausibility has not yet been convincingly demonstrated (nor their potential biosignatures explicated). Further constraints on the development and persistence of life likely exist, although they are less precisely enumerable.

Unity of biochemistry:

Life on Earth has a common ancestor. This idea is based on the “unity of biochemistry,” which is the fact that all life has the same biochemical and molecular characteristics: the same nucleotide bases, the same 20 amino acids (along with selenocysteine), the same genetic code, lipids with straight, methylbranched chains, and metabolic energetics that use phosphate anhydrides and thioesters. This unity of biochemistry is reflected in the global phylogenetic tree. The question is whether life on other planetary bodies would exhibit a similar sort of unity of biochemistry and if Darwinian selection would allow the most fit genes to survive there too.

Is biology universal?

We readily accept that the concepts of physics and chemistry apply throughout the cosmos and are valid for all time, but should this not make us wonder whether biology is universal as well, and not just a special feature that only applies to planet Earth?

We are however left with a fundamental gap in understanding how these molecules become ‘alive’. If there are alien civilizations at a comparable stage of evolution, one might expect that they do not differ that much from our own. However, with the Sun just about half-way through its lifetime as a main-sequence star, with about 4.5 billion years remaining, that ‘comparable stage’ might constitute a rather short transient episode, and advanced extra-terrestrial life might be inconceivable to us in its complexity, just as human life is to amoebae.

The Earth looks like a big, beautiful blue marble from space. Our home planet is certainly one of the most beautiful planets in the solar system, and it is also the only planet we know that supports life in any form. Other planets in our solar system may turn out to have life in some form, but, at best, it would probably be very tiny forms of life, such as microbes.

The Earth is unique in our Solar System most notably because it harbours complex, intelligent life that has survived and evolved through 3.9 billion years of the planet’s 4.5 billion year history. There are a number of conditions that make life possible on Earth. Among them are:

-1. Liquid water near the surface;

-2. A level of incoming radiation from space, filtered through our atmosphere, that is neither too much nor too little;

-3. A stable planetary orbit around the Sun;

-4. The presence of gaseous atmosphere and liquid water ocean;

-5. Enough internal heat from the planet’s molten core to allow plate tectonics (which are important for maintaining the balance of the carbon cycle);

-6. Having Jupiter as a neighbour that protects us from comets and asteroids;

-7. The presence of a large moon that stabilizes tilt (keeping the seasons mild) and the tides;

-8. The relative absence of impacts from asteroids or other matter flying through space, after an initial bombardment period early in the Earth’s history;

-9. Our current position relative to the Sun, which provides us heat and energy, and

-10. The evolution of the process of photosynthesis within microbial life forms at a certain point in Earth’s history, which in turn enriched the atmosphere with oxygen, enabling life to evolve.

No other planet has this delicate balance of conditions, making Earth rare indeed. But each planet has its own set of geochemical and atmospheric conditions that make it unique. Just because we don’t see Earth-like conditions on other planets does not mean life, as we know it, cannot exist there. Understanding where life might have developed in the solar system requires comprehension of how life arose on Earth.

Discovery of river channels on Mars, possible fossil evidence of ancient microorganisms in a meteorite from Mars, hints of water ice on the Moon and Mercury, oceans on Europa and Enceladus, organic materials and an atmosphere on Titan, and cryovolcanism seen on Enceladus and Triton suggest that the solar system might not be as inhospitable to the development of life as was believed immediately following the results of the Viking Landers.

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From our experience on Earth, we know that if life once originated, it seems that it can adapt to all kinds of extreme environmental conditions as shown in Table below.

Limits for life as we know it (Cockell 1999):

Parameters	Limits for life as we know it
Temperature (◦C)	<0–113
Surface pressure (MPa)	>100
Acidity (pH)	>12
UV radiation	On early Earth 200–400nm range of the 25% less luminous Sun reached the surface
Atmospheric composition	Pure CO2 can be tolerated by some organisms. High N2 will not prevent life
Water availability, liquid H2O on the surface	Liquid H2O should be present but some halophilic organisms live in high (4–5M) NaCl

Whereas the limits of life have changed in some ways over the past few decades due to global warming, there has been a more radical change in our appreciation of where microbial ecosystems can be found. Notable examples of the discovery of unexpected microbial ecosystems include endolithic microorganisms in the Antarctic cold desert, hot deep-sea vents, cool deep-sea vents, deep in basalt, deep below the subsurface, and in an ice-covered Antarctic Lake that has been sealed for thousands of years. Several aspects of these recently discovered ecosystems are worth comment: first, the organisms found are not alien and map in expected areas of the tree of life; second, with the exception of the high-temperature vents, the organisms do not greatly extend the limits of life derived from more mundane and accessible ecosystems; third, the organisms themselves do not find these unusual environments extreme and typically are well adapted to the conditions under which they live; and fourth, the organisms in these environments do not in general control the physical environment (temperature and pressure) with their own metabolic activity but rather live in locations where the local physical conditions are suitable even when these environments are nestled within larger inhospitable areas. The lesson to be learned from these discoveries is that microbial life is extremely adept at locating places to live, and we have not been adept at anticipating how small environments can be habitable in otherwise barren locations: microbial life is more clever than we are. This is a factor that should inform our consideration of habitability of exoplanets.

The basic habitability requirements for life as we know are constrained by the following main factors:

-1. a certain time span where a celestial body can accumulate enough building blocks necessary for the origin of life,

-2. liquid water which is in contact with these building blocks,

-3. external and internal environmental conditions that allow liquid water to exist, on a celestial body over a time span necessary that allows life to evolve.

After life has originated, depending on the evolution of the planetary environment, it may evolve to form complex multi-cellular life forms or it may remain as microbial life which may adapt to extreme environmental planetary conditions.

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Evolutionary history of producing a habitable Earth:

Figure below shows origin of life timeline:

Early universe with first stars:

Soon after the Big Bang, which occurred roughly 14 Gya, the only chemical elements present in the universe were hydrogen, helium, and lithium, the three lightest atoms in the periodic table. These elements gradually came together to form stars. These early stars were massive and short-lived, producing all the heavier elements through stellar nucleosynthesis. Carbon, currently the fourth most abundant chemical element in the universe (after hydrogen, helium, and oxygen), was formed mainly in white dwarf stars, particularly those bigger than twice the mass of the sun. As these stars reached the end of their lifecycles, they ejected these heavier elements, among them carbon and oxygen, throughout the universe. These heavier elements allowed for the formation of new objects, including rocky planets and other bodies. According to the nebular hypothesis, the formation and evolution of the Solar System began 4.6 Gya with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.

Emergence of Earth:

The Earth was formed 4.54 Gya. The Hadean Earth (from its formation until 4 Gya) was at first inhospitable to any living organisms. During its formation, the Earth lost a significant part of its initial mass, and consequentially lacked the gravity to hold molecular hydrogen and the bulk of the original inert gases. The atmosphere consisted largely of water vapor, nitrogen, and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about 5.5. The Hadean atmosphere has been characterized as a “gigantic, productive outdoor chemical laboratory,” similar to volcanic gases today which still support some abiotic chemistry.

Oceans may have appeared as soon as 200 million years after the Earth formed, in a near-boiling (100 deg C) reducing environment, as the pH of 5.8 rose rapidly toward neutral. This scenario has found support from the dating of 4.404 Gya zircon crystals from metamorphosed quartzite of Mount Narryer in Western Australia. Despite the likely increased volcanism, the Earth may have been a water world between 4.4 and 4.3 Gya, with little if any continental crust, a turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun, from cosmic radiation, and from continued asteroid and comet impacts.

The Late Heavy Bombardment hypothesis posits that the Hadean environment between 4.28 and 3.8 Gya was highly hazardous to life. Following the Nice model, changes in the orbits of the giant planets may have bombarded the Earth with asteroids and comets that pockmarked the Moon and inner planets. Frequent collisions would have made photosynthesis unviable. The periods between such devastating events give time windows for the possible origin of life in early environments. If the deep marine hydrothermal setting was the site for the origin of life, then abiogenesis could have happened as early as 4.0-4.2 Gya. If the site was at the surface of the Earth, abiogenesis could have occurred only between 3.7 and 4.0 Gya. However, new lunar surveys and samples have led scientists, including an architect of the Nice model, to deemphasize the significance of the Late Heavy Bombardment.

If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from late impacts and the then high levels of ultraviolet radiation from the sun. Geothermically heated oceanic crust could have yielded far more organic compounds through deep hydrothermal vents than the Miller–Urey experiments indicated. The available energy is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live. These modern organisms may be among the closest surviving relatives of the LUCA.

Earliest evidence of life:

Life existed on Earth more than 3.5 Gya, during the Eoarchean when sufficient crust had solidified following the molten Hadean. The earliest physical evidence of life so far found consists of microfossils in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation rocks at least 3.77 and possibly 4.28 Gya. The micro-organisms lived within hydrothermal vent precipitates, soon after the 4.4 Gya formation of oceans during the Hadean. The microbes resembled modern hydrothermal vent bacteria, supporting the view that abiogenesis began in such an environment.

Biogenic graphite has been found in 3.7 Gya metasedimentary rocks from southwestern Greenland and in microbial mat fossils from 3.49 Gya Western Australian sandstone. Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya, have shown biogenic carbon isotopes. In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 Gya. In the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing sandstone in a fossilized beach, with rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen. Zircons from Western Australia imply that life existed on Earth at least 4.1 Gya. The Pilbara region of Western Australia contains the Dresser Formation with rocks 3.48 Gya, including layered structures called stromatolites. Their modern counterparts are created by photosynthetic micro-organisms including cyanobacteria. These lie within undeformed hydrothermal-sedimentary strata; their texture indicates a biogenic origin. Parts of the Dresser formation preserve hot springs on land, but other regions seem to have been shallow seas. A molecular clock analysis suggests the LUCA emerged prior to the Late Heavy Bombardment (3.9 Gya).

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How did life originate on earth:

The origin of life has been one of humanity’s most compelling enquiries since the cradle of civilization. Innumerable creation myths have tried to shed light on this essential issue without the limitations that a scientific approach to the issue would entail. Until the 19th century, the theory of spontaneous generation was widely accepted, since it was the most comprehensive way to conceive how maggots in rotting meat, or mice in grain, could appear from apparently thin air. Using the scientific method, Louis Pasteur disproved this theory by showing that small organisms (later known as microorganisms) are ubiquitous and cannot emerge in strictly isolated sterile organic media. We should note that inorganic media— including minerals—were not mentioned (Pasteur 1862; Leduc 1911; Ligon 2002). Yet, Pasteur’s findings did not go unchallenged (Strick 1988). Darwin, for example, had been convinced that “the intimate relation of Life with laws of chemical combination, and the universality of latter render spontaneous generation not improbable” (Peretó et al. 2009). In turn, Darwin continued to distance himself from the view and Pasteur himself was said to have second thoughts towards the end of his life (Strick 1988). It is significant to record that the views of Pasteur and his acolytes had a negative effect on mineral-based hypotheses. Moreover, notwithstanding Goldschmidt’s posthumous publication (Goldschmidt 1952), no other significant mineral-based hypothesis was proposed until Graham Cairns-Smith—partly influenced by Bernal’s focus upon the likely significance of clays and mineral surfaces to the emergence of life (Bernal 1949)—published his ‘Genetic takeover: and the mineral origins of life’ (Cairns-Smith 1982). Indeed, Leduc’s lonely plea that: “Without the idea of spontaneous generation and a physical theory of life, the doctrine of evolution is a mutilated hypothesis without unity or cohesion” went largely unheard until the present century when it has been rejuvenated under the rubric of “chemobrionics” (Barge et al. 2015). Last century’s thought was dominated by the organic soup hypothesis of Haldane, Oparin, Miller, and Orgel, which gave birth to the RNA world hypothesis—a common view to this day.

Abiogenesis:

In biology, abiogenesis is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes as seen in the figure below. Many proposals have been made for different stages of the process.

Figure above shows stages in the origin of life range from the well-understood, such as the habitable Earth and the abiotic synthesis of simple molecules, to the largely unknown, like the derivation of the last universal common ancestor (LUCA) with its complex molecular functionalities.

Trying to figure out how biology emerged on the planet we know best is the province of “origin of life” studies. There are two main hypotheses for how clumps of chemistry became lumps of biology—a process called abiogenesis. One holds that RNA arose able to make more of itself, because that’s what it does, and that it could also catalyze other chemical reactions. Over time that replication led to beings whose makeup relied on that genetic code. The “metabolism-first” framework, on the other hand, posits that chemical reactions organized in a self-sustaining way. Those compound communities and their chemical reactions grew more complex and eventually spit out genetic code.

Figure below shows how the steps to get from simple molecules to life are ordered differently in the “genetics first” and “metabolism first” hypotheses of abiogenesis.

Those two main hypotheses aren’t mutually exclusive but we don’t understand how biology got started.

The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from biology and chemistry, with more recent approaches attempting a synthesis of many sciences. Life functions through the specialized chemistry of carbon and water, and builds largely upon four key families of chemicals: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and nucleic acid DNA and RNA for the mechanisms of heredity. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world, although other self-replicating molecules may have preceded RNA. RNA is basic, DNA is sophisticated. It took a long time for life on earth to switch from a purely RNA based genome to the more stable DNA. RNA is still essential for life processes. It is clear that RNA, or other coding polymers, were of central importance for the emergence of life. Despite this, the idea that a self-replicating polymer can, on its own, “invent” even a simple network of energy-dissipating reactions (i.e. metabolism) represents a substantial leap of faith with little evidence to back it up.

The classic 1952 Miller–Urey experiment demonstrated that most amino acids, the chemical constituents of proteins, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. External sources of energy may have triggered these reactions, including lightning, radiation, atmospheric entries of micro-meteorites and implosion of bubbles in sea and ocean waves. Other approaches (“metabolism-first” hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.

A genomics approach has sought to characterise the last universal common ancestor (LUCA) of modern organisms by identifying the genes shared by Archaea and Bacteria, members of the two major branches of life (where the Eukaryotes belong to the archaean branch in the two-domain system). 355 genes appear to be common to all life; their nature implies that the LUCA was anaerobic with the Wood–Ljungdahl pathway, deriving energy by chemiosmosis, and maintaining its hereditary material with DNA, the genetic code, and ribosomes. Although the LUCA lived over 4 billion years ago (4 Gya), researchers do not believe it was the first form of life. Earlier cells might have had a leaky membrane and been powered by a naturally occurring proton gradient near a deep-sea white smoker hydrothermal vent.

Earth remains the only place in the universe known to harbor life, and fossil evidence from the Earth informs most studies of abiogenesis. The Earth was formed 4.54 Gya; the earliest undisputed evidence of life on Earth dates from at least 3.5 Gya. Fossil micro-organisms appear to have lived within hydrothermal vent precipitates dated 3.77 to 4.28 Gya from Quebec, soon after ocean formation 4.4 Gya during the Hadean.

Fresh clues hint at how the first living organisms arose from inanimate matter:

Researchers have found a way that the genetic molecule RNA could have formed from chemicals present on the early earth.
Other studies have supported the hypothesis that primitive cells containing molecules similar to RNA could assemble spontaneously, reproduce and evolve, giving rise to all life.
Scientists are now aiming at creating fully self-replicating artificial organisms in the laboratory—essentially giving life a second start to understand how it could have started the first time.

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Why does Earth have an atmosphere?

In short, our atmosphere is here because of gravity. When Earth formed, about 4.5 billion years ago, the molten planet barely had an atmosphere. But as the world cooled, its atmosphere formed, largely from gases spewed out of volcanoes, according to the Smithsonian Environmental Research Center (SERC). This ancient atmosphere was very different from today’s; it had hydrogen sulfide, methane and 10 to 200 times as much carbon dioxide as the modern atmosphere does, according to SERC. The Earth started out with an atmosphere a bit like [that of] Venus, with nitrogen, carbon dioxide, maybe methane. Life then began somehow, almost certainly in the bottom of an ocean somewhere. After around 3 billion years, the photosynthetic system evolved, meaning that single-celled organisms used the sun’s energy to turn molecules of carbon dioxide and water into sugar and oxygen gas. This dramatically increased oxygen levels.

Nowadays, Earth’s atmosphere consists of approximately 80 percent nitrogen and 20 percent oxygen. That atmosphere is also home to argon, carbon dioxide, water vapor and numerous other gases, according to the National Center for Atmospheric Research (NCAR). It’s a good thing these gases are there. Our atmosphere protects the Earth from the harsh rays of the sun and reduces temperature extremes, acting like a duvet wrapped around the planet. Meanwhile, the greenhouse effect means that energy from the sun that reaches Earth gets waylaid in the atmosphere, absorbed and released by greenhouse gases, according to the NCAR. There are several different types of greenhouse gases; the major ones are carbon dioxide, water vapor, methane and nitrous oxide. Without the greenhouse effect, Earth’s temperature would be below freezing. However, today, greenhouse gases are out of control. As humans release more carbon dioxide into the atmosphere, Earth’s greenhouse effect gets stronger, according to NCAR. In turn, the planet’s climate gets warmer.

Intriguingly, no other planet in the universe has an atmosphere like Earth’s. Mars and Venus have atmospheres, but they cannot support life (or, at least, not Earth-like life), because they don’t have enough oxygen. Indeed, Venus’ atmosphere is mainly carbon dioxide with clouds of sulfuric acid, the ‘air’ is so thick and hot that no human could breathe there. According to NASA, the thick carbon dioxide atmosphere of Venus traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system. Surface temperatures there are hot enough to melt lead. The fact that Earth has an atmosphere is extremely unusual in respect of the planets in the solar system, in that it’s very different from any of the other planets. For example, the pressure of Venus is about 90 atmospheres, the equivalent to diving 3,000 feet (914 meters) beneath the ocean on Earth. The original Russian spaceships that went there [to Venus] just recorded for a few seconds and then got crushed. Nobody ever really understood how hot it was.

So, Earth’s atmosphere is life — and without it, life as we know it wouldn’t exist. Earth needed the right atmosphere [for life] to get started. It has created that atmosphere, and it has created circumstances to live in that atmosphere. The atmosphere is a totally integral part of the biological system.

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Why is there water on Earth?

Water is essential to life as we know it and it seems completely normal to have water all around us. Yet Earth is the only known planet to be covered by oceans. Do we know exactly where its water came from?

This is not a simple question: it was long thought that Earth formed dry – without water, because of its proximity to the Sun and the high temperatures when the solar system formed. In this model, water could have been brought to Earth by comets or asteroids colliding with the Earth. Such a complex origin for water would likely mean that our planet is unique in the universe.

However, in a 2020 study, it was shown that water – or at least its components, hydrogen and oxygen – may have been present in the rocks that initially formed the Earth. If that is so indeed, other “blue planets” with liquid water are more likely to exist elsewhere. This does not tell us when the oceans appeared on Earth’s surface, but we now know that Earth’s water was not necessarily delivered by hydrated bodies that formed very far from the Sun. However, we do not yet understand in what form(s) and by what process hydrogen was incorporated and stored in rocks of the inner solar system. The presence of hydrogen in inner solar system rocks is particularly important because it could have been a water source for the other rocky planets (Mercury, Venus, and Mars). Similar rocks could then represent a source of water for planets orbiting other suns, a condition to develop life, at least life as we know it.

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Panspermia:

This idea dates back to Anaxagoras in the 5th century BC is panspermia, the idea that life exists throughout the universe, distributed by meteoroids, asteroids, comets and planetoids. It does not attempt to explain how life originated in itself, but shifts the origin of life on Earth to another heavenly body. The advantage is that life is not required to have formed on each planet it occurs on, but rather in a more limited set of locations (potentially even a single location), and then spread about the galaxy to other star systems via cometary or meteorite impact.

Panspermia is the hypothesis that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraft carrying unintended contamination by microorganisms. Panspermia is a fringe theory with little support amongst mainstream scientists. Critics argue that it does not answer the question of the origin of life but merely places it on another celestial body. It is also criticized because it cannot be tested experimentally. Panspermia proposes that microscopic lifeforms which can survive the effects of space (such as extremophiles), can become trapped in debris ejected into space after collisions between planets and small Solar System bodies that harbor life. Panspermia studies concentrate not on how life began but on methods that may distribute it in the Universe.

Panspermia requires:

-1. that organic molecules originated in space (perhaps to be distributed to Earth)

-2. that life originated from these molecules, extraterrestrially

-3. that this extraterrestrial life was transported to Earth.

Researchers just announced the discovery of the nucleobase uracil—one of the building blocks of RNA—in a sample from the asteroid Ryugu. This isn’t the first time a nucleobase has been discovered in a sample of a space object, but it’s the first time we’ve been able to rule out contamination from Earth as an explanation for its presence. The creation and distribution of organic molecules from space is now uncontroversial; it is known as pseudo-panspermia. Pseudo-panspermia (sometimes called soft panspermia or molecular panspermia) is the well-attested hypothesis that many of the pre-biotic organic building-blocks of life originated in space, became incorporated in the solar nebula from which planets condensed, and were further—and continuously—distributed to planetary surfaces where life then emerged. The existence of extraterrestrial life is unconfirmed but scientifically possible.

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Carbon-based lifeforms:

Most life forms do not use all the elements in the periodic table. In fact, an organism may incorporate just a handful of elements in large quantities. In nature, the predominant chemical elements are known as CHNOPS, or Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulphur. These are the most common elements found in all organic molecules on Earth. They are often referred to as the ingredients of life, as their presence is found from the largest mammals to the smallest paramecium.

All life on earth is made from the commonest elements in the universe, including carbon, and compounds made with oxygen, nitrogen, and hydrogen. These are common because many stars are, and have been, of a size sufficiently large to synthesise those elements (our sun gets up to the first two from hydrogen- helium and some lithium). Having gone through their cycle from formation to explosion, they have distributed these very reactive elements through their vicinity, and when gathered up in our own solar system, they have become the building blocks of all life, from the simplest to the most complex.

Carbon makes more compounds than all the others put together- about 10 million. The enormous diversity of carbon-containing compounds, known as organic compounds, has led to a distinction between them and compounds that do not contain carbon, known as inorganic compounds. The branch of chemistry that studies organic compounds is known as organic chemistry. All life forms therefore require constant infusions of carbon in many forms- in sugars, starches, proteins, in order to grow and live for their life span. Some of those necessities can be produced in our bodies, most have to be taken from other sources- but they are all organic/carbon-based compounds.

Carbon is the 15th most abundant element in the Earth’s crust, and the fourth most abundant element in the universe by mass, after hydrogen, helium, and oxygen. Carbon’s widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms. In a 2018 study, carbon was found to compose approximately 550 billion tons of all life on Earth. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen.

The most important characteristics of carbon as a basis for the chemistry of life are that each carbon atom is capable of forming up to four valence bonds with other atoms simultaneously, and that the energy required to make or break a bond with a carbon atom is at an appropriate level for building large and complex molecules which may be both stable and reactive. Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long macromolecules and polymers in a process known as catenation. “What we normally think of as ‘life’ is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus”, as per Stephen Hawking in a 2008 lecture, “carbon […] has the richest chemistry.”

It would be impossible for life on earth to exist without carbon. Carbon is the main component of sugars, proteins, fats, DNA, muscle tissue, pretty much everything in your body. The reason carbon is so special is down to the electron configuration of the individual atoms. Electrons exist in concentric ‘shells’ around the central nucleus and carbon has four electrons in its outermost shell. Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule. As the most stable thing for an atom to have is eight electrons, this means that each carbon can form four bonds with surrounding atoms by the sharing of two electrons; one from the carbon and one from surrounding atom. The ability to form four bonds isn’t restricted to carbon though, it’s a property of every atom with four outer electrons, including silicon, tin and lead. What’s special about carbon, and the reason that silicon-based lifeforms are restricted to science fiction (and lead-based lifeforms are hardly ever mentioned) is that it can form double-bonds which share more than one electron with another atom. Why is carbon able to do this while silicon can’t? The answer lies in the size. Carbon is the smallest of all the atoms with four outermost electrons, which means that the electrons in the above-and-below orbitals are close enough to overlap and form that second bond. For silicon however, there are more electron orbitals in the way, the entire atom is bigger, and it is almost impossible for the outer orbitals to get close enough to form a double bond. This is why carbon dioxide is a small gaseous molecule consisting of two oxygens both forming a double bond with a single carbon while silicon dioxide is a massive behemoth of a molecule made of huge numbers of alternating oxygen and silicon atoms and is more commonly known as sand. Note that an orbital is a region of space where there is a high probability of finding an electron.

You can just about get silicon-silicon double bonds if you try hard, but they are fairly unstable and will take any chance they can to lose that double-bond in favour of forming another single one. Carbon-carbon double bonds on the other hand form naturally and easily, and are crucial for every living organism on earth. If there were to be silicon-based lifeforms, the sheer chemistry of their atoms means that they would have to be built along very different lines to life on earth.

Figure below shows Elements that make up Human Body:

Oxygen makes up around 65% of the total mass of a human. The second most common elements are carbon and hydrogen, making up around 18% and 10% of the human body by mass respectively. Since our bodies are 60% water, it is easy to see why oxygen and hydrogen are in abundance. Carbon is found in organic compounds in the body such as fats, carbohydrates, proteins, and nucleic acids. Hydrogen is found in water and in many organic compounds. Similarly, nitrogen is found in nucleic acids and proteins. In human DNA, nitrogen forms a key component of the genetic code. Phosphorus is found in the molecule ATP, which is the primary energy carrier of the body. It is also found in the human bones. Calcium makes up around 1.5% of the human body by mass and is abundant in human bones, proteins, and muscles.

Can forms of life be based on something else other than carbon?

We have discussed a lot about carbon and how its presence in organic compounds led to the term “carbon-based life.” Carbon is regarded as the building block of life, but this notion leads to something referred to as Carbon Chauvinism. This is a belief that carbon-based life applies to the whole universe. In other words, if aliens exist, they would also be based on carbon.

However, there is an element other than carbon that can sustain similar types of bonds. It is situated right below carbon in the periodic table – silicon. Silicon is also capable of forming four covalent bonds, just like carbon, and this element has been used as a building block of extra-terrestrial life in many science fiction creations. Similar to carbon and oxygen, silicon is abundant on Earth. Most of us have interacted with the oxidized form of silicon, called silica, at some point in our lives in the form of sand.

When carbon oxidizes, it turns into carbon dioxide gas. Our body oxidizes carbon to produce energy, giving off carbon dioxide as a waste product. However, when silicon is oxidized, it turns into a solid – sand. Silicon bonds are also more unstable than carbon bonds. Another reason why there are no silicon-based organisms is that silicon cannot use water as a solvent in the same way that carbon can. It would require a completely different solvent, such as methane, which is not stable in normal conditions.

Life on Earth is capable of chemically manipulating silicon. For example, microscopic particles of silicon dioxide called phytoliths are found in some plants, and a type of photosynthetic algae called diatoms incorporates silicon dioxide in their skeletons. However, there are no known natural instances of life on Earth combining silicon and carbon together into molecules.

Researchers have succeeded in creating synthesized molecules made up of both silicon and carbon. These compounds are used in products such as pharmaceuticals, adhesives, paints, and fungicides. Scientists have also recently found a way to use microbes to chemically bond carbon and silicon together.

Even though silicon-based life on Earth is not probable, we cannot rule out the fact that it might not be the case on other planets, where the atmospheric conditions are much different than that of earth.

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Science fiction has popularized the idea of silicon-based life, since silicon resides in the same group as carbon in the periodic table. This analogy breaks down when one examines the details, however. The chemical bond of silicon dioxide is too strong, while the silicon–silicon bond is too weak. Silicon dioxide (quartz) is an overly stable sink of silicon and is insoluble in water. These properties prevent silicon from forming a variety of complex molecules as carbon does. This expectation is consistent with what astronomers find when pointing their telescopes at seemingly uninteresting parts of space: an abundance of organic molecules, ranging from methanol and glycine (an amino acid) to fullerene (the “buckyball” with 60 carbon atoms). The building blocks of life, as we understand them on Earth, are commonly found elsewhere in the cosmos—preassembled. Darwin’s “warm little pond” idea of organic molecules forming in a primeval soup on Earth may need rethinking.

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Non-chirality of silicon compounds:

Silicon compounds offer little “handedness.” Consider your right and left hands. In an ideal world, both have the same composition and structure, but they cannot be converted from one to the other. They are mirror images, or in more technical terms, have a definite chirality (or handedness). A large number of the compounds formed by carbon come in mirror-image copies, but that is not true for silicon compounds. Many of the important biological molecules have a definite handedness. Almost all amino acids are left-handed, where all the sugars tend to be right-handed. This handedness plays a critical role in how these molecules react with other substances. For example, one handedness of naproxen helps relieve arthritis pain, while the opposite handedness causes liver poisoning (while not relieving any pain)! From a biochemical perspective, the functioning of life seems to depend on organic molecules having a specific handedness. These large, complicated molecules do their job with great precision only because they have a property called “handedness.” When any one enzyme “mates” with compounds it is helping to react, the two molecular shapes fit together like a lock and key, or a shake of hands. In fact, many carbon-based molecules take advantage of right and left-hand forms. For instance, nature chose the same stable six-carbon carbohydrate to store energy both in our livers (in the form of the polymer called glycogen) and in trees (in the form of the polymer cellulose). Glycogen and cellulose differ mainly in the handedness of a single carbon atom, which forms when the carbohydrate polymerizes, or forms a chain. Cellulose has the most stable form of the two possibilities; glycogen is the next most stable. Because humans don’t have enzymes to break cellulose down into its basic carbohydrate, we cannot utilize it as food. But many lower life-forms, such as bacteria, can.

In short, handedness is the characteristic that provides a variety of biomolecules with their ability to recognize and regulate sundry biological processes. And silicon doesn’t form many compounds having handedness. Thus, it would be difficult for a silicon-based life-form to achieve all of the wonderful regulating and recognition functions that carbon-based enzymes perform for us.

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Section-4

Introduction to extraterrestrial life (ETL):

In the shadowy expanse of the cosmos, a question lingers—echoing across time and space—do we share this universe with others, or are we solitary inhabitants of a cosmic stage?

Extraterrestrial life (ETL) is the term used to define any form of life that may exist and originate outside the planet Earth, the only place in the universe known to support life. Its existence is currently hypothetical; there is no evidence of extraterrestrial life that has been widely accepted by the scientific community. The putative study and theorization of ETL is known as astrobiology or exobiology, and the term “exopolitics” may be used to denote the study of political relations between humanity and extraterrestrial civilizations. If extraterrestrial life exists, it could range from simple microorganisms and multicellular organisms similar to animals or plants, to complex alien intelligences akin to humans. When scientists talk about extraterrestrial life, they consider all those types. Although it is possible that extraterrestrial life may have other configurations, scientists use the hierarchy of lifeforms from Earth for simplicity, as it is the only one known to exist. Life on Earth is quite ubiquitous across the planet and has adapted over time to almost all the available environments in it, even the most hostile ones. As a result, it is inferred that life in other celestial bodies may be equally adaptive. Extraterrestrial life forms, especially intelligent ones, are often referred to in popular culture as “aliens” or “ETs.” The hypothesis that life exists on worlds other than our own is found in human culture from the Ancient Age until our day. There is no doubt that to find forms of life on other planets and, above all, to communicate with extraterrestrial intelligence would represent one of the most extraordinary experiences in all of human history.

Extraterrestrial Intelligence is intelligent life that developed somewhere other than the earth. Such life has not yet been discovered. However, scientific research, including astronomy, biology, planetary science and studies of fossils here on earth have led many scientists to conclude that such life may exist on planets orbiting at least some of the hundreds of billions of stars in our Milky Way Galaxy. Today, some researchers are trying to find evidence for extraterrestrial intelligence. This effort is often called SETI, which stands for Search for Extraterrestrial Intelligence. SETI researchers decided that looking for evidence of their technology might be the best way to discover other intelligent life in the Galaxy. They decided to use large radio telescopes to search the sky over a wide range of radio frequencies.

Though schools of thought in antiquity differed on whether extraterrestrial life existed, by the Middle Ages, the Aristotelian worldview of a unified, finite cosmos without extraterrestrials was most influential, though there were such dissenters as Nicholas of Cusa. That would change as the Copernican revolution progressed. Scholars such as Bruno, Kepler, Galileo, and Descartes would argue for a Copernican system of a moving Earth. Cartesian and Newtonian physics would eventually lead to a view of the universe in which the Earth was one of many planets in one of many solar systems extended in space. As this cosmological model was developing, so too were notions of extraterrestrial life.

According to Big Bang interpretations, the universe as a whole was initially too hot to allow life. 15 million years later, it cooled to temperate levels, but the elements that make up living things did not exist yet. The only freely available elements at that point were hydrogen and helium. Carbon and oxygen (and later, water) would not appear until 50 million years later, created through stellar fusion. At that point, the difficulty for life to appear was not the temperature, but the scarcity of free heavy elements. Planetary systems emerged, and the first organic compounds may have formed in the protoplanetary disk of dust grains that would eventually create rocky planets like Earth. Although Earth was in a molten state after its birth and may have burned any organics that fell in it, it would have been more receptive once it cooled down. Once the right conditions on Earth were met, life started by a chemical process known as abiogenesis. Alternatively, life may have formed less frequently, then spread – by meteoroids, for example – between habitable planets in a process called panspermia.

There is an area around a star, the circumstellar habitable zone or “Goldilocks zone”, where water may be at the right temperature to exist in liquid form at a planetary surface. This area is neither too close to the star, where water would become steam, nor too far away, where water would be frozen as a rock. However, although useful as an approximation, planetary habitability is complex and defined by several factors. Being in the habitable zone is not enough for a planet to be habitable, not even to actually have such liquid water. Venus is located in the habitable zone of the Solar System but does not have liquid water because of the conditions of its atmosphere. Jovian planets or Gas Giants are not considered habitable even if they orbit close enough to their stars as hot Jupiters, due to crushing atmospheric pressures. The actual distances for the habitable zones vary according to the type of star, and even the solar activity of each specific star influences the local habitability. The type of star also defines the time the habitable zone will exist, as its presence and limits will change along with the star’s stellar evolution.

The search for life beyond the Solar System is a significant motivator for the detection and characterization of extrasolar planets around nearby stars. We are poised at the transition between exoplanet detection and demographic studies and the detailed characterization of exoplanet atmospheres and surfaces. Transit and radial velocity surveys have confirmed the existence of thousands of exoplanets (Akeson et al., 2013; Batalha, 2014; Morton et al., 2016) with well over a dozen located within the circumstellar habitable zones (HZs) of their host stars (e.g., Kane et al., 2016). Planets with masses and radii consistent with rocky compositions and likely to contain secondary, volcanically outgassed atmospheres have been found in nearby stellar systems (Berta-Thompson et al., 2015; Wright et al., 2016), some of which reside in the HZ of their host star such as Proxima Centauri b (Anglada-Escudé et al., 2016); TRAPPIST-1 e, f, and g (Gillon et al., 2017); and LHS 1140b (Dittmann et al., 2017).

Those planets that transit their stars are excellent candidates for atmospheric characterization through transmission spectroscopy with James Webb Space Telescope (JWST). Planets with sufficient planet–star separations will likewise be excellent targets for direct-imaging spectroscopy. Space-based telescope missions with the capability of measuring directly imaged spectra of potentially habitable exoplanets are in their science-definition stages (e.g., Dalcanton et al., 2015; Mennesson et al., 2016). Ground-based observers are also devising instrumentation and techniques for current and future observatories that will have the capacity to image Earth-sized planets around nearby stars (Kawahara et al., 2012; Snellen et al., 2013, 2015; Lovis et al., 2016).

For exoplanets – planets around other stars – the era opens with NASA’s James Webb Space Telescope. Instruments aboard the spacecraft are detecting the composition of atmospheres on exoplanets. The James Webb Space Telescope, launched in 2021, could get the first glimpses: the mix of gases in the atmospheres of Earth-sized exoplanets. Webb, or a similar spacecraft in the future, could pick up signs of an atmosphere like our own – oxygen, carbon dioxide, methane. A strong indication of possible life. Future telescopes might even pick up signs of photosynthesis – the transformation of light into chemical energy by plants – or even gases or molecules suggesting the presence of animal life. Intelligent, technological life might create atmospheric pollution, as it does on our planet, also detectable from afar. Of course, the best we might be able to manage is an estimate of probability. Still, an exoplanet with, say, a 95 percent probability of life would be a game changer of historic proportions. As the power of telescopes increases in the years ahead, future advanced instruments could capture possible signs of life – “biosignatures” – from a planet light-years away. Within our solar system, the Perseverance rover on Mars is gathering rock samples for eventual return to Earth, so scientists can probe them for signs of life. And the coming Europa Clipper mission will visit an icy moon of Jupiter. Its goal: to determine whether conditions on that moon would allow life to thrive in its global ocean, buried beneath a global ice shell.

Many astronomers are no longer asking whether there is life elsewhere in the Universe. The question on their minds is instead: when will we find it? Many are optimistic of detecting life signs on a faraway world within our lifetimes – possibly in the next few years. And one scientist, leading a mission to Jupiter, goes as far as saying it would be “surprising” if there was no life on one of the planet’s icy moons. Nasa’s James Webb Space Telescope (JWST) recently detected tantalising hints at life on a planet outside our Solar System – and it has many more worlds in its sights. Numerous missions that are either under way or about to begin mark a new space race for the biggest scientific discovery of all time. “We live in an infinite Universe, with infinite stars and planets. And it’s been obvious to many of us that we can’t be the only intelligent life out there,” says Prof Catherine Heymans, Scotland’s Astronomer Royal. We now have the technology and the capability to answer the question of whether we are alone in the cosmos.

About two-thirds of Americans (65%) say their best guess is that intelligent life exists on other planets, according to a Pew Research Center survey. Most Americans say intelligent life exists outside Earth and don’t see UFOs as a major security threat. NASA, to a big disappointment for the UFO enthusiasts, have categorically denied having evidence of the presence of life beyond Earth. “No life beyond Earth has ever been found; there is no evidence that alien life has ever visited our planet,” a blog on NASA’s website reads. However, to investigate signs of life on other space bodies, as one of the US agency’s key goals is the search for life in the universe, it has an astrobiology programme. It uses missions such as the Transiting Exoplanet Survey Satellite (TESS), Hubble Space Telescope, James Webb Space Telescope to spot evidence of life in space. Recently NASA commissioned an independent team to examine unidentified anomalous phenomena (UAP) – observations of events in the sky that cannot be identified as aircraft or as known natural phenomena. The Pentagon too has opened an office called the All-domain Anomaly Resolution Office to investigate around 510 reports of UFOs.

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A Brief Glance at History:

The historical debate about a possible plurality of inhabited worlds has been widely documented (cf. Crowe, 1988; Dick, 1982 and 1996; review articles by Crowe, 1997, and Dick, 1993). In the Ancient Age the atomists were probably the first to hypothesize the existence of extraterrestrial life. Their mechanistic philosophy assigned to the infinite number of atoms in the cosmos the capacity to give rise to an infinite number of bodies in a multitude of possible combinations, hence also beyond the Earth. Epicurus (341-270 B.C.), and then more importantly Lucretius (99-55 B.C.), affirmed a kind of “Principle of Plenitude,” according to which all the potentialities of matter were destined to be realized sooner or later, in this way giving rise to a world the perfection of which would be proportional to the richness of the existence it contained. The question regarding possible inhabitants of the moon —a question intuitive and spontaneous given the proximity and the large apparent size of our satellite —appears in the works of various classical authors, including Plutarch (45-125 C.E.). Within his De facie quae in orbe lunae apparet, a small treatise of philosophical cosmology on the difference between the properties of the Earth and the moon, the Greek writer presents a debate about the origin of the shadowy spots apparent on the lunar surface.

In the Middle Ages, Christianity was not opposed to the idea that God could have created other worlds, even ones more perfect than our own, but the theme did not directly concern the possibility of their being inhabited. In the cosmology of De docta ignorantia, Nicholas of Cusa (1401-1464) alludes to possible inhabitants of other worlds (which he naïvely placed on the stars). He also tried to systematize from a philosophical point of view the relations such worlds would have with the Earth and its perfections, as well as that between the nature of their inhabitants and our intellectual nature. In a reflection shared by many of our contemporaries, the Cardinal-philosopher concluded that we simply cannot know anything about such comparisons: “The inhabitants of other stars, wherever they are, do not have any proportion with the inhabitants of our world, also if their whole region is in a concealed proportion with our own, for the finality of the universe […]. But, since this region remains unknown to us, also its inhabitants remain completely unknown to us.” (Book. II, ch. 12). Giordano Bruno (1548-1600), the renaissance interpreter of the “Principle of Plenitude,” hypothesized the presence of life diffused throughout the whole universe, not only in the form of inhabited stars and planets, but also as a vital principle able to provide a soul to the stars, planets, comets, and indeed to the whole universe. Galileo (1564-1642) and Kepler (1571-1630) never addressed the theme directly, but understood that the heliocentric system placed the Earth in a condition of greater similarity with other solar planets. As had Plutarch and not without irony, both of them asked themselves whether the visible and regular spots on the surface of the moon could have been the work of intelligent inhabitants (cf. C. Sinigaglia, Lo “scherzo” di Plutarco e il “sogno” di Keplero, in Colombo et al., 1999, pp. 155-168).

By the middle of the 17th century, thanks to the use of the optical telescope as a scientific instrument for astronomical observation, an immense number of stars invisible to the naked eye were now revealed, and thus interest in the theme of life in the universe experienced a rebirth. The rapid diffusion of works in favor of a plurality of inhabited worlds stands as proof. For example, the work of Bernard le Bovier de Fontenelle (1657-1757), Entretiens sur la pluralité des mondes (1686), appeared in dozens of editions and translations, and the posthumous work of Christian Huygens (1629-1695), Kosmotheoros, sive de terris coelestibus earumque ornatu conjecturae (1698), was quickly translated into five languages.

The progressive widening of horizons caused by the scientific observation of the cosmos stimulated astronomers to publish works concerning the possibility of forms of life beyond the confines of the Earth. Initially William Herschel (1738-1822), well-known for his studies on the spatial distribution of stars aimed at drawing the overall structure of our Milky Way, then Richard Proctor (Other Worlds Than Ours: The Plurality of Worlds Studied under the Light of Recent Scientific Researches, 1871), and above all Camille Flammarion (La pluralité des mondes habités, 1862), contributed to the debate within the scientific world throughout the 19th Century. The work of the French astronomer experienced an extraordinary diffusion, with over 30 editions in fewer than twenty years and in print without interruption until 1921. It was again an astronomer, the Italian Giovanni Schiaparelli (1835-1910), who provoked interest in the possibility of intelligent life on the planet Mars with his famous observations of “channels” on the red planet’s surface, regular structures to which attention had earlier been drawn by Angelo Secchi (1818-1878), a Jesuit astronomer. The writings of Schiaparelli on the planet Mars, (re-edited in Italian with the title La vita sul pianeta Marte: tre scritti su Marte e i marziani, Milano, 1998), together with those of Proctor and Flammarion, brought about a cultural phenomenon that ended up generically identifying inhabitants of other worlds with the term “Martian.” The position of a non-astronomer, Alfred R. Wallace (1823-1913), a naturalist and an original supporter with Darwin of the theory of evolution by natural selection, must also be recalled as part of the debate between the 19th and 20th centuries. In his work, Man’s Place in the Universe: A Study of the Results of Scientific Research in Relation to the Unity or Plurality of Worlds (1903). Wallace prepared a vigorous defense of an anthropocentric universe, in open disagreement with the pluralist position. This essay, which enjoyed wide diffusion due to the scientific environment where it originated, provided a number of arguments in defense of the uniqueness of human life within the cosmos.

From the middle of the 20th century, the progress of radio-astronomy and the initiation of space research, together with physical images of a universe of unsuspected dimensions of space and time, offered a vision of man’s place in the cosmos that logically raised the question of the possibility of extraterrestrial intelligence. Works by scientists such as H. Shapley, Of Stars and Men (Boston, 1958), and of Shklovskii and Sagan, Intelligent Life in the Universe (San Francisco, 1966), exercised great influence. General interest in the theme, however, has been sustained above all through other phenomena, such as science-fiction literature and the cinema. In the more narrowly scientific realm, the 19th century’s enthusiasm for a possible “close encounter” with other inhabitants of the solar system has been replaced by the methodical research for elementary life forms or pre-biotic material in environments similar to our solar system, not to mention the initiation of long-term programs in radio-astronomic exploration of more remote environments. At the same time, the opportunity was not lost to send “messages in a bottle,” such as the plate with an image of a human couple and some coded scientific data placed on the automatic probes Pioneer 10 and 11 (launched in 1971), the first to venture outside the solar system; digitalized images and sounds of planet Earth on the similar Voyager probes (1977); and a radio transmission in binary code sent towards the galactic globular cluster M13 by the Arecibo radio telescope (1974).

The enthusiasm toward the possibility of alien life continued well into the 20th century. Indeed, the roughly three centuries from the Scientific Revolution through the beginning of the modern era of solar system probes were essentially the zenith for belief in extraterrestrials in the West. Many astronomers and other secular thinkers, at least some religious thinkers, and much of the general public were largely satisfied that aliens were a reality. This trend was finally tempered as actual probes visited potential alien abodes in the solar system. The moon was decisively ruled out as a possibility while Venus and Mars, long the two main candidates for extraterrestrials, showed no obvious evidence of current life. The other large moons of our system which have been visited appear similarly lifeless, though the interesting geothermic forces observed (Io’s volcanism, Europa’s ocean, Titan’s thick atmosphere) have underscored how broad the range of potentially habitable environments may be. Although the hypothesis of a deliberate cosmic silence of advanced extraterrestrials is also a possibility, the failure of the SETI program to detect anything resembling an intelligent radio signal after four decades of effort has partially dimmed the optimism that prevailed at the beginning of the space age. Emboldened critics view the search for extraterrestrials as unscientific, despite the fact the SETI program is not the result of a continuous, dedicated search but instead utilizes what resources and manpower it can, when it can.

Thus, the three decades preceding the turn of the second millennium saw a crossroads reached in beliefs in alien life. The prospect of ubiquitous, intelligent, space-faring civilizations in our solar system appears increasingly dubious to many scientists. Still, in the words of SETI’s Frank Drake, “All we know for sure is that the sky is not littered with powerful microwave transmitters.” Drake has also noted that it is entirely possible advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes and giant strides in detection methods have allowed science to begin delineating habitability criteria on other worlds and to confirm that, at least, other planets are plentiful though aliens remain a question mark.

In 2000, geologist and paleontologist Peter Ward and astrobiologist Donald Brownlee published a book entitled Rare Earth: Why Complex Life is Uncommon in the Universe. In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the universe, while microbial life is common in the universe. The possible existence of primitive (microbial) life outside of Earth is much less controversial to mainstream scientists although at present no direct evidence of such life has been found. Indirect evidence has been offered for the current existence of primitive life on the planet Mars. However, the conclusions that should be drawn from such evidence remain in debate.

Scientists are directly searching for evidence of unicellular life within the solar system, carrying out studies on the surface of Mars and examining meteors that have fallen to Earth. A mission is also proposed to Europa, one of Jupiter’s moons with a possible liquid water layer under its surface, which might contain life. There is some limited evidence that microbial life might possibly exist or have existed on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Recently, Circadian rhythms have been allegedly discovered in Viking data. The interpretation is controversial. Independently in 1996 structures resembling nanobacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is also controversial and scientific debate continues.

In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA’s Ames Research Center, based their claims on methane signatures found in Mars’ atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists’ claims, and Stoker herself backed off from her initial assertions. Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey conducted at the conference in which the European Space Agency presented its findings, 75 percent of the scientists in attendance reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there.

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere that is not in chemical equilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near to their star are necessary before this can be used to analyze extrasolar planets. It is theorised that any technological society in space will be transmitting information. Projects such as SETI are conducting an astronomical search for radio activity that would confirm the presence of intelligent life. A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical as well as infrared spectrum; laser signals have the advantage of not “smearing” in the interstellar medium and may prove more conducive to communication between the stars. And while other communication techniques including laser transmission and interstellar spaceflight have been discussed seriously and may not be infeasible, the measure of effectiveness is the amount of information communicated per unit cost, resulting with the radio as method of choice.

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Expert’s view whether they believe aliens exist or not:

Jonti Horner

Astrobiologist Jonti Horner thinks aliens exist, but wonders whether they’re close enough for us to discover them. The sheer number of planets in our galaxy alone makes it seem unlikely that Earth is the only planet that harbours life. However, our galaxy is 100,000 light years from side to side, and the great distances between other potentially life-hosting planets would make it near impossible for us to hear alien signals – at least at the moment.

Steven Tingay

Astrophysicist Steven Tingay thinks aliens exist, but is clear to define the term ‘alien’ to mean all manner of life resident on places on other than Earth. This includes bacteria found somewhere other than our planet. Potential life could be primitive or advanced, while there’s also the possibility that it plays by a completely different set of rules.

Helen Maynard-Casely

Planetary Scientist Helen Maynard-Casely believes it’s a matter of time until we find something akin to biology somewhere other than on Earth. She cites the fact that we’re finding more and more pockets in our solar system that may be hospitable to life as we understand it, such as the under oceans of two of Jupiter’s large moons. Maynard-Casely also acknowledges that potential alien life may not conform to our “rules”, and considers whether potential life can contact us as an entirely different matter.

Rebecca Allen

Space Technology Expert Rebecca Allen thinks aliens exists, but states that it’s unlikely that they resemble humans. The word ‘alien’ conjures up an image of a human-like creature; however, the most predominant forms of life on Earth are much smaller, older and hardier: microorganisms. These organisms can survive in places where life has no business existing, and if aliens do exist, it’s likely to be in this form.

Martin Van-Kranendonk

Astrobiologist Martin Van-Kranendonk cites the lack of empirical data as indication that there is no life outside of Earth that’s not related to human activity. However, he concedes that our knowledge is limited; we have not investigated the whole of the universe and we don’t know what might constitute life in another chemical system. Reconciling these points, Van-Kranendonk concludes that a more expanded answer may be that we just don’t know, and may in fact never know.

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Distinct Types of Extraterrestrial Life:

The possibilities for extraterrestrial life are diverse and can be categorised into various types based on the conditions, environments, and forms of life that could exist beyond Earth:

-1. Microbial Life: This type of life includes simple, single-celled organisms like bacteria and archaea. Microbial life might exist in extreme environments such as subsurface oceans on icy moons like Europa or Enceladus, where conditions might be conducive to liquid water despite the extreme cold.

-2. Extremophiles: These are organisms capable of thriving in extreme conditions on Earth, such as extreme heat, cold, acidity, or high radiation. Extremophiles offer insight into the potential habitability of environments like hydrothermal vents on the ocean floor or acidic lakes on other planets.

-3. Complex Life: More advanced forms of life could exist, potentially resembling plants, animals, or even ecosystems similar to those on Earth. These forms might inhabit habitable exoplanets within the habitable zone of their host stars, where conditions allow for liquid water to exist on the surface.

-4. Intelligent Life: Extraterrestrial civilisations with advanced cognitive capabilities and the ability to manipulate their environment could exist. This type of life might engage in complex communication, technology development, and space exploration.

-5. Silicon-Based Life: While carbon is the foundation of life on Earth, some speculate that life forms based on other elements, such as silicon, could exist under different environmental conditions. However, silicon-based life would require unique chemical and physical properties.

-6. Non-Biological Life: Hypothetical forms of life might not rely on the same biochemistry as Earth based organisms. These could include self-replicating nanobots, artificial intelligence, or other non-biological entities that exhibit characteristics of life.

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Universal criteria for ETL:

No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are contingent, in the sense that they are the result of evolutionary accidents such that elsewhere a different sequence of events might have led to different properties of life. In this respect the discovery of even a single example of extraterrestrial life, no matter how elementary in form or substance, would represent a fundamental revolution in science. Do a vast array of biological themes and counterpoints exist in the universe, or are there places with living fugues, compared with which Earth’s one tune is a bit thin and reedy? Or is Earth’s the only tune around?

Life on Earth, structurally based on carbon, hydrogen, nitrogen, and other elements, uses water as its interaction medium. Phosphorus, as phosphate bound to an organic residue, is required for energy storage and transport; sulfur is involved in the three-dimensional configuration of protein molecules; and other elements are present in smaller concentrations. Must these particular atoms be the atoms of life everywhere, or might there be a wide range of atomic possibilities in extraterrestrial organisms? What are the general physical constraints on extraterrestrial life?

In approaching these questions, several criteria can be used. The major atoms should tend to have a high cosmic abundance. Structural molecules of organisms at the temperature of the planet in question should not be so extremely stable that chemical reactions are impossible, but neither should they be extremely unstable, or else the organism would fall to pieces. A medium for molecular interaction must be present. Solids are inappropriate because of their inertness. The medium, most likely a liquid but possibly a very dense gas, must be stable in a number of respects. It should have a large temperature range (for a liquid, the temperature difference between freezing point and boiling point should be large). The liquid should be difficult to vaporize and to freeze; in general, it should be difficult to change its temperature. The interaction medium needs to be an excellent solvent. A fluid phase must be present on the planet in question, for material must cycle to the organism as food and away from the organism as waste.

Stars like the Sun are born spinning rapidly, which creates a strong magnetic field that can erupt violently, bombarding their planetary systems with charged particles and harmful radiation. Over billions of years, the rotation of the star gradually slows as its magnetic field drags through a wind flowing from its surface, a process known as magnetic braking. In our own solar system, life’s transition from the oceans onto land occurred several hundred million years ago, coinciding with the time that magnetic braking began to weaken in the Sun. Young stars bombard their planets with radiation and charged particles that are hostile to the development of complex life, but older stars appear to provide a more stable environment. The planet should therefore have an atmosphere and some liquid near the surface, although not necessarily a water ocean. If the intensity of ultraviolet light or charged particles from its sun (host star) is intense at the planetary surface, then some area, perhaps below the surface, should be shielded from this radiation (although some forms or intensity of radiation might permit useful chemical reactions to occur). Finally, it is imperative that conditions allow the existence of autotrophy (the ability of an organism to synthesize at least some of its own nutrients) or other means of net production of necessary compounds.

Thermodynamically, photosynthesis based on stellar radiation may be the optimal source of energy for extraterrestrial life. Photosynthetic organisms and the radiation they receive are not in thermodynamic equilibrium. On Earth, for example, a green plant may have a temperature of about 300 K (23 °C, or 73 °F); the Sun’s temperature is about 6,000 K. (K = kelvin. On the Kelvin temperature scale, in which 0 K [−273 °C, or −460 °F] is absolute zero, 273 K [0 °C, or 32 °F] is the freezing point of water, and 373 K [100 °C, or 212 °F] is the boiling point of water at one atmosphere pressure.) Photosynthetic processes are possible because energy is transported from a hotter object (the Sun) to a cooler object (Earth). Were the source of radiation at the same or at a colder temperature than the photosynthesizer, no photosynthetic activity would be possible. For this reason, the idea that a subterranean green plant will photosynthesize by use of thermal infrared radiation emitted by its surroundings is untenable. Equally unfeasible is the idea that a cold star, with a surface temperature similar to that of Earth, could sustain photosynthetic organisms.

One can use these conditions to establish the limits for the chemical requirements of life. When atoms chemically combine, the energy necessary to separate them is called the bond energy, and the measure of this energy determines how tightly the two atoms are bound to each other. Bond energies generally vary from about 10 electron volts (eV) to about 0.03 eV. Covalent bonds, where electrons are shared between atoms, tend to be more energetic than hydrogen bonds, where a hydrogen atom is shared between atoms, and hydrogen bonds in turn are more energetic than van der Waals forces, which arise from the attraction of the electrons of one atom for the nucleus of another. Atoms, free or bound, move with an average kinetic energy corresponding to about 0.02 eV. The higher the temperature, the more atoms move with energy sufficient to break a given bond spontaneously. The energy-rich phosphate bonds in adenosine triphosphate (ATP), about as energetic as the hydrogen bonds, are in fact of relatively low energy. Cells store large numbers of these bonds to drive a molecular degradation or synthesis. One expects the energy currency on high-temperature worlds to be much more energetic per bond and on low-temperature worlds to be much less energetic per bond.

In The Fitness of the Environment (1913), American biochemist Lawrence Joseph Henderson first stressed the advantages of carbon and water for life in terms of comparative chemistry. Henderson was struck by the fact that the very atoms needed are exactly those that are around. It remains a remarkable fact that the atoms most useful for life have very high cosmic abundances.

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Biosignature and technosignature of ETL:

The search for life elsewhere in the universe focuses on identifying biosignatures that would indicate the presence of extraterrestrial life. An extension of the search for biosignatures is the search for “technosignatures” (Tarter 2007), which specifically focuses on identifying remotely observable signatures of extraterrestrial technology.

The search for remotely detectable biosignatures has developed greatly since the discovery of exoplanets (Wolszczan & Frail 1992; Mayor & Queloz 1995). Since that event, the field has tackled a number of scientific questions and challenges including the coupling of climate models to atmospheric chemistry networks for the production of synthetic observations (Seager et al. 2012; Grenfell 2017; Kaltenegger 2017; Catling et al. 2018; Fujii et al. 2018; Meadows et al. 2018; Schwieterman et al. 2018; Walker et al. 2018; Lammer et al. 2019), the exploration of alternative pathways for biological processes such as photosynthesis (Wolstencroft & Raven 2002; Kiang et al. 2007; Schwieterman et al. 2018; Lingam & Loeb 2020a), and the ongoing development of agnostic biosignatures such as markers of chemical complexity (Johnson et al. 2018; Walker et al. 2018; Marshall et al. 2021).

Searches for remotely detectable technology have benefited from renewed energy in recent years due to a confluence of several events, including the discovery that habitable-zone rocky planets are ubiquitous in the universe (Burke et al. 2015; Dressing & Charbonneau 2015; Hsu et al. 2018; Kopparapu et al. 2018; Mulders et al. 2018; Bryson et al. 2021), the resulting development of a robust astrobiology program focused on technosignatures, the Breakthrough Listen Initiative—which has pledged $100 million over 10 yr toward the search for technosignatures (Worden et al. 2017)—and a renewed interest from the US Congress and NASA in funding technosignature research. This renewed investment has cultivated a much broader range of avenues for the detection of extraterrestrial life.

Both fields (technosignature and biosignature searches) have developed expectations and theoretical frameworks for the abundance of signs of life in the universe. Of course, an objective, quantitative comparison of the actual relative abundances of technosignatures and biosignatures is difficult because it depends on details of extraterrestrial life that we cannot know for certain until we have some examples to learn from. The logic is that technology—and its attendant technosignatures—differs in fundamental ways from biology—and its biosignatures—in that it can spread far beyond its origin in space, time, and scope (e.g., Walters et al. 1980; Papagiannis 1982; Freitas 1983), and this difference gives technosignatures almost unlimited potential for key metrics associated with exolife searches: abundance, longevity, detectability, and ambiguity. The intuition suggested by the Drake equation implies that technology should be less prevalent than biology in the galaxy and it can lead one to the erroneous conclusion that technosignatures must be poorer search targets than biosignatures. There is no incontrovertible reason that technology could not be more abundant, longer-lived, more detectable, and less ambiguous than biosignatures. It has been appreciated for decades in the SETI community that technosignatures could be more abundant, longer-lived, more detectable, and less ambiguous than biosignatures.

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Earth has not been the same planet over its history, so are exoplanets:

Earth is the only planet we know of with life. But Earth has been many different planets over its history. Those are the alternative Earths. Would we recognize a living Earth, for instance, before oxygen was abundant enough in the atmosphere to be detected? Life-forms that did not rely on oxygen thrived for billions of years before an oxygenated atmosphere would have registered on the instruments of an observer many light-years away. And after life began producing oxygen, its accumulation in the atmosphere was likely low enough to evade detection for billions of years. It’s even possible that oxygen would have remained undetectable until perhaps as recently as 800 million years ago, long after the earliest appearance of complex life – cells with a central nucleus – and about the same time as the earliest animal life. We have to use chemical measurements of ancient rocks, which provide a record of the past, as well as computer models, to produce a kind of catalog of gaseous profiles of Earth’s many phases. Using such a platform, we can imagine possibilities on distant planets, even if very different from anything in Earth’s archives. If Webb and future space telescopes capture matching profiles in the atmosphere of an exoplanet, it could be a strong sign of a “biosphere” – a world marked by environmental conditions and changes that drive, and are driven by, some form of life. The ultimate objective is to understand how a planet can develop and sustain a detectable biosphere – not only to know that [life] could be there, but that it is there.

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The approach to detecting life beyond the Earth:

The search for life is one of the most active fields in space science and involves a wide variety of scientific disciplines, including planetary science, astronomy and astrophysics, chemistry, biology, chemistry, geoscience, and so forth. Four categories can be used to help guide the approach to detecting life beyond the Earth.

In situ detection in the solar system of life as we know it (e.g., Mars),
In situ detection in the solar system of life as we don’t know it (e.g., Titan),
Remote detection on exoplanets of life as we know it (e.g., “exo-Earths”), and
Remote detection on exoplanets of life as we don’t know it (target unknown).

Each region has its own characteristic set of biosignatures and will require a different set of technologies, instruments, knowledge, and expertise to determine whether life can or does exist in each environment.

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Today, in 2024, there are three main ways we’re looking for extraterrestrial life:

-1. We’re exploring worlds in our Solar System, including Mars, Venus, Titan, Europa, and Pluto, remotely, with fly-by missions, orbiters, landers, and even rovers, searching for evidence of past or even present simple life.

-2. We’re examining exoplanets, searching for evidence that there’s life on them, from the surface to the atmosphere and beyond, based on observable signatures of color, seasonal change, and atmospheric contents.

-3. And by looking for any signals that would reveal the presence of intelligent aliens: through efforts like SETI and Breakthrough Listen.

All three approaches have their advantages and disadvantages, but most scientists believe that it’s the second option that’s most likely to deliver our first success.

A question often asked is whether we’ll detect the first signs of life on another body within our solar system, or on an exoplanet – a planet orbiting another star. Exploration of the solar system has the advantage of landing on planets, moons, or asteroids, and collecting samples for analysis. For the planets beyond our solar system, remote detection of signs of life will have to suffice. Still, we might have good reason to expect the first detection will come from an exoplanet. Although solar system planets are more directly accessible, finding life among them poses enormous technical challenges – whether we’re seeking life on Mars or on Jupiter’s moon, Europa, or Saturn’s Enceladus. The best hope on Mars is the subsurface. How long will it take before we’re able to drill into the subsurface? For Enceladus and Europa, we’re talking about being in a subsurface ocean below kilometers of ice. How soon will it be before we actually get into those? It becomes more an issue of access and not of greater probability. Exoplanets, on the other hand, despite the challenges of remote detection, offer a vast number of targets: thousands of planets confirmed so far in our galaxy, which likely contains hundreds of billions. And the right technology to conduct such a search is just coming online. NASA’s James Webb Space Telescope is already adding to inventories of ingredients in exoplanet atmospheres; more powerful and sensitive observatories are being readied for the future search for life signs – also called biosignatures. Hopes are also high for the next generation of ground-based telescopes, massive instruments 100 or 130 feet (30 or 40 meters) wide.

If there aren’t intelligent, actively broadcasting civilizations nearby, SETI won’t deliver any positive results. But if even a small fraction of worlds that exist with Earth-like properties have life on them, exoplanet studies can deliver a success where the other two options won’t. And we’ve come a very long way in our studies of exoplanets: we have more than 5000 known, confirmed exoplanets within the Milky Way, where we know the mass, radius, and orbital period of most confirmed worlds.

Although more than 5,000 confirmed exoplanets are known, with more than half of them uncovered by Kepler, there are no true analogues of the planets found in our Solar System. Jupiter-analogues, Earth-analogues, and Mercury-analogues all remain elusive with current technology. The overwhelming majority of planets found via the transit method are close to their parent star, are ~10% the radius (or, equivalently, ~1% the surface area) of their parent star or more, and are orbiting low-mass, small-sized stars as seen in the figure below:

So far, telescopes have revealed exoplanets come in many flavors, some rocky, some gaseous. They include “super-Earths,” which might or might not be scaled-up, rocky worlds, and “mini-Neptunes,” junior versions of our own Neptune – two planet types that, though common in the galaxy, are strange to us because they don’t occur in our solar system. Add to the menagerie “hot Jupiters” and “hot Saturns,” in tight, scorching orbits around their stars, and rogue planets floating freely through space without a parent star.

Unfortunately, this isn’t enough to inform us about whether any of these worlds are inhabited. To make that determination, we need more than that. We’d need to know things like:

Does the exoplanet have an atmosphere?
Does it have clouds, precipitation, and weather cycles?
Do its continents green-and-brown with the seasons, like they do on Earth?
Does it have gases or gas combinations in its atmosphere that hint at biological activity, and do they show seasonal variations like Earth’s CO2 levels do?

On the cutting edge of performing these measurements, today, are the space-based JWST and ground-based 10-meter class telescopes, performing direct exoplanet imaging and transit spectroscopy. Unfortunately, this isn’t sufficient technology to reach our goal of measuring the properties of Earth-sized planets in Earth-like orbits around Sun-like stars. For direct imaging studies, we can take pictures of planets that are the size of Jupiter and that are more than about Saturn’s distance from the Sun: good for gas giant worlds, but not so great for looking for life on rocky planets. For transit spectroscopy, we can see the light that filters through the atmospheres of super-Earth-sized worlds around red dwarf stars, but Earth-sized planets around Sun-like stars are well beyond the reach of current technology.

The Roman Space Telescope, expected to launch in 2027, may discover some 100,000 more of these distant worlds, in addition to testing new technology for directly imaging exoplanets. Future, even more powerful space telescopes could search exoplanet atmospheres directly for signs of life – what astrobiologists call biosignatures.

When starlight passes through a transiting exoplanet’s atmosphere, signatures are imprinted. Depending on the wavelength and intensity of both emission and absorption features, the presence or absence of various atomic and molecular species within an exoplanet’s atmosphere can be revealed through the technique of transit spectroscopy as seen in the figure above. JWST cannot get spectra for Earth-sized planets around Sun-like stars, but Habitable Worlds Observatory finally will.

It’s a promising start, but one we need to build on if we hope to attain the ultimate success of finding and characterizing an inhabited planet. Currently, we’re building the next generation of ground-based telescopes, ushering in the era of 30-meter class telescopes with the Giant Magellan Telescope (GMT) and the Extremely Large Telescope (ELT), and looking forward to NASA’s next astrophysics flagship mission: the Nancy Roman Telescope, which will have the same capabilities as Hubble but with superior instrumentation, a field-of-view that’s 50-100 times as great as Hubble’s, and a coronagraph that allows us to image planets within the glare of their parent star’s light that are about 1000 times fainter than JWST can see. Even with these advances, however, we’ll only get Earth-sized planets around the nearest red dwarf stars and super-Earth or mini-Neptune sized planets around Sun-like stars. To image a truly Earth-like planet, an improved observatory with still greater capabilities is required.

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Signs of ETL:

There are various signs of alien life scientists are most interested in investigating — with the understanding that there are plenty of other observations and data, too, that could determine whether life exists on a different world.

-1. Water

Life on Earth fundamentally requires H2O. Where there is water, organic molecules can come together and form living systems. These, in turn, reproduce and pass down genetic material. That’s why astronomers are so obsessed with finding water on other moons and planets.

What makes water so vital? It has chemical properties that no other natural substance in the world can emulate. It takes a lot of energy to change the temperature of water — so it does a great job of insulating bodies from the cold while keeping them cool under heat. It’s excellent at carrying nutrients into cells while expelling waste and toxins. It can withstand sharp pressure shifts. It’s really good at dissolving other others substances. Simply put, life as we know it can’t exist without water. This is why the discovery of liquid water on Mars was so big. Though surface of Mars is likely lacking life, there’s hope we might find signs of ancient Martian organisms — or that the planet could be home to future forms of life.

-2. Gas biosignatures

Liquid isn’t the only state of matter that matters. It’s not always pleasant, but it’s a reality that living things on Earth produce gas. The large amounts of specific gases in our atmosphere act as biosignatures of life. Inorganic geochemical processes can produce gas — but concentrations of certain gases would be a good sign of life on another planet.

Oxygen is the biggest signature on Earth, and methane is a close second. But other biosignatures include any kind of carbon-based gas. And really, besides the noble gases, life on Earth is produces every single gas known to man.

If you have instruments that can analyze the chemical composition of another world’s atmosphere, you’re in a good position to deduce whether biosignatures are present and the likelihood there’s life. A big problem, however, is making sure that organisms produced those biosignatures.

-3. Radio waves

Here’s where we distinguish the search for just any signs of alien life and the search for intelligence. If aliens are anything like us, chances are good they can harness radio waves for communication and scientific purposes. Nikola Tesla was one of the first people to suggest aliens might try to reach us through radio transmission. As our radio telescopes have improved, the possibility of stumbling on E.T.’s radio are better than ever.

One of the most promising radio telescopes is the Square Kilometer Array (SKA) under construction in Australia and South America. When completed, it will be 50 times more sensitive than any other radio instrument, capable of scanning the sky 10,000 times faster than we can now. Wherever the radio waves passing through our solar system might originate — be it inside the Milky Way, or from a galaxy dozens of light-years away — this array could pick them up.

That highlights the biggest problem with looking for radio waves — they may be coming from light-years away, potentially millions of years old. We’d be listening in on the ancient past. Successfully sending a response back would take much longer than humanity’s lifetime.

-4. Heavy elements

It stands to reason that intelligent life would rely on the same heavy elements we use to construct infrastructure and technology in our sentient civilization. We’re not simply talking about metals like gold and iron and aluminum. We’re talking bigger. Nuclear. Stephen Hawking once observed that “when intelligent life gets smart enough to send signals into space, it is also busying itself with stockpiling nuclear bombs.” In that case, that species needs to deal with nuclear waste. Nuclear material collected in unusually large concentrations on a planet — or even out in space — might be a sign of an intelligent civilization nearby. A fortuitous sign, but we would want to be a little cautious that introductions don’t inadvertently trigger an interstellar nuclear war.

-5. Artifacts

If Mars was overflowing in vast oceans at some point in its ancient history, then perhaps some form of life existed on the red planet. And if this was intelligent life, there must be some sign it that still remains. That’s the hope among some scientists looking to find alien artifacts sitting on Mars or some other planet or moon. These could be ruins of an ancient city or small tools hidden away in a cave. Or anything else in between. Looking for alien artifacts would actually not be too dissimilar from how archaeologists study early humans. Furthermore, artifacts aren’t necessarily a sign that species has gone extinct. They may have migrated to another planet, and what remains are leftovers from a failed or lost colony.

-6. Technostructures

Lastly, the best and most direct sign of intelligent life would be finding what are called “technostructures” — signs of technology that don’t include radio messages. These could be small, like the space probes we’ve also sent off into space — or incredibly massive, like alien megastructures. For example, Dyson sphere is a megastructure that could theoretically harvest all the energy of a star and supply an advanced alien civilization. In essence, a technostructure would show us there’s a species of life out there that’s at least as smart as humans are in the 21st century. The possibility of finding a technostructure with alien origins is extremely low.

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Possibility about potential alien life:

-1. Research revealed the presence of phosphine in the upper clouds of Venus. Phosphine is a rare gas that’s associated with living organisms on Earth. The finding doesn’t prove alien life, but rather suggests that life may have the potential to exist in places previously thought unlikely. Venus’s surface is too hellish to be habitable, but its clouds could be mild enough to harbour communities of microbial life. But that discovery has been mired in controversy after independent reanalysis found issues with the data.

-2. The Milky Way may be teeming with ocean world. For instance, Jupiter’s moon is believed to host seas under its icy shell, while Saturn’s moon features geysers spewing from its exterior. Ocean worlds may not be uncommon on exoplanets.

-3. Some microbes can live without oxygen, and searching for organisms elsewhere in the Universe need not be based on finding conditions that resemble Earth.

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Section-5

Astrobiology and astrobotany:

Astrobiology is a scientific field within the life and environmental sciences that studies the origins, early evolution, distribution, and future of life in the universe by investigating its deterministic conditions and contingent events. As a discipline, astrobiology is founded on the premise that life may exist beyond Earth. Research in astrobiology comprises three main areas: the study of habitable environments in the Solar System and beyond, the search for planetary biosignatures of past or present extraterrestrial life, and the study of the origin and early evolution of life on Earth.

Regarding habitable environments, astrobiology investigates potential locations beyond Earth that could support life, such as Mars, Europa, and exoplanets, through research into the extremophiles populating austere environments on Earth, like volcanic and deep sea environments. Research within this topic is conducted utilising the methodology of the geosciences, especially geobiology, for astrobiological applications.

The search for biosignatures involves the identification of signs of past or present life in the form of organic compounds, isotopic ratios, or microbial fossils. Research within this topic is conducted utilising the methodology of planetary and environmental science, especially atmospheric science, for astrobiological applications, and is often conducted through remote sensing and in situ missions.

Astrobiology also concerns the study of the origin and early evolution of life on Earth to try to understand the conditions that are necessary for life to form on other planets. This research seeks to understand how life emerged from non-living matter and how it evolved to become the diverse array of organisms we see today. Research within this topic is conducted utilising the methodology of paleosciences, especially paleobiology, for astrobiological applications.

Astrobiology, also called exobiology or xenobiology, is a multidisciplinary field dealing with the nature, existence, and search for extraterrestrial life (life beyond Earth). Astrobiology encompasses areas of biology, astronomy, and geology. Astrobiology research sponsored by NASA focuses on three basic questions: How does life begin and evolve? Does life exist elsewhere in the Universe? How do we search for life in the Universe? Over the past 50 years, astrobiologists have uncovered myriad clues to answering these Big Questions. Accordingly, the discipline of astrobiology embraces the search for potentially inhabited planets beyond our Solar System, the exploration of Mars and the outer planets, laboratory and field investigations of the origins and early evolution of life, and studies of the potential of life to adapt to future challenges, both on Earth and in space. Interdisciplinary research is required that combines molecular biology, ecology, planetary science, astronomy, information science, space exploration technologies, and related disciplines. The broad interdisciplinary character of astrobiology compels us to strive to achieve the most comprehensive and inclusive understanding of biological, planetary, and cosmic phenomena.

Although no compelling evidence of extraterrestrial life has yet been found, the possibility that biota might be a common feature of the universe has been strengthened by the discovery of extrasolar planets (planets around other stars), by the strong suspicion that several moons of Jupiter and Saturn might have vast reserves of liquid water, and by the existence of microorganisms called extremophiles that are tolerant of environmental extremes. The first development indicates that habitats for life may be numerous. The second suggests that even in the solar system there may be other worlds on which life evolved. The third suggests that life can arise under a wide range of conditions. The principal areas of astrobiology research can be classified as (1) understanding the conditions under which life can arise, (2) looking for habitable worlds, and (3) searching for evidence of life.

For life like that on Earth (based on complex carbon compounds) to exist, a world must have liquid water. Because planets either too close to or too far from their host stars will be at temperatures that cause water either to boil or to freeze, astrobiologists define a “habitable zone,” a range of orbital distances within which planets can support liquid water on their surfaces. In the solar system, only Earth is inside the Sun’s habitable zone. However, photographs and other data from spacecraft orbiting Mars indicate that water once flowed on the surface of the red planet and is still present in large quantities underground. Consequently, there is a sustained international effort to use robotic probes to examine Mars for evidence of past, and even present, life that could have retreated to subsurface, liquid aquifers.

Also, discoveries primarily due to the Galileo space probe (launched in 1989) suggest that some of the moons of Jupiter—principally Europa but also Ganymede and Callisto—as well as Saturn’s moon Enceladus, might have long-lived liquid oceans under their icy outer skins. These oceans can be kept warm despite their great distance from the Sun because of gravitational interactions between the moons and their host planet, and they might support the kind of life found in deep sea vents on Earth.

Even Titan, a large moon of Saturn with a thick atmosphere, might conceivably have some unusual biology on its cold surface, where lakes of liquid methane and ethane may exist. The European space probe Huygens landed on Titan on January 14, 2005, and saw signs of liquid flow on its surface. Such discoveries as these have strongly promoted the emergence of astrobiology as a field of study by broadening the range of possible extraterrestrial habitats far beyond the conventional notion of a “habitable zone.”

An additional impetus has been the discovery since 1995 of hundreds of extrasolar planets around other normal stars. Most of these are giant worlds, similar to Jupiter and therefore unlikely to be suitable for life themselves, although they could have moons on which life might arise. However, this work has shown that at least 5 to 10 percent (and possibly as much as 50 percent or more) of all Sun-like stars have planets, implying many billions of solar systems in the Milky Way Galaxy. The discovery of these planets has encouraged astrobiology and in particular has motivated proposals for several space-based telescopes designed to search for smaller, Earth-size worlds and if such worlds are found, to analyze spectrally the light reflected by the planets’ atmospheres in the hope of detecting oxygen, methane, or other substances that would indicate the presence of biota. While no one can say with certainty what sort of life might be turned up by these experiments, the usual assumption is that it will be microbial, as single-celled life is adaptable to a wide range of environments and requires less energy. However, telescopic searches for extraterrestrial intelligence (SETI) are also part of astrobiology’s extensive research palette.

Astrobiology has made huge strides just in the past 10 years, due to space flight opportunities, new technology, and advances in molecular biology, among other developments. And astrobiology has been turbocharged by the rapid growth of the field of exoplanet science – the search for planets around other stars and the quest to define “habitability” and identify so-called biosignatures that could signal the existence of life on such planets. In recognition of the intersections between astrobiology and exoplanet science, NASA created a Nexus for Exoplanet System Science (NExSS) in 2015, a research coordination network intended to bring together astrobiologists, astrophysicists, Earth scientists and heliophysicists in a systems approach to studying planetary habitability. Led by the Ames Research Center, the NASA Exoplanet Science Institute, and the Goddard Institute for Space Studies, NExSS will help organize the search for life on exoplanets from participating research teams and acquire new knowledge about exoplanets and extrasolar planetary systems.

Spacecraft have flown by, orbited around, or landed on Mercury, Venus, Mars, Jupiter and several of its moons, Saturn and several of its moons, the dwarf planet Pluto and its moons, and the dwarf planet Ceres. A myriad of spacecraft have orbited Earth to study the home planet. Comparative planetology is a thriving field. Ocean worlds in our solar system – in particular, Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan – are top targets for astrobiological investigations of prebiotic chemistry, habitability, and possible life. Many astrobiologists are exploring the possibility of extant life in the deep subsurface of Mars.

At the same time that research into the origin, evolution, and distribution of life as we know is revealing that life is highly adaptable and resilient, these same lines of research are helping to reveal how life and its environment are deeply interdependent. Some key lines of research in this area – such as understanding the timing and mechanics of the rise of oxygen in the atmosphere of early Earth; the role of the environment in the production of organic molecules; and the co-evolution of climates, atmospheres, interiors, and biospheres – are improving understanding of the evolution of habitability and life on Earth and prospects for the evolution of habitability and life elsewhere, contributing to understanding of global climate history and evolution, and at the same time complicating the further study of life, terrestrial or otherwise.

Astrovirology, or our understanding of viruses in astrobiology, is another avenue astrobiologists have been moving toward, yet very little is known. Viruses co-occurring with archaea and bacteria on Earth’s biosphere express highly diverse structural and genomic sequences. These are vital in biogeochemical cycles in terrestrial ecosystems and evolution, mediating horizontal gene transfer and influencing microbial community dynamics. Overall, astrovirologists hypothesize that viruses are as vital in other planet ecosystems as they are paramount contributors on Earth. Furthermore, viral signatures may be pivotal in searching for life in other biospheres and understanding their evolutionary mechanics. Nowadays, two main field priorities are 1) viruses that inhabit extreme analog environments characterization and 2) virus-detection experiments in ancient oceans (Europa and Enceladus) using flight instruments to detect viral particles or sequences.

Novel biological activity signature advances shed light on future frontiers for life detection missions. Since the Viking age, the astrobiology community has gained a palpable awareness about defining experimental protocols in the search for life on other worlds and the guiding principles needed to interpret generated data. In the coming years, several missions will be launched to answer fundamental astrobiology queries: how planets form, evolve, and support life. Current and planned planetary missions will examine extraterrestrial environments’ physical and chemical characteristics. Furthermore, space agencies (NASA, ESA, CNSA) are expected to develop biosignature strategies for Mars, Europa, and Enceladus soon.

Astrophysicists… spent decades studying and searching for black holes before accumulating today’s compelling evidence that they exist. The same can be said for the search for room-temperature superconductors, proton decay, violations of special relativity, or for that matter the Higgs boson. Indeed, much of the most important and exciting research in astronomy and physics is concerned exactly with the study of objects or phenomena whose existence has not been demonstrated—and that may, in fact, turn out not to exist. In this sense astrobiology merely confronts what is a familiar, even commonplace situation in many of its sister sciences. In other words, future progress in this field will consist of developing ways to hunt for possible technosignatures and determining in what form these signatures cannot be ruled out as natural phenomena. They begin by considering the extensive work that has been done in the field of radio astronomy.

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Astrobiology in the solar system and beyond:

The Viking lander was the first mission to land successfully on Mars and perform a chemical analysis of its surface. Since Viking became the first mission to successfully touch down on Mars, there have been a wealth of missions on the Red Planet’s surface and in its orbit providing crucial information for astrobiologists. These missions include the now-retired rovers Sojourner, Spirit, Opportunity, and Curiosity rover and the currently active Perseverance rover which touched down on the Martian surface in 2021. The Perseverance rover, part of NASA’s Mars 2020 mission, is currently providing important data relevant to astrobiology research from its stomping ground on the Red Planet the Jezero crater. This 28-mile (45-kilometer) wide crater is believed to have flooded with water around 3.5 billion years ago. With water considered a key element for life, Perseverance is looking for biosignatures that indicate life once existed in this region of Mars. The rover has recently begun dropping sample tubes on the surface of the Red Planet which will be picked up by the joint NASA/ESA Mars Sample Return Mission in the near future. Before this mission goes into operation the Perseverance and Curiosity rovers perform in-situ chemical analysis of Martian rock to provide information for astrobiologists.

There is another way astrobiologists can get their hands on samples from Mars without waiting for a sample return mission, however. Meteorites from the Red Planet are frequently deposited on the surface of our planet with scientists discovering a wealth of organic compounds within these rocks launched into space by violent events on Mars. Though these organic compounds comprised of carbon, nitrogen, oxygen, and a few other elements, are associated with biological processes, they aren’t direct evidence of life as they can also be created by non-biological or “abiotic” processes.

Further afield in the solar system than Earth’s neighbor Mars, astrobiologists are interested in the moons of Jupiter which are currently being studied in depth by the Juno spacecraft. Among Jupiter’s four largest moons, referred to as the Galilean moon because they were discovered by Galileo Galilei in the 1600s, Europa is one of the most promising prospects for discovering life elsewhere in the solar system. The Europa Clipper spacecraft will soon be paying a visit to Europa to discover if the icy Jovian moon could harbor the conditions suitable for life. In particular, astrobiologists want to know if Europa’s icy shell hides subsurface lakes similar to those beneath Antarctica’s ice sheet. The Europa Clipper is set to launch in late 2024 arriving at Jupiter 5 years later when it will make 45 flybys of the massive Jovian moon aiming to use its suite of 9 science instruments to determine the depth and saltiness of Europa’s ocean. Additionally, the spacecraft will also look for molecules in the atmosphere of the Jovian moon deposited there by eruptions of icy water. Astrobiologists will be particularly interested to discover the presence of complex organic molecules that could indicate the processes associated with simple forms of life are occurring at Europa. The moons of Jupiter’s fellow gas giant Saturn are also of great interest to astrobiologists. In 2022 NASA-supported astrobiologists used geochemical models and data from the Cassini mission to find Enceladus’ subsurface waters could be rich in dissolved phosphorus. As this element is essential for life this suggests the oceans of Enceladus could be habitable.

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Astrobiology and exoplanets:

Outside the solar system, astrobiologists are beginning to study the atmospheres of so-called ‘extrasolar planets’ or ‘exoplanets’ to determine the kind of elements and chemical compounds that comprise them. The investigation of exoplanets is a relatively new science but is growing at a staggering rate. This is exemplified by the fact that the first exoplanet around a sun-like star was discovered in 1995 by the team of Michel Mayor and Didier Queloz and in 28 years the “exoplanet catalogue” had grown to over 5,000 confirmed worlds.

In December 2021 the James Webb Space Telescope (JWST), the most powerful telescope ever built by humanity launched and quickly began to revolutionize astronomy and space science. NASA says the JWST’s impact on astrobiology comes from its ability to observe the formation of stars from their first stages to the formation of planetary systems thus allowing astrobiologists to observe the kind of elements that are present as planets form. The JWST is also capable of measuring the physical and chemical properties of planetary systems and thus allowing astrobiologists to investigate the potential for life in those systems.

The first JWST results included a detailed investigation of the atmosphere of an exoplanet, the gas giant WASP-96 b. The space telescope then examined the atmosphere of the Saturn-like exoplanet WASP-39b returning what NASA says is “the first molecular and chemical profile of a distant world’s skies.” A boiling Saturn-like planet 700 light-years away from the sun has become the best-explored planet outside our solar system. The James Webb Space Telescope’s measurements of the planet’s atmosphere have revealed unprecedented details of its chemistry, and even allowed astronomers to test methods for detecting alien life. The exoplanet WASP-39b, which orbits a star in the constellation Virgo, made headlines when the James Webb Space Telescope (Webb or JWST) found carbon dioxide in its atmosphere. It was the first ever such detection and experts hailed the finding as a major breakthrough. The observations revealed that WASP-39b is shrouded in thick clouds containing sulfur and silicates. These chemicals interact with the light of the parent star, producing sulfur dioxide in a reaction similar to the one that produces ozone in Earth’s atmosphere. Though neither of these planets is capable of hosting life, the results set the stage for the JWST’s investigations of the atmospheres of rocky or terrestrial worlds similar to Earth. The JWST followed these exoplanet breakthroughs by discovering its first world orbiting another star in January 2023.

The study of exoplanets and their atmospheres and thus astrobiology received another significant boost at the start of 2023 when NASA unveiled plans for a future telescope to succeed the JWST. The primary goal of this telescope, the Habitable Worlds Observatory (HWO), will be to search for the signs of life on Earth-like worlds. The new space telescope could be operational as soon as the 2036, and will be designed for robotic servicing and upgrades, which means not only could it operate for decades, but its observing power could improve with age. The HWO should be capable of detecting the signs of life on 25 nearby Earth-like worlds, the statistic minimum that astrobiologists and other scientists need to determine if life is common in the Milky Way.

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Astrobotany:

Astrobotany is an applied sub-discipline of botany that is the study of plants in space environments. It is a branch of astrobiology and botany. Astrobotany concerns both the study of extraterrestrial vegetation discovery, as well as research into the growth of terrestrial vegetation in outer space by humans. It has been a subject of study that plants may be grown in outer space typically in a weightless but pressurized controlled environment in specific space gardens. In the context of human spaceflight, they can be consumed as food and/or provide a refreshing atmosphere. Plants can metabolize carbon dioxide in the air to produce valuable oxygen, and can help control cabin humidity. Growing plants in space may provide a psychological benefit to human spaceflight crews.

The first challenge in growing plants in space is how to get plants to grow without gravity. This runs into difficulties regarding the effects of gravity on root development, providing appropriate types of lighting, and other challenges. In particular, the nutrient supply to root as well as the nutrient biogeochemical cycles, and the microbiological interactions in soil-based substrates are particularly complex, but have been shown to make possible space farming in hypo- and micro-gravity. NASA plans to grow plants in space to help feed astronauts, and to provide psychological benefits for long-term space flight.

Known terrestrial plants grown in space:

Arabidopsis (Thale cress)
Bok choy (Tokyo Bekana) (Chinese cabbage)
Tulips
Kalanchoe
Flax
Onions, peas, radishes, lettuce, wheat, garlic, cucumbers, parsley, potato, and dill
Cinnamon basil
Cabbage
Zinnia hybrida (‘Profusion’ var.)
Red romaine lettuce (‘Outredgeous’ var.)
Sunflower
Ceratopteris richardii
Brachypodium distachyon

Some plants, like tobacco and morning glory, have not been directly grown in space but have been subjected to space environments and then germinated and grown on Earth.

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Vegetation red edge (VRE):

The vegetation red edge (VRE) is a biosignature of near-infrared wavelengths that is observable through telescopic observation of Earth, and has increased in strength as evolution has made vegetative life more complex. On Earth, this phenomenon has been detected through analysis of planetshine on the Moon, which can show a reflection spectrum that spikes at 700 nm. In an article published in Nature in 1990, Sagan et al. described Galileo’s detection of infrared light radiating from Earth as evidence of “widespread biological activity” on earth, with evidence of photosynthesis a particularly strong factor. The increase-in-strength of Earth’s VRE biosignature has been assessed through modelling of early Earth radiation. Mosses and ferns, which were dominant on Earth in the Ordovician and Carboniferous periods, produce weaker detectable infrared radiation spikes at 700 nm than modern Earth vegetation. Astrobotanists focused on extraterrestrial vegetation have thus theorized that by using these same models, it could be possible to measure whether exoplanets in their respective Goldilocks zones currently hold vegetation, and by comparing VRE biosignatures to modelled historic Earth radiation, estimate the complexity of this vegetation.

There are a number of obstacles to the detection of exoplanetary VREs:

Galileo’s detection of Earth’s VRE was facilitated by the satellite’s physical proximity to Earth; up until the launch of the James Webb Space Telescope in December 2021, telescopic technology was not yet advanced enough to detect the telltale infrared radiation spikes of VRE in distant exoplanet systems.
Heavy cloud cover has been observed to be detrimental to the detection of VRE, as more cloud cover increases overall albedo, which makes it more difficult to detect radiation wavelength variety. In addition, clouds are detrimental to surface observation, leading to an estimation of ≥20% vegetation cover AND cloud-free surface present as the minimum for detectable exoplanetary VRE visible from telescopes on Earth.
Certain minerals have been shown to demonstrate similar sharp edge reflective spectra as light-harvesting photosynthetic pigments. This means that mineral origins for VRE-like effects must first be ruled-out before a biological explanation can be confirmed. This may be difficult to achieve from Earth as minerals in finer regolith particle form demonstrate different reflective characteristics than large crystal forms found on Earth. One suggestion made by Sara Seager et al. is to use atmospheric measurements to determine the level of atmospheric oxygen, which if high would rule out surface abundance of non-oxidised minerals.

Vegetation searches:

After Galileo’s 1990 fly-by demonstrating the VRE effect on Earth, astrobotanical interest in extraterrestrial vegetation has mainly focused on examining the feasibility of VRE detection, and a number of projects have been proposed:

Both the European Space Agency Darwin project and NASA Terrestrial Planet Finder were cited as projects that could have analyzed exoplanetary VRE biosignatures before being cancelled in 2007 and 2011, respectively.
The ESO Extremely Large Telescope, set to launch in 2028, has also been cited as another telescope that will be able to detect exoplanetary VRE biosignatures.
Future NASA space telescopes, such as the Habitable Exoplanet Imaging Mission, have been planned with the capacity to examine for VRE biosignatures.

The James Webb Space Telescope has been searching the TRAPPIST-1 exoplanet system since 2021 for signs of extraterrestrial vegetation through capturing atmospheric data, including a VRE biosignature, that is made visible when TRAPPIST-1’s exoplanets pass across the face of the star. NASA have judged three of TRAPPIST-1’s rocky exoplanets (1e, 1f, and 1g) as within the habitable zone for liquid water (and other biological matter, such as vegetation).

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Exopsychology:

Astrobiology, the study of the origins of life on Earth and the search for life elsewhere in the universe (for example, by analyzing chemical signatures of far-away planets), and the Search for Extraterrestrial Intelligence (SETI) are now part of mainstream science, pursued at both NASA and prestigious universities worldwide. Inasmuch as psychology is essentially a branch of applied biology, it should come as no surprise that some brave behavioral scientists have risked the ridicule of their peers to extend the investigation of non-earthly lifeforms into the realm of psychology, with research called Exopsychology. The ultimate goal of this research, according to German exopsychologist Niklas Dobler, is not only to contribute to SETI, but to better understand ourselves by exploring not only what we are, but what we are not. For instance, rigorously challenging assumptions about extraterrestrial motivations rooted in anthropocentrism (projecting human motivations onto non-humans), might help us do a better job relating to other humans who are very different from us, or to gain a deeper understanding of animal behavior.

At first glance, getting inside the minds of extraterrestrials without using ourselves as a reference would seem impossible because we lack the mental building blocks with which to construct utterly alien mindsets. The challenge is analogous to trying to imagine a color you’ve never seen before. Close your eyes and try it. Without mental building blocks for a previously unseen color, it’s nearly impossible to create one in your head. Imagining motivations and predicting behaviors of intelligent entities, who are wildly different from us, and who, for reasons of their own, have travelled here—either themselves or through probes—would be exceedingly difficult. Exopsychology is a new field exploring the possible cognitions and motivations of extraterrestrial entities. The biggest challenge, as with studying animal behavior, is to overcome anthropocentrism.

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Section-6

Exoplanets:

One of the most intriguing fields within astronomy is exoplanetology, the study of exoplanets. An “exoplanet” is a planet outside our solar system. Exoplanets were considered extremely rare until the 1990s, but thanks to advances in telescope technology, we now know that they are common throughout the universe. It is estimated that the Milky Way galaxy contains more exoplanets than stars. The first confirmed detection of an exoplanet occurred in 1992, and since then, advancements in technology and observational techniques have led to the identification of thousands of these alien worlds. In the thousands of years humanity has been contemplating the cosmos, we are the first people to know one thing for sure: The stars beyond our Sun are teeming with planets. They come in many varieties, and a good chunk of them are around the size of Earth.

An exoplanet is a planet outside our solar system, usually orbiting another star. They are also sometimes called “extrasolar planets,” “extra-” implying that they are outside of our solar system.

Exoplanets are subjected to rigorous characterization efforts once discovered to reveal their secrets. The analysis of light emitted or absorbed by a celestial body, known as spectroscopy, is a powerful tool in exoplanetology. Scientists can deduce an exoplanet’s chemical composition, atmospheric conditions, and even the presence of life-enabling elements like water and organic molecules by studying its light spectrum.

Over the past several decades, the National Aeronautics and Space Administration (NASA), other space agencies, and the astronomical community have discovered multiple Earth-like planets. However, this type of celestial body must first meet the requirement of being an exoplanet, also called extrasolar planet. Aside from being situated outside our solar system, an exoplanet must orbit other stars. This criterion is the same as what astronomical authorities define what a planet should be in general. According to NASA, a celestial body can be considered as a planet if it has an adequate and large mass to have enough gravity, forcing it to have a spherical shape. In addition, its size must be big enough for its gravity to clear away other objects with similar size near its orbit around the Sun. As of November 24, 2023, there are 5,539 confirmed exoplanets with 10,009 candidates and 4,128 planetary systems, according to the US space agency.

Exoplanets List:

There are thousands of confirmed exoplanets outside our solar system, as mentioned earlier. However, there are some extrasolar planets that are unique than others due to being potentially habitable planets. Below are the 10 most Earth-like exoplanets out there:

Gliese 667CC
Kepler-22B
Kepler-69C
Kepler-62F
Kepler-186F
Kepler-442B
Kepler-452B
Kepler-1649C
Proxima Centauri B
Trappist-1E

Since no one has ever travelled to any exoplanets before, questions have been raised as to how planetary scientists determine whether an exoplanet is habitable or not. According to NASA, Earth-like planets must be roughly the same size as our planet. In addition, they should orbit their stars’ habitable zones, meaning their distance from their stars must be right, not having extreme heat or cold. Exoplanetary environments are also observed for potential signs of habitability or biosignatures through the James Webb Space Telescope and other powerful space telescopes and observatories. The detection of these habitable exoplanets could provide us new insight into the distribution of and potential for life in the universe, according to researchers from the University of Washington.

With current telescopes, astronomers can directly measure a planet’s distance to its star and the time it takes it to complete an orbit. Those measurements can help scientists infer whether a planet is within a habitable zone. But there’s been no way to directly confirm whether a planet is indeed habitable, meaning that liquid water exists on its surface. Across our own solar system, scientists can detect the presence of liquid oceans by observing “glints” — flashes of sunlight that reflect off liquid surfaces. These glints, or specular reflections, have been observed, for instance, on Saturn’s largest moon, Titan, which helped to confirm the moon’s large lakes. Detecting a similar glimmer in far-off planets, however, is out of reach with current technologies.

The closest exoplanet to Earth is Proxima Centauri b, which is about four light-years away. Its mass is consistent with it being slightly larger than Earth. Proxima Centauri b orbits the nearest star to the Sun (1.3 pc away). Proxima Centauri b has a minimum mass (msini) of 1.27 M⊕, a semi-major axis of 0.05 AU, a period of 11 days, a radial velocity semi-amplitude of 1.4 m/s, and does not appear to transit. Based on what we know about exoplanets, and planets in our solar system similar in mass to Earth, it is most likely a rocky planet. Proxima Centauri b orbits in the “habitable zone” of its star, which means it could have liquid water on its surface– if it has an atmosphere which could support it. Calculations suggest that the one side of the exoplanet Proxima b faces its star all the time, which would make for a very different atmosphere and weather than we have on Earth. Its parent star, Proxima Centauri, is a dim red dwarf star that gives off about 600 times less light than our sun. Proxima Centauri is the closest star to the Earth after our sun, but it is still about 9,000 times further than planet Neptune.

The successes of discovering exoplanets in recent decades seem to be telling us that the galaxy is teeming with trillions of exoplanets, but finding them isn’t easy. Planets are typically billions of times fainter than the stars they orbit, and they are incredibly distant. The challenges of observing extrasolar planets stem from four basic facts:

Planets don’t produce any light of their own, except when they’re recently formed (i.e. young).
They are an enormous distance from us.
They are lost in the blinding glare of their parent stars.
Their sizes and masses are typically much, much smaller than that of the stars they orbit.

Most exoplanets are found through indirect methods: measuring the dimming of a star that happens to have a planet pass in front of it, or monitoring the spectrum of a star for the tell-tale signs of a planet pulling on its star and causing its light to subtly Doppler shift. NASA’s Kepler Space Telescope finds thousands of planets by observing “transits,” the slight dimming of light from a star when its tiny planet passes between it and our telescopes. Other methods include gravitational lensing, the “wobble method” (Doppler Spectroscopy, or sometimes the Radial Velocity Method) and direct imaging.

Many people thought that other solar systems were like our own – a few small rocky planets closer to the sun, and some giant planets further out – and that it would, therefore, be nearly impossible to find exoplanets because our tools aren’t sensitive enough to see into those kinds of systems. This was such a popular idea that people working in the field early on had trouble getting access to telescopes and funding. There were tentative early discoveries but they didn’t match expectations, so they didn’t really change the field that much. Then, the 1995 paper, from Michel Mayor and Didier Queloz – that led to their winning the Nobel Prize in 2019 – strongly argued that we really were seeing exoplanets. Another half-dozen exoplanet discoveries came right after because they had just been sitting in peoples’ closets, unanalyzed, waiting for this kind of strong argument. It turns out, also, the universe seems to favor small planets and, so, as the techniques got more sensitive, they’ve found more and more.

Michel Mayor shared Nobel Prize in 2019 along with Didier Queloz for discovering an exoplanet orbiting a solar-type star using doppler spectroscopy in October 1995. Using novel instruments at the Haute-Provence Observatory in southern France, they detected a gas giant similar to Jupiter, which they named 51 Pegasi b. It was a combination of really carefully ruling out other explanations and of having the confidence to assert that they found an exoplanet. Their measurements required colleagues to accept a planet – now called 51 Pegasi b – unlike anything they had imagined: hot, Jupiter-sized, closer to its sun than Earth is to ours, and with an orbit of less than five days. Along the way, Mayor and Queloz had to rule out other possibilities, such as the suggestion that their measurements were actually showing a star that was expanding and contracting, or that they had found something larger orbiting a star and were merely observing it from an odd angle that made the orbiting object seem planet-sized. It also helped that many others made similar measurements, so unexpected exoplanets started to become more likely than some weird chance alignment.

There are two main methods that we discover planets by right now: the Doppler method and the transit method. Both of these are indirect ways of “seeing” planets, which means we are observing their effects but not the planets themselves. Seeing planets directly is very hard because they are so close to their stars and so much fainter by comparison.

The Doppler method measures how the planet’s gravity tugs on the star that it’s orbiting day after day, year after year. We can’t see the thing that’s pulling on the star but we can calculate it by Doppler Spectroscopy. This was the technique that was used by Michel Mayor and Didier Queloz who were awarded the Nobel Prize in 2019.

The transit method involves measuring changes in light from the star. If a planet passes in front of a star, it will block some of the light from the star, causing it to dim. (If you were looking at our solar system from far away in just the right direction, you’d see our sun get about 1 percent fainter every 12 years when Jupiter gets in the way.) For this to work, though, you have to get very lucky – the planet and star have to line up just so. If you’re not feeling mega-lucky, then you have to look at tens of thousands or hundreds of thousands of stars to find the few that are lined up just right. With modern big digital cameras and modern computing, that’s possible. Automated software finds the possible planets, then astronomers figure out which ones are real and interesting. Because it’s so automated and computerized, that’s the way most planets have been discovered so far.

Both of these methods work best when planets are close to their star. In a universe full of solar systems just like our own they would almost never work. The first amazing surprise about exoplanets is that there are so many planets of all kinds and sizes so close to their stars. We can calculate their mass or radius, maybe their density and a little bit of vague information about their atmosphere. We can also sometimes estimate their age.

With modern telescopes and instruments, if the light from the star passed through a transiting planet’s atmosphere before it gets to you, you can learn something about its atmospheric composition. Right now, for that to work, it has to be a big planet – at least Neptune-sized – and you have to see it transit many times. By analyzing that light, we can find evidence of individual molecules in the planet’s atmosphere – like carbon monoxide or water vapor or methane – and learn things about the temperature of the planet or the pressure in its atmosphere. As for age, you can usually tell if a star is really young, and that means its planets (if it has any) will also be young.

Earth-like means Earth-sized:

Right now, every time you see the word “Earth-like” you should replace it with the word “Earth-sized,” because that’s what we’re measuring. Here’s why that matters: Venus is an Earth-sized planet but isn’t Earth-like in other ways we care about. Given that we can’t measure the composition for an Earth-sized planet or for planets that are orbiting stars like our own sun, there’s certainly no evidence for a truly Earth-like planet. The transit method can get some measurements of exoplanets – specifically hot versions of Neptune. In those, we do see the same elements that are present that are present in our solar system, like water and carbon dioxide. So that might imply that smaller planets are Earth-like.

A study of Kepler space telescope data shows that probably all sun-like stars have planets of some kind, and that small Earth-size worlds are more common than gas giants like Jupiter. Astronomers studying data from the Kepler space telescope estimate that 17 percent of stars in the Milky Way galaxy have planets about the size of Earth. This means that about one in six stars has an Earth-size companion exoplanet. As there are about 100 billion stars in the galaxy, there are at least 17 billion Earth-size worlds in this galaxy alone. Our own star possesses two Earth-size worlds. Earth itself, and the uninhabitable planet Venus, with its thick carbon dioxide atmosphere and a surface temperature of 870 degrees F (465 degrees C). The research was done by the Harvard-Smithsonian Center for Astrophysics.

There are telescopes in space, on the ground and even in the air that are being used to hunt exoplanets. NASA has telescopes in space currently studying exoplanets – one observatory dedicated to discovering exoplanets (the Transiting Exoplanet Survey Satellite), and Hubble, a powerful, general-purpose observatory that conduct a wide range of astronomical observations, including exoplanet science. More than two dozen telescopes on the ground are being used to discover and characterize exoplanets, ranging from small robotic observatories to large telescopes like the Keck Observatories in Hawaii. Even SOFIA, NASA’s infrared observatory built into a Boeing 747-SP airplane, has conducted some exoplanet observations.

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Methods to detect exoplanets:

There are at least five major methodologies that can be used to detect extrasolar planets and these are listed as follows: Transit Method, Doppler Shift, Gravitational Microlensing, Astrometric Measurement, and Direct Detection.

Transit photometry: Transit method for detecting exoplanet:

The transit phenomenon is similar to a solar eclipse. When a planet goes between us and its host star, it blocks out a little bit of the star’s light. It is technically demanding to precisely measure the minute changes in the stellar light, but this procedure happens to be the most efficient way of identifying several thousand exoplanets, including those by NASA’s Kepler and TESS missions. It also has value-added advantages in collecting candidates for follow-up characterization of the chemical composition and physical make-up of the planets’ atmospheres during their transits.

If a star’s brightness temporarily dips, then something probably passed in front of it as seen from Earth. If it dips in regular intervals and by the same amount each time, that’s usually caused by a planet. While the radial velocity method provides information about a planet’s mass, the photometric method can determine the planet’s radius. If a planet crosses (transits) in front of its parent star’s disk, then the observed visual brightness of the star drops by a small amount, depending on the relative sizes of the star and the planet. For example, in the case of HD 209458, the star dims by 1.7%. However, most transit signals are considerably smaller; for example, an Earth-size planet transiting a Sun-like star produces a dimming of only 80 parts per million (0.008 percent).

Figure above shows transit method by which astronomers can tell a lot about the size and composition of a planet just by observing the light it blocks as it orbits its star.

If, during a transit, a star’s light passes through a world’s skies, then it carries information about the chemical makeup of that planet’s atmosphere — information that, with the right instruments, astronomers can decode. This is useful for all sorts of reasons, including working out if a world is potentially habitable to biology of any variety, or even to search for hints of possible biosignatures themselves — chemicals that can be produced (sometimes exclusively) by life.

This technique could also be used to find the pollution from an alien civilization. Nitrogen dioxide, for example, is made by forest fires, volcanoes, lightning, and other natural sources. But much of Earth’s nitrogen dioxide comes from the burning of fossil fuels, particularly from road vehicles. Detecting that on an exoplanet may hint at the presence of a fossil fuel-burning civilization that has yet to move exclusively onto sources of futuristic clean energy, like nuclear fusion. Like many biosignatures with both natural and artificial sources, the detection of plenty of nitrogen dioxide wouldn’t be a slam-dunk confirmation of an alien intelligence.

Other chemicals would sound a clearer alarm, such as chlorofluorocarbons (CFCs). These are found in aerosol sprays, packing materials, solvents, refrigerants, and more; they ate away at the ozone layer before being broadly banned across the globe by the Montreal Protocol. There is no natural process that can produce CFCs. It is not inconceivable that, as the James Webb Space Telescope is examining an exoplanet for biosignatures, it detects the presence of CFCs.

SETI scientists are not just interested in the information carried by that starlight; they are also curious about the total amount of starlight they are receiving. When an object passes in front of the star, its apparent brightness dips. And it’s possible the dip could be caused by something other than a planet — something much more implausible but considerably more fantastic. Science fiction is full of alien megastructures, unfathomably giant objects like world-sized space stations or colossal orbs surrounding stars to siphon off an almost endless supply of solar energy. There is always a chance that a transit reveals the existence of something decidedly nonnatural around a distant star — a detection that could be followed up by targeted radio SETI work.

Some transits have already raised astronomers’ eyebrows. The chaotic, sporadic dimming around Tabby’s Star (named after an American astronomer who led the team that discovered the star’s weird light fluctuations), for example, cannot be explained by the periodic orbit of a planet. Nobody can confidently explain the cause of these shenanigans, but various hypotheses have been suggested, including the shattered remnants of a planet, swarms of comets, and, yes, an alien megastructure — a type of optical technosignature. Although few scientists are betting on an extraterrestrial intelligence explanation, ongoing work has still to conclusively rule it out.

To establish a new transiting exoplanet, one should first check that the observed photometric signal is due to a planet or to a false-positive scenario. For that, different kind of complementary observations can be used:

Ground-based high-resolution photometry (Deeg et al. 2009) or centroid measurement (Bryson et al. 2013) to reject background eclipsing binary contaminating the target’s Point Spread Function (PSF) (Almenara et al. 2009). Adaptive optics images (Adams et al. 2012) or speckle observations (Howell et al. 2011) can also be used to constrain, closer to the star, the presence of a contaminant.
Infrared photometry to constrain the presence of a contaminating star with a different color than the target (Fressin et al. 2012).
High-resolution spectroscopy to identify multiple stellar systems (e.g. Santerne et al. 2012).

Precise radial-velocity (RV) observations can be used to measure the mass of the transiting object. If this transiting object has a mass compatible with the planet’s mass range, the planet is therefore established (e.g. Santerne et al. 2011b,c).

Disadvantages of transit method:

This method has two major disadvantages.

-1. First, planetary transits are observable only when the planet’s orbit happens to be perfectly aligned from the astronomers’ vantage point. The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit (in small stars, the radius of the planet is also an important factor). About 10% of planets with small orbits have such an alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a Sun-sized star at 1 AU, the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot guarantee that any particular star is not a host to planets. However, by scanning large areas of the sky containing thousands or even hundreds of thousands of stars at once, transit surveys can find more extrasolar planets than the radial-velocity method. Several surveys have taken that approach, such as the ground-based MEarth Project, SuperWASP, KELT, and HATNet, as well as the space-based COROT, Kepler and TESS missions. The transit method has also the advantage of detecting planets around stars that are located a few thousand light years away. The most distant planets detected by Sagittarius Window Eclipsing Extrasolar Planet Search are located near the galactic center. However, reliable follow-up observations of these stars are nearly impossible with current technology.

-2. The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the Kepler mission could be as high as 40% in single-planet systems. For this reason, a star with a single transit detection requires additional confirmation, typically from the radial-velocity method or orbital brightness modulation method. The radial velocity method is especially necessary for Jupiter-sized or larger planets, as objects of that size encompass not only planets, but also brown dwarfs and even small stars. As the false positive rate is very low in stars with two or more planet candidates, such detections often can be validated without extensive follow-up observations. Some can also be confirmed through the transit timing variation method.

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Doppler spectroscopy method:

Our sun is over 300,000 times heavier than Earth. While Earth and the Sun are both attracted to each other gravitationally, the effect on the Earth is much more easily seen because of Earth’s lower mass. However, not all planets and stars are like this. Jupiter, for instance, is only 1000 times lighter than the sun. When we look at some other solar systems we have found, there are planets much heavier than Jupiter that orbit their stars at a closer distance than Mercury orbits our sun. When the planet’s mass becomes significant compared to the star’s, it becomes more obvious that the planet and the star are orbiting together. They move together around a single central point. This is true in our solar system as well, but it’s easier to see in star systems where the planets are heavier and closer in to their stars. We could look for the star to move in the sky, but most stars are so far away that we wouldn’t be able to see their motion in this way. Luckily, we can use another technique.

The movement of a light source, such as a star, changes the frequencies of light that the star emits. This is the same Doppler effect that makes a fire engine’s siren change in pitch as it moves past you (higher pitch as it moves toward you, lower as it moves away). When the star moves towards us, it releases higher-frequency light. When it moves away, the light is of lower frequency. The difference is small, but our instruments are so sensitive to frequency that we can detect a speed of just half a meter per second! If we see this type of rhythmic change in the light frequency emitted by a star, we know the star is being orbited by a significant body. This approach for detecting extrasolar planets is known as Doppler Spectroscopy, or sometimes the Radial Velocity Method. Because the star moves around in a circle, this is also sometimes called the Wobble Method.

According to scientists “the Doppler Effect is the apparent change in the wavelength of radiation caused by the motion of the source”. In application of this principle, NASA scientists were able to discover that “precise measurement of the velocity or change of position of stars tells us the extent of the star’s movement induced by a planet’s gravitational tug”. It is a clever way of determining if there is a planet that orbits a particular star. However, there can be other factors that are creating the Doppler Shift. But more importantly there is no way to clarify if this planet resembles the Earth in terms of size and if it orbits within a habitable zone. The Doppler Shift technique can easily detect giant-sized planets orbiting close to a star such as those with masses similar to Jupiter.

A star with a planet will move in its own small orbit in response to the planet’s gravity. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star’s spectral lines due to the Doppler effect. The radial-velocity method measures these variations in order to confirm the presence of the planet using the binary mass function.

The speed of the star around the system’s center of mass is much smaller than that of the planet, because the radius of its orbit around the center of mass is so small. (For example, the Sun moves by about 13 m/s due to Jupiter, but only about 9 cm/s due to Earth). However, velocity variations down to 3 m/s or even somewhat less can be detected with modern spectrometers, such as the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrometer at the ESO 3.6 meter telescope in La Silla Observatory, Chile, the HIRES spectrometer at the Keck telescopes or EXPRES at the Lowell Discovery Telescope. An especially simple and inexpensive method for measuring radial velocity is “externally dispersed interferometry”.

Until around 2012, the radial-velocity method (also known as Doppler spectroscopy) was by far the most productive technique used by planet hunters. (After 2012, the transit method from the Kepler spacecraft overtook it in number.) The radial velocity signal is distance independent, but requires high signal-to-noise ratio spectra to achieve high precision, and so is generally used only for relatively nearby stars, out to about 160 light-years from Earth, to find lower-mass planets. It is also not possible to simultaneously observe many target stars at a time with a single telescope. Planets of Jovian mass can be detectable around stars up to a few thousand light years away. This method easily finds massive planets that are close to stars. Modern spectrographs can also easily detect Jupiter-mass planets orbiting 10 astronomical units away from the parent star, but detection of those planets requires many years of observation. Earth-mass planets are currently detectable only in very small orbits around low-mass stars, e.g. Proxima b.

It is easier to detect planets around low-mass stars, for two reasons: First, these stars are more affected by gravitational tug from planets. The second reason is that low-mass main-sequence stars generally rotate relatively slowly. Fast rotation makes spectral-line data less clear because half of the star quickly rotates away from observer’s viewpoint while the other half approaches. Detecting planets around more massive stars is easier if the star has left the main sequence, because leaving the main sequence slows down the star’s rotation.

Like all methods, the wobble method has its limits. The most obvious is that the smaller the planet, and the farther away it is from its central star, the smaller the effect on the star. Earth’s gravitational effect on the Sun, for example, would be beyond the detection limits of our current spectroscopes. The Doppler Effect is also much more potent when a star is moving toward or away from us in the sky, rather than around in a circle. If the orbital plane of the planet happens to line up with the line-of-sight of the observer, then the measured variation in the star’s radial velocity is the true value. It is hard or impossible for us to detect solar systems that are tilted at the wrong angle with respect to us. Sometimes Doppler spectroscopy produces false signals, especially in multi-planet and multi-star systems. Magnetic fields and certain types of stellar activity can also give false signals. When the host star has multiple planets, false signals can also arise from having insufficient data, so that multiple solutions can fit the data, as stars are not generally observed continuously. Some of the false signals can be eliminated by analyzing the stability of the planetary system, conducting photometry analysis on the host star and knowing its rotation period and stellar activity cycle periods.

Even when the wobble method works, it only tells us that a planet is present, and some information about its mass and orbital distance. We do not learn about the size of the planet, which also means we cannot determine its density. The transit method provides more information about the planet, but it does not give us information about the planet’s mass. The wobble method and transit method work best in conjunction with one another.

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Astrometric Measurement method:

When astronomers in the Kepler Mission “use astrometry, they look for a minute but regular wobble in a star’s position”. This methodology, however, requires sophisticated telescopes that ideally must be launched into space to overcome the effect of the interference of the Earth’s atmosphere. Although the Astrometric Measurement is a refined technique in the sense that it can detect even the slight wobble of the star in relation to its interaction with an orbiting planet, it has one major limitation. The main weakness is that it cannot help astronomers determine the exact size of the extrasolar planet. Nevertheless, the Astrometric Measurement enables astronomers to increase the number of extrasolar planets that they can detect and analyse. It is therefore imperative to have another tool at their disposal.

Gravitational Microlensing method:

Another strategy that utilises the behaviour of light from a star in connection to a planet orbiting around it is the Gravitational Microlensing. The idea came from Einstein’s theory of general relativity and it is based on the principle that “light rays bend when passing through space that is warped by the presence of a massive object such as a star”. When one star in the sky appears to pass nearly in front of another, the light rays of the background source star become bent due to the gravitational “attraction” of the foreground star. This star is then a virtual magnifying glass, amplifying the brightness of the background source star, so we refer to the foreground star as the lens star. If the lens star harbors a planetary system, then those planets can also act as lenses, each one producing a short deviation in the brightness of the source. Thus we discover the presence of each exoplanet, and measure its mass and separation from its star. Microlensing is the only known method capable of discovering planets at truly great distances from the Earth and is capable of finding the smallest of exoplanets. Whereas the Radial Velocity Method is effective when looking for planets up to 100 light years from Earth and Transit Photometry can detect planets hundreds of light-years away, microlensing can find planets that are thousands of light-years away.

While most other methods have a detection bias towards smaller planets, the microlensing method is the most sensitive means of detecting planets that are around 1-10 astronomical units (AU) away from Sun-like stars. Microlensing is also the only proven means of detecting low-mass planets in wider orbits, where both the transit method and radial velocity are ineffective. This technique will tell us how common Earth- like planets are, and will guide the design of future exoplanet imaging missions. More than 20 planets have been discovered from the ground using this technique.

The Transit Method must be used to complement the Doppler Shift method, Astrometric Measurement method and Gravitational Microlensing.

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Direct imaging:

The most straightforward way is the direct imaging method. You just look up into the sky at the right place and right time, as ancient Greek and Chinese astronomers did when they spotted the planets in our solar system. Since they move relative to the Sun and background stars, our ancestral observers named them planets. It is difficult to see Mars, Jupiter and Saturn during the day because the Sun’s light is too bright. Similarly, from our vantage point, exoplanets are very close to their host star, which usually outshines them billions of times over. The technical challenge with this approach is the attempt to effectively block the star’s light so that the reflected light from the planets can peek through.

Planets are extremely faint light sources compared to stars, and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is very difficult to detect and resolve them directly from their host star. Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead. It is easier to obtain images when the star system is relatively near to the Sun, and when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation; images have then been made in the infrared, where the planet is brighter than it is at visible wavelengths. Coronagraphs are used to block light from the star, while leaving the planet visible. Direct imaging of an Earth-like exoplanet requires extreme optothermal stability. During the accretion phase of planetary formation, the star-planet contrast may be even better in H alpha than it is in infrared – an H alpha survey is currently underway.

Figure above shows direct image of exoplanets around the star HR 8799 using a Vortex coronagraph on a 1.5m portion of the Hale telescope.

Direct imaging can give only loose constraints of the planet’s mass, which is derived from the age of the star and the temperature of the planet. Mass can vary considerably, as planets can form several million years after the star has formed. The cooler the planet is, the less the planet’s mass needs to be. In some cases it is possible to give reasonable constraints to the radius of a planet based on planet’s temperature, its apparent brightness, and its distance from Earth. The spectra emitted from planets do not have to be separated from the star, which eases determining the chemical composition of planets.

Sometimes observations at multiple wavelengths are needed to rule out the planet being a brown dwarf. Direct imaging can be used to accurately measure the planet’s orbit around the star. Unlike the majority of other methods, direct imaging works better with planets with face-on orbits rather than edge-on orbits, as a planet in a face-on orbit is observable during the entirety of the planet’s orbit, while planets with edge-on orbits are most easily observable during their period of largest apparent separation from the parent star.

The planets detected through direct imaging currently fall into two categories. First, planets are found around stars more massive than the Sun which are young enough to have protoplanetary disks. The second category consists of possible sub-brown dwarfs found around very dim stars, or brown dwarfs which are at least 100 AU away from their parent stars.

So far, few exoplanets have been directly imaged – when pixels of light are captured from the planet itself. Very large, very young planets still glowing from the heat of formation are, so far, the only ones to be imaged this way. But planets past their youth, lit up only by their stars, would be targeted for direct imaging by space telescopes now in the conceptual phase. Some of these would use a type of starlight-blocking technology called a coronagraph. This system of masks, prisms, mirrors, and filters inside a telescope blots out the light of a star, revealing the planets in orbit around it. Another possible technology would deploy a “starshade,” a sunflower-shaped spacecraft as big as a baseball diamond, some 25,000 miles (40,000 kilometers) ahead of a space telescope. The starshade also would block starlight, allowing the telescope to capture direct images of a star’s suite of planets.

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How many Earth-Like Planets are out there?

Scientists from UC Berkeley and the University of Hawaii issued a study that 22 percent of Sun-like stars may harbour planets roughly the size of Earth in their habitable zones that have been over-looked because these planets are harder to detect. One of the study’s co-authors, Andrew Howard started “With about 100 billion stars in our Milky Way galaxy, that’s about 20 billion such planets, … That’s a few Earth-sized planets for every human being on the planet Earth.”

The estimates of potential earth-like planets vary among astronomers and exobiologists (those who study extraterrestrial life). NASA estimates conclude that there are more than 100 billion Earth-like planets based on the assumption that our universe has 500 billion stars like our own. That figure contemplates nearly identical conditions for life to evolve on these planets. Evidence derived from new powerful telescopes, including Kepler’s exploration of the Milky Way, and various space probes in our own solar system have shown that the water is more common-place that thought, and the organic building blocks of life are abundant.

Beyond the Milky Way, the numbers of planets that could support life, as we know it, are really overwhelming. Astronomers at the University of Auckland claim that there are around 100 billion habitable, Earth-like planets in the Milky Way. Multiplied by the 500 billion plus galaxies in the universe, they estimate around 50,000,000,000,000,000,000,000 (5×10^22) habitable planets, or 50 sextillions in the universe. Of course, forms of life could potentially evolve without Earth eco-systems would exponentially change that estimate to even a greater number.

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How do we name exoplanets?

Exoplanet names can look long and complicated at first, especially when compared to names like Venus and Mars. However, there is a logic behind their naming system that is important to how scientists catalogue thousands of planets. Astronomers differentiate between the alphanumeric “designations” and alphabetical “proper names.” All stars and exoplanets have designations, but very few have proper names.

The first part of an exoplanet name is usually the telescope or survey that discovered it. The number is the order in which the star was catalogued by position. The lowercase letter stands for the planet, in the order in which the planet was found. The first planet found is always named b, with ensuing planets named c, d, e, f and so on. The star that the exoplanet orbits is usually the undeclared “A” of the system, which can be useful if the system contains many stars, which themselves may be designated B or C. (Stars get capital letters; planets receive lowercase designations.) If a bunch of exoplanets around the same star are found at once, the planet closest to its star is named b with more distant planets named c, d, e and so on.

An example of an exoplanet name is Kepler-16b, where “Kepler” is the name of the telescope that observed the system, 16 is the order in which the star was catalogued and “b” is the closest planet to the star. If we were naming Earth as an exoplanet, it would be called Sun d (Sun is the name of our star, and Earth is the third planet, starting with b, Mercury).

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Section-7

Planet habitability:

As the existence of life beyond Earth is unknown, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and Solar System which appear favorable to life’s flourishing. Of particular interest are those factors that have sustained complex, multicellular organisms on Earth and not just simpler, unicellular creatures. Research and theory in this regard is a component of a number of natural sciences, such as astronomy, planetary science and the emerging discipline of astrobiology.

Discussing the habitability of a celestial body is very closely connected to the discussion of life outside Earth. Life of course cannot ignore its connection to habitability, but the reverse is not automatically true. The word “habitable” means “suitable to live in (or on)”, and life cannot originate without habitability. But as an abandoned house might be perfectly suitable to live in (habitable) this does not necessarily mean there is somebody living in it. The question to ask in regard to celestial bodies is not only “Is it habitable?”, but also “Could life have originated and evolved there?” A planet may be defined as habitable if it has an atmosphere and is warm enough to support the existence of liquid water on its surface. Such a world has the basic set of conditions that allow it to develop life similar to ours, which is carbon-based and has water as its universal solvent. While this definition is suitably vague to allow a fairly broad range of possibilities, it does not address the question as to whether any life that does form will become either complex or intelligent.

The notion of the habitable zone has its origins in what we know about the planetary trio of Venus, Earth, and Mars. With surface temperatures exceeding 400 degrees Celsius, present-day Venus is a scorching inferno largely devoid of water. Its hostile temperatures are a direct consequence of its thick atmosphere being dominated by carbon dioxide—a powerful greenhouse gas that makes up less than one part in a thousand of the atmospheres of Earth. Mars is a study in contrasts, having a thin atmosphere, large temperature swings, and an average surface temperature that is well below the freezing point of water. Earth sits between these two extremes, so it is convenient to visualize Earth as residing within a zone of habitability, flanked by Venus and Mars.

Besides the potential habitats in the Solar System, more than 5000 exoplanets have been detected to date. The current status of exoplanet characterization shows a surprisingly diverse set of mainly giant planets. Some of their properties have been measured using photons from the host star, a background star, or a mixture of the star and planet. These indirect techniques include radial velocity, micro-lensing, transits, and astrometry (Beaugé et al. 2008; Fridlund and Kaltenegger 2008; Rauer and Erikson 2008). Earth is until now the only example of a known habitable planet. Compared to other terrestrial planets in our Solar System, Earth is unique: it has liquid water on its surface, an atmosphere with a greenhouse effect that keeps its surface above freezing, and the right mass to maintain tectonics (e.g., Kasting et al. 1993).

All living things have one thing in common. All living things are carbon-based and that they need water to survive. Water is therefore important to sustain life. There is no need to elaborate the reason why water is a crucial ingredient in a habitable planet. Water must be used as a primary requirement in the search for Earth-like planets.

Scientists pointed out that water remains in liquid form if the planet is in a distance that would enable it to “receive sufficient radiation from the star to maintain the effective surface temperature of the planet above the melting point of water, 273 K”. This is an important statement because water can appear in three basic forms, liquid, gas, and solid.

It is imperative that water remains in a liquid state in order for life to exist. However, if one considers the environmental conditions outside planet Earth, one will find that water cannot remain liquid beyond what is called the habitable zone.

The habitable zone or HZ is the optimum condition and “this condition defines a habitable zone around a star and it is the range of distances from which an orbiting planet will have liquid water on its surface”.

In the case of the Earth, “the habitable zone is determined by two factors: the effective surface temperature of a planet as determined by the flux arriving form the local star and the radiation-trapping efficiency atmosphere around the planet”. This is an interesting definition because it also takes into consideration the necessity to trap heat because of the presence of an atmosphere. The HZ is an important topic of discussion when it comes to the search for life outside planet Earth.

Habitable zones are those regions from a star at which liquid water could potentially exist on the surface of orbiting planets, according to NASA. The concept of habitable zones, also referred to as the Goldilocks’ zones, suggests that these areas could be optimal for the development of life.

When searching for habitable exoplanets, scientists focus on exoplanets that are similar to Earth, although the definition of “similar” can be subjective. Most of the Earth-sized exoplanets discovered so far are in orbit around red-dwarf stars, while Earth-sized planets in wide orbits around Sun-like stars are harder to detect.

The potential for life-bearing planets in the habitable zones is wider for hotter stars, and smaller, dimmer red dwarfs have much tighter habitable zones, which are exposed to high levels of X-ray and ultraviolet (UV) radiation. The exoplanets in the narrow habitable zone around a red dwarf star are very close to the star and receive extreme levels of X-ray and UV radiation, which can be hundreds of thousands of times more intense than what Earth receives from the Sun. Therefore, the search for habitable exoplanets requires taking into account the type of star that the planet orbits.

In astrobiology, the Goldilocks zone refers to the habitable zone (HZ) around a star. The Earth is in the middle of which is called the habitable zone or Goldilocks zone. The Goldilocks principle is named by analogy to the children’s story, The Three Bears, in which a little girl named Goldilocks tastes three different bowls of porridge, and she finds that she prefers porridge that is neither too hot nor too cold but has just the right temperature. The Goldilocks principle states that something must fall within certain margins, as opposed to reaching extremes. In planetary science, the “Goldilocks zone” is the terminology for the band around a sun where temperatures are neither too hot nor too cold for liquid water to exist. As Stephen Hawking put it, “like Goldilocks, the development of intelligent life requires that planetary temperatures be ‘just right'”. The Rare Earth Hypothesis uses the Goldilocks principle in the argument that a planet must be neither too far away from nor too close to a star and galactic center to support life, while either extreme would result in a planet incapable of supporting life. Such a planet is colloquially called a “Goldilocks Planet”.

The definition of “habitable zone” is the distance from a star at which liquid water could exist on orbiting planets’ surfaces. Habitable zones are also known as Goldilocks’ zones, where conditions might be just right – neither too hot nor too cold – for life as seen in the figure below.

Paul Davies has argued for the extension of the principle to cover the selection of our universe from a (postulated) multiverse: “observers arise only in those universes where, like Goldilocks’s porridge, things are by accident ‘just right'”.

Earth is until now the only example of a known habitable planet. Compared to other terrestrial planets in our Solar System, Earth is unique: it has liquid water on its surface, an atmosphere with a greenhouse effect that keeps its surface above freezing, and the right mass to maintain tectonics (e.g., Kasting et al. 1993). Earth orbits its host star—our Sun—within a region that is called the habitable zone (HZ)—the region where an Earth analog planet can maintain liquid water on its surface is shown in figure below.

Figure above shows the HZ (upper panel) and the chemistry composition (lower panel) of an Earth-analog atmosphere as a function of distance from its host star. The dashed-dotted line represents the surface temperature of the planet and the dashed lines correspond to the inner edge of the HZ where the greenhouse conditions vaporize the whole water reservoir.

The classical concept of the HZ was first proposed by Huang (1959, 1960) and has been modeled by several authors (e.g., Rasool and deBergh 1970; Hart 1979; Kasting et al. 1993). The differences in the calculations are the climatic constraints imposed on the limits of the HZ. In all cases the stellar habitable zone is a spherical shell around a main sequence star where a planet with an atmosphere can support liquid water at a given time. The width and distance of this shell depends on the stellar luminosity that evolves during the star’s lifetime. The continuously habitable zone (CHZ) has been introduced as the zone that remains habitable around a star during a given period of time (Hart 1978).

Liquid water seems to be an important requirement for habitability. Liquid water has been pointed out as the best solvent for life to emerge and evolve in. Some of the important characteristics of liquid water as a solvent include: a large dipole moment, the capability to form hydrogen bonds, to stabilize macromolecules, to orient hydrophobic–hydrophilic molecules, etc. Water is an abundant compound in our galaxy, it can be found in different environments from cold dense molecular clouds to hot stellar atmospheres (e.g., Cernicharo and Crovisier 2005). Water is liquid at a large range of temperatures and pressures and it is a strong polar–nonpolar solvent. This dichotomy is essential for maintaining stable biomolecular and cellular structures (DesMarais et al. 2002) and there are a large number of organisms that are capable of living in water.

As planets are being discovered around other stars by the thousands, several scientific disciplines, including astronomy, planetary science, and biochemistry, are converging, with the goal of locating and identifying life elsewhere in the Universe. We are engaged in a search for habitability—conditions suitable for life—even though we lack a clear definition of what life is. We are hunting for something we cannot yet sharply define. Nevertheless, we can make informed inferences about what life requires. From what we know about life on Earth, liquid water appears to be an essential ingredient. If an exoplanet orbits at the appropriate range of distances from its star to allow liquid water to exist on its surface, then it is said to be in the habitable zone— it is not too hot, not too cold, purportedly just right for living things.

The Habitable Exoplanets Catalogue uses estimated surface temperature range to classify exoplanets:

-hypopsychroplanets – very cold (<−50 °C)

-psychroplanets – cold (<−50 to 0 °C)

-mesoplanets – medium temperature (0–50 °C; not to be confused with the other definition of mesoplanets)

-thermoplanets – hot (50-100 °C)

-hyperthermoplanets – (> 100 °C)

Mesoplanets would be ideal for complex life, whereas hypopsychroplanets and hyperthermoplanets might only support extremophilic life.

The Habitable Exoplanets Catalogue uses the following terms to classify exoplanets in terms of mass, from least to greatest: asteroidan, mercurian, subterran, terran, superterran, neptunian, and jovian.

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Habitability factors:

Habitable zone, the orbital region around a star in which an Earth-like planet can possess liquid water on its surface and possibly support life. Liquid water is essential to all life on Earth, and so the definition of a habitable zone is based on the hypothesis that extraterrestrial life would share this requirement. This is a very conservative (but observationally useful) definition, as a planet’s surface temperature depends not only on its proximity to its star but also on such factors as its atmospheric greenhouse gases, its reflectivity, and its atmospheric or oceanic circulation. Moreover, internal energy sources such as radioactive decay and tidal heating can warm a planet’s surface to the melting point of water. These energy sources can also maintain subsurface reservoirs of liquid water, so a planet could contain life without being within its star’s habitable zone. Earth, for instance, has a thriving subsurface biosphere, albeit one that is composed almost exclusively of simple organisms that can survive in oxygen-poor environments. Jupiter’s moon Europa has a liquid water ocean tens of kilometres below its surface that may well be habitable for some organisms.

About 40 planets, including the nearest extrasolar planet, Proxima Centauri b, and three planets in the TRAPPIST-1 system, have been found that are both roughly Earth-sized and orbiting within the habitable zones of their stars. Astronomers have also used simulations of the climates of other extrasolar planets such as Kepler-452b to determine that they could have surface water under the right climatic conditions.

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Atmosphere and habitability:

Evaluation of potential habitability is assisted by the concept of the Habitable Zone (HZ), defined as the range of distances, or annulus, around a star that would allow a planet with a given atmosphere to maintain surface liquid water. The habitable zone is also an atmosphere-specific concept. Three types of atmospheric gases strongly influence a body’s surface temperature.

First, we need an incondensible greenhouse gas —one that stays in its gaseous form over the range of temperatures found in the atmosphere. On Earth, this role is played by carbon dioxide.

Second, we need a condensible greenhouse gas, which exists in both gaseous and liquid forms. Water is the condensible greenhouse gas of our atmosphere and is the lynchpin of the hydrological cycle.

The boundaries of the habitable zone are determined by what happens to the condensible and incondensible greenhouse gases at different distances from the parent star. The inner boundary of the habitable zone is the distance at which the condensible greenhouse gas cannot condense, and the outer boundary of the habitable zone is the distance at which the incondensible greenhouse gas can condense. If the Earth were located too close to the Sun, then higher temperatures would result in more water existing as vapor, which in turn would lead to further warming. The planet would compensate for this greenhouse warming by emitting more infrared radiation and by shedding heat, but at some point there would be so much water vapor in the atmosphere that it would become opaque to infrared radiation. At that point, the cooling of the atmosphere would be overwhelmed by heating, leading to a runaway greenhouse effect. Venus is believed to have suffered this fate.

The third ingredient needed is an inert gas, and its role is subtle. On Earth, the primary inert gas is molecular nitrogen. It does not contribute to greenhouse warming, because a nitrogen molecule has an even distribution of electric charge across it. Quantum physics tells us that such molecules are largely incapable of absorbing radiation. Counterintuitively, despite being the dominant gas by mass, molecular nitrogen is transparent to the radiation received and emitted by Earth. However, as the atmosphere warms and accumulates water vapor, water and nitrogen molecules collide. Absorption of units of light or radiation, known as photons, must match the discrete energy levels within a water molecule. When water and nitrogen molecules collide, deficits or surpluses of energy are exchanged. Known as pressure broadening, this effect increases the extent to which the water molecules may absorb radiation. Molecular nitrogen does not directly absorb light, but it influences how the greenhouse gases do so.

Inert gases also set a characteristic distance in the atmosphere known as the pressure scale height, which determines whether an atmosphere is puffy or compact. Hydrogen-dominated atmospheres tend to be puffier than their nitrogen-dominated counterparts. Furthermore, inert gases may participate in the chemistry involving greenhouse gases and can alter their abundances.

Figure below depicts habitable zone as a function of interaction of host star temperature and distance of planet from host star.

If we were to move Earth farther from the Sun, then at some point carbon dioxide would condense out of its atmosphere. As this greenhouse gas was removed, the atmosphere would cool and the overall temperature would drop. The outer boundary of the habitable zone is the distance from the Sun at which the atmosphere becomes too cool to support liquid water on the surface of the body. At high pressures, nitrogen molecules may form transient pairs, which have an uneven distribution of electric charge across them. These pairs produce a weak greenhouse effect known as collision-induced absorption. One imagines that the loss of gaseous carbon dioxide may be compensated for by packing more molecular nitrogen into the atmosphere, but there is a limit to the mileage gained, because the nitrogen also condenses out, at some point, when the temperature becomes too low.

Once we understand how greenhouse gases control the habitable-zone boundaries, we may imagine different flavors of habitable zones. Molecular nitrogen may be swapped out for molecular hydrogen, which has a considerably lower condensation temperature: tens of kelvin, rather than about a hundred. For planets with hydrogen-rich atmospheres, the outer boundary of the habitable zone may extend several times as far from the star, because molecular hydrogen compensates for the loss of the incondensible greenhouse gas through collision-induced absorption, thereby warding off its condensation. Water and carbon dioxide may be exchanged for other greenhouse gases, which could absorb and reradiate heat at other wavelengths or frequencies. Generally, a greenhouse gas is effective only if it is absorbent at wavelengths over which the planet is emitting radiation. A greenhouse gas that favors the absorption of blue light is useless if the planet emits only red light.

A fascinating example of a place with alternative atmospheric chemistry is found on Titan, a moon of Saturn that is about 40 percent of the size of Earth and has a fully functioning atmosphere. As in Earth’s atmosphere, the inert gas is molecular nitrogen, and methane is a greenhouse gas. But unlike on Earth, where methane exists only in gaseous form, it is a condensible greenhouse gas on Titan because of the considerably lower temperatures. Instead of carbon dioxide, the incondensible greenhouse gas is molecular hydrogen, which plays a negligible role on Earth. Molecular hydrogen warms the atmosphere of Titan via collision-induced absorption. Titan is hardly in the habitable zone for liquid water, but it would be in the habitable zone for liquid methane!

Without knowledge of the major molecules of an exoplanet’s atmosphere, we can only speculate whether it resides in the habitable zone for liquid water. It is akin to assuming that the exoplanet has an atmosphere exactly like Earth’s, consisting of nitrogen, water, and carbon dioxide—in precisely the same relative amounts, summing up to exactly the same total mass. Declaring a freshly detected exoplanet to be in the “habitable zone” amounts to little more than media spin if its atmospheric composition is unknown. Even professional astronomers sometimes forget this fact.

One of the most promising worlds in which to search for life in our Solar System illustrates why the habitable zone concept may be incomplete. Europa, one of Jupiter’s moons, sits outside of the traditional habitable zone. It has no atmosphere, and water is not liquid at its surface. However, a body of evidence suggests that a deep ocean exists beneath its icy surface, which may host life. Unfortunately, even if subsurface habitats for life are common on exoplanets, they are currently invisible to astronomers. Current technology largely restricts us to characterizing the atmospheres of exoplanets that are Jupiter-like in size. As technology advances, astronomers expect to decipher the atmospheres of smaller, Earth-like exoplanets.

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Boundaries of habitable zones:

The inner boundary of a habitable zone is where water would be lost as a result of a runaway greenhouse effect, in which greenhouse gases in a planet’s atmosphere would trap incoming infrared radiation, leading to the planet’s becoming hotter and hotter until the water boiled away. The outer boundary is where such greenhouse warming would not be able to maintain surface temperatures above freezing anywhere on the planet. Astronomers have calculated the extent of the habitable zone for many different types of stars. For example, at present, the habitable zone of the Sun is estimated to extend from about 0.9 to 1.5 astronomical units (the distance between Earth and the Sun).

The location of a star’s habitable zone depends upon its luminosity. Because a star’s luminosity increases with time, both the inner and outer boundaries of its habitable zone move outward. Thus, a planet that is in the habitable zone when a star is young may subsequently become too hot. Venus may have been such a planet; however, because it is geologically active, its current surface is too young to show any evidence that a more clement climate may have existed billions of years ago. Other planets could be too cold for liquid water to exist when their star is young but might warm up enough to have liquid water on their surface later as their star’s luminosity increases. This may happen to Mars a few billion years hence. Thus, the most promising region to find Earth-like life would be in a “continuously habitable zone,” where liquid water could have been present from early in the star’s life up to the current epoch. The continuously habitable zone of the Sun (from four billion years ago to the present) is from about 0.9 to 1.2 astronomical units.

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Faint young Sun problem:

Earth has had liquid water on its surface for much of the past four billion years. However, four billion years ago the Sun’s luminosity was only about 75 percent as intense as it is at present, and climate models suggest that Earth should have been frozen over at such a low solar luminosity. This apparent disagreement between theory and observation is known as the “faint young Sun problem.” Another planet to which the faint young Sun problem might apply is Mars. On that planet the oldest regions of the surface show signs of running water while younger regions do not, which suggests that Mars had a warmer and thicker atmosphere in the past, when the Sun was less luminous, than it has now that the Sun is brighter. The warmth of Earth and Mars during their early periods (and thus the solution to the faint young Sun problem) can be attributed to the presence of abundant greenhouse gases in their atmospheres, with carbon dioxide, water, and possibly ammonia and methane playing major roles.

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Type of star and planet habitability:

With the flurry of recent discoveries made with the Kepler Space Telescope, it is routine to encounter media reports of “habitable-zone exoplanets”—sometimes accompanied by speculation on what types of life forms may exist on them—using a conception of the habitable zone that extrapolates directly from what we know about our own Solar System. The habitable zone is a star-specific concept (besides atmospheric specific). Stars exist in a variety of sizes and masses. The location of a star’s habitable zone also depends upon its mass. Smaller stars like the Sun survive far longer than do high-mass stars. High-mass stars have lifetimes of only millions of years, whereas advanced life took billions of years to develop on Earth. Thus, even if Earth-like planets formed around high-mass stars at distances where liquid water was stable, it is unlikely that benign conditions would exist long enough on these planets for life to form and evolve into advanced organisms. At the other end of the mass spectrum, the smallest, faintest stars can last for trillions of years. However, these cool dwarf stars emit almost all of their luminosity at infrared wavelengths, which may be difficult for life to harness, and they typically display larger luminosity variations than do Sun-type stars. In addition, in order for a planet to remain within the habitable zone of a faint star, it would have to orbit so close that tidal locking to the host star is likely causing the same hemisphere always to face the star (just as the Moon’s near side always faces Earth). As a result, there would be no day-night cycle, and the planet’s atmosphere, unless it was sufficiently thick, would freeze onto the surface of the cold, perpetually dark hemisphere. (However, if the planet had a sufficiently massive atmosphere, winds would redistribute heat and the atmosphere would not freeze.) Moreover, the high temperatures within the habitable zones of faint stars suggest that such planets are likely to lack the atmospheric gases required by life.

The spectral class of a star indicates its photospheric temperature, which (for main-sequence stars) correlates to overall mass. The appropriate spectral range for habitable stars is considered to be “late F” or “G”, to “mid-K”. This corresponds to temperatures of a little more than 7,000 K down to a little less than 4,000 K (6,700 °C to 3,700 °C); the Sun, a G2 star at 5,777 K, is well within these bounds. This spectral range probably accounts for between 5% and 10% of stars in the local Milky Way galaxy. The most common types of stars in the Universe are not like our Sun, but instead have masses between 10 and 50 percent of it. These red dwarfs have cooler temperatures than our Sun and radiate far less energy, which means that if a planet of Earth’s size were to maintain the same range of atmospheric temperatures it would have to orbit such stars more closely.

Figure above compares the characteristics of three classes of stars in our galaxy: Sunlike stars are classified as G stars; stars less massive and cooler than our Sun are K dwarfs; and even fainter and cooler stars are the reddish M dwarfs.

Life on other planets might be like nothing on Earth – it could be life as we don’t know it. But it makes sense, at least at first, to search for something more familiar. Life as we know it should be easier to find. And “the light’s better” in the habitable zone, or the area around a star where planetary surface temperatures could allow the pooling of water. Other similarities to Earth come into sharper focus in the search for life. Many rocky planets have been detected in Earth’s size-range: a point in favor of possible life. Based on what we’ve observed in our own solar system, large, gaseous worlds like Jupiter seem far less likely to offer habitable conditions. But most of these Earth-sized worlds have been detected orbiting red-dwarf stars; Earth-sized planets in wide orbits around Sun-like stars are much harder to detect. Yet these red-dwarfs have a potentially deadly habit, especially in their younger years: Powerful flares tend to erupt with some frequency from their surfaces. These could sterilize closely orbiting planets where life had only begun to get a toehold. That’s a strike against possible life.

Because our Sun has nurtured life on Earth for nearly 4 billion years, conventional wisdom would suggest that stars like it would be prime candidates in the search for other potentially habitable worlds. G-type yellow stars like our Sun, however, are shorter-lived and less common in our galaxy. A 2020 study found that about half of Sun-like stars could host rocky, potentially habitable planets. Specifically, they estimated with that, on average, the nearest habitable zone planet around G and K-type stars is about 6 parsecs away, and there are about 4 rocky planets around G and K-type stars within 10 parsecs (32.6 light years) of the Sun.

Stars slightly cooler and less luminous than our Sun — called orange dwarfs — are considered by some scientists as potentially better for advanced life. They can burn steadily for tens of billions of years. This opens up a vast timescape for biological evolution to pursue an infinity of experiments for yielding robust life forms. And, for every star like our Sun there are three times as many orange dwarfs in the Milky Way.

K dwarfs, are the true Goldilocks stars. K-dwarf stars are in the ‘sweet spot,’ with properties intermediate between the rarer, more luminous, but shorter-lived solar-type stars (G stars) and the more numerous red dwarf stars (M stars). The K stars, especially the warmer ones, have the best of all worlds. If you are looking for planets with habitability, the abundance of K stars pump up your chances of finding life.

Our Sun is a stable, long-lasting, and metal-rich star:

Our Sun is the most important source of energy for life on Earth. It’s also a stable and long-lasting star.

Stars more massive than the Sun live shorter, usually not long enough for planets to develop life. In general, the larger a star, the shorter its life. The Sun is a G-type main-sequence star (G2V) based on spectral class and is informally referred to as a yellow dwarf. It is stable (even less active compared to its siblings), long-lasting, and metal-rich. The Sun is also unusually metal-rich for a star of its age and type. One possibility is that the Sun formed in a part of the Milky Way Galaxy that had an abundance of metals and then migrated to its current position. Metal-rich stars are more likely to have planets orbiting around them. Furthermore, all life forms require certain core chemical elements needed for biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, often represented by the acronym CHNOPS. Our solar system contains a lot of these metals (all elements heavier than hydrogen and helium are called metals in astronomy).

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Mass and size of planet vis-à-vis habitability:

Low-mass planets are poor candidates for life for two reasons.

First, their lesser gravity makes atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces (for example, Mars, with its thin atmosphere, is colder than the Earth would be if it were at a similar distance from the Sun), and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less dense than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure, 4.56 mm Hg (608 Pa), does not occur. In addition, a lessened pressure reduces the range of temperatures at which water is liquid.

Secondly, smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. Plate tectonics appear particularly crucial, at least on Earth: not only does the process recycle important chemicals and minerals, it also fosters bio-diversity through continent creation and increased environmental complexity and helps create the convective cells necessary to generate Earth’s magnetic field.

“Low mass” is partly a relative label: the Earth is low mass when compared to the Solar System’s gas giants, but it is the largest, by diameter and mass, and the densest of all terrestrial bodies. It is large enough to retain an atmosphere through gravity alone and large enough that its molten core remains a heat engine, driving the diverse geology of the surface (the decay of radioactive elements within a planet’s core is the other significant component of planetary heating). Mars, by contrast, is nearly (or perhaps totally) geologically dead and has lost much of its atmosphere. Thus it would be fair to infer that the lower mass limit for habitability lies somewhere between that of Mars and that of Earth or Venus: 0.3 Earth masses has been offered as a rough dividing line for habitable planets. However, a 2008 study by the Harvard-Smithsonian Center for Astrophysics suggests that the dividing line may be higher. Earth may in fact lie on the lower boundary of habitability: if it were any smaller, plate tectonics would be impossible. Venus, which has 85% of Earth’s mass, shows no signs of tectonic activity. Conversely, “super-Earths”, terrestrial planets with higher masses than Earth, would have higher levels of plate tectonics and thus be firmly placed in the habitable range.

Exceptional circumstances do offer exceptional cases: Jupiter’s moon Io (which is smaller than any of the terrestrial planets) is volcanically dynamic because of the gravitational stresses induced by its orbit, and its neighbor Europa may have a liquid ocean or icy slush underneath a frozen shell also due to power generated from orbiting a gas giant.

Saturn’s Titan, meanwhile, has an outside chance of harbouring life, as it has retained a thick atmosphere and has liquid methane seas on its surface. Organic-chemical reactions that only require minimum energy are possible in these seas, but whether any living system can be based on such minimal reactions is unclear, and would seem unlikely. These satellites are exceptions, but they prove that mass, as a criterion for habitability, cannot necessarily be considered definitive at this stage of our understanding.

A larger planet is likely to have a more massive atmosphere. A combination of higher escape velocity to retain lighter atoms, and extensive outgassing from enhanced plate tectonics may greatly increase the atmospheric pressure and temperature at the surface compared to Earth. The enhanced greenhouse effect of such a heavy atmosphere would tend to suggest that the habitable zone should be further out from the central star for such massive planets.

Finally, a larger planet is likely to have a large iron core. This allows for a magnetic field to protect the planet from stellar wind and cosmic radiation, which otherwise would tend to strip away planetary atmosphere and to bombard living things with ionized particles. Mass is not the only criterion for producing a magnetic field—as the planet must also rotate fast enough to produce a dynamo effect within its core—but it is a significant component of the process.

The mass of a potentially habitable exoplanet is between 0.1 and 5.0 Earth masses. However it is possible for a habitable world to have a mass as low as 0.0268 Earth Masses. The radius of a potentially habitable exoplanet would range between 0.5 and 1.5 Earth radii.

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Orbit and rotation of planet vis-à-vis habitability:

As with other criteria, stability is the critical consideration in evaluating the effect of orbital and rotational characteristics on planetary habitability. Orbital eccentricity is the difference between a planet’s farthest and closest approach to its parent star divided by the sum of said distances. It is a ratio describing the shape of the elliptical orbit. The greater the eccentricity the greater the temperature fluctuation on a planet’s surface. Although they are adaptive, living organisms can stand only so much variation, particularly if the fluctuations overlap both the freezing point and boiling point of the planet’s main biotic solvent (e.g., water on Earth). If, for example, Earth’s oceans were alternately boiling and freezing solid, it is difficult to imagine life as we know it having evolved. The more complex the organism, the greater the temperature sensitivity. The Earth’s orbit is almost perfectly circular, with an eccentricity of less than 0.02; other planets in the Solar System (with the exception of Mercury) have eccentricities that are similarly benign.

Habitability is also influenced by the architecture of the planetary system around a star. The evolution and stability of these systems are determined by gravitational dynamics, which drive the orbital evolution of terrestrial planets. Data collected on the orbital eccentricities of extrasolar planets has surprised most researchers: 90% have an orbital eccentricity greater than that found within the Solar System, and the average is fully 0.25. This means that the vast majority of planets have highly eccentric orbits and of these, even if their average distance from their star is deemed to be within the HZ, they nonetheless would be spending only a small portion of their time within the zone.

A planet’s movement around its rotational axis must also meet certain criteria if life is to have the opportunity to evolve. A first assumption is that the planet should have moderate seasons. If there is little or no axial tilt (or obliquity) relative to the perpendicular of the ecliptic, seasons will not occur and a main stimulant to biospheric dynamism will disappear. The planet would also be colder than it would be with a significant tilt: when the greatest intensity of radiation is always within a few degrees of the equator, warm weather cannot move poleward and a planet’s climate becomes dominated by colder polar weather systems.

If a planet is radically tilted, seasons will be extreme and make it more difficult for a biosphere to achieve homeostasis. The axial tilt of the Earth is higher now (in the Quaternary) than it has been in the past, coinciding with reduced polar ice, warmer temperatures and less seasonal variation. Scientists do not know whether this trend will continue indefinitely with further increases in axial tilt.

The exact effects of these changes can only be computer modelled at present, and studies have shown that even extreme tilts of up to 85 degrees do not absolutely preclude life “provided it does not occupy continental surfaces plagued seasonally by the highest temperature.” Not only the mean axial tilt, but also its variation over time must be considered. The Earth’s tilt varies between 21.5 and 24.5 degrees over 41,000 years. A more drastic variation, or a much shorter periodicity, would induce climatic effects such as variations in seasonal severity.

Other orbital considerations include:

-The planet should rotate relatively quickly so that the day-night cycle is not overlong. If a day takes years, the temperature differential between the day and night side will be pronounced, and problems similar to those noted with extreme orbital eccentricity will come to the fore.

-The planet also should rotate quickly enough so that a magnetic dynamo may be started in its iron core to produce a magnetic field.

-Change in the direction of the axis rotation (precession) should not be pronounced. In itself, precession need not affect habitability as it changes the direction of the tilt, not its degree. However, precession tends to accentuate variations caused by other orbital deviations. Precession on Earth occurs over a 26,000-year cycle.

The Earth’s Moon appears to play a crucial role in moderating the Earth’s climate by stabilising the axial tilt. It has been suggested that a chaotic tilt may be a “deal-breaker” in terms of habitability—i.e. a satellite the size of the Moon is not only helpful but required to produce stability. In the case of the Earth, the sole Moon is sufficiently massive and orbits so as to significantly contribute to ocean tides, which in turn aids the dynamic churning of Earth’s large liquid water oceans. These lunar forces not only help ensure that the oceans do not stagnate, but also play a critical role in Earth’s dynamic climate.

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Energy source:

An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism”.

It is assumed that any life elsewhere in the universe would require an energy source. Previously, it was assumed that this would necessarily be from a sun-like star, however with developments within extremophile research contemporary astrobiological research often focuses on identifying environments that have the potential to support life based on the availability of an energy source, such as the presence of volcanic activity on a planet or moon that could provide a source of heat and energy. It is important to note that these assumptions are based on our current understanding of life on Earth and the conditions under which it can exist. As our understanding of life and the potential for it to exist in different environments evolves, these assumptions may change.

Light probably is not directly required for life to arise, however, except as it may be involved in the formation of organic compounds during the accretion of a planetary system. On the other hand, the biological use of light energy, photosynthesis, may be a prerequisite for persistence of planetary life over billions of years. The reason for this conjecture is that light provides a continuous and relatively inexhaustible source of energy. Life that depends only on chemical energy inevitably will fail as resources diminish and cannot be renewed. Nonetheless, we know that life occurs in Earth’s crust, away from the direct influence of light, and that many organisms have metabolisms that function independently of light. Thus, the outer boundary of the potentially habitable zone extends into the far reaches of the solar system, to any rocky body with internal heating, regardless of its distance from the sun. Life can persist in the absence of light by using inorganic energy sources, as do lithotrophic organisms, or organic sources deposited in planetary interiors during their accretion, as do heterotrophs. Therefore, rather than proximity to the sun, it seems more useful to define the habitable zone for life in terms of the chemical and physical conditions that are expected to be required for life.

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High metallicity:

Metals are the building blocks of “rocky” planets. In the Milky Way, or in any other galaxy, the amount of metals in the interstellar medium varies with location. Please note that all elements other than (heavier than) hydrogen and helium are called metals in astronomy. Most metals are produced in nuclear reactions in the cores of massive stars and distributed through supernovae explosions into the interstellar medium. Some heavy elements like silver, gold, platinum, cesium, and uranium are produced by violent collisions like neutron star mergers. According to a study, around 4.6 billion years ago, two neutron stars collided near the early Solar System. 0.3% of the Earth’s heaviest elements have been created by this event.

While the bulk of material in any star is hydrogen and helium, there is a significant variation in the amount of heavier elements (metals). A high proportion of metals in a star correlate to the amount of heavy material initially available in the protoplanetary disk. A smaller amount of metal makes the formation of planets much less likely, under the solar nebula theory of planetary system formation. Any planets that did form around a metal-poor star would probably be low in mass, and thus unfavorable for life. Spectroscopic studies of systems where exoplanets have been found to date confirm the relationship between high metal content and planet formation: “Stars with planets, or at least with planets similar to the ones we are finding today, are clearly more metal rich than stars without planetary companions.” This relationship between high metallicity and planet formation also means that habitable systems are more likely to be found around stars of younger generations, since stars that formed early in the universe’s history have low metal content.

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Terrestrial vs gas planets:

Whether a planet will emerge as habitable depends on the sequence of events that led to its formation, which could include the production of organic molecules in molecular clouds and protoplanetary disks, delivery of materials during and after planetary accretion, and the orbital location in the planetary system. The chief assumption about habitable planets is that they are terrestrial. Such planets, roughly within one order of magnitude of Earth mass, are primarily composed of silicate rocks, and have not accreted the gaseous outer layers of hydrogen and helium found on gas giants. The possibility that life could evolve in the cloud tops of giant gas planets has not been decisively ruled out, though it is considered unlikely, as they have no surface and their gravity is enormous. The natural satellites of giant gas planets, meanwhile, remain valid candidates for hosting life.

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Geochemistry:

It is generally assumed that any extraterrestrial life that might exist will be based on the same fundamental biochemistry as found on Earth, as the four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen, are also the most common chemically reactive elements in the universe. Indeed, simple biogenic compounds, such as very simple amino acids such as glycine, have been found in meteorites and in the interstellar medium. These four elements together comprise over 96% of Earth’s collective biomass. Carbon has an unparalleled ability to bond with itself and to form a massive array of intricate and varied structures, making it an ideal material for the complex mechanisms that form living cells. Hydrogen and oxygen, in the form of water, compose the solvent in which biological processes take place and in which the first reactions occurred that led to life’s emergence. The energy released in the formation of powerful covalent bonds between carbon and oxygen, available by oxidizing organic compounds, is the fuel of all complex life-forms. These four elements together make up amino acids, which in turn are the building blocks of proteins, the substance of living tissue. In addition, neither sulfur (required for the building of proteins) nor phosphorus (needed for the formation of DNA, RNA, and the adenosine phosphates essential to metabolism) are rare.

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Ecological factors:

Current ecological approaches for predicting the potential habitability use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation.

Some habitability factors
Water	· Activity of liquid water · Past or future liquid (ice) inventories · Salinity, pH, and Eh of available water
Chemical environment	Nutrients: · C, H, N, O, P, S, essential metals, essential micronutrients · Fixed nitrogen · Availability/mineralogy Toxin abundances and lethality: · Heavy metals (e.g. Zn, Ni, Cu, Cr, As, Cd, etc.; some are essential, but toxic at high levels) · Globally distributed oxidizing soils
Energy for metabolism	Solar (surface and near-surface only) Geochemical (subsurface) · Oxidants · Reductants · Redox gradients
Conducive physical conditions	· Temperature · Extreme diurnal temperature fluctuations · Low pressure · Strong ultraviolet germicidal irradiation · Galactic cosmic radiation and solar particle events (long-term accumulated effects) · Solar UV-induced volatile oxidants, e.g. O ₂⁻, O⁻, H₂O₂, O₃ · Climate and its variability (geography, seasons, diurnal, and eventually, obliquity variations) · Substrate (soil processes, rock microenvironments, dust composition, shielding) · High CO₂ concentrations in the global atmosphere · Transport (aeolian, ground water flow, surface water, glacial)

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Galactic habitable zone:

The concept of a stellar habitable zone has been extended to a planet’s location in the Milky Way Galaxy. Near the centre of the Milky Way, stars are typically much closer to one another than they are farther out on the spiral arms, where the Sun is located. At the galactic centre, therefore, phenomena such as supernovae might present a greater hazard to life than they would in the region where Earth is located. On the other hand, in the outer regions of the Milky Way beyond the location of Earth, there are fewer stars. Since the bulk of a terrestrial planet is composed of chemical elements that were produced within stars, the material out of which new stars are being formed may not have enough of those elements necessary for Earth-like planets to grow. Considerations of this type have led to the concept of a galactic habitable zone, analogous to a stellar habitable zone. The concept of a galaxy’s habitable zone may well be viable, but the extent and boundaries of such a region are far more difficult to quantify than those of a star’s habitable zone.

Our knowledge of galaxy types and their distribution suggests that life as we know it can only exist in about 10% of all galaxies. Our galaxy, the Milky Way is one of them, it is a galaxy that is suitable for life. The safest environments for life similar to that on Earth are the lowest-density regions in the outskirts of large galaxies like the Milky Way. The main reason is extremely energetic explosions that have been observed in distant galaxies, called Gamma-ray bursts. They are the brightest electromagnetic events known to occur in the Universe. The gamma-ray bursts are extremely dangerous events. A GRB within a few parsecs, with its energy directed towards Earth, will mostly damage life by raising the UV levels; during the burst itself and for a few years thereafter. Some types of galaxies are too compact (stars so close to each other). These are not suitable for life, too. According to a 2015 study, elliptical galaxies are the most habitable in the cosmos, though.

In a nutshell, what makes a planet habitable is a much more complex question than having a planet located at the right distance from its host star so that water can be liquid on its surface: various geophysical and geodynamical aspects, the radiation, and the host star’s plasma environment can influence the evolution of planets and life, if it originated. Liquid water is a necessary but not sufficient condition for life as we know it, as habitability is a function of a multitude of environmental parameters.

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Features that make a planet habitable:

-1. Right distance from their star

Obviously, Earth needs to orbit in the Sun’s habitable zone. Many exoplanetary systems have planets much closer to their star, since young, full-grown planets tend to migrate inwards while embedded in the protoplanetary disc. Earth did not: it grew slowly, remaining too small to migrate during the disc’s lifetime, probably because Jupiter blocked dust inflow from the outer Solar System. Then again, if Jupiter had migrated inwards, Earth would still have ended up close to (or even in) the Sun, or it might have been kicked out into interstellar space. The fact Jupiter didn’t migrate is likely to be due to the gravitational influence of a second giant planet in the system – Saturn.

-2. More or less circular orbit

Most exoplanet giants orbit their parent stars on elongated orbits. If that was the case for the giants in our Solar System, Earth’s orbit would have become eccentric too. A stretched-out orbit means a varying climate, which could prevent the evolution of complex organisms, and even the origin of life. We’ve been lucky that Jupiter remained on a rather circular path, despite its complex interactions with other planets.

-3. Stable rotational axis

We know that’s what happens to Mars, and these huge changes in axial tilt angle may have contributed to the loss of most of the Red Planet’s atmosphere and water. Without Earth’s more or less stable obliquity, life might have become extinct long ago, or it may never even have started. So what prevents our planet’s axis from flipping in every possible direction? It’s the stabilising influence of its large Moon.

-4. Some water, but not too much

If Earth had become a water world, a deep layer of high-pressure, high-density ‘tetragonal’ ice would have formed at the bottom of the ocean, separating the liquid water from the mineral-laden crust. Without erosion-driven organics from the crust, the ocean would have remained sterile.

-5. No hydrogen-rich atmosphere

The solar nebula was more than 70% hydrogen. If Earth had formed rapidly, it would have accreted a thick, dense atmosphere of hydrogen (and helium), ending up resembling the many ‘mini-Neptunes’ found among exoplanets. Life may have formed on such a world, but the oxygen produced by the first cyanobacteria would then combine with hydrogen to form water, and without an oxygen-rich atmosphere many complex life forms (humans included) would never evolve. Apparently, Earth formed slowly and relatively late, after much of the solar nebula had already dissipated.

-6. Plate tectonics

Thanks to Earth’s plate tectonics and associated volcanism, our planet has been able to regulate its climate, despite the fact that the Sun was much fainter long ago and has increased in brightness ever since. The well-known greenhouse gas CO2 (carbon dioxide) is brought into the atmosphere by volcanoes, but washed away by rain storms, which are more frequent in warmer climates. Eventually, plate tectonics returns CO2 back into Earth’s lithosphere. Thus, the CO2 cycle acts like a thermostat. So why does Earth have plate tectonics while Venus doesn’t? It’s most likely because of the giant impact from which the Moon was born.

-7. Magnetic field

Earth’s global magnetic field shields surface life from the lethal effects of charged particles in the solar wind and in cosmic rays. The field is generated in the planet’s molten outer core. Again, you may wonder why Venus (similar in size to Earth) does not have a magnetic field. The theory is that the giant Moon-forming impact messed up the nicely layered interior structure of Earth. As a result, both the core and the mantle became prone to convective motions, with a magnetic field and plate tectonics as a result, respectively. Without this cosmic catastrophe, our planet may well have remained a barren rock.

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What makes a planet habitable? A 2009 study:

This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. Authors discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, authors discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.

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Planet habitability index: a 2018 study:

Méndez and colleagues presented planet habitability index in 2018 at the Lunar and Planetary Science Conference in The Woodlands, Texas. The researchers’ equation includes five variables in judging a planet’s habitability: the amount of starlight a planet receives, the planet’s radius, its reflectiveness, the fraction of surface covered with ocean and the atmospheric density. Determining those last three variables is beyond the reach of current telescopes. “This framework tells you exactly what you have to measure,” Méndez says of his team’s habitability index. And he thinks the technology needed could be 10 to 20 years away. Other astronomers question such an index’s usefulness, given other factors that could influence habitability including an active geological cycle, a molten core, plate tectonics, volcanoes to emit gases into the atmosphere and a magnetic field to protect it from stellar flares. Those factors also can’t be detected from Earth. Some may never be. “You can make a long list,” says Harvard astronomer Charbonneau. “It doesn’t matter if there are microbes hunkered down on a tidally heated moon if I can’t detect them with my telescope.”

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Many habitable environments:

Recent discoveries suggest that the solar system and broader Milky Way galaxy teem with environments that could support life as we know it. For example, oceans of liquid water slosh beneath the icy shells of the Jupiter moons Europa and Ganymede, as well as that of the Saturn satellite Enceladus. Oceans covered much of Mars in the ancient past, and seasonal dark streaks observed on the Red Planet’s surface today may be caused by salty flowing water. Further, NASA’s Curiosity rover has found carbon-containing organic molecules and “fixed” nitrogen, basic ingredients necessary for Earth-like life, on the Martian surface. Farther afield, observations by NASA’s Kepler space telescope suggest that nearly every star in the sky hosts planets — and many of these worlds may be habitable. Indeed, Kepler’s work has shown that rocky worlds like Earth and Mars are probably more common throughout the galaxy than gas giants such as Saturn and Jupiter. And just as the solar system is awash in water, so is the greater galaxy, said Paul Hertz, director of NASA’s Astrophysics Division. The Milky Way is “a soggy place,” Hertz said. “We can see water in the interstellar clouds from which planetary systems and stellar systems form. We can see water in the disks of debris that are going to become planetary systems around other stars, and we can even see comets being dissipated in other solar systems as [their] star evaporates them.”

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Forget habitability, just look for life:

“If you want to know for certain that you can pack your bags and go there, you need a biosignature” — a sign that life has altered the chemistry of the planet’s atmosphere, says Kane. “If we detect unambiguous biosignatures, that means by definition the planet is habitable, because something is living there.” NASA is now evaluating plans for two telescopes which will be able to scan the skies of Earthlike exoplanets. The Habitable Exoplanet Observatory, or HabEx, would take photos of a dozen or so Earthlike exoplanets orbiting sunlike stars. The Large UV/Optical/IR Surveyor, nicknamed LUVOIR, would do the same, but for up to 100 planets. Both telescopes would look for the chemical imprints of life in the planets’ atmospheres. Both LUVOIR and HabEx are proposed to observe potentially habitable planets and search for potential biosignatures. However, HabEx will be optimized for planets while enabling a broader range of general astrophysical observations. LUVOIR, on the other hand, will be a general observatory for a variety of astrophysical goals, including exoplanets. The two missions also have different levels of ambition. HabEx aims to search for planets around enough stars to have a very good chance at characterizing at least one rocky planet in the habitable zone of another star. LUVOIR, on the other hand, will attempt to characterize dozens of such worlds. LUVOIR will also be able to constrain the abundance of any property on those worlds, including a biosignature or combination of biosignatures, to a level of ~10 percent. What the next set of instruments is being designed to do is unravel what’s in the atmospheres of these truly Earthlike planets. Still, the concept of a habitable zone is useful for designing projects like HabEx and LUVOIR. Focusing on a particular distance from a star tells engineers how big to make a telescope. If signs of life are finally found, arguments over the term “habitable zone” might just fade away. Studying actual aliens will give scientists something much more important to talk about.

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Note:

A full discussion of habitability would require reviewing the complex interplay among instellation, atmospheric dynamics, greenhouse gases, planetary tectonics, orbital stability, ice-albedo feedbacks, the remote detectability of these processes, and many other topics that are beyond the scope of this article.

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Planet super-habitability:

A superhabitable planet is a hypothetical type of exoplanet or exomoon that may be better suited than Earth for the emergence and evolution of life. The concept was introduced in 2014 by René Heller and John Armstrong, who have criticized the language used in the search for habitable planets and proposed clarifications. According to Heller and Armstrong, knowing whether or not a planet is in its host star’s habitable zone (HZ) is insufficient to determine its habitability: It is not clear why Earth should offer the most suitable physicochemical parameters to living organisms, as “planets could be non-Earth-like, yet offer more suitable conditions for the emergence and evolution of life than Earth did or does.” While still assuming that life requires water, they hypothesize that Earth may not represent the optimal planetary habitability conditions for maximum biodiversity; in other words, they define a superhabitable world as a terrestrial planet or moon that could support more diverse flora and fauna than there are on Earth, as it would empirically show that its environment is more hospitable to life.

Heller and Armstrong also point out that not all rocky planets in a habitable zone (HZ) may be habitable, and that tidal heating can render terrestrial or icy worlds habitable beyond the stellar HZ, such as in Europa’s internal ocean. The authors propose that in order to identify a habitable—or superhabitable—planet, a characterization concept is required that is biocentric rather than geo- or anthropocentric. Heller and Armstrong proposed to establish a profile for exoplanets according to stellar type, mass and location in their planetary system, among other features. According to these authors, such superhabitable worlds would likely be larger, warmer, and older than Earth, and orbiting K-type main-sequence stars.

Heller and Armstrong proposed that a series of basic characteristics are required to classify an exoplanet or exomoon as superhabitable; for size, it is required to be about 2 Earth masses, and 1.3 Earth radii will provide an optimal size for plate tectonics. In addition, it would have a greater gravitational attraction that would increase retention of gases during the planet’s formation. It is therefore likely that they have a denser atmosphere that will offer greater concentration of oxygen and greenhouse gases, which in turn raise the average temperature to optimum levels for plant life to about 25 °C (77 °F). A denser atmosphere may also influence the surface relief, making it more regular and decreasing the size of the ocean basins, which would improve diversity of marine life in shallow waters.

Other factors to consider are the type of star in the system. Our sun is actually not the best kind of star for hosting a planet with lots of life on it. Orange dwarf stars are about 50% more common than yellow dwarfs in the Milky Way. K-type stars and low-luminosity G-type stars, collectively referred to as orange dwarfs, are less massive than the Sun, and are stable on the main sequence for a very long time (18 to 34 billion years, compared to 10 billion for the Sun, a G2V star), giving more time for the emergence of life and evolution. Since complex life took about 3.5 billion years to appear on Earth, the longer lifetimes of orange dwarf stars could give planets within their habitable zones more time to develop life and accrue biodiversity. In addition, orange dwarfs emit less ultraviolet radiation (which can damage DNA and thus hamper the emergence of nucleic acid based life) than stars like the Sun.

A study led by Washington State University scientist Dirk Schulze-Makuch recently published in the journal Astrobiology details characteristics of potential “superhabitable” planets which include those that are older, a little larger, slightly warmer and possibly wetter than Earth. Life could also more easily thrive on planets that circle more slowly changing stars with longer lifespans than our sun. The 24 top contenders for superhabitable planets are all more than 100 light years away, but Schulze-Makuch said the study could help focus future observation efforts, such as from NASA’s James Web Space Telescope, the LUVIOR space observatory and the European Space Agency’s PLATO space telescope.

Schulze-Makuch and team identified 24 potentially superhabitable planets. None of these worlds met all the criteria the researchers drew up for superhabitable planets, but one did meet at least two — KOI 5715.01. KOI (Kepler Object of Interest) 5725.01 is a planet about 5.5 billion years old and 1.8 to 2.4 times Earth’s diameter orbiting an orange dwarf about 2,965 light-years away. It might have an average surface temperature about 4.3 degrees F (2.4 degrees C) cooler than that of Earth, but if it has more greenhouse gases than Earth to trap heat, it might be superhabitable, the researchers wrote. Schulze-Makuch’s own favorite potentially superhabitable world from these 24 was KOI 5554.01. This planet is about 6.5 billion years old, with a diameter 0.72 to 1.29 times that of Earth, orbiting a yellow dwarf about 700 light-years from Earth.

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Section-8

Rare earth hypothesis: Why complex intelligent life only on earth:

A lot of people believe that it is a simple matter of math that life exists elsewhere in the Universe: there are hundreds of billions of stars in any given galaxy and most of those stars, like our own, appear to have multiple planets orbiting them. Surely if just a tiny percentage of those planets were habitable then there could be hundreds of millions of planets supporting life. Sadly, it is just not that simple. Although our planet is teaming with life, it is also exceedingly unlikely we are here at all, given all of the required factors that have had to come together to make life happen. It takes a lot more than just being habitable.

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligence) required an improbable combination of astrophysical and geological events and circumstances. According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare throughout the universe as a whole. The term “Rare Earth” originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.

In the 1970s and 1980s, Carl Sagan and Frank Drake, among others, argued that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred spiral galaxy. From the principle of mediocrity (extended from the Copernican principle), they argued that the evolution of life on Earth, including human beings, was also typical, and therefore that the universe teems with complex life. However, Ward and Brownlee argue that planets, planetary systems, and galactic regions that are as accommodating for complex life as are the Earth, the Solar System, and our own galactic region are not typical at all, but actually exceedingly rare.

Requirements for complex life:

The Rare Earth hypothesis argues that the evolution of biological complexity anywhere in the universe requires the coincidence of a large number of fortuitous circumstances, including, among others, a galactic habitable zone; a central star and planetary system having the requisite character (i.e. a circumstellar habitable zone); a terrestrial planet of the right mass; the advantage of one or more gas giant guardians like Jupiter and possibly a large natural satellite to shield the planet from frequent impact events; conditions needed to ensure the planet has a magnetosphere and plate tectonics; a chemistry similar to that present in the Earth’s lithosphere, atmosphere, and oceans; the influence of periodic “evolutionary pumps” such as massive glaciations and bolide impacts; and whatever factors may have led to the emergence of eukaryotic cells, sexual reproduction, and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human beings and of human intelligence may have required yet further specific events and circumstances, all of which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates. In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges.

Arguments for rare earth hypothesis:

-1. The right orbital distance from the right type of star:

According to the hypothesis, Earth has an improbable orbit in the very narrow habitable zone (dark green) around the Sun as seen in the figure below.

The terrestrial example suggests that complex life requires liquid water, the maintenance of which requires an orbital distance neither too close nor too far from the central star, another scale of habitable zone or Goldilocks principle. The habitable zone varies with the star’s type and age.

For advanced life, the star must also be highly stable, which is typical of middle star life, about 4.6 billion years old. Proper metallicity and size are also important to stability. The Sun has a low (0.1%) luminosity variation. To date, no solar twin star, with an exact match of the Sun’s luminosity variation, has been found, though some come close. The star must also have no stellar companions, as in binary systems, which would disrupt the orbits of any planets. Estimates suggest 50% or more of all star systems are binary. The habitable zone for a main sequence star very gradually moves out over its lifespan until the star becomes a white dwarf and the habitable zone vanishes.

The liquid water and other gases available in the habitable zone bring the benefit of the greenhouse effect. Even though the Earth’s atmosphere contains a water vapor concentration from 0% (in arid regions) to 4% (in rainforest and ocean regions) and – as of November 2022 – only 417.2 parts per million of CO2, these small amounts suffice to raise the average surface temperature by about 40 °C, with the dominant contribution being due to water vapor.

Rocky planets must orbit within the habitable zone for life to form. Although the habitable zone of such hot stars as Sirius or Vega is wide, hot stars also emit much more ultraviolet radiation that ionizes any planetary atmosphere. Such stars may also become red giants before advanced life evolves on their planets. These considerations rule out the massive and powerful stars of type F6 to O as homes to evolved metazoan life.

Conversely, small red dwarf stars have small habitable zones wherein planets are in tidal lock, with one very hot side always facing the star and another very cold side always facing away, and they are also at increased risk of solar flares. As such, it is disputed whether they can support life. Rare Earth proponents claim that only stars from F7 to K1 types are hospitable. Such stars are rare: G type stars such as the Sun (between the hotter F and cooler K) comprise only 9% of the hydrogen-burning stars in the Milky Way.

Such aged stars as red giants and white dwarfs are also unlikely to support life. Red giants are common in globular clusters and elliptical galaxies. White dwarfs are mostly dying stars that have already completed their red giant phase. Stars that become red giants expand into or overheat the habitable zones of their youth and middle age (though theoretically planets at much greater distances may then become habitable).

An energy output that varies with the lifetime of the star will likely prevent life. A sudden decrease, even if brief, may freeze the water of orbiting planets, and a significant increase may evaporate it and cause a greenhouse effect that prevents the oceans from reforming.

All known life requires the complex chemistry of metallic elements. The absorption spectrum of a star reveals the presence of metals within, and studies of stellar spectra reveal that many, perhaps most, stars are poor in metals. Because heavy metals originate in supernova explosions, metallicity increases in the universe over time. Low metallicity characterizes the early universe: globular clusters and other stars that formed when the universe was young, stars in most galaxies other than large spirals, and stars in the outer regions of all galaxies. Metal-rich central stars capable of supporting complex life are therefore believed to be most common in the less dense regions of the larger spiral galaxies—where radiation also happens to be weak.

Solar System’s location avoids the galaxy’s perilous inner regions. There are relatively few stars near the Sun. This reduces risks to Earth (or any other planet in the Solar System) from gravitational tugs, supernovae, or gamma-ray bursts (all observed GRBs have originated from outside the Milky Way galaxy, though). The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms. Most stars the same age as our Sun have more elliptical orbits. Spiral arms are dangerous places because massive star supernovae are concentrated there, and giant molecular clouds can perturb the Oort cloud comets leading to more comet showers in the inner Solar System. The Oort cloud is theorized to be a vast cloud of icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 AU (0.03 to 3.2 light-years). Our position in the Milky Way galaxy places us in a spot where there is less astronomical “activity” such as radiation from supernovas. Many other planets throughout the galaxies are not so lucky, with planets being constantly destroyed by supernovas, red giants, black holes, gamma ray bursts and more…it’s a dangerous universe for a young planet.

Supernovae pose a great danger to the development of complex life. A supernova is a transient astronomical event that occurs during the last stellar evolutionary stages of a massive star‘s life, whose dramatic and catastrophic destruction is marked by one final, titanic explosion. If one occurred within 10 parsecs of Earth the high-energy photons and protons would obliterate the ozone, leaving land animals unprotected from the Sun’s ultraviolet radiation (marine life would be largely unaffected). But, the closer the supernova, the higher the risk. Scientists also associate supernovae with gamma-ray bursts.

-2. The right arrangement of planets around the star:

Rare Earth argues that without such an arrangement, in particular the presence of the massive gas giant Jupiter (the fifth planet from the Sun and the largest), complex life on Earth would not have arisen.

Rare Earth proponents argue that a planetary system capable of sustaining complex life must be structured more or less like the Solar System, with small, rocky inner planets and massive outer gas giants. Without the protection of such “celestial vacuum cleaner” planets with strong gravitational pulls, other planets would be subject to more frequent catastrophic asteroid collisions.

Observations of exoplanets have shown that arrangements of planets similar to the Solar System are rare. Most planetary systems have super-Earths, several times larger than Earth, close to their star, whereas the Solar System’s inner region has only a few small rocky planets and none inside Mercury’s orbit. Only 10% of stars have giant planets similar to Jupiter and Saturn, and those few rarely have stable, nearly circular orbits distant from their star. Konstantin Batygin and colleagues argue that these features can be explained if, early in the history of the Solar System, Jupiter and Saturn drifted towards the Sun, sending showers of planetesimals towards the super-Earths which sent them spiralling into the Sun, and ferrying icy building blocks into the terrestrial region of the Solar System which provided the building blocks for the rocky planets. The two giant planets then drifted out again to their present positions. In the view of Batygin and his colleagues: “The concatenation of chance events required for this delicate choreography suggest that small, Earth-like rocky planets – and perhaps life itself – could be rare throughout the cosmos.”

-3. A continuously stable orbit:

Rare Earth argues that a gas giant also must not be too close to a body where life is developing. Close placement of one or more gas giants could disrupt the orbit of a potential life-bearing planet, either directly or by drifting into the habitable zone.

Newtonian dynamics can produce chaotic planetary orbits, especially in a system having large planets at high orbital eccentricity.

The need for stable orbits rules out stars with planetary systems that contain large planets with orbits close to the host star (called “hot Jupiters”). It is believed that hot Jupiters have migrated inwards to their current orbits. In the process, they would have catastrophically disrupted the orbits of any planets in the habitable zone. To exacerbate matters, hot Jupiters are much more common orbiting F and G class stars.

-4. A terrestrial planet of the right size:

Rare Earth argues that complex life cannot exist on large gaseous planets like Jupiter and Saturn or Uranus and Neptune, or smaller planets such as Mars and Mercury.

The Rare Earth hypothesis argues that life requires terrestrial planets like Earth, and since gas giants lack such a surface, that complex life cannot arise there.

A planet that is too small cannot maintain much atmosphere, rendering its surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent. A planet that is too large will retain too dense an atmosphere, like Venus. Although Venus is similar in size and mass to Earth, its surface atmospheric pressure is 92 times that of Earth, and its surface temperature is 735 K (462 °C; 863 °F). The early Earth once had a similar atmosphere, but may have lost it in the giant impact event which formed the Moon.

The planet must rotate at the right speed. The speed at which a newly-formed planet rotates depends on the speed at which the clouds of dust and matter from which the planet formed were initially moving. Some planets spin too fast for it to be likely that life has evolved there, because of the extreme effects of its rotational speed on that planet’s environment. Similarly, if a planet rotates too slowly, then it becomes too hot during the day for too long, and too cold during the night for too long for it to be likely life will emerge

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An increasing number of extrasolar planet discoveries are being made, with 5,506 planets in 4,065 planetary systems known as of 1 October 2023. Rare Earth proponents argue life cannot arise outside Sun-like systems, due to tidal locking and ionizing radiation outside the F7–K1 range. However, some exobiologists have suggested that stars outside this range may give rise to life under the right circumstances; this possibility is a central point of contention to the theory because these late-K and M category stars make up about 82% of all hydrogen-burning stars.

Current technology limits the testing of important Rare Earth criteria: surface water, tectonic plates, a large moon and biosignatures are currently undetectable. Though planets the size of Earth are difficult to detect and classify, scientists now think that rocky planets are common around Sun-like stars. The Earth Similarity Index (ESI) of mass, radius and temperature provides a means of measurement, but falls short of the full Rare Earth criteria.

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Section-9

Origin of extraterrestrial life:

Two of the most fundamental questions in all of biology are whether life exists on other worlds and what form such life may take. While a simple explanation for life on other worlds may involve some variant of “panspermia” (e.g., see Kawaguchi 2019), wherein extraterrestrial life is not fully independent of life on our planet, more intriguing prospects exist wherein extraterrestrial life is truly independent. Because we have only observed life on our planet, all of which shares common ancestry, imagining unrelated extraterrestrial life forms and how they may differ from those on Earth poses a challenge. Much of the field of astrobiology focuses on the potential detection of life on other worlds and what the physical and chemical nature of that life may be. The evolutionary aspects of astrobiology often focus on the “origin of life” and/ or possible relationships to life on Earth. Less attention has been paid to how heredity and evolution may operate on such extraterrestrial life. While some classical evolutionary biologists have weighed in on astrobiology at times (e.g., Simpson 1964; Mayr 1993), our understanding of inheritance and evolution in hypothetical extraterrestrial life necessarily remains severely constrained by what we have observed on Earth limiting what we can imagine elsewhere. It is essential to approach the question of extraterrestrial life with scientific caution, recognizing that we currently have limited knowledge and evidence. Continued advancements in technology, space exploration, and the study of exoplanets will provide valuable insights into the potential existence of life beyond our galaxy.

Can life originate, evolve, or survive in extraterrestrial environments? Such fundamental questions motivate scientists to search for life beyond Earth. Astrobiology is a relatively new branch of space-related science merging astronomy and biology. Searching for habitable environments is quintessential when investigating extraterrestrial life. Nowadays, nearly 200 planets and satellites in the solar system and more than 5000 exoplanets orbiting stars in the universe have been discovered, inspiring an exploration mission concerning planetary environment diversity that may host life. However, Earth remains the only known living planetary body that can guide us to these answers.

Early philosophers believed Earth was the centre of the universe, and that the gods essentially focused on us. The question of life elsewhere in the universe must have seemed absurd to them. However, when Galileo confirmed once and for all that we were a small and insignificant part of the universe, the question resurfaced. Indeed, Galileo himself felt sure there was life on Mars, based on what he thought were artificial canals on its surface. But serious studies of planets in the 20th century made it clear life was not possible on other planets in our solar system. It was only in the late 20th century that we began to study the moons of other planets, and now, some harbour a suspicion that there just may be primitive life on some of the moons of these planets and in the interior of Mars. But that is still a suspicion. However, the question has acquired a new edge since NASA sent out satellite Kepler. The Kepler Mission has completely revolutionised the subject. In just one small, fixed region of the sky, Kepler found thousands of planets around a similar number of stars. This has made us suspect that as many as 80% of all stars may harbour planets. Amongst Kepler’s find was an Earth-like planet. Kepler 22b is the first planet roughly the size of the Earth (it is 2.4 times larger) that is within the habitable zone of a star. Its host star is slightly smaller and cooler than the Sun, and is some 600 light years from us. Its ‘year’ is 290 earth days long. If its surface is solid, it will have a very comfortable temperature of about 20° Celsius (the Earth is 30° Celsius). On the basis of this, scientists estimate that around one in five Sun-like stars have an “Earth-sized” planet in the habitable zone, so the nearest would be expected to be within 12 light-years distance from Earth.

When the universe was born, it consisted almost entirely of hydrogen. All the other elements we see around us were created in the cores of stars. As the stars died, they dispersed these elements in the neighbouring regions. Hence, if all the life-giving atoms and molecules are to be made, they will exist in residual material around small, second or third generation stars. These materials will then form the planets that support life around other stars. Such planets will need a very coherent environment where the temperature differences and environment are stable. The star itself will have to be stable over a long period of time. How common it is, is a matter of opinion and debate. Some even argue that planets are not needed for this, but the conventional wisdom is that planets are necessary to precipitate life. So how did life arise in the universe? Various unsatisfactory ideas have been suggested. The most common of these is that it arose by chance. But the probability that all molecules of life arose suddenly is very small. It seems more probable that there was a certain build-up of increasingly complex molecules of all kinds from where molecules that support life replicated themselves more efficiently compared to random molecules. It seems likely that all organic molecules (molecules that contain carbon) have properties that we do not yet know of which seem to allow them to organise themselves into self-replicating molecules and their companion molecules. It is even suggested that there may be some water soluble minerals that stick to rocks and could have helped facilitate the accumulation of such molecules. Two scientists, Stanley Miller and Harold Urey at the University of Chicago, conducted a famous experiment in 1953, where they put soil, water and other ingredients that must have existed in the early Earth’s environment, and passed electric current through them as would be produced by lightening. They found that such process spontaneously created a lot of organic molecules, many of which are useful amino acids that help life. So, it is quite likely that through a cumulative effect of several million ‘Miller-Urey’ experiments, the Earth accumulated life-supporting molecules.

Life as we know it here on Earth also requires a magnetic field and an atmosphere, both of which protect it from the lethal radiation our parent star, the sun, emits. Earth’s magnetic field—generated by its rotating iron core—deflects the solar wind, a continuous stream of high-speed, high-energy particles coming out of the sun. (As those particles careen by the edges of Earth’s atmosphere, they sometime create the phenomenon we call the Northern Lights.) Without the magnetic field there, the solar wind might destroy all life on Earth.

As for Earth’s atmosphere, it protects life because the water, carbon dioxide and other gases in it absorb solar radiation in its harmful ultraviolet-light form. The parent stars of other solar systems would emit radiation as well, and the planets orbiting them would need the same kind of protection.

Extraterrestrial intelligence is extraterrestrial life that is capable of thinking. Work in the new field of astrobiology has provided some evidence that evolution of other intelligent species in the Milky Way Galaxy is not utterly improbable. In particular, more than 5,000 extrasolar planets have been detected, and underground water is likely present on Mars and on some of the moons of the outer solar system. These efforts suggest that there could be many worlds on which life, and occasionally intelligent life, might arise. Searches for radio signals or optical flashes from other star systems that would indicate the presence of extraterrestrial intelligence have so far proved fruitless, but detection of such signals would have an enormous scientific and cultural impact.

Requirements For Extraterrestrial Life:

Star:

For a star to be able to host habitable planets, it must:

have a life span of at least one billion years.
not be a flare star.
not be too close to a cosmic explosion, such as a supernova

Planet:

For a planet to be habitable by life similar to that on Earth, it must:

have a temperature that stays between -15°C, below which chemicals start reacting too slowly for life to occur and water freezes, and 115°C, above which protein and carbohydrate molecules, as well as genetic material, start to decompose and water evaporates.
be at just the right distance from the star so it gets enough energy from the star.
have continuous liquid water on its surface.
enough mass for radioactive heating, which allows for geological activity.
a thick atmosphere to protect the planet, stabilize its climate, and provide sufficient pressure to hold liquid water. However, it must not be so thick that it creates a greenhouse effect that makes the planet too hot.
not be too close to massive planets which can divert asteroids towards or strongly perturb its orbit.
have a massive planet well outside of its orbit that diverts dangerously large asteroids away from it.

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Argument for extraterrestrial life/intelligence:

The universe is vast and contains billions of galaxies, each with billions of stars.
Many of these stars have planets orbiting them, and some of these planets may be similar to Earth in size and composition.
The conditions that are necessary for life, such as liquid water and an atmosphere that contains oxygen, are not unique to Earth.
Life on Earth has evolved to survive in a wide range of environments, from the scorching deserts to the freezing polar regions.

Based on these factors, it seems likely that there are other planets in the universe that also have life. However, it is also possible that life on other planets is very different from life on Earth. We may never know for sure if we are alone in the universe, but the search for extraterrestrial life is an exciting and important scientific endeavour.

Three arguments grounded in science bolster the conjecture that extraterrestrial life is surely out there somewhere: Big Numbers. the Copernican principle and Extremophiles surviving in harsh environments.

1. The Big Numbers argument notes that our galaxy, the Milky Way, has something like 400 billion stars, and it’s just one of untold billions of galaxies in a universe that might be infinite. Moreover, in the past 30 years, astronomers have discovered that planets of all shapes and sizes are common in the universe. With so much turf out there, even the most frowny-faced sceptic must admit it’s hard to run the numbers in a 13.8 billion-year-old universe like ours and wind up with just one self-aware, technological, telescope-constructing species.

2. The Copernican principle is inspired by 16th-century astronomer Copernicus, whose revolutionary model of the solar system put the sun and not Earth at the center. The principle suggests that, in the same way that Earth is not in a privileged place in the universe, humanity should not presume itself special, or unique. The universe is not about us, and what happened on this planet over the past 4 billion years could happen elsewhere. Otherwise, you have to believe that Earth is a miracle. It’s just never true that you only find one example of something in nature. The argument for the existence of extraterrestrial life/intelligence is based on the so-called principle of mediocrity extended from the Copernican principle and widely believed by astronomers states that the properties and evolution of the solar system are not unusual in any important way. Consequently, the processes on Earth that led to life, and eventually to thinking beings, could have occurred throughout the cosmos.

The most important assumptions in this argument are that

(1) planets capable of spawning life are common,

(2) biota will spring up on such worlds, and

(3) the workings of natural selection on planets with life will at least occasionally produce intelligent species.

To date, only the first of these assumptions has been proven. However, astronomers have found several small rocky planets that, like Earth, are the right distance from their stars to have atmospheres and oceans able to support life. Unlike the efforts that have detected massive, Jupiter-size planets by measuring the wobble they induce in their parent stars, the search for smaller worlds involves looking for the slight dimming of a star that occurs if an Earth-size planet passes in front of it. The U.S. satellite Kepler, launched in 2009, found thousands of planets, more than 20 of which are Earth-sized planets in the habitable zone where liquid water can survive on the surface, by observing such transits. Another approach is to construct space-based telescopes that can analyze the light reflected from the atmospheres of planets around other stars, in a search for gases such as oxygen or methane that are indicators of biological activity. In addition, space probes are trying to find evidence that the conditions for life might have emerged on Mars or other worlds in the solar system, thus addressing assumption 2. Proof of assumption 3, that thinking beings will evolve on some of the worlds with life, requires finding direct evidence. This evidence might be encounters, discovery of physical artifacts, or the detection of signals. Claims of encounters are problematic. Despite decades of reports involving unidentified flying objects, crashed spacecraft, crop circles, and abductions, most scientists remain unconvinced that any of these are adequate proof of visiting aliens.

3. Extremophiles thriving in mimic outer space environments exhibit traits that preponderate our comprehension regarding the possibility of life elsewhere:

Hardy Microbes (extremophiles) hint at possibilities for Extraterrestrial Life:

Based on what we know, several planetary bodies exhibit extinct or extant life potential. Prokaryotic life dominates our planet’s evolutionary history, evolving to occupy every possible environmental habitat, including various extreme environments. Common Earth life forms have traditionally taught us about terrestrial boundaries and abilities. We now appreciate living organisms’ physiological and biochemical capabilities as it illuminates an extensive origin, evolution, and future for Earth-like beings in our solar system and beyond, primarily due to an ever-increasing awareness of extremophile varieties over the past 50 years. Extremophiles can survive in a myriad of planetary environments and present relevant characteristics advancing our understanding of potential life elsewhere and in situ life detection. Thus, extremophilic microbes, especially those thriving under multiple extremes (polyextremophiles), represent a vital research avenue for astrobiological and space exploration. Furthermore, many extremophiles are ideal astrobiology models, aiding in finding indigenous extraterrestrial life or potential life-produced metabolites outside Earth.

To ascertain extraterrestrial life, we must first define boundary conditions where life can thrive. Outer space presents severely harsh and inhabitable environmental conditions deleterious for life growth, including high radiation doses, extreme temperatures, different gravity, pressure, pH, salinity, energy source, and nutrient scarcity. Nevertheless, as microbial life can flourish within broad physicochemical spectrums and extremely inhospitable habitats on Earth, they may be capable of surviving space’s harsh conditions. Thus, understanding living extremophiles’ molecular mechanisms and unique physiological characteristics is paramount for defining Earth’s boundary life limits and identifying conditions likely to originate or support life on other planetary bodies.

Studies of extreme ecosystems on Earth can guide the search for Martian life and may reveal the fundamental limits of biology.

On the Red Planet, NASA’s Mars rover Perseverance is searching for fossils and traces of alien biochemistry in Jezero Crater, an ancient lake bed thought to have once offered habitable conditions for microbial life. Back home, microbiologists are investigating oxygen-poor environments that may mimic the habitat of early Mars. This two-pronged approach of grounding scientists’ extraterrestrial extrapolations with studies of Earthly analogues could help clarify the bedrock limits for life on rocky planets, greatly aiding the development and execution of future extraterrestrial missions.

The Mars Analogues for Space Exploration project (MASE) was a four-year-long effort that used Earth to understand Mars by analyzing five types of harsh but habitable terrestrial environments that may resemble those that once—or even now—existed on our neighboring planet. Its funding concluded in 2017, but MASE researchers continue to publish results about the habitability of Mars. The study sites included a sulfidic spring, a briny mine, an acidic lake and river, and permafrost. Because of the extreme conditions in these environments, organisms that live here are called extremophiles.

Extremophile research was pioneered by the late Thomas Brock, a microbiologist at the University of Wisconsin–Madison. He found, against all expectations, that certain hardy microbes could thrive in geothermal springs hot enough to poach an egg. The microbiologist’s curiosity led to the isolation of a molecule—from a heat-loving bacterium—that is now used in laboratories across the world to amplify and sequence DNA. Brock passed away in April 2021, but his legacy lives on. Brock published his extremophile findings in April 1969, mere months before humans first walked on the moon. This paved the way for astrobiology, the study of life in all its forms on this planet and elsewhere in the universe. Astrobiology is not about making money off of space travel but it is about basic science and answering a single, timeless question: Does life exist beyond Earth?

Life on the Edge: Bioprospecting Extremophiles for Astrobiology, a 2023 study:

Discovering exoplanets and satellites in habitable zones within and beyond our solar system has sparked intrigue in planetary setting varieties that could support life. Based on our understanding of life on Earth, we can shed light on the origin, evolution, and future of Earth-like organisms in the galaxy and predict extinct or extant extraterrestrial life. Hence, extremophiles thriving in mimic outer space environments are particularly interesting as they exhibit traits that preponderate our comprehension regarding the possibility of life elsewhere and in situ life detection. Additionally, many extremophiles have been used for astrobiological research model organisms to unveil native alien life or possible life-produced metabolites outside Earth. Laboratory-based simulation chambers mimic this outer space condition, helping researchers study life beyond Earth in near identical conditions and understand molecular mechanisms for survival. This review summarizes relevant studies with isolated microorganisms from extreme analog Earth environments, harnessing them as promising astrobiological model candidates for pursuing life potentialities in other planetary bodies. Authors also highlight the necessity of environmental simulation chamber approaches for mimicking extraterrestrial habitats.

Microbes living in atmospheres different from that of Earth:

A recent study by Seager and colleagues discovered several microbes can grow in 100% pure hydrogen environments. In this study, scientists grew single-celled microbes, E. coli and yeast, in glass bottles with a nutrient broth food source. They changed the available gases in the bottles to reflect possible atmospheric conditions on other planets. In particular, they grew the microbes under pure hydrogen or helium gas, nitrogen-carbon dioxide gas mix, or normal atmospheric gases found on earth. Interestingly, they found that all conditions supported the life of both microbes tested. These findings show that microbes that do not normally live in these atmospheric conditions can adapt to survive and grow, which supports the notion that other lifeforms could also exist in similar conditions.

However, these results were not all that surprising because researchers have previously found some microbes surviving in some of the most extreme microenvironments on Earth. It is also important to note that these experiments provided the microbes with a rich food source, so having a hydrogen atmosphere alone likely is not enough to support life. A planet would also potentially need a water source for the exchange of food in the form of chemicals and nutrients. Regardless, this study broadens the known range of conditions that allow for life, which scientists can use to look for indications of microbial alien life.

Microorganisms tested in outer space:

The survival of some microorganisms exposed to outer space has been studied using both simulated facilities and low Earth orbit exposures. Bacteria were some of the first organisms investigated, when in 1960 a Russian satellite carried Escherichia coli, Staphylococcus, and Enterobacter aerogenes into orbit. Many kinds of microorganisms have been selected for exposure experiments since. Experiments of the adaption of microbes in space have yielded unpredictable results. While sometimes the microorganism may weaken, they can also increase in their disease-causing potency.

It is possible to classify these microorganisms into two groups, the human-borne and the extremophiles. Studying the human-borne microorganisms is significant for human welfare and future crewed missions in space, whilst the extremophiles are vital for studying the physiological requirements of survival in space. NASA has pointed out that normal adults have ten times as many microbial cells as human cells in their bodies. They are also nearly everywhere in the environment and, although normally invisible, can form slimy biofilms.

Extremophiles have adapted to live in some of the most extreme environments on Earth. This includes hypersaline lakes, arid regions, deep sea, acidic sites, cold and dry polar regions and permafrost. The existence of extremophiles has led to the speculation that microorganisms could survive the harsh conditions of extraterrestrial environments and be used as model organisms to understand the fate of biological systems in these environments. The focus of many experiments has been to investigate the possible survival of organisms inside rocks (lithopanspermia), or their survival on Mars for understanding the likelihood of past or present life on that planet. Research and testing of microorganisms in outer space could eventually be applied for directed panspermia or terraforming.

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Universal vs alternative biochemistry:

Finster and fellow scientists who speculate on the existence and nature of alien life usually follow one of two lines of reasoning. Some are confident in the universal nature of biochemistry, according to which chemical and physical constraints make it highly probable that life elsewhere in the universe follows the same general principles as terrestrial life and uses similar building blocks for macromolecules, although extraterrestrial life might have some biochemical peculiarities (Pace, 2001).

By contrast, fans of alternative biochemistry have shown much creativity in describing how life could have evolved in habitats very different from those on Earth, by using a combination of atoms other than carbon to build molecular structures, solvent systems besides water and a variety of energy sources (Bains, 2004; Benner et al, 2004; Schulze-Makuch & Irwin, 2006). Silicon-based polymers, liquid ammonia or ammonia–water mixtures, and geothermal energy or electromagnetic fields are some of the commonly proposed solutions for sustaining such truly alien life forms. “We propose that the only absolute requirements [for life] are a thermodynamic disequilibrium and temperatures consistent with chemical bonding,” stated Steven Benner and colleagues from the University of Florida at Gainesville, USA (Benner et al, 2004). “We must be careful [when equating] ‘water/ice’ and ‘life’,” remarked Philippe Blondel from the University of Bath, UK, who co-edited a book reviewing current knowledge of our solar system (Blondel & Mason, 2006). “The recent discoveries on Earth itself, like chemosynthetic life forms near hydrothermal vents in the deep oceans, or dormant microorganisms in the extremely low temperatures below Antarctica, have shown how adaptable life can be.”

But if life is possible in the absence of water, then the quest for living organisms in space rises to another level of complexity. Places such as Venus and Titan, where water is virtually absent, thus become possible habitats for life. Furthermore, if alien life diverges radically from terrestrial organisms, on which experimental basis will scientists recognize it? Conversely, if microbial organisms on, say, Mars share a substantial fraction of biochemical features with their Earth brethren, how will astrobiologists be able to distinguish them without any doubts about interplanetary contamination? To complicate matters, some astrobiologists also consider the possibility that life arose just once in our solar system—or even in the entire universe—and was then dispersed by meteorites, which would further reduce the ability to discriminate between alien and terrestrial origins (Pace, 2001; Schulze-Makuch & Irwin, 2006).

For the time being, however, water is considered the source of life, and any planets and moons in our solar system that are likely to host water become obvious targets for exploration. At the forefront, owing to its proximity and similarity to Earth, is Mars. Next in line is Europa, one of the icy moons of Jupiter, where a deep ocean could lie below the frozen surface. Are Mars and Europa the more likely candidates to host ‘possible life’? Sadly, there is no answer at the moment, in the absence of past or present life or signatures of life to observe. However, the basic materials for water/carbon life forms are all there, distributed in many places around the solar system. The continuing discoveries of planetary exploration show us there are plenty of opportunities for life to have evolved, or to evolve, in different places.

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Unusual habitability:

The traditional astronomical definition of a habitable planet is one that’s in the so-called Goldilocks zone. That means the planet surface temperature is one where water is able to exist in liquid form. It’s not too hot, it’s not too cold. But is that too restrictive, especially since life is found in some pretty extreme environments right here on Earth. There’s plenty of places on Earth where microbes are totally happy and we would die immediately. We have life on Earth that exist deep in the oceans that’s not part of a photosynthetic food chain. We have life inside deep rock. We have life that can handle higher than boiling point of water, lower than the freezing point of water. So the bounds on life on Earth are pretty wide. What we consider habitable for life here on Earth might not be the same for other planets in our solar system, or even in the rest of the galaxy, and that could have major implications for what our understanding of life is. Take the exoplanet K2-18b. It’s 124 light years away from Earth and was first discovered in 2015. Recently NASA announced that the James Webb Space Telescope spotted signs of carbon dioxide and methane there, which suggests it might be an ocean World, and an ocean World with all the elements for life could be habitable. But habitable for some life forms doesn’t necessarily mean that humans could survive there.

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Alternative biochemistry:

Part of the difficulty in searching for life of any sort is that scientists don’t agree on how life started in the first place—or what life even is. One good attempt at a definition came in 2011 from geneticist Edward Trifonov, who collated more than 100 interpretations of the word “life” and distilled them into one overarching idea: it’s “self-reproduction with variations.” NASA formulated a similar working definition years earlier, in the mid-1990s, and still uses it to design astrobiology studies. Life, according to this formulation, “is a self-sustaining chemical system capable of Darwinian evolution.”

Neither of those classical definitions requires a particular chemistry. On Earth, of course, life runs on DNA: deoxyribonucleic acid. DNA is made up of two twisted strands, each comprising alternating sugar and phosphate groups. Stuck to every sugar is a base—the As (adenine), Gs (guanine), Cs (cytosine), and Ts (thymine). Together the bases and sugar-phosphates form nucleotides; DNA itself is a nucleic acid. RNA is kind of like single-stranded DNA—among other things, it helps translate DNA’s instructions into actual protein production. The simple letters in a genetic sequence, strung together in a laddered order, carry all the information needed to make you, squirrels and sea anemones. DNA can replicate, and DNA from different organisms of same species can mix and meld to form a new organism that can replicate itself in turn. If biology elsewhere relied on this same chemistry, it would be life as we know it.

Scientists assume all forms of life would need some way to pass down biological instructions whose shifts could also help the species evolve over time. But it’s conceivable that aliens might not make these instructions out of the same chemicals as ours—or in the same shape. For instance, starting in the 1990s, Northwestern University researchers made SNAs, spherical nucleic acids. Alien life could have genetic code with, say, different bases. NASA-supported 2019 research, from the Foundation for Applied Molecular Evolution, successfully created synthetic DNA that used the four old-school bases and four new ones: P, Z, B and S. Scientists have also altered the strand part of genetic code, creating XNA—where X means anything goes—that uses a molecule such as cyclohexene (CeNA) or glycol (GNA), rather than deoxyribose. Big thinkers have long suggested that rather than using carbon as a base, as all these molecules do, perhaps alien life might use the functionally similar element silicon—meaning it wouldn’t have nucleic acids at all but other molecules that perhaps play the same role.

Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons it may be chemically similar, though it is also possible that there are organisms with quite different chemistries – for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water. The possibility of life-forms being based on “alternative” biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction. The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

Alternate chirality:

Just as a person can be left-handed or right-handed, so too can organic molecules. These molecules are mirror images of one another, but life, for whatever reason, wound up using one side or the other, which is called chirality. Amino acids, for instance, are “left-handed,” while the sugars in RNA and DNA are “right-handed.” For these molecules to interact with one another, they have to be of the correct kind of chirality; if protein chains are made with mixed-chirality amino acids, they simply don’t work. But a protein chain constructed from right-handed amino acids, the opposite of what life on Earth uses, would work perfectly fine. Alien life might evolve to use the opposite chirality as Earth.

Almost all of Earthly amino acids have a L form and sugars have a D form. But, life in some other planet could adopt L-L, D-L or D-D.

Replacing carbon:

Silicon: Silicon is the most frequent candidate as a replacement for carbon, but molecules created with long sequences of silicon (those are called silanes) tends to be much less chemically stable than their carbon counterparts. Further, silicon is much more common than carbon in Earth, and even with that, life here is carbon-based. Anyway, it remains as a possible candidate.
Silicone: Alternate sequences of silicon and oxygen (aka silicones) are much more stable than long sequences of just silicon, so silicones tend to work better than silanes. Polysilanols are the silicone compounds analogue to carbon-based sugars, and they are soluble in liquid nitrogen.
Boron: Boron sequences, called boranes are highly explosive in Earth’s atmosphere, but might be viable in some other planet. However, boron is relatively too rare to be seen as a viable alternative.
Sulfur or Phosphorus: Those also are able to create long chains in some situations, however they tend to be even less stable than silanes.
Metals: Some metals mixed with oxygen are very capable of creating significantly complex molecules. However, many metals are relatively rare, so you only get a few options left, namely titanium, aluminium, magnesium and iron, which are even more abundant than carbon.

Replacing water:

Water is essential because it is a liquid capable of acting as a solvent to a large set of substances and also has a pretty large liquid temperature range. Few substances have those characteristics, but there are some, namely:

Ammonia: In low temperatures (i.e. planets which orbit reasonably far from their host stars), ammonia becomes a liquid capable of act as a solvent to a large plethora of substances and has a reasonable large range of liquid temperature. The liquid range is not as large as water is, but in those cold planets where it remains liquid, maybe the temperature does not variate as much as it does in Earth. In high pressures environments, however, ammonia has a liquid range even larger than water. Also, ammonia dissolves many metals even better than water does.
Hydrogen fluoride: Has a suitable liquid temperature range, similar to water, but in colder temperatures. It is also able to act as a solvent for many substances. However, it is considered too rare to be a viable candidate.
Hydrogen sulfide: A good solvent, but not as good as water or ammonia. However, mixed with a small proportion of hydrogen fluoride, it becomes great. Unfortunately, it features a narrow range of liquid temperature, but it might be workable at high pressures where its liquid range is better.
Methane and other simple Hydrocarbons: Titan has lakes made from hydrocarbons, mostly ethane and methane. Darrell Strobel from John Hopkins University suggests that Titan actually features hydrocarbon-based life, which combines hydrocarbons to hydrogen, reducing ethane and acetylene to methane (something analogous to what many organisms here in Earth do in order to breath). Water is a stronger solvent than methane, however this is not necessarily a con to methane, since this might mean that it is more able than water to selectively preserve molecules useful for biologic reactions. Also, computer models shows that cell membranes based on carbon, hydrogen and nitrogen are capable of working in liquid methane (Earth’s cell membranes uses carbon, hydrogen, oxygen and phosphorus). Also, accordingly to Chris McKey, methane-based life could be more common in the universe than water-based life.
Silicon dioxide: In Earth this becomes glass or sand, a solid. However, at higher temperatures (higher than what we see in Venus), it becomes a liquid. It still needs high pressure to keep a possible workable liquid range that is not too hot for organic reactions.

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Alternative looks:

Based on these speculations and numerous theories on the origin of life gives rise to one of the many-asked questions what would aliens look like us? The scientific community, as of now, does not have a consensus on what aliens might look like. One popular idea is that aliens might resemble Earth’s life forms, as they would have evolved in similar conditions, such as the presence of water and a suitable temperature range. This would mean that aliens might have similar characteristics to Earth’s life forms, such as having similar biochemistry and being based on a similar genetic code. Another possibility is that aliens might be vastly different from Earth’s life forms, as they would have evolved in different conditions, such as a different atmosphere, temperature range, or radiation environment. Some scientists have even suggested that aliens might not be based on carbon, the element that forms the basis of all known life on Earth, but on silicon or other chemical elements. Given the vastness of the universe, it is possible that aliens could take on a wide range of different forms and characteristics.

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In no environment is a life based primarily around silicon chemistry a plausible option:

On the Potential of Silicon as a Building Block for Life, a 2020 study:

Despite more than one hundred years of work on organosilicon chemistry, the basis for the plausibility of silicon-based life has never been systematically addressed nor objectively reviewed. Authors provide a comprehensive assessment of the possibility of silicon-based biochemistry, based on a review of what is known and what has been modeled, even including speculative work. Authors assess whether or not silicon chemistry meets the requirements for chemical diversity and reactivity as compared to carbon. To expand the possibility of plausible silicon biochemistry, they explore silicon’s chemical complexity in diverse solvents found in planetary environments, including water, cryosolvents, and sulfuric acid. In no environment is a life based primarily around silicon chemistry a plausible option. Authors find that in a water-rich environment silicon’s chemical capacity is highly limited due to ubiquitous silica formation; silicon can likely only be used as a rare and specialized heteroatom. Cryosolvents (e.g., liquid N2) provide extremely low solubility of all molecules, including organosilicons. Sulfuric acid, surprisingly, appears to be able to support a much larger diversity of organosilicon chemistry than water.

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Life on a different planet may use DNA:

Life on Earth pretty early on settled on deoxyribonucleic acid organized in chains of base pairs as the means to code for the construction of proteins which make up a lifeform. It also has the benefit that it can be biologically copied relatively simply and accurately. But why would life that has evolved separately from Earth life end up with the same solution to the problem of coding for construction of proteins and inheritance of such coding?

The specific base pairs in DNA are cytosine (“C”, C4H5N3O), guanine (“G”, C5H5N5O), adenine (“A”, C5H5N5) and thymine (“T”, C5H6N2O2). These happen to be able to form the fairly well-known “double helix” DNA structure. Here already we can see a strong dependence on an environment rich in carbon, nitrogen and oxygen (as well as hydrogen), which works well on Earth and with Earth life.

Assuming that life develops independently (no common origin) on different planets, possibly in different solar systems, each able to support some kind of life which may be either similar to or dissimilar from Earth life, is there any plausible reason, or plausible set of criteria, why life would happen onto specifically DNA (as used by Earth life) on different planets? Or is it simply a random chance thing and there is no reason whatsoever why alien life wouldn’t just as well happen onto something utterly and completely different that solves the same problem?

Because those nucleotides occur naturally.

Recently scientists spotted the building blocks of RNA at the center of the Milky Way. RNA, or ribonucleic acid, a molecule similar to DNA and it is present in all living cells. The team of researchers discovered the building blocks of RNA in a molecular cloud in our galaxy. Such building blocks have also been discovered on asteroids. Most notably, Japanese researchers discovered more than 20 amino acids on the space rock Ryugu, which is more than 200 million miles (320 million kilometers) from Earth. Scientists made the detection by studying samples retrieved from the near-Earth asteroid by the Japan Aerospace Exploration Agency’s (JAXA) Hayabusa spacecraft, which landed on Ryugu in 2018. According to Kensei Kobayashi, a professor emeritus of astrobiology at Yokohama National University, “Proving amino acids exist in the subsurface of asteroids increases the likelihood that the compounds arrived on Earth from space. This means that amino acids could likely be found on other planets and natural satellites – a clue that “life could have been born in more places in the Universe than previously thought.

We’ve been finding the building blocks of DNA on meteorites for a while now. According to NASA there’s a good chance they occur naturally. The team found adenine and guanine, which are components of DNA called nucleobases, as well as hypoxanthine and xanthine. …Hypoxanthine and xanthine are not found in DNA, but are used in other biological processes. Also, in two of the meteorites, the team discovered for the first time trace amounts of three molecules related to nucleobases: purine, 2,6-diaminopurine, and 6,8-diaminopurine; the latter two almost never used in biology. These compounds have the same core molecule as nucleobases but with a structure added or removed. However, if asteroids are behaving like chemical ‘factories’ cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid.

And we’ve been able to produce them in the lab using non-biological reactions.

The team found these nucleobases — both the biological and non-biological ones — were produced in a completely non-biological reaction. In the lab, an identical suite of nucleobases and nucleobase analogs were generated in non-biological chemical reactions containing hydrogen cyanide, ammonia, and water. This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial.

NASA scientists studying the origin of life have reproduced uracil, cytosine, and thymine, three key components of our hereditary material, in the laboratory. They discovered that an ice sample containing pyrimidine exposed to ultraviolet radiation under space-like conditions produces these essential ingredients of life.

Rather than being a fluke that happened once on Earth, DNA, or at least its building blocks, appear to occur naturally. So another planet would have the same chemical base pairs available for proto-life to produce DNA.

DNA and RNA are very good at what they do: encode the blueprints for an organism very efficiently, accurately enough to ensure stability, but allowing sufficient inaccuracies for evolutionary variations to respond to changes in the environment. It’s so good at what it does that despite billions of years of evolution and endless variety, no life on Earth does anything else. This implies that even if several competing forms of life arise on another planet, DNA/RNA based life will win.

Table above shows percentage of elements in life on earth compared with those in interstellar frost and comets. Almost similar pattern seen proving the point that life on earth follows cosmic abundance of elements. Due to high cosmic abundance, these elements have had more opportunities to interact with each other and laws of chemistry allowed such interactions, ultimately leading to life. And corollary would be extraterrestrial life following similar pattern.

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Extraterrestrial Life may not be Alien: the evolutionary convergence:

Consider flight, since that’s the most famous example of convergence. If you live on a planet with an atmosphere, or even with an ocean or some other fluid, if you want to get from one place to another through that fluid, there’s only a handful of ways to do it. You can jump. You can float, if you’re lighter than the medium that you’re in. The only other way is aerodynamically, with a wing, to generate lift. Those are the mechanics of moving through a fluid medium. On Earth, flight evolved four different times in four different groups: in birds and bats and pterosaurs and insects. The fact that they all use wings isn’t because they evolved on Earth; it’s because it was advantageous to fly, and wings are just about the only way to fly. And so we can expect these constraints to be operating everywhere in the universe.

Coincidences of evolutionary (and even cosmic) history will always affect the details of animal shape and appearance. We have four limbs only because it was a four-finned fish that crawled out of the sea almost 400 million years ago. We could easily have had six limbs, or even eight, if evolutionary history had played out differently. So there will never really be close similarity between us and our equivalent species on an alien planet. But some things are just so tightly constrained that there aren’t really many alternative ways to do things.

Stephen Jay Gould, the noted paleontologist and evolutionary theorist, famously wrote about the idea of “replaying the tape of life” and letting life evolve over again. Gould imagined that the outcomes would be different; we would be unlikely to end up with Homo sapiens, for example. But it sounds like you’re arguing that, while any one specific outcome is unlikely, the same kinds of innovations would crop up again and again? That’s absolutely right. There’s this big argument between Stephen Jay Gould and Simon Conway Morris [of Cambridge]: Is it going to be different every time you replay the tape? Is it going to be the same every time? But obviously, the correct answer is: It will be different, but many things will be the same. And the things that will be the same are those things that are constrained either by the laws of physics or by the laws of evolution. There are mathematical rules that govern the way evolution works.

Cohen and Stewart argue against a conception of extraterrestrial life that assumes life can only evolve in environments similar to Earth (the so-called Rare Earth hypothesis), and that extraterrestrial lifeforms will converge toward characteristics similar to those of life on Earth. They suggest that any investigation of extraterrestrial life relying on these assumptions is overly restrictive, and it is possible to make a scientific and rational study of the possibility of life forms that are so different from life on Earth that we may not even recognise them as life in the first instance.

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Section-10

Technology for detection of extraterrestrial life:

As our technology has become more sophisticated, so has the search for other potential life in the universe. Some new factors to consider are based on advanced radio telescopes and on Kepler data (Kepler is a space observatory launched by NASA to discover Earth-size planets orbiting other stars) that habitable exoplanets similar to Earth are much more common than originally thought, and the universe is more expansive than we have believed. In August of 2022, in an exciting development, astronomers captured the first direct image of an exoplanet with the recently deployed James Webb Space Telescope. The earlier model Hubble Space Telescope, which was also a marvel, could take images of exoplanets but the Webb has far more capabilities to allow for infrared exploration of exoplanets that can detect water and carbon dioxide signatures in a planet’s atmosphere. There are other events bringing light to the possibilities of life elsewhere too. Recently scientists discovered an exoplanet ocean world called TOI-1452 b, just 100 light-years from Earth. A paper on the discovery says that the entire planet is covered by a thick layer of water and that it’s located far enough from its star to possibly support life. This discovery, along with recent discoveries of other potential exoplanets that can harbour life has led many leading astronomers to conclude that we are not alone in the universe.

Figure above depicts some major international efforts to search for extraterrestrial life. Clockwise from top left:

The search for extrasolar planets (image: Kepler telescope)
Listening for extraterrestrial signals indicating intelligence (image: Allen array)
Robotic exploration of the Solar System (image: Curiosity rover on Mars)

Perhaps the greatest advances in all of astrophysics have come from NASA’s flagship missions, which gave us revolutionary views with Hubble and JWST, among others.
The next flagship mission, the Nancy Roman Telescope, is already being built, but there were four proposals to choose from for the one after that, as recommended to the Astro2020 decadal committee.
The top priority has now been chosen and is being designed: NASA’s Habitable Worlds Observatory. The goal is no smaller than to find inhabited planets beyond Earth.

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Find water in the solar system:

While on Earth we can see water and handle it with our bare hands, detecting water or finding evidence of past water from far away is much more difficult. Optical telescopes that collect visible light and provide visual images of distant bodies only give us some indication of the brightness and large-scale shapes and structures of large regions. Brighter regions, especially near the north or south pole of a planet or moon could indicate reflections of frozen water (think of how shiny ice can be when you walk down the street in winter). However, when it’s cold enough, even carbon dioxide (a gas at room temperature on Earth) forms a reflective solid. As such, optical telescopes alone cannot confirm the presence of water.

Optical telescopes can also give some indication of mountains and valleys on other planets, but the large distance from Earth makes it very difficult to determine the size and structure of smaller geological features, like those that may have been formed with the help of flowing water. Placing a camera closer to a planet via an orbiting spacecraft allows scientists to collect much higher resolution images of the surface. Spacecraft that can land and even drive on the planet’s surface (called rovers), allow humans to “move” around a planet to look at the size and shape of rocks ranging from larger boulders to tiny pebbles.

Figure above shows Extraterrestrial exploration. There are many different ways that we search for water on extraterrestrial bodies, such as planets, moons, and asteroids. These include landers, rovers, and various varieties of telescopes.

In addition to camera images, scientists also indirectly understand what material is on a planet by measuring the reflectance of light off the surface. Different materials, including water, absorb and reflect different wavelengths of light – both visible light that humans see as different colors, and light that we cannot see with our eyes. The latter includes ultraviolet (shorter wavelengths than visible that can give you a sunburn) and infrared (longer wavelengths than visible that can be used to heat food in your microwave). The relative intensity of reflection of different wavelengths is together called a spectrum, which is measured to narrow down the possible range of materials on the surface of an extraterrestrial body.

Additional instruments can be designed to detect other wavelengths of light or elementary particles like neutrons emanating from the surface of an extraterrestrial body. These signals can be further interpreted to detect atomic elements such as hydrogen (H), which is one component of water. Often, the data from multiple instruments and camera images must be put together to actually determine what’s on or even just below the surface.

Additionally, landers and rovers can collect samples from the surface of the planet to be placed in an analysis chamber that can determine the chemical composition and types of minerals in the sample, such as clay minerals that likely formed in a liquid water environment. The capabilities of such chambers are limited by size, power requirements, and remote control from Earth. (On Earth, multiple samples could be moved from machine to machine and modified with human hands that have more dexterity than a robot arm.) Spacecraft can also collect a sample and return it to Earth for more detailed analysis. However, such sample return missions are very expensive and difficult to perform owing to the need to land on the surface of an extraterrestrial body, escape its gravitational pull, and return to Earth.

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Spacecrafts:

Our definition of habitable environments continues to expand. Off the Earth we’ve only begun to look. Spacecraft have flown by, orbited around, or landed on Mercury, Venus, Mars, Jupiter and several of its moons, Saturn and several of its moons, the dwarf planet Pluto and its moons, and the dwarf planet Ceres. A myriad of spacecrafts have orbited Earth to study the home planet. Comparative planetology is a thriving field. Ocean worlds in our solar system – in particular, Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan – are top targets for astrobiological investigations of prebiotic chemistry, habitability, and possible life. Many astrobiologists are exploring the possibility of extant life in the deep subsurface of Mars. NASA has sent five rovers and four landers to the surface of Mars. Additionally, orbiters have been outfitted with some amazing cameras to take pictures of the whole surface of the Red Planet. But we’ve only explored a tiny fraction of Mars. And that’s only one of the promising bodies to look for life in our solar system.

Starting in the early 1960s, both the United States and the Soviet Union launched a multitude of robotic deep-space probes to learn more about the other planets and satellites of the solar system. Carrying television cameras, detectors, and an assortment of other instruments, these probes sent back impressive amounts of scientific data and close-up pictures. Among the most successful missions were those involving the U.S. Messenger flybys of Mercury (2008–15), the Soviet Venera probes to Venus (1967–83), the U.S. Mars Exploration Rover landings on Mars (2004–18), and the U.S. Voyager 2 flybys of Jupiter, Saturn, Uranus, and Neptune (1979–89). When the Voyager 2 probe flew past Neptune and its moons in August 1989, every known major planet had been explored by spacecraft. Many long-held views, particularly those about the outer planets, were altered by the findings of the Voyager probe. These findings included the discovery of several rings and six additional satellites around Neptune, all of which are undetectable to ground-based telescopes.

Nearly 50 years ago, Viking 1 and Viking 2 became NASA’s first spacecraft to search for life on another planet, with each mission conducting inconclusive soil experiments to detect microbes on Mars. Today, their latest successor, the Perseverance rover, continues these efforts on the Red Planet as it investigates Jezero crater. Perseverance rover on Mars is gathering rock samples for eventual return to Earth, so scientists can probe them for signs of life. And the coming Europa Clipper mission will visit an icy moon of Jupiter. Its goal: to determine whether conditions on that moon would allow life to thrive in its global ocean, buried beneath a global ice shell.

In the decades since the Viking missions, astrobiology techniques have advanced dramatically. These developments have focused largely on identifying organics because carbon is an essential building block for life as we currently understand it. Like Viking 1 and 2, NASA’s Curiosity rover uses a technique called gas chromatography–mass spectrometry as part of its Sample Analysis at Mars tool to study organic compounds. Although Curiosity’s updated tool is gentler on materials, the method requires heating samples, risking the degradation of organics and a loss of molecular information. To safely probe organic molecules, the new mini Orbitrap device uses a fundamentally different approach called laser desorption mass spectrometry (LDMS). Rather than heating molecules, LDMS applies a high-energy laser to remove tiny fragments of material, which are then analyzed using the miniaturized Orbitrap.

Recently NASA announced two missions to Venus: DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging) and VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy). These will be the first NASA missions to Venus, often referred to as Earth’s twin, since 1990, and will increase our understanding of the planet and its evolution. This information will help SETI scientists better characterize habitable zones around stars and climate change on planets. DAVINCI+ will send a probe into Venus’ atmosphere to better determine its composition at various altitudes and take images of Venus’ “tesserae,” land masses akin to Earth’s continents. VERITAS is an orbiter that will better map the planet’s surface, identify rock types on the surface, and determine if active volcanoes are releasing water vapor or phosphine into the atmosphere.

Space probes:

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Telescope:

The first known practical telescopes were refracting telescopes with glass lenses and were invented in the Netherlands at the beginning of the 17th century. They were used for both terrestrial applications and astronomy. The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s.

Space telescope:

A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launched in 1968, and the Soviet Orion 1 ultraviolet telescope aboard space station Salyut 1 in 1971. Space telescopes avoid the filtering and distortion (scintillation) of electromagnetic radiation which they observe, and avoid light pollution which ground-based observatories encounter. They are divided into two types: Satellites which map the entire sky (astronomical survey), and satellites which focus on selected astronomical objects or parts of the sky and beyond. Space telescopes are distinct from Earth imaging satellites, which point toward Earth for satellite imaging, applied for weather analysis, espionage, and other types of information gathering.

Since the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observed from the Earth’s surface. These bands are visible – near-infrared and a portion of the radio-wave part of the spectrum. For this reason there are no X-ray or far-infrared ground-based telescopes as these have to be observed from orbit. Even if a wavelength is observable from the ground, it might still be advantageous to place a telescope on a satellite due to issues such as clouds, astronomical seeing and light pollution. The disadvantages of launching a space telescope include cost, size, maintainability and upgradability.

Space telescopes “see” by using mirrors to collect and focus light from distant stars. The bigger the mirror, the more details the telescope can see. It’s very difficult to launch a giant, heavy mirror into space. So, engineers gave the Webb telescope 18 smaller mirrors that fit together like a puzzle. The mirrors fold up inside the rocket, then unfold to form one large mirror in orbit.

Some examples of space telescopes from NASA are the Hubble Space Telescope that detects visible light, ultraviolet, and near-infrared wavelengths, the Spitzer Space Telescope that detects infrared radiation, and the Kepler Space Telescope that discovered thousands of exoplanets. The latest telescope that was launched was the James Webb Space Telescope on December 25th, 2021 in Kourou, French Guiana. The Webb telescope detects infrared light.

Figure below shows some space observatories and their wavelength working ranges, as of 2005:

Advantages:

Performing astronomy from ground-based observatories on Earth is limited by the filtering and distortion of electromagnetic radiation (scintillation or twinkling) due to the atmosphere. A telescope orbiting Earth outside the atmosphere is subject neither to twinkling nor to light pollution from artificial light sources on Earth. As a result, the angular resolution of space telescopes is often much higher than a ground-based telescope with a similar aperture. Many larger terrestrial telescopes, however, reduce atmospheric effects with adaptive optics.

Space-based astronomy is more important for frequency ranges that are outside the optical window and the radio window, the only two wavelength ranges of the electromagnetic spectrum that are not severely attenuated by the atmosphere. For example, X-ray astronomy is nearly impossible when done from Earth, and has reached its current importance in astronomy only due to orbiting X-ray telescopes such as the Chandra X-ray Observatory and the XMM-Newton observatory. Infrared and ultraviolet are also largely blocked.

Disadvantages:

Space telescopes are much more expensive to build than ground-based telescopes. Due to their location, space telescopes are also extremely difficult to maintain. The Hubble Space Telescope was serviced by the Space Shuttle, but most space telescopes cannot be serviced at all.

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Computers:

Besides the telescope itself, the electronic computer has become the astronomer’s most important tool. Indeed, the computer has revolutionized the use of the telescope to the point where the collection of observational data is now completely automated. The astronomer need only identify the object to be observed, and the rest is carried out by the computer and auxiliary electronic equipment.

A telescope can be set to observe automatically by means of electronic sensors appropriately placed on the telescope axis. Precise quartz or atomic clocks send signals to the computer, which in turn activates the telescope sensors to collect data at the proper time. The computer not only makes possible more efficient use of telescope time but also permits a more detailed analysis of the data collected than could have been done manually. Data analysis that would have taken a lifetime or longer to complete with a mechanical calculator can now be done within hours or even minutes with a high-speed computer.

Improved means of recording and storing computer data also have contributed to astronomical research. Optical disc data-storage technology, such as the CD-ROM (compact disc read-only memory) or the DVD-ROM (digital video disc read-only memory), has provided astronomers with the ability to store and retrieve vast amounts of telescopic and other astronomical data.

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Great Observatories:

Great Observatories, a semiformal grouping of four U.S. satellite observatories that had separate origins: the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope. The grouping came about because the four would provide unprecedented spatial and temporal coverage across much of the electromagnetic spectrum from gamma rays (Compton) through X-rays (Chandra) and visible light (Hubble) to the infrared (Spitzer). The four provided much sharper views of the universe than had been available previously. (Radio was not included in the Great Observatories. The long wavelength of radio waves required much larger satellites than were possible at that time, and most radio wavelengths can be detected from the ground.)

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NASA’s exoplanet space telescopes:

Thousands of exoplanets have been discovered and confirmed orbiting other stars. The first evidence of exoplanets dates to 1917 when Van Maanen identified the first polluted white dwarf, however, the first confirmed detection of an exoplanet would not come until the 1990s. The discovery of exoplanets grew exponentially in the years to follow with the launch of the Kepler Space Telescope.

The Kepler mission was specifically designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone (also called the “Godilocks zone,” the area around a star where rocky planets could have liquid water on the surface) and determine the fraction of stars that might have such planets around them. After the second of Kepler’s four gyroscope-like wheels failed in 2013, Kepler completed its prime mission that November and began its extended mission, K2. The spacecraft was retired in 2018, but Kepler data are still being used to find exoplanets (more than 2,700 confirmed so far).

NASA’s Spitzer Space Telescope (2013-2020) was not designed to search for exoplanets, but its infrared instruments made it an excellent exoplanet explorer. It was used in the notable discovery of the TRAPPIST-1 system. In 2018 the Transiting Exoplanet Survey Satellite (TESS) was launched as a successor to Kepler to discover exoplanets in orbit around the brightest dwarf stars, the most common star type in our galaxy. NASA’s James Webb Space Telescope and the future Nancy Grace Roman Space Telescope hold great promise for what we can learn from exoplanets. Through spectroscopy, reading light signatures for information, astronomers hope to learn more about planet atmospheres and the conditions of the planets themselves.

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Hubble Space Telescope (HST):

The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versatile, renowned both as a vital research tool and as a public relations boon for astronomy. The Hubble telescope is named after astronomer Edwin Hubble and is one of NASA’s Great Observatories. The Space Telescope Science Institute (STScI) selects Hubble’s targets and processes the resulting data, while the Goddard Space Flight Center (GSFC) controls the spacecraft.

Hubble Space Telescope (HST) is the first sophisticated optical observatory placed into orbit around Earth. Earth’s atmosphere obscures ground-based astronomers’ view of celestial objects by absorbing or distorting light rays from them. A telescope stationed in outer space is entirely above the atmosphere, however, and receives images of much greater brightness, clarity, and detail than do ground-based telescopes with comparable optics.

The HST is a large reflecting telescope whose mirror optics gather light from celestial objects and direct it into two cameras and two spectrographs (which separate radiation into a spectrum and record the spectrum). The HST has a 2.4-metre (94-inch) primary mirror, a smaller secondary mirror, and various recording instruments that can detect visible, ultraviolet, and infrared light. The most important of these instruments, the wide-field planetary camera, can take either wide-field or high-resolution images of the planets and of galactic and extragalactic objects. This camera is designed to achieve image resolutions 10 times greater than that of even the largest Earth-based telescope. A faint-object camera can detect an object 50 times fainter than anything observable by any ground-based telescope; a faint-object spectrograph gathers data on the object’s chemical composition. A high-resolution spectrograph receives distant objects’ ultraviolet light that cannot reach Earth because of atmospheric absorption.

Only 40 light-years away — a stone’s throw on the scale of our galaxy — several Earth-sized planets orbit the red dwarf star TRAPPIST-1. Four of the planets lie in the star’s habitable zone, a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces. Astronomers using NASA’s Hubble Space Telescope have conducted the first spectroscopic survey of these worlds. Hubble reveals that at least three of the exoplanets do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune. This means the atmospheres may be more shallow and rich in heavier gases like those found in Earth’s atmosphere, such as carbon dioxide, methane, and oxygen.

The HST is scheduled to remain operational through at least 2021, after which it is expected to be replaced by the James Webb Space Telescope, equipped with a mirror seven times larger than that of the HST.

The HST’s discoveries have revolutionized astronomy. Observations of Cepheid variables in nearby galaxies allowed the first accurate determination of Hubble’s constant, which is the rate of the universe’s expansion. The HST photographed young stars with disks that will eventually become planetary systems. The Hubble Deep Field, a photograph of about 1,500 galaxies, revealed galactic evolution over nearly the entire history of the universe. Within the solar system, the HST was also used to discover Hydra and Nix, two moons of the dwarf planet Pluto.

Figure below shows Hubble Space Telescope’s Ultra-Deep Field. Virtually every point of light in this image is a galaxy, each composed of billions of stars.

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Kepler space telescope (KST):

The Kepler space telescope is a space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orbit. The principal investigator was William J. Borucki. After nine and a half years of operation, the telescope’s reaction control system fuel was depleted, and NASA announced its retirement on October 30, 2018. Kepler mission was launched in 2009 to search for planets that transit (cross in front of) their host stars. The resulting dimming of the host stars is detectable by measuring their brightness, and Kepler monitored the brightness of 150,000 stars every 30 min for 4 y. The most easily detectable planets in the Kepler survey are those that are relatively large and orbit close to their host stars, especially those stars having lower intrinsic brightness fluctuations (noise). These large, close-in worlds dominate the list of known exoplanets. However, the Kepler brightness measurements can be analyzed and debiased to reveal the diversity of planets, including smaller ones, in our Milky Way Galaxy.

Figure below shows various exoplanets discovered by KST.

Kepler Space Telescope has played a significant role in finding the pantheon of planets with similarities to Earth.

Designed to survey a portion of Earth’s region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets, Kepler’s sole scientific instrument is a photometer that continually monitored the brightness of approximately 150,000 main sequence stars in a fixed field of view. These data were transmitted to Earth, then analyzed to detect periodic dimming caused by exoplanets that cross in front of their host star. Only planets whose orbits are seen edge-on from Earth could be detected. As of November 2018, Kepler has discovered 5,011 exoplanet candidates and 2,662 confirmed exoplanets. Kepler orbits the Sun, which avoids Earth occultations, stray light, and gravitational perturbations and torques inherent in an Earth orbit.

Are places where life might evolve common in the universe or vanishingly rare, leaving us effectively without hope of ever knowing whether another living world exists? Kepler’s answer was unequivocal. There are more planets than there are stars, and at least a quarter are Earth-size planets in their star’s so-called habitable zone, where conditions are neither too hot nor too cold for life. With a minimum of 100 billion stars in the Milky Way, that means there are at least 25 billion places where life could conceivably take hold in our galaxy alone—and our galaxy is one among trillions. It’s no wonder that Kepler, which ran out of fuel in 2018, is regarded almost with reverence by astronomers. Kepler was the greatest step forward in the Copernican revolution since Copernicus. It’s changed the way we approach one of the great mysteries of existence. The question is no longer, is there life beyond Earth? It’s a pretty sure bet there is. The question now is, how do we find it?

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Transiting Exoplanet Survey Satellite (TESS):

Transiting Exoplanet Survey Satellite (TESS) is an MIT-led NASA space telescope launched in 2018. Like Kepler, TESS looks for a slight dimming in the luminosity of a star when a planet passes—transits—in front of it. TESS is scanning nearly the whole sky, with the goal of identifying about 50 exoplanets with rocky surfaces like Earth’s that could be investigated by more powerful telescopes coming on line, beginning with the James Webb Space Telescope, which NASA launched in 2021. TESS can detect minor, rocky planets in the habitable zone of their host star, where conditions are favorable for liquid water to exist. This makes TESS an essential tool in the search for potentially habitable exoplanets. TESS can also cover a sky area 400 times larger than that monitored by Kepler.

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The James Webb Space Telescope (JWST):

The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. Its high-resolution and high-sensitivity instruments allow it to view objects too old, distant, or faint for the Hubble Space Telescope. This enables investigations across many fields of astronomy and cosmology, such as observation of the first stars and the formation of the first galaxies, and detailed atmospheric characterization of potentially habitable exoplanets. The Webb was launched on 25 December 2021 on an Ariane 5 rocket from Kourou, French Guiana. In January 2022 it arrived at its destination, a solar orbit near the Sun–Earth L2 Lagrange point, about 1.5 million kilometers (930,000 mi) from Earth. The telescope’s first image was released to the public on 11 July 2022.

The James Webb Space Telescope is the largest, most powerful space telescope ever built. It will allow scientists to look at what our universe was like about 200 million years after the Big Bang. The telescope will be able to capture images of some of the first galaxies ever formed. It will also be able to observe objects in our solar system from Mars outward, look inside dust clouds to see where new stars and planets are forming and examine the atmospheres of planets orbiting other stars.

The Webb telescope is as tall as a 3-story building and as long as a tennis court! It is so big that it has to fold origami-style to fit inside the rocket to launch. The telescope will unfold, sunshield first, once in space.

Webb’s primary mirror consists of 18 hexagonal mirror segments made of gold-plated beryllium, which together create a 6.5-meter-diameter (21 ft) mirror, compared with Hubble’s 2.4 m (7 ft 10 in). This gives Webb a light-collecting area of about 25 square meters, about six times that of Hubble. Unlike Hubble, which observes in the near ultraviolet and visible (0.1 to 0.8 μm), and near infrared (0.8–2.5 μm) spectra, Webb observes a lower frequency range, from long-wavelength visible light (red) through mid-infrared (0.6–28.3 μm). The telescope must be kept extremely cold, below 50 K (−223 °C; −370 °F), so that the infrared light emitted by the telescope itself does not interfere with the collected light. Its five-layer sunshield protects it from warming by the Sun, Earth, and Moon.

The James Webb Space Telescope has four key goals:

to search for light from the first stars and galaxies that formed in the universe after the Big Bang
to study galaxy formation and evolution
to understand star formation and planet formation
to study planetary systems and the origins of life

These goals can be accomplished more effectively by observation in near-infrared light rather than light in the visible part of the spectrum. For this reason, Webb’s instruments will not measure visible or ultraviolet light like the Hubble Telescope, but will have a much greater capacity to perform infrared astronomy.

The James Webb Space Telescope is capable of analysing tiny flecks of light from the atmospheres of distant planets. Nasa’s James Webb Space Telescope may have discovered tentative evidence of a sign of life on a faraway planet. It may have detected a molecule called dimethyl sulphide (DMS). On Earth, at least, this is only produced by life. The researchers stress that the detection on the planet 120 light years away is “not robust” and more data is needed to confirm its presence. Researchers have also detected methane and CO2 in the planet’s atmosphere. Detection of these gases could mean the planet, named K2-18b, has a water ocean.

JWST is able to analyse the light that passes through the faraway planet’s atmosphere. That light contains the chemical signature of molecules in its atmosphere. The details can be deciphered by splitting the light into its constituent frequencies – rather like a prism creating a rainbow spectrum. If parts of the resulting spectrum are missing, it has been absorbed by chemicals in the planet’s atmosphere, enabling researchers to discover its composition. The feat is all the more remarkable because the planet is more than 1.1 million billion km away, so the amount of light reaching the space telescope is tiny.

NASA is using advanced tools, like the James Webb Space Telescope, to search for signs of life beyond Earth. The focus is on detecting biosignatures, and they are developing a scale to interpret evidence. Key markers of potential life include chemical systems capable of evolution, liquid water, energy sources, and atmospheric gas imbalances. The presence of environmental “gradients” also indicates potential life-hosting environments.

NASA is now evaluating plans for two space telescopes which will be able to scan the skies of Earthlike exoplanets. The Habitable Exoplanet Imaging Mission (HabEx) and the Large UV/Optical/Infrared Surveyor (LUVOIR) could potentially detect life on exoplanets. [vide supra]

The Earth as a Transiting Exoplanet:

The James Webb Space Telescope (JWST) will enable the search for and characterization of terrestrial exoplanet atmospheres in the habitable zone via transmission spectroscopy. In the new study, researchers took a spectrum of Earth’s atmosphere and deliberately decreased the quality of the data to mimic how it would look to an observer dozens of light-years away. The team then used a computer model, which replicated JWST’s sensor capabilities, to see if the spacecraft could detect the key biosignatures and technosignatures from the dataset, such as methane and oxygen, produced by biological life, and nitrogen dioxide and chlorofluorocarbons (CFCs), which are produced by humans. The results show that JWST could likely detect all the key markers of non-intelligent and intelligent life in our planet’s atmosphere. The researchers noted that the quality of the altered dataset is roughly equivalent to JWST observations of planets from TRAPPIST-1 — a star system containing seven exoplanets that orbit a red dwarf star around 40 light-years from Earth. This suggests the telescope should be able to detect life or alien civilizations on exoplanets within 40 light-years of Earth. But the team believes JWST could possibly detect signs of extraterrestrial life up to 50 light-years from Earth.

Only around 20 exoplanets have been officially discovered within a 50-light-year radius of Earth, but based on the number of suspected stars in this region of space, experts predict that there may actually be as many as 4,000 exoplanets within JWST’s reach, according to Project EDEN, an international astronomical collaboration dedicated to finding potentially habitable planets close to Earth. However, this doesn’t guarantee that JWST would be able to detect life on other planets.

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The Nancy Grace Roman Space Telescope:

The Nancy Grace Roman Space Telescope (shortened as Roman or the Roman Space Telescope, and formerly the Wide-Field Infrared Survey Telescope or WFIRST) is a NASA infrared space telescope in development and scheduled to launch by May 2027. The Roman Space Telescope is based on an existing 2.4 m (7.9 ft) wide field of view primary mirror and will carry two scientific instruments. The Wide-Field Instrument (WFI) is a 300.8-megapixel multi-band visible and near-infrared camera, providing a sharpness of images comparable to that achieved by the Hubble Space Telescope over a 0.28 square degree field of view, 100 times larger than imaging cameras on the Hubble. The Coronagraphic Instrument (CGI) is a high-contrast, small field of view camera and spectrometer covering visible and near-infrared wavelengths using novel starlight-suppression technology.

The Nancy Grace Roman Space Telescope began life as the Wide Field Infrared Survey Telescope (WFIRST) in 2010, only gaining its current name a decade later when in May 2020 it was renamed in honor of Nancy Grace Roman, a pioneering scientist who served as NASA’s first chief astronomer from 1961 to 1963. Stated objectives of this space telescope include a search for extra-solar planets using gravitational microlensing, along with probing the chronology of the universe and growth of cosmic structure, with the end goal of measuring the effects of dark energy, the consistency of general relativity, and the curvature of spacetime.

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Habitable Worlds Observatory (HWO):

Planning is well underway for NASA’s Habitable Worlds Observatory (HWO), which will scour the atmospheres of planets outside the solar system for telltale signs of alien life. Recently a workshop was held at the California Institute for Technology (Caltech) at which scientists and engineers discussed the state of technology that could be employed by the HWO, one of NASA’s next big telescope projects after the James Webb Space Telescope (JWST). The hunt for signs of life in the atmospheres of planets outside the solar system orbiting distant stars — exoplanets — is akin to hunting for a needle in a cosmic haystack. After all, NASA estimates there are several billion Earth-size planets sitting in the habitable zones of their stars, which regions with the right temperatures to allow liquid water to exist. And that’s in the Milky Way alone. Yet, scientists at least have a good idea of what they should be hunting for as well as knowledge of signs that would potentially indicate life. “We want to probe the atmospheres of these exoplanets to look for oxygen, methane, water vapor, and other chemicals that could signal the presence of life,” NASA’s Exoplanet Exploration Program chief technologist, Nick Siegler, said in a statement. “We aren’t going to see little green men but rather spectral signatures of these key chemicals, or what we call biosignatures.”

The HWO was first proposed as a top priority by the Decadal Survey on Astronomy and Astrophysics 2020 (Astro2020), a roadmap of goals for the astronomy community to take on over the coming decade. This is because, in addition to hunting for signs of life outside the solar system and helping astronomers understand entire planetary systems, the observatory will also play a major role in astrophysics investigations. Though the mission is set to launch in the late 2030s or early 2040s, advancing technologies the telescope will use now could help prevent cost overruns later down the line, according to Dmitry Mawet, member of the HWO Technical Assessment Group (TAG).

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Spectroscopy:

Every chemical compound absorbs a unique set of wavelengths of light. (We see leaves as green, for instance, because chlorophyll is a light-hungry molecule that absorbs red and blue, so the only light reflected is green.) In the same way, compounds in a transiting planet’s upper atmosphere might leave their spectral fingerprints in starlight passing through. Theoretically, if there are gases in a planet’s atmosphere from living creatures, we could see the evidence in the light that reaches us. There’s an outside chance a rocky planet orbits a star close enough for the Webb telescope to capture sufficient light to investigate it for signs of life. But most scientists think we’ll need to wait for the next generation of space telescopes. Electromagnetic radiation carries a lot of information about the nature of stars and other astronomical objects. To extract this information, however, astronomers must be able to study the amounts of energy we receive at different wavelengths of light in fine detail.

Properties of Light:

Light exhibits certain behaviors that are important to the design of telescopes and other instruments. For example, light can be reflected from a surface. If the surface is smooth and shiny, as with a mirror, the direction of the reflected light beam can be calculated accurately from knowledge of the shape of the reflecting surface. Light is also bent, or refracted, when it passes from one kind of transparent material into another—say, from the air into a glass lens. Reflection and refraction of light are the basic properties that make possible all optical instruments (devices that help us to see things better)—from eyeglasses to giant astronomical telescopes. Such instruments are generally combinations of glass lenses, which bend light according to the principles of refraction, and curved mirrors, which depend on the properties of reflection. Small optical devices, such as eyeglasses or binoculars, generally use lenses, whereas large telescopes depend almost entirely on mirrors for their main optical elements.

In 1672, in the first paper that he submitted to the Royal Society, Sir Isaac Newton described an experiment in which he permitted sunlight to pass through a small hole and then through a prism. Newton found that sunlight, which looks white to us, is actually made up of a mixture of all the colors of the rainbow as seen in the figure below:

When we pass a beam of white sunlight through a prism, we see a rainbow-colored band of light that we call a continuous spectrum. Figure above shows how light is separated into different colors with a prism—a piece of glass in the shape of a triangle with refracting surfaces. Upon entering one face of the prism, the path of the light is refracted (bent), but not all of the colors are bent by the same amount. The bending of the beam depends on the wavelength of the light as well as the properties of the material, and as a result, different wavelengths (or colors of light) are bent by different amounts and therefore follow slightly different paths through the prism. The violet light is bent more than the red. This phenomenon is called dispersion and explains Newton’s rainbow experiment. Upon leaving the opposite face of the prism, the light is bent again and further dispersed. If the light leaving the prism is focused on a screen (see figure below), the different wavelengths or colors that make up white light are lined up side by side just like a rainbow. (In fact, a rainbow is formed by the dispersion of light though raindrops) Because this array of colors is a spectrum of light, the instrument used to disperse the light and form the spectrum is called a spectrometer.

Figure above shows Continuous Spectrum. When white light passes through a prism, it is dispersed and forms a continuous spectrum of all the colors. Although it is hard to see in this printed version, in a well-dispersed spectrum, many subtle gradations in color are visible as your eye scans from one end (violet) to the other (red).

The Value of Stellar Spectra:

When Newton described the laws of refraction and dispersion in optics, and observed the solar spectrum, all he could see was a continuous band of colors. If the spectrum of the white light from the Sun and stars were simply a continuous rainbow of colors, astronomers would have little interest in the detailed study of a star’s spectrum once they had learned its average surface temperature. In 1802, however, William Wollaston built an improved spectrometer that included a lens to focus the Sun’s spectrum on a screen. With this device, Wollaston saw that the colors were not spread out uniformly, but instead, some ranges of color were missing, appearing as dark bands in the solar spectrum. He mistakenly attributed these lines to natural boundaries between the colors. In 1815, German physicist Joseph Fraunhofer, upon a more careful examination of the solar spectrum, found about 600 such dark lines (missing colors), which led scientists to rule out the boundary hypothesis.

Figure above shows Visible Spectrum of the Sun. Our star’s spectrum is crossed by dark lines produced by atoms in the solar atmosphere that absorb light at certain wavelengths. This figure shows an absorption spectrum.

Later, researchers found that similar dark lines could be produced in the spectra (“spectra” is the plural of “spectrum”) of artificial light sources. They did this by passing their light through various apparently transparent substances—usually containers with just a bit of thin gas in them.

These gases turned out not to be transparent at all colors: they were quite opaque at a few sharply defined wavelengths. Something in each gas had to be absorbing just a few colors of light and no others. All gases did this, but each different element absorbed a different set of colors and thus showed different dark lines. If the gas in a container consisted of two elements, then light passing through it was missing the colors (showing dark lines) for both of the elements. So it became clear that certain lines in the spectrum “go with” certain elements. This discovery was one of the most important steps forward in the history of astronomy.

What would happen if there were no continuous spectrum for our gases to remove light from? What if, instead, we heated the same thin gases until they were hot enough to glow with their own light? When the gases were heated, a spectrometer revealed no continuous spectrum, but several separate bright lines. That is, these hot gases emitted light only at certain specific wavelengths or colors.

When the gas was pure hydrogen, it would emit one pattern of colors; when it was pure sodium, it would emit a different pattern. A mixture of hydrogen and sodium emitted both sets of spectral lines. The colors the gases emitted when they were heated were the very same colors as those they had absorbed when a continuous source of light was behind them. From such experiments, scientists began to see that different substances showed distinctive spectral signatures by which their presence could be detected (see figure below). Just as your signature allows the bank to identify you, the unique pattern of colors for each type of atom (its spectrum) can help us identify which element or elements are in a gas.

Figure above shows Continuous Spectrum and Line Spectra from Different Elements. Each type of glowing gas (each element) produces its own unique pattern of lines, so the composition of a gas can be identified by its spectrum. The spectra of sodium, hydrogen, calcium, and mercury gases are shown here. This figure shows the emission spectrum of a number of common elements along with an example of a continuous spectrum.

Types of Spectra:

In these experiments, then, there were three different types of spectra.

-1. A continuous spectrum (formed when a solid or very dense gas gives off radiation) is an array of all wavelengths or colors of the rainbow. A continuous spectrum can serve as a backdrop from which the atoms of much less dense gas can absorb light.

-2. A dark line, or absorption spectrum, consists of a series or pattern of dark lines—missing colors—superimposed upon the continuous spectrum of a source.

-3. A bright line, or emission spectrum, appears as a pattern or series of bright lines; it consists of light in which only certain discrete wavelengths are present.

When we have a hot, thin gas, each particular chemical element or compound produces its own characteristic pattern of spectral lines—its spectral signature. No two types of atoms or molecules give the same patterns. In other words, each particular gas can absorb or emit only certain wavelengths of the light peculiar to that gas. In contrast, absorption spectra occur when passing white light through a cool, thin gas. The temperature and other conditions determine whether the lines are bright or dark (whether light is emitted or absorbed), but the wavelengths of the lines for any element are the same in either case. It is the precise pattern of wavelengths that makes the signature of each element unique. Liquids and solids can also generate spectral lines or bands, but they are broader and less well defined—and hence, more difficult to interpret. Spectral analysis, however, can be quite useful. It can, for example, be applied to light reflected off the surface of a nearby asteroid as well as to light from a distant galaxy.

The dark lines in the solar spectrum thus give evidence of certain chemical elements between us and the Sun absorbing those wavelengths of sunlight. Because the space between us and the Sun is pretty empty, astronomers realized that the atoms doing the absorbing must be in a thin atmosphere of cooler gas around the Sun. This outer atmosphere is not all that different from the rest of the Sun, just thinner and cooler. Thus, we can use what we learn about its composition as an indicator of what the whole Sun is made of. Similarly, we can use the presence of absorption and emission lines to analyze the composition of other stars and clouds of gas in space.

Such analysis of spectra is the key to modern astronomy. Only in this way can we “sample” the stars, which are too far away for us to visit. Encoded in the electromagnetic radiation from celestial objects is clear information about the chemical makeup of these objects. Only by understanding what the stars were made of could astronomers begin to form theories about what made them shine and how they evolved.

In 1860, German physicist Gustav Kirchhoff became the first person to use spectroscopy to identify an element in the Sun when he found the spectral signature of sodium gas. In the years that followed, astronomers found many other chemical elements in the Sun and stars. In fact, the element helium was found first in the Sun from its spectrum and only later identified on Earth. (The word “helium” comes from helios, the Greek name for the Sun.)

The ultimate goal of astrobiology is to find evidence for life on planets outside of our solar system. To do this, we must gather as much information as possible about other planets including orbital parameters (the distance from a planet to its star and how circular its orbit is), bulk composition (its proportions of gasses, water, and land mass), and the atmosphere of the planet. Life has altered Earth’s atmosphere substantially from what we otherwise find on planets devoid of life, where chemical and physical processes alone shape the planet’s atmosphere. If life exists on other planets, it will likely have altered their atmospheres as well. Ultimately we need larger telescopes to characterize the atmospheres of extrasolar planets through a technique called spectroscopy. Light interacts with molecules in very predictable ways, allowing us to determine what is in an atmosphere just by simply observing absorption and emission in either the starlight reflected off of a planet or in the heat radiated by the planet. Spectra measured with these improved telescopes are the key to detecting life on other planets as they will allow us to detect molecules in the atmospheres of planets and even test for surface features that may indicate life.

Figure above shows Spectrum of Earth as seen from Saturn.

For the past century, astronomers have used spectroscopy to learn more about stars, galaxies, super massive black holes, and the planets in our own Solar System. Only in the last two decades, with powerful new telescopes, cameras, and computers, have we finally achieved the precision necessary to measure the spectra of exoplanets. The first spectrum of an exoplanet, published in 2002, was taken using the Hubble Space Telescope and showed evidence of vaporized sodium in the atmosphere of the exoplanet HD 209458 b. There are major prospects in the next few decades to observe the spectra of planets and hunt for potential “biosignatures.” Biosignatures are the spectral markers of molecules that might be necessary for, or produced by, life as we know it.

There are three ways to measure a planet’s spectrum: 1) look for light bouncing off the planet’s surface or atmosphere (reflection spectroscopy), 2) observe the light produced by the heat of the planet itself (thermal emission spectroscopy), or 3) watch light pass through the planet’s atmosphere (transmission spectroscopy).

Transmission spectroscopy:

Graphic below showing how light can be split into different wavelengths and that if it has passed through an atmosphere some wavelengths will be missing – depending on what was in the atmosphere – which scientists can then interpret.

Transmission spectra, the “light fingerprints” of planets, are created when a planet’s atmosphere absorbs certain colors of starlight and allows others to pass through. These spectra are essential tools that scientists use to decipher the composition of an atmosphere from afar. The ultimate goal of NASA’s Exoplanet Program is to find unmistakable signs of current life. Exoplanets’ own skies could hold such signs, waiting to be revealed by detailed analysis of the atmospheres of planets well beyond our solar system. When we analyze light shot by a star through the atmosphere of a distant planet, a technique known as transmission spectroscopy, the effect looks like a barcode. The slices missing from the light spectrum tell us which ingredients are present in the alien atmosphere. One pattern of black gaps might indicate methane, another, oxygen. Seeing those together could be a strong argument for the presence of life. Or we might read a barcode that shows the burning of hydrocarbons; in other words, smog.

Spectroscopy lines of an exoplanet is seen in the figure below:

Light from exoplanets, if passed through a prism, can be spread out into a rainbow of colors called a spectrum. Different colors correspond to different wavelengths of light. Missing colors show up as black lines, indicating specific gases are present, because each gas absorbs light in a specific wavelength (or color).

Astronomical spectroscopy is used to measure three major bands of radiation in the electromagnetic spectrum: visible light, radio waves, and X-rays. While all spectroscopy looks at specific bands of the spectrum, different methods are required to acquire the signal depending on the frequency. Ozone (O3) and molecular oxygen (O2) absorb light with wavelengths under 300 nm, meaning that X-ray and ultraviolet spectroscopy require the use of a space telescope. Radio signals have much longer wavelengths than optical signals, and require the use of antennas or radio dishes. Infrared light is absorbed by atmospheric water and carbon dioxide, so while the equipment is similar to that used in optical spectroscopy, space telescope is required to record much of the infrared spectrum.

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Finding extraterrestrial life using ground-based high-dispersion spectroscopy, a 2013 study:

Exoplanet observations promise one day to unveil the presence of extraterrestrial life. Atmospheric compounds in strong chemical disequilibrium would point to large-scale biological activity just as oxygen and methane do in the Earth’s atmosphere. The cancellation of both the Terrestrial Planet Finder and Darwin missions means that it is unlikely that a dedicated space telescope to search for biomarker gases in exoplanet atmospheres will be launched within the next 25 years. Here authors show that ground-based telescopes provide a strong alternative for finding biomarkers in exoplanet atmospheres through transit observations. Recent results on hot Jupiters show the enormous potential of high-dispersion spectroscopy to separate the extraterrestrial and telluric signals making use of the Doppler shift of the planet. The transmission signal of oxygen from an Earthtwin orbiting a small red dwarf star is only a factor 3 smaller than that of carbon monoxide recently detected in the hot Jupiter Tau Boötis b, albeit such a star will be orders of magnitude fainter. Authors show that if Earth-like planets are common, the planned extremely large telescopes can detect oxygen within a few dozen transits. Ultimately, large arrays of dedicated flux collector telescopes equipped with high-dispersion spectrographs can provide the large collecting area needed to perform a statistical study of life-bearing planets in the solar neighborhood.

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Starlight suppression for spectroscopy:

The main exoplanet discovery tools—the radial velocity and transit techniques, which have discovered more than 95 percent of the more than 5000 known exoplanets—will not be the techniques to directly image exoplanets, which is needed to get a reflected light spectrum. Spectroscopy will be hard because there simply aren’t many photons available to use, but it will not be the biggest problem. The biggest problem will be suppressing the light from the stars, which can be 10 billion times brighter than a rocky planet in the habitable zone of a Sun-like star. Starlight suppression could be done in one of the following three ways: internal occulters (i.e., coronagraphs), external occulters (i.e., starshades), and nulling interferometers. The latter option is the least technologically mature of the options and one that NASA is not currently pursuing.

The space telescope’s ability to characterize the atmospheres of exoplanets, and therefore look for signatures that could indicate life, depends on technologies that block the glare from a distant star. There are two main ways of blocking the star’s light: a small mask internal to the telescope, known as a coronagraph, and a large mask external to the telescope, known as a starshade. In space, starshades would unfurl into a giant sunflower-shaped structure.

In both cases, the light of stars is blocked so that faint starlight reflecting off a nearby planet is revealed. The process is similar to holding your hand up to block the sun while snapping a picture of your smiling friends. By directly capturing the light of a planet, researchers can then use other instruments called spectrometers to scrutinize that light in search of the chemical signatures. If any life is present on a planet orbiting a distant star, then the collective inhales and exhales of that life might be detectable in the form of biosignatures.

Coronagraph:

While the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) both have coronagraphs, WFIRST will be the first space telescope with a coronagraph (or possibly a starshade) specifically designed for directly imaging exoplanets. WFIRST’s Wide-Field Instrument (WFI) will arguably help answer questions in three of the biggest astrophysical areas—dark matter, dark energy, and exoplanets (via microlensing and coronagraphy). The telescope’s coronagraph instrument (CGI) will be used for the direct imaging and spectroscopy of exoplanets. WFIRST is in its formulation phase (Phase A) at this time. The project, telescope, and WFI are managed by NASA Goddard Space Flight Center, while the CGI is managed by JPL. The project has now also been directed to study the compatibility of a starshade with WFIRST. The current state of the art for coronagraphs is the Gemini Planet Imager (GPI) and the Very Large Telescope Spectro-Polarimetric High-contrast Exoplanet Research instrument (VLT SPHERE). WFIRST would improve upon their contrast ratio capability by 2 to 3 orders of magnitude and also improve upon the ability to probe smaller planet-star separations. Further technological advancement would be required to observe rocky planets in the habitable zone of stars at a distance of 10 parsec (pc) and further.

Figure above shows how coronagraphs developed at Caltech will help astronomers search for molecular bio-signatures on exoplanets. Coronagraphs block a star’s light, making orbiting planets easier to see. High-resolution spectrometers would help further isolate a planet’s light, and could reveal molecules in the planet’s atmosphere.

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Starshades:

There are two separate spacecraft with separate propulsion systems. When aligned, the starshade blocked the star’s light, revealing the reflected light of the planets. The starshade possesses a petal-like shape which serves to reorient the diffraction, creating a dark shadow for the telescope. A starshade is a simpler method than the coronagraph because the starshade is doing all the work. It drastically reduces wavefront-control requirements on sensitivity, segment phasing, and other corrections. It has a higher tolerance for error as long as the starshade performs as designed. The starshade would be tens of meters across and tens of thousands of kilometers away. The starshade needs to be able to deploy and position its petals and maintain its physical stability, suppress the starlight, and fly in formation with a telescope separated from it by tens of thousands of kilometers and maintain the telescope’s lateral offset within acceptable limits. A starshade optical demonstration performed by Northrop Grumman in the Nevada desert was able to detect a simulated planet 100 million times fainter. Another experiment used a baseline of 2.4 km with a solar telescope to block out Arcturus and observe background stars. Another test, currently ongoing at Princeton University, has exceeded a contrast ratio of 10^−8 at a single wavelength of 632 nanometers.

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Instrumentation:

The emerging field of astrobiology combines the key characteristics of astronomy, biology, and geology and as such, so does the instrumentation. Within our solar system the focus resides on Mars, Europa, and Titan, due to the presence or assumed presence of water (cf. Europa) and organics (cf. Titan). Search and analysis of organics, search for water as prerequisite for life, search for habitable zones by remote sensing and in situ search instrumentation are summarized in Table below. Due to the harsh conditions on most of the planetary surfaces the search is focusing on subsurface niches (e.g., with GPR), requiring mobility and subsurface penetration capabilities (e.g., drills or moles), equipped with sampling mechanism and dedicated sensors for analysis of prebiotic chemistry, as well as for the search for past or present life.

Instrument examples for astrobiology payloads suites for remote or in situ sensing:

Remote sensing	In situ
• High-resolution imager (visual)	• Panoramic camera
• Stereo camera	• Laser mass spectrometer
• Spectral imager (UV, NIR, FIR, visual)	• LIBS/Raman spectrometer
• Subsurface radar	• Alpha particle X-ray spectrometer • X-ray diffractometer
• X-ray, gamma ray and neutron spectrometer	• UV spectrometer
• Microwave sounder, submillimeter sounder	• Attenuated total reflection spectrometer
• Magnetometer	• Mössbauer spectrometer
• Thermal infrared radiometer	• Infrared Fourier spectrometer
	• Gas-chromatograph and mass spectrometer
	• Microscopic imager
	• Heat flow and physical properties package
	• (Miniature) thermal emission spectrometer
	• Radon exhalation
	• Magnetometer
	• Seismometer
	• Ground penetrating radar (surface deployed)
	• Drill system
	• Life marker chip
	• Radiation monitor
	• Permittivity probe
	• Environmental package

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Radio astronomy:

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. Radio astronomy is study of celestial bodies by examination of the radio-frequency energy they emit or reflect. Radio waves penetrate much of the gas and dust in space, as well as the clouds of planetary atmospheres, and pass through Earth’s atmosphere with little distortion. Astronomers around the world use radio telescopes to observe the naturally occurring radio waves that come from stars, planets, galaxies, clouds of dust, and molecules of gas.

All astronomy is about observing waves of light. Stars, galaxies and gas clouds in space emit visible light as well as light from other parts of the electromagnetic spectrum in the form of radio waves, gamma rays, X-rays, and infrared radiation. Optical telescopes – telescopes that collect visible light – show us shining stars, glowing gas and dark dust but this doesn’t give us the whole picture of what’s happening in space. Telescopes tuned to different parts of the electromagnetic spectrum can reveal hidden objects in space; the resulting images can then be combined to give a more complete picture. Radio waves from space were first detected in the 1930s but little was done to follow them up until after the Second World War. In the post-war period CSIRO scientists and engineers were among the pioneers of radio astronomy. Radio telescopes detect and amplify radio waves from space, turning them into signals that astronomers use to enhance our understanding of the Universe.

Figure above shows Radio telescope that are often used by SETI projects.

A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects, just as optical telescopes are the main observing instrument used in traditional optical astronomy which studies the light wave portion of the spectrum coming from astronomical objects. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night.

Radio telescopes are directional radio antennas that typically employ a large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Unlike an optical telescope, which produces a magnified image of the patch of sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies. In some newer radio telescope designs, a single dish contains an array of several receivers; this is known as a focal-plane array. By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed.

In much the same way that you tune the radio to a particular station, radio astronomers can tune their telescopes to pick up radio waves millions of light years from Earth. Just as optical telescopes collect visible light, bring it to a focus, amplify it and make it available for analysis by various instruments, so do radio telescopes collect weak radio light waves, bring it to a focus, amplify it and make it available for analysis. Using sophisticated computer programming, they can unravel signals to study the birth and death of stars, the formation of galaxies and the various kinds of matter in the Universe. Radio astronomers process the masses of information collected by a telescope. To help make sense of the strings of numbers, they convert the numbers into pictures. Each number represents information from a specific point in space. Often they have colours assigned to the numbers corresponding to the amount of information they represent. Astronomers then combine the colours to make a picture, visualising the information to reveal some of the characteristics of objects in the Universe.

Many radio frequencies penetrate Earth’s atmosphere quite well, and this led to radio telescopes that investigate the cosmos using large radio antennas. Naturally occurring radio waves are extremely weak by the time they reach us from space. A cell phone signal is a billion billion times more powerful than the cosmic waves our telescopes detect. Since astronomical radio sources such as planets, stars, nebulas and galaxies are very far away, the radio waves coming from them are extremely weak, so radio telescopes require very large antennas to collect enough radio energy to study them, and extremely sensitive receiving equipment. Radio telescopes are typically large parabolic (“dish”) antennas similar to those employed in tracking and communicating with satellites and space probes. They may be used individually or linked together electronically in an array. Radio observatories are preferentially located far from major centers of population to avoid electromagnetic interference (EMI) from radio, television, radar, motor vehicles, and other man-made electronic devices. Human endeavours emit considerable electromagnetic radiation as a byproduct of communications such as television and radio. These signals would be easy to recognize as artificial due to their repetitive nature and narrow bandwidths. Earth has been sending radio waves from broadcasts into space for over 100 years. These signals have reached over 1,000 stars, most notably Vega, Aldebaran, Barnard’s Star, Sirius, and Proxima Centauri. If intelligent alien life exists on any planet orbiting these nearby stars, these signals could be heard and deciphered, even though some of the signal is garbled by the Earth’s ionosphere.

Some of the more notable frequency bands used by radio telescopes include:

Every frequency in the United States National Radio Quiet Zone
Channel 37: 608 to 614 MHz
The “Hydrogen line”, also known as the “21 centimeter line”: 1,420.40575177 MHz, used by many radio telescopes including The Big Ear in its discovery of the Wow! signal
1,406 MHz and 430 MHz
The Waterhole: 1,420 to 1,666 MHz
The Arecibo Observatory had several receivers that together covered the whole 1–10 GHz range.
The Wilkinson Microwave Anisotropy Probe mapped the Cosmic microwave background radiation in 5 different frequency bands, centered on 23 GHz, 33 GHz, 41 GHz, 61 GHz, and 94 GHz.

Radio astronomy has changed the way we view the Universe and dramatically increased our knowledge of it, for example:

Astronomers trying to identify the source of interference in a radio antenna in the 1960s discovered the Cosmic Microwave Background Radiation, the afterglow of the Big Bang.
Cold clouds of gas found in interstellar space emit radio waves at distinct wavelengths. As hydrogen is the most abundant element in the Universe and is common in galaxies, radio astronomers use its characteristic emission to map out the structure of galaxies.
Radio astronomy has also detected many new types of objects including pulsars, the rapidly spinning remnants of supernova explosions that send out regular flashes of radio waves much like the beam from a lighthouse. Our Parkes radio telescope, Murriyang, has detected over half of the more than 2000 known pulsars.

Figure above shows The Circinus galaxy as seen at different wavelengths: cold hydrogen gas (coloured blue), the fuel for star formation, was mapped using one of CSIRO’s radio telescopes; the warm dust of space (coloured red) and stars (shown in green) were mapped using data from mid-infrared instruments. When combined, these three images reveal gas and stars in the inner disk and spiral arms of the galaxy.

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Radioastronomy to radio wave technosignature detection:

Many international radio telescopes are currently being used for radio SETI searches, including the Low Frequency Array (LOFAR) in Europe, the Murchison Widefield Array (MWA) in Australia, and the Lovell Telescope in the United Kingdom.

When it comes right down to it, only a extremely narrowband astronomical radio source could be said to have an artificial origin, since broadband radio transmissions are a common occurrence in our galaxy. As a result, SETI researchers have conducted surveys that looked for both continuous wave and pulse radio sources that could not be explained by natural phenomena. A good example of this is the famous “WOW.” signal that was detected on August 15th, 1977, by astronomer Jerry R. Ehman using the Big Ear radio telescope at Ohio State University. In the course of surveying the Sagittarius constellation, near the M55 globular cluster, the telescope noted a sudden jump in radio transmissions. Unfortunately, multiple follow-up surveys were unable to find any further indications of radio signals from this source. This and other examples characterize the painstaking and difficult work that comes with searching for radio wave technosignatures, which has been characterized as looking for a needle in the “cosmic haystack”. Examples of existing survey instruments and methods include the SETI Institute’s Allen Telescope Array, the Arecibo Observatory, the Robert C. Byrd Green Bank Telescope, the Parkes Telescope, and the Very Large Array (VLA), the SETI@home project and Breakthrough Listen. But given that the volume of space that has been searched for both continuous and pulsed radio searches, the current upper limits on radio wave signatures are quite weak.

The National Radio Astronomy Observatory (NRAO) and the privately-funded SETI Institute announced an agreement to collaborate on new systems to add SETI capabilities to radio telescopes operated by NRAO. The first project will develop a system to piggyback on the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) that will provide data to a state-of-the-art technosignature search system. As the VLA conducts its usual scientific observations, this new system will allow for an additional and important use for the data we’re already collecting. Determining whether we are alone in the Universe as technologically capable life is among the most compelling questions in science, and NRAO telescopes can play a major role in answering it. The SETI Institute will develop and install an interface on the VLA permitting unprecedented access to the rich data stream continuously produced by the telescope as it scans the sky. This interface will allow us to conduct a powerful, wide-area SETI survey that will be vastly more complete than any previous such search. Similarly, optical and near-infrared light (NIL) signals also need to be compressed in terms of frequency and time in order to be considered artificial in origin. Here, examples include the Near-Infrared Optical SETI (NIROSETI) instrument, the Very Energetic Radiation Imaging Telescope Array System (VERITAS), the Near-Earth Object Wide-field Survey Explorer (NEOWISE), and the Keck/High Resolution Echelle Spectrometer (HIRES).

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Allen Telescope Array:

Figure above shows Allen Telescope Array in California dedicated to the search for extraterrestrial intelligence.

The SETI Institute collaborated with the Radio Astronomy Laboratory at the Berkeley SETI Research Center to develop a specialized radio telescope array for SETI studies, similar to a mini-cyclops array. Formerly known as the One Hectare Telescope (1HT), the concept was renamed the “Allen Telescope Array” (ATA) after the project’s benefactor, Paul Allen. Its sensitivity is designed to be equivalent to a single large dish more than 100 meters in diameter, if fully completed. Presently, the array has 42 operational dishes at the Hat Creek Radio Observatory in rural northern California. SETI Institute’s Center for SETI Research (CSR) uses ATA in the search for extraterrestrial intelligence, observing 12 hours a day, 7 days a week. From 2007 to 2015, ATA identified hundreds of millions of technological signals. So far, all these signals have been assigned the status of noise or radio frequency interference because a) they appear to be generated by satellites or Earth-based transmitters, or b) they disappeared before the threshold time limit of ~1 hour. Researchers in CSR are working on ways to reduce the threshold time limit, and to expand ATA’s capabilities for detection of signals that may have embedded messages. ATA is well suited to the search for extraterrestrial intelligence (SETI) and to discovery of astronomical radio sources, such as heretofore unexplained non-repeating, possibly extragalactic, pulses known as fast radio bursts or FRBs. The Allen Telescope Array was originally designed to cover frequencies between 500 and 10,000 MHz, which is more than five times the range searched in Project Phoenix. Thanks to a generous donation from Franklin Antonio (co-founder and Chief Scientist at Qualcomm) the ATA receivers are now being replaced with new systems that will cover 1,000 to 15,000 MHz, and offer improved sensitivity and greater reliability.

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FAST (Five-hundred-meter Aperture Spherical Telescope):

China’s 500 meter Aperture Spherical Telescope (FAST) lists detecting interstellar communication signals as part of its science mission. It is funded by the National Development and Reform Commission (NDRC) and managed by the National Astronomical observatories (NAOC) of the Chinese Academy of Sciences (CAS). FAST is the first radio observatory built with SETI as a core scientific goal. FAST consists of a fixed 500 m (1,600 ft) diameter spherical dish constructed in a natural depression sinkhole caused by karst processes in the region. It is the world’s largest filled-aperture radio telescope. According to its website, FAST can search to 28 light-years, and is able to reach 1,400 stars. If the transmitter’s radiated power were to be increased to 1,000,000 MW, FAST would be able to reach one million stars. This is compared to the former Arecibo 305 meter telescope detection distance of 18 light-years.

On 14 June 2022, astronomers, working with China’s FAST telescope, reported the possibility of having detected artificial (presumably alien) signals, but cautioned that further studies were required to determine if a natural radio interference may be the source. On 18 June 2022, Dan Werthimer, chief scientist for several SETI-related projects, reportedly noted, “These signals are from radio interference; they are due to radio pollution from earthlings, not from E.T.”.

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Breakthrough Listen:

Breakthrough Listen is a ten-year initiative with $100 million funding begun in July 2015 to actively search for intelligent extraterrestrial communications in the universe, in a substantially expanded way, using resources that had not previously been extensively used for the purpose. It has been described as the most comprehensive search for alien communications to date. The science program for Breakthrough Listen is based at Berkeley SETI Research Center, located in the Astronomy Department at the University of California, Berkeley.

Announced in July 2015, the project is observing for thousands of hours every year on two major radio telescopes, the Green Bank Observatory in West Virginia, and the Parkes Observatory in Australia. Previously, only about 24 to 36 hours of telescope time per year were used in the search for alien life. Furthermore, the Automated Planet Finder at Lick Observatory is searching for optical signals coming from laser transmissions. The massive data rates from the radio telescopes (24 GB/s at Green Bank) necessitated the construction of dedicated hardware at the telescopes to perform the bulk of the analysis. Some of the data are also analyzed by volunteers in the SETI@home volunteer computing network. Founder of modern SETI Frank Drake was one of the scientists on the project’s advisory committee.

In October 2019, Breakthrough Listen started a collaboration with scientists from the TESS team (Transiting Exoplanet Survey Satellite) to look for signs of advanced extraterrestrial life. Thousands of new planets found by TESS will be scanned for technosignatures by Breakthrough Listen partner facilities across the globe. Data from TESS monitoring of stars will also be searched for anomalies.

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Section-11

Biosignatures:

A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, molecule, or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical or chemical structures and its use of free energy and the production of biomass and wastes. A biosignature can provide evidence for living organisms outside the Earth and can be directly or indirectly detected by searching for their unique byproducts. Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life. The usefulness of a biosignature is determined not only by the probability of life creating it but also by the improbability of non-biological (abiotic) processes producing it. Concluding that evidence of an extraterrestrial life form (past or present) has been discovered requires proving that a possible biosignature was produced by the activities or remains of life. As with most scientific discoveries, discovery of a biosignature will require evidence building up until no other explanation exists.

Possible examples of a biosignature include complex organic molecules or structures whose formation is virtually unachievable in the absence of life:

-1. Cellular and extracellular morphologies

-2. Biomolecules in rocks

-3. Bio-organic molecular structures

-4. Chirality

-5. Biogenic minerals

-6. Biogenic isotope patterns in minerals and organic compounds

-7. Atmospheric gases

-8. Photosynthetic pigments

Life detection techniques:

To understand how life emerged on Earth, interdisciplinary research is essential, as the emergence of life relied on many physical, chemical, and biological processes. As such, research has been devoted to understanding each facet in relation to the origins of terrestrial life, contributing to modern astrobiology research. Nevertheless, while understanding the origins of terrestrial life is a worthy goal, full understanding is not possible in the near term; it may not even be fully achievable at all. Thus, another important aspect of understanding the origins of life focuses on the potential emergence and existence of extraterrestrial life. Various extraterrestrial biosignatures based on terrestrial life have been proposed, assuming that extraterrestrial life takes similar forms and behaviors as Earth-based life (terrestrial life is the only known life currently, making this a reasonable assumption); each biosignature, if found extraterrestrially, could give evidence toward the existence of extraterrestrial life. In particular, a variety of techniques are needed to analyze the entire range of potential biosignatures at different scales (Figure below). Furthermore, recent work has highlighted the need for considering agnostic and/or universal biosignatures, as alternative forms of life may also exist. As such, a substantial amount of research has gone into detecting biosignatures using the current suite of available technologies, whether coupled with space missions or performed on Earth.

Figure above shows types of biosignatures categorized by scale, showing that different instruments and different types of analyses are required to analyze the entire range of potential biosignatures.

The techniques for unambiguous detection of extraterrestrial life, widely known as life detection techniques, are drawn from various fields allowing for a wide suite of analytical technologies whose applications are normally limited to only a few scientific disciplines. For example, chemists, astrochemists, and geochemists use spectrometers to detect chemical biosignatures during in situ analyses. Physicists, planetary scientists, geobiologists, and geologists use microscopy-based imaging to visualize in situ biosignatures. Biologists and geneticists are developing sequencing/bioinformatics methods to study genetic biosignatures. In each of these fields, researchers are also developing frameworks for criteria of what proper biosignatures should be, and how one can claim detection of life via these biosignatures. Life detection technologies are thus inspired by such proven analytical technologies taken from various scientific disciplines, and the rich diversity and breadth of potential life detection technologies can only be enhanced by further exploration of available and proven technologies in all astrobiology-related fields (into which almost all scientific fields fall).

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Exoplanet biosignature definition:

A biosignature is nominally defined as an “object, substance, and/or pattern whose origin specifically requires a biological agent” (Des Marais and Walter, 1999; Des Marais et al., 2008). A sign of life from an exoplanet may manifest itself as a spectroscopic signal (or signals), a measurement that will have a stated uncertainty and potentially a range of explanations (including measurement error). That signal may be used to infer the presence of a gas or surface feature, which then may be interpreted as originating from a living process. As a matter of definition, we may ask whether the biosignature is the measured spectral signature or the inferred presence of the gas based on that signature. Or, rather, is the biosignature a further inference that a living process must have been involved in the production of the gas or surface feature, perhaps through the collection of additional remotely sensed information? If latter, what level of certainty is required to designate the feature(s) a “biosignature?” In other words, can something be considered a biosignature if there is a nonzero probability that it is not produced by life?

We ought to know what are the known remotely observable biosignatures, the processes that produce them, and their known non-biological sources. There are three types of biosignatures that astrobiologists have proposed as markers for life on other planets, all of which must be remotely detected since exoplanets orbit distant stars that we cannot reach in person. These include gaseous by-products of life that can be detected in the atmosphere, such as oxygen produced by photosynthesis, as on Earth. Another marker uses surface biosignatures, such as life-induced changes in the absorption and reflection of light on the surface of a planet, such as the red-edge caused when plants absorb red light during photosynthesis but reflect infrared light that is not used. Time-dependent fluctuations in gaseous or surface biosignatures, such as biologically modulated changes in the Earth’s atmosphere that occur during different seasons can also be an indication of life.

Researchers are using Earth to guide our search for life on other planets because it is the only known example we have. Rather than being constrained to a study of present-day life, they use geological and geochemical analyses to examine the billions of years that life survived, evolved, and thrived on Earth under conditions that are very different than today’s, hence the concept of ‘alternative Earths’.

Typical biosignatures are atmospheric gases, such as oxygen in the presence of methane. There are also surface biosignatures, such as the “red edge,” which is due to the phenomenon that Earth’s plants are highly reflective in the near-infrared. Other types of “edges” may be possible with different pigments, which may or may not be related to photosynthesis (e.g., UV protection). Temporal biosignatures are also possible, such as daily or seasonal changes. An example is the seasonal change in abundance of CO2 in Earth’s atmosphere. A large disequilibrium could also indicate signs of life. The classic example is Earth’s high abundance of both O2 and CH4. Since methane’s lifetime in the atmosphere is just 10 years, methane’s high abundance in the presence of O2 indicates an active source of the gas, and in the case of Earth, that is due to life. Some recent work showed that the largest Gibbs energy disequilibrium on Earth is the fact that Earth has both N2 and O2 with an ocean. Without life, this would end up as nitrate dissolved in the ocean.

There are three terms useful in thinking about biosignatures. First, an “antibiosignature” is an aspect of the planetary environment that suggests that life is not present, such as abundant CO on Mars, which would be an attractive energy source for life if it were there. A “false positive” is an abiotic source for a potential biosignature, such as O2 being produced by photolysis of H2O or CO2. A “false negative” is when processes on the planet work to reduce the detectability of a biosignature, such as oxidation on a planet’s surface. The complexities of searching for life on planets that are too far away to visit includes phenomena called false positives and false negatives. The search for life using biosignatures is not as simple as looking for a single molecule or compound. Atmospheric oxygen, for example, could be a sign of life, but there are many nonbiological ways that oxygen gas could be produced on an exoplanet. Conversely, it is possible that life could exist in the absence of oxygen gas, similar to early life on Earth or portions of the oceans today.

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Biosignature categories:

There is currently no universally accepted scheme for classifying the vast array of potential exoplanet biosignatures. For convenience, we group biosignatures into three broad categories following a suggestion by Meadows (2006, 2008): gaseous, surface, and temporal biosignatures as seen in the figure below. In this scheme, gaseous biosignatures are direct or indirect products of metabolism, surface biosignatures are spectral features imparted on radiation reflected or scattered by organisms, and temporal biosignatures are modulations in measurable quantities that can be linked to the actions and time-dependent patterns of a biosphere.

Figure above shows summary of gaseous, surface, and temporal biosignatures.

Left panel: gaseous biosignatures are direct or indirect products of biological processes. One example is molecular O2 generated as a by-product of photosynthesis that is then photochemically processed into O3 in the stratosphere.

Middle panel: surface biosignatures are the spectral signatures imparted by reflected light that interacts directly with living material. One example is the well-known VRE produced by plants and the associated NDVI used for mapping surface vegetation on Earth (Tucker, 1979).

Right panel: time-dependent changes in observable quantities, including gas concentrations or surface albedo features, may represent a temporal biosignature if they can be linked to the response of a biosphere to a seasonal or diurnal change. An example is the seasonal oscillation of CO2 as a response to the seasonal growth and decay of vegetation (e.g., Keeling et al., 1976).

NDVI, Normalized Difference Vegetation Index; O2, oxygen; O3, ozone; VRE, vegetation red edge.

Potential signs of life on exoplanets includes gaseous, surface, and temporal biosignatures. The most detectable signs of life will likely result from a photosynthetic biosphere. Biosignature gases in Earth-like (N2-H2O-CO2) atmospheres include O2, O3, CH4, C2H6, N2O, CH3Cl, CH3SH, DMS, and DMDS, although any individual gas alone is likely insufficient for biosignature confirmation due to potential false positive scenarios. Organic aerosols may be suggestive of life in atmospheres high in CO2, while NH3 may be a biosignature gas in H2-dominated terrestrial atmospheres, providing false positives can be ruled out. Overlaps between absorbing wavelengths of key gases (e.g., O3 with CO2 and CH3Cl) and the potential abiotic production of certain gases caution against reliance on any single spectral feature, and indicate a wide spectral range is necessary for biosignature characterization. The interpretation of gaseous signatures will depend on the redox state of the atmosphere, which will determine which disequilibrium signatures are feasible.

The environmental context will also play a key role in interpreting potential gaseous biosignatures. For example, the N2-O2-ocean disequilibrium signature requires the detection of surface liquid water, perhaps through glint. The most well-studied surface biosignature continues to be the VRE. Detecting an exact analogue to Earth’s disk-averaged VRE signature will likely require 1% spectrophotometric precision and ∼10% or more cloud-free surface coverage of exo-vegetation. Analogues to the VRE, “edge” biosignatures, may be produced by photosynthetic or nonphotosynthetic pigments or structures and occur throughout the visible and NIR spectrum. Linear and circular polarization signatures and contextual information could be used to rule out false positives for surface biosignatures. Temporal biosignatures may include seasonal modulation in biologically mediated gases such as CO2 or O2, changes in surface signatures such as analogues to the VRE, or direct emission of light by organisms (e.g., bioluminescence, fluorescence). In general, temporal biosignatures are less well studied than gaseous or surface biosignatures and additional work is necessary to elucidate the range of applicability for this category of signatures.

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Viability of biosignature:

Determining whether a potential biosignature is worth investigating is a fundamentally complicated process. Scientists must consider any and every possible alternate explanation before concluding that something is a true biosignature. This includes investigating the minute details that make other planets unique and understanding when there is a deviation from the expected non-biological processes present on a planet. In the case of a planet with life, it is possible that these differences can be extremely small or not present at all, adding to the difficulties of discovering a biosignature.

The major question in exoplanet biosignature is how to detect life at great distances. In this case, life must have a global impact to be observable. Identifying biosignatures requires three things: reliability that the signature is indeed biological, survivability of the potential biosignature, and the detectability of the possible signature. An alternative way to search for life would be to look for a disequilibrium or some sort of unexpected planetary process that cannot be explained by abiotic processes.

Years of scientific studies have culminated in three criteria that a potential biosignature must meet to be considered viable for further research: Reliability, survivability, and detectability.

-1. Reliability:

A biosignature must be able to dominate over all other processes that may produce similar physical, spectral, and chemical features. When investigating a potential biosignature, scientists must carefully consider all other possible origins of the biosignature in question. Many forms of life are known to mimic geochemical reactions. One of the theories on the origin of life involves molecules figuring out how to catalyse geochemical reactions to exploit the energy being released by them. In a case such as this, scientists might search for a disequilibrium in the geochemical cycle, which would point to a reaction happening more or less often than it should. A disequilibrium such as this could be interpreted as an indication of life.

-2. Survivability:

A biosignature must be able to last for long enough so that a probe, telescope, or human can be able to detect it. A consequence of a biological organism’s use of metabolic reactions for energy is the production of metabolic waste. In addition, the structure of an organism can be preserved as a fossil and we know that some fossils on Earth are as old as 3.5 billion years. The byproducts can make excellent biosignatures since they provide direct evidence for life. However, in order to be a viable biosignature, a byproduct must subsequently remain intact so that scientists may discover it.

-3. Detectability:

A biosignature must be detectable with the current technology to be relevant in scientific investigation. This seems to be an obvious statement, however, there are many scenarios in which life may be present on a planet yet remain undetectable because of human-caused limitations.

False positives:

Every possible biosignature is associated with its own set of unique false positive mechanisms or non-biological processes that can mimic the detectable feature of a biosignature. An important example is using oxygen as a biosignature. On Earth, the majority of life is centred around oxygen. It is a byproduct of photosynthesis and is subsequently used by other life forms to breathe. Oxygen is also readily detectable in spectra, with multiple bands across a relatively wide wavelength range, therefore, it makes a very good biosignature. However, finding oxygen alone in a planet’s atmosphere is not enough to confirm a biosignature because of the false-positive mechanisms associated with it. One possibility is that oxygen can build up abiotically via photolysis if there is a low inventory of non-condensable gasses or if it loses a lot of water. Finding and distinguishing a biosignature from its potential false-positive mechanisms is one of the most complicated parts of testing for viability because it relies on human ingenuity to break an abiotic-biological degeneracy, if nature allows.

Figure below shows False positive mechanisms for oxygen on a variety of planet scenarios:

The molecules in each large rectangle represent the main contributors to a spectrum of the planet’s atmosphere. The molecules circled in yellow represent the molecules that would help confirm a false positive biosignature if they were detected. Furthermore, the molecules crossed out in red would help confirm a false positive biosignature if they were not detected.

In 1976, two probes from NASA landed on Mars to conduct the first experiments in search of life beyond Earth. The Viking 1 and 2 landers were looking for evidence of living Martian microbes. They treated soil samples with nutrients or other compounds that microbes could metabolize and then monitored for molecules that indicated active biochemistry. Initial results had scientists excited: one experiment detected radiolabeled gases emitted from samples treated with carbon-14-labeled nutrients. If information from other experiments on board the two Viking landers had not been available, this set of data would almost certainly have been interpreted as presumptive evidence for biology. But other instruments on the Viking landers detected only trace amounts of organic molecules—like chloro- and dichloromethane. The lack of complex molecules, organic or otherwise, precluded a biological explanation for the radiolabeling results. Other experiments run by the landers were inconclusive at best. After many years of intense debate, the scientific community eventually concluded that non-living, or abiotic, processes—like unknown oxidants in the soil—were a more likely explanation for the Viking results. These experimental results demonstrated just how challenging it can be to identify physical signs of life, or biosignatures, much less make a definitive claim for having found life on another planet. The Viking missions led scientists to develop new techniques for evaluating biosignatures and instrumentation for detecting them.

False negatives:

Opposite to false positives, false negative biosignatures arise in a scenario where life may be present on another planet, but some processes on that planet make potential biosignatures undetectable. This is an ongoing problem and area of research in preparation for future telescopes that will be capable of observing exoplanetary atmospheres.

Human limitations:

There are many ways in which humans may limit the viability of a potential biosignature. The resolution of a telescope becomes important when vetting certain false-positive mechanisms, and many current telescopes do not have the capabilities to observe at the resolution needed to investigate some of these. In addition, probes and telescopes are worked on by huge collaborations of scientists with varying interests. As a result, new probes and telescopes carry a variety of instruments that are a compromise to everyone’s unique inputs. For a different type of scientist to detect something unrelated to biosignatures, a sacrifice may have to be made in the capability of an instrument to search for biosignatures.

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Control experiment detected life on earth!!!

Earth from Galileo!!!

In 1990, the late Carl Sagan convinced NASA to use its Galileo spacecraft as a demonstration to try and detect life on Earth. Galileo was sent to Jupiter on a roundabout flight path that took it to Venus where it got a gravity assist, then back to Earth for a second boost from this planet, giving it enough speed to reach all the way to Jupiter. This presented a unique opportunity where one of our probes was approaching Earth from afar and made a close flyby of our planet the way we might send a probe to look for signs of life on another world. Galileo passed within 600 miles of Earth in 1990.

Figure below shows Earth and Moon as seen by the Galileo spacecraft from a distance of 6 million km away.

Galileo results:

Galileo was kitted out with a variety of instruments designed to study the atmosphere and space environment of Jupiter and its moons. These included imaging cameras, spectrometers (which break down light by wavelength) and a radio experiment. Scientists did not presume any characteristics of life on Earth ab initio (from the beginning), but attempted to derive their conclusions just from the data. The near infra-red mapping spectrometer (NIMS) instrument detected gaseous water distributed throughout the terrestrial atmosphere, ice at the poles and large expanses of liquid water “of oceanic dimensions”. It also recorded temperatures ranging from -30°C to +18°C.

Evidence for life? Not yet. The detection of liquid water and a water weather system was a necessary, but not sufficient argument.

NIMS also detected high concentrations of oxygen and methane in the Earth’s atmosphere, as compared to other known planets. Both of these are highly reactive gases that would rapidly react with other chemicals and dissipate in a short period of time. In an O2-rich atmosphere, the observed ∼1 ppm CH4 would be oxidized to CO2 and H2O within 10 years, needing fast replenishment to accumulate at the observed levels. The only way for such concentrations of these species to be upheld were if they were continuously replenished by some means — again suggesting, but not proving, life. Other instruments on the spacecraft detected the presence of an ozone layer, shielding the surface from damaging UV radiation from the Sun. Had O2, a diatomic molecule, not been detected owing to its lack of significant vibrational absorption, its presence could have been inferred from its photochemical product O3, detected more definitively by Galileo’s UV spectrometer.

One might imagine that a simple look through the camera might be enough to spot life. But the images showed oceans, deserts, clouds, ice and darker regions in South America which, only with prior knowledge, we know of course to be rain forests. However, once combined with more spectrometry, a distinct absorption of red light was found to overlay the darker regions, which was “strongly suggestive” of light being absorbed by photosynthetic plant life. No minerals were known to absorb light in exactly this fashion.

The highest resolution images taken, as dictated by the flyby geometry, were of the deserts of central Australia and the ice sheets of Antarctica. Hence none of the images taken showed cities or clear examples of agriculture. The spacecraft also flew by the planet at closest approach during the daytime, so lights from cities at night were not visible either.

Of greater interest though was Galileo’s plasma wave radio experiment. The cosmos is full of natural radio emission, however most of it is broadband. That is to say, the emission from a given natural source occurs across many frequencies. Artificial radio sources, by contrast, are produced in a narrow band: an everyday example is the meticulous tuning of an analogue radio required to find a station amidst the static. Galileo detected consistent narrowband radio emission from Earth at fixed frequencies. This could only have come from a technological civilisation, and would only be detectable within the last century. If our alien spacecraft had made the same flyby of Earth at any time in the few billion years prior to the 20th century then it would have seen no definitive evidence of a civilisation on Earth at all.

It is perhaps no surprise then that, as yet, no evidence for extra-terrestrial life has been found. Even a spacecraft flying within a thousand kilometres of human civilisation on Earth is not guaranteed to detect it. Control experiments like this are therefore critical in informing the search for life elsewhere.

In the present era, humanity has now discovered over 5,000 planets around other stars, and we have even detected the presence of water in the atmospheres of some planets. Sagan’s experiment shows this is not enough by itself.

A strong case for life elsewhere will likely require a combination of mutually supporting evidence, such as light absorption by photosynthesis-like processes, narrowband radio emission, modest temperatures and weather and chemical traces in the atmosphere which are hard to explain by non-biological means. As we move into the era of instruments such as the James Webb space telescope, Sagan’s experiment remains as informative now as it was 30 years ago. When Sagan and his colleagues pointed Galileo at Earth, they invented a scientific framework for looking for signs of life on these other worlds — one that has permeated every search for such biosignatures since. Life is the last, not first, inference to draw when seeing something unusual on another planet. Extraordinary claims require extraordinary evidence.

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Ladder of life detection:

Before scientists can start to look for molecular signs of life, they need to define what life is. NASA’s working definition is “a self-sustaining chemical system capable of Darwinian evolution.” NASA scientists see life as a system of molecules that can reproduce, store information, and generate energy through metabolizing molecules in its environment. NASA researchers have used that definition to establish a system for assessing whether a molecule or material from outer space—or even ancient Earth—is a biosignature. They call this framework the Ladder of Life Detection (Astrobiology 2018, DOI: 10.1089/ast.2017.1773). Developed by a research team led by Marc Neveu, an astrobiologist with the University of Maryland, College Park, and the Goddard Space Flight Center, the ladder consists of rungs corresponding to key features that scientists might look for in life, going from ones that are not strongly indicative of life to those that are. Neveu et al. (2018) formulated the Ladder of Life Detection, a tool designed to guide life detection investigations during robotic astrobiology space missions. This tool draws from examples of previous life detection experiments to provide current and future efforts with decision rules that allow for the rejection of all abiotic explanations, pointing to a life detection claim as a “last resort hypothesis” (Sagan et al., 1993).

“The key starting point here is that life has many features, but no single feature is a telltale sign of life in and of itself,” Neveu says. He thinks the ladder can help scientists think about how to compile a chain of evidence in a “practical way.” For example, amino acids are the building blocks of proteins on Earth. If scientists found these molecules on another planet, that would correspond to the rung for potential biomolecule components. But that’s only if amino acids can’t be produced by any non-living systems on that planet. A chemical hint of life can be deemed a biosignature only if the compound deviates from abiotic distributions, the authors write, meaning its presence or abundance doesn’t make sense given the planet’s general geochemistry.

Briefly, for a life detection claim to be convincing, the measurement or sets of measurements must be sensitive, contamination-free, and repeatable; the features being measured must be detectable, preservable (survivable), reliable (distinctly different from abiotic background), and compatible with our understanding of life; ultimately, all other abiotic hypotheses must be rejected (Neveu et al., 2018). These criteria are often met not only with one particular analytical instrument, but by using multiple sets of measurements deriving from a variety of technologies that are able to provide the contextual background information needed to assess the biosignature’s presence and provenance as well as the unlikelihood of its production through abiotic means alone. This necessitates a checks-and-balances approach between scientific measurements and instrumentation that enhances the certainty of a potential biosignature while decreasing the likelihood of false positive and false negative interpretations (NAS, 2019; Neveu et al., 2018; Chou et al., 2021). In some cases, statistical frameworks can also help establish the qualitative criteria necessary to confirm a series of measurements as life, measurements which can be evaluated using tools such as Bayesian hypothesis testing (Johnson et al., 2018; Lorenz, 2019; Pohorille and Sokolowska, 2020).

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NASA scientists propose new ‘alien life evidence’ scale:

The newly proposed alien-life evidence scale was outlined in a study published online 2021 in the journal Nature that was led by NASA chief scientist Jim Green. The scale includes seven levels, which are subject to change depending on the type of environment involved and how the scientific community responds. The NASA Confidence of Life Detection (CoLD) Scale is a seven-level framework designed to help scientists evaluate and communicate the progress made in life detection research, both among themselves and with the public. The CoLD Scale is based on NASA’s Technology Readiness Level (TRL) scale, which is used to characterize the maturation of instruments from concept to implementation in missions. The CoLD Scale aims to provide a standardized method for assessing claims about extraterrestrial life and to facilitate clear communication of findings to the public.

The seven levels of the CoLD Scale act as benchmarks that must be met before proceeding to the next step. The levels are as follows:

For a Mars mission, for example, finding hints of a signature of life would register at Level 1 on the scale, and showing that the discovery was not due to contamination by Earth life would raise it to Level 2. The highest levels include verifying signs of life with several instruments (Level 6) and in different locations on a world (Level 7).

The CoLD Scale is intended to be a starting point for a larger conversation within the scientific community and to help guide future missions and technologies in the search for extraterrestrial life. It is also hoped that the CoLD Scale will prevent scientists from “crying wolf” and making overly cavalier claims about the discovery of extraterrestrial life, as it requires more stringent tests to progress up the scale. The scale can also help redirect heated scientific debates and provide a common language for discussing the search for life in the universe.

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Earth biosphere as guide to search extraterrestrial life:

Our observations suggest increasingly that Earth-size planets orbiting within the habitable zone may be common in the Galaxy—current estimates suggest that more than 40% of stars have at least one. But are any of them inhabited? With no ability to send probes there to sample, we will have to derive the answer from the light and other radiation that come to us from these faraway systems. What types of observations might constitute good evidence for life?

To be sure, we need to look for robust biospheres (atmospheres, surfaces, and/or oceans) capable of creating planet-scale change. Earth hosts such a biosphere: the composition of our atmosphere and the spectrum of light reflected from our planet differ considerably from what would be expected in the absence of life. Presently, Earth is the only body in our solar system for which this is true, despite the possibility that habitable conditions might prevail in the subsurface of Mars or inside the icy moons of the outer solar system. Even if life exists on these worlds, it is very unlikely that it could yield planet-scale changes that are both telescopically observable and clearly biological in origin.

What makes Earth “special” among the potentially habitable worlds in our solar system is that it has a photosynthetic biosphere. This requires the presence of liquid water at the planet’s surface, where organisms have direct access to sunlight. The habitable zone concept focuses on this requirement for surface liquid water—even though we know that subsurface habitable conditions could prevail at more distant orbits—exactly because these worlds would have biospheres detectable at a distance.

Indeed, plants and photosynthetic microorganisms are so abundant at Earth’s surface that they affect the color of the light that our planet reflects out into space—we appear greener in visible wavelengths and reflect more near-infrared light than we otherwise would. Moreover, photosynthesis has changed Earth’s atmosphere at a large scale—more than 20% of our atmosphere comes from the photosynthetic waste product, oxygen. Such high levels would be very difficult to explain in the absence of life. Other gases, such as nitrous oxide and methane, when found simultaneously with oxygen, have also been suggested as possible indicators of life. When sufficiently abundant in an atmosphere, such gases could be detected by their effect on the spectrum of light that a planet emits or reflects. Astronomers today are beginning to have the capability of detecting the spectrum of the atmospheres of some planets orbiting other stars. Astronomers have thus concluded that, at least initially, a search for life outside our solar system should focus on exoplanets that are as much like Earth as possible—roughly Earth-size planets orbiting in the habitable zone—and look for the presence of gases in the atmosphere or colors in the visible spectrum that are hard to explain except by the presence of biology. Simple, right? In reality, the search for exoplanet life poses many challenges.

If we manage to separate out a clean signal from the planet and find some features in the light spectrum that might be indicative of life, we will need to work hard to think of any nonbiological process that might account for them. “Life is the hypothesis of last resort,” noted astronomer Carl Sagan—meaning that we must exhaust all other explanations for what we see before claiming to have found evidence of extraterrestrial biology. This requires some understanding of what processes might operate on worlds that we will know relatively little about; what we find on Earth can serve as a guide but also has potential to lead us astray (Figure below).

Figure above shows Spectrum of Light Transmitted through Earth’s Atmosphere. This graph shows wavelengths ranging from ultraviolet (far left) to infrared. The many downward “spikes” come from absorption of particular wavelengths by molecules in Earth’s atmosphere. Some of these compounds, like water and the combination oxygen/ozone and methane, might reveal Earth as both habitable and inhabited. We will have to rely on this sort of information to seek life on exoplanets, but our spectra will be of much poorer quality than this one, in part because we will receive so little light from the planet.

To discern if these gases and other biomarkers are in other planets’ atmospheres, telescopes measure the spectra of light radiated from the planet as heat (known as the infrared or IR region) as well as starlight reflected by the planet (known as the visible light region since this is the range of light our eyes can detect).

In the IR region we can see spectral features indicative of molecules such as carbon dioxide, water, ozone, methane, ammonia and nitrous oxide. Methane, nitrous oxide and ammonia are produced on Earth primarily by bacteria. Ozone is produced higher up in Earth’s atmosphere when high energy light breaks apart oxygen gas, which then recombines to form ozone. So ozone can serve as a proxy for the presence of oxygen. In addition to the possible detection of atmospheric species, IR (which is radiated from objects with high temperature) also provide a measurement of the planet’s surface temperature, telling us if it can support liquid water, but only for planets with a thin atmosphere like Earth or Mars.

Primary features of interest in the visible and near-infrared (the region in between the visible and IR) portion of the spectrum are water, ozone, oxygen, carbon dioxide, and methane. With new spectroscopy techniques, the visible region of the spectrum may also allow us to detect the presence of oceans and/or continents (Cowan et al. 2009; Palle 2010). Finally, global vegetation may also be detectable through something analogous to the Vegetation Red Edge on Earth (Kiang et al. 2008), which results from the fact plants strongly reflect red light.

It would be extremely difficult to account for the abundance of oxygen in Earth’s atmosphere except by the presence of biology. But it has been hypothesized that oxygen could build up to substantial levels on planets orbiting M-dwarf stars through the action of ultraviolet radiation on the atmosphere—with no need for biology. It will be critical to understand where such “false positives” might exist in carrying out our search.

We need to understand that we might not be able to detect biospheres even if they exist. Life has flourished on Earth for perhaps 3.5 billion years, but the atmospheric “biosignatures” that, today, would supply good evidence for life to distant astronomers have not been present for all of that time. Oxygen, for example, accumulated to detectable levels in our atmosphere only a little over 2 billion years ago. Could life on Earth have been detected before that time? Scientists are working actively to understand what additional features might have provided evidence of life on Earth during that early history, and thereby help our chances of finding life beyond.

Note:

The amount of starlight received per unit area of a planet’s surface (per square meter, for example) decreases with the square of the distance from the star. Thus, when the orbital distance doubles, the illumination decreases by 4 times (2^2), and when the orbital distance increases tenfold, the illumination decreases by 100 times (10^2). Venus and Mars orbit the sun at about 72% and 152% of Earth’s orbital distance, respectively, so Venus receives about 1/(0.72)^2 = 1.92 (about twice) and Mars about 1/(1.52)^2 = 0.43 (about half) as much light per square meter of planet surface as Earth does.

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Detecting ancient earth-like planets easier than identifying modern Earth:

A team of astronomers from Cornell University has made a breakthrough in the ongoing search for extraterrestrial life. By delving into Earth’s distant past, they’ve identified a time when the chemical signatures of life – specifically those from the age of the dinosaurs – were more discernible than they are today. This revelation could significantly refine our search for life on distant exoplanets.

The Earth’s Phanerozoic Eon, encompassing the last 540 million years of our planet’s history, was the period that saw dinosaurs roam and thrive. During this epoch, the levels of atmospheric oxygen were much higher — between 10% to 35% — compared to the current 21%. Consequently, the biosignature pairs of oxygen and methane, and ozone and methane, were more robust and detectable.

Transmission spectra, the “light fingerprints” of planets, are created when a planet’s atmosphere absorbs certain colors of starlight and allows others to pass through. These spectra are essential tools that scientists use to decipher the composition of an atmosphere from afar. The research shows that Earth’s historical transmission spectra during the dinosaur age would have been more distinct. Theoretically, it would have been easier for an extraterrestrial civilization to detect live on Earth during the Jurassic period than during modern times. This is due to our current atmospheric signatures. By using Earth’s ancient atmospheric models, we could improve our ability to spot signs of life on other planets and planets with atmospheres resembling Earth’s prehistoric times might not only harbor simple life forms, but also more complex organisms.

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Agnostic biosignatures:

Because the only form of known life is that on Earth, the search for biosignatures is heavily influenced by the products that life produces on Earth. However, life that is different than life on Earth may still produce biosignatures that are detectable by humans, even though nothing is known about their specific biology. This form of biosignature is called an “agnostic biosignature” because it is independent of the form of life that produces it. It is widely agreed that all life–no matter how different it is from life on Earth–needs a source of energy to thrive. This must involve some sort of chemical disequilibrium, which can be exploited for metabolism. Geological processes are independent of life, and if scientists can constrain the geology well enough on another planet, then they know what the particular geologic equilibrium for that planet should be. A deviation from geological equilibrium can be interpreted as an atmospheric disequilibrium and agnostic biosignature.

Methane can be produced both with and without life. It wouldn’t be a convincing biosignature on its own. But finding methane and oxygen together would be hugely exciting; it’s very difficult to produce that combination without life. Likewise finding methane along with the right amounts of other gases, such as carbon dioxide, would be hard to explain without life. Watching how an exoplanet atmosphere changes over time might also provide valuable context that could strengthen otherwise weak biosignatures. Seasonal variations in the concentration of ozone, for example, could be a fingerprint of life. Of course, if you’re looking for individual gases like oxygen or methane, then built into that are assumptions about what type of life is elsewhere. So some scientists are developing agnostic biosignatures that don’t assume alien biochemistry will be anything like Earth’s biochemistry. One possible agnostic biosignature is an exoplanet atmosphere’s degree of chemical “surprisingness”—what scientists call chemical disequilibrium.

An atmosphere close to equilibrium would be chemically uninteresting, a bit like a closed flask of gas in a laboratory. Of course, no planet is as boring as a lab flask. Chemical reactions in a planet’s atmosphere can be powered by their stars, and geological processes like volcanic activity can increase disequilibrium, and thus increase the chemical surprisingness of the atmosphere. Life can also push planets away from equilibrium. And assuming that alien life produces gases of some kind, they could push a planet’s atmosphere much further from equilibrium than it would be otherwise.

Yet disequilibrium alone “is not an unambiguous indicator,” says Krissansen-Totton. In 2016, he and his colleagues calculated the thermal disequilibrium of the atmosphere of every planet in the Solar System and Saturn’s moon Titan. By this measure, the Earth’s atmosphere stood out as extreme—but only if the oceans were built into the calculations. Ignoring its interactions with the ocean, the Earth’s atmosphere is actually closer to equilibrium than the atmosphere of Mars. Still, even if it might not point to biology, finding an exoplanet atmosphere far from equilibrium would tell astronomers that something interesting is happening, Krissansen-Totton says, something that’s “modifying the atmosphere in a dramatic way that we need to understand.”

David Kinney, a philosopher of science at Yale University, recently worked with biophysicist Chris Kempes of the Santa Fe Institute to develop a new way of detecting possible agnostic biosignatures. It’s a deceptively simple idea: To find life, look for the weirdest planets. If no assumptions are made about what alien life is like, practically any gas could be a biosignature in the right context. In 2016, MIT astrophysicist Sara Seager and colleagues proposed a list of about 14,000 molecules for consideration as possible biosignatures. Kinney and Kempes developed their assessment method by using that list of compounds, along with methods inspired by machine learning algorithms designed to recognize the odd-image-out in a set. This led to a way to precisely define and score the “weirdness” of a hypothetical exoplanet’s atmosphere compared to a set of other hypothetical atmospheres. Kinney and Kempes argue that the weirdest atmospheres in a set are the most likely to host life. This rests on a few basic assumptions: Life in the universe is rare, it leaves traces in planetary atmospheres, and it’s hard to mimic those traces without life. Of course, those assumptions might turn out to be false, Kinney says. But “if we want to make no assumptions at all,” he adds, “then I think it’s very hard to make any kind of scientific progress, let alone in the area with such severe uncertainty as this one.”

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Antibiosignatures:

In the same way that detecting a biosignature would be a significant discovery about a planet, finding evidence that life is not present can also be an important discovery about a planet. Life relies on redox imbalances to metabolize the resources available into energy. The evidence that nothing on an earth is taking advantage of the “free lunch” available due to an observed redox imbalance is called antibiosignatures. The Martian atmosphere contains high abundances of photochemically produced CO and H2, which are reducing molecules. Mars’ atmosphere is otherwise mostly oxidizing, leading to a source of untapped energy that life could exploit if it used a metabolism compatible with one or both of these reducing molecules. Because these molecules can be observed, scientists use this as evidence for an antibiosignature. Scientists have used this concept as an argument against life on Mars.

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Molecular oxygen as biosignature:

There is no shortage of the most common elements in known living organisms, such as carbon, hydrogen, nitrogen and oxygen, on exoplanets. By the most basic standards, there is even no lack of known habitable planets. However, there is a scarcity of easily detectable signs of life. It may be more fruitful to first look for circumstantial indicators of living species. For instance, when you go to the Tibetan Plateau to spot snow leopards, it is much easier to sift through their footprints over extensive rugged terrains. On Earth, the most easily identifiable ‘footprint of life’ is the vast amount of molecular oxygen in the atmosphere. These molecules are not the necessary ingredients, but waste byproducts, of microbial life. Biological processes dramatically enriched the molecular oxygen content of Earth’s atmosphere during the Archean Eon. If we completely sterilized all life forms on Earth today, the molecular oxygen in the Earth’s atmosphere would quickly plummet because oxygen is very reactive with other elements. A good example is Mars, where the oxygen rusted the iron on the surface to make its top soil appear reddish.

Most planets in our solar system have some oxygen in their lower atmospheres, but Earth has much more, about 21 percent. This is because so many organisms have been busy turning light, water, and carbon dioxide into sugar and oxygen—the process called photosynthesis—for the past 3.8 billion years.

Figure below shows a 10-minute, infrared exposure of Earth taken from the moon during the Apollo 16 mission. The bright yellow is “dayglow” from atomic oxygen (O). On the dark side, “nightglow” bands, arising from atomic oxygen ions (O+) in the ionosphere, can be seen near the equator.

On Earth today, excess oxygen molecules, in the form of O2, float upward. When the O2 gets about 150 kilometers above the Earth’s surface, ultraviolet light splits it in two. The single oxygen atoms float higher, into the ionosphere, where more ultraviolet light and x-rays from the sun rip electrons from their outer shells, leaving charged oxygen zipping through the air. The abundance of O2 near the Earth’s surface—so different than the other planets—leads to an abundance of O+ high in the sky. This finding suggests that scientists seeking extraterrestrial life could perhaps narrow their search area.

Astronomers choose molecular oxygen as a biomarker because it is relatively easy to detect. Oxygen is the third most abundant element in the universe, though it does not commonly exist as free molecules in the atmosphere of most stars and planets. Traces of oxygen molecules are not difficult to pick up from the infrared waveband. They are even possible to detect in visible light if they exist in the form of ozone. Most astronomers believe that if we detect a high concentration of molecular oxygen in the atmosphere of a planet, there is a reasonable chance it is an indicator of carbon-based life on the surface of that planet. Meanwhile, if a planet has an ozone layer in its atmosphere, we may even be able to detect the traces with ground-based telescopes. Rich supplies of oxygen can also come from other sources such as photo-dissociated water molecules. It is important to distinguish between oxygen molecules related to biological activities and those from other sources. The scenario is extremely complicated because it involves cross-disciplinary interpretation of competing biological, chemical, physical, geological and atmospheric processes.

Besides molecular oxygen, we need to observe other biosignatures as well. Even with the most powerful ground-based and space-borne telescopes, state-of-the-art instruments and software, it will take time to observe biosignatures on a handful of carefully selected habitable planets on a one-by-one basis. Maybe 10 years from now, we will have sufficient data to say that potential biosignatures have been found in either 10%, 1% or 0.1% of all the planets we have examined. We can then go on to compare the newly found biomarkers’ dependence on the location of planets, the age and composition of their host stars, etc. with respect to the habitable zones. In this pursuit, both the positive detection and stringent upper limits of biosignatures will have profound implications on the ubiquity of life, at least in the microbial form, elsewhere in the universe.

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Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment, a 2018 study:

Authors describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here they provide an in-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth’s history and (2) we now know of several potential planetary mechanisms that can generate abundant O2 without life being present. Consequently, our ability to interpret both the presence and absence of O2 in an exoplanetary spectrum relies on understanding the environmental context. Here authors examine the coevolution of life with the early Earth’s environment to identify how the interplay of sources and sinks may have suppressed O2 release into the atmosphere for several billion years, producing a false negative for biologically generated O2. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. Authors review the most recent knowledge of false positives for O2, planetary processes that may generate abundant atmospheric O2 without a biosphere. They provide examples of how future photometric, spectroscopic, and time-dependent observations of O2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions.

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The case and context for atmospheric methane as an exoplanet biosignature, a 2022 study:

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, authors argue that—in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH4 that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO2 with comparatively little CO.

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Color spectra of Earth-based microscopic organisms as biosignature:

Scientists at Cornell University rounded up 137 microorganisms and catalogued how each life-form uniquely reflects sunlight. This database of individual reflection fingerprints, which is available to anyone, might help astronomers identify similar microscopic life-forms on distant alien planets. “This database gives us the first glimpse at what diverse worlds out there could look like,” Lisa Kaltenegger, professor of astronomy and director of Cornell University’s new Institute for Pale Blue Dots, said in a statement. If alien beings living on a planet many light-years away from Earth were to point their telescopes at our planet, they might be able to catch a glimpse of sunlight bouncing off its surface. This reflected light is also known as the planet’s spectrum, or basically, its color. Earth’s spectrum would be a shade of green, according Kaltenegger, because a significant portion of our planet’s surface is covered in green plant life. Based on this observation, those alien astronomers could potentially deduce that life exists on our planet. Scientists on Earth would like to look at the spectra of alien planets in the hope of identifying signs of life. Astronomers are just starting to see the spectra of planets outside our solar system, and have plans to build even better tools for this task. The goal of the new catalogue, say the authors, is to provide planet hunters and astronomers with a baseline comparison: this is what the color spectra of Earth-based microscopic organisms would look like from afar. Astronomers can, hypothetically, compare these spectra of known organisms with those seen on other planets. Studying the spectra of life on Earth might give them an idea of what the spectra of life will look like on other planets.

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Chirality as biosignature:

Hold up your hands in front of your face. For most people, they will be mirrored copies of each other: You can hold them palm-to-palm and they will match up, but you cannot superimpose them. Molecules also exhibit this handedness, or chirality. They come structured in two mirrored, non-superimposable forms. And it’s a fascinating quirk of life that almost all biomolecules will only work in one of their two forms. Natural amino acids – the building blocks of proteins – are almost always left-handed, or sinistral. Natural sugars like those that make up RNA and DNA, on the other hand, are almost always right-handed, or dextral. If you replace any of these molecules with the other form, the whole system breaks down.

This quirk is called homochirality. We’re not sure why it happens, but it’s thought to be a key property of life. And back in 2021, scientists detected molecular homochirality from a helicopter flying at a velocity of 70 kilometers per hour (43.5 mph) at an altitude of 2 kilometers (1.2 miles). Why would they do such a thing, you ask? To see if we can detect molecular homochirality on other planets, in the search for extraterrestrial life. Even here on Earth being able to measure this signal from altitude would be useful, since it can reveal information about the health of plants.

When light is reflected by biological matter, a part of the light’s electromagnetic waves will travel in either clockwise or counterclockwise spirals. This phenomenon is called circular polarization and is caused by the biological matter’s homochirality. Similar spirals of light are not produced by abiotic non-living nature. As you might expect, however, this signal is extremely faint. The circular polarization of vegetation makes up less than 1 percent of the light reflected. One type of instrument that can detect the signal of polarized light is called a spectropolarimeter, which uses special sensors to separate the polarized fraction.

For several years, Patty and his team have been working on a highly sensitive spectropolarimeter for detecting the circular polarization of vegetation. Called TreePol, it could positively detect circular polarization from several kilometers away. Then, they adapted TreePol for flight, with upgraded spectrographs and added temperature control for the optics. This new design is called FlyPol. When Patty and his team took to the air above Val-de-Travers and Le Locle in Switzerland with FlyPol, the improvement offered by these upgrades became immediately apparent. It wasn’t just that FlyPol could isolate the circular polarization signal and differentiate it from abiotic surfaces, such as asphalt roads. The team could use it to differentiate between various types of vegetation, such as grass, forests and even algae in lakes – all from a fast-moving helicopter. This could open up a whole new way to monitor the health of various vegetative ecosystems, and maybe even coral reefs, the researchers said. But they’re not done refining it yet. They want to take it to a velocity of roughly 27,580 km/h and an altitude of 400 kilometers – low Earth orbit. “The next step we hope to take is to perform similar detections from the International Space Station (ISS), looking down at the Earth,” said astrophysicist Brice-Olivier Demory of the University of Bern and MERMOZ. At that altitude, the resolution wouldn’t be as fine – maybe 6 to 7 kilometers – but it will be able to help the researchers refine their spectropolarimeter, and see how well it works on more extreme scales. That will allow us to assess the detectability of planetary-scale biosignatures. This step will be decisive to enable the search for life in and beyond our Solar System using polarization.

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Complexity of molecules as biosignature:

In a paper published in Nature Communications in 2021, a team of scientists describes a universal approach to life detection including a system that could easily be flown on a space probe to find life in our solar system. The key innovation enabling these advances is the application of a new theory called assembly theory, developed by Professor Lee Cronin and his team at the University of Glasgow, with a team of scientists from Arizona State University collaborating on the theory development. Applied to molecules, assembly theory identifies molecules as biosignatures because of what life does, not what life is. ‘Our system is the first falsifiable hypothesis for life detection and is based on the idea that only living systems can produce complex molecules that could not form randomly in any abundance, and this allows us to sidestep the problem of defining life,’ said Cronin, Regius Professor of Chemistry at the University of Glasgow.

Their team developed an algorithm that could assign a complexity score to a given molecule, which they call the molecular assembly (MA) number. The MA number is based on the number of bonds needed to make the molecule. Simply put, large biogenic molecules would have a bigger MA number than smaller molecules or large molecules that aren’t biogenic. This one is the first complexity measure that is experimentally measurable. And that makes the algorithm especially powerful because it could be proven and tested in the lab by instruments that could be incorporated on future space missions. The method enables identifying life without the need for any prior knowledge of its biochemistry. It can therefore be used to search for alien life in future NASA missions, and it is informing an entire new experimental and theoretical approach to finally reveal the nature of what life is in the universe, and how it can emerge from lifeless chemicals.

In their experiments, the team used their method to assign MA numbers to a database containing about 2.5 million molecules. Next, they used a sample subset of about 100 small molecules and small protein fragments (peptides) to experimentally verify the expected correlation between the MA number and the number of fragments generated by a widely used lab instrument that verifies the structure of molecules, called a mass spectrometer. Mass spectrometers ionize a sample, breaking the molecule into bits, and then count the number of unique parts. The larger the number of unique parts, the larger the MA number. Collaboration with NASA, ASU and the Glasgow team, led by Cronin, reveals that the system works with samples from all over the Earth and extraterrestrial samples. The samples included a bit of the Murchison meteorite (not of biological origin), and fossil-containing lake sediment samples from the Holocene (30,000 years ago) and the mid-Miocene (14 million years ago).

The team was able to show that life is the only process that can make molecules with high MA numbers. And there is a MA threshold that they were able to demonstrate that — once crossed — indicates life was necessary to produce the molecule. ‘Living and non-living systems are set apart by the degree to which they can reliably, and in detectable abundances, assemble highly complex molecular structures,’ said Doug Moore, postdoctoral research associate at the Beyond Center at ASU and co-author of the study. ‘We set out to show that this is the case and propose a biosignature that is both biochemically agnostic and practically useful.’ A life-detection instrument based on this method could be deployed on missions to extraterrestrial locations to detect biosignatures or detect the emergence of de novo artificial life in the lab. This is important because “developing an approach that cannot produce false positives is vital for the discovery of life beyond Earth, an event that will only happen once in human history,” Cronin said.

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Atmospheric carbon depletion as a tracer of water oceans and biomass on temperate terrestrial exoplanets, a 2023 study:

Life on Earth is obvious. It shapes most aspects of our environment, including the composition of the atmosphere above us, and of the ocean and rocks below us. Its continuity across several billion years is likely regulated by Gaian cycles, a complex system of geological, atmospheric and biological balances understood and nowadays investigated as “Earth’s systems”. Life on Earth has a truly global effect. In other words, Life on Earth is planet-shaping. Planet-shaping Life is really what astronomers are after. Should Life have arisen on Venus, on Mars, Europa or Enceladus, whilst extremely fascinating, it is clearly not as planet-shaping, simply because it is not nearly as obvious in affecting what can be observed and measured about these celestial objects. Such Life would not be detectable on exoplanets, whereas planet-shaping Life is. In this perspective, authors outline how atmospheric carbon depletion, particular a carbon dioxide depletion, is a logical tracer of planet-shaping Life, and is able to reveal its global impact using remote-sensing methods of observation. An atmosphere such as Earth’s becomes depleted in carbon thanks to the action of extensive amounts of surface liquid water, and/or by intense biological processes. Plate tectonics buries carbon away from the atmosphere, causing atmospheric depletion of CO2 over geological timescales. The researchers propose that if a terrestrial planet has substantially less carbon dioxide in its atmosphere compared to other planets in the same system, it could be a sign of liquid water — and possibly life — on that planet’s surface.

The conventional observables to identify a habitable or inhabited environment in exoplanets, such as an ocean glint or abundant atmospheric O2, will be challenging to detect with present or upcoming observatories. Here authors suggest a new signature. A low carbon abundance in the atmosphere of a temperate rocky planet, relative to other planets of the same system, traces the presence of substantial amount of liquid water, plate tectonic and/or biomass. Authors show that JWST can already perform such a search in some selected systems like TRAPPIST-1 via the CO2 band at 4.3µm, which falls in a spectral sweet spot where the overall noise budget and the effect of cloud/hazes are optimal. Authors propose a 3-step strategy for transiting exoplanets: 1) detection of an atmosphere around temperate terrestrial planets in ∼ 10 transits for the most favorable systems, (2) assessment of atmospheric carbon depletion in ∼ 40 transits, (3) measurements of O3 abundance to disentangle between a water- vs biomass-supported carbon depletion in ∼ 100 transits. The concept of carbon depletion as a signature for habitability is also applicable for next-generation direct imaging telescopes.

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Section-12

SETI and technosignatures:

SETI stands for the Search for Extraterrestrial Intelligence. It is a project which started in 1959 to search for radio signals from intelligent life in space. The SETI project uses radio telescopes from around the world to scan the sky and look for special patterns in radio waves which could have been sent by another civilization in space. Radio telescopes are used because radio waves can travel very far in space without being absorbed by the thick clouds of gas and dust which lie in many regions of space. Also, radio telescopes can be used both day and night. We have been sending out our own radio waves into space for over sixty years. All of our radio and television signals travel into space at the speed of light and may one day be detected by another civilization in space. Seti focuses on the radio part of the electromagnetic spectrum. But as we have no idea of what’s out there, we should clearly explore all wavebands, including the optical and X-ray parts. One aspect of SETI research involves searching for “noise” or anomalies in the electromagnetic spectrum that could indicate intelligent life. This is what is considered a shortcut to finding extraterrestrial life. For example, SETI researchers use radio telescopes to search for unusual radio signals and intentional communication from an intelligent extraterrestrial civilization. They also use other instruments to search for different types of anomalies, such as unusual patterns (not created by nature) in the visible light spectrum or other wavelengths of electromagnetic radiation. Rather than just listening for radio transmission, we should also be alert to other evidence of non-natural phenomena or activity. These include artificial structures built around stars to absorb their energy (Dyson spheres) or artificially created molecules, such as chlorofluorocarbons in planet atmospheres. These chemicals are greenhouse gasses that can’t be created by natural processes, meaning they could be a sign of “terraforming” (changing a planet to make it more habitable) or industrial pollution.

The Search for Extra-Terrestrial Intelligence (SETI) Institute was founded by Carl Sagan and Jill Tarter, two astronomers who believe there’s more to interplanetary life than us. SETI’s mission is “to explore, understand, and explain the origin and nature of life in the universe and the evolution of intelligence.” The Institute works with NASA and the National Science Foundation as a research contractor to pool resources and explore the possibility of intelligent life on other planets. Aside from optical and radio wave signals, SETI uses a laser detection system to look for signs of alien technology.

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Radio searches:

The most promising scheme for finding extraterrestrial intelligence is to search for electromagnetic signals, more particularly radio or light, that may be beamed toward Earth from other worlds, either inadvertently (in the same way that Earth leaks television and radar signals into space) or as a deliberate beacon signal. Physical law implies that interstellar travel requires enormous amounts of energy or long travel times. Sending signals, on the other hand, requires only modest energy expenditure, and the messages travel at the speed of light. Projects to look for such signals are known as the search for extraterrestrial intelligence (SETI). The first modern SETI experiment was American astronomer Frank Drake’s Project Ozma, which took place in 1960. Drake used a radio telescope (essentially a large antenna) in an attempt to uncover signals from nearby Sun-like stars. In 1961 Drake proposed what is now known as the Drake equation, which estimates the number of signaling worlds in the Milky Way Galaxy. This number is the product of terms that define the frequency of habitable planets, the fraction of habitable planets upon which intelligent life will arise, and the length of time sophisticated societies will transmit signals. Because many of these terms are unknown, the Drake equation is more useful in defining the problems of detecting extraterrestrial intelligence than in predicting when, if ever, this will happen.

By the mid-1970s the technology used in SETI programs had advanced enough for the National Aeronautics and Space Administration to begin SETI projects, but concerns about wasteful government spending led Congress to end these programs in 1993. However, SETI projects funded by private donors (in the United States) continued. One such search was Project Phoenix, which began in 1995 and ended in 2004. In 1995, the SETI Institute started Project Phoenix, which used three of the most powerful radio telescopes in the world: the Green Bank radio telescope in West Virginia, USA; the Arecibo telescope in Puerto Rico; and the Parkes radio telescope in NSW, Australia. During its initial phase, Project Phoenix used the Parkes telescope to search for signals coming from 202 Sun-like stars as distant as 155 light years away. By the end of its operations, Project Phoenix had scanned a total of 800 ‘nearby’ (up to 240 light years away) stars for signs of life. The project detected some cosmic noises, but none that could be attributed to aliens.

Other radio SETI experiments, such as Project SERENDIP V (begun in 2009 by the University of California at Berkeley) and Australia’s Southern SERENDIP (begun in 1998 by the University of Western Sydney at Macarthur), scan large tracts of the sky and make no assumption about the directions from which signals might come. The former uses the Green Bank Telescope and, until its collapse in 2020, the Arecibo telescope, and the latter (which ended in 2005) was carried out with the 64-metre (210-foot) telescope near Parkes, New South Wales. Such sky surveys are generally less sensitive than targeted searches of individual stars, but they are able to “piggyback” onto telescopes that are already engaged in making conventional astronomical observations, thus securing a large amount of search time. In contrast, targeted searches such as Project Phoenix require exclusive telescope access.

In 2007 a new instrument, jointly built by the SETI Institute and the University of California at Berkeley and designed for round-the-clock SETI observations, began operation in northeastern California. The Allen Telescope Array (ATA, named after its principal funder, American technologist Paul Allen) has 42 small (6 metres [20 feet] in diameter) antennas. When complete, the ATA will have 350 antennas and be hundreds of times faster than previous experiments in the search for transmissions from other worlds.

Beginning in 2016, the Breakthrough Listen project began a 10-year survey of the one million closest stars, the nearest 100 galaxies, the plane of the Milky Way Galaxy, and the galactic centre using the Parkes telescope and the 100-metre (328-foot) telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia. That same year the largest single-dish radio telescope in the world, the Five-hundred-meter Aperture Spherical Radio Telescope in China, began operation and had searching for extraterrestrial intelligence as one of its objectives.

Since 1999 some of the data collected by Project SERENDIP (and since 2016, Breakthrough Listen) has been distributed on the Web for use by volunteers who have downloaded a free screen saver, SETI@home. The screen saver searches the data for signals and sends its results back to Berkeley. Because the screen saver is used by several million people, enormous computational power is available to look for a variety of signal types. Results from the home processing are compared with subsequent observations to see if detected signals appear more than once, suggesting that they may warrant further confirmation study.

Nearly all radio SETI searches have used receivers tuned to the microwave band near 1,420 megahertz. This is the frequency of natural emission from hydrogen and is a spot on the radio dial that would be known by any technically competent civilization. The experiments hunt for narrowband signals (typically 1 hertz wide or less) that would be distinct from the broadband radio emissions naturally produced by objects such as pulsars and interstellar gas. Receivers used for SETI contain sophisticated digital devices that can simultaneously measure radio energy in many millions of narrowband channels.

Laser SETI or optical SETI:

While the ATA searches the skies for radio signals, LaserSETI will examine star systems for laser flashes that could indicate the presence of an advanced civilization. Repetitive laser pulses may be the most promising method for detecting alien life because its quick and can travel across vast distances. Due to these reasons, scientists are not ruling out that aliens might be using a similar method to try to contact us, or other civilizations. Recently the SETI Institute’s Eliot Guillum began setting up the latest LaserSETI hardware in Haleakala, Hawaii. SETI searches for light pulses are also under way at a number of institutions, including the University of California at Berkeley as well as Lick Observatory and Harvard University. The Berkeley and Lick experiments investigate nearby star systems, and the Harvard effort scans all the sky that is visible from Massachusetts. Sensitive photomultiplier tubes are affixed to conventional mirror telescopes and are configured to look for flashes of light lasting a nanosecond (a billionth of a second) or less. Such flashes could be produced by extraterrestrial societies using high-powered pulsed lasers in a deliberate effort to signal other worlds. By concentrating the energy of the laser into a brief pulse, the transmitting civilization could ensure that the signal momentarily outshines the natural light from its own sun. SETI is currently testing this method via its LaserSETI project but so far, there has been no sign of alien life.

SETI works best when telescopes double-check each other:

The Search for Extraterrestrial Intelligence (SETI) has evolved considerably in the past 60 years since the first experiment was conducted. This was Project Ozma, which was conducted in 1960 by Dr. Frank Drake and his colleagues using the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia. While the experiment did not reveal any radio signals from space, it established the foundation upon which all future SETI is based. Like Ozma, the vast majority of these experiments have searched for possible technosignatures in the radio spectrum.

Unfortunately, this search has always been plagued by the problem of radio interference from Earth-based radio antennas and satellites in orbit, which can potentially flood SETI surveys with false positives. In a recent study published in The Astronomical Journal, an international team of astronomers (including researchers with Breakthrough Listen) recommended that future technosignature searches rely on multi-site simultaneous observations. This has the potential of eliminating interference from terrestrial sources and narrowing the search for extraterrestrial radio signals.

Results of SETI experiments:

No confirmed extraterrestrial signals have yet been found by SETI experiments. Early searches, which were unable to quickly determine whether an emission was terrestrial or extraterrestrial in origin, would frequently find candidate signals. The most famous of these was the so-called “Wow” signal, measured by a SETI experiment at Ohio State University in 1977. Subsequent observations failed to find this signal again, and so the Wow signal, as well as other similar detections, is not considered a good candidate for being extraterrestrial.

Two-way communication:

Most SETI experiments do not transmit signals into space. Because the distance even to nearby extraterrestrial intelligence could be hundreds or thousands of light-years, two-way communication would be tedious. For this reason, SETI experiments focus on finding signals that could have been deliberately transmitted or could be the result of inadvertent emission from extraterrestrial civilizations.

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Criticism of SETI:

As various SETI projects have progressed, some have criticized early claims by researchers as being too “euphoric”. For example, Peter Schenkel, while remaining a supporter of SETI projects, wrote in 2006 that: In light of new findings and insights, it seems appropriate to put excessive euphoria to rest and to take a more down-to-earth view […] We should quietly admit that the early estimates—that there may be a million, a hundred thousand, or ten thousand advanced extraterrestrial civilizations in our galaxy—may no longer be tenable.

Critics claim that the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability, as explained in a 2009 editorial in Nature, which said: Seti… has always sat at the edge of mainstream astronomy. This is partly because, no matter how scientifically rigorous its practitioners try to be, SETI can’t escape an association with UFO believers and other such crackpots. But it is also because SETI is arguably not a falsifiable experiment. Regardless of how exhaustively the Galaxy is searched, the null result of radio silence doesn’t rule out the existence of alien civilizations. It means only that those civilizations might not be using radio to communicate. Nature added that SETI was “marked by a hope, bordering on faith” that aliens were aiming signals at us, that a hypothetical alien SETI project looking at Earth with “similar faith” would be “sorely disappointed”, despite our many untargeted radar and TV signals, and our few targeted Active SETI radio signals denounced by those fearing aliens, and that it had difficulties attracting even sympathetic working scientists and government funding because it was “an effort so likely to turn up nothing”. However, Nature also added, “Nonetheless, a small SETI effort is well worth supporting, especially given the enormous implications if it did succeed” and that “happily, a handful of wealthy technologists and other private donors have proved willing to provide that support”.

George Basalla, Emeritus Professor of History at the University of Delaware, is a critic of SETI who argued in 2006 that “extraterrestrials discussed by scientists are as imaginary as the spirits and gods of religion or myth”, and was in turn criticized by Milan M. Ćirković for, among other things, being unable to distinguish between “SETI believers” and “scientists engaged in SETI”, who are often sceptical (especially about quick detection), such as Freeman Dyson and, at least in their later years, Iosif Shklovsky and Sebastian von Hoerner, and for ignoring the difference between the knowledge underlying the arguments of modern scientists and those of ancient Greek thinkers.

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SETI vs SETA (search for extraterrestrial artifacts):

In contrast to SETI, SETA allows astronomers to dig deep into the past. They don’t have to hope to catch a radio signal from a civilization that is active at the same time we’re listening. In fact, multiple civilizations could have come and gone throughout the galaxy, each one leaving something behind in our solar system before fading from existence (or moving on to something more interesting).

In a new paper, astronomers proposed a strategy for using existing telescopes, surveys and planetary probes to hunt for signs of past alien visits. They discuss three categories of remnants we might detect.

The first category is regular surface artifacts — dead and leftover spacecraft, probes and even just trash. In humanity’s comparatively short time exploring the solar system, we’ve littered the moon and Mars with dozens of defunct spacecraft and random pieces of junk, so it’s not implausible to suspect that a visiting alien civilization would do the same. Plus, because many surfaces in the solar system do not experience weathering or volcanism, an artifact left there could be noticeable for billions of years.

Along with surface artifacts, there can be spacecraft hanging out in interplanetary space — a category commonly known as “lurkers.” They might wait in a stable gravitational Lagrange point or orbit some distant moon. They might be active, monitoring and recording interesting things happening in the solar system, or they might be long dead and not look much different from an asteroid or a comet.

Lastly, we might encounter interstellar artifacts, ones that are never meant to stay put in any one star system but aimlessly wander the galaxy, traipsing from one system to another. We already have several means of detecting these kinds of artifacts, like broad astronomical survey telescopes and planetary missions.

Speaking of travel, we might have the capability with existing surveys to find evidence of interstellar and interplanetary adventures. For example, any interstellar spacecraft worth its salt will need some method of propulsion. And because even aliens have to obey Newton’s laws, there will have to be some kind of exhaust to propel the spacecraft. The faster we want the craft to go, the more powerful its exhaust will have to be, potentially making it visible to the James Webb Space Telescope or the Chandra X-ray Observatory. Interstellar travel could also involve laser propulsion via lightsail, which could be detectable. Or we might find evidence from more subtle clues, like gravitational anomalies — orbits of small objects that don’t quite make sense because they might have been perturbed by a passing craft.

Lastly, we can search for signs of past interference, rather than just passive observation, in the solar system. If aliens opened up a strip mine on Mercury, for example, we would still be able to see it today. Or if heavy equipment is still active, it would have a bit of waste heat associated with it, which would stand out against the radiation emitted by the surface of a planet or moon. Lastly, we may be able to find geochemical anomalies — the result of tinkering with chemical processes on a world (or just outright pollution).

The authors highlighted how we could use current and planned observatories and solar system probes to hunt for these artifacts without having to change their mission parameters. If we’re already scanning the surfaces of planets and imaging large swaths of the solar system, we can piggyback on these campaigns to search for evidence of extraterrestrial life.

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Technosignatures:

Technosignature is defined as any measurable property or effect that provides scientific evidence of past or present technology, and it is analogous with biosignature which provide evidence of past or present life, intelligent or not. Some authors prefer to exclude radio transmissions from the definition, but such restrictive usage is not widespread. Jill Tarter has proposed that the search for extraterrestrial intelligence (SETI) be renamed “the search for technosignatures”. The principal challenge in conducting SETI in the radio domain is developing a generalized technique to reject human radio frequency interference (RFI). In a recent study authors present the most comprehensive deep-learning based technosignature search to date, returning 8 promising ETI signals of interest for re-observation as part of the Breakthrough Listen initiative.

Figure below illustrates various types of technosignatures including atmospheric, optical, and radio technosignatures.

Atmospheric technosignatures may include obviously artificial molecules such as sulfur hexafluoride (SF6) in addition to common molecules expected for an inhabited terrestrial planet, such as oxygen (O2), carbon dioxide (CO2), and methane (CH4). The top left inset shows the absorption cross-sections of SF6.

Optical technosignatures include highly collimated laser pulses that can outshine the host star at narrow wavelengths (i.e., Optical SETI). The middle left inset illustrates the narrow power distribution of an optical (green) laser pulse.

Active radio beacons or passive radio leakage from the planetary surface, orbit, or elsewhere in the stellar system would be recognizably artificial (i.e., traditional SETI). The bottom left inset illustrates the narrow distribution of power versus frequency anticipated for an artificial radio signal.

Additional potentially detectable technosignatures in this planetary system include artificial lighting on the planetary nightside, recognizable spectral breaks from solar arrays on the planet’s moon, and anomalous transit signatures from the orbiting habitats and satellite arrays.

Just as astrobiologists have a catalogue of tell-tale signs of life on other planets called biosignatures, SETI researchers have their own list of things that would indicate the existence of intelligent life beyond Earth. These are known as “technosignatures”. Figure below shows major technosignatures as outlined in a 2021 scientific review.

Figure below shows capabilities for detecting technosignatures with recent, ongoing, and future missions and facilities:

Cells coloured green indicate there is at least one stellar system where the given technosignature could be detectable with the mission or observatory. A green cell further indicates there is a peer-reviewed publication that has evaluated the hypothetical detectability of that technosignature.

A yellow cell indicates the potential detectability of that technosignature in at least one stellar system, but that further study is needed.

A red cell indicates that the given technosignature is not detectable with that observatory or mission architecture for any stellar systems.

Note that all ground-based instrumentation in the ground-based photometry and ground-based spectroscopy categories are included, although specific observatory-instrument combinations may only access a subset of indicated technosignatures. For example, the Gemini Planet Imager (GPI) could plausibly detect optical beacons, but no other indicated technosignatures.

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Humans’ “first contact” with aliens is likely to be with a civilization much more technologically advanced than ours, according to a new NASA-funded study into the search for intelligent extraterrestrial life (SETI). According to the paper published in the specialized journal Acta Astronautica in 2021, the easiest way to detect extraterrestrial civilizations is by searching for “technosignatures”—evidence for the use of technology or industrial activity in other parts of the Universe. Technosignatures, many of which are based on how Earth might look now, or in the past or future, to alien onlookers, include:

Radio signals, such as the Arecibo message we humans sent in the direction of globular star cluster M13 on November 16, 1974.
The presence of industrial pollution in the atmosphere of a planet. For example, the presence of nitrogen dioxide—as studied recently by the same team of researchers—or the wholly artificial chlorofluorocarbons (CFCs), both of which are evidence for there being a technologically advanced civilization on Earth.
Large swarms of satellites around a planet.
Gigantic space engineering around exoplanets, such as heat shields or “Dyson spheres” that harvest solar energy from the local star.
Crash sites on the Moon or Mars of probes that might have been sent here in a distant past.

However, the study—which was funded by NASA Goddard’s Sellers Exoplanet Environments Collaboration (SEEC) and the NASA Exobiology program—argues that our search for technosignatures would likely only be successful at finding much more advanced technology than humans can currently create. “It seems unlikely that civilizations with a relatively low level of technological development would enter into contact with each other, since that would require either very high sensitivities or highly visible engineering,” reads the paper. “Less advanced civilizations lack the sensitivity needed to detect other civilizations unless they have built very large or luminous structures.” In short, we don’t yet have instruments sensitive enough to definitively find “another Earth” by detecting an alien civilization outright. That’s despite huge advances in our astronomical instrumentation in the past decade that have revolutionized the science of discovery and study of exoplanets, which now number 5,000+. “For us to detect such signals at interstellar distances with our current sensitivities, such signals would need to be stronger than those produced by current human civilization, particularly the unintentional ones,” read the paper. “Only those species that have constructed or developed technology is much larger or more luminous than any of our own can be detected with our current astronomical infrastructure.” So we’re looking for massive, unmistakable signs of alien civilisations far more advanced that we are.

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Radio wave technosignature:

Natural sources of radio waves have distinct fingerprints. Namely: Those sources broadcast across a wide range of frequencies. They emit signals that can be picked up on many stations of an astronomer’s (jacked-up) radio dial. An artificial source of radio waves (i.e., an alien transmitter beaming out a message) would look very different. Think about humanity’s own radio communications. When you want to listen to a particular radio station, you must tune into a very specific frequency. That is essentially what radio SETI research is: a hunt for coherent transmissions broadcast on an extremely narrow range of frequencies (dubbed “narrowband”). Nature just cannot do that. Narrowing down a broadcast to a particular frequency, or a few frequencies, requires machinery — with essentially no exceptions.

That means this quest is fundamentally straightforward. Scientists are looking for “stuff you don’t see normally coming from stars and galaxies,” says Michael Garrett, the director of the Jodrell Bank Centre for Astrophysics. “Anything that you don’t expect nature to produce.” But that’s easier said than done. Nature has temporarily hoodwinked astrophysicists in the past. Take pulsars. Today, scientists know that they are the rapidly spinning, hyperdense corpses of stars, emitting beams of radiation from two poles like a deity’s lighthouse. But that wasn’t always the case.

The flamboyant behavior of pulsars was first theorized about in 1967. In 1968, a different group of scientists discovered the signal from a pulsar for the very first time, but they didn’t know exactly what it was; the regularity of the radiation bursts seemed so non-random that, for a moment, astronomers could not entirely rule out an artificially generated signal as a possibility, even dubbing the source LGM1 — “little green men 1”. But later that year, another scientist connected the regular rhythm of LGM1 with the pre-existing star carcass lighthouse theory, and LGM1 was understood to be a natural phenomenon, not a beacon of an alien design. There are always caveats, and nature is always capable of surprises. But a non-random narrowband radio signal coming from space is an excellent place to start but the likelihood of crafting this sort of order out of such chaos is infinitesimally small — and even if you find one, you can check to make sure it isn’t a fluke of nature.

Since the 1960s, the most common method of SETI has involved searching the cosmos for radio signals that are artificial in origin. The first such experiment was Project Ozma (April to July 1960), led by famed Cornell astrophysicist Frank Drake (creator of the Drake Equation). This survey relied on the 25-meter dish at the National Radio Astronomy Observatory in Green Bank, West Virginia, to monitor Epsilon Eridani and Tau Ceti at frequencies of about 400 kHz around 1.42 GHz. These searches have since expanded to cover larger areas of the night sky, wider frequency ranges, and greater signal diversity.

In the 1960s, the idea was to focus on a region around a well-known frequency where neutral hydrogen emits radiation in interstellar space, 1.42 GHz. Since this natural emission is prevalent throughout the galaxy, the idea is that any intelligent civilization would know about it, and potentially target this frequency for transmission to maximize the chance of detection. Since then, especially as technology has rapidly advanced, radio SETI has expanded along all axes of measurement. We now can take measurements across a bandwidth of multiple GHz instantaneously. As storage has improved, we can collect huge amounts of data, allowing higher resolution observations in both time and frequency directions. By the same token, we’ve done surveys of nearby stars and other direction in the galaxy, to maximize exposure to potentially interesting directions in the sky. Another major change has been the incorporation of machine learning-based algorithms designed to find transmissions amid the radio background noise of the cosmos and correct for radio frequency interference (RFI).

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Frequency of radio waves for Interstellar communication:

Morrison and Cocconi considered how well the types of electromagnetic waves passed through interstellar space. While space is usually described as a vacuum, it does contain some gas and dust. Over great distances that gas and dust absorb many types of radiation but radio waves pass through nearly unimpeded. For interstellar communication, a particular range of radio frequencies, “microwaves” from 1 GHz to 10 GHz, are particularly good choices. At lower frequencies our galaxy emits prodigious amounts of radio waves creating a loud background of noise. At higher frequencies the Earth’s atmosphere, and presumably the atmosphere of other Earth-like planets, absorbs and emits broad ranges of radio frequencies. The result is a quiet “Microwave Window” through which efficient radio communication is possible.

Microwave window is depicted in figure below:

The Microwave Window has another interesting feature to recommend it as a place for interstellar communication: the “Water Hole.” Some atoms and molecules in space emit radio waves at particular frequencies. Hydrogen atoms emit at 1420 MHz (a wavelength of 21 cm). Hydroxyl molecules, composed of one atom of hydrogen and one atom of oxygen (OH), emit at four specific radio frequencies ranging from 1612 MHz to 1720 MHz. When a hydrogen atom combines with a hydroxyl molecule it forms a molecule of water, the most essential molecule for life as we know it. Thus, the range of frequencies from 1420 to 1720 MHz is called the Water Hole. It has been a popular frequency range for many SETI programs. The frequency between 1GHZ and 10GHZ is suited for interstellar communication, but those in the range of 1GHZ – 2GHZ seem the most appropriate ones. Optimal frequencies for communication are supposed to be the neutral hydrogen lines (1420) and the hydroxyl lines (1,612 MHZ and 1,615 MHZ, 1,667, and 1,720 MHZ).

At SETI, astronomers use the Allen Telescope Array (ATA) of 42 radio antennas to “listen” for signals over a range of radio frequencies, tuned to “hear” the regions around 20,000 red dwarf stars (a broad term describing stars smaller than our sun and in a certain spectral range) that are closest to Earth. Investigating red dwarf stars for life-supporting worlds is a relatively recent development at SETI. In the past, stars that were more like our own sun — a yellow dwarf — were thought to be the most likely candidates to host planets harboring life. But over the last few decades, astronomers have determined that many red dwarf stars host planets that could be at the right distance from the star to be habitable.

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Researchers have scanned 14 Worlds from the Kepler Mission for radio technosignatures:

The team selected 14 systems from the Kepler catalogue and examined them for technosignatures. For the sake of their study, the team conducted an L-band radio survey of these 14 planetary systems. Specifically, they looked for signs of radio waves in the 1.15 to 1.73 gigahertz (GHz) range. At those frequencies, their study is sensitive to Arecibo-class transmitters located within 450 light-years of Earth. So if any of these systems have civilizations capable of building radio observatories comparable to Arecibo, the team hoped to find out! They searched for signals that are narrow (< 10 Hz) in the frequency domain. Such signals are technosignatures because natural sources do not emit such narrowband signals… they identified approximately 850,000 candidate signals, of which 19 were of particular interest. Ultimately, none of these signals were attributable to an extraterrestrial source.

What they found was that of the 850,000 candidate signals, about 99% of them were automatically ruled out because they were quickly determined to be the result of human-generated radio-frequency interference (RFI). Of the remaining candidates, another 99% were also flagged as anthropogenic because their frequencies overlapped with other known sources of RFI – such as GPS systems, satellites, etc.

The 19 candidate signals that remained were heavily scrutinized, but none could be attributed to an extraterrestrial source. It would therefore be no exaggeration to say that the hunt for ETI is still in its infancy, and our efforts are definitely beginning to pick up speed. There is literally a Universe of possibilities out there and to think that there are no other civilizations that are also looking for us seems downright unfathomable. To quote the late and great Carl Sagan: “The Universe is a pretty big place. If it’s just us, seems like an awful waste of space.”

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Differentiate between Earth- and space-based radio signals:

In a significant advancement for the Search for Extraterrestrial Intelligence (SETI), researchers from the University of California, Berkeley have devised a new technique for detecting potential alien radio signals. This technique involves analyzing signals for signs of having traversed interstellar space, thereby ruling out Earth-based radio interference.

Breakthrough Listen, the most comprehensive SETI search project, monitors the northern and southern skies for technosignatures using radio telescopes. It also focuses on thousands of individual stars in the plane of the Milky Way galaxy, which is considered the most likely direction for a civilization to send a signal. For more than 60 years, SETI researchers have scanned the skies in search of signals that look different from the typical radio emissions of stars and cataclysmic events, such as supernovas. One key distinction is that natural cosmic sources of radio waves produce a broad range of wavelengths — that is, broadband radio waves — whereas technical civilizations, like our own, produce narrowband radio signals.

Because of the huge background of narrowband radio bursts from human activity on Earth, finding a signal from outer space is like looking for a needle in a haystack. So far, no narrowband radio signals from outside our solar system have been confirmed, though Breakthrough Listen found one interesting candidate — dubbed BLC1 — in 2020. Later analysis determined that it was almost certainly due to radio interference.

Researchers realized that real signals from extraterrestrial civilizations should exhibit features caused by passage through the ISM (interstellar medium) that could help discriminate between Earth- and space-based radio signals. Thanks to past research describing how the cold plasma in the interstellar medium, primarily free electrons, affect signals from radio sources such as pulsars, astronomers now have a good idea how the ISM affects narrowband radio signals. Such signals tend to rise and fall in amplitude over time — that is, they scintillate. This is because the signals are slightly refracted, or bent, by the intervening cold plasma, so that when the radio waves eventually reach Earth by different paths, the waves interfere, both positively and negatively. Our atmosphere produces a similar scintillation, or twinkle, that affects the pinprick of optical light from a star. Planets, which are not point sources of light, do not twinkle.

Researcher developed a computer algorithm, available as a Python script, that analyzes the scintillation of narrowband signals and plucks out those that dim and brighten over periods of less than a minute, indicating they’ve passed through the ISM. This implies that we could use a suitably tuned pipeline to unambiguously identify artificial emission from distant sources vis-a-vis terrestrial interference.

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Low Frequency Array (Lofar):

Previous technosignature surveys have included only the radio frequency band above 600 MHz, leaving lower frequencies virtually unexplored. That’s despite the fact that everyday communication services such as air traffic control, marine emergency broadcasting and FM radio stations all emit this type of low-frequency radiation on Earth. The reason it hasn’t been explored is that telescopes that operate at these frequencies are rather new. And lower-frequency radio waves have less energy, meaning they can be more challenging to detect but low-frequency observation boasts a major advantage in having large fields of view compared with their higher-frequency siblings. That’s because the area of the sky covered decreases with higher frequencies. The Low-Frequency Array (LOFAR) is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. The Low Frequency Array (Lofar) is the world’s most sensitive low-frequency telescope, operating from 10-250 MHz. It’s composed of 52 radio telescopes with more on the way, spread across Europe. These telescopes can reach a high resolution when used in unison.

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Fast Radio Burst (FRB):

In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging for a fraction of a millisecond caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB releases as much energy in a millisecond as the Sun puts out in days. While extremely energetic at their source, the strength of the signal reaching Earth has been described as 1,000 times less than from a mobile phone on the Moon.

FRBs are some of the most mysterious celestial phenomena ever studied ever since they were first discovered in 2007, and astronomers have made incredible strides in both understanding their potential origins and the number of FRBs that exist in the universe. This includes discovering that most FRBs come from outside our Milky Way Galaxy. However, in 2020, astronomers found one source of FRBs was from a magnetar within our own Milky Way Galaxy. Also, while FRB 20121102A is designated as the first known repeating FRB, a 2023 study identified 25 regularly repeating FRBs found using the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which is located in British Colombia, Canada, and has found more than 1000 FRBs to date.

A recent study published in Nature Astronomy examines the discovery of what astronomers are dubbing “ultra-fast radio bursts”, a new type of fast radio bursts (FRBs) that the team determined lasts for a mind-boggling ten millionths of a second or less. Traditionally, FRBs have been found to last only thousandths of a second, but this study builds on a 2021 study that hypothesized FRBs could possibly last for millionths of a second. This also comes after astronomers recently announced the discovery of the oldest and farthest FRB ever observed, approximately 8 billion light-years from Earth. What set this particular FRB apart was its unparalleled distance; it emerged from a galaxy so remote that the waves had journeyed across the universe for an astounding eight billion years to finally reach our telescopes. Furthermore, in terms of energy, this event ranked among the most potent ever observed. Astonishingly, in the blink of an eye, it unleashed energy equivalent to a staggering 30 years’ worth of emissions from our Sun.

Recently the team at the Allen Telescope array announced an observation of a bright double-peaked Fast Radio Burst (FRB) from the repeating source known as FRB20201124A. This observation was the first FRB detected with the ATA, which has been undergoing extensive upgrades for both its receivers and digital signal processing hardware.

The exact origin and cause of the FRBs is still the subject of investigation; proposals for their origin range from a rapidly rotating neutron star and a black hole, to extraterrestrial intelligence. Professor Loeb thought it was alien technology.

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Breakthrough Listen Investigation for Periodic Spectral Signals (BLIPSS):

BLIPSS is a collaborative effort between Cornell, the SETI Institute and Breakthrough Listen. The project significantly enhances the probability of capturing evidence of extraterrestrial technology by focusing on the central region of the Milky Way, known for its dense concentration of stars and potentially habitable exoplanets. The center of the Milky Way would also be an ideal place for aliens to place a beacon to contact large swaths of the galaxy. BLIPSS utilizes the Fast Folding Algorithm (FFA) to search for channel-wide periodic signals in radio dynamic spectra. Using a ground-based radio telescope in West Virginia, BLIPSS has focused upon a sliver of the sky less than one-200th of the area covered by the moon, stretching toward the center of the Milky Way roughly 27,000 light years away. This area contains about 8 million stars. If extraterrestrial life forms exist, they presumably would populate rocky planets orbiting in what is called the habitable zone, or Goldilocks zone, around a star – not too hot and not too cold. Efforts to detect alien technological signatures previously have focused on a narrowband radio signal type concentrated in a limited frequency range or on single unusual transmissions. The new initiative focuses on a different signal type that perhaps could enable advanced civilizations to communicate across the vast distances of interstellar space. These wideband pulsating signals for which the scientists are monitoring feature repetitive patterns – a series of pulses repeating every 11 to 100 seconds and spread across a few kilohertz, similar to pulses used in radar transmission. No aliens yet have been detected in the monitoring efforts.

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Why no radio technosignature so far?

For over sixty years, amateur and professional astronomers have been monitoring the sky in the search for extraterrestrial intelligence (SETI). So far, to no avail. But how should we read the absence of alien radio signals? Is it time we stop looking? Or should we double down and look harder, peering ever deeper into our galaxy?

A recent statistical analysis of the sixty-year silence suggests a simple, optimistic explanation and urges the SETI community to continue searching, but to stay patient, as the chances for detecting signals in the coming sixty years are slim.

The prevailing explanations for the absence of electromagnetic signals from extraterrestrial societies fall into two extreme categories, says Claudio Grimaldi from EPFL’s Laboratory of Statistical Biophysics. The “optimistic” camp holds that we’ve been using detectors that are not sensitive enough or missed incoming signals because we’ve been pointing our radio telescopes in the wrong direction. The “pessimistic” camp, on the other hand, interprets the silence as indicating the absence of alien life in our galaxy.

According to Grimaldi’s study, published in The Astronomical Journal, there’s a third explanation. “We’ve only been looking for 60 years. Earth could simply be in a bubble that just happens to be devoid of radio waves emitted by extraterrestrial life,” he says.

No one method is flawless. On one hand, radio waves are a tempting way to communicate with extraterrestrials because these signals fit in a convenient gap in the electromagnetic spectrum called the “water hole” — a frequency between 1420 and 1720 megahertz that’s relatively free of cosmic background noise. On the other hand, radio waves broaden as they travel, meaning any message we send will become more diluted the farther from Earth it gets. Laser light does not have this problem — however, laser signals require incredible precision, and are unlikely to reach any alien observers unless we target our message directly to their star system. Both methods have their advantages — and neither are perfect.

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Pollutants as technosignature:

Alien life may one day be found not from radio signals beamed across the cosmos but from an all-too-familiar side-effect of civilisation: pollution. Pollutants such as nitrogen dioxide (NO2) and chlorofluorocarbons (CFCs) are mostly formed by industrial activity and could be a good pointer towards pollutants from civilisation. On Earth, various appliances including aerosol cans and fridges released CFCs gases for years, in huge volumes, before we realised that CFCs were eroding the ozone layer. Any advanced extraterrestrial civilisation is likely to have transformed its host planet with industry. SETI researchers have proposed that we could look for their non-natural waste products such as chlorofluorocarbons (CFCs), which can persist in the atmosphere for tens of thousands of years. Astrophysicist Avi Loeb of Harvard University has also suggested light pollution on the night side of an extrasolar planet as a possible sign of technological civilizations.

In 2014, González Abad co-authored a paper that discussed the possibility of finding aliens via CFC emissions. The researchers calculated that, if the concentration of these gases in the atmosphere of a distant planet reached roughly 10 times their concentration on Earth, it might be possible to detect their presence using the James Webb Space Telescope, which became operational in 2022. Crucially, CFCs could remain in a planet’s atmosphere for tens of thousands of years, meaning that an alien civilisation would not necessarily have to produce them for very long in order to leave a trace of CFC-related activity. The chlorine in Earth’s atmosphere today is there due to the emission of CFCs in past decades. These gases are banned worldwide today, though there’s plenty of work still to do if we are to eliminate them entirely. Detection of CFCs with the James Webb Space Telescope might be possible if the polluted planet were orbiting a small white dwarf star, González Abad and his co-authors suggest, since that would increase the chances of useful levels of light reaching Earth. Scientists are able to look for CFCs, and various other chemicals in faraway planets’ atmospheres, by studying the spectra – or specific wavelengths of light – reflected off alien worlds. Since some light is absorbed by chemicals while some passes through, the precise character of the onward light can reveal what chemicals are present on a distant body. Truly intelligent lifeforms might not, in the end, produce pollutant-based technosignatures, even CFCs, for long stretches in their history – perhaps only for fleeting moments before they clean up their activities. If so, we would have to be lucky to be looking for such signatures at just the right time.

The waxing and waning of NO2 concentrations could give us a clue as to the levels of industrial activity occurring on an alien world. Unlike CFCs, NO2 doesn’t hang around in the atmosphere for thousands of years, which could make it harder to find on other planets. However, on the flipside, the waxing and waning of NO2 concentrations could give us a clue as to the levels of industrial activity occurring on an alien world. Arney explains that she and her colleagues got the idea for their paper after NO2 levels in Earth’s atmosphere fell sharply during Covid-19 lockdowns. Ground measurements revealed that NO2 plummeted by around 30% in some countries that had strict lockdowns – and reductions in NO2 emissions were also observed by satellites orbiting the Earth. Besides the short life of NO2, however, there’s also the issue that there are quite a lot of natural sources that produce it – from lightning to wildfires, so finding it might not be hard proof that an alien civilisation had developed internal combustion engines, for instance, or any other NO2-spewing technology. That said, Arney and her co-authors argue that, on Earth, natural sources would not on their own produce enough NO2 to make it detectable from afar. Therefore, observing it in the atmosphere of another planet might actually hint that industrial activity of some kind is afoot.

Besides the James Webb Space Telescope, which now orbits the sun, there’s the European Southern Observatory’s Extremely Large Telescope, a ground-based facility in Chile that is due to become operational in 2028. Nasa is currently planning a space-based telescope called the Large Ultraviolet Optical Infrared Surveyor, or Luvoir, for the 2030s. Arney and her colleagues considered the capabilities of this device this when calculating how they might detect NO2 on an alien planet. And finally there’s the Habitable Worlds Observatory, a telescope specifically designed to hunt for biosignatures – and perhaps technosignatures, by extension – in the late 2030s or 2040s. But optical telescopes have their limitations while radio emissions could be detected “across the galaxy” meaning we might have a higher chance of spotting them.

Since the possible technosignatures that have been suggested so far are essentially all based on pollutants that humans have produced, there’s a risk that we assume alien civilisations will be highly similar to our own, when there’s no guarantee that that would be the case. As Madhusudhan says, “expect the unexpected” – rather than taking an anthropocentric view of life across the Universe. With this in mind, SETI is increasingly focused on a search for anomalies in data rather than specific traces that we might assume could be left by an alien population. Anything that’s weird in a dataset of cosmic observations could be the smoking gun we need.

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Other technosignatures:

-1. Solar panels:

Stars are one of the most potent energy generators that exist. Since we harness so much energy from the Sun, our host star, it would make sense that other civilisations would operate similarly. If a civilization uses a lot of solar panels, the light that is reflected from the planet would have a certain spectral signature—a measurement of the wavelengths of light that are reflected or absorbed—indicating the presence of those solar collectors. The researchers will determine the spectral signatures of large-scale planetary solar energy collection.

-2. Dyson spheres:

Around the same time as astronomers began scouring stars for radio signals, the physicist Freeman Dyson suggested another potential technosignature. Dyson reasoned that to satisfy its ever-increasing energy needs, an advanced alien civilisation would build an enormous solar power plant around its host star. This would heat up and generate an infrared glow in excess of what you would expect from an unadorned star – a glow that we could see from Earth. These hypothetical megastructures are now known as Dyson spheres. Searching for them remains a minority sport, but some researchers have recently begun to step up the hunt by figuring out how to distinguish a genuine Dyson sphere from a star shrouded in dust. When it comes to looking for megastructures (such as Dyson spheres), astronomers focus on both waste heat from stars and dips in their luminosity (obscurations). In the case of the former, surveys have been conducted that looked for excess infrared energy coming from nearby stars. This could be seen as an indication that starlight is being captured by technology (such as solar panels). Consistent with the laws of thermodynamics, some of this energy would be radiated away as “waste” heat. In the case of the latter, obscurations have been studied using data from the Kepler and K2 missions to see if they could indicate the presence of massive orbiting structures – in the same way that they were used to confirm planetary transits and the existence of exoplanets. Similarly, surveys have been conducted of other galaxies using the Wide-field Infrared Survey Explorer (WISE) and Two Micron All-Sky Survey (2MASS) to look for signs of obscurations. Other ongoing searches are being conducted with the Infrared Astronomical Satellite (IRAS) and the Vanishing & Appearing Sources during a Century of Observations (VASCO).

-3. Particle colliders:

If intelligent aliens are, like us, curious about the fundamental forces of nature, they might have built a particle collider that makes our Large Hadron Collider look puny. An accelerator powered by a black hole, for instance, would produce super-high-energy neutrinos, particles that could be detected from Earth.

-4. Apocalypse:

Any advanced civilisation runs the risk of destroying itself, and the fallout might be visible to distant observers. Nuclear bombs would release flashes of gamma rays, but they would be fleeting and the resulting dust would be hard to distinguish from that produced by an asteroid strike.

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Atmospheric oxygen as technosignature:

Oxygen’s significance extends beyond biology and into the realm of advanced technology on a cosmic scale. Adam Frank of the University of Rochester and Amedeo Balbi, Associate Professor of Astronomy and Astrophysics at the University of Roma Tor Vergata, Italy, delve into this connection in their thought-provoking new study; The oxygen bottleneck for technospheres, 2023. Their research highlights the intricate relationship between atmospheric oxygen and the emergence of advanced technology on distant planets.

The duo introduces the concept of “technospheres,” vast domains of advanced technology emitting unique signs, or “technosignatures,” indicative of extraterrestrial intelligence. They argue that oxygen is not only vital for respiration and metabolism in multicellular organisms but also essential for developing fire — a cornerstone of technological civilizations. On Earth, the evolution of technology has hinged on the ability to utilize open-air combustion — a process where fuel and an oxidant, typically oxygen, combine to create fire. From cooking and metal forging to energy harnessing, combustion has been pivotal in shaping industrial societies. The researchers trace Earth’s historical trajectory, discovering that controlled fire use and subsequent metallurgical advancements were only feasible when atmospheric oxygen levels hit or surpassed 18 percent. This finding implies that only planets with significant oxygen concentrations can develop advanced technospheres capable of leaving detectable technosignatures. “You might be able to get biology — you might even be able to get intelligent creatures — in a world that doesn’t have oxygen,” Frank says, “but without a ready source of fire, you’re never going to develop higher technology because higher technology requires fuel and melting.” Interestingly, the oxygen levels needed to biologically sustain complex life and intelligence are lower than those required for technology. Thus, while a species might evolve in an oxygen-deficient world, it is unlikely to progress into a technological species, the study suggests. Frank elaborates on this bottleneck, stating that high oxygen levels are a prerequisite for a technological species. Without it, all other conditions may align, but technological advancement remains unachievable. Frank concludes by emphasizing the need to focus on planets with high oxygen levels, as their atmospheres could be a significant indicator in locating potential technosignatures. “Targeting planets with high oxygen levels should be prioritized because the presence or absence of high oxygen levels in exoplanet atmospheres could be a major clue in finding potential technosignatures,” Frank says.

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Kardashev scale:

The Kardashev scale is a method of measuring a civilization’s level of technological advancement based on the amount of energy it is capable of using. The measure was proposed by Soviet astronomer Nikolai Kardashev (1932–2019) in 1964 and was named after him. The Kardashev scale proposes that a civilization may eventually start consuming energy directly from its local star. This would require giant structures built next to it, called Dyson-spheres. Those speculative structures would cause an excess infrared radiation, that telescopes may notice. The infrared radiation is typical of young stars, surrounded by dusty protoplanetary disks that will eventually form planets. An older star such as the Sun would have no natural reason to have excess infrared radiation. The presence of heavy elements in a star’s light-spectrum is another potential technosignature; such elements would (in theory) be found if the star were being used as an incinerator/repository for nuclear waste products.

Kardashev proposed what would become known as the Kardashev scale, in which increasingly technologically developed civilizations harness the total energy of first a planet (Level I), then a star (Level II) and then an entire galaxy (Level III). In principle, the latter two levels would be achievable via Dyson swarms of solar-energy collectors around a civilization’s home star, and then around every star and black hole in their galaxy. According to the Kardashev scale, a Type II civilization could harness 4 x 10^26 watts; a Type III civilization could reach 4 x 10^37 watts.

A Type I civilization is able to access all the energy available on its planet and store it for consumption. Hypothetically, they should also be able to control natural events such as earthquakes, volcanic eruptions, etc.
A Type II civilization can directly consume a star’s energy, most likely through the use of a Dyson sphere.
A Type III civilization is able to capture all the energy emitted by its galaxy, and every object within it, such as every star, black hole, etc.

Several scientists have conducted various searches for possible civilizations, but with no conclusive results. However, based on these criteria, unusual objects, now known to be either pulsars or quasars, were identified.

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AI and the Detection of Extraterrestrial Life:

Traditional SETI searches suffer from two limitations: First, they assume intelligent aliens (if they exist) are trying to talk directly to us. Second, they assume that we’d recognize those messages if we found them. Recent advances in artificial intelligence (AI) are opening up exciting ways to reexamine all that data in search of subtle anomalies that have been overlooked. This idea is at the heart of a new SETI strategy: scanning for anomalous patterns that are not necessarily communication signals, but rather are the by-products of a technologically advanced civilization going about its business. The goal is to develop a versatile and intelligent anomaly engine that can work out which data values and interconnecting patterns are unusual when compared with a baseline.

This strategy helps mitigate a great struggle of SETI to date: the natural tension between making assumptions about what you are looking for so that you can search efficiently, balanced against the intuition that our definition of technology is very nascent indeed and so the less we assume the better.

The AI anomaly engine assumes only that the activities of an alien civilization might have some detectable effect on our observable universe. Best of all, the anomaly engine is a win-win proposition: Even if a strange observation has nothing to do with alien technology, it demands an explanation that could expand our understanding of the natural universe.

For example, in the early morning of July 25, 2001, a powerful burst of radio energy, less than 5 milliseconds in duration, swept through the Solar System and washed over the Southern Hemisphere of Earth. This extraordinary event went unnoticed for more than six years. It was not until November 2007 that astronomer Duncan Lorimer and his research student stumbled across the evidence for this intense spike of radio energy. The evidence had been hiding in plain sight among the mountains of archived data from the Parkes radio telescope in Australia. Perhaps an AI anomaly engine could have found the evidence for these fast radio bursts much sooner. And what is more important, there may be other surprises that human eyes have missed that are still waiting to be uncovered in data archives.

Indeed, as the capabilities of AI improve, new computer applications, such as deep-learning models, are expected to intelligently isolate similar anomalies within the huge data archives that have been collected across space science disciplines.

However, anomaly detection within multivariant data remains a dark art for even the best human experts, so developing an intelligent and flexible anomaly engine will be no easy feat. One approach is to train a deep neural network to be an autocorrelator that finds unusual examples of data. The input data must be compressed down to flow through a pinch-point in the neural net, like sand flowing through the waist of an hourglass. The more often this AI system is shown data of a similar nature, the better it becomes at compressing and accurately restoring the information. But if it is shown data that are unusual in some way, output would be poorly replicated and could be flagged as anomalous.

The problem is that these simple autocorrelators work best within a narrow domain of data and still lack the broad flexibility we need. However, AI research is making great strides forward.

One of the biggest challenges today is moving the search for signs of technology beyond just radio signals. We still want to look at all the sky all the time, at all wavelengths including pulses of laser light that might be used for communication. Another challenge is short-lived “transient” signals, one-time events that can be bright and energetic. Mixed among the many natural sources for such signals, like gamma-ray bursts or supernovae, might be artificial transients from distant civilizations – an engineered signal lasting less than a few minutes. But teasing them apart likely would require enormous amounts of computer time. Artificial intelligence could prove an ally in such searches. Sophisticated algorithms can sort through large amounts of data for patterns that could indicate an engineered signal. And AI searches likely would have fewer of the possible biases of human analysts, who might tend to focus their search on types of signals they’ve defined in advance, or view as more likely.

Machine learning technique reveals a sample’s biological or non-biological origin with 90% accuracy:

In recent experiments, Robert Hazen and his colleagues took 134 living and non-living samples (including petroleum, carbon-rich meteorites, ancient fossils, and a wasp that flew into their lab), vaporized them, and spread out their chemical constituents. Roughly 500,000 different attributes were identified within each sample’s molecular makeup and run through a machine-learning program. “When we look at those 500,000 attributes, there are patterns that are unique to living things and patterns unique to non-living things,” says Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science. After the software was trained on 70% of the specimens, the technique was able to recognize with 90% accuracy which of the remaining samples had a biological origin. The device that is used to spread out the chemical components of the samples is around seven inches long, small enough to be sent on missions to nearby ocean worlds like Jupiter’s Europa or Saturn’s Enceladus. NASA’s Perseverance rover carried a similar instrument to Mars, so Hazen thinks his team’s machine-learning algorithm could be adapted to sift through its data and hunt for organisms past or present there. And because it relies on molecular relationships rather than detecting specific organic chemicals like DNA or amino acids, which may not be used in other biospheres, the method could allow scientists to look for life entirely unlike what we have on Earth.

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Section-13

Search methods and studies for detecting extraterrestrial life/intelligence:

Although science has revealed much about our Universe, life beyond Earth remains elusive. Deep under the sea, around hydrothermal vents, where no sunlight reaches, life still thrives on Earth. How to create life from non-life is one of the great open questions in science today, but if life can exist down here, perhaps undersea on Europa or Enceladus, there’s life, too.

Chart below shows different projects searching for life in space:

As powerful as Nasa’s JWST is, it has its limits. Earth’s size and proximity to the Sun enable it to support life. But JWST wouldn’t be able to detect faraway planets as small as Earth (K2-18b is eight times bigger) or as close to their parent stars, because of the glare. So, Nasa is planning the Habitable Worlds Observatory (HWO), scheduled for the 2030s. Using what is effectively a high-tech sunshield, it minimises light from the star which a planet orbits. That means it will be able to spot and sample the atmospheres of planets similar to our own. Also coming online later this decade is the European Southern Observatory (ESO)’s Extremely Large Telescope (ELT), which will be on the ground, looking up at the crystal-clear skies of the Chilean desert. It has the largest mirror of any instrument built, 39-metres in diameter, and so can see vastly more detail at planetary atmospheres than its predecessors. All three of these atmosphere-analysing telescopes make use of a technique, used by chemists for hundreds of years, to discern the chemicals inside materials from the light they give off. They are so incredibly powerful that they can do this from the tiny pin prick of light from the atmosphere of a planet orbiting a star, hundreds of light years away.

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Earth life, Earth-kin life, and Earth-independent life:

It is helpful when discussing the detectability of extraterrestrial life to divide potential discoveries into three possible categories.

(1) Earth life:

The discovery of Earth life is that of microorganisms that are the same species as those on Earth. Such a detection would usually be expected to be due to Earth contamination. Variations that may have evolved from the common ancestor in extraterrestrial environments are considered here in the separate category of “Earth-kin life.”

(2) Earth-kin life:

This is extraterrestrial life that shares a common ancestor with life on Earth. It is not due to contamination from current life on Earth but also not an independent emergence of life from abiotic processes. Earth-kin life includes cases related to the panspermia hypothesis in which life migrated from Earth to Mars in ancient times, migrated in the reverse direction, or life from the same species or a shared common ancestor that originated in a third place within the universe arrived on both Earth and Mars.

(3) Earth-independent life:

This final category is an entirely separate emergence of life whereby extraterrestrial life began from non-living systems entirely independently from life on Earth.

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There are several main methods for searching for life outside Earth.

The first approach is to look for signs of any kind of life at all, intelligent or otherwise. This is by far the most common method in astronomy, and is usually targeted at other star systems — for example, looking for life-generated chemicals in the atmospheres of other worlds. But the search for extraterrestrial life also considers places within the solar system, like the surface of Mars and the hydrocarbon-rich atmosphere of Saturn’s moon Titan.

Another approach is to look specifically for intelligent life, because presumably, intelligent aliens are capable of making their presence known far more easily than a microbe is. For instance, we can look for communicative aliens that are broadcasting their existence in radio or optical wavelengths. Looking for artificial radio signals is the bread and butter of the search for extraterrestrial intelligence (SETI).

But intelligent aliens may also leave other clues. If they become capable of building so-called megastructures, like Dyson swarms, then we can detect those megastructures in searches of other systems. For example, enough large structures around a star would alter the light we see and could be a sign of intelligent activity.

So far, all searches for extraterrestrial life have come up empty.

But there is another avenue that is relatively unexplored: the search for extraterrestrial artifacts (SETA). The idea behind this approach is that if aliens become advanced enough, they might want to explore the galaxy, either by themselves or through robotic spacecraft. In the roughly 4.5 billion-year history of the solar system, these aliens would have had plenty of time to swing by our neighborhood and maybe leave a mark.

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Extraterrestrial life detection can be divided in 4 groups:

In situ detection of life as we know it,
In situ detection of life as we don’t know it,
Remote detection of life as we know it, and
Remote detection of life as we don’t know it.

-1. In situ detection of life as we know it:

Significant developments have been made in the field of DNA sequencing and analog environments. Current instrumentation could be used in upcoming missions on the in situ detection of life. There are some plans on the horizon: the Mars sample return mission, its associated analytical tools, and a holographic microscope. Planetary targets for the next generation of missions are also clear: Mars, Venus, and the icy ocean moons.

There is a strong emphasis on equipment to extract and handle samples. We need deep drills in order to test the material below what has currently been examined. Tools to assist in the capture and analysis of sprays (i.e., plumes) are also desired. Instruments that can perform clumped isotope analysis are also needed, although some already exist that could analyze the returned Martian samples. The list of molecular types that an instrument should be able to detect include: native fluorescents, adenosine tri-phosphate (ATP), dipicolinic acid (which is in spores), lipids, and more. These instruments should be capable of capillary microchip electrofluorosis and fluorescent immunoassay experiments. Any instrument sent into space must also be radiation hardened to prevent degradation, especially on longer missions.

The detection of life requires a spatially resolved analysis that combines multiple techniques. In a sample return mission, particularly from the outer solar system, the issue of how to preserve ices needs to be solved. Any sample return mission needs to have multiple laboratories independently analyze the samples as a hedge against false positives or negatives. Any mission focusing on the detection of life must also have collaboration between the instruments being flown.

-2. In situ detection of life as we don’t know it:

There are three steps in detecting life: finding it, seeing it, and determining its composition. We have to focus on looking for carbon and water as (likely) universal requirements for life. After finding life, we would want to see it do something. This can also be problematic. For example, Brownian motion of a particle could be easily deceiving. Maybe we would have to see it do something more interesting, like dividing and replicating. Testing for life also depends on what kind of life it is—micro versus macro and extant versus extinct. Macro life would probably be pretty easy to find no matter what it was made of. Finding extant microbes might be possible. The biggest difficulty, of course, would be detecting extinct microbes.

Everybody agree that a thermal equilibrium is necessary. The energy used by known life is chemical or visible light. There could be abnormal sources of energy for life, such as ionizing gamma radiation. This type of life might also be extant on Earth. A von Neumann automaton (capable of self-replication) should be classified as living or, at the very least, a biosignature. Any planet with life would likely have a biosphere filled with different species. Patterns and textures created by this biosphere could itself be a biosignature.

-3. Remote detection of life as we know it:

Atmospheric spectroscopy would show external observers that Earth is inhabited. At a minimum, therefore, the goal should be having the ability to detect and recognize Earth life.

More work needs to be done to determine the ratios of trace gases that can be biosignatures, but which can also be produced abiotically (e.g., CH4, O2, O3, and N2O). This requires knowledge of the environmental context to avoid false positives that could be caused by geochemical or photochemical processes. A broader environmental context requires knowledge of the planetary architecture and correlations between different parameters, which could inform the interpretation of biosignatures. In atmospheres with a potential biosignature gas (e.g., O2), certain other gases could instead indicate an abiotic origin. The feasibility of using isotopic measurements needs to be explored (e.g., 13C/1 C and D/H ratios), including how to interpret the results. However, this would require very-high-resolution spectroscopy (R ~ 100,000). Biosignature gases might also have seasonal changes modulated by life, which could potentially be measured. Clouds and aerosols might obscure biosignatures, but they also provide information about the planet’s geophysical and atmospheric processes and the planet’s potential habitability. Another factor in a planet’s habitability is tectonic and volcanic activity. Sulfur gases could be used to infer these properties about the planet. Polarimetry could also be informative in terms of both biological chirality and scattering processes in the atmosphere.

Technological advancements are required to enable or enhance the scientific return of future missions. The wavelength range is one of the most important properties of any instrument. It determines which molecules you can detect. Because many molecules have overlapping spectral lines and bands, multi-band measurements should be pursued, particularly if done alongside low-resolution spectroscopy. Detector technologies and telescope size limit the wavelength range accessible for each target (which is also a function of the angular separation between planet and star), so new technologies or larger telescopes would expand the number of observable targets. The noise sources, such as exozodiacal light, also vary as a function of wavelength. Improvements to cooling technology would keep thermal noise down in the near- and mid-infrared, enhancing the science return at these wavelengths.

Another technological advancement that could increase scientific return is high-resolution spectroscopy (R ~ 10,000-100,000), which is necessary for uniquely fingerprinting molecules (and especially isotopes) and their mixing ratios. Pushing high-resolution spectroscopy into space requires miniaturization of existing and developing technology. This will be difficult and expensive. A potential partial solution is to use high-resolution facilities on the ground and complement them with lower-resolution instruments in space.

And more advanced technologies should also be pursued. A photon detector that can resolve energies in the ultraviolet, visible, and infrared regions of the spectrum could vastly reduce the noise and technical hurdles of data reduction. Coronagraph technology, currently under development, has a throughput problem. Only 1 to 3 percent of the total light gets through. This also needs to be improved.

Note:

Resolution in optical spectroscopy is the minimum wavenumber, wavelength or frequency difference between two lines in a spectrum that can be distinguished. Resolving power, R, is given by the transition wavenumber, wavelength or frequency, divided by the resolution.

-4. Remote detection of life as we don’t know it:

Life as we don’t know it is known as “weird life.” We have National Research Council (NRC) report The Limits of Organic Life in Planetary Systems, which has become known as the “Weird Life Report”. It focused on carbon- and water-based life rather than life made out of neutronium or interstellar clouds.

Weird life allows for an expansion of the definition of “habitable.” The habitable zone could be much wider and weirder. Examples include a planet outside the conventional habitable zone with a large greenhouse effect from H2, an ultra-cold ocean world (e.g., an ocean composed of water plus ammonia and salt), and a very hot world with a few habitable locations (e.g., the clouds of Venus). This wider range of planets allows for a wider range of the planetary system’s possible architecture and evolution. However, the search for life doesn’t have to happen only on other bodies. Earth itself could harbour weird life.

One research goal is to move away from looking for just an Earth-like world in an Earth-like orbit around a Sun-like star, but to look for other combinations of planets and environments that could support life. It is not enough to just identify geochemistry of these alternative types of planets. Inputs, outputs, and rates of production also need to be modeled to check for detectability.

Another research goal is to further explore energy capture, specifically the relationship between photon flux, energy per photon, plausible photon capture mechanisms and efficiencies, and oxidants and reductants available in the environment. This relates to looking for a “blip” in the data at certain wavelengths. The terrestrial “red edge” is just one example of such a blip. Any dips, edges, peaks, or other blips need to be examined in the search for life. A biological origin of a weird blip might be ruled out in this way. Revisiting early Earth could be a useful exercise to explore whether different photosynthetic or energy capture processes were used.

An obvious sign of life would be any sort of technosignature, such as gases that are very unlikely to be formed naturally. Other indications of life would be large-scale differences from what is expected. For example, a Mars-sized planet in a Mars-like orbit, but with the climate of Los Angeles. Even more bizarre examples of technosignatures include rearranging planetary systems, Dyson spheres, Alderson disks, von Neumann probes, and machine civilizations.

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Studies to detect extraterrestrial life/intelligence:

-1. Planetary Mass Spectrometry for Agnostic Life Detection in the Solar System, a 2021 study:

For the past fifty years of space exploration, mass spectrometry has provided unique chemical and physical insights on the characteristics of other planetary bodies in the Solar System. A variety of mass spectrometer types, including magnetic sector, quadrupole, time-of-flight, and ion trap, have and will continue to deepen our understanding of the formation and evolution of exploration targets like the surfaces and atmospheres of planets and their moons. An important impetus for the continuing exploration of Mars, Europa, Enceladus, Titan, and Venus involves assessing the habitability of solar system bodies and, ultimately, the search for life—a monumental effort that can be advanced by mass spectrometry. Modern flight-capable mass spectrometers, in combination with various sample processing, separation, and ionization techniques enable sensitive detection of chemical biosignatures. While our canonical knowledge of biosignatures is rooted in Terran-based examples, agnostic approaches in astrobiology can cast a wider net, to search for signs of life that may not be based on Terran-like biochemistry. Here, authors delve into the search for extraterrestrial chemical and morphological biosignatures and examine several possible approaches to agnostic life detection using mass spectrometry. Authors discuss how future missions can help ensure that our search strategies are inclusive of unfamiliar life forms.

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-2. Comparative study of methods for detecting extraterrestrial life in exploration mission of Mars and the solar system, a 2022 study:

The detection and analysis of extraterrestrial life are important issues of space science. Mars is among the most important planets to explore for extraterrestrial life, owing both to its physical properties and to its ancient and present environments as revealed by previous exploration missions. In this paper, authors present a comparative study of methods for detecting extraterrestrial life and life-related substances. To this end, they have classified and summarized the characteristics targeted for the detection of extraterrestrial life in solar system exploration mission and the methods used to evaluate them. A summary table is presented. Authors conclude that at this moment (i) there is no realistic single detection method capable of concluding the discovery of extraterrestrial life, (ii) no single method has an advantage over the others in all respects, and (iii) there is no single method capable of distinguishing extraterrestrial life from terrestrial life. Therefore, a combination of complementary methods is essential. Authors emphasize the importance of endeavouring to detect extraterrestrial life without overlooking possible alien life forms, even at the cost of tolerating false positives. Summaries of both the targets and the detection methods should be updated continuously, and comparative studies of both should be pursued. Although this study assumes Mars to be a model site for the primary environment for life searches, both the targets and detection methods described herein will also be useful for searching for extraterrestrial life in any celestial environment and for the initial inspection of returned samples.

A useful point of view is that the primary goal of in situ analysis in space exploration must not overlook the potential of life (with suppressing false positives to the greatest possible extent), rather than reaching the final conclusion if the structure found on other celestial body is life or not. With this point of view, the primary role of in situ analysis should be a means for selecting samples to be returned to Earth without overlooking the potential of life as efficiently as possible. Fluorescence microscopy holds several advantages for life detection: the observation of morphology is possible, and numerous characteristics such as metabolism, various biomolecular components, and metabolic enzymes can be observed with a single instrument using adequate fluorescent dyes, including a high enough sensitivity to detect a single cell. Other microscopy techniques, such as electron microscopes, can observe the morphology as well, although it is difficult to obtain information on the cell’s components. Although spectrophotometers can perform various analyses, their detection sensitivity is significantly lower. From the viewpoint of planetary protection, it is likewise important not to overlook the possibility of life. If life is overlooked at the location, and the absence of life is concluded, serious problems can arise in terms of the safety of manned activities and pollution by Earth-derived microorganisms.

Potential disadvantages of fluorescence microscopy involve the occurrence of false positives. However, false positives can be suppressed by the use of appropriate fluorescent dyes and adequate conditions based on preliminary experiments in a ground-based laboratory. Nevertheless, it is difficult to eliminate false positives completely in experiments for unknown extraterrestrial samples. Furthermore, the fluorescence microscope is not suitable for obtaining detailed information, such as the molecular weight of the constituent, although it can provide information on the category of the component. Therefore, fluorescence microscopes and instruments capable of measuring molecular weight, such as mass spectrometers, are complementary. Such complementary combinations of in situ analyses will improve the efficient selection of samples to be returned, suitable for analysis by on-ground scientific researchers.

This paper summarizes various methods that can be employed to detect present life in space. Most of the biochemical compounds discussed in this paper are fragile. They degrade quickly after the host microorganism has died and they have been liberated into the environment. The (rather immediate) deleterious effects of UV light and (over a longer period) of ionizing radiation on target biomolecules are important issues. In this context, it is important to obtain a sample that contains living organisms. This might be achieved by collecting the sample at a site that fulfills the requirements for the organisms to grow: an adequate environment for the organisms to survive, containing sufficient water activity, energy sources, and the needed elements and molecules. The organics contained in living organisms can be distinguished from organics originating from space in that the former exhibit catalytic activity enclosed by a membrane. The organics in terrestrial living organisms include limited numbers of molecular species of amino acids and nucleobases, which are more varied in meteorites (Koga and Naraoka, 2017; Callahan et al., 2011). Once the organisms are dead, deterioration of the organic compounds occurs, producing degraded organic compounds such as those with low enantiomeric excess, kerogen, and PAHs. It may still be possible to distinguish between the deteriorated organics originating from living organisms and those originating from meteorites and other space origins, since the organic compounds found in meteorites exhibit characteristics specific to their space origin (Koga and Naraoka, 2017; Callahan et al., 2011). It is important to emphasize that, in addition to their molecular content, morphological information is important for distinguishing between dead fossilized and living organisms. For example, confocal laser scanning microscopy and Raman imagery have been used to resolve Precambrian microfossils (Schopf and Kudryavtsev, 2009).

After the first occurrence of life, microorganisms had dominated the Earth for billions of years (Joseph et al., 2022; Joseph et al., 2019). Longer time periods witnessed the emergence of larger and multicellular organisms. Microorganisms are more numerous in number and have spread across wider areas than the large organisms present on Earth (Bar-On et al., 2018; Whitman et al., 1998). Accordingly, a higher number of microorganisms is expected if larger organisms are to be present. However, the existence of microorganisms does not imply the presence of large organisms. Therefore, in this study, authors described life exploration targeting microorganisms. They note that it is highly unlikely that solar system (except terrestrial) organisms could have evolved beyond microorganisms; even the evolution of anaerobic photosynthesis on Mars is problematic (Joseph et al., 2019). Nevertheless, the observation of macroscopic objects has unique importance for understanding unknown worlds based on observational evidence. A microscope is not required for observing macroscopic life-related objects: if the object to be observed is in an appropriate position, information can be obtained with a camera. The targeted characteristics may be movement, growth, change of color, and proliferation of the object.

The methods described in this study are applicable to extraterrestrial life exploration in other celestial bodies. Potential exploration sites are the Martian surface (including atmospheric dust and underground samples), the Venus atmosphere (Cockell, 1999; Limaye et al., 2018; Sasaki et al., 2022), icy objects (like moons of Jupiter and Saturn), small bodies (like asteroids), and the Moon. In principle, the methods described in this article are also applicable to the returned samples. An analysis that does not overlook life is useful as a method for the preliminary analysis of a returned sample. The small size of the instrument is advantageous for the initial analysis of the return sample, which is performed in a limited and enclosed area. If the preliminary analysis of the sample reveals that it is free of microorganisms, it is possible to move the sample out of the enclosed space. Then, it will be possible to carry out elaborate experiments using a larger device, as there will be no restrictions on its size. Any type of any number of characteristics can be analyzed in the ground laboratory following preliminary analysis.

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-3. Applying Novel Techniques from Physical and Biological Sciences to Life Detection, a 2023 study:

One of the primary goals of the space exploration community is to unambiguously detect past or present life outside of Earth. As such, a number of so-called life detection technologies, instruments, and approaches have been applied as part of past, current, and future space missions. As astrobiology is a truly interdisciplinary field within the realm of space exploration with major contributions from physical and biological sciences (among others), recently there has been development of a number of relevant techniques from scientific fields that have yet to be fully applied to extraterrestrial life detection. As a culmination of the 2021 Blue Marble Space Institute of Science (BMSIS) Young Scientist Program (YSP), authors present a number of techniques drawn from various fields (including, but not limited to, chemistry, materials science, biology, nanotechnology, medical science, astrophysics, and more) that either have been or have the potential to be applied to life detection research. These techniques broadly fall under three categories: instrumentation for in situ measurements of biosignatures within the solar system, calculations or observational techniques for remote measurements of exoplanet biosignatures, and technosignatures. Authors hope that this primer serves to inspire the field to consider applying more potential technologies from adjacent fields into any of these three categories of life detection.

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Section-14

Search for extraterrestrial life in solar system:

The Solar System has a wide variety of planets, dwarf planets, and moons, and each one is studied for its potential to host life. Each one has its own specific conditions that may benefit or harm life. So far, the only lifeforms found are those from Earth. No extraterrestrial intelligence other than humans exists or has ever existed within the Solar System. Astrobiologist Mary Voytek points out that it would be unlikely to find large ecosystems, as they would have already been detected by now.

The inner Solar System is likely devoid of life. However, Venus is still of interest to astrobiologists, as it is a terrestrial planet that was likely similar to Earth in its early stages and developed in a different way. There is a greenhouse effect, the surface is the hottest in the Solar System, sulfuric acid clouds, all surface liquid water is lost, and it has a thick carbon-dioxide atmosphere with huge pressure. Comparing both helps to understand the precise differences that lead to beneficial or harmful conditions for life. And despite the conditions against life on Venus, there are suspicions that microbial lifeforms may still survive in high-altitude clouds.

Mars is a cold and almost airless desert, inhospitable to life. However, recent studies revealed that water on Mars used to be quite abundant, forming rivers, lakes, and perhaps even oceans. Mars may have been habitable back then, and life on Mars may have been possible. But when the planetary core ceased to generate a magnetic field, solar winds removed the atmosphere and the planet became vulnerable to solar radiation. Ancient lifeforms may still have left fossilised remains, and microbes may still survive deep underground.

Gas giants are unlikely to contain life. The most distant bodies are in permanent cold and locked in deep-freeze, but cannot be ruled out completely. Although the ice giants themselves are not likely to have life, there is much hope to find it in some of the many moons of those planets. Europa, from the Jovian system, has a subsurface ocean below a thick layer of ice. Ganymede and Callisto also have subsurface oceans, but life is less likely in them because water is sandwiched between layers of solid ice. Europa would have contact between the ocean and the rocky surface, which helps the chemical reactions. It may be difficult to dig so deep in order to study those oceans, though. Enceladus, a tiny moon of Saturn with another subsurface ocean, may not need to be dug, as it releases water to space in eruption columns. The space probe Cassini flew inside one of those, but could not make a full study because NASA did not expect this phenomenon and did not equip the probe to study ocean water. Still, it could detect complex organic molecules, salts, evidence of hydrothermal activity, hydrogen, and methane.

Titan is the only celestial body in the Solar System besides Earth that has liquid bodies on the surface. It has rivers, lakes, and rain of hydrocarbons, methane, and ethane, and even a cycle similar to Earth’s water cycle. This special context encourages speculations about lifeforms with different biochemistry, but the cold temperatures would make such chemistry take place at a very slow pace. Water is rock-solid on the surface, but Titan has a subsurface ocean like other moons. However, it is too deep and it would be very difficult to access it for study.

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Missions inside the Solar System:

The Viking missions to Mars:

The Viking missions to Mars in the 1970s conducted the first experiments which were explicitly designed to look for biosignatures on another planet. Each of the two Viking landers carried three life-detection experiments which looked for signs of metabolism; however, the results were declared inconclusive.

Mars Science Laboratory:

The Curiosity rover from the Mars Science Laboratory mission is currently assessing the potential past and present habitability of the Martian environment and is attempting to detect biosignatures on the surface of Mars. Considering the MSL instrument payload package, the following classes of biosignatures are within the MSL detection window: organism morphologies (cells, body fossils, casts), biofabrics (including microbial mats), diagnostic organic molecules, isotopic signatures, evidence of biomineralization and bioalteration, spatial patterns in chemistry, and biogenic gases. The Curiosity rover targets outcrops to maximize the probability of detecting ‘fossilized’ organic matter preserved in sedimentary deposits.

ExoMars Orbiter:

The 2016 ExoMars Trace Gas Orbiter (TGO) is a Mars telecommunications orbiter and atmospheric gas analyzer mission. It delivered the Schiaparelli EDM lander and then began to settle into its science orbit to map the sources of methane on Mars and other gases, and in doing so, will help select the landing site for the Rosalind Franklin rover to be launched in 2028. The primary objective of the Rosalind Franklin rover mission is the search for biosignatures on the surface and subsurface by using a drill able to collect samples down to a depth of 2 metres (6.6 ft), away from the destructive radiation that bathes the surface.

Mars 2020 Rover:

The Mars 2020 rover, the perseverance rover, which launched in 2020, is intended to investigate an astrobiologically relevant ancient environment on Mars, investigate its surface geological processes and history, including the assessment of its past habitability, the possibility of past life on Mars, and potential for preservation of biosignatures within accessible geological materials. In addition, it will cache the most interesting samples for possible future transport to Earth.

Titan Dragonfly:

NASA’s Dragonfly lander/aircraft concept is proposed to launch in 2028 and would seek evidence of biosignatures on the organic-rich surface and atmosphere of Titan, as well as study its possible prebiotic primordial soup. Titan is the largest moon of Saturn and is widely believed to have a large subsurface ocean consisting of a salty brine. In addition, scientists believe that Titan may have the conditions necessary to promote prebiotic chemistry, making it a prime candidate for biosignature discovery.

Europa Clipper:

NASA’s Europa Clipper probe is designed as a flyby mission to Jupiter’s smallest Galilean moon, Europa. Set to launch in 2024, this probe will investigate the potential for habitability on Europa. Europa is one of the best candidates for biosignature discovery in the Solar System because of the scientific consensus that it retains a subsurface ocean, with two to three times the volume of water on Earth. Evidence for this subsurface ocean includes:

Voyager 1 (1979): The first close-up photos of Europa are taken. Scientists propose that a subsurface ocean could cause the tectonic-like marks on the surface.
Galileo (1997): The magnetometer aboard this probe detected a subtle change in the magnetic field near Europa. This was later interpreted as a disruption in the expected magnetic field due to the current induction in a conducting layer on Europa. The composition of this conducting layer is consistent with a salty subsurface ocean.
Hubble Space Telescope (2012): An image was taken of Europa which showed evidence for a plume of water vapor coming off the surface.

The Europa Clipper probe will carry instruments to help confirm the existence and composition of a subsurface ocean and thick icy layer. In addition, it will map the surface to study features that may point to tectonic activity due to a subsurface ocean.

Enceladus mission:

Although there are no set plans to search for biosignatures on Saturn’s sixth-largest moon, Enceladus, the prospects of biosignature discovery there are exciting enough to warrant several mission concepts that may be funded in the future. Similar to Jupiter’s moon Europa, there is much evidence for a subsurface ocean to also exist on Enceladus. Plumes of water vapor were first observed in 2005 by the Cassini mission and were later determined to contain salt as well as organic compounds. Cassini–Huygens, commonly called Cassini, was a space-research mission by NASA, the European Space Agency, and the Italian Space Agency to send a space probe to study the planet Saturn and its system, including its rings and natural satellites. In 2014, more evidence was presented using gravimetric measurements on Enceladus to conclude that there is in fact a large reservoir of water underneath an icy surface. Mission design concepts include:

Enceladus Life Finder (ELF)
Enceladus Life Signatures and Habitability
Enceladus Organic Analyzer
Enceladus Explorer (En-Ex)
Explorer of Enceladus and Titan (E2T)
Journey to Enceladus and Titan (JET)
Life Investigation For Enceladus (LIFE)
Testing the Habitability of Enceladus’s Ocean (THEO)

All of these concept missions have similar science goals: To assess the habitability of Enceladus and search for biosignatures, in line with the strategic map for exploring the ocean-world Enceladus.

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A brief survey of life’s prospects on the moons and planets of the solar system:

In the solar system there are many different environments that could contain significant clues to the origin of life and perhaps even life itself. However, there is not yet definitive evidence for or against extraterrestrial life on these planets.

The Moon and Mercury:

The Moon’s surface is inhospitable to life of any sort. Diurnal temperatures range from about 100 K (−173 °C, or −279 °F) to about 400 K (127 °C, or 261 °F). In the absence of either an atmosphere or a magnetic field, ultraviolet light and charged particles from the Sun penetrate unimpeded to the lunar surface. In less than an hour, they deliver a dose lethal to the most radiation-resistant bacteria known. The subsurface environment of the Moon is not nearly so inclement. Ultraviolet light and solar protons do not penetrate more than 1 metre (3.3 feet) below the surface, and the temperature is maintained at a relatively constant value of about 230 K (−43 °C, or −45 °F). Nevertheless, the absence of any surface fluid, atmosphere, or liquid to cycle matter and energy makes prospects for life dim.

The environment of Mercury is rather like that of the Moon. Its surface temperatures range from about 100 K to about 620 K (347 °C, or 657 °F), but, about 1 metre below the surface, the temperature is constant at roughly room temperature. However, the absence of any significant atmosphere, the unlikelihood of bodies of liquid, and the intense solar radiation make the prospect for life on Mercury remote.

Venus:

In our solar system, Venus is the most similar to Earth with respect to mass and size, and it is also the planet physically closest to Earth. Theoretically, Venus should be as habitable as Earth, right? No. Venus suffered a fate that Earth did not: being slightly too close to the sun and too slow to spin. The average surface temperature of Venus is approximately 750 K (477 °C, or 891 °F). Even at the poles or on the tops of the highest Venusian mountains, surface temperatures do not fall below 400 K (127 °C, or 261 °F). The temperatures on Venus are too hot for Earth-style life. However, carbon dioxide, sunlight, and water (according to the results of the Venera space vehicles) are found in the clouds of Venus. These three are the prerequisites for photosynthesis. Molecular nitrogen also is expected at the cloud level, and some minerals are likely convectively raised to the cloud level from surface dust. The surface air pressure on the planet Venus may be 75 or 100 times that on Earth. The cloud pressures are about the same as those on the surface of Earth, and the temperatures in the lower clouds also are quite Earth-like. Although highly acidic by virtue of their sulfur, the lower clouds of Venus are the most Earth-like extraterrestrial environment known. No organisms on Earth lead a completely airborne existence, so most scientists dispute the possibility that organisms exist buoyed in the clouds of Venus.

Life on Mars is discussed below in a separate segment.

Jovian planets:

The Jovian planets are Jupiter, Saturn, Uranus, and Neptune. They orbit far from the sun. These planets have no solid surfaces and are essentially large balls of gas composed primarily of hydrogen and helium. They are much larger than the terrestrial planets (Earth, Mercury, Venus, and Mars).

The atmosphere of Jupiter is composed of hydrogen, helium, methane, ammonia, some neon, and water vapour. These are exactly the gases used in experiments that simulate the early Earth. Laboratory and computer experiments have been performed on the application of energy to simulated Jovian atmospheres. Immediate gas-phase products include significant quantities of hydrogen cyanide and acetylene. More-complex organic molecules, including aromatic hydrocarbons, are formed in lower yields. The clouds of Jupiter are vividly coloured, and their hue may be attributable to organic compounds. An apparent absorption feature near 260 nanometres in Jupiter’s ultraviolet spectrum may be due to aromatic hydrocarbons or even due to nucleotide bases. Jupiter may be a vast planetary brew that has operated for 4.5 billion years as a laboratory of organic chemistry.

The other Jovian planets, Saturn, Uranus, and Neptune, resemble Jupiter, although less is known about them. Their cloud-top temperatures progressively decrease with distance from the Sun. Microwave studies of Saturn indicate that the atmospheric temperature increases with depth below the clouds. A similar situation is expected to exist on Jupiter, Uranus, and Neptune. These planets of the solar system are associated with many natural satellites. Some, such as Titan, a satellite of Saturn, and Io, a satellite of Jupiter, have atmospheres. Despite the relative suitability for life’s preconditions, no evidence is known for life on the outer planets or their satellites.

Europa, other Jovian moons, comets, and asteroids:

Europa, the fourth largest satellite of Jupiter, may be the best candidate for extraterrestrial life in the solar system. The Galileo orbiter revealed a crust of water ice and a complex surface on this moon. Optical imaging, thermographic temperature probes, and magnetic field measurements support the strong inference that a liquid saltwater ocean surges beneath the frozen crust. A wisp of an oxygen atmosphere has also been detected by spectrographic techniques. Furthermore, since organic molecules including methane and nitrogen-rich gases such as ammonia abound on Jupiter and some of its other moons, such “prebiotic chemicals” are highly likely to be present on Europa. The Galileo flyby also detected abundant sulfuric acid, a potential chemical power source, on the surface of Europa. (Such discoveries in the Jovian planets inspire further investigation of the limits to diversity of life on Earth. Lakes such as Vostok in Antarctica reside under more than 3 km [2 miles] of ice. Studies of bacteria in these lakes and of water seeps within cavities in granitic and carbonate rocks provide models for the viability of possible Earth-like life-forms on Europa and other Jovian moons.)

Io is the most volcanically active place in the solar system, and Ganymede and Callisto may also have water ice under their surfaces. The immense tidal influence of Jupiter regularly pumps energy into these planetary systems. Now that it has become clear that chemoautotrophic life-forms do not require sunlight as sources of energy, some scientists argue that a shift of focus from Mars and the other inner planets is in order. The outer planets’ satellites, especially Europa and Saturn’s Titan, promise new insights into the search for extraterrestrial life in the solar system. In 2008, for example, the Cassini spacecraft reported several hundred lakes and seas of organic materials on Titan, dozens of which contain more liquid hydrocarbon (such as methane and ethane) than all of Earth’s oil and gas reserves combined.

Tens of thousands of comets, as well as some thousands of asteroids and asteroidal fragments revolving about the Sun between the orbits of Mars and Jupiter, contain organic molecules. The asteroids are the presumed sources of the carbonaceous chondrites’ organic matter. Pluto has a predominantly nitrogen atmosphere covering a surface of frozen nitrogen, carbon dioxide, and methane. The intense cold and paucity of solar radiation on Pluto and the lack of atmosphere and liquid waters on the asteroids argue against the likelihood of finding life on these bodies.

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Life on Mars:

In the early days of Mars exploration, NASA found ample evidence of water on Mars. Using rovers and analyzing collected sediments on the surface of Mars, scientists deduced that Mars was once wet and covered with plenty of rivers, lakes and oceans, just like Earth. This means that, approximately 3.5 billion years ago, Mars had a similar climate to Earth. But today, Mars is a dry desert and unfit to sustain life. How did this planet, a neighbor to Earth, change from a potentially habitable planet to one with conditions too harsh for life? A recent study suggests an answer to that question: Mars may be too small to hold onto sufficient amounts of water. Mars is a lot smaller than our home planet—only slightly more than half the size of Earth. The scientists found that larger planets, like Earth, have stronger gravitational pulls and thus can more easily hold evaporated molecules like water, while smaller bodies like Mars cannot. The atmosphere of Mars is mostly carbon dioxide, the surface of the planet is too cold to sustain human life, and the planet’s gravity is a mere 38% of Earth’s. Plus, the atmosphere on Mars is equivalent to about 1% of the Earth’s atmosphere at sea level. The red planet Mars was once like Earth, only it currently has no magnetic field and no greenhouse gases, so it has no liquid state, which means it cannot support life. Mars is known as the “Red Planet” because it appears faintly reddish/orange when viewed in the night sky. This reddish color comes from the abundance of iron minerals and dust on the Martian surface.

Mars was one of the first extraterrestrial bodies that captured human attention due to a belief in liquid water flowing across the surface with accompanying life. In the early 1780s, the astronomer William Herschel calculated the rotation period of Mars and the planet’s tilt to be similar to Earth’s, leading him to conclude the analogy between Mars and the Earth is, perhaps, by far the greatest in the whole solar system and that its inhabitants probably enjoy a situation in many respects similar to ours. Images of Mars from telescopes showed changing brightness on the planet over time, which some scientists believed could be water or vegetation growing and receding seasonally, as happens on Earth. Despite these early hopes in the capacity of Mars to house both water and life, scientific evidence indicated otherwise. By the early 20th century, most astronomers agreed that Mars must be much drier and colder than Earth, and that no complex forms of life could survive there. Light collected from the planet showed primarily a carbon dioxide atmosphere with no detectable amounts of oxygen or water vapor at the time. (Note that modern data shows trace amounts of oxygen and water vapor.) Because of the cold temperature and thin atmosphere on Mars (0.6% that of Earth), liquid water is unstable almost everywhere on the planet’s surface.

During the coming decades, the search for evidence of extinct or extant life on Mars will be a central focus of both the National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) as a flotilla of spacecraft explore the Martian surface and return Martian samples back to Earth for comprehensive state-of-the-art analyses. The challenges are daunting. Missions to Mars are costly and risky, as evidenced by the recent losses of the Mars Climate Orbiter and Mars Polar Lander spacecraft. Collecting and returning samples to Earth, while appealing because of the direct hands-on analytical advantages they provide, are constrained by the amount of material that can be returned and sample containment issues related to potential biological hazards associated with possible extant Martian organisms being transported back to Earth. Spacecraft-based robotic instruments designed to carry out direct analyses for evidence of water, prebiotic chemistry, or biologically derived molecules are constrained by mass and power limitations, as well as challenges associated with obtaining samples suitable for analysis.

There is still considerable uncertainty about whether water in amounts sufficient to support life has ever existed on Mars. Evidence suggests that liquid water flowed, accumulated, and evaporated on Mars billions of years ago and perhaps even more recently. One of the major goals in the Mars exploration program is to provide a better assessment of the amount of water present on the planet. This appraisal is important not only for evaluating the possibility that life may have once existed, or still exists, on Mars, but also for providing information about potential in situ resources available for future human explorers.

The possible presence of organic compounds on Mars is also uncertain. Using a pyrolysis procedure, in combination with a gas chromatograph/mass spectrometer (GCMS), Viking did not detect any organic compounds above a level of a few parts per billion in near surface samples at two different landing sites. However, it is now apparent that the Viking pyrolysis GCMS instruments would not have detected the presence of millions of bacterial cells in 1 g of soil. In addition, oxidation reactions involving organic compounds on the Martian surface would likely produce non-volatile products that also would not have been detected by the Viking GCMS.

Meteorites from Mars have been extensively investigated to assess whether they contain organic compounds possibly derived from life. Unfortunately, contamination of Martian meteorites by terrestrial organic compounds greatly compromise these investigations. If there is any indigenous organic material present in Martian meteorites, it appears to be derived from the infall of carbonaceous meteorites rather than from Martian biology. The problems associated with terrestrial contamination underscores the importance of doing in situ organic compound analyses on Mars before samples are returned to Earth, where even under the best of circumstances they will be exposed to some level of terrestrial contamination.

Because amino acids are the building blocks of proteins and enzymes in terrestrial organisms, they are excellent target compounds in the search for life on Mars and elsewhere. While it is not certain that Martian biology would use the exact same set of amino acids as life on Earth, their ubiquity as constituents of organic material in the solar system suggests that amino acids would have been available for incorporation into living entities on Mars just as they were on Earth. Amino acids derived from either extinct or extant life, and from the infall of meteorites and cosmic dust, could be present on the surface of Mars.

Another class of organic compounds of interest are polycyclic aromatic hydrocarbons (PAHs). Although PAHs have no known role in biochemistry on Earth, they can be produced from the long-term (tens of millions of years or more) degradation of biologically derived organic compounds. PAHs have been identified in the interstellar medium and in carbonaceous meteorites. They may be the most abundant single class of organic compounds in the universe. Given the infall of meteorites and cosmic dust throughout the history of Mars, PAHs could be one of the organic components of the Martian surface, especially if retrieved samples contained fragments of carbonaceous chondrites.

Mudstone on Mars:

Since the visit of Viking, our understanding of Mars has deepened spectacularly. Orbiting spacecraft have provided ever-more detailed images of the surface and detected the presence of minerals that could have formed only in the presence of liquid water. Two bold surface missions, the Mars Exploration Rovers Spirit and Opportunity (2004), followed by the much larger Curiosity Rover (2012), confirmed these remote-sensing data. All three rovers found abundant evidence for a past history of liquid water, revealed not only from the mineralogy of rocks they analyzed, but also from the unique layering of rock formations.

Curiosity has gone a step beyond evidence for water and confirmed the existence of habitable environments on ancient Mars. “Habitable” means not only that liquid water was present, but that life’s requirements for energy and elemental raw materials could also have been met. The strongest evidence of an ancient habitable environment came from analyzing a very fine-grained rock called a mudstone—a rock type that is widespread on Earth but was unknown on Mars until Curiosity found it (see figure below). The mudstone can tell us a great deal about the wet environments in which they formed. The Perseverance rover is collecting samples of sedimentary rock in a former lakebed, to later return to Earth for laboratory analysis.

Figure above shows Mudstone. Shown are the first holes drilled by NASA’s Curiosity Mars rover into a mudstone, with “fresh” drill-pilings around the holes. Notice the difference in color between the red ancient martian surface and the gray newly exposed rock powder that came from the drill holes. Each drill hole is about 0.6 inch (1.6 cm) in diameter.

The Curiosity rover started in Yellowknife Bay in the crater basin and drove partway up Mt. Sharp. Along the way, it drilled many holes and analyzed their composition as seen in the figure below:

Figure above shows composition of Martian samples in Gale Crater along Curiosity’s path.

Five decades of robotic exploration have allowed us to develop a picture of how Mars evolved through time. Early Mars had epochs of warmer and wetter conditions that would have been conducive to life at the surface. However, Mars eventually lost much of its early atmosphere and the surface water began to dry up. As that happened, the ever-shrinking reservoirs of liquid water on the Martian surface became saltier and more acidic, until the surface finally had no significant liquid water and was bathed in harsh solar radiation. The surface thus became uninhabitable, but this might not be the case for the planet overall.

Reservoirs of ice and liquid water could still exist underground, where pressure and temperature conditions make it stable. There is recent evidence to suggest that liquid water (probably very salty water) can occasionally (and briefly) flow on the surface even today. Thus, Mars might even have habitable conditions in the present day, but of a much different sort than we normally think of on Earth.

In 2015, data collected from the Mars Reconnaissance Orbiter suggested that recurring dark streaks on the Martian surface were due to liquid brine (water mixed with salt) existing just below the surface during the warmer season on Mars (-10 F) that would make its way to the surface for a brief time. However, additional data in 2017 cast this hypothesis into doubt, suggesting instead that these dark streaks are primarily due to grains of sand and dust cascading down very steep slopes on the rugged planet.

As mentioned above, liquid water is generally unstable on the Martian surface. However, it can exist under certain conditions below the surface. Specifically, in 2018 scientists using radar data from the European Space Agency’s Mars Express orbiter identified a brine liquid-water lake a mile below the southern ice cap of Mars. The high levels of salt and frigid temperatures would make it difficult for most known lifeforms to survive. However, even on Earth we have discovered organisms that thrive in salty, though not so cold, conditions. While these recent data give strong evidence of a form of liquid water on Mars, to find direct evidence of present or past life in that water, missions would have to be sent to drill into the Martian surface. Such drilling would be incredibly difficult, and there are no current public or private plans for such a mission. Additionally, scientists are hesitant to perform such missions out of fear of contaminating or destroying any possible life that might be detected. Last, if humans are to ever go to Mars, it will likely be easier for them to melt surface or near-surface ice for survival than to dig deep into the ground to purify this salt-water mixture.

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Life in the Outer Solar System:

Evidence points to oceans on other planets and moons, even within our own solar system. But Earth is the only known planet (or moon) to have consistent, stable bodies of liquid water on its surface. In our solar system, Earth orbits around the sun in an area called the habitable zone. The temperature within this zone, along with an ample amount of atmospheric pressure, allow water to be liquid for long periods of time. Scientists believe several moons within our solar system have significant subsurface liquid water deposits. Saturn’s moon Enceladus and Jupiter’s moon Europa are two examples. Both appear to have salty, liquid oceans covered with thick layers of ice at the surface. Scientists have observed water plumes erupting from Enceladus, and believe similar plumes can be found on Europa. The existence of these geysers also tells scientists that these moons have a source of energy, perhaps from gravitational forces or radiation — energy that keeps the oceans liquid under the ice and could even support life.

The massive gas and ice giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune—are almost certainly not habitable for life as we know it, but some of their moons might be. Although these worlds in the outer solar system contain abundant water, they receive so little warming sunlight in their distant orbits that it was long believed they would be “geologically dead” balls of hard-frozen ice and rock. But missions to the outer solar system have found something much more interesting. Jupiter’s moon Europa revealed itself to the Voyager and Galileo missions as an active world whose icy surface apparently conceals an ocean with a depth of tens to perhaps a hundred kilometers. As the moon orbits Jupiter, the planet’s massive gravity creates tides on Europa—just as our own Moon’s gravity creates our ocean tides—and the friction of all that pushing and pulling generates enough heat to keep the water in liquid form. Similar tides act upon other moons if they orbit close to the planet. Scientists now think that six or more of the outer solar system’s icy moons may harbor liquid water oceans for the same reason. Among these, Europa and Enceladus, a moon of Saturn, have thus far been of greatest interest to astrobiologists.

Europa:

Europa has probably had an ocean for most or all of its history, but habitability requires more than just liquid water. Life also requires energy, and because sunlight does not penetrate below the kilometers-thick ice crust of Europa, this would have to be chemical energy. One of Europa’s key attributes from an astrobiology perspective is that its ocean is most likely in direct contact with an underlying rocky mantle, and the interaction of water and rocks—especially at high temperatures, as within Earth’s hydrothermal vent systems—yields a reducing chemistry (where molecules tend to give up electrons readily) that is like one half of a chemical battery. To complete the battery and provide energy that could be used by life requires that an oxidizing chemistry (where molecules tend to accept electrons readily) also be available. On Earth, when chemically reducing vent fluids meet oxygen-containing seawater, the energy that becomes available often supports thriving communities of microorganisms and animals on the sea floor, far from the light of the Sun. The Galileo mission found that Europa’s icy surface does contain an abundance of oxidizing chemicals. This means that availability of energy to support life depends very much on whether the chemistry of the surface and the ocean can mix, despite the kilometers of ice in between. That Europa’s ice crust appears geologically “young” (only tens of millions of years old, on average) and that it is active makes it tantalizing to think that such mixing might indeed occur. Understanding whether and how much exchange occurs between the surface and ocean of Europa will be a key science objective of future missions to Europa, and a major step forward in understanding whether this moon could be a cradle of life.

Figure above shows Jupiter’s Moon Europa, as imaged by NASA’s Galileo Mission. The relative scarcity of craters on Europa suggests a surface that is “geologically young,” and the network of colored ridges and cracks suggests constant activity and motion. Galileo’s instruments also strongly suggested the presence of a massive ocean of salty liquid water beneath the icy crust.

Potential Biosignatures on Europa:

Below a thick ice shell model (~15 km) for Europa, on top of a thick ocean of water (~100 km) is depicted, to examine exchange processes and how potential biosignatures might be preserved on the surface of Europa (see figure below). The top layer is composed of brittle ice. Underneath this surface is a layer of ductile, convective ice.

Figure above shows possible habitable regions, sites of biosignature preservation, and sites of potential biosignature emplacement on Europa.

The seafloor region could potentially be habitable, but that the ice-water interface could also be a chemically rich and potentially habitable interface. Oxidants from the surface could mix with reductants delivered from ocean currents. Fractures and diapirs could provide pathways to deliver material up or down. This could lead to several regions within or at the boundaries of the icy crust that could be habitable.

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Enceladus:

In 2005, the Cassini mission performed a close flyby of a small (500-kilometer diameter) moon of Saturn, Enceladus (see figure below), and made a remarkable discovery. Plumes of gas and icy material were venting from the moon’s south polar region at a collective rate of about 250 kilograms of material per second. Several observations, including the discovery of salts associated with the icy material, suggest that their source is a liquid water ocean beneath tens of kilometers of ice. Although it remains to be shown definitively whether the ocean is local or global, transient or long-lived, it does appear to be in contact, and to have reacted, with a rocky interior. Like Europa, Enceladus is an ice-covered moon with a subsurface ocean of liquid water. As on Europa, this is probably a necessary—though not sufficient—condition for habitability. What makes Enceladus so enticing to planetary scientists, though, are those plumes of material that seem to come directly from its ocean: samples of the interior are there for the taking by any spacecraft sent flying through. For a future mission, such samples could yield evidence not only of whether Enceladus is habitable but, indeed, of whether it is home to life.

Figure above shows image of Saturn’s Moon Enceladus from NASA’s Cassini Mission. The south polar region was found to have multiple plumes of ice and gas that, combined, are venting about 250 kilograms of material per second into space. Such features suggest that Enceladus, like Europa, has a sub-ice ocean.

Enceladus orbits Saturn and first came to the attention of scientists as a potentially habitable world following the surprise discovery of enormous geysers near the moon’s south pole. These jets of water escape from large cracks on the surface and, given Enceladus’ weak gravitational field, spray out into space. They are clear evidence of an underground store of liquid water. Not only was water detected in these geysers but also an array of organic molecules and, crucially, tiny grains of rocky silicate particles that can only be present if the sub-surface ocean water was in physical contact with the rocky ocean floor at a temperature of at least 90 degrees Celsius. This is very strong evidence for the existence of hydrothermal vents on the ocean floor, providing the chemistry needed for life and localised sources of energy.

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Titan:

Saturn’s big moon Titan is very different from both Enceladus and Europa (see figure below). Although it may host a liquid water layer deep within its interior, it is the surface of Titan and its unusual chemistry that makes this moon such an interesting place. Titan’s thick atmosphere—the only one among moons in the solar system—is composed mostly of nitrogen but also of about 5% methane. In the upper atmosphere, the Sun’s ultraviolet light breaks apart and recombines these molecules into more complex organic compounds that are collectively known as tholins. The tholins shroud Titan in an orange haze, and imagery from Cassini and from the Huygens probe that descended to Titan’s surface show that heavier particles appear to accumulate on the surface, even forming “dunes” that are cut and sculpted by flows of liquid hydrocarbons (such as liquid methane). Some scientists see this organic chemical factory as a natural laboratory that may yield some clues about the solar system’s early chemistry—perhaps even chemistry that could support the origin of life.

Figure above shows image of Saturn’s Moon Titan from NASA’s Cassini Mission.

(a) The hazy orange glow comes from Titan’s thick atmosphere (the only one known among the moons of the solar system). That atmosphere is mostly nitrogen but also contains methane and potentially a variety of complex organic compounds. The bright spot near the top of the image is sunlight reflected from a very flat surface—almost certainly a liquid. We see this effect, called “glint,” when sunlight reflects off the surface of a lake or ocean.

(b) Cassini radar imagery shows what look very much like landforms and lakes on the surface of Titan. But the surface lakes and oceans of Titan are not water; they are probably made of liquid hydrocarbons like methane and ethane.

Titan is thought to be a prebiotic environment rich in complex organic compounds, but its surface is in a deep freeze at −179 °C (−290.2 °F; 94.1 K) so it is currently understood that life cannot exist on the moon’s frigid surface. However, Titan seems to contain a global ocean beneath its ice shell, and within this ocean, conditions are potentially suitable for microbial life.

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Section-15

Search for extraterrestrial life on exoplanets: Life beyond solar system:

The list of exoplanets is increasing rapidly with a diversity of masses, orbital distances, and star types. The long list motivates us to consider which of these worlds could support life and what type of life could live there. The only approach to answering these questions is based on observations of life on Earth. Compared with astronomical targets, life on Earth is easily studied and our knowledge of it is extensive––but it is not complete. The most important area in which we lack knowledge about life on Earth is its origin. We have no consensus theory for the origin of life nor do we know the timing or location. What we do know about life on Earth is what it is made of, and we know its ecological requirements and limits. Thus, it is not surprising that most of the discussions related to life on exoplanets focus on the requirements for life rather than its origin.

Our understanding of life on exoplanets and exomoons must be based on what we know about life on Earth. Liquid water is the common ecological requirement for Earth life. Temperature on an exoplanet is the first parameter to consider both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Life needs some water, but deserts show that even a little can be enough. Only a small amount of light from the central star is required to provide for photosynthesis. Some nitrogen must be present for life and the presence of oxygen would be a good indicator of photosynthesis and possibly complex life.

Given the general requirements for life, the elemental composition of life, and the environmental limits for life, we can consider how to assess the habitability of the environment on an exoplanet. It may seem logical to focus on primary production because without that there cannot be an ecosystem. However, it is possible that photochemical processes in an exoplanet atmosphere play the role of primary production as has been suggested for Titan. Primary production means the synthesis of biological materials from inorganic carbon dioxide. Carbon dioxide is referred to as an inorganic compound because organic molecules don’t just contain carbon. They contain carbon bonded to hydrogen.

Many of the limits to life such as pH and salinity are unlikely to be extreme over an entire world. As on Earth they would shape the distribution of life on a world but not its possible occurrence and are therefore not considered further. The key parameters that could be extreme over an entire world and the order in which they may limit any life on an exoplanet are listed in Table below.

Checklist for habitability of an extrasolar planet:

Requirement	Note
1. Temperature and state of water	T between -15 °C and 122 °C, P > ∼0.01 atmospheres
2. Water availability	Few days per y of rain, fog, snow, or RH > 80%
3. Light and redox energy sources
4. UV and Ionizing radiation	Limits exemplified by D. radiodurans
5. Nitrogen	Enough N₂ for fixation or fixed nitrogen present
6. O₂	Over 0.01 atmospheres need to support complex life

The most important parameter for Earth-like life is the presence of liquid water, which directly depends on pressure and temperature. Temperature is key both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems.

As the number of known exoplanets and exomoons expands we will certainly find worlds that resemble the Earth to varying extent. Based on our understanding of life on Earth we can present a checklist for speculating on the possibilities of life on these distant worlds. (i) Is the temperature between −15 °C and 122 °C, and a total pressure high enough to keep water liquid water stable (P > ∼0.01 atmospheres)? (ii) If the world is arid, are there at last a few days per year of rain, fog, snow, or RH > 80%? (iii) Are there adequate light or geothermal energy sources––light determined by distance from the star, geothermal energy estimated by bulk density? (iv) Are the UV and ionizing radiation below the (very high) limits of microbial tolerance? (v) Is there a biologically available source of nitrogen? (vi) If O2 is present at over 0.01 atmospheres there could be complex life, and the presence of O2 is convincing indicator of photosynthetic life on Earth-like worlds.

At 4.2 light-years (1.3 parsecs, 40 trillion km, or 25 trillion miles) away from Earth, the closest potentially habitable exoplanet is Proxima Centauri b, which was discovered in 2016. This means it would take more than 18,264 years to get there if a vessel could consistently travel as fast as the Juno spacecraft (250,000 kilometers per hour or 150,000 miles per hour). It is currently not feasible to send humans or even probes to search for biosignatures outside of the Solar System. The only way to search for biosignatures outside of the Solar System is by observing exoplanets with telescopes.

There have been no plausible or confirmed biosignature detections outside of the Solar System. Despite this, it is a rapidly growing field of research due to the prospects of the next generation of telescopes. The James Webb Space Telescope, which launched in December 2021, will be a promising next step in the search for biosignatures. Although its wavelength range and resolution will not be compatible with some of the more important atmospheric biosignature gas bands like oxygen, it will still be able to detect some evidence for oxygen false positive mechanisms.

The new generation of ground-based 30-meter class telescopes (Thirty Meter Telescope and Extremely Large Telescope) will have the ability to take high-resolution spectra of exoplanet atmospheres at a variety of wavelengths. These telescopes will be capable of distinguishing some of the more difficult false positive mechanisms such as the abiotic buildup of oxygen via photolysis. In addition, their large collecting area will enable high angular resolution, making direct imaging studies more feasible.

An interferometer is used to study interference effects between two or more waves. A spectrometer is used to study the array of wavelengths, i.e. the “spectrum,” emitted by a single source of waves. Spectrometry measures the absorbance or fluorescence of compounds as a function of wavelength. What is measured in an interferometer is the shift in fringes produced when light is made to travel different distances and recombined. To extend our search for life to extrasolar planetary systems, we must rely on infrared-based remote sensing technology to search for key molecules like water and chemically incompatible gases such as methane, carbon dioxide, and ozone in the atmospheres of extrasolar planets. But, the intense, blinding light from the host star presents a difficult obstacle for remote sensing instrumentation. This obstacle can be overcome, however, using interferometer techniques. With coordinated telescopes working in tandem and broadband destructive interference methods, the central starlight could be blackened out, or nulled, while leaving the dim reflected planet’s light unaffected. Of course, internal occulters (i.e., coronagraphs) and external occulters (i.e., starshades) are technologically mature methods to occlude glare from star.

To carry out spectral analyses of the atmospheres of extrasolar planets, infrared interferometer telescopes would need to be space-based systems. To reduce the background from our solar system’s zodiacal light, it may be necessary to make these observations at orbital distances much greater than at near Earth orbit. All of this is very, very expensive, with price tags approaching the $500 million range, but it should be worth it. A space-based infrared interferometry system looking at our solar system from a distant star would be able to detect most of the planets in our solar system, including Earth, and determine the chemical composition of their atmospheres. Just like the Galileo spacecraft observed when it passed by Earth, finding an extrasolar planet with an atmosphere containing both ozone (and by implication oxygen) and methane would be an indication that not only does life exist there, but that it has likely had a long evolutionary history. Finding an Earth-like extrasolar planet with an atmosphere rich in methane and water would suggest the possibility for the prebiotic chemistry needed to set the stage for the origin of life. Futuristic space-based interferometers consisting of perhaps a 100 or more coordinated telescopes might even be able to determine the type of organic material present on the surfaces of promising extrasolar planets.

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Exoplanet detection:

According to NASA estimates there are at least 100 billion stars in the Milky Way, of which about 4 billion are sunlike. If only 7 percent of those stars have habitable planets — a seriously conservative estimate — there could be as many as 300 million potentially habitable Earths out there in the whole Milky Way alone.

On the spectrum of stars, our sun is what’s known as a yellow dwarf. It’s bright, and not terribly large compared to the largest stars in our galaxy. Yet, even middling stars like our sun aren’t all that common. Our local stellar neighborhood — and probably the universe as a whole — is filled with many more low-mass stars. There are 20 yellow dwarf stars like our sun nearby and 250 M-dwarfs, a variety of star so small and dim that, despite their abundance, they can’t be seen with the naked eye. Over last few years, every single low-mass star we’ve studied appears to have at least one planet. Usually, they have more than one. For a typical M-dwarf, there tends to be 2.5 planets. One in four of the stars has a planet the same size and temperature as Earth in the habitable zone. Given the number of M-dwarfs in the local region, there should be at least 60 potentially Earth-like planets in habitable zones within 32 or so light-years from here, and perhaps many more. To date, most of our exoplanet data comes from NASA’s Kepler spacecraft, which has focused its search for planets on large M-dwarf stars. In the near future, when the small and medium-sized M-dwarfs are studied, we may discover that closer to one in three stars have an Earth-like planet in the habitable zone. Apart from their abundance, these low-mass stars offer other advantages to researchers who study potentially habitable exoplanets. M-dwarf planets have tight orbits around their stars because the habitable zones are close in, giving scientists opportunities to view their transits every few weeks. It is during these transits, when the exoplanets pass in front of their stars, that we have the best opportunity to study their atmospheres for signs of life. Because the rise of oxygen in Earth’s atmosphere corresponded with the appearance of life, we frequently use that particular molecule as a marker for the presence of life elsewhere. Also, oxygen likes to interact with other chemicals. If we discover a planet where oxygen is still hanging around in the atmosphere, something, possibly life, is actively making it. So, the search for life will focus on elements and molecules like hydrogen, oxygen, and methane. However, there is a downside to this approach. A planet’s atmosphere is only 1 percent the size of the planet. The size of the signal is tiny. You need to collect at least one trillion photons to be very certain that you are truly looking at oxygen. The good news is that a new generation of telescopes designed for planetary exploration and astrobiology will help us gather those photons.

Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets have been observed directly, with even fewer being resolved from their host star. Instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. As of 2016, several different indirect methods have yielded success as seen in the figure below.

Since the Nobel-Prize discovery, planets have been found everywhere. The two most successful methods are the radial velocity and transit techniques. The first measures the reflex motion of a star around a system’s center of mass. This can reveal a planet’s orbit, and a lower limit to its mass (depending on the orientation of the orbit). For the second method, the orientation of the planet’s orbit needs to be nearly edge-on. The planet then regularly occults a part of the star, resulting in a small dip in the amount of starlight that we see. This reveals the size and orbit of the planet, and in combination with the radial velocity method, also its mass and mean density – providing first clues about its composition.

Studying the atmospheres of exoplanets is a whole different ball-game. Almost all planets found to date have been discovered without identifying a single photon from the planets themselves. For atmospheric studies this can no longer be the case – planet light needs to be separated from that of the star, with the latter being many orders of magnitude brighter. There are basically two families of methods to accomplish this. The first has so far been the most successful and involves transiting systems, making use of temporal variations in how we see the planet. When a planet transits a star, in addition to occulting part of the stellar surface, starlight also filters through the planet atmosphere, leaving an imprint of atomic and molecular absorption and scattering. In addition, half an orbit later, the planet is occulted by the star, meaning that for a few hours the planet’s light (either intrinsic thermal emission or starlight reflected off the planet’s atmosphere) is missing and can be accounted for. Also during the rest of the orbit, varying parts of the dayside and nightside of the planet are visible, resulting in variations that reveal its heat distribution which can constrain global climate circulation models.

Making direct images of exoplanets, by angularly separating the planet from the star in the sky, is still very difficult. Ground-based telescopes require adaptive optics to mitigate the disturbing influence of our atmosphere, to approach their theoretical angular resolution. This is needed in combination with coronagraphy to block the star light such that a faint orbiting planet can be seen. Telescopes in space have the advantage not to need the former, but are costly, and are therefore smaller and have significantly less resolution, limiting their performance.

Most atmospheric characterization has been limited to warm gas giant planets, which have the largest atmospheric scale heights and therefore strongest transit signals, and emit most thermal emission. For direct imaging this needs to be accompanied by a large enough orbital distance to assure angular separation from their host stars so that young gas giants, which are still hot from their formation, can be probed. So far, several different molecules, such as carbon monoxide and water have been identified with both families of methods, in addition to several atoms and ions, and evidence for Rayleigh scattering and the presence of clouds and hazes. Also, vertical and longitudinal temperature structures have been measured, including thermal inversions and atmospheric escape processes.

Figure above shows schematic relation between the composition and structure of an exoplanet atmosphere and the observed Transmission spectrum. Starlight travels through the planet atmosphere and is being scattered by molecules at certain wavelengths (top), is largely unaltered because of the small scale height of the atmosphere (middle), or absorbed by clouds (bottom).

Detecting terrestrial planets:

The transit and radial velocity techniques have detected the bulk of the known exoplanets thus far. Although both techniques will continue to add more terrestrial candidates around M type stars, the radial velocity technique is the most likely technique to detect Earth-like planets around G type stars, needed to form the target list for the Habitable Worlds Observatory (HWO) mission. This will require measuring Star’s Radial Velocity due to the planet of <50 cm/s and more likely 10-20 cm/s. And if the Earth-like planet is in the habitable zone, this velocity change would occur over half a year, so the rate of change is extremely small.

There have been over 5400 exoplanets confirmed to date. Figure below shows the mass vs orbital period distribution of the known planets. Data is color coded to highlight the detection technique used.

Figure above shows 5400+ confirmed exoplanets as of the 11th of August 2023 on a Jupiter mass vs orbital period plot. The Earth is shown on the plot to indicate where an Earth-like planet around a sun-like star would appear. The blue region highlights the terrestrial regime. Very few Earth mass/size planets have been detected thus far. Only a handful are in the habitable zone of their host star. To detect extraterrestrial life, we must first detect more terrestrial like planets in the habitable zones of their host stars. In particular, finding planets around solar type stars (G stars) is critical for studying systems similar to our own to understand our place in the Universe. Spectroscopy of the exoplanet atmosphere is critical to revealing a wealth of information (composition and abundance, spin rate, weather patterns, etc). The field of exoplanetary sciences is now focusing efforts on characterization of these systems by for example providing direct exoplanet spectroscopy. By understanding more about the known exoplanets, we can refine planetary formation and evolution models and better understand where life is likely to exist.

Finding an analog of Earth:

A prime target for all these methods is one of the most sought-after and elusive exoplanets of all: an Earth-sized world orbiting a Sun-like star, with a “year” – an orbit – comparable to our own. It might seem puzzling that, with the head-spinning variety among the thousands of exoplanets confirmed so far, a world checking all these boxes still hasn’t turned up. But our inability to find such a world is not so mysterious when you consider the technology we have at our disposal. The telescopes and the instruments we attach to them, both in space and on the ground, have made astonishing progress since the early days of the 1990s. They’ve also run up against stubborn limits.

Exoplanets tens or hundreds of light years away are usually much too dim to see, lost in the glare of their stars. Light-blocking technology might one day overcome this barrier, but – aside from those young, self-luminous planets – hasn’t done so yet.

Space telescopes already are powerful enough to pick up transits by Earth-sized planets around Sun-like stars. But they would have to wait far too long to confirm planets with long-period orbits. If the planet has a year comparable to Earth’s, for instance, they’d have to wait on the order of 365 days to see a second transit. That turned out to be out of reach for the history-making Kepler space telescope, and none of the telescopes launched since then can do so, either.

Many small, rocky planets in Earth’s size range have been discovered, like the seven roughly Earth-sized planets orbiting a star called TRAPPIST-1. But all found so far orbit red-dwarf stars – smaller, cooler versions of our own Sun. While some of these planets might be habitable – though very close to their stars, the lower temperature could allow water to pool on their surfaces – their “years” are typically only a few days long. Red dwarfs also have a bad habit of erupting with potentially sterilizing flares, especially in their younger years. That could be a disqualifying feature for the habitability of closely orbiting planets, like moths flying too near the flame.

The strangest planets, perhaps, are those we see elsewhere that are not found in our system. “Super-Earths,” or planets as much as 1.8 times as big around as Earth, appear to be fairly common in the galaxy. Are they scaled-up, rocky worlds, like giant Earths. The term “super-Earth” is also used by astronomers to refer to planets bigger than Earth-like planets (from 0.8 to 1.2 Earth-radius), but smaller than mini-Neptunes (from 2 to 4 Earth-radii). Another type, often called “mini-Neptunes,” are probably about what they sound like: gaseous worlds smaller than our own Neptune. We’re also mystified by what’s not out there. Between these two size ranges – super-Earths and mini Neptunes – seems to be a kind of demographic desert: very few planets. It’s been called the “Fulton gap,” after B.J. Fulton, a scientist who outlined their absence in a 2017 paper.

Figure below depicts exoplanet types:

The qualifications for a planet to be a candidate for alien life:

To start, scientists look for exoplanets orbiting a sun-like star, similar to how Earth orbits our sun. In the 1990’s, NASA launched the Kepler Space Telescope, which allowed scientists for the first time to find exoplanets orbiting stars. Even though we only have small glimpses of exoplanets in the Milky Way, scientists estimate that up to 50% of stars might have an orbiting exoplanet, which still leaves billions of candidate exoplanets.

After identifying exoplanets that orbit a sun, scientists must then narrow potential candidates for exoplanets that are also orbiting their suns within a habitable zone. If a planet orbits too close to a sun, the proximity causes extreme heat on the planet’s surface. Conversely, orbiting too far from a sun causes extreme cold temperatures. Thus, a habitable zone is just right: not too close, but not too far either. This allows for moderate temperatures that can sustain liquid water, which as we know from evolution on Earth, is integral for life.

To find out more about an exoplanet candidate, scientists use telescopes to discover what gases make up the distant planets’ atmospheres and whether they are compatible with life. For example, we know that, on Earth, life can exist in an atmosphere primarily made up of nitrogen and oxygen but too much carbon dioxide is dangerous. The atmospheres of exoplanets can be probed using a technique called spectroscopy.

At first thought, you may think to look only for exoplanets with nitrogen- and oxygen-rich atmospheres like Earth, but scientists are also interested in finding hydrogen-dominated atmospheres. This may seem odd since there are only trace amounts of hydrogen in Earth’s atmosphere, but scientists have good reasons. Hydrogen gas is really light, so it forms a fluffy atmosphere that can be more easily probed for other evidence of life. Think of it this way: hydrogen is light like cotton balls, but heavier gases like nitrogen are dense like marbles. It is much easier to move through and find a tiny object in a room full of cotton balls than marbles. This other evidence of life is actually more gases called ‘biosignature’ gases. Biosignature gases, like ammonia, nitrous oxide, and oxygen are gases that can only be made from past or present organisms. In other words, they are the clues that a living being is living or has lived on that planet. These biosignature gases might make up only a tiny portion of total gases, which would explain why denser, heavier gases would make it harder to detect them.

Figure below depicts atmospheric density in the search for life.

Planet A (left) has a dense atmosphere (shown in blue) made up of heavy gases, which obscures the biosignature gases (blue circles). In contrast, Planet B’s (right) atmosphere has a less dense atmosphere made up of lighter gases like hydrogen. Light atmospheres can be probed more easily for biosignature gases produced by microbes.

While powerful, our current telescopes only allow for a glimpse of exoplanet atmospheres. Given these capabilities, some scientists are interested in identifying planets with hydrogen-dominated atmospheres, a feature that can be gleaned with the current technology. However, what’s the point of probing hydrogen-dominated atmospheres? If we humans cannot survive in hydrogen-dominated atmospheres, can any life live under these harsh atmospheres? It turns out that certain microbes can.

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Habitable conditions could exist on planets so different from Earth:

The Cambridge researchers identified a new class of exoplanet, called Hycean planets, that, much like Earth, are covered in oceans and have atmospheres rich with hydrogen, an element that is essential for life. Many of the planets are “bigger and hotter than Earth”— up to 2.6 times larger than our planet and reaching atmospheric temperatures up to nearly 200 degrees Celsius, or 392 degrees Fahrenheit. Hyceans are further categorized as either “dark” or “cold,” with dark worlds only having habitable conditions on their permanent night sides and cold worlds receiving just a little radiation from the stars they orbit. But researchers believe that they could support microbial lifeforms that are similar to those found in the extreme aquatic environments on Earth, and that Hycean planets are likely common throughout space. These planets are numerous and, thanks to their size, easier to find than small, rocky, Earth-like planets. They are also thought to be quite warm, making it possible for microbial life to thrive beneath their surface. Hycean worlds are named after their two vital features: hydrogen atmospheres (whence the hy-) and oceans (hence the -cean).

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Kepler system:

The diagram below made by NASA very briefly compares the new habitable zone discovered by the Kepler spacecraft and our solar system. The zone may have life beyond Earth.

Kepler spots a planet when it comes between its cameras and the parent star, a phenomenon called a transit. It takes at least three transits of a planet for the scientists to announce a planet candidate, which is then confirmed through other observations. For a planet that has a revolutionary period of one year like the Earth, it takes three years for three transits. Kepler gazes at only one portion of the sky, but its eyes can spot objects as far away as 3,000 light years. Since the galaxy is 25,000 light years across, we are still looking at a small part of the sky. And Kepler cannot find out the composition of the planets; we need other methods for this.

A Kepler object of interest (KOI) is a star observed by the Kepler space telescope that is suspected of hosting one or more transiting planets. While it has been estimated that 90% of the KOI transit candidates are true planets, it is expected that some of the KOIs will be false positives, i.e., not actual transiting planets. The majority of these false positives are anticipated to be eclipsing binaries which, while spatially much more distant and thus dimmer than the foreground KOI, are too close to the KOI on the sky for the Kepler telescope to differentiate. On the other hand, statistical fluctuations in the data are expected to contribute less than one false positive event in the entire set of 150,000 stars being observed by Kepler.

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Characteristics of super-habitable exoplanets:

The exoplanets may have life outside of Earth and enter the list of possible super-habitables. They have at least some of the characteristics sought by researchers to define a planet with acceptable conditions, which are, in short:

They are in orbit around a K-class dwarf star;
Estimated age between 5 and 8 billion years;
Up to 1,6 times more massive and 10% larger than Earth;
Up to 5°C higher average surface temperature;
Moist atmosphere with up to 30% oxygen and the other 70% inert gases;
Roughly equivalent division of land areas and water area;
Large moon that is at a medium distance;
It has plate tectonics as well as a strong magnetic field.

Well, even if we can’t dream too high about fictional delusions, it’s interesting to know that there are planets so far away that they can contain life beyond Earth. Possibly neither you nor me nor our great-grandchildren will live to see any of them in more detail.

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Earth-like world:

Though there are some basic requirements for a world to be considered Earth-like, overall, it’s difficult to narrow down a definition. We know that mass is key when it comes to identifying Earth-like worlds: Some research suggests that objects just 1.6 times the size of Earth are more likely to be gaseous, making them less similar to our planet. Usually astronomers categorize planets by how massive they are. There’s no category below Earth, so even if there’s something Mercury-sized, we would still call that an Earth. Mass is critical to spotting worlds like ours, but there are other factors to consider, such as a star’s habitable zone: the not-too-hot, not-too-cold region where liquid water can exist on a planet’s surface. That’s where things get a little more nuanced. The world should be able to host water and have an atmosphere — not as thin as Mars, but not a puffy, Neptune-like atmosphere. Even with those guidelines, there’s a gray area. For example, some researchers would argue that a planet can’t be Earth-like unless it orbits around a star similar to ours, that is, a G-type main sequence star. Unpacking what it truly means to be “Earth-like” evokes a host of scientific and philosophical questions. Although defining that term is pretty complicated, there are some worlds that appear to fit the description, or at the very least, parts of it.

Earth-like worlds of interest:

The first exoplanets — worlds orbiting stars besides our own — were discovered in the 1990s. In the grand scheme of, well, everything, that’s very recent. It’s impressive how much scientists have been able to uncover about these distant worlds in such a short time. With the help of ground and space-based telescopes, researchers have found about 5,000 confirmed exoplanets and thousands of other candidates. A relatively small number of these worlds bear similarities to Earth.

Some of them are:

-1. KEPLER–186F

In 2014, NASA’s Kepler Space Telescope discovered the first Earth-sized world in the habitable zone of another star. The planet, dubbed Kepler-186f, is located within the Kepler-186 system about 500 light-years from Earth. While Kepler-186f receives only one-third as much energy from its star as Earth does from the sun, it is at most 10% larger than Earth and looks to be located in the habitable zone of its star, albeit on the zone’s periphery. The red dwarf parent star of Kepler-186f prevents the alien world from being a real Earth twin.

-2. KEPLER-452B

NASA considers exoplanet Kepler-452b and its star to be the closest analog to our planet and Sun so far. Though it’s 60% larger than Earth in diameter, Kepler-452b is thought to be rocky and within the habitable zone of a G-type star similar to ours. The parent star of Kepler-452b is 10 percent bigger than the sun. The exoplanet Kepler-452b orbits in the habitable zone and shares many similarities with our earth.

The planet is about 1,800 light-years (550 pc) away from the Solar System. At the speed of the New Horizons spacecraft, at about 59,000 km/h (16,000 m/s; 37,000 mph), it would take approximately 30 million years to get there. So what makes Kepler-452b so great? NASA called it “Earth 2.0” for a reason. It’s a “Goldilocks planet,” meaning it sits in the habitable zone of its star, where the temperatures are not too hot or cold for liquid water to form. Kepler-452b is the most Earth-like planet ever discovered, a place with just enough sunlight to possibly support the crops and house plants of life forms like ourselves.

-3. TRAPPIST-1

Located about 40 light-years away, the TRAPPIST-1 system has quickly — and rightfully — garnered a lot of interest. Not only do all seven planets in the system appear to be Earth-sized and rocky, three of them are located in the habitable zone of their star. While liquid water could theoretically pool on TRAPPIST-1e, f and g, the other planets are probably too close or far from their sun to be in this “goldilocks zone.” While TRAPPIST-1 hosts some interesting prospects, its planets are not Earth’s long-lost twins. Some research based on computer modeling suggests even the habitable TRAPPIST-1 planets may have developed like Venus, making them too hot to host water. TRAPPIST-1e may be the only planet in the system still hospitable to life, but without more data, it’s impossible to confirm.

-4. GLIESE 667CC

According to NASA’s Jet Propulsion Laboratory, this exoplanet is at least 4.5 times as big as Earth and is only 22 light-years away. Although Gliese 667Cc completes one orbit of its host star in just 28 days, the exoplanet is believed to be in the habitable zone because its star is a red dwarf that is much cooler than the sun. Gliese 667Cc, which was found using the 3.6-meter telescope of the European Southern Observatory in Chile, may circle too closely to the red dwarf, however, and could be roasted by flares.

-5. KEPLER 22B

Kepler-22b’s 290-day orbit is fairly similar to Earth’s 365-day orbit. The exoplanet orbits a G-class star, which is similar to our sun but smaller and colder than the sun that the Earth orbits.

-6. KEPLER-69C

The planet orbits its sun once every 242 days, placing it in a position similar to Venus’ in our solar system. The host star of Kepler-69c is around 80% as bright as the sun, hence the planet seems to be in the habitable zone. Kepler-69c is about 2,700 light-years away and is over 70% bigger than Earth. As a result, scientists are once again uncertain about its composition.

-7. KEPLER-62 F

According to NASA, this planet is about 40% bigger than Earth and revolves around a star that is significantly cooler than the sun. However, Kepler-62f is directly within the habitable zone thanks to its 267-day orbit. Kepler-62 orbits its red dwarf star more closely than the sun does to the sun, although the star emits much less light. Given its distance of 1,200 light-years and size, Kepler-62f is in the neighborhood of potentially rocky planets that might have oceans.

-8. KEPLER-442 B

According to NASA, this exoplanet is 33 percent bigger than Earth and revolves around its star every 112 days. 2015 saw the announcement of the discovery of Kepler-442, a star 1,194 light-years from Earth. One study discovered that this exoplanet might get enough light to support a substantial biosphere, which was published in the Monthly Notices of the Royal Astronomical Society in 2021. The possibility that several planets would be able to perform photosynthesis was examined by the researchers. They discovered that the radiation Kepler-442b receives from its star is adequate.

-9. KEPLER-1649 C

Scientists found Kepler 1649 c after reanalyzing data from NASA’s Kepler space telescope. It was discovered that the exoplanet orbited in its star’s habitable zone and was around the same size as Earth. According to NASA, a computer algorithm mistakenly recognised the astronomical body during the initial data collection from the telescope, but in 2020 it was found to be a planet.

-10. PROXIMA CENTAURI B

According to NASA Exoplanet Exploration, Proxima Centauri b is the closest exoplanet to Earth and is only four light-years away. The exoplanet’s mass is 1.27 times greater than Earth’s. It was found in 2016. The exoplanet is exposed to intense UV radiation while being near the Proxima Centauri star’s habitable zone. This is due to its short orbital period of 11.2 days and near proximity to its parent star.

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Milky Way could be teeming with ocean worlds:

Ocean worlds, which are classified as those having significant amounts of water on or just beneath their surfaces, are surprisingly common in the solar system. Earth is obviously one such place, but Jupiter’s moon Europa is thought to host vast seas under its icy shell and Saturn’s moon Enceladus is known to have watery geysers spewing from its exterior. Momentum is in fact building in the astronomy community to send a probe that could land on either satellite sometime in the 2030s and check if any living things might lurk under their shells. As for ocean worlds beyond our sun, in a study released recently, researchers looked at 53 exoplanets similar in size to Earth and analyzed variables including their size, density, orbit, surface temperature, mass and distance from their star. The scientists conclude that, of the 53, roughly a quarter might have the right conditions to be considered ocean worlds, suggesting that such places could be relatively common in the galaxy.

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Life on exomoons:

We only know of one planet in our universe that harbours life: Earth. So it’s logical that humanity has focused its search for life beyond our world on planetary bodies. But a new study suggests that we may be leaving out a larger chunk of the cosmos: moons. In a new study published in The Astrophysical Journal, researchers suggest that there are 121 giant exoplanets — worlds in orbit around other stars — that could host a potentially habitable moon. Using data from NASA’s Kepler space telescope, which has confirmed thousands of exoplanets, scientists modelled how frequent giant, gas-like planets — like Jupiter and Saturn — might also form in a star’s habitable zone, a region where liquid water can exist on a planet’s surface. From there, the researchers determined how many of those planets might have just one moon, arriving at 121. While we get energy from the sun, scientists believe that exomoons could get energy not only from its star, but also from the giant exoplanet. This, they say make them potentially “super-habitable.” The idea of planets providing additional energy to their moons comes from observations in our solar system: Jupiter and Saturn do just that for their moons. Some moons have an elliptical orbit (not quite circular) around their planet that causes a gravitational interaction. The moon is squeezed as it goes between being closer and then further away. This, called tidal flexing, causes heating. It’s much the same way if you took a stress ball and kept squeezing it: it would generate some heating (not just from the heat of your hand). Also, as is the case with the moons of Jupiter and Saturn, an exoplanet’s magnetism could protect moons from cosmic radiation, thus increasing its potential habitability.

While the finding life on exomoons is a possibility, the reality is, it’s going to take time. For one, we haven’t been able to detect any biosignatures — signs that a something possesses the ingredients for life — on any exoplanets thus far. In the search for exoplanets, scientists look for small dips in a star’s light that indicates a planet is crossing it, called a transit. But a moon would be much smaller and more difficult to detect in this manner. Observations of exomoons in the search for life is very challenging, because the radiation coming from the exomoon will be mingled with the planet. The combined light of the exomoon and planet is already dwarfed by the light of the host star. Also, we haven’t even confirmed the existence of an exomoon. Just one exoplanet holds any promise at all, Kepler 1625b, which lies 4,000 light-years from Earth. But in a study published in 2019, lead author Rene Heller, an astrophysicist at the Max Planck Institute in Germany, concluded that the jury was still out.

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Section-16

Odds of finding extraterrestrial life:

Most astronomy-related astrobiology research falls into the category of extrasolar planet (exoplanet) detection, the hypothesis being that if life arose on Earth, then it could also arise on other planets with similar characteristics. To that end, a number of instruments designed to detect Earth-sized exoplanets have been considered both on ground and in space. The goal of these missions is not only to detect Earth-sized planets but also to directly detect light from the planet so that it may be studied spectroscopically. By examining planetary spectra, it would be possible to determine the basic composition of an extrasolar planet’s atmosphere and/or surface. Given this knowledge, it may be possible to assess the likelihood of life being found on that planet.

Ever since humans acknowledged the enormity of the universe, we have intuited that life must exist somewhere, either in our galaxy or some galaxy far, far away. If the universe contains billions of galaxies, and if each galaxy contains billions of stars, and if a fraction of those stars have Earth-like planets, then hundreds — maybe even thousands — of alien civilizations must exist across the cosmos. For a while, science contented itself with the logic alone. Then, in 1995, astronomers located the first planets outside our solar system. Since then, they’ve detected nearly 5000 of these extra-solar planets. Although most are large, hot planets similar to Jupiter (which is why they’re easier to find), smaller, Earth-like planets are beginning to reveal themselves. These discoveries have served as an affirmation for those involved with the search for extraterrestrial intelligent life, or SETI. Harvard physicist and SETI leader Paul Horowitz boldly stated in a 1996 interview with TIME Magazine, “Intelligent life in the universe? Guaranteed. And yet his enthusiasm must be tempered by what scientists call the Fermi Paradox. This paradox, first articulated by nuclear physicist Enrico Fermi in 1950, asks the following questions: If extraterrestrials are so common, why haven’t they visited? Why haven’t they communicated with us? Or, finally, why haven’t they left behind some residue of their existence, such as heat or light or some other electromagnetic offal?

Intelligent extraterrestrial life is uncommon:

Remember, only the existence of intelligent beings is relevant. Primitive life may yet be discovered on Mars; perhaps even multicellular animals will be found on a nearby extrasolar planet. These revolutionary discoveries would help us reconstruct how life on Earth evolved, but unless a species that is capable of conscious, independent thought and has the ability to communicate exists, we will still be alone—with no one to teach or learn from, no one to save us from ourselves (and no one to battle against). Intelligent life, for the purposes of this discussion, means life able to communicate between stars; this implies having something like radio technology. Our own society, by this definition, is only about 100 years old. If intelligent life is common in a universe that is 13.8 billion years old, then surely we are among the youngest forms in existence. As the physicist Enrico Fermi famously observed, however, the fact that there is no other known intelligent life indicates that the assumption is wrong—intelligent life is not common. Cosmologist Paul Davies explores this absence in detail in his 2010 book, The Eerie Silence: Renewing Our Search for Alien Intelligence.

Perhaps extraterrestrial life isn’t so common after all. Or perhaps extraterrestrial life that gives rise to advanced civilizations isn’t so common. If only astronomers could quantify those odds. If only they had a formula that accounted for all of the right variables related to extraterrestrial life. As it turns out, they do. In 1961, as a way to help convene the first serious conference on SETI, radio astronomer Frank Drake presented a formula, now known as the Drake Equation, that estimates the number of potential intelligent civilizations in our galaxy. The formula has generated much controversy, mainly because it leads to widely variable results. And yet it remains our one best way to quantify just how many extraterrestrials are out there trying to communicate.

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy. The equation was formulated in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI). The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. It is more properly thought of as an approximation than as a serious attempt to determine a precise number.

Trying to calculate the probability that extraterrestrial life exists in the universe is actually quite complicated. The universe isn’t a static environment. Stars are born, they live and they die. Some stars form in association with planets. Others don’t. Only some of those planets have the right conditions to support life. Life is a tricky variable in its own right. Some planets might support complex organic molecules — proteins and nucleic acids — and nothing else. Other planets might support simple, single-celled organisms. And still others might support multicellular organisms, including those advanced enough to develop the technologies to travel or send signals into outer space. Finally, even organisms that have adapted extremely well to their environments don’t last forever. As both the dinosaurs and the Roman Empire illustrate here on Earth, all dynasties come to an end, be it cataclysmic or otherwise.

Frank Drake had to account for all of these variables in developing a formula to quantify the odds of finding extraterrestrial life. His first task was deciding what he wanted to calculate. First, he limited his thinking to extraterrestrials in our home galaxy — and only those that might be capable of interstellar communication. Then he inserted a mathematical factor to account for all of the conditions required to enable such civilizations to evolve. The result is the following formula:

N = R × fp × ne × fl × fi × fc × L

Where:

– N denotes the count of civilisations within the Milky Way with whom interaction could occur.

– R stands for the average pace of star formation in our galaxy.

– fp represents the proportion of stars hosting planetary systems.

– ne signifies the average number of potentially habitable planets per star with planetary systems.

– fl denotes the fraction of planets where life emerges.

– fi represents the fraction of life-bearing planets that evolve intelligent life.

– fc signifies the fraction of intelligent civilisations that acquire the capability to communicate across interstellar spaces.

– L symbolises the timespan over which such civilisations can sustain communication.

Each element in the equation represents a crucial factor contributing to the overarching estimation of communicative extraterrestrial civilisations. The assigned values to these elements can considerably vary, reflecting the current extent of scientific knowledge and comprehension.

The only variable known with any degree of certainty is the rate of stellar formation, R. In the Milky Way, a typical spiral galaxy, new stars form at a rate of roughly four per year. The variable astronomers feel most uncertain about is L, the length of time a civilization remains detectable. A variety of estimates have been used for L, ranging from 10 years to 10 million years.

Astronomers can make educated guesses about the rest of the variables. For example, of the nine planets in our solar system, only four are what astronomers call terrestrial planets — those that have a solid surface. Of those terrestrial planets, only Earth supports life. If we take our solar system as representative, then we might argue that ne equals 1/4 or 0.25. Similar guesses have been made about the other variables and, interestingly, they all end up having very similar values, usually in a range between 0.1 and 1.0. So, a typical calculation might look like this:

N = 4 x 0.5 x 0.25 x 0.2 x 0.2 x 0.2 x 3,000,000

which gives us a value of 12,000 civilizations in our galaxy.

Drake’s original calculations were very close to this value for N. When he ran the numbers, he predicted that there might be 10,000 detectable civilizations in the Milky Way. Carl Sagan, a leader in the SETI movement until he passed away in 1996, was even more generous when he suggested that 1 million civilizations might exist in the galaxy.

No wonder astronomers were so optimistic when they started searching diligently for extraterrestrial life in the 1960s.

Armed with an estimate of the number of communicative civilizations in our galaxy, SETI scientists set out to find them. They had two basic options: face-to-face communication or long-distance communication. The former scenario required that extraterrestrials visit humans or vice versa. This seemed highly unlikely given the distances between our solar system and other stars in the Milky Way. The latter scenario involved radio broadcasts, either sending or receiving electromagnetic signals through space.

In 1974, astronomers intentionally transmitted a 210-byte message from the Arecibo Observatory in Puerto Rico in the hopes of signaling a civilization in the globular star cluster M13. The message contained fundamental information about humans and our corner of the universe, such as the atomic numbers of key elements and the chemical structure of DNA. But this sort of active communication has been rare. Astronomers mostly rely on passive communication — listening for transmissions sent by alien civilizations.

A radio telescope is the tool of choice for such listening experiments because it’s designed to detect longer-wavelength energy that optical telescopes can’t see. In radio astronomy, a giant dish is pointed to a nearby, sunlike star and tuned to the microwave region of the electromagnetic spectrum. The microwave frequency band, between 1,000 megahertz and 3,000 megahertz (MHz), is ideal because it’s less contaminated with unwanted noise. It also contains an emission line — 1,420 MHz — that astronomers can hear as a persistent hiss across the galaxy. This narrow line corresponds to energy transformations taking place in neutral hydrogen. As a primordial element of the universe, hydrogen should be known to all intergalactic civilizations, making it an ideal marker. Several teams from around the world have been systematically listening to stars across the Milky Way and adjacent galaxies since 1960.

Despite their collective efforts, no SETI search has received a confirmed, extraterrestrial signal. Our telescopes have picked up a few unexplained and intriguing signals, such as the so-called “Wow” signal detected by researchers at Ohio State University in 1977, but no transmission has been repeated in such a way that it provides indisputable evidence of extraterrestrial life. All of which brings us back to the Fermi Paradox: If thousands of civilizations in the Milky Way galaxy, why haven’t we detected them?

Since Drake and Sagan made their estimates, astronomers have become more conservative. Paul Horowitz, who boldly guaranteed the existence of extraterrestrial life, has generated more modest results from the Drake Equation, finding that N may be closer to 1,000 civilizations. But even that figure may be too large.

In 2002, Skeptic magazine publisher Michael Shermer argued that astronomers weren’t being critical enough in their evaluation of L, the length of time a civilization remains detectable. Looking at 60 civilizations that have existed on Earth since the dawn of humanity, Shermer came up with a value for L that ranged from 304.5 years to 420.6 years. If you plug these numbers into the Drake Equation, you find that N equals 2.44 and 3.36, respectively. Tweak the numbers some more, and you can easily get N to fall to one or even lower. Suddenly, the odds of hearing from an extraterrestrial life form are considerably lower.

The Drake equation has been used by both optimists and pessimists, with wildly differing results. The first scientific meeting on the search for extraterrestrial intelligence (SETI), which had 10 attendees including Frank Drake and Carl Sagan, speculated that the number of civilizations was roughly between 1,000 and 100,000,000 civilizations in the Milky Way galaxy. Conversely, Frank Tipler and John D. Barrow used pessimistic numbers and speculated that the average number of civilizations in a galaxy is much less than one. Almost all arguments involving the Drake equation suffer from the overconfidence effect, a common error of probabilistic reasoning about low-probability events, by guessing specific numbers for likelihoods of events whose mechanism is not yet understood, such as the likelihood of abiogenesis on an Earth-like planet, with current likelihood estimates varying over many hundreds of orders of magnitude. An analysis that takes into account some of the uncertainty associated with this lack of understanding has been carried out by Anders Sandberg, Eric Drexler and Toby Ord, and suggests “a substantial ex ante probability of there being no other intelligent life in our observable universe”.

Even the most enthusiastic SETI supporters are troubled by the lack of results produced by more than 40 years of “listening” to the cosmic airwaves. And yet most of that search has been confined to our home galaxy. Even if there are only three or four civilizations per galaxy, there are billions and billions of galaxies. This tilts the odds again in favor of finding extraterrestrial life, which is why many SETI astronomers take the same approach to their work as lottery players: You can’t win if you don’t play.

The implications of the Drake Equation:

-1. Existence of Extraterrestrial Life: The Drake Equation encourages the consideration of various astronomical, biological, and societal factors that play a role in the emergence of life on other planets. As our understanding of extremophiles and the potential for life in extreme environments grows, the probability of life beyond Earth increases.

-2. Galactic Diversity: The equation underscores the potential diversity of intelligent civilisations in the galaxy, ranging from primitive forms of life to highly advanced societies. It prompts us to contemplate possible logical, cultural, and societal development.

-3. Search for Extraterrestrial Intelligence (SETI): The Drake Equation influenced the development of the Search for Extraterrestrial Intelligence (SETI) initiative, which involves actively seeking signals or signs of intelligent life beyond Earth. It guides the focus of such endeavours by suggesting areas where the likelihood of contact could be higher.

-4. Limits of Communication: The equation reminds us that even if intelligent civilisations exist, the factors related to the capability of interstellar communication might be a significant bottleneck. Advanced technologies may be required to bridge the vast distances between stars.

-5. Longevity of Civilizations: The longevity factor (L) in the equation raises questions about the sustainability and longevity of technological civilisations. It suggests that the ability to communicate across cosmic distances depends on civilisations’ stabilised period.

It’s important to note that the Drake Equation is not a definitive solution to the question of extraterrestrial life. It is, however, a valuable framework for fostering discussions, guiding research, and encouraging a systematic exploration of the factors that could influence the probability of intelligent life beyond Earth.

Criticism of Drake equation:

Criticism of the Drake equation follows mostly from the observation that several terms in the equation are largely or entirely based on conjecture. Star formation rates are well-known, and the incidence of planets has a sound theoretical and observational basis, but the other terms in the equation become very speculative. The uncertainties revolve around the present day understanding of the evolution of life, intelligence, and civilization, not physics. No statistical estimates are possible for some of the parameters, where only one example is known. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.

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Scientists debate likelihood of finding life on other planets by 2042 in 2015:

In a debate hosted by the Department of Astronomy & Astrophysics, six scientists argued whether remote sensing will reveal evidence of extant life on an exoplanet—any planet outside of our solar system—by the end of 2042. The debate held in fall 2015 was an event for AstroChicago 123.

The scientists arguing for the discovery of extra-terrestrial life in the near future centered on the ideas that life is versatile, that living organisms create noticeable biosignatures by changing their environment’s chemical makeup, and that with the increasing number of earth-like planets found through ventures like the Kepler mission, it shouldn’t be too long before a planet with the right signs is found.

The opposition focused on branching lines of logic. In addition to the possibility of false positives on biosignatures and the unlikelihood of humanity devoting serious resources to finding life, a paradox by Nobel laureate Enrico Fermi argues that if there’s life among billions of planets, some should be advanced enough to have reached Earth already.

In the discussion, three researchers defended each side.

Dorian Abbot, associate professor of geophysical sciences, framed the arguments for finding extra-terrestrial life in the near future, or the “yes” side. First, he described how microbial life was common and able to survive in extreme conditions on Earth. This meant that, with the raw materials essential for living matter being abundant in our universe, life could likely survive on planets within habitable zones.

Leslie Rogers, assistant professor of astronomy and astrophysics, explained how all life modifies its environment, and that biosignatures such as oxygen and ammonia would be positive evidence toward the existence of life on an exoplanet. It would only take one thousandth of the biomass in the Earth’s ocean to produce a noticeable amount of ammonia.

Kreidberg said that NASA currently has the technology to find these signatures, and had a list of planets that were promising candidates for life.

Kite, framing the arguments of the “no” side, explained that at the lab and on the planetary scale, life does not arise spontaneously—the exception being Earth, which proved that life was rare. He further explained that there is no combination of atmospheric signatures that cannot be explained through non-biological processes.

Daniel Fabrycky, assistant professor of astronomy and astrophysics, argued from the standpoint of Fermi’s paradox—that with the abundance of earth-like planets, including many much older than Earth, an alien civilization should have reached the stage of interstellar travel and made some contact with Earth. Following the theory further, Fabrycky argued that confirming existence of life on exoplanets would mean there’s a higher probability of intelligent alien civilizations. He also noted the fact that over billions of years no intelligent aliens have made it to the point of reaching Earth, meaning humanity has dismal prospects for space exploration and expansion. The last argument against finding life by 2042, put forth by Jacob Bean, an assistant professor of astronomy and astrophysics, was that humanity had too many political hurdles to overcome. Should humanity devote serious funding to developing remote-sensing technology to find extant life over other projects, it might find evidence that life exists elsewhere in the universe. However, Bean expressed doubt that the astrophysics community could band together, let alone the nation, to agree to this mission and overcome the technological difficulty.

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New Equation for abiogenesis on a planet, a 2016 study:

The Drake equation, originally penned by astronomer Frank Drake in the 1960s, laid out a series of terms estimating how many intelligent extraterrestrial civilizations likely exist in the Milky Way. The equation takes into account factors such as the rate of star formation in the galaxy, the fraction of planets where life emerges, and the fraction of that life that gains intelligence and the capability to broadcast its presence into space. Over the years, the equation has acted as a road map for researchers searching for communications signals created by intelligent civilizations beyond Earth.

Scharf and his co-author Lee Cronin, a chemist at the University of Glasgow in Scotland, hope to provide a similar road map to researchers trying to work out how — and how often — life forms on a given planet. The new equation breaks down the process of abiogenesis — the formation of life from non-living components — into a series of simpler factors. Those factors incorporate the planet’s conditions, the ingredients needed to form life and the likelihood of those ingredients getting into the right configuration for life to emerge. As with the Drake equation, each of the terms is straightforward to describe, but each hides additional complexity and room for new research.

Figure below shows new equation:

The average number of origin-of-life events for a given planet = (number of building blocks on planet) × 1/(average [mean] number of building blocks needed per “organism”) × (availability of building blocks during time t) × (probability of assembly in a given time) × time.

On the left, the equation considers the average (mean) expected number of origin-of-life events for a given planet. To get there, it takes into account the number of potential “building blocks” for life on the planet, the average number of building blocks needed to create a living system, the availability of those building blocks during a given time and the probability of that assembly happening during that time.

On Earth, building blocks for life take the form of amino acids, lipids and certain essential metals. Somewhere else, though, an entirely different set of ingredients could create enough complexity to form life — the equation doesn’t assume any specific set is necessary. For instance, if you know the size of a planet and its composition, you can begin to estimate how many potential building blocks for life there are on the planet. To calculate whether those building blocks are actually available to form life, you’d have to know more about the conditions on the planet, such as its temperature, which could render some of the blocks unusable or inaccessible. For example, these blocks could be unusable or inaccessible if they’re always in gaseous form or if water is not readily available — although future research might show that life could emerge in more scenarios than scientists currently know about.

The value Pa, which is the probability that life will assemble out of those particular building blocks over a given time, is murkier — and much more interesting. If the value of Pa is very low, it’s extremely unlikely that life will form even when the ingredients are there — potentially explaining why humans haven’t yet happened to create life in the lab, even if scientists have used the right ingredients. But a planet-wide “lab” would increase the odds that life-creating events will occur.

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Evolution on cosmic level, a 2020 study:

A new study led by the University of Nottingham and published in The Astrophysical Journal has taken a new approach. Using the assumption that intelligent life forms on other planets in a similar way as it does on Earth, researchers have obtained an estimate for the number of intelligent communicating civilizations within our own galaxy -the Milky Way. They calculate that there could be over 30 active communicating intelligent civilizations in our home Galaxy.

Professor of Astrophysics at the University of Nottingham, Christopher Conselice who led the research, explains: “There should be at least a few dozen active civilizations in our Galaxy under the assumption that it takes 5 billion years for intelligent life to form on other planets, as on Earth.” Conselice also explains that, “The idea is looking at evolution, but on a cosmic scale. We call this calculation the Astrobiological Copernican Limit.”

First author Tom Westby explains: The classic method for estimating the number of intelligent civilizations relies on making guesses of values relating to life, whereby opinions about such matters vary quite substantially. This new study simplifies these assumptions using new data, giving us a solid estimate of the number of civilizations in our Galaxy.

The two Astrobiological Copernican limits are that intelligent life forms in less than 5 billion years, or after about 5 billion years—similar to on Earth where a communicating civilization formed after 4.5 billion years. In the strong criteria, whereby a metal content equal to that of the Sun is needed (the Sun is relatively speaking quite metal rich), authors calculate that there should be around 36 active civilizations in our Galaxy. The research shows that the number of civilizations depends strongly on how long they are actively sending out signals of their existence into space, such as radio transmissions from satellites, television, etc. If other technological civilizations last as long as ours which is currently 100 years old, then there will be about 36 ongoing intelligent technical civilizations throughout our Galaxy.

However, the average distance to these civilizations would be 17,000 light-years away, making detection and communication very difficult with our present technology. It is also possible that we are the only civilization within our Galaxy unless the survival times of civilizations like our own are long.

Professor Conselice says: “Our new research suggests that searches for extraterrestrial intelligent civilizations not only reveals the existence of how life forms, but also gives us clues for how long our own civilization will last. If we find that intelligent life is common then this would reveal that our civilization could exist for much longer than a few hundred years, alternatively if we find that there are no active civilizations in our Galaxy it is a bad sign for our own long-term existence. By searching for extraterrestrial intelligent life—even if we find nothing—we are discovering our own future and fate.”

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Earth’s intelligent life is probably rare:

Intelligent life would probably not evolve that often if you reran Earth a few hundred times:

An Objective Bayesian Analysis of Life’s Early Start and Our Late Arrival, a 2020 study:

What ‘Bayesian analysis’ means:

There are two main approaches to statistics: frequentist and Bayesian.

When news networks announce who just won a presidential election, meteorologists predict the weather, and public health officials estimate coronavirus infection rates from limited samples, they’re usually using frequentist approaches. In other words, they use the limited information they have to judge what the truth about the world most likely is.

Bayesian analysis more closely resembles the way human beings actually think. Bayesian analysis is a statistical paradigm that answers research questions about unknown parameters using probability statements. For example, what is the probability that the average male height is between 70 and 80 inches or that the average female height is between 60 and 70 inches? Bayesian analysis answers the following question: “What is the probability of the hypothesis given the measured data?” Frequentist statistics accepts or rejects the null hypotheses, but Bayesian statistics estimates the ratio of probabilities of two different hypotheses. Bayesian models incorporate prior knowledge into the analysis, updating hypotheses probabilities as more data become available.

We know from the geological record that life started relatively quickly, as soon as our planet’s environment was stable enough to support it. We also know that the first multicellular organism, which eventually produced today’s technological civilization, took far longer to evolve, approximately 4 billion years. But despite knowing when life first appeared on Earth, scientists still do not understand how life occurred, which has important implications for the likelihood of finding life elsewhere in the universe.

In a paper published in the Proceeding of the National Academy of Sciences, David Kipping, an assistant professor in Columbia’s Department of Astronomy, shows how an analysis using a statistical technique called Bayesian inference could shed light on how complex extraterrestrial life might evolve in alien worlds. “The rapid emergence of life and the late evolution of humanity, in the context of the timeline of evolution, are certainly suggestive,” Kipping said. “But in this study it’s possible to actually quantify what the facts tell us.” To conduct his analysis, Kipping used the chronology of the earliest evidence for life and the evolution of humanity. He asked how often we would expect life and intelligence to re-emerge if Earth’s history were to repeat, re-running the clock over and over again.

He framed the problem in terms of four possible answers: Life is common and often develops intelligence, life is rare but often develops intelligence, life is common and rarely develops intelligence and, finally, life is rare and rarely develops intelligence.

This method of Bayesian statistical inference — used to update the probability for a hypothesis as evidence or information becomes available — states prior beliefs about the system being modeled, which are then combined with data to cast probabilities of outcomes. “The technique is akin to betting odds,” Kipping said. “It encourages the repeated testing of new evidence against your position, in essence a positive feedback loop of refining your estimates of likelihood of an event.”

From these four hypotheses, Kipping used Bayesian mathematical formulas to weigh the models against one another. “In Bayesian inference, prior probability distributions always need to be selected,” Kipping said. “But a key result here is that when one compares the rare-life versus common-life scenarios, the common-life scenario is always at least nine times more likely than the rare one.” The analysis is based on evidence that life emerged within 300 million years of the formation of the Earth’s oceans as found in carbon-13-depleted zircon deposits, a very fast start in the context of Earth’s lifetime.

Kipping’s conclusion is that if planets with similar conditions and evolutionary time lines to Earth are common, then the analysis suggests that life should have little problem spontaneously emerging on other planets. And what are the odds that these extraterrestrial lives could be complex, differentiated and intelligent? Here, Kipping’s inquiry is less assured, finding just 3:2 odds in favor of intelligent life.

This result stems from humanity’s relatively late appearance in Earth’s habitable window, suggesting that its development was neither an easy nor ensured process. “If we played Earth’s history again, the emergence of intelligence is actually somewhat unlikely,” he said. “The analysis can’t provide certainties or guarantees, only statistical probabilities based on what happened here on Earth,” Kipping said. “Yet encouragingly, the case for a universe teeming with life emerges as the favored bet. The search for intelligent life in worlds beyond Earth should be by no means discouraged.”

It’s estimated that there could be in excess of 10 septillion (that’s 10,000,000,000,000,000,000,000,000) planets in the observable universe. And if any of those worlds are exact twins of Earth, the odds that they also host life are nine times better than the odds they’re lifeless and barren.

Isabelle Winder, a biologist, archaeologist and expert in primate and human evolution at Bangor University in the United Kingdom ⁠— who wasn’t involved in Kipping’s research ⁠— said his history of life on Earth is basically correct. Still, that’s not a lot of data, certainly not enough for frequentist analysis. (We’ve only run one “Earth” experiment, and have no other similar planets to compare ourselves to yet.) But a Bayesian analysis offers some clarity. While Kipping’s paper makes reasonable assumptions and simplifications about how life works, it’s important to recognize that they are assumptions and simplifications, Winder said. Sure, intelligent life probably can emerge only some time after life itself, and life itself probably requires a habitable planet, and so on.

But Kipping’s paper only looks at when life first emerged and when intelligence first emerged after the planet became habitable, Winder said. The paper doesn’t care if life and intelligent life emerged more than once, though they might have. The paper also doesn’t care what form those life-forms take. That’s reasonable for the purposes of making a mathematical model, she said. But the details of what habitability, life and intelligence look like are trickier than the paper suggests, she said.

Before the Cambrian explosion 541 million years ago, life was relatively simple. For billions of years, the fossil record suggests Earth was inhabited by just individual cells or small colonies. Then, during the Cambrian explosion, life rapidly diversified. Within tens of millions of years, nearly every current animal body plan (including that of vertebrates) emerged. And hordes of creatures with body plans totally unlike anything seen today also flourished, suggesting alternative, alien-seeming evolutionary routes that life might have taken. Then, a massive extinction event 488 million years ago wiped out much of that diversity of life, narrowing animal life down to what we see today.

Kipping’s paper addresses the issue by abstracting it away, Winder said. In whatever manner intelligence develops in an Earth rerun, his model only cares about the first time it emerges. And it assumes that so far on this Earth, it’s happened only once, with humans. Very likely, she said, the common ancestors of humans and other apes might meet our definitions of intelligence. And we don’t know for sure, she said, that intelligence has emerged only once on Earth. If Earth were rerun, the results might be so different from our current reality that we’d have trouble recognizing “intelligence.”

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The Timing of Evolutionary Transitions Suggests Intelligent Life is Rare, a 2021 study:

Life on Earth has undergone a number of major evolutionary transitions (Smith and Szathmary, 1997). These include abiogenesis, as well as the emergence of increasingly complex forms of life such as eukaryotic, multicellular, and intelligent life. Some transitions seem to have occurred only once in Earth’s history, suggesting a hypothesis reminiscent of Gould’s remark that if the “tape of life” were to be rerun, “the chance becomes vanishingly small that anything like human intelligence” would occur (Gould, 1990). It took approximately 4.5 billion years for a series of evolutionary transitions resulting in intelligent life to unfold on Earth. In another billion years, the increasing luminosity of the Sun will make Earth uninhabitable for complex life. Intelligence therefore emerged late in Earth’s lifetime.

It is unknown how abundant extraterrestrial life is, or whether such life might be complex or intelligent. On Earth, the emergence of complex intelligent life required a preceding series of evolutionary transitions such as abiogenesis, eukaryogenesis, and the evolution of sexual reproduction, multicellularity, and intelligence itself. Some of these transitions could have been extraordinarily improbable, even in conducive environments. The emergence of intelligent life late in Earth’s lifetime is thought to be evidence for a handful of rare evolutionary transitions, but the timing of other evolutionary transitions in the fossil record is yet to be analyzed in a similar framework. Using a simplified Bayesian model that combines uninformative priors and the timing of evolutionary transitions, authors demonstrate that expected evolutionary transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude. Authors results corroborate the original argument suggested by Brandon Carter that intelligent life in the Universe is exceptionally rare, assuming that intelligent life elsewhere requires analogous evolutionary transitions. Arriving at the opposite conclusion would require exceptionally conservative priors, evidence for much earlier transitions, multiple instances of transitions, or an alternative model that can explain why evolutionary transitions took hundreds of millions of years without appealing to rare chance events. Although the model is simple, it provides an initial basis for evaluating how varying biological assumptions and fossil record data impact the probability of evolving intelligent life, and also provides a number of testable predictions, such as that some biological paradoxes will remain unresolved and that planets orbiting M dwarf stars are uninhabitable.

In this paper, scientists from Oxford’s Future of Humanity Institute, theorize that as life evolved on earth, in many cases it depended on a series of unlikely “revolutionary transitions.” Given how late intelligent life evolved on this planet, the chances of similar developments happening on other planets, before they are no longer able to sustain life, were highly unlikely, they said.

To reach their conclusions, the scientist looked at statistical models to determine the probability that the sequence of evolutionary transitions that occurred on Earth, could occur elsewhere. The paper points to the fact that it took more than a billion years for life to advance from prokaryotic (single-cell organisms) to eukaryotes (organisms with a nucleus) means that such a step is highly unlikely. It also notes that humans have only existed on Earth for about the last 6 million years, with homo sapiens, only arriving some 200,000 years ago. However, authors noted that just because scenarios of intelligent life on other plants were improbable, doesn’t mean humans should stop searching.

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New Theory Suggests that the Origin of Life on Earth-Like Planets is Likely, a 2022 paper:

Does the presence of life on Earth provide any insight into the likelihood that abiogenesis—the process by which life first emerges from inorganic substances—occurs elsewhere? That is a question that has baffled scientists for a while, as well as everyone else inclined to think about it.

Astrophysicist Brandon Carter makes the widely accepted claim that the selection effect of our own existence limits our ability to observe. Nothing can be concluded about the likelihood of life existing elsewhere based on the fact that we had to end up on a planet where abiogenesis took place. He claimed that understanding life on this earth had, at best, neutral value. Another way to look at it is to say that because Earth wasn’t chosen at random from the group of all Earth-like planets, it can’t be seen as a typical Earth-like planet.

However, a recent paper by retired astrophysicist and University of Arkansas mathematics instructor Daniel Whitmire argues that Carter’s logic was flawed. Whitmire contends that Carter’s theory suffers from “The Old Evidence Problem” in Bayesian Confirmation Theory, which is used to update a theory or hypothesis in light of new evidence, despite the fact that it has gained widespread acceptance.

After giving a few examples of how this formula is employed to calculate probabilities and what role old evidence plays, Whitmire turns to what he calls the conception analogy.

As he explains, “One could argue, like Carter, that I exist regardless of whether my conception was hard or easy, and so nothing can be inferred about whether my conception was hard or easy from my existence alone.”

In this analogy, “hard” means contraception was used. “Easy” means no contraception was used. In each case, Whitmire assigns values to these propositions.

Whitmire continues, “However, my existence is old evidence and must be treated as such. When this is done the conclusion is that it is much more probable that my conception was easy. In the abiogenesis case of interest, it’s the same thing. The existence of life on Earth is old evidence and just like in the conception analogy the probability that abiogenesis is easy is much more probable.”

In other words, the evidence of life on Earth is not of neutral value in making the case for life on similar planets. As such, our life suggests that life is more likely to emerge on other Earth-like planets — maybe even on the recent “super-Earth” type planet, LP 890-9b, discovered 100 light years away.

So according to this paper, the existence of life on Earth provides proof that abiogenesis is relatively easy on planets similar to Earth, refuting the “Carter argument” conclusion.

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If Earth is average, we should find Extraterrestrial Life within 60 Light-Years, a 2023 study:

In a recent study, Professor Piero Madau introduced a mathematical framework for calculating the population of habitable planets within 100 parsecs (326 light-years) of our Sun. Assuming Earth and the Solar System are representative of the norm, Madau calculated that this volume of space could contain as much as 11,000 Earth-sized terrestrial (aka. rocky) exoplanets that orbit within their stars’ habitable zones (HZs). Prof. Madau is a professor of astronomy and astrophysics at the University of California, Santa Cruz (UCSC).

For his study, Madau considered how time-dependent factors have played a vital role in the emergence of life in our Universe. This includes the star formation history of our galaxy, the enrichment of the interstellar medium (ISM) by heavy elements (forged in the interior of the first population of stars), the formation of planets, and the distribution of water and organic molecules between planets. As Madau explained that the central role of time and age are not explicitly stressed in the Drake Equation. The Drake equation amounts to a useful pedagogical summary of the factors (probabilities) that may affect the likelihood of detecting life-bearing worlds – and eventually technologically advanced extraterrestrial civilizations – around us today. But that likelihood and those factors depend, among other quantities, on the star formation and chemical enrichment history of the local Galactic disk, as well as on the timeline of the emergence of simple microbial and eventually complex life.

Earth is a relative newcomer to our galaxy, having formed with our Sun roughly 4.5 billion years ago (making it less than 33% the age of the Universe). Life, meanwhile, took about 500 million years to emerge from the primordial conditions that existed on Earth ca. 4 billion years ago. About 500 million years after that, photosynthesis emerged in the form of single-celled organisms that metabolized carbon dioxide and produced oxygen gas as a byproduct. This gradually altered the chemical makeup of our atmosphere, triggering the Great Oxidation Event about 2.4 billion years ago and the eventual rise of complex life forms. A long and complex process of chemical and biological evolution followed, eventually leading to conditions suitable for complex life and the emergence of all known species. Given the importance of these time-dependent steps, Madau argues that the Drake Equation is only part of the story. Looking beyond it, he created a mathematical framework to estimate when “temperate terrestrial planets” (TTPs) formed in our corner of the galaxy and microbial life could have emerged.

This framework allows astronomers to determine which potential target stars (based on mass, age, and metallicity) may be optimal candidates in the search for atmospheric biosignatures. As Madau described it, his approach consists of considering the local population of long-lived stars, exoplanets, and TTPs as a series of mathematical equations, which can be solved numerically as a function of time: These equations describe the changing rates of star, metal, giant, and rocky planets, and habitable world formation over the history of the solar neighborhood, the locale where more detailed calculations are justified by an avalanche of new data from space-based and ground-based facilities and the target of current and next-generation stellar and planetary surveys. The equations are statistical in nature, i.e. they do not describe the birth and evolution of individual planetary systems but rather the changing (over time) population (by number) of TTPs within 100 parsecs of the Sun.

Ultimately, Madau’s analysis showed that within 100 parsecs of the Sun, there may be as many as 10,000 rocky planets orbiting with their star’s HZs. He also found that the formation of TTPs near our Solar System was likely episodic, starting with a burst of star formation roughly 10-11 billion years ago, followed by another event that peaked about 5 billion years ago that produced the Solar System. Another interesting takeaway from Madau’s mathematical framework indicates that most TTPs within 100 parsecs are likely older than the Solar System, confirming that we are a relative latecomer to the party!

Equally interesting are the implications this study could have on the search for extraterrestrial life. Using the generally accepted timeline of the emergence of life on Earth (abiogenesis) and applying a conservative estimate of the prevalence of life on other planets – the fl parameter of the Drake Equation – Madau’s framework also indicated how far away the closest exoplanet harboring life could be: So, if microbial life arose as soon as it did on Earth in more than 1% of TTPs (and that is a big if), then one expects the closest, life-harboring Earth-like planet to be less than 20 pc away [65 light-years], he said. This may be cause for some cautious optimism in the search for habitability markers and biosignatures by the next generation of large ground-based facilities and instrumentation. Needless to say, biosignatures are going to be extremely challenging to detect. And it is also possible that life may be so rare that there are no biosignatures within a kpc or more for us to detect.

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Section-17

Why extraterrestrial life/intelligence hard to find:

Based on observations from the Hubble Space Telescope, there are nearly 2 trillion galaxies in the observable universe. It is estimated that at least ten per cent of all Sun-like stars have a system of planets, i.e. there are 6.25×10^18 stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25 billion life-supporting planetary systems in the observable universe. A 2013 study based on results from the Kepler spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets. The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the Fermi paradox. The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, “If life is so easy, someone from somewhere must have come calling by now.” Italian-American physicist Enrico Fermi’s name is associated with the paradox because of a casual conversation in the summer of 1950 with fellow physicists Edward Teller, Herbert York, and Emil Konopinski. While walking to lunch, the men discussed recent UFO reports and the possibility of faster-than-light travel. The conversation moved on to other topics, until during lunch Fermi blurted out, “But where is everybody?” (although the exact quote is uncertain).

Explanations for the Fermi paradox and why extraterrestrial life/intelligence hard to find:

-1. Rarity of intelligent life

Those who think that intelligent extraterrestrial life is (nearly) impossible argue that the conditions needed for the evolution of life—or at least the evolution of biological complexity—are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis, a rejection of the mediocrity principle, complex multicellular life is regarded as exceedingly unusual.

The Rare Earth hypothesis argues that the evolution of biological complexity requires a host of fortuitous circumstances, such as a galactic habitable zone, a star and planet(s) having the requisite conditions, such as enough of a continuous habitable zone, the advantage of a giant guardian like Jupiter and a large moon, conditions needed to ensure the planet has a magnetosphere and plate tectonics, the chemistry of the lithosphere, atmosphere, and oceans, the role of “evolutionary pumps” such as massive glaciation and rare bolide impacts. The rare Earth hypothesis, posits that perhaps life really is special and unique on the cosmic scale. In this scenario, life is so incredibly rare that we may be among the first creatures or any kind to arise in the Milky Way. In other words, the circumstances that led to the emergence of life on Earth are so special that even with trillions of other worlds, life happened essentially only once.

-2. Great filter hypothesis

The great filter hypothesis postulates that all life must overcome certain challenges, and at least one hurdle is nearly impossible to clear. Before a civilization is able to reach the level of intelligence necessary for space colonization, it gets “filtered” by some external circumstance and ceases to exist. So, while we may find bacteria or other simple organisms throughout the galaxy, we’re unlikely to meet anybody capable of conversation.

-3. Intelligent alien species have not developed advanced technologies.

It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate. Along with non-intelligent life, such civilizations would also be very difficult to detect. A trip using conventional rockets would take hundreds of thousands of years to reach the nearest stars.

To skeptics, the fact that in the history of life on the Earth only one species has developed a civilization to the point of being capable of spaceflight and radio technology lends more credence to the idea that technologically advanced civilizations are rare in the universe.

Another hypothesis in this category is the “Water World hypothesis”. According to author and scientist David Brin: “it turns out that our Earth skates the very inner edge of our sun’s continuously habitable—or ‘Goldilocks’—zone. And Earth may be anomalous. It may be that because we are so close to our sun, we have an anomalously oxygen-rich atmosphere, and we have anomalously little ocean for a water world. In other words, 32 percent continental mass may be high among water worlds…” Brin continues, “In which case, the evolution of creatures like us, with hands and fire and all that sort of thing, may be rare in the galaxy. In which case, when we do build starships and head out there, perhaps we’ll find lots and lots of life worlds, but they’re all like Polynesia. We’ll find lots and lots of intelligent lifeforms out there, but they’re all dolphins, whales, squids, who could never build their own starships.

-4. Civilizations only broadcast detectable signals for a brief period of time

It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. The usual assumption is that civilizations outgrow radio through technological advancement. However, there could be other leakage such as that from microwaves used to transmit power from solar satellites to ground receivers.

Regarding the first point, radio leakage from a planet is only likely to get weaker as a civilization advance and its communications technology gets better. Earth itself is increasingly switching from broadcasts to leakage-free cables and fiber optics, and from primitive but obvious carrier-wave broadcasts to subtler, hard-to-recognize spread-spectrum transmissions.

More hypothetically, advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by technologies not developed or used by mankind. Some scientists have hypothesized that advanced civilizations may send neutrino signals. If such signals exist, they could be detectable by neutrino detectors that are now under construction for other goals.

-5. Alien life may be too incomprehensible

Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human mathematics is parochial to Earth and not shared by other life, though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods).

Physiology might also cause a communication barrier. Carl Sagan speculated that an alien species might have a thought process orders of magnitude slower (or faster) than that of humans. A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.

Another thought is that technological civilizations invariably experience a technological singularity and attain a post-biological character. Hypothetical civilizations of this sort may have advanced drastically enough to render communication impossible.

-6. Alien species may have only settled part of the galaxy

Astrophysicist Adam Frank, along with co-authors such as astronomer Jason Wright, ran a variety of simulations in which they varied such factors as settlement lifespans, fractions of suitable planets, and recharge times between launches. They found many of their simulations seemingly resulted in a “third category” in which the Milky Way remains partially settled indefinitely. The abstract to their 2019 paper states, “These results break the link between Hart’s famous ‘Fact A’ (no interstellar visitors on Earth now) and the conclusion that humans must, therefore, be the only technological civilization in the galaxy.

-7. Humans have not listened properly

There are some assumptions that underlie the SETI programs that may cause searchers to miss signals that are present. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) frequencies, which would make them hard to distinguish from background noise. Signals might be sent from non-main sequence star systems that humans search with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.

The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable universe), the limited amount of resources committed to SETI, and the sensitivity of modern instruments. SETI estimates, for instance, that with a radio telescope as sensitive as the Arecibo Observatory, Earth’s television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance. However, this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.

Thus, to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than humanity’s own; that one of SETI’s programs is listening to the correct frequencies from the right regions of space; or that aliens are deliberately sending focused transmissions in Earth’s general direction.

-8. Intelligent life may be too far away

The universe is so vast that it might still contain many Earth-like planets, but if such planets exist, they are likely to be separated from each other by many thousands of light-years. Such distances may preclude communication among any intelligent species that may evolve on such planets, which would solve the Fermi paradox. Important factor to consider is the vastness of the universe and how little of it we have explored so far. The nearest star to our own, Proxima Centauri, is over four light years away, and there are billions of other stars in our galaxy alone. To explore even a tiny fraction of the universe would take an enormous amount of time and resources, and we have only just begun to scratch the surface. It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication. Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years. If two civilizations are separated by several thousand light-years, it is possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. It has been suggested that this problem might be ameliorated somewhat if contact and communication is made through a Bracewell probe. In this case at least one partner in the exchange may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present, and humanity has undertaken similar activities like the Arecibo message, which could transfer information about Earth’s intelligent species, even if it never yields a response or does not yield a response in time for humanity to receive it. It is possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it.

-9. We are not looking hard enough

In a dazzling new book, “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth,” the astrophysicist Avi Loeb offers a forceful rejoinder to Fermi. Loeb, a professor at Harvard, argues that the absence of evidence regarding life elsewhere is not evidence of its absence. What if the reason we haven’t come across life beyond Earth is the same reason I can never find my keys when I’m in a hurry — not because they don’t exist but because I did a slapdash job looking for them.

Loeb argues, we are not looking hard enough. Other areas of physics, especially abstruse mathematical concepts like supersymmetry, are showered with funding and academic respect, while one of the most profound questions humanity has ever pondered — Are we alone? — lingers largely on the sidelines. Besides lack of resources, Loeb says the search for aliens has been hampered by risk aversion and groupthink. Young scientists rarely push boundaries because those who do so risk making mistakes, and mistakes don’t advance careers.

-10. Biosignature gases are difficult or too low to detect

To a distant observer peering through a telescope, even Earth would not have shown signs of life through most of its past. Despite the fact that our planet was teeming with mostly microscopic life for three billion years, levels of oxygen and methane — gases often produced by metabolizing organisms — would have been too low to be noticed from afar. This means that today’s scientists on Earth might not be able to detect commonly assumed signs of extraterrestrial life, and they might give up on planets that are actually inhabited, according to a new study in the journal Astrobiology. ‘There are huge swaths of time throughout Earth’s history during which it would’ve been difficult to see the presence of these metabolisms even though we know from the rock record that they were around. It’s a sobering thing,’ said Christopher Reinhard, an Earth scientist at the Georgia Institute of Technology in Atlanta, and lead author of the study.

Scientists envision using oxygen, ozone and methane in a planet’s atmosphere as key indicators of life. But there are problems with this approach. The gases are tough to detect with current technology, and their presence is suggestive, but not conclusive, evidence of living organisms. Even for an Earth-like planet, the search for life beyond our solar system turns out to be even tougher than previously thought. It’s becoming clear that you need more than one chemical signature, and at best you will just get a statistical estimate of whether there’s life.

Oxygen has often been considered the primary sign of life for astrobiologists, since it’s hard to produce large quantities of oxygen gas without the presence of biological processes, like photosynthesis. But life in Earth’s ocean predates detectable oxygen by more than a billion years, as the gas takes time to build up in the atmosphere. Ozone, which consists of three oxygen atoms, helps to shield nascent lifeforms on a planet’s surface from ultraviolet radiation — if there’s life down there at all. Methane is problematic as well, as it’s produced and consumed by biological processes in the ocean but can also be generated by geological processes. Because oxygen and methane levels were so low for so long on Earth according to Reinhard’s models, until a few hundred million years ago a distant alien astronomer would’ve had few hints that life exists here. Earthling astronomers face the same problem when searching for life-friendly planets beyond the solar system. It’s not just that our favorite worlds in the “habitable zone” around their stars might be uninhabited; we could be prematurely ruling out many other worlds that actually do host life.

-11. Our satellites could disrupt our ability to observe the stars from Earth – and search for life beyond it.

Astronomers have raised concerns that large groups of satellites could disrupt our ability to observe the stars from Earth – and search for life beyond it. Recently BlueWalker 3, a prototype for a constellation of more than one hundred satellites for mobile communications, was launched into low-Earth orbit (LEO). Observations taken within weeks of the launch showed that the satellite was among the brightest objects in the sky. Several companies are planning to launch ‘constellations’ of satellites – groups of potentially hundreds of satellites that can deliver mobile or broadband services anywhere in the world, following in the footsteps of SpaceX’s Starlink and EutelSat’s OneWeb. However, these satellites need to be in low-Earth orbit and can be relatively large, so their potential to disrupt night-sky observations is a concern.

Besides visible observations, BlueWalker 3 could also interfere with radio astronomy, since it uses wavelengths close to those that radio telescopes use to observe the universe and scan the skies for signs of extraterrestrial life.

The interference of these satellites in astronomical observations could severely hamper progress in our understanding of the cosmos.

The effects may not be confined to ground-based observations. A study found that almost 3% of Hubble images were found to contain satellite trails.

As a result, experts have called for consideration of their side effects and recommended efforts to minimise their impact on astronomy.

-12. Wait

Now, a new paper written by Amri Wandel at the Hebrew University of Jerusalem and published in the preprint database arXiv puts forward a new explanation: Because we have only recently arrived on the cosmic scene, in the sense of being able to broadcast our presence through radio transmissions, maybe we just need to wait a bit.

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Limitations in the Exploration of Extraterrestrial Life:

Investigating the possibility of extraterrestrial life is a complex and multifaceted endeavour that offers profound insights into our understanding of the universe and our place within it. However, like any scientific inquiry, this research has limitations. Here are some of the critical limitations inherent in exploring extraterrestrial life.

Lack of Empirical Evidence: One of the most significant limitations in this field is the lack of empirical evidence of extraterrestrial life. Despite the advancements in telescopes, spectroscopy, and other detection methods, we have yet to discover definitive signs of life beyond Earth. This absence of concrete evidence makes it challenging to conclude the existence of extraterrestrial beings or the diversity of life forms that may exist.
Sample Size and Scope: Current knowledge is primarily based on our understanding of life on Earth, which represents just one data point. Extrapolating from Earth’s biology to other planets or celestial bodies might not be accurate. The sample size of known life forms is limited, and our assumptions about the conditions necessary for life might not be universally applicable.
Communication and Understanding: Even if we were to discover extraterrestrial life, communication could be a significant barrier. The limitations of our current understanding of potential alien languages, modes of communication, or even the physical forms of these beings could hinder effective interaction.
Interstellar Distances: The vast distances between stars and galaxies present a practical challenge for interstellar exploration and communication. The limitations of our current propulsion technologies and the immense timescales involved make it difficult to establish direct contact with potential extraterrestrial civilisations.
Inherent Bias and Anthropomorphism: Our conceptualisation of extraterrestrial life often tends to be influenced by our Earth-based experiences and perspectives. This anthropocentric bias might hinder our ability to recognise or understand forms of life that are drastically different from what we are familiar with.
Technological Constraints: Current technology limits our ability to explore distant planets and quickly identify biosignatures. Limitations in telescope capabilities, spectroscopic techniques, and space mission budgets can hinder our ability to make definitive discoveries.
Changing Paradigms: Our understanding of life and the universe constantly evolves. The frameworks and assumptions we have today might undergo significant shifts with the advancement of scientific knowledge, rendering current theories and predictions obsolete.

In conclusion, exploring extraterrestrial life is a captivating and intellectually stimulating pursuit. However, it is essential to approach the subject with a balanced understanding of its limitations. The abovementioned limitations remind us of the complexities inherent in this field and the need for ongoing research, open-mindedness, and innovative approaches to address these challenges.

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What’s the best scientific evidence we’ve found for the existence of alien life?

“There’s just no evidence right now,” says Professor Sara Seager, an astrophysicist and planetary scientist at the Massachusetts Institute of Technology and author of “The Smallest Lights in the Universe: A Memoir” (Crown, 2020). Seager believes we’d confirm the existence of alien life only with sample-return missions — collecting samples from another planet or moon and bringing them back to Earth for study, in-situ measurements or “incredibly futuristic” technology, like a solar gravitational lens telescope, a theoretical instrument that would use the sun’s gravity to magnify light from distant planets. “With all the tools we have now, or that we could build with enough money, it’s sample return, because I don’t see anything else being definitive,” Seager said.

Nikku Madhusudhan, a professor of astrophysics and exoplanetary science at the University of Cambridge, gave a similar response, saying, “I don’t think we have clear evidence just yet of any kind for alien life.”

There is, however, good reason to hold out hope that the evidence will eventually come, even if it isn’t personally delivered by a little green man. Madhusudhan says that “there are hints here and there” and “there is evidence for habitable conditions.” In other words, there are signs that certain planets and moons could harbour life, but we haven’t found evidence of life in these places yet.

There may be hundreds of millions of habitable planets in our galaxy alone. Scientists consider planets capable of hosting life if they sit in the so-called habitable zone, the distance from a star where it’s possible for a rocky planet to have liquid water on the surface, an essential ingredient for life on Earth. Planets and moons outside of the habitable zone aren’t necessarily inhospitable to life either. For example, Jupiter’s moon Europa isn’t in the sun’s habitable zone, but has a saltwater ocean beneath its icy crust that may be able to host life. But we have to keep in mind that space is incredibly vast. It would take humans more than a million years to visit K2-18 b with traditional rocket propulsion. Even sending our fastest probe to the nearest known exoplanet, Proxima Centauri b, would take thousands of years. The planets and moons within our solar system are right on our doorstep by comparison, with probe travel times ranging from some years to mere months.

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Section-18

Asteroids, meteoroids, space dust, UFOs and extraterrestrial life:

An asteroid is a small rocky object that orbits the Sun. Asteroids, which orbit the sun but are much smaller than planets, are among the oldest objects in the solar system and therefore may help explain how Earth formed and evolved. The main difference between asteroids and comets is their composition, as in, what they are made of. Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material. Both asteroids and comets were formed early in the history of the solar system about 4.5 billion years ago. A meteoroid is a “space rock” smaller than an asteroid. A meteorite is a meteoroid that enters Earth’s atmosphere but does not burn up entirely, instead surviving to crash into the surface. A meteor is a meteoroid, burns up upon entering Earth’s atmosphere.

Meteorites provide a record of the chemical processes that occurred in the solar system before life began on Earth. Carbonaceous chondrites are a rare class of meteorite and are composed of various groups (e.g., CI group, CM group, and CR group) according to their composition and petrography. They are known to contain a diverse suite of organic compounds including many that are essential in contemporary biology. Amino acids, which are the monomers of proteins, have been extensively studied in meteorites. An extraterrestrial origin for most of the amino acids detected in carbonaceous chondrites has been firmly established based on three factors: the detection of racemic amino acid mixtures (i.e., equal mixtures of D and L amino acids), wide structural diversity (including the presence of many nonprotein amino acids that are rare or non-existent in the biosphere), and non-terrestrial values for compound-specific deuterium, carbon, and nitrogen isotope measurements. In contrast to amino acids, nucleobases in meteorites have been far less studied. All terrestrial organisms depend on nucleic acids (RNA and DNA), which use pyrimidine and purine nucleobases to encode genetic information. Carbon-rich meteorites may have been important sources of organic compounds required for the emergence of life on the early Earth; however, the origin and formation of nucleobases in meteorites has been debated for over 50 y.

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DNA building blocks found in meteorites were created in space:

NASA-funded researchers have evidence that some building blocks of DNA, the molecule that carries the genetic instructions for life, found in meteorites were likely created in space. The research gives support to the theory that a “kit” of ready-made parts created in space and delivered to Earth by meteorite and comet impacts assisted the origin of life. “People have been discovering components of DNA in meteorites since the 1960’s, but researchers were unsure whether they were really created in space or if instead they came from contamination by terrestrial life,” said Dr. Michael Callahan of NASA’s Goddard Space Flight Center, Greenbelt, Md. “For the first time, we have three lines of evidence that together give us confidence that these DNA building blocks actually were created in space.”

The discovery adds to a growing body of evidence that the chemistry inside asteroids and comets is capable of making building blocks of essential biological molecules. For example, previously, these scientists at the Goddard Astrobiology Analytical Laboratory have found amino acids in samples of comet Wild 2 from NASA’s Stardust mission, and in various carbon-rich meteorites. Amino acids are used to make proteins, the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. In the new work, the Goddard team ground up samples of twelve carbon-rich meteorites, nine of which were recovered from Antarctica. They extracted each sample with a solution of formic acid and ran them through a liquid chromatograph, an instrument that separates a mixture of compounds. They further analyzed the samples with a mass spectrometer, which helps determine the chemical structure of compounds. The team found adenine and guanine, which are components of DNA called nucleobases, as well as hypoxanthine and xanthine. DNA resembles a spiral ladder; adenine and guanine connect with two other nucleobases to form the rungs of the ladder. They are part of the code that tells the cellular machinery which proteins to make. Hypoxanthine and xanthine are not found in DNA, but are used in other biological processes. Also, in two of the meteorites, the team discovered for the first time trace amounts of three molecules related to nucleobases: purine, 2,6-diaminopurine, and 6,8-diaminopurine; the latter two almost never used in biology. These compounds have the same core molecule as nucleobases but with a structure added or removed.

Three lines of evidence:

-1. It’s these nucleobase-related molecules, called nucleobase analogs, which provide the first piece of evidence that the compounds in the meteorites came from space and not terrestrial contamination. “You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they’re not used in biology, aside from one report of 2,6-diaminopurine occurring in a virus (cyanophage S-2L),” said Callahan. “However, if asteroids are behaving like chemical ‘factories’ cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid.”

-2. The second piece of evidence involved research to further rule out the possibility of terrestrial contamination as a source of these molecules. The team also analyzed an eight-kilogram (17.64-pound) sample of ice from Antarctica, where most of the meteorites in the study were found, with the same methods used on the meteorites. The amounts of the two nucleobases, plus hypoxanthine and xanthine, found in the ice were much lower — parts per trillion — than in the meteorites, where they were generally present at several parts per billion. More significantly, none of the nucleobase analogs were detected in the ice sample. One of the meteorites with nucleobase analog molecules fell in Australia, and the team also analyzed a soil sample collected near the fall site. As with the ice sample, the soil sample had none of the nucleobase analog molecules present in the meteorite.

-3. Thirdly, the team found these nucleobases — both the biological and non-biological ones — were produced in a completely non-biological reaction. “In the lab, an identical suite of nucleobases and nucleobase analogs were generated in non-biological chemical reactions containing hydrogen cyanide, ammonia, and water. This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial,” says Callahan. “In fact, there seems to be a ‘goldilocks’ class of meteorite, the so-called CM2 meteorites, where conditions are just right to make more of these molecules,” adds Callahan.

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Oumuamua:

On October 19, 2017, a Canadian astronomer named Robert Weryk was reviewing images captured by a telescope known as Pan-starrs1 when he noticed something strange. The telescope is situated atop Haleakalā, a ten-thousand-foot volcanic peak on the island of Maui, and it scans the sky each night, recording the results with the world’s highest-definition camera. It’s designed to hunt for “near-Earth objects,” which are mostly asteroids whose paths bring them into our planet’s astronomical neighborhood and which travel at an average velocity of some forty thousand miles an hour. The dot of light that caught Weryk’s attention was moving more than four times that speed, at almost two hundred thousand miles per hour.

Weryk alerted colleagues, who began tracking the dot from other observatories. The more they looked, the more puzzling its behavior seemed. The object was small, with an area roughly that of a city block. As it tumbled through space, its brightness varied so much—by a factor of ten—that it had to have a very odd shape. Instead of swinging around the sun on an elliptical path, it was zipping away more or less in a straight line. The bright dot, astronomers concluded, was something never before seen. It was an “interstellar object”—a visitor from far beyond the solar system that was just passing through. The interstellar visitor was shaped like a cigar. It was about 10 times more reflective than asteroids in our solar system. Most curiously, as it zoomed away from the sun, it sped up faster than it should have based on the pure physics of its trajectory. In the dry nomenclature of the International Astronomical Union, it became known as 1I/2017 U1. More evocatively, it was dubbed ‘Oumuamua (figure below), from the Hawaiian, meaning, roughly, “scout.”

Even interstellar objects have to obey the law of gravity, but Oumuamua raced along as if propelled by an extra force. Comets get an added kick thanks to the gases they throw off, which form their signature tails. Oumuamua, though, didn’t have a tail. Nor did the telescopes trained on it find evidence of any of the by-products normally associated with outgassing, like water vapor or dust.

As astronomers pored over the data, they excluded one theory after another. ‘Oumuamua’s weird motion couldn’t be accounted for by a collision with another object, or by interactions with the solar wind, or by a phenomenon that’s known, after a nineteenth-century Polish engineer, as the Yarkovsky effect. One group of researchers decided that the best explanation was that 1I/2017 U1 was a “miniature comet” whose tail had gone undetected because of its “unusual chemical composition.” Another group argued that ‘Oumuamua was composed mostly of frozen hydrogen. This hypothesis—a variation on the mini-comet idea—had the advantage of explaining the object’s peculiar shape. By the time it reached our solar system, it had mostly melted away, like an ice cube on the sidewalk. Nearly all scientists believe that ʻOumuamua probably originates from outside the solar system. It is an asteroid- or comet-like object that has left another star and travelled through interstellar space—we saw it as it zipped by us. But not everyone agrees. Avi Loeb, a Harvard professor of astronomy, suggested in a recent book that it is indeed an alien spaceship. But how feasible is this? And how come most scientists disagree with the claim?

Researchers estimate that the Milky Way should contain around 100 million billion billion comets and asteroids ejected from other planetary systems, and that one of these should pass through our solar system every year or so. So it makes sense that ‘Oumuamua could be one of these. We spotted another in 2020– “Borisov”—which suggests they are as common as we predict. What made Oumuamua particularly interesting was that it didn’t follow the orbit you would expect—its trajectory shows it has some extra non-gravitational force acting on it. This is not too unusual. The pressure of solar radiation or gas or particles driven out as an object warms up close to the Sun can give extra force, and we see this with comets all the time. Experts on comets and the solar system have explored various explanations for this. Given this was a small, dark object passing us very quickly before disappearing, the images we were able to get weren’t wonderful, and so it is difficult to be sure.

In “Extraterrestrial,” Loeb lays out his reasoning as follows. The only way to make sense of Oumuamua’s strange acceleration, without resorting to some sort of undetectable outgassing, is to assume that the object was propelled by solar radiation—essentially, photons bouncing off its surface. And the only way the object could be propelled by solar radiation is if it were extremely thin—no thicker than a millimetre—with a very low density and a comparatively large surface area. Such an object would function as a sail—one powered by light, rather than by wind. The natural world doesn’t produce sails; people do. Thus, Loeb writes, “Oumuamua must have been designed, built, and launched by an extraterrestrial intelligence.” He argues the non-gravitational acceleration is a sign of “deliberate” manoeuvring. This argument seems largely to be based on the fact that Oumuamua lacks a fuzzy envelope (“coma”) and a comet-like tail, which are usual signatures of comets undergoing non-gravitational acceleration (although jets from particular spots cannot be ruled out). He may or may not be right, and there is no way of proving or disproving this idea.

But claims like this, especially from experienced scientists are disliked by the scientific community for many reasons. To stop ourselves jumping to weird and wonderful conclusions every time we come across something strange, science has several sanity checks.

One is Occam’s razor, which tells us to look for the simplest solutions that raise the fewest new questions. Is this a natural object of the type that we suspect to be extremely common in the Milky Way, or is it aliens? Aliens raise a whole set of supplementary questions (who, why, from where?) which means Occam’s razor tells us to reject it, at least until all simpler explanations are exhausted.

Another sanity check is the general rule that “extraordinary claims require extraordinary evidence”. A not quite completely understood acceleration is not extraordinary evidence, as there are many plausible explanations for it.

Yet another check is the often sluggish but usually reliable peer-review system, in which scientists publish their findings in scientific journals where their claims can be assessed and critiqued by experts in their field.

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Martian meteorite ALH84001

The meteorite had escaped from Mars 16 million years ago when an asteroid or comet collided with the planet and blasted out a crater. The 2-kilogram fragment of Martian rock then moved in an elliptical orbit around the Sun until it was swept up by the Earth about 13,000 years ago. It landed in glacial Antarctica, where it remained until 1984, when a meteorite-hunting party picked it up it in the Allan Hills. The specimen was designated ALH84001. At first, no one suspected that it came from Mars. About ten years later, scientists examined ALH84001 more closely and found that it was not an ordinary meteorite, but one of the so-called SNC meteorites, which come from Mars. Meteorites of this class all contain traces of gas having a composition identical to the Martian atmosphere. Each of the dozen other SNC Martian meteorites then known had crystallized within the last 1.3 billion years, after Mars had become a frozen desert. But ALH84001 was over 4 billion years old, and had presumably existed at a time when liquid water was common on the surface of Mars. Liquid water is essential to life as we know it. For that reason ALH84001 attracted the attention of a team of scientists led by David McKay of NASA’s Johnson Space Flight Center, who thought that the rock might preserve microscopic and chemical evidence of ancient life on Mars.

To avoid the possible terrestrial contaminants picked up by the meteorite in Antarctica, the team obtained their samples from the solid interior volume of the rock. They found that cracks within the meteorite contain orange-tinted carbonate globules, which resemble limestone cave deposits. This sort of material can form only in the presence of liquid water. McKay and his colleagues found three kinds of evidence that they interpreted in terms of ancient microbial life on Mars:

The globules contained traces of complex organic compounds called polycyclic aromatic hydrocarbons (PAHs), which might be the decay products of microbes.
The globules contained microscopic grains of magnetite (a magnetic iron oxide) and of iron sulfide, two compounds rarely found together in the presence of carbonates, unless produced by bacterial metabolism.
The carbonate globules, when examined with an electron microscope, were found to be covered in places with large numbers of worm-like forms that resemble fossilized bacteria.

McKay and his colleagues conceded from the start that any one of these lines of evidence could be interpreted without recourse to biology. However they judged that the presence of all three in association with the carbonate globules made a persuasive case for ancient life on Mars. Other scientists at once began to subject the evidence to intense critical scrutiny, which is an expected and essential part of the scientific process.

Some geochemists found evidence that the carbonate globules were formed at temperatures up to 300º C, too hot for the survival of any known microbial life on Earth. But others concluded that the globules may have formed at temperatures below 100º C. This now seems to be the case.

What about the PAHs?

Chemical theory and experiments show that organic (carbon-based) molecules are formed non-biologically within giant interstellar clouds of gas and dust. The solar system was born in such an environment. Organic molecules produce PAHs when heated, so these materials would have been present on the Earth and on Mars from the beginning. Ordinary soot contains PAHs. McKay and his colleagues soon conceded that the organic carbon evidence for fossil life on Mars was weak.

The evidence from the magnetite and iron sulfide grains is more substantial. In size and shape, most of the magnetite grains closely resemble those produced by terrestrial bacteria. No one has yet demonstrated a non-biological formation mechanism for such grains in association with iron sulfide. The evidence so far suggests that such materials require a biological origin.

What about the objects said to resemble fossil bacteria?

Microscopists contend that shape alone is often misleading, because non-biological processes can produce objects that superficially resemble bacteria. The size of the supposed Martian fossils is also a contentious issue. Many of the worm-like objects in ALH84001 are just a few tens of nanometers across, or about a tenth the size of the smallest-known bacteria on Earth. But the minimum amount of molecular “equipment” needed to keep a bacterium alive (including the DNA and ribosomes to translate the genetic code into proteins) would require a volume equal to a 200-nanometer sphere. In other words, these so-called Martian fossils are just too small to have ever been alive.

How can we be sure that any hypothetical Martian life would use the same kind of biochemistry as terrestrial bacteria? Does all life have to be based on molecules as large and complex as DNA?

Some scientists have reported finding so-called “nanobacteria” in a wide range of environments. These mysterious objects are as small as the alleged Martian microbes, and are conceivably living organisms, or fragments of organisms. Before the evolution of DNA, ribosomes, and complex proteins on Earth, simpler ancestral life forms must have existed. Those primitive organisms would have lost out in competition by the far more complex bacteria that later evolved on Earth, but their fossilized remains might still be found. And perhaps such things have been found in the Martian meteorite.

For several years, the general current of scientific opinion was running against the biological interpretation of the evidence from ALH84001. But recent studies of the magnetite grains provide increased support to a biological origin. Scientists continue to study ALH84001 and the other Martian meteorites. And they are planning a mission to bring samples back from Mars. While the evidence from ALH84001 remains controversial, it has without question stimulated a major new effort in the search for life, ancient or extant, on Mars.

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Ryugu:

162173 Ryugu, provisional designation 1999 JU3, is a near-Earth object and a potentially hazardous asteroid of the Apollo group. It measures approximately 900 metres (3,000 ft) in diameter. Ryugu orbits the Sun at a distance of 0.96–1.41 AU once every 16 months. The Japanese spacecraft Hayabusa2 was launched in 2014, touched down on Ryugu twice, despite its extremely rocky surface, and successfully collected data and samples during the 1.5 years after it arrived there in June 2018. In the first touchdown in February 2019, it collected surface dust samples. In July, it collected underground samples from the asteroid for the first time in space history after landing in a crater that it had earlier created by blasting the asteroid’s surface. The spacecraft then began a journey back home, dropping the capsule containing the soil from the asteroid over Australia in December 2020. However, that was not the end of the mission for the probe. After dropping the capsule, it returned to space and is heading to another distant small asteroid called 1998KY26 on a journey slated to take 10 years. The samples showed the presence of organic compounds, such as uracil (one of the four components in RNA) and vitamin B3. This suggests that such molecules of prebiotic interest are commonly formed in carbonaceous asteroids including Ryugu and were delivered to the early Earth.

Japanese researchers have for the first time discovered amino acids — key ingredients for life — in an asteroid flying in space. They identified 20 amino acids in the samples returned from the asteroid Ryugu by the Hyabusa2 mission. The findings confirm the scientific claims of the asteroid that it contains traces of carbon and organic matter. These amino acids have previously been detected in the asteroids that fell on Earth. However, they were barely quantified as they were lost during entry through Earth’s atmosphere that burns and creates plasma. The discovery of 20 of these key ingredients confirms the presence of organic material in these remnants from the creation of the solar system. “The Ryugu material is the most primitive material in the solar system we have ever studied. Ruygu is a CI chondrite asteroid, a type of stony carbon-rich asteroid with a chemical composition that is the most similar to that of the sun. These asteroids, rich in water and organic material, are a possible source of the seeds of life delivered to the nascent Earth billions of years ago,” Hisayoshi Yurimoto, a geoscience professor at Hokkaido University said.

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Bennu:

Bennu is a carbonaceous asteroid in the Apollo group discovered by the LINEAR Project on 11 September 1999. It is a potentially hazardous object that is listed on the Sentry Risk Table and has the highest cumulative rating on the Palermo Technical Impact Hazard Scale. It has a cumulative 1-in-1,750 chance of impacting Earth between 2178 and 2290 with the greatest risk being on 24 September 2182. It is named after Bennu, the ancient Egyptian mythological bird associated with the Sun, creation, and rebirth. 101955 Bennu has a mean diameter of 490 m and has been observed extensively by the Arecibo Observatory planetary radar and the Goldstone Deep Space Network. Bennu orbits the Sun with a period of 1.19 years (435 days) as of 2022. Earth gets as close as about 480,000 km (0.0032 au) from its orbit around 23 to 25 September.

Bennu was the target of the OSIRIS-REx (short for the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) mission that returned samples of the asteroid to Earth. The spacecraft, launched in September 2016, arrived at the asteroid two years later and mapped its surface in detail, seeking potential sample collection sites. Analysis of the orbits allowed calculation of Bennu’s mass and its distribution. In October 2020, OSIRIS-REx briefly touched down and collected a sample of the asteroid’s surface. The sample, collected from the 4.5 billion-year-old near-Earth asteroid Bennu in October 2020 by NASA’s OSIRIS-REx mission, arrived on Earth in a capsule on September 24, 2023, dropping from the spacecraft and landing in the Utah desert. The spacecraft didn’t land, but continued on to a new mission, OSIRIS-APEX, to explore asteroid Apophis.

On 11 October 2023, the recovered capsule was opened to reveal a “first look” at the asteroid sample contents. The rocks and dust contain water and a large amount of carbon, which suggests that asteroids may have delivered the building blocks of life to Earth. The sample is nearly 5% carbon by weight, making it one of the highest concentrations of carbon to be studied in an asteroid. The carbon and water molecules are exactly the kinds of material that scientists wanted to find. They’re crucial elements in the formation of our own planet. And they’re going to help us determine the origin of elements that could have led to life. The analysis also revealed sulfide minerals, a critical element for planetary evolution and biology, iron oxide minerals called magnetite that react to magnetic fields, and other minerals that could be important for organic evolution. The material also hosts high amounts of magnesium, sodium and phosphorus, a combination that so far puzzles the team.

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Extraterrestrial protein Hemolithin:

Hemolithin is a proposed protein containing iron and lithium, of extraterrestrial origin, according to an unpublished preprint. The result has not been published in any peer-reviewed scientific journal. The protein was purportedly found inside two CV3 meteorites, Allende and Acfer-086, by a team of scientists led by Harvard University biochemist Julie McGeoch. The report of the discovery was met with some skepticism and suggestions that the researchers had extrapolated too far from incomplete data. According to the researchers’ mass spectrometry, hemolithin is largely composed of glycine and hydroxyglycine amino acids. The researchers noted that the protein was related to “very high extraterrestrial” ratios of Deuterium/Hydrogen (D/H); such high D/H ratios are not found anywhere on Earth, but are “consistent with long-period comets” and suggest, as reported, “that the protein was formed in the proto-solar disc or perhaps even earlier, in interstellar molecular clouds that existed long before the Sun’s birth”. Its deuterium to hydrogen ratio is 26 times terrestrial which is consistent with it having formed in an interstellar molecular cloud, or later in the protoplanetary disk at the start of the Solar System 4.567 billion years ago. The elements hydrogen, lithium, carbon, oxygen, nitrogen and iron that it is composed of, were all available for the first time 13 billion years ago after the first generation of massive stars ended in nucleosynthetic events.

Given that some meteorites contain stardust grains that are older than our solar system, it is not outlandish to imagine that they could also preserve proteins that date back billions of years. Hemolithin is a particularly intriguing example because it might be able to split water into its constituent oxygen and hydrogen parts, which is a process that played a major role in the development of life on Earth. Hemolithin’s water-splitting behavior is only speculation at this point. If true, this could be a chemical energy source, which is the most important ingredient for a biochemical process leading on to life.

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Cosmic dust:

Cosmic dust – also called extraterrestrial dust, space dust, or star dust – is dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 μm), such as micrometeoroids. Larger particles are called meteoroids. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust (as in the zodiacal cloud), and circumplanetary dust (as in a planetary ring). There are several methods to obtain space dust measurement. This space dust rains down on us every day, hitting the planet at speeds of between 22,400 mph and 157,000 mph (36,000 to 253,000 km/h) and small particles floating at least 93 miles (150 kilometers) above Earth’s surface could theoretically get knocked into space by this wandering dust. It’s unclear if microbes could survive such violent collisions.

In the Solar System, interplanetary dust causes the zodiacal light. Solar System dust includes comet dust, planetary dust (like from Mars), asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. Thousands of tons of cosmic dust are estimated to reach Earth’s surface every year, with most grains having a mass between 10^−16 kg (0.1 pg) and 10^−4 kg (0.1 g). The density of the dust cloud through which the Earth is traveling is approximately 10^−6 dust grains/m3.

Cosmic dust contains some complex organic compounds (amorphous organic solids with a mixed aromatic–aliphatic structure) that could be created naturally, and rapidly, by stars. A smaller fraction of dust in space is “stardust” consisting of larger refractory minerals that condensed as matter left by stars. Interstellar dust particles were collected by the Stardust spacecraft and samples were returned to Earth in 2006.

Space dust that regularly hits Earth could contain proof of extraterrestrial life:

If we want to find evidence for alien life, we don’t need to keep looking for chemicals in exoplanet atmospheres or distant radio signals, says a Japanese astronomer. Instead, we should be studying the thousands of micrometer-sized bits of interstellar dust that hit Earth every year. University of Tokyo Professor Tomonori Totani proposed the new approach in a paper in which he suggests bits of space dust could contain the remains of single-celled organisms or other chemical evidence of life. This dust would, of course, have to make it through the inhospitable gauntlet of its own solar system and millions of years of travel through the void of space without being intercepted first. Despite the incredibly low chance that pieces of dust ejected from an exoplanet millions of years ago actually make it to Earth, Totani believes around 100,000 pieces of dust worth a look land on our planet every year. “Given there are many unknowns involved, this estimate could be too high or too low, but the means to explore it already exist so it seems like a worthwhile pursuit,” Totani said.

Plenty of space rocks hit Earth each year, leaving behind meteorites filled with potential scientific discoveries. Unfortunately, in the process of entering the atmosphere, heat burns off a lot of material – like traces of biological life. But the same rule doesn’t apply to dust particles smaller than 10-100 micrometers, which Totani said are able to survive atmospheric entry without generating much heat, meaning biosignatures on or in the dust could survive the trip. Micrometeorites have been found both in snow and ice samples from Antarctica, and deep-sea sediment, Totani said in the paper. He also noted, however, that identifying interstellar dust wouldn’t be easy. Extrasolar particles scattered by giant planets and then bound to the Solar System may be difficult to distinguish from particles ejected from Earth, even if they contain biosignatures. To catch these particles, Totani suggests trying to do so in space, instead of waiting for them to fall to Earth, by using material like aerogel, a super-lightweight foam made from silica that’s 95 percent air. NASA has already applied aerogel to such a purpose, in fact: During the Stardust mission in the early 2000s, aerogel was used to catch bits of dust coming off a comet in a manner similar to what Totani proposes.

Solid grains ejected from terrestrial exoplanets as a probe of the abundance of life in the Milky Way, a 2023 study:

Searching for extrasolar biosignatures is important to understand life on Earth and its origin. Astronomical observations of exoplanets may find such signatures, but it is difficult and may be impossible to claim unambiguous detection of life by remote sensing of exoplanet atmospheres. Here, another approach is considered: collecting grains ejected by asteroid impacts from exoplanets in the Milky Way and then traveling to the Solar System. The optimal grain size for this purpose is around 1 µm, and though uncertainty is large, about 10^5 such grains are expected to be accreting on Earth every year, which may contain biosignatures of life that existed on their home planets. These grains may be collected by detectors placed in space, or extracted from Antarctic ice or deep-sea sediments, depending on future technological developments.

An important issue not discussed here is whether biosignatures are preserved until the exoplanet particles reach Earth. They may be damaged at various stages, including launches from the home planets, exposure to radiation and cosmic rays in interstellar space, entry to Earth, and weathering in Earth environments. Microbial carcasses would be most vulnerable to damage, while microfossils and biominerals would be more likely to be preserved. It is important to investigate and choose the best biosignatures for this purpose, which should be abundant on terrestrial planets harboring life and identifiable after a long travel from their home. Identifying grains of extrasolar origin would not be easy after eliminating the possibility of terrestrial or Solar-System origin.

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UFO:

Ufology is the investigation of unidentified flying objects (UFOs) by people who believe that they may be of extraordinary origins (most frequently of extraterrestrial alien visitors). While there are instances of government, private, and fringe science investigations of UFOs, ufology is generally regarded by skeptics and science educators as an example of pseudoscience. Unidentified anomalous phenomena (UAP) is a term that is used in place of UFOs (unidentified flying objects). In 2021, astronomer Avi Loeb launched The Galileo Project which intends to collect and report scientific evidence of extraterrestrials or extraterrestrial technology on or near Earth via telescopic observations. In Germany, the University of Würzburg is developing intelligent sensors that can help detect and analyze aerial objects in hopes of applying such technology to UAP. A 2021 Gallup poll found that belief among Americans in some UFOs being extraterrestrial spacecraft grew between 2019 and 2021 from 33% to 41%. Gallup cited increased coverage in mainstream news and scrutiny from government authorities as a factor in changing attitudes towards UFOs. In 2022, NASA announced a nine-month study starting in fall to help establish a road map for investigating UAP – or for reconnaissance of the publicly available data it might use for such research. In 2023, the RAND Corporation, published a study reviewing 101,151 public reports of UAP sightings in the United States from 1998 to 2022. The models used to conduct the analysis showed that reports of UAP sightings were less likely within 30 km of weather stations, 60 km of civilian airports, and in more–densely populated areas, while rural areas tended to have a higher rate of UAP reports. The most consistent and statistically significant finding was that reports of UAP sightings were more likely to occur in areas within 30 km of military operations areas, where routine military training occurs.

NASA didn’t find Aliens as of September 2023:

76 years after the infamous Roswell incident, when a high-altitude balloon—or something—crashed in southeastern New Mexico, the US National Aeronautics and Space Administration (NASA) has now officially weighed in on UFO sightings. Don’t get too excited: They haven’t proven, or disproven, the existence of aliens. Instead, the report released recently by the agency’s independent study team describes how NASA should assess new reports of “unidentified anomalous phenomena” (UAP), a term that federal agencies use in place of UFOs (unidentified flying objects). It stresses that the agency should make use of machine learning and artificial intelligence as analytical tools, but that first it needs higher quality data to analyze. The top takeaway from the study is that there is a lot more to learn. The NASA study did not find any evidence that UAP have an extraterrestrial origin, but we don’t know what these UAP are.

The main problem is that while there are plenty of eyewitness accounts of strange lights in the sky, very little high-quality, standardized data has been collected from these incidents. Most sightings involve a fleeting encounter—and perhaps only a single opportunity for photographs. As the report puts it: “The nature of science is to explore the unknown, and data is the language scientists use to discover our universe’s secrets. Despite numerous accounts and visuals, the absence of consistent, detailed, and curated observations means we do not presently have the body of data needed to make definitive, scientific conclusions about UAP.” Analysis, it continues, “is hampered by poor sensor calibration, the lack of multiple measurements, the lack of sensor metadata, and the lack of baseline data.”

The team’s new report lays out specific recommendations for how to improve data collection. Among them: using sensors aboard NASA’s fleet of Earth-observing satellites that monitor atmospheric and oceanic conditions to look for corroborating evidence and to rule out natural causes, using Synthetic Aperture Radar satellites to “provide critical validation of any truly anomalous properties, such as rapid acceleration or high-G maneuvers,” and using the NEXRAD Doppler radar network “for distinguishing interesting objects from airborne clutter.”

Pentagon’s UFO sightings:

After an 18-month tenure as head of the All-domain Anomaly Resolution Office (AARO), the Pentagon’s UFO chief, Sean Kirkpatrick has stepped down. Kirkpatrick, who delayed his retirement in the pursuit of evidence regarding extraterrestrial phenomena, recently asserted that numerous reported sightings of unidentified flying objects (UFOs) over the United States could be attributed to either foreign nations or aliens, with the latter being the more concerning possibility. During his brief directorship, Kirkpatrick delved into over 800 cases, issuing a cautionary alert about the existence of UFOs. “If we don’t prove it’s aliens, then what we’re finding is evidence of other people doing stuff in our own backyard, and that’s not good,” Kirkpatrick said. While concrete evidence of extraterrestrial activity remains elusive, he emphasized that the alternative, indicating foreign entities operating within U.S. airspace, is a disconcerting prospect.

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Section-19

Communications with extraterrestrial intelligence (ETI):

If a person is lost in the wilderness, they have two options. They can search for civilization, or they could make themselves easy to spot by building a fire or writing HELP in big letters. For scientists interested in the question of whether intelligent aliens exist, the options are much the same. For over 70 years, astronomers have been scanning for radio or optical signals from other civilizations in the search for extraterrestrial intelligence, called SETI. We could group a signal into two types – intentional or unintentional. An intentional message would be a message directed at us (or whoever is listening). An unintentional message would be a case where we “overhear” the aliens. Most scientists are confident that life exists on many of the millions of potentially habitable worlds in the Milky Way galaxy. Astronomers also think there is a decent chance some life forms have developed intelligence and technology. But no signals from another civilization have ever been detected, a mystery that is called “The Great Silence.” While SETI has long been a part of mainstream science, METI, or messaging extraterrestrial intelligence, has been less common.

The search for extraterrestrial intelligence (SETI) has provoked many critical discussions on technical and philosophical levels (Cirkovi´c 2013´). It is much debated whether contact with ETI would benefit or harm humanity (Baum et al. 2011; Shostak 2014; Brin 2014; Billingham & Benford 2014), and whether mankind should (Benford 2014; Gertz 2016b) or should not (Zaitsev 2011; Vakoch 2016) keep quiet in order to protect Earth from threats, or even “cloak” our planet using lasers to compensate for Earth’s transit signatures (Kipping & Teachey 2016).

One of the scenarios considered in the literature is the reception of an ETI message through electromagnetic radiation, e.g. through a radio telescope (Cocconi & Morrison 1959). Alternatively, a message might be found in the form of, or through, an alien probe, as first suggested by Bracewell (1960). It was suggested to search the solar system for non-terrestrial artifacts (Papagiannis 1995; Tough & Lemarchand 2004; Haqq-Misra & Kopparapu 2012), particularly for starships (Martin & Bond 1980) in addition to classical SETI (Gertz 2016a). In our solar system, probes are speculated to be in geocentric, selenocentric, Earth-Moon libration, and Earth-Moon halo orbits (Freitas & Valdes 1980; Valdes & Freitas 1983; Freitas 1983), or buried on the moon (Clarke & Kubrick 1993). Alternative ideas include the Kuiper belt (Loeb & Turner 2012), general technosignatures (Wright 2017), or even “footprints of alien technology on Earth” (Davies 2012).

Active SETI (Active Search for Extra-Terrestrial Intelligence) is the attempt to send messages to intelligent extraterrestrial life. Active SETI messages are predominantly sent in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message. Active SETI is also known as METI (Messaging to Extra-Terrestrial Intelligence). Whether or not to conduct Active SETI, as well as the tone of any message, is a highly controversial topic. Active SETI has primarily been criticized due to the perceived risk of revealing the location of the Earth to alien civilizations, without some process of prior international consultation. That is, Active SETI does not meet the criteria for informed consent in a mass experiment involving human subjects and, potentially, nonhuman sentient subjects.

The below projects have targeted stars between 17 and 69 light-years from the Earth. The exception is the Arecibo message, which targeted globular cluster M13, approximately 24,000 light-years away. The first interstellar message to reach its destination was the Altair (Morimoto – Hirabayashi) Message, which likely reached its target in 1999.

The Morse Message (1962)
Arecibo Message (1974)
Cosmic Call 1 (1999)
Teen Age Message (2001)
Cosmic Call 2 (2003)
Across the Universe (2008)
Hello from Earth (2009)
Wow! Reply (2012)
Lone Signal (2013)
A Simple Response to an Elemental Message (2016)

The communication with extraterrestrial intelligence (CETI) is a branch of the search for extraterrestrial intelligence (SETI) that focuses on composing and deciphering interstellar messages that theoretically could be understood by another technological civilization. The best-known CETI experiment of its kind was the 1974 Arecibo message composed by Frank Drake. CETI research has focused on four broad areas: mathematical languages, pictorial systems such as the Arecibo message, algorithmic communication systems (ACETI), and computational approaches to detecting and deciphering “natural” language communication. There remain many undeciphered writing systems in human communication, such as Linear A, discovered by archaeologists. Much of the research effort is directed at how to overcome similar problems of decipherment that arise in many scenarios of interplanetary communication.

The “Wow!” Signal:

The WOW! signal was a strong narrowband radio signal detected by the SETI Institute’s Big Ear radio telescope in Ohio on August 15, 1977. See figure below. It was 30 times louder than the background noise and, unlike natural radio sources like quasars, it only hit one frequency on the radio spectrum. The signal appeared to be of extraterrestrial origin and was detected only once, despite attempts to observe it again.

The WOW! signal received its name from astronomer Jerry Ehmann, who was studying the data from the Big Ear telescope at the time and circled the signal on the printout, writing the word “WOW!” next to it. The signal was considered one of the most promising candidates for a sign of extraterrestrial origin, and it has generated a great deal of interest and speculation over the years. Despite efforts to determine the source of the Wow! It was never detected again, and its origin remains a mystery. Some scientists believe that it could have been caused by extraterrestrial intelligence, while others propose more natural explanations, such as a passing comet or a glitch in the telescope’s equipment.

Arecibo message:

Despite the mystery surrounding the WOW! Signal, there is hope for future communication with extraterrestrial life. If there is extraterrestrial life, they may be intelligent enough to have their own SETI program. Unfortunately, most of Earth’s transmissions are not strong enough to be detected. However, The Arecibo broadcast proved to be an exception. See figure below. It was a 3-minute-long message that was transmitted into space from the Arecibo Observatory in Puerto Rico in 1974. The message was sent as a radio signal aimed at M13, a star cluster located about 25,000 light-years from Earth.

The message was encoded in the form of a series of 1s and 0s. It contained information about human beings and the Earth, including our DNA structure, the numbers 1 through 10, a diagram of the solar system, and a depiction of a human being. The message was transmitted in the hopes that an intelligent extraterrestrial civilization would receive it and provide them with a basic understanding of human life and our place in the universe. The Arecibo broadcast was the first attempt to intentionally transmit a message into space, and it has generated great interest and debate over the years. Some people believe it is essential to try communicating with other intelligent civilizations, while others argue that it could be dangerous to contact extraterrestrial beings.

Other than the Arecibo broadcast, there is also another form of communication: the voyager spacecraft. They carry a “Golden Record,” a phonograph record with a message for any extraterrestrial life that might find it (figure below), including images and sounds of Earth and its inhabitants and instructions on how to play the record.

The Voyager spacecraft carry a variety of scientific instruments, including cameras, spectrometers, and magnetometers, that have been used to study the planets, moons, and interplanetary space. They sent back a wealth of data and images that have greatly expanded our understanding of the outer solar system.

The Voyager missions discovered active volcanoes on Jupiter’s moon Io, revealed Saturn’s complex atmosphere and weather patterns, and found that some of Saturn’s moons have subsurface oceans, making them potential locations for life. The Voyager spacecrafts continue to transmit data back to Earth, despite being more than 22 billion kilometers away from the Earth.

Beacon in the Galaxy (Updated Arecibo message):

An updated, binary-coded message has been developed for transmission to extraterrestrial intelligences in the Milky Way galaxy. The proposed message includes basic mathematical and physical concepts to establish a universal means of communication followed by information on the biochemical composition of life on Earth, the Solar System’s time-stamped position in the Milky Way relative to known globular clusters, as well as digitized depictions of the Solar System, and Earth’s surface. The message concludes with digitized images of the human form, along with an invitation for any receiving intelligences to respond. Calculation of the optimal timing during a given calendar year is specified for potential future transmission from both the Five-hundred-meter Aperture Spherical radio Telescope in China and the SETI Institute’s Allen Telescope Array in northern California to a selected region of the Milky Way which has been proposed as the most likely location for life to have developed. These powerful new beacons, the successors to the Arecibo radio telescope which transmitted the 1974 message upon which this expanded communication is in part based, can carry forward Arecibo’s legacy into the 21st century with this equally well-constructed communication from Earth’s technological civilization.

Figure above shows a sample from a new message intended to be sent toward potential intelligent extraterrestrials in the galaxy. It is a pictorial representation of binary and decimal systems in the Beacon in the Galaxy message.

The new message, called the “Beacon in the Galaxy,” conveys more information about mathematics and science; a map of the Earth; more detailed male and female human figures; the structure and composition of the Earth; and an invitation to reply. Like the Arecibo message, researchers created it as a bitmap, which is a way to use binary to create a pixelated image. The motivation for the design was to deliver the maximum amount of information about our society and the human species in the minimal amount of message. With improvements in digital technology, we can do much better than the [Arecibo message] in 1974.

But there’s no guarantee extraterrestrial life will understand the message—or that those listening will be peaceful. Some scientists, including theoretical physicist Stephen Hawking, have warned against actively reaching out to other life forms, saying they may have no problem wiping out humanity. “We don’t know much about aliens, but we know about humans,” Hawking said in 2015. “If you look at history, contact between humans and less intelligent organisms have often been disastrous from their point of view, and encounters between civilizations with advanced versus primitive technologies have gone badly for the less advanced.”

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Extraterrestrial Message basics:

Every interstellar message must address two fundamental questions: what to say and how to say it. Nearly all the messages that humans have broadcast into space so far start by establishing common ground with a basic lesson in science and mathematics, two topics that are presumably familiar to both ourselves and extraterrestrials. If a civilization beyond our planet is capable of building a radio telescope to receive our message, it probably knows a thing or two about physics. A far messier question is how to encode these concepts into the communiqué. Human languages are out of the question for obvious reasons, but so are our numeral systems. Though the concept of numbers is nearly universal, the way we depict them as numerals is entirely arbitrary. This is why many attempts, including “Beacon in the Galaxy,” opt to design their letter as a bitmap, a way to use binary code to create a pixelated image.

The bitmap design philosophy for interstellar communication stretches back to the Arecibo message. It is a logical approach—the on/off, present/absent nature of a binary seems like it would be recognized by any intelligent species. But the strategy is not without its shortcomings. When pioneering search for extraterrestrial intelligence (SETI) scientist Frank Drake designed a prototype of the Arecibo message, he sent the binary message by post to some colleagues, including several Nobel laureates. None of them were able to understand its contents, and only one figured out that the binary was meant to be a bitmap. If some of the smartest humans struggle to understand this form of encoding a message, it seems unlikely that an extraterrestrial would fare any better. Furthermore, it is not even clear that space aliens will be able to see the images contained within the message if they do receive it.

“One of the key ideas is that, because vision has evolved independently many times on Earth, that means aliens will have it, too,” says Douglas Vakoch, president of METI (Messaging Extraterrestrial Intelligence) International, a nonprofit devoted to researching how to communicate with other life-forms. “But that’s a big ‘if,’ and even if they can see, there is so much culture embedded in the way we represent objects. Does that mean we should rule out pictures? Absolutely not. It means we should not naively assume that our representations are going to be intelligible.”

Messaging extraterrestrials has always occupied a controversial position in the broader SETI community, which is mostly focused on listening for alien transmissions rather than sending out our own. To detractors of “active SETI,” the practice is a waste of time at best and an existentially dangerous gamble at worst. There are billions of targets to choose from, and the odds that we send a message to the right planet at the right time are dismally low. Plus, we have no idea who may be listening. What if we give our address to an alien species that lives on a diet of bipedal hominins? “I don’t live in fear of an invading horde, but other people do. And just because I don’t share their fear doesn’t make their concerns irrelevant,” says Sheri Wells-Jensen, an associate professor of English at Bowling Green State University and an expert on the linguistic and cultural issues associated with interstellar message design. “Just because it would be difficult to achieve global consensus on what to send or whether we should send doesn’t mean we shouldn’t do it. It is our responsibility to struggle with this and include as many people as possible.”

Despite the pitfalls, many insist that the potential rewards of active SETI far outweigh the risks. First contact would be one of the most momentous occasions in the history of our species, the argument goes, and if we just wait around for someone to call us, it may never happen. As for the risk of annihilation by a malevolent space alien: We blew our cover long ago. Any extraterrestrial capable of traveling to Earth would be more than capable of detecting evidence of life in the chemical signatures of our atmosphere or the electromagnetic radiation that has been leaking from our radios, televisions and radar systems for the past century.

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Difficulties in communicating with extraterrestrial intelligence:

-1. Time

Eighteen years from now we might get a call from aliens. In 2017, a powerful radio transmission was aimed at exoplanet GJ 273b, thought to be able to support life. Its message, sent by the alien-hunting group Messaging Extraterrestrial Intelligence International, contained instructions on how to understand Earthling math, music and time. If it lands on intelligent alien ears once it arrives in about a decade, E.T. now has our number. Humans eager to make friends in other star systems might be disappointed to learn that any developing relationship will likely resemble a phenomenally slow pen pal correspondence, rather than one conducted at the speed of text or email—never mind light. There’s considerable distance between us and, for instance, GJ 273b: 12.36 light-years to be precise. At that distance, it will take a dozen years for our message to arrive and then another dozen for us to receive the return message. It would be 2041, at the earliest, by the time we get a reply. And, GJ 273b is one of the closer exoplanets (a planet that orbits a star other than the sun). There are only 12 stars within 10 light years of Earth around which exoplanets could circle. That means any exchange of information would take place across at least 20 years and more likely many decades.

-2. The constraints of communication

It is assumed that if we can build a radio telescope, extraterrestrials would understand our math, understand our science but that may not be plausible. In the early 1970s, NASA sent the “Pioneer Plaques” aboard the Pioneer spacecraft, the first ships designed to achieve escape velocity from our solar system. The plaques, intended to be a message to any extraterrestrials that might find them, contained drawings of nude humans, and pictorial symbols representing our solar system and a hydrogen atom. But even if intelligent extraterrestrial beings understood, for example, the chemical composition of the universe, they might have such a radically different way of representing it that our pictograms would be completely uninterpretable.

-3. Language issue

If we found a signal that we did think was from extraterrestrials, the really hard part would now begin – to analyze the signal. But before we get too excited – remember – we can’t communicate with elephants or cats. How can we even hope to communicate with an intelligence from another planet?

In human languages, we have concepts like nouns, verbs, singular and plural, and words to express the passage of time. But are these words a result of our neuroanatomy? Do we communicate about time in a certain way because we are inhabiting a rather flat portion of space-time? Do we have the concepts of numbers because we see ourselves as distinct entities from one another? We can’t peer into alien psychology and understanding – so right now, we just don’t know. Similarities that connect human languages over time may not connect our languages and that of ET.

One thing that might help however, is Zipf’s Law. Zipf’s Law shows up in many areas in the social world and in the physical sciences, and even in the most general definition of language. Named for linguist George Kingsley Zipf, who around 1935 was the first to draw attention to this phenomenon, the law examines the frequency of words in natural language and how the most common word occurs twice as often as the second most frequent word, three times as often as the subsequent word and so on until the least frequent word. The word in the position n appears 1/n times as often as the most frequent one. This has worked for many human languages and has even been used for dolphins – showing that, while we may not understand it, the squeaks and squawks of dolphins aren’t just sounds – they are a language of sorts. The same concept could be applied to an alien signal.

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Interstellar communication message decontamination is impossible: a 2018 paper:

A complex message from space may require the use of computers to display, analyze and understand. Such a message cannot be decontaminated with certainty, and technical risks remain which can pose an existential threat. Complex messages would need to be destroyed in the risk averse case.

While it has been argued that sustainable ETI is unlikely to be harmful (Baum et al. 2011), we cannot exclude this possibility. After all, it is cheaper for ETI to send a malicious message to eradicate humans compared to sending battleships. If ETI exist, there will be a plurality of good and bad civilizations. Perhaps there are few bad ETI, but we cannot know for sure the intentions of the senders of a message. Consequently, there have been calls that SETI signals need to be “decontaminated” (Carrigan 2004, 2006).

In this paper, authors show that it is impossible to decontaminate a message with certainty. Instead, complex messages would need to be destroyed after reception in the risk averse case.

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Section-20

Why search extraterrestrial life/intelligence:

The reasons to explore the universe are as vast and varied as the reasons to explore the forests, the mountains, or the sea. Since the dawn of humanity, people have explored to learn about the world around them, find new resources, and improve their existence. NASA’s exploration vision is anchored in providing value for humanity by answering some of the most fundamental questions: Why are we here? How did it all begin? Are we all alone? What comes next? And, as an addendum to that: How can we make our lives better?

Extraterrestrial life exploration is one of the most important issues in space science. If extraterrestrial life were to be detected, the general conception on the universe will change, and findings will initiate a totally new biology addressing extraterrestrial life, different from the current biology addressing only the life form present on Earth. The negative result of life not being detected in a certain parameter space is also significant not only for science, but also for space utilization and planetary protection because the result will provide information on safety issues.

If you ask any of SETI members to explain their motivations, a common refrain is that the scientific revelations of discovering ET would be unparalleled. If they were to discover that there is life out there — intelligent life that has forged a civilization — it would first mean that biology is not a fluke. Instead, it is something that can take root on many worlds; something that does not merely arise but repeatedly produces thinking, technological, curious creatures, those that may wish to share their knowledge of the universe, and their way of traversing or surviving it, with others. And if this civilization existed on a world very different from Earth, it would demonstrate that the largely unliveable cosmos is populated by myriad different isles of habitability.

Hunting for extraterrestrial biology differs from most science in that its hypothesis can’t be disproven. Most researchers think there must be life elsewhere in the cosmos, and polls show that the public generally agrees. But unlike the majority of research assertions, there’s no way to demonstrate that such life doesn’t exist. The hypothesis of a universe laced by biology can’t be falsified. So, by the conventions of science, you could say that our experiments to find other life – whether microbes in the Solar System or extraterrestrials on a planet hundreds of light-years away – are not really experiments: they’re searches. They’re exploration. As a social activity, exploration has been essential to survival. The ancient Egyptians weren’t terribly interested in lands beyond the shores of the Nile, leading to ossification of their culture and its eventual subjugation by the Greeks and the Romans. The Renaissance, which marked the transition from feudalism to modernity, might have stalled if it hadn’t been accompanied by the Age of Discovery.

There is also the matter of simple curiosity – a seemingly lightweight word. Curiosity has led to good things: clever inventions and major revelations in natural science, psychology, medicine, social behavior, and just about everything else. You might also argue that finding extraterrestrial biology will give us cosmic context; we’ll have insight into our own importance. People have suggested that finding intelligent beings on distant planets would be good news for us, demonstrating that Homo sapiens is not inevitably fated to self-destruction. If the aliens can survive their own technology, so can we. Evolution has built into our natures our wish to learn something new. And sure, you could point out that there’s obvious survival value in wanting to know what lies beyond the nearest chain of hills.

The search for exoplanets and life can help us understand earthly problems, such as climate change:

If we look at Venus and Mars, we can already anticipate some consequences of global climate changes. It was a lot warmer here on Earth during the Cretaceous period, but the climate transitions in geological history happened over a really long period of time. What is alarming about today is the astonishing rate at which carbon dioxide and other greenhouse gases accumulate in the atmosphere, largely due to human factors. Climatic models have predicted major changes in weather patterns and coastal boundaries within our children’s generation. There is a sense of urgency to quell this trend. Although current political discussions by world leaders have mostly focused on cutting carbon emissions, it is also important to explore carbon capture technologies. We can get some inspiration in this regard by understanding atmospheric circulation and climatic dynamics on other planets. For instance, the greenhouse effect on Venus is so strong that rain evaporates before it reaches the ground. We do not want this to happen on Earth, because it would cut off a major channel of carbon return from the Earth’s atmosphere to the soil and rocks on Earth’s surface. We need to unravel the complex interplay between air, ocean, sea ice, land surface, atmosphere chemistry and biological and industrial activities on a global scale as well as to understand the long-term carbon deposit-and-release geophysical cycle in and beneath the Earth’s rocky crust. This comprehensive knowledge will be useful for strategizing effective, holistic approaches to quench and hopefully to thwart the current trend of rapid global warming.

Finding extraterrestrial intelligence would show us an ideal version of our future selves:

Alien civilization must have encountered difficulties like the ones that face us today. Wars, climate change, pollution, decreasing biodiversity, and so on. The mere fact that they have survived would indicate that they have a functioning social structure that can handle and avoid crises (or at least that they are able to recover from them), and that they have a complex social system that regulates risks and destructive behavior. Finding extraterrestrial intelligence, in other words, would show us an ideal version of our future selves—a more cohesive society, one that’s better at stewarding its planet and less murderous of each other. To search for such a civilization indicates a hope that those versions of intelligence are out there, stably surfing through the universe. These beings may not have kept their Earth-twin planet pristine. Neither would it be that Ur-Earth for which astronomers feel hiraeth. But it would be a place—a real place—that would look nice on a poster. “It may not be perfect,” these ideal aliens may beam in a message to us, “but it will always be home.”

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Moral of the story:

-1. There are an estimated 400 billion stars in our galaxy Milky Way and an estimated two trillion galaxies in the universe. The formation of planetary systems arises from the existence of nebulae of stars. If the star is too hot, radiation pressure will disperse its nebula rapidly, leaving, if anything, small atmosphere-less planets, or a system of millions of tiny asteroids. Below 7000 deg Celsius temperature, stars retain enough of their nebulae to form planets (provided they have not used up their nebulae in forming double or multiple sun systems). The number of such stars is between one and ten per cent of the total number of stars, suggesting that there are billions of solar systems in our galaxy alone. Multiplying it with two trillion galaxies in the universe, that makes for trillions upon trillions of worlds on which life could, in theory at least, have taken hold. It seems likely that quite a high fraction of stars have Earth-like planets, i.e., rocky planets with a temperature that allows liquid water on surface.

-2. Our Sun is a stable, long-lasting, and metal-rich star:

Our Sun is the most important source of energy for life on Earth. The Sun is a G-type main-sequence star (G2V) based on spectral class and is informally referred to as a yellow dwarf. It is stable (even less active compared to its siblings), long-lasting, and metal-rich. The Sun is also unusually metal-rich for a star of its age and type. One possibility is that the Sun formed in a part of the Milky Way Galaxy that had an abundance of metals and then migrated to its current position. Metal-rich stars are more likely to have planets orbiting around them. Furthermore, all life forms require certain core chemical elements needed for biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur, often represented by the acronym CHNOPS. Our solar system contains a lot of these metals (all elements heavier than hydrogen and helium are called metals in astronomy). However, our Sun is actually not the best kind of star for hosting a planet with lots of life on it.

-3. Main sequence stars are those that fuse hydrogen into helium in their cores. The Morgan–Keenan system classifies stars based on their spectral characteristics. Our Sun is a yellow dwarf G-type star. The sun worked for us because it has a balanced mass, not so big that it burns out quickly and not so small that it doesn’t produce enough energy. The most common class of star out there in the Universe — red dwarf (M-class) stars make up 75-80% of all stars — and there are all sorts of reasons why life is unlikely to exist there. The analyses carried out by the researchers point to dwarfs of type K as the most promising in maintaining life outside Earth. This is because they are not prone to radical changes and proton explosions, like the M-class ones, and they last longer than the G-class ones, contributing to the development of a broader biodiversity. K-type stars and low-luminosity G-type stars, collectively referred to as orange dwarfs, are less massive than the Sun, and are stable on the main sequence for a very long time (18 to 34 billion years, compared to 10 billion for the Sun, a G2V star), giving more time for the emergence of life and evolution. Since complex life took about 4.5 billion years to appear on Earth, the longer lifetimes of orange dwarf stars could give planets within their habitable zones more time to develop life and accrue biodiversity. In addition, orange dwarfs emit less ultraviolet radiation (which can damage DNA and thus hamper the emergence of nucleic acid based life) than stars like the Sun.

-4. The Sun is our local host star, and all the other stars are also enormous balls of glowing gas that generate vast amounts of energy by nuclear fusion reactions deep within. The other stars look faint only because they are very far away. We are able to see the nearby planets in our skies only because they reflect the light of our local host star, the Sun. If the planets were much farther away, the tiny amount of light they reflect would usually not be visible to us. The exoplanets we have so far discovered orbiting other stars were found from the pull their gravity exerts on their parent stars, or from the light they block from their stars when they pass in front of them. We can’t see most of these exoplanets directly, although a few are now being imaged directly.

-5. Life and extraterrestrial life:

-6. Astrobiology, also called exobiology or xenobiology, is a multidisciplinary field that studies the origins, early evolution, distribution, and future of life in the universe by investigating its deterministic conditions and contingent events. As a discipline, astrobiology is founded on the premise that life may exist beyond Earth. Research in astrobiology comprises three main areas: the study of habitable environments in the Solar System and beyond, the search for planetary biosignatures of past or present extraterrestrial life, and the study of the origin and early evolution of life on Earth. Astrobiology encompasses areas of biology, astronomy, and geology. Astrobiology and the Search for Extraterrestrial Intelligence (SETI) are now part of mainstream science, pursued at both NASA and prestigious universities worldwide.

-7. Many people make a distinction between the origin of life and the evolution of life. In this view, biological evolution refers to the gradual development of the diversity of living things from a common ancestor, while the ultimate origin of life is a separate question. However, without the idea of spontaneous generation and a physical theory of life, the doctrine of evolution is a mutilated hypothesis without unity or cohesion. Therefore, origin of life and evolution of life are inseparable and represent continuum from non-living molecules to living molecules to living organisms.

-8. Extraterrestrial life detection can be divided in 4 groups:

In situ detection of life as we know it (e.g. Mars)
In situ detection of life as we don’t know it (e.g. Titan)
Remote detection of life as we know it (e.g. exo-earth)
Remote detection of life as we don’t know it (target unknown)

Search for life also depends on what kind of life it is—micro versus macro and extant versus extinct. Macro life would probably be pretty easy to find no matter what it was made of. Finding extant microbes might be possible. The biggest difficulty, of course, would be detecting extinct microbes.

-9. The Drake equation, originally penned by astronomer Frank Drake in the 1960s, laid out a series of terms estimating how many intelligent extraterrestrial civilizations likely exist in the Milky Way Galaxy. The equation takes into account factors such as the rate of star formation in the galaxy, the fraction of planets where life emerges, and the fraction of that life that gains intelligence and the capability to broadcast its presence into space. Over the years, the equation has acted as a road map for researchers searching for communications signals created by intelligent civilizations beyond Earth. Criticism of the Drake equation follows mostly from the observation that several terms in the equation are largely or entirely based on conjecture. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.

-10. Life on Earth, structurally based on carbon, hydrogen, nitrogen, oxygen and other elements, uses water as its interaction medium. Life functions through the specialized chemistry of carbon and water, and builds largely upon four key families of chemicals: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and nucleic acid DNA and RNA for the mechanisms of heredity. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules.

Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulphur are often referred to as the ingredients of life, as their presence is found from the largest mammals to the smallest paramecium. Since the most abundant element, cosmically, is hydrogen, the atmosphere of the early protoplanets of any system must contain much hydrogen and hydrogen compounds. The hydrogen compounds of carbon, nitrogen, and oxygen are probably the most abundant hydrogen compounds in the proto-atmosphere. They are, respectively, methane, CH4, ammonia, NH3, and water vapor, H20.

Research showed that when hydrogen, methane, ammonia, and water vapor are mixed together, and supplied with energy, some fundamental organic compounds are produced that precisely are the molecules necessary to form life as we know it. And it can be shown that seas of water would be most efficient in collecting and preserving these bio-molecules. Researchers have found a way that the genetic molecule RNA could have formed from chemicals present on the early earth. These nucleic acids have some unusual properties; so far as we know, ones not found in any other molecules. They can not only construct other identical molecules from the matter floating in the sea around it, but if changed in some way can also construct copies of its changed structure. Such a mutating, self-reproducing molecule or collection of molecules must undergo natural selection. For these reasons, it must be identified as the first living being on the planet in question. Studies have supported the hypothesis that primitive cells containing molecules similar to RNA could assemble spontaneously, reproduce and evolve, giving rise to all life. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA/ RNA to define and control their form.

Thus, there may be millions of planets in this galaxy alone on which flourish organisms at least biochemically akin to ourselves. On the other hand, due to natural selection, these organisms must be well adapted, each to its own environment. Since even slight differences in the environment eventually cause extreme differences in the structure of organisms, we should not accept extraterrestrial lifeforms to resemble anything familiar. But there is reason to believe they are out there. Just because we don’t see Earth-like conditions on other planets does not mean life, as we know it, cannot exist there.

-11. Panspermia posits that life exists throughout the universe, distributed by meteoroids, asteroids, comets and planetoids, and extraterrestrial life was transported to Earth thereby shifts the origin of life on Earth to another heavenly body. Panspermia is a fringe theory with little support amongst mainstream scientists.

Pseudo-panspermia posits that many of the pre-biotic organic building-blocks of life originated in space, became incorporated in the solar nebula from which planets condensed, and were further—and continuously—distributed to planetary surfaces where life then emerged. Pseudo-panspermia is a well-supported hypothesis in the origin of life.

-12. Role of water in life:

The liquid solvent for existence of life is likely to be water, both because of its cosmic abundance (it is one of the most cosmically abundance molecules, consisting of the first [H] and third [O] most abundant elements) and its distinct physicochemical properties that make it highly suitable for mediating macromolecular interactions. It has chemical properties that no other natural substance in the world can emulate. It takes a lot of energy to change the temperature of water — so it does a great job of insulating bodies from the cold while keeping them cool under heat. It’s excellent at carrying nutrients into cells while expelling waste and toxins. It can withstand sharp pressure shifts. It’s really good at dissolving others substances. Liquid water has been pointed out as the best solvent for life to emerge and evolve in. Some of the important characteristics of liquid water as a solvent include: a large dipole moment, the capability to form hydrogen bonds, to stabilize macromolecules, to orient hydrophobic–hydrophilic molecules, etc. Water plays an essential role in protein folding, protein substrate binding, enzyme actions, the rapid transport of protons in aqueous solution, maintaining the structural stability of proteins and DNA/RNA, and the inhomogeneous segregation of salt ions at cellular interfaces. Where there is water, organic molecules can come together and form living systems. Simply put, life as we know it can’t exist without water.

-13. Role of carbon in life:

Carbon’s high cosmic abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms.

The most important characteristics of carbon as a basis for the chemistry of life are that each carbon atom is capable of forming up to four valence bonds with other atoms simultaneously, and that the energy required to make or break a bond with a carbon atom is at an appropriate level for building large and complex molecules which may be both stable and reactive. The ability to form four bonds isn’t restricted to carbon though, it’s a property of every atom with four outer electrons, including silicon, tin and lead. What’s special about carbon, and the reason that silicon-based lifeforms are restricted to science fiction (and lead-based lifeforms are hardly ever mentioned) is that it can form double-bonds which share more than one electron with another atom.

Why is carbon able to do this while silicon can’t? The answer lies in the size. Carbon is the smallest of all the atoms with four outermost electrons, which means that the electrons in the above-and-below orbitals are close enough to overlap and form that second bond. For silicon however, there are more electron orbitals in the way, the entire atom is bigger, and it is almost impossible for the outer orbitals to get close enough to form a double bond. This is why carbon dioxide is a small gaseous molecule consisting of two oxygens both forming a double bond with a single carbon while silicon dioxide is a massive behemoth of a molecule made of huge numbers of alternating oxygen and silicon atoms and is more commonly known as sand.

It would be impossible for life on earth to exist without carbon. Carbon is the main component of sugars, proteins, fats, DNA, muscle tissue, pretty much everything in your body.

-14. Energy for life:

From basic thermodynamic considerations it is clear that life requires a source of energy. Thermodynamically, photosynthesis based on stellar radiation may be the optimal source of energy for extraterrestrial life. Photosynthetic organisms and the radiation they receive are not in thermodynamic equilibrium. On Earth, for example, a green plant may have a temperature of about 300 K (23 °C, or 73 °F); the Sun’s temperature is about 6,000 K. Photosynthetic processes are possible because energy is transported from a hotter object (the Sun) to a cooler object (Earth). Were the source of radiation at the same or at a colder temperature than the photosynthesizer, no photosynthetic activity would be possible. For this reason, the idea that a subterranean green plant will photosynthesize by use of thermal infrared radiation emitted by its surroundings is untenable. Equally unfeasible is the idea that a cold star, with a surface temperature similar to that of Earth, could sustain photosynthetic organisms.

Life requires energy, and because sunlight does not penetrate to sea floor, this would have to be chemical energy. The interaction of water and rocks—especially at high temperatures, as within Earth’s hydrothermal vent systems—yields a reducing chemistry (where molecules tend to give up electrons readily) that is like one half of a chemical battery. To complete the battery and provide energy that could be used by life requires that an oxidizing chemistry (where molecules tend to accept electrons readily) also be available. On Earth, when chemically reducing vent fluids meet oxygen-containing seawater, the energy that becomes available often supports thriving communities of microorganisms and animals on the sea floor, far from the light of the Sun. Now that it has become clear that chemoautotrophic life-forms do not require sunlight as sources of energy, some scientists argue to focus on subsurface water oceans or reservoirs inside the moons of Solar System gas giants and more than 335 exoplanets.

-15. Today, in 2024, there are three main ways we’re looking for extraterrestrial life:

[1. We’re exploring worlds in our Solar System, including Mars, Venus, Titan, Europa, and Enceladus, remotely, with fly-by missions, orbiters, landers, and even rovers, searching for evidence of past or even present simple life. Spacecraft have flown by, orbited around, or landed on Mercury, Venus, Mars, Jupiter and several of its moons, Saturn and several of its moons, the dwarf planet Pluto and its moons, and the dwarf planet Ceres. Ocean worlds in our solar system – in particular, Jupiter’s moon Europa and Saturn’s moons Enceladus and Titan – are top targets for astrobiological investigations of prebiotic chemistry, habitability, and possible life. Many astrobiologists are exploring the possibility of extant life in the deep subsurface of Mars. Perseverance rover on Mars is gathering rock samples for eventual return to Earth, so scientists can probe them for signs of life. And the coming Europa Clipper mission will visit an icy moon of Jupiter. Its goal: to determine whether conditions on that moon would allow life to thrive in its global ocean, buried beneath a global ice shell.

[2. We’re examining exoplanets, searching for evidence that there’s life on them, from the surface to the atmosphere and beyond, based on observable signatures of color, seasonal change, and atmospheric contents. An exoplanet is a planet outside our solar system, usually orbiting another star. As of November 24, 2023, there are 5,539 confirmed exoplanets. The closest exoplanet to Earth is Proxima Centauri b, which is about four light-years away. Few of them are in habitable zones neither too hot or too cold, neither too light to lack atmosphere or magnetic field nor too heavy that gravity becomes unbearable; these exoplanets are suitable for extraterrestrial life.

[3. And by looking for any signals/artifacts that would reveal the presence of intelligent aliens: through efforts like SETI and SETA.

-16. The chief assumption about habitable planets is that they are terrestrial. Terrestrial planets have a compact, rocky surface like Earth’s terra firma. Such planets, roughly within one order of magnitude of Earth mass, are primarily composed of silicate rocks, and have not accreted the gaseous outer layers of hydrogen and helium found on gas giants. Life requires terrestrial planets like Earth, and since gas giants lack such a surface, complex life cannot arise there. The possibility that life could evolve in the cloud tops of giant gas planets has not been decisively ruled out, though it is considered unlikely, as they have no surface and their gravity is enormous. The natural moons of giant gas planets, meanwhile, remain valid candidates for hosting life.

-17. Habitability:

Because planets either too close to or too far from their host stars will be at temperatures that cause water either to boil or to freeze, astrobiologists define a “habitable zone,” a range of orbital distances within which planets can support liquid water on their surfaces. Habitable zones are those regions from a star at which liquid water could potentially exist on the surface of orbiting planets, and these areas could be optimal for the development of life. If an exoplanet orbits at the appropriate range of distances from its star to allow liquid water to exist on its surface, then it is said to be in the habitable zone— it is not too hot, not too cold, purportedly just right for living things.

The habitable zone is also an atmosphere-specific concept.

Declaring a freshly detected exoplanet to be in the “habitable zone” amounts to little more than media spin if its atmospheric composition is unknown. Even professional astronomers sometimes forget this fact. Thus, habitable zone is defined as the range of distances, or annulus, around a star that would allow a planet with a given atmosphere to maintain surface liquid water. This is a very conservative (but observationally useful) definition, as a planet’s surface temperature depends not only on its proximity to its star but also on such factors as its atmospheric greenhouse gases, its reflectivity, and its atmospheric or oceanic circulation. Moreover, internal energy sources such as radioactive decay and tidal heating can warm a planet’s surface to the melting point of water. A planet may be defined as habitable if it has an atmosphere and is warm enough to support the existence of liquid water on its surface. Such a world has the basic set of conditions that allow it to develop life similar to ours, which is carbon-based and has water as its universal solvent. It is imperative that water remains in a liquid state in order for life to exist. Earth is until now the only example of a known habitable planet. Kepler, launched in 2009, found thousands of planets, more than 20 of which are Earth-sized planets in the habitable zone where liquid water can survive on the surface.

The habitable zone is also a star-specific concept (besides atmospheric specific).

Stars exist in a variety of sizes and masses. The location of a star’s habitable zone also depends upon its mass. Smaller stars like the Sun survive far longer than do high-mass stars. High-mass stars have lifetimes of only millions of years, whereas advanced life took billions of years to develop on Earth. Thus, even if Earth-like planets formed around high-mass stars at distances where liquid water was stable, it is unlikely that benign conditions would exist long enough on these planets for life to form and evolve into advanced organisms. At the other end of the mass spectrum, the smallest, faintest stars can last for trillions of years. However, these cool dwarf stars emit almost all of their luminosity at infrared wavelengths, which may be difficult for life to harness, and they typically display larger luminosity variations than do Sun-type stars. In addition, in order for a planet to remain within the habitable zone of a faint star, it would have to orbit so close that tidal locking to the host star is likely causing the same hemisphere always to face the star (just as the Moon’s near side always faces Earth). As a result, there would be no day-night cycle, and the planet’s atmosphere, unless it was sufficiently thick, would freeze onto the surface of the cold, perpetually dark hemisphere.

-18. Low-mass planets/ small planets are poor candidates for life.

Their lesser gravity makes atmosphere retention difficult. Constituent molecules are more likely to reach escape velocity and be lost to space when buffeted by solar wind or stirred by collision. Planets without a thick atmosphere lack the matter necessary for primal biochemistry, have little insulation and poor heat transfer across their surfaces, and provide less protection against meteoroids and high-frequency radiation. Further, where an atmosphere is less dense than 0.006 Earth atmospheres, water cannot exist in liquid form as the required atmospheric pressure, 4.56 mm Hg (608 Pa), does not occur. In addition, a lessened pressure reduces the range of temperatures at which water is liquid.

Smaller planets have smaller diameters and thus higher surface-to-volume ratios than their larger cousins. Such bodies tend to lose the energy left over from their formation quickly and end up geologically dead, lacking the volcanoes, earthquakes and tectonic activity which supply the surface with life-sustaining material and the atmosphere with temperature moderators like carbon dioxide. A planet that is too small cannot maintain much atmosphere, rendering its surface temperature low and variable and oceans impossible. A small planet will also tend to have a rough surface, with large mountains and deep canyons. The core will cool faster, and plate tectonics may be brief or entirely absent.

The mass of a potentially habitable exoplanet is between 0.1 and 5.0 Earth masses. The radius of a potentially habitable exoplanet would range between 0.5 and 1.5 Earth radii.

-19. We have concept of a galactic habitable zone, analogous to a stellar habitable zone. The concept of a stellar habitable zone has been extended to a planet’s location in the Milky Way Galaxy. Near the centre of the Milky Way, stars are typically much closer to one another than they are farther out on the spiral arms, where the Sun is located. At the galactic centre, therefore, phenomena such as supernovae might present a greater hazard to life than they would in the region where Earth is located. On the other hand, in the outer regions of the Milky Way beyond the location of Earth, there are fewer stars. Since the bulk of a terrestrial planet is composed of chemical elements that were produced within stars, the material out of which new stars are being formed may not have enough of those elements necessary for Earth-like planets to grow. Considerations of this type have led to the concept of a galactic habitable zone, analogous to a stellar habitable zone.

-20. NASA has defined the principal habitability criteria as “extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism”. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. If we detect unambiguous biosignatures, that means by definition the planet is habitable, because something is living there, then arguments over the term “habitable zone” might just fade away.

-21. Exoplanet detection:

It’s pretty rare for astronomers to see an exoplanet through their telescopes the way you might see Mars through a telescope from Earth. That’s called direct imaging, and only a handful of exoplanets have been found this way (and these tend to be young gas giant planets orbiting very far from their stars). Planets are extremely faint light sources compared to stars, and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is very difficult to detect and resolve them directly from their host star. Planets orbiting far enough from stars to be resolved reflect very little starlight, so such planets are detected through their thermal emission instead. So far, few exoplanets have been directly imaged – when pixels of light are captured from the planet itself. Very large, very young planets still glowing from the heat of formation are, so far, the only ones to be imaged this way. But planets past their youth, lit up only by their stars, would be targeted for direct imaging by space telescopes now in the conceptual phase, who would be using coronagraphs or starshades to block the glare from a star.

Most exoplanets are found through indirect methods: they are indirect ways of “seeing” planets, which means we are observing their effects but not the planets themselves. For example, measuring the dimming of a star that happens to have a planet pass in front of it, or monitoring the spectrum of a star for the tell-tale signs of a planet pulling on its star and causing its light to subtly Doppler shift. Both of these methods work best when planets are close to their star. For solar systems just like our own they would almost never work. NASA’s Kepler Space Telescope finds thousands of planets by observing “transits,” the slight dimming of light from a star when its tiny planet passes between it and our telescopes. Other indirect methods include Gravitational Microlensing, Astrometric Measurement etc.

Transit method:

If a star’s brightness temporarily dips, then something probably passed in front of it as seen from Earth. If it dips in regular intervals and by the same amount each time, that’s usually caused by a planet. This is transit method for exoplanet detection. But for this to work, though, you have to get very lucky – the planet and star have to line up just so. If you’re not feeling mega-lucky, then you have to look at tens of thousands or hundreds of thousands of stars to find the few that are lined up just right. With modern big digital cameras and modern computing, that’s possible. Automated software finds the possible planets, then astronomers figure out which ones are real and interesting. Because it’s so automated and computerized, that’s the way most planets have been discovered so far. While the radial velocity method provides information about a planet’s mass, the transit method can determine the planet’s radius. Transit method also has value-added advantages in collecting candidates for follow-up characterization of the chemical composition and physical make-up of the planets’ atmospheres during their transits. The transit method has the advantage of detecting planets around stars that are located a few thousand light years away.

Radial velocity or doppler spectroscopy method:

The radial-velocity method for detecting exoplanets relies on the fact that a star does not remain completely stationary when it is orbited by a planet. The star moves, ever so slightly, in a small circle or ellipse, responding to the gravitational tug of its smaller companion. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star’s spectral lines due to the Doppler effect. The movement of a light source, such as a star, changes the frequencies of light that the star emits. This is the same Doppler effect that makes a fire engine’s siren change in pitch as it moves past you (higher pitch as it moves toward you, lower as it moves away). When the star moves towards us, it releases higher-frequency light. When it moves away, the light is of lower frequency. If we see this type of rhythmic change in the light frequency emitted by a star, we know the star is being orbited by a significant body. This approach for detecting extrasolar planets is known as Doppler Spectroscopy, or sometimes the Radial Velocity Method. Because the star moves around in a circle, this is also sometimes called the Wobble Method. The Doppler Shift technique can easily detect giant-sized planets orbiting close to a star such as those with masses similar to Jupiter. Remember the Sun moves by about 13 m/s due to Jupiter, but only about 9 cm/s due to Earth and velocity variations down to 3 m/s or even somewhat less can be detected with modern spectrometers.

Transit method reveals the size and orbit of the planet, and in combination with the radial velocity method, also its mass and mean density – providing first clues about its composition. The transit and radial velocity techniques have detected the bulk of the known exoplanets thus far. Although both techniques will continue to add more terrestrial candidates around M type stars, the radial velocity technique is the most likely technique to detect Earth-like planets around G type stars.

-22. Telescope:

A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally it was an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects – an optical telescope. Nowadays, the word “telescope” is defined as wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors. The computer has revolutionized the use of the telescope to the point where the collection of observational data is now completely automated. The astronomer need only identify the object to be observed, and the rest is carried out by the computer and auxiliary electronic equipment. The computer not only makes possible more efficient use of telescope time but also permits a more detailed analysis of the data collected than could have been done manually.

Space telescope:

A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Space telescopes avoid the filtering and distortion (scintillation) of electromagnetic radiation (by atmosphere) which they observe, and avoid light pollution which ground-based observatories encounter. Space telescopes are distinct from Earth imaging satellites, which point toward Earth for satellite imaging, weather analysis, espionage, and other types of information gathering.

Space-based astronomy is more important for frequency ranges that are outside the optical window and the radio window, the only two wavelength ranges of the electromagnetic spectrum that are not severely attenuated by the atmosphere. Many larger terrestrial telescopes, however, reduce atmospheric effects with adaptive optics.

-23. Radio astronomy and Radio telescope:

Radio astronomy is study of celestial bodies by examination of the radio-frequency energy they emit or reflect. Radio waves penetrate much of the gas and dust in space, as well as the clouds of planetary atmospheres, and pass through Earth’s atmosphere with little distortion. Astronomers around the world use radio telescopes to observe the naturally occurring radio waves that come from stars, planets, galaxies, clouds of dust, and molecules of gas.

Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects, just as optical telescopes are the main observing instrument used in traditional optical astronomy which studies the light wave portion of the spectrum coming from astronomical objects. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night. Optical telescopes have their limitations while radio emissions could be detected “across the galaxy” meaning we might have a higher chance of spotting them. Just as optical telescopes collect visible light, bring it to a focus, amplify it and make it available for analysis by various instruments, so do radio telescopes collect weak radio light waves, bring it to a focus, amplify it and make it available for analysis. Using sophisticated computer programming, they can unravel signals to study the birth and death of stars, the formation of galaxies and the various kinds of matter in the Universe.

Many radio frequencies penetrate Earth’s atmosphere quite well, and this led to radio telescopes that investigate the cosmos using large radio antennas. The long wavelength of radio waves required much larger satellites but most radio wavelengths can be detected from the ground. Naturally occurring radio waves are extremely weak by the time they reach us from space. A cell phone signal is a billion billion times more powerful than the cosmic waves our telescopes detect. Since astronomical radio sources such as planets, stars, nebulas and galaxies are very far away, the radio waves coming from them are extremely weak, so radio telescopes require very large antennas to collect enough radio energy to study them, and extremely sensitive receiving equipment.

Radio observatories are preferentially located far from major centers of population to avoid electromagnetic interference (EMI) from radio, television, radar, motor vehicles, and other man-made electronic devices. Human endeavours emit considerable electromagnetic radiation as a byproduct of communications such as television and radio. These signals would be easy to recognize as artificial due to their repetitive nature and narrow bandwidths.

-24. Kepler space telescope (KST):

The Kepler space telescope is a space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Designed to survey a portion of Earth’s region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets, Kepler’s sole scientific instrument is a photometer that continually monitored the brightness of approximately 150,000 main sequence stars in a fixed field of view. These data were transmitted to Earth, then analyzed to detect periodic dimming caused by exoplanets that cross in front of their host star. Only planets whose orbits are seen edge-on from Earth could be detected. As of June 16, 2023, the Kepler space telescope and its follow-up observations have detected 2,778 confirmed planets, including hot Jupiters, super-Earths, circumbinary planets, and planets located in the circumstellar habitable zones of their host stars. Kepler’s answer is unequivocal. There are more planets than there are stars, and at least a quarter are Earth-size planets in their star’s so-called habitable zone, where conditions are neither too hot nor too cold for life. With a minimum of 100 billion stars in the Milky Way, that means there are at least 25 billion places where life could conceivably take hold in our galaxy alone—and our galaxy is one among trillions. Kepler was the greatest step forward in the Copernican revolution since Copernicus. It’s changed the way we approach one of the great mysteries of existence. The question is no longer, is there life beyond Earth? It’s a pretty sure bet there is. The question now is, how do we find it?

-25. James Webb Space Telescope:

The James Webb Space Telescope (JWST) is a space telescope launched in 2021 and designed to conduct infrared astronomy. Its high-resolution and high-sensitivity instruments enables investigations across many fields of astronomy and cosmology, such as observation of the first stars and the formation of the first galaxies, and detailed atmospheric characterization of potentially habitable exoplanets. The James Webb Space Telescope (JWST) will enable the search for and characterization of terrestrial exoplanet atmospheres in the habitable zone via transmission spectroscopy. JWST could likely detect all the key markers of non-intelligent and intelligent life in exoplanet’s atmosphere, detecting the key biosignatures and technosignatures from the dataset, such as methane and oxygen, produced by biological life, and nitrogen dioxide and chlorofluorocarbons (CFCs), which are produced by aliens creating pollution.

Researchers have detected methane and CO2 in the planet’s atmosphere via transition spectroscopy. Detection of these gases could mean the planet, named K2-18b, has a water ocean. K2-18 b, an exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth.

-26. Spectroscopy for studying atmosphere of exoplanets to detect extraterrestrial life:

Life has altered Earth’s atmosphere substantially from what we otherwise find on planets devoid of life, where chemical and physical processes alone shape the planet’s atmosphere. If life exists on other planets, it will likely have altered their atmospheres as well. The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere that is not in chemical equilibrium, which is relatively easy to determine from a distance by spectroscopy. Ultimately, we need larger telescopes to characterize the atmospheres of extrasolar planets through a technique called spectroscopy. Light interacts with molecules in very predictable ways, allowing us to determine what is in an atmosphere just by simply observing absorption and emission in either the starlight reflected off of a planet or in the heat radiated by the planet. Spectra measured with these improved telescopes are the key to detecting life on other planets as they will allow us to detect molecules in the atmospheres of planets and even test for surface features that may indicate life. Only in the last two decades, with powerful new telescopes, cameras, and computers, have we finally achieved the precision necessary to measure the spectra of exoplanets. Extraterrestrial life can be found not only by using space telescope based spectroscopy but also using ground-based high-dispersion spectroscopy.

Astronomical spectroscopy is used to measure three major bands of radiation in the electromagnetic spectrum: visible light, radio waves, and X-rays. While all spectroscopy looks at specific bands of the spectrum, different methods are required to acquire the signal depending on the frequency. Ozone (O3) and molecular oxygen (O2) absorb light with wavelengths under 300 nm, meaning that X-ray and ultraviolet spectroscopy require the use of a space telescope. Infrared light is absorbed by atmospheric water and carbon dioxide, so while the equipment is similar to that used in optical spectroscopy, space telescope is required to record much of the infrared spectrum.

There are basically two families of methods to study atmosphere of exoplanet.

[1. The first has so far been the most successful and involves transiting systems, making use of temporal variations in how we see the planet. When a planet transits a star, in addition to occulting part of the stellar surface, starlight also filters through the planet atmosphere, leaving an imprint of atomic and molecular absorption and scattering. In addition, half an orbit later, the planet is occulted by the star, meaning that for a few hours the planet’s light (either intrinsic thermal emission or starlight reflected off the planet’s atmosphere) is missing and can be accounted for. Also during the rest of the orbit, varying parts of the dayside and nightside of the planet are visible, resulting in variations that reveal its heat distribution which can constrain global climate circulation models. Those planets that transit their stars are excellent candidates for atmospheric characterization through transmission spectroscopy with James Webb Space Telescope (JWST).

[2. Second is making direct images of exoplanets, by angularly separating the planet from the star in the sky. Planets with sufficient planet–star separations will be excellent targets for direct-imaging spectroscopy to observe the light produced by the heat of the planet itself (thermal emission spectroscopy).

-27. The biggest problem in studying atmosphere of exoplanets is suppressing the light from the stars, which can be 10 billion times brighter than light reflected of a rocky planet in the habitable zone of a Sun-like star. For atmospheric studies planet light needs to be separated from that of the star, with the latter being many orders of magnitude brighter. The space telescope’s ability to characterize the atmospheres of exoplanets, and therefore look for signatures that could indicate life, depends on technologies that block the glare from a distant star. There are two main ways of blocking the star’s light: a small mask internal to the telescope, known as a coronagraph, and a large mask external to the telescope, known as a starshade. In space, starshades would unfurl into a giant sunflower-shaped structure. In both cases, the light of stars is blocked so that faint starlight reflecting off a nearby planet is revealed.

Third method is by using interferometer techniques. With coordinated telescopes working in tandem and broadband destructive interference methods, the central starlight could be blackened out, or nulled, while leaving the dim reflected planet’s light unaffected.

-28. Our current technology is the space-based JWST and ground-based 10-meter class telescopes, performing direct exoplanet imaging and transit spectroscopy. Unfortunately, this isn’t sufficient technology to reach our goal of measuring the properties of Earth-sized planets in Earth-like orbits around Sun-like stars. For direct imaging studies, we can take pictures of planets that are the size of Jupiter and that are more than about Saturn’s distance from the Sun: good for gas giant worlds, but not so great for looking for life on rocky planets. For transit spectroscopy, we can see the light that filters through the atmospheres of super-Earth-sized worlds around red dwarf stars, but Earth-sized planets around Sun-like stars are well beyond the reach of current technology. We have not yet found Earth-sized planets in Earth-like orbits around Sun-like stars due to limitation of technology. Remember 22 percent of Sun-like stars may harbour planets roughly the size of Earth in their habitable zones that have been over-looked because these planets are harder to detect. JWST wouldn’t be able to detect faraway planets as small as Earth (K2-18b is eight times bigger) or as close to their parent stars, because of the glare. So, NASA is planning the Habitable Worlds Observatory (HWO), scheduled for the 2030s. Using what is effectively a high-tech sunshield, it minimises light from the star which a planet orbits. That means it will be able to spot and sample the atmospheres of planets similar to our own.

-29. Biosignature:

A biosignature is any substance – such as an element, isotope, molecule, or phenomenon that provides scientific evidence of past or present life. The search for biosignatures involves the identification of signs of past or present life in the form of organic compounds, isotopic ratios, or microbial fossils. Key markers of potential life include chemical systems capable of evolution, liquid water, energy sources, and atmospheric gas imbalances. The presence of environmental “gradients” also indicates potential life-hosting environments. Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life. To search for life in extrasolar planetary systems, we must rely on infrared-based remote sensing technology to search for key molecules like water and chemically incompatible gases such as methane, carbon dioxide, and ozone in the atmospheres of extrasolar planets. A sign of life from an exoplanet may manifest itself as a spectroscopic signal (or signals), a measurement that will have a stated uncertainty and potentially a range of explanations (including measurement error). That signal may be used to infer the presence of a gas or surface feature, which then may be interpreted as originating from a living process.

Every possible biosignature is associated with its own set of unique false positive mechanisms or non-biological processes that can mimic the detectable feature of a biosignature. Opposite to false positives, false negative biosignatures arise in a scenario where life may be present on another planet, but some processes on that planet make potential biosignatures undetectable. The search for life using biosignatures is not as simple as looking for a single molecule or compound. Atmospheric oxygen, for example, could be a sign of life, but there are many nonbiological ways that oxygen gas could be produced on an exoplanet. Conversely, it is possible that life could exist in the absence of oxygen gas, similar to early life on Earth or portions of the oceans today.

-30. VRE:

The vegetation red edge (VRE) is a biosignature of near-infrared wavelengths that is observable through telescopic observation of Earth, and has increased in strength as evolution has made vegetative life more complex. Astrobotanists focused on extraterrestrial vegetation have thus theorized that by using these same models, it could be possible to measure whether exoplanets in their respective Goldilocks zones currently hold vegetation, and by comparing VRE biosignatures to modelled historic Earth radiation, estimate the complexity of this vegetation. The James Webb Space Telescope has been searching the TRAPPIST-1 exoplanet for signs of VRE through capturing atmospheric data.

-31. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, —in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2. Hence methane with CO2 in atmosphere with comparatively little CO is biosignature of life.

-32. Photosynthesis has changed Earth’s atmosphere at a large scale—more than 20% of our atmosphere comes from the photosynthetic waste product, oxygen. Such high levels would be very difficult to explain in the absence of life. Astronomers choose molecular oxygen as a biomarker because it is relatively easy to detect. You need to collect at least one trillion photons to be very certain that you are truly looking at oxygen. The good news is that a new generation of telescopes designed for planetary exploration and astrobiology will help us gather those photons. Traces of oxygen molecules are not difficult to pick up from the infrared waveband. They are even possible to detect in visible light if they exist in the form of ozone. Most astronomers believe that if we detect a high concentration of molecular oxygen in the atmosphere of a planet, there is a reasonable chance it is an indicator of carbon-based life on the surface of that planet. Meanwhile, if a planet has an ozone layer in its atmosphere, we may even be able to detect the traces with ground-based telescopes. Rich supplies of oxygen can also come from other sources such as photo-dissociated water molecules. It is important to distinguish between oxygen molecules related to biological activities and those from other sources. Typical biosignatures are atmospheric gases, such as oxygen in the presence of methane. Finding methane and oxygen together would be hugely exciting; it’s very difficult to produce that combination without life.

-33. An atmosphere such as Earth’s becomes depleted in carbon dioxide thanks to the action of extensive amounts of surface liquid water, and/or by intense biological processes. Plate tectonics buries carbon away from the atmosphere, causing atmospheric depletion of CO2 over geological timescales. The researchers propose that if a terrestrial planet has substantially less carbon dioxide in its atmosphere compared to other planets in the same system, it could be a sign of liquid water — and possibly life — on that planet’s surface.

-34. Only living systems can produce complex molecules that could not form randomly in any abundance abiotically. Living and non-living systems are set apart by the degree to which they can reliably, and in detectable abundances, assemble highly complex molecular structures, hence it is proposed as a biosignature that is both biochemically agnostic and practically useful. A life-detection instrument based on this method could be deployed on missions to extraterrestrial locations to detect biosignatures or detect the emergence of de novo artificial life in the lab.

-35. At 4.2 light-years (1.3 parsecs, 40 trillion km, or 25 trillion miles) away from Earth, the closest potentially habitable exoplanet is Proxima Centauri b, which was discovered in 2016. This means it would take more than 18,264 years to get there if a vessel could consistently travel as fast as the Juno spacecraft (250,000 kilometers per hour or 150,000 miles per hour). It is currently not feasible to send humans or even probes to search for biosignatures outside of the Solar System. The only way to search for biosignatures outside of the Solar System is by observing exoplanets with telescopes.

-36. If we want to find evidence for alien life, we don’t need to keep looking for chemicals in exoplanet atmospheres or distant radio signals Instead, we should be studying asteroids, meteoroids and the thousands of micrometer-sized bits of interstellar dust that hit Earth every year. An important issue is whether biosignatures are preserved until the exoplanet particles reach Earth. They may be damaged at various stages, including launches from the home planets, exposure to radiation and cosmic rays in interstellar space, entry to Earth, and weathering in Earth environments. Microbial carcasses would be most vulnerable to damage, while microfossils and biominerals would be more likely to be preserved. It is important to investigate and choose the best biosignatures for this purpose, which should be abundant on terrestrial planets harbouring life and identifiable after a long travel from their home.

-37. Technosignature is defined as any measurable property or effect that provides scientific evidence of past or present technology, and it is analogous with biosignature which provide evidence of past or present life, intelligent or not. Just as astrobiologists have a catalogue of tell-tale signs of life on other planets called biosignatures, SETI researchers have their own list of things that would indicate the existence of intelligent life beyond Earth. These are known as “technosignatures”.

-38. SETI:

Some researchers are trying to find evidence for extraterrestrial intelligence. This effort is often called SETI, which stands for Search for Extraterrestrial Intelligence. SETI researchers decided that looking for evidence of their technology might be the best way to discover other intelligent life in the Galaxy. The SETI project uses radio telescopes from around the world to scan the sky and look for special patterns in radio waves which could have been sent by another civilization in space. SETI is using radio waves as technosignature of alien civilization. Radio telescopes are used because radio waves can travel very far in space without being absorbed by the thick clouds of gas and dust which lie in many regions of space. Also, radio telescopes can be used both day and night. Many radio telescopes are currently being used for radio SETI searches including Allen Telescope Array, the Arecibo Observatory, the Robert C. Byrd Green Bank Telescope, the Parkes Telescope, the Very Large Array (VLA), the Low Frequency Array (LOFAR) in Europe, the Murchison Widefield Array (MWA) in Australia, and the Lovell Telescope in the United Kingdom.

The frequency between 1GHZ and 10GHZ is suited for interstellar communication, but those in the range of 1GHZ – 2GHZ seem the most appropriate ones. Optimal frequencies for communication are supposed to be the neutral hydrogen lines (1,420 MHZ) and the hydroxyl lines (1,612 MHZ and 1,615 MHZ, 1,667, and 1,720 MHZ).

Determining whether we are alone in the Universe as technologically capable life is among the most compelling questions in science, and a specialized radio telescope array can play a major role in answering it.

SETI search has always been plagued by the problem of radio interference from Earth-based radio antennas and satellites in orbit, which can potentially flood SETI surveys with false positives. About 99.6% of radio signals have been dismissed as radio interference. Most signals have been assigned the status of noise or radio frequency interference because a) they appear to be generated by satellites or Earth-based transmitters, or b) they disappeared before the threshold time limit of ~1 hour.

-39. Most SETI experiments do not transmit signals into space. Why?

[1. Because the distance even to nearby extraterrestrial intelligence could be hundreds or thousands of light-years, two-way communication would be tedious. For this reason, SETI experiments focus on finding signals that could have been deliberately transmitted or could be the result of inadvertent emission from extraterrestrial civilizations.

[2. Because transmission of ‘we are here’ type beacons comes with the danger of potentially inviting aliens with unknown intentions to the Earth. Deliberate transmissions to potential aliens from Earth should be considered only if by global consensus humankind deems it safe and appropriate.

-40. No confirmed extraterrestrial signals have yet been found by SETI experiments. Why?

[1. The “optimistic” camp holds that we’ve been using detectors that are not sensitive enough or missed incoming signals because we’ve been pointing our radio telescopes in the wrong direction.

[2. The “pessimistic” camp, on the other hand, interprets the silence as indicating the absence of intelligent alien life in our galaxy.

[3. We’ve only been looking for 60 years. Earth could simply be in a bubble that just happens to be devoid of radio waves emitted by extraterrestrial life.

[4. Radio waves broaden as they travel, meaning any message/signal sent by alien civilization intentionally or inadvertently will become more diluted the farther it gets from its source.

[5. Techno-signatures require significant energy to be visible across interstellar space and thus intentional signals might be concentrated in frequency, in time, or in space, to be found in mutually obvious places.

[6. It may be that while alien species with intelligence exist, they are primitive or have not reached the level of technological advancement necessary to communicate.

[7. It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far away for meaningful communication.

[8. SETI has difficulty attracting scientists and government funding because it is an effort so likely to turn up nothing and money spent on SETI would be wasteful government spending. The Laser Interferometer Gravitational-Wave Observatory (LIGO) discovered gravitational waves only after the National Science Foundation (NSF) invested $1.1 billion in it. Similarly, we should expect to find evidence for extraterrestrial life only after we invest major funds in a search. It would be most appropriate to allocate taxpayer funds to the search for our cosmic neighbors, given the major impact that such a discovery would have on society—far exceeding the implication of discovering gravitational waves. Should humanity devote serious funding to developing remote-sensing technology to find extant life over other projects, it might find evidence that life exists elsewhere in the universe.

-41. SETA (search for extraterrestrial artifacts):

In contrast to SETI, SETA (search for extraterrestrial artifacts) allows astronomers to dig deep into the past. They don’t have to hope to catch a radio signal from a civilization that is active at the same time we’re listening. In fact, multiple civilizations could have come and gone throughout the galaxy, each one leaving something behind in our solar system before fading from existence (or moving on to something more interesting). If Mars was overflowing in vast oceans at some point in its ancient history, then perhaps some form of life existed on the red planet. And if this was intelligent life, there must be some sign it that still remains. That’s the hope among some scientists looking to find alien artifacts sitting on Mars or some other planet or moon. These could be ruins of an ancient city or small tools hidden away in a cave. Or anything else in between. Furthermore, artifacts aren’t necessarily a sign that species has gone extinct. They may have migrated to another planet, and what remains are leftovers from a failed or lost colony.

-42. Alien life may one day be found not from radio signals beamed across the cosmos but from an all-too-familiar side-effect of civilisation: pollution. Pollutants such as nitrogen dioxide (NO2) and chlorofluorocarbons (CFCs) are mostly formed by industrial activity and could be a good pointer towards pollutants from civilisation. If the concentration of these gases in the atmosphere of a distant planet reached roughly 10 times their concentration on Earth, it might be possible to detect their presence using the James Webb Space Telescope, which became operational in 2022. Scientists are able to look for CFCs, and various other chemicals in faraway planets’ atmospheres, by studying transmission spectroscopy.

-43. On Earth, the evolution of technology has hinged on the ability to utilize open-air combustion — a process where fuel and an oxidant, typically oxygen, combine to create fire. From cooking and metal forging to energy harnessing, combustion has been pivotal in shaping industrial societies. Controlled fire use and subsequent metallurgical advancements were only feasible when atmospheric oxygen levels hit or surpassed 18 percent. This finding implies that only planets with significant oxygen concentrations can develop advanced technology capable of leaving detectable technosignatures. Interestingly, the oxygen levels needed to biologically sustain complex life and intelligence are lower than those required for technology. Thus, while a species might evolve in an oxygen-deficient world, it is unlikely to progress into a technological species. Earlier we discussed oxygen as biosignature but now it became technosignature.

-44. AI can help detection of Extraterrestrial Life:

[1. One aspect of SETI research involves searching for “noise” or anomalies in the electromagnetic spectrum that could indicate intelligent life. AI anomaly engine can detect anomalies within the huge data archives that have been collected across space science disciplines.

[2. Sophisticated AI algorithms can sort through large amounts of data for patterns that could indicate an engineered signal.

[3. Organizations such as the SETI Institute search the cosmos for potential forms of communication. They started with radio waves, and now search for laser pulses as well. The challenge for this search is that there are natural sources of such signals as well, such as gamma-ray bursts and supernovae, and the difference between a natural signal and an artificial one would be in its specific patterns. Astronomers intend to use artificial intelligence for this, as it can manage large amounts of data and is devoid of biases and preconceptions.

[4. A newly developed method based on artificial intelligence (AI) is capable of detecting subtle differences in molecular patterns that indicate biological signals — even in samples hundreds of millions of years old. Better yet, the mechanism reveals a sample’s biological or non-biological origin with 90% accuracy. And because it relies on molecular relationships rather than detecting specific organic chemicals like DNA or amino acids, which may not be used in other biospheres, the method could allow scientists to look for life entirely unlike what we have on Earth.

-45. Biosignature vs Technosignature:

The intuition suggested by the Drake equation implies that technology should be less prevalent than biology in the galaxy and it can lead one to the erroneous conclusion that technosignatures must be poorer search targets than biosignatures. An objective, quantitative comparison of the actual relative abundances of technosignatures and biosignatures is difficult because it depends on details of extraterrestrial life that we cannot know for certain until we have some examples to learn from. The logic is that technology—and its attendant technosignatures—differs in fundamental ways from biology—and its biosignatures—in that it can spread far beyond its origin in space, time, and scope, and this difference gives technosignatures almost unlimited potential for key metrics associated with extraterrestrial life searches: abundance, longevity, detectability, and ambiguity. It has been appreciated for decades in the SETI community that technosignatures could be more abundant, longer-lived, more detectable, and less ambiguous than biosignatures.

-46. Since the visit of Viking, our understanding of Mars has deepened spectacularly. Orbiting spacecraft have provided ever-more detailed images of the surface and detected the presence of minerals that could have formed only in the presence of liquid water. Two bold surface missions, the Mars Exploration Rovers Spirit and Opportunity (2004), followed by the much larger Curiosity Rover (2012), confirmed these remote-sensing data. All three rovers found abundant evidence for a past history of liquid water, revealed not only from the mineralogy of rocks they analyzed, but also from the unique layering of rock formations.

Collecting and returning samples to Earth, while appealing because of the direct hands-on analytical advantages they provide, are constrained by the amount of material that can be returned and sample containment issues related to potential biological hazards associated with possible extant Martian organisms being transported back to Earth. The problems associated with terrestrial contamination underscores the importance of doing in situ organic compound analyses on Mars before samples are returned to Earth, where even under the best of circumstances they will be exposed to some level of terrestrial contamination.

If alien life diverges radically from terrestrial organisms, on which experimental basis will scientists recognize it? Conversely, if microbial organisms on, say, Mars share a substantial fraction of biochemical features with their Earth brethren, how will astrobiologists be able to distinguish them without any doubts about interplanetary contamination?

-47. Europa, the fourth largest satellite of Jupiter, may be the best candidate for extraterrestrial life in the solar system. The Galileo orbiter revealed a crust of water ice and a complex surface on this moon. Optical imaging, thermographic temperature probes, and magnetic field measurements support the strong inference that a liquid saltwater ocean surges beneath the frozen crust. A wisp of an oxygen atmosphere has also been detected by spectrographic techniques. Furthermore, since organic molecules including methane and nitrogen-rich gases such as ammonia abound on Jupiter and some of its other moons, such “prebiotic chemicals” are highly likely to be present on Europa. The Galileo flyby also detected abundant sulfuric acid, a potential chemical power source, on the surface of Europa. (Such discoveries in the Jovian planets inspire further investigation of the limits to diversity of life on Earth. Lakes such as Vostok in Antarctica reside under more than 3 km [2 miles] of ice. Studies of bacteria in these lakes and of water seeps within cavities in granitic and carbonate rocks provide models for the viability of possible Earth-like life-forms on Europa and other Jovian moons.)

-48. In the present era, humanity has now discovered over 5,000 planets around other stars, and we have even detected the presence of water in the atmospheres of some planets. Sagan’s Galileo experiment shows this is not enough by itself. A strong case for life elsewhere will likely require a combination of mutually supporting evidence, such as light absorption by photosynthesis-like processes, narrowband radio emission, modest temperatures and weather, and chemical traces in the atmosphere which are hard to explain by non-biological means. Life is the last, not first, inference to draw when seeing something unusual on another planet. Extraordinary claims require extraordinary evidence.

-49. Fermi paradox:

The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilisations and the lack of evidence for such civilisations is known as the Fermi paradox. The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. There have been many attempts to resolve the Fermi paradox, such as suggesting that intelligent extraterrestrial beings are extremely rare, that the lifetime of such civilizations is short, or that they exist but (for various reasons) humans see no evidence.

-50. UFOs sighting is not conclusively shown to be extraterrestrial in origin. There is no evidence to show that UFOs are driven by aliens. Unidentified anomalous phenomena (UAP) is a term that is used in place of UFOs (unidentified flying objects). The main problem is that while there are plenty of eyewitness accounts of strange lights in the sky, very little high-quality, standardized data has been collected from these incidents. Most sightings involve a fleeting encounter—and perhaps only a single opportunity for photographs. Despite numerous accounts and visuals, the absence of consistent, detailed, and curated observations means we do not presently have the body of data needed to make definitive, scientific conclusions about UAP. While concrete evidence of extraterrestrial activity remains elusive, the alternative, indicating foreign entities operating within our airspace, is a disturbing prospect. It could well be unidentified foreign object and not unidentified alien object.

-51. Why search extraterrestrial life:

[1. No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are contingent, in the sense that they are the result of evolutionary accidents such that elsewhere a different sequence of events might have led to different properties of life. In this respect the discovery of even a single example of extraterrestrial life, no matter how elementary in form or substance, would represent a fundamental revolution in science.

[2. If we were to discover that there is life out there — intelligent life that has forged a civilization — it would first mean that biology is not a fluke. Instead, it is something that can take root on many worlds; something that does not merely arise but repeatedly produces thinking, technological, curious creatures, those that may wish to share their knowledge of the universe, and their way of traversing or surviving it, with others. And if this civilization existed on a world very different from Earth, it would demonstrate that the largely unliveable cosmos is populated by myriad different isles of habitability.

[3. The negative result of life not being detected in a certain parameter space is also significant not only for science, but also for space utilization and planetary protection because the result will provide information on safety issues.

-52. What’s the best scientific evidence we’ve found for the existence of extraterrestrial life?

The sobering reality is that there isn’t any yet. There’s no scientific evidence for aliens in the declassified UFO videos, in mutilated cows whose injuries are blamed on extraterrestrial activities or in purported alien bodies. Nor is there any such evidence in the formal academic research. There is, however, good reason to hold out hope that the evidence will eventually come, even if it isn’t personally delivered by a little green man. There are signs that certain planets and moons could harbour life, but we haven’t found evidence of life in these places yet. But we have to keep in mind that space is incredibly vast. It would take humans more than a million years to visit K2-18 b with traditional rocket propulsion. Even sending our fastest probe to the nearest known exoplanet, Proxima Centauri b, would take thousands of years. The planets and moons within our solar system are right on our doorstep by comparison, with probe travel times ranging from some years to mere months. Our best hope could be finding microbial life in planets and moons of our solar system itself.

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My theory on extraterrestrial life (life beyond earth):

Research questions are as follows:

[1. Does extraterrestrial life (simple or complex) exist?

[2. Is extraterrestrial life carbon-based and has liquid water as its universal solvent (earth-like)?

[3. Is extraterrestrial life intelligent?

Basic axiom:

Biology is not a fluke that happened once on Earth. Biology is universal and cannot exist only on earth.

Biological phenomena emerge from and must be consistent with the principles of chemistry, physics, and math. In other words, chemistry and physics constrain how biological organism can behave or evolve. We readily accept that the concepts of physics and chemistry apply throughout the universe and are valid for all time, and as a corollary, concepts of biology are universal as well, and not just a special feature that only applies to planet Earth. We believe in the universality of the laws of physics and chemistry, so why biology be an exception? We believe that matter is made up of electrons, protons and neutrons; and certain combinations of them generate various elements like hydrogen, oxygen, nitrogen and carbon with varied properties. They all obeys laws of physics and chemistry all over universe and not only on earth. And their combinations in certain way make molecules that acquire property of self-replication with variation, whom we call it as life, and these molecules follows laws of biology in addition to laws of physics and chemistry. Biology arose from physics and chemistry on earth, and it can arise anywhere in universe that allows environment for such self-replicating molecules with variation to exist. So extraterrestrial life (primitive or microbial) ought to exist somewhere in universe no matter we find it or not. Existence of extraterrestrial life cannot be challenged logically no matter whether we find it or not.

Anything that has capacity for self-replication with variation, genetic continuity (information storage & propagation), evolution and utilize energy from environment, is life, no matter at molecular level or at higher level. This definition would apply not only to life on Earth, but also to life on another planet. Extraterrestrial life is the term used to define any form of life (intelligent or otherwise) that may exist and originate outside the planet Earth. Extraterrestrial Intelligence is intelligent life that developed somewhere other than the Earth. Life on Earth, structurally based on carbon, hydrogen, nitrogen, oxygen and other elements, uses water as its interaction medium. It is not a coincidence that atoms most useful for life have very high cosmic abundances. Due to high cosmic abundance, these elements have had more opportunities to interact with each other and laws of chemistry allowed such interactions, ultimately leading to life. The way carbon, hydrogen, oxygen, nitrogen, and other elements have combined to form life on earth is the ‘only way’ they can combine to form life and there is no alternative way these elements can combine as such combination is constrained by laws of physics and chemistry; so, life anywhere in universe ought to have similar combination of similar elements. So, any extraterrestrial life that might exist will be based on the same four elements most vital for life, carbon, hydrogen, oxygen, and nitrogen. I am confident in the universal nature of biochemistry, according to which chemical and physical constraints make it highly probable that life elsewhere in the universe follows the same general principles as earthly (terrestrial) life and uses similar building blocks for macromolecules, although extraterrestrial life might have some biochemical peculiarities.

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Silicon based life is fiction.

Why silicon-based life is not possible:

[1. When carbon oxidizes, it turns into carbon dioxide gas. Our body oxidizes carbon to produce energy, giving off carbon dioxide as a waste product. However, when silicon is oxidized, it turns into a solid – sand.

[2. Silicon bonds are more unstable than carbon bonds. You can get silicon-silicon double bonds if you try hard, but they are fairly unstable and they easily lose that double-bond in favour of forming another single one. Carbon-carbon double bonds on the other hand form naturally and easily, and are crucial for every living organism on earth.

[3. Another reason why there are no silicon-based organisms is that silicon cannot use water as a solvent in the same way that carbon can. It would require a completely different solvent, such as methane, which is not stable in normal conditions.

[4. The chemical bond of silicon dioxide is too strong, while the silicon–silicon bond is too weak. Silicon dioxide is an overly stable sink of silicon and is insoluble in water. These properties prevent silicon from forming a variety of complex molecules as carbon does.

[5. From a biochemical perspective, the functioning of life seems to depend on organic molecules having a specific handedness or chirality. These large, complicated molecules do their job with great precision only because they have a property called “handedness.” Handedness is the characteristic that provides a variety of biomolecules with ability to recognize and regulate sundry biological processes. And silicon doesn’t form many compounds having handedness. Thus, it would be difficult for a silicon-based life-form to achieve all of the wonderful regulating and recognition functions that carbon-based biomolecules perform for us.

[6. Silicon is much more common than carbon on Earth, and even with that, life here is carbon-based.

[7. A 2020 study found that a life based primarily around silicon chemistry is not a plausible option in any environment.

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Scientific reasons for the existence of extraterrestrial life:

Various factors have played a vital role in the emergence of life in our Universe. This includes the star formation history of our galaxy, the enrichment of the interstellar medium (ISM) by heavy elements (forged in the interior of the first population of stars), the formation of planets, and the distribution of water and organic molecules between planets. Although no compelling evidence of extraterrestrial life has yet been found, the possibility that biota might be a common feature of the universe has been strengthened by the discovery of extrasolar planets (planets around other stars), by the strong suspicion that several moons of Jupiter and Saturn might have vast reserves of liquid water, and by the existence of microorganisms called extremophiles that are tolerant of environmental extremes. The first development indicates that habitats for life may be numerous. The second suggests that even in the solar system there may be other worlds on which life evolved. The third suggests that life can arise under a wide range of conditions.

I enumerate various reasons grounded in science that bolster the conjecture that extraterrestrial life is surely out there somewhere:

[1. Big numbers

The universe is vast and contains billions of galaxies, each with billions of stars. Many of these stars have planets orbiting them, and some of these planets may be similar to Earth in size and composition. The conditions that are necessary for life, such as liquid water and an atmosphere that contains oxygen, are not unique to Earth. It’s estimated that there could be in excess of 10 septillion (that’s 10,000,000,000,000,000,000,000,000) planets in the observable universe. And if any of those worlds are exact twins of Earth, the odds that they also host life are nine times better than the odds they’re lifeless and barren, according to a scientific study.

Around between 300 million to 40 billion Earth-like planets are estimated to inhabit our galaxy, making the potential for life extremely high. Over the past decade, planet hunters have found hundreds of exoplanets, many of them gas giants like Jupiter. But new techniques for planetary detection have allowed them to detect smaller, rocky worlds like Earth. Some are even in the “Goldilocks zone” around their stars, meaning they orbit at a distance that could produce temperatures similar to those on Earth for surface liquid water to exist. Given how common exoplanets beyond our solar system have turned out to be, it seems likely that one of them plays host to some form of life.

[2. Growing evidence that oceans and lakes are common, at least in our solar system

Life on Earth originated in the ocean, so it follows that this might be the case on other worlds. Now there is strong evidence that water once flowed freely on Mars, and Saturn’s moon Titan has seas of methane as well as rivers flowing across its surface. Jupiter’s moon Europa is believed to be one, massive ocean, warmed by the moon’s core and completely covered in a thick, protective layer of ice. Europa contains both a source of radiation strong enough to lead to chemical reactions and evidence of an ocean similar to Earth’s. Similarly, the conditions of Saturn’s moon, Enceladus, could support microbial life with its liquid ocean and hydrothermal reactions. While many speculate that the life we uncover on these moons will likely be microbial, it would be the first step in learning how to search for more complex lifeforms within our galaxy.

[3. Time window

The time window during which life evolved on earth is at most 0.5 billion years, and could be as little as 0.1 billion years (100 million years). Although this may seem like a long time, it is much shorter than the age of the planet. If the time for life to evolve on Earth is in any way typical, then we expect that there are many other planets that are old enough for life to have had a good chance of originating.

[4. Copernican principle

The Copernican principle is inspired by 16th-century astronomer Copernicus, whose revolutionary model of the solar system put the sun and not Earth at the center. The principle suggests that, in the same way that Earth is not in a privileged place in the universe, humanity should not presume itself special, or unique. The universe is not about us, and what happened on this planet over the past 4 billion years could happen elsewhere. In the 1970s and 1980s, Carl Sagan and Frank Drake, among others, argued that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common barred spiral galaxy. From the principle of mediocrity (extended from the Copernican principle), they argued that the evolution of life on Earth, including human beings, was also typical, and therefore the universe teems with complex life.

[5. Extremophiles surviving in harsh environments

One reason to be optimistic that life might be common elsewhere is that life occurs in a very wide range of environments on Earth, with micro-organisms (bacteria and archaea) being particularly versatile in their habitat. Organisms that live in conditions that appear ‘extreme’ to us are known as extremophiles. For example, organisms are found in boiling hot springs at close to 100 °C, crevices in sea ice at close to 0 °C, and lakes of extremely high salt concentration. Although most organisms would die in these extreme conditions, the fact that some organisms can survive there suggests that evolution often finds a way to solve the challenges posed by unusual environments. Typical conditions on other planets might be very different from those on Earth, but the study of extremophiles on Earth tells us that we should not be too narrow in our expectations of what other kinds of planets might be able to support life.

The existence of extremophiles has led to the speculation that microorganisms could survive the harsh conditions of extraterrestrial environments. Outer space presents severely harsh and inhabitable environmental conditions deleterious for life growth, including high radiation doses, extreme temperatures, different gravity, pressure, pH, salinity, energy source, and nutrient scarcity. Nevertheless, as extremophiles can flourish within broad physicochemical spectrums and extremely inhospitable habitats on Earth, they may be capable of surviving space’s harsh conditions. Extremophiles can survive in a myriad of planetary environments and present relevant characteristics advancing our understanding of potential life elsewhere.

We’ve found creatures who live without oxygen around the edges of super-heated volcanic vents at the bottom of the ocean, and we’ve found life in the brackish pools of the high Andes, as well as the ice-covered lakes of the arctic. There are even tiny creatures called tardigrades that can survive in the vacuum of space. So we have direct evidence that life can thrive in pockets of alien atmosphere on Earth. In other words, we know life can survive in conditions we’ve seen on other planets and moons. We just haven’t found it yet.

[6. Sheer diversity and tenacity of life on Earth

Not only did life evolve on Earth under extremely difficult conditions, but it somehow managed to survive mega-volcanoes, meteorite strikes, ice ages, droughts, ocean acidification, and radical atmospheric changes. We’ve also seen incredible diversity of life on our planet in a relatively short period of time, geologically speaking. Life is pretty tenacious and it could take hold on Mars, or in another star system.

[7. Elements in life follow cosmic abundance of elements:

When percentage of various elements in life on earth is compared with those in interstellar frost and comets, almost similar pattern is seen, proving the point that elements in life on earth follows cosmic abundance of elements. Due to high cosmic abundance, these elements have had more opportunities to interact with each other and laws of chemistry allowed such interactions, ultimately leading to life. And corollary would be extraterrestrial life following similar pattern.

[8. We should be open-minded as we search for life elsewhere

The human brain has plenty of constraints. We are misled by cognitive biases, optical illusions and inattentional blindness to things we don’t expect to see. One question that has always dogged research into alien creatures is whether or not we could recognize life that is so different from what we encounter here on Earth. Scholars have long urged us to expect the unexpected, trying not to let theory too heavily influence what we count as significant. Life on other planets might not leave the same biological signatures as terrestrial organisms, making them difficult to spot from our vantage point. And, as Claire Webb, an anthropology and history of science student at the Massachusetts Institute of Technology says, we must train ourselves to “make the familiar strange,” looking at ourselves through an alien lens in an effort to constantly reexamine our own assumptions. That way, we might be able to better understand ourselves through the eyes of another and perhaps meet creatures on other worlds on their own terms rather than ours.

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Scientific reasons for the resemblance of extraterrestrial life to life on earth:

I believe that life in different exoplanets would be using the same DNA/RNA as life on earth for the following reasons:

[1. Presence of nucleotides in asteroids and meteorites:

We’ve been finding the building blocks of DNA/RNA on various meteorites for a while now. Strong evidence suggest that DNA building blocks found in meteorites were created in space and not contamination by terrestrial life. Researchers have found adenine and guanine, which are components of DNA called nucleobases, as well as hypoxanthine and xanthine. Hypoxanthine and xanthine are not found in DNA, but are used in other biological processes. Researchers just announced the discovery of the nucleobase uracil—one of the building blocks of RNA—in a sample from the asteroid Ryugu. Also, in two of the meteorites, researchers discovered for the first-time trace amounts of three molecules related to nucleobases: purine, 2,6-diaminopurine, and 6,8-diaminopurine; the latter two almost never used in biology. These compounds have the same core molecule as nucleobases but with a structure added or removed.

People have been discovering components of DNA in meteorites since the 1960’s, but researchers were unsure whether they were really created in space or if instead they came from contamination by terrestrial life. Now we have three lines of evidence that together give us confidence that these DNA/RNA building blocks actually were created in space.

Three lines of evidence:

(1). It’s these nucleobase-related molecules, called nucleobase analogs, which provide the first piece of evidence that the compounds in the meteorites came from space and not terrestrial contamination. You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they’re not used in biology, aside from one report of 2,6-diaminopurine occurring in a virus (cyanophage S-2L). However, if asteroids are behaving like chemical ‘factories’ cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid.

(2). The second piece of evidence involved research to further rule out the possibility of terrestrial contamination as a source of these molecules. The team analyzed an eight-kilogram (17.64-pound) sample of ice from Antarctica, where most of the meteorites in the study were found, with the same methods used on the meteorites. The amounts of the two nucleobases, plus hypoxanthine and xanthine, found in the ice were much lower — parts per trillion — than in the meteorites, where they were generally present at several parts per billion. More significantly, none of the nucleobase analogs were detected in the ice sample. One of the meteorites with nucleobase analog molecules fell in Australia, and the team also analyzed a soil sample collected near the fall site. As with the ice sample, the soil sample had none of the nucleobase analog molecules present in the meteorite.

(3). Thirdly, the team found these nucleobases — both the biological and non-biological ones — were produced in a completely non-biological reaction. In the lab, an identical suite of nucleobases and nucleobase analogs were generated in non-biological chemical reactions containing hydrogen cyanide, ammonia, and water. This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial. In fact, there seems to be a ‘goldilocks’ class of meteorite, the so-called CM2 meteorites, where conditions are just right to make more of these molecules. NASA scientists studying the origin of life have reproduced uracil, cytosine, and thymine, three key components of our hereditary material, in the laboratory. They discovered that an ice sample containing pyrimidine exposed to ultraviolet radiation under space-like conditions produces these essential ingredients of life.

[2. Pseudo-panspermia is a fact:

The sample, collected from the 4.5-billion-year-old near-Earth asteroid Bennu in October 2020 by NASA’s OSIRIS-REx mission, arrived on Earth in a capsule on September 24, 2023. The rocks and dust contain water and a large amount of carbon, which suggests that asteroids may have delivered the building blocks of life to Earth. The creation and distribution of organic molecules from space is now uncontroversial; it is known as pseudo-panspermia. Pseudo-panspermia is the well-attested hypothesis that many of the pre-biotic organic building-blocks of life originated in space, became incorporated in the solar nebula from which planets condensed, and were further—and continuously—distributed to planetary surfaces where life then emerged. Rather than being a fluke that happened once on Earth, DNA/RNA, or at least their building blocks, appear to occur naturally. So another planet would have the same chemical base pairs available for proto-life to produce DNA/RNA.

[3. Efficiency of DNA/RNA in promoting evolutionary variations:

[4. Amino acids in asteroids and meteorites:

Amino acids, which are the monomers of proteins, have been extensively studied in meteorites. An extraterrestrial origin for most of the amino acids detected in carbonaceous chondrites has been firmly established based on three factors: the detection of racemic amino acid mixtures (i.e., equal mixtures of D and L amino acids), wide structural diversity (including the presence of many nonprotein amino acids that are rare or non-existent in the biosphere), and non-terrestrial values for compound-specific deuterium, carbon, and nitrogen isotope measurements. Japanese researchers have for the first time discovered amino acids — key ingredients for life — in an asteroid flying in space. They identified 20 amino acids in the samples returned from the asteroid Ryugu by the Hyabusa2 mission.

[5. Evidence of chemical precursors to life on other planets and moons:

Life on Earth probably evolved from chemical reactions that eventually formed cellular membranes and proto-DNA. But those original chemical reactions may have started with complex organic compounds — such as nucleic acids, proteins, carbohydrates, and lipids — in the atmosphere and ocean. There is evidence that these “precursors to life” exist on other worlds already. Titan has some in its atmosphere, and astronomers have spotted them in the rich environment of the Orion Nebula too. Again, we haven’t actually found life, but we’ve found the ingredients that many scientists believe contributed to the development of life on Earth. If those ingredients are common throughout the universe, it’s likely that life has emerged in places other than our home planet.

[6. Polycyclic aromatic hydrocarbons (PAHs) found in interstellar medium and meteorites:

PAHs have been identified in the interstellar medium and in carbonaceous meteorites. They may be the most abundant single class of organic compounds in the universe. More than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics — a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively. PAHs assumed to be abundant in the primordial soup of the early Earth, played a major role in the origin of life by mediating the synthesis of RNA molecules.

[7. The evolutionary convergence:

No planet will have a complex form of life that popped into existence all on its own. Whatever life is like on an alien planet, it must have begun simply. Now, it could be that it remained simple; that’s possible. Probable, even, on many planets. But if life is to achieve any kind of complexity, the only way that complexity can accumulate is if favorable changes and innovations are retained and unfavourable ones are lost — and that’s precisely evolution by natural selection. Because some evolutionary challenges are truly universal, life throughout the cosmos may share certain features. Wherever organisms confront similar environmental challenges, they may come up with similar adaptive solutions. We expect to see this throughout the universe. There are mathematical rules that govern the way evolution works. Just as we have laws of physics applied universally, for example speed of light cannot be exceeded; we have laws of evolution applied universally. Though Darwin and his contemporaries were hardly thinking of life on exoplanets when they came up with evolutionary theory, it does suggest that where life can take hold, it will. And if you consider that our environment isn’t just planets, but also solar systems and interstellar space, an unorthodox interpretation of evolutionary theory suggests that life will adapt to outer space too.

In a nutshell:

There is actually no lack of the building blocks of life; the number of molecules fundamental to Earth’s biochemistry that have already been found in the interstellar medium, planetary atmospheres and on the surfaces of comets, asteroids, meteorites and interplanetary dust particles is surprisingly rather large. We are however left with a fundamental gap in understanding how these molecules could become ‘alive’; and in my view they could become alive on any planet if suitable environment exists. If there are alien civilizations at a comparable stage of evolution, one might expect that they do not differ that much from our own although they might look different from us having different body structures, for example, eyes on back of head in addition to front eyes, four arms instead of two arms etc.

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Scientific reasons for the rarity of extraterrestrial intelligence:

Extraterrestrial intelligence is extraterrestrial life that is capable of thinking. Remember, only the existence of intelligent beings is relevant. Primitive life (extant or extinct) may be discovered on Mars; perhaps even multicellular animals will be found on a nearby extrasolar planet. These revolutionary discoveries would help us reconstruct how life on Earth evolved, but unless a species that is capable of conscious, independent thought and has the ability to communicate exists, we will still be alone—with no one to teach or learn from, no one to save us from ourselves (and no one to battle against). Intelligent life, for the purposes of this discussion, means life able to communicate between stars; this implies having something like radio technology. Our own society, by this definition, is only about 100 years old. If intelligent life is common in a universe that is 13.8 billion years old, then surely we are among the youngest forms in existence.

Reasons why extraterrestrial intelligence is rare:

[1. Rare earth hypothesis

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity such as sexually reproducing, multicellular organisms on Earth (and, subsequently, human intelligence) required an improbable combination of astrophysical and geological events and circumstances. According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare throughout the universe as a whole. In order for a small rocky planet to support complex life, the values of several variables must fall within narrow ranges

The Rare Earth hypothesis argues that the evolution of biological complexity anywhere in the universe requires the coincidence of a large number of fortuitous circumstances, including, among others, a galactic habitable zone; a central star and planetary system having the requisite character (i.e. a circumstellar habitable zone); a terrestrial planet of the right mass; the advantage of one or more gas giant guardians like Jupiter and possibly a large natural satellite to shield the planet from frequent impact events; conditions needed to ensure the planet has a magnetosphere and plate tectonics; a chemistry similar to that present in the Earth’s lithosphere, atmosphere, and oceans; the influence of periodic “evolutionary pumps” such as massive glaciations and bolide impacts; and whatever factors may have led to the emergence of eukaryotic cells, sexual reproduction, and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human beings and of human intelligence may have required yet further specific events and circumstances, all of which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates. The conditions under which complex intelligent life on Earth was created are almost impossible to replicate. An emerging theory from Germany posits that life on Earth may have started from a meteorite impact – one that had to have been incredibly precise to generate the molecules that now make up our world. Other theories, while differing in the “how,” agree that the merging of cells responsible for Earth’s intelligent beings is extremely rare – most likely a one-time event.

[2. Evolution favours survival and reproduction, and not intelligence unless intelligence help survival [you may be intelligent but if your intelligence does not help you survive then what’s the use of such intelligence?]

Many biologists think abiogenesis is much more difficult than many astronomers think, and no one knows how it happened on Earth. While some scientists suspect that life inevitably progresses toward complication and intelligence, that’s a human-centric bias. We don’t know if intelligence is a winning evolutionary strategy. After all, some of the oldest species on Earth, including cyanobacteria (3.5 million years old), coelacanths (65 million years old) and crocodiles (55 million years old), are not smart by human standards. They definitely wouldn’t be able build a radio telescope or wonder if they were alone in the universe. Nevertheless, they persist, arguably better than we have.

[3. The Great Filter Hypothesis

Our lone existence in the universe can be explained by The Great Filter. The theory contends that before a civilization is able to reach the level of intelligence necessary for space colonization, it gets “filtered” by some external circumstance and ceases to exist. On Earth alone, there’s been evidence of at least five mass extinctions, and there are plenty of logical reasons this happens: disease outbreak, climate change, natural disaster – the list goes on. While humans may (or may not) have dodged this inevitable filter, it’s unlikely that our alien counterparts could have done the same.

[4. The Timing of Evolutionary Transitions

It takes 5 billion years for intelligent life to form on other planets, as on Earth, assuming that intelligent life forms on other planets in a similar way as it does on Earth. On Earth, the emergence of complex intelligent life required a preceding series of evolutionary transitions such as abiogenesis, eukaryogenesis, and the evolution of sexual reproduction, multicellularity, and intelligence itself. Some of these transitions could have been extraordinarily improbable, even in conducive environments. We know from the geological record that life started relatively quickly, as soon as our planet’s environment was stable enough to support it. Life took about 500 million years to emerge from the primordial conditions that existed on Earth ca. 4.5 billion years ago. About 500 million years after that, photosynthesis emerged in the form of single-celled organisms that metabolized carbon dioxide and produced oxygen gas as a byproduct. This gradually altered the chemical makeup of our atmosphere, triggering the Great Oxidation Event about 2.4 billion years ago and the eventual rise of complex life forms. A long and complex process of chemical and biological evolution followed, eventually leading to conditions suitable for complex life and the emergence of all known species. The existence of life on Earth provides proof that abiogenesis is relatively easy on planets similar to Earth. But it took more than a billion years for life to advance from prokaryotic (single-cell organisms) to eukaryotes (organisms with a nucleus) means that such a step is highly unlikely. It took approximately 4.5 billion years for a series of evolutionary transitions resulting in intelligent life to unfold on Earth. In another billion years, the increasing luminosity of the Sun will make Earth uninhabitable for complex life. Intelligence therefore emerged late in Earth’s lifetime. Given how late intelligent life evolved on this planet, the chances of similar developments happening on other planets, before they are no longer able to sustain life, were/are highly unlikely. The Timing of Evolutionary Transitions suggests Intelligent Life is rare. Earth-like intelligent life is probably rare but nonintelligent life is common extraterrestrially.

[5. Rarity of intelligence even on earth:

In the history of life on the Earth only one species has developed a civilization to the point of being capable of spaceflight and radio technology lends credence to the idea that technologically advanced civilizations are rare in the universe.

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Biggest problem in search of extraterrestrial life:

Of course, we have technological constraints. Our current technology limits our ability to explore distant planets and quickly identify biosignatures. Limitations in telescope capabilities, spectroscopic techniques, and space mission budgets can hinder our ability to make definitive discoveries. Then we have anthropocentric bias that might hinder our ability to recognise or understand forms of life that are drastically different from what we are familiar with.

But the most important hurdles are vast interstellar distances and lifespan of human.

At 4.2 light-years (1.3 parsecs, 40 trillion km, or 25 trillion miles) away from Earth, the closest potentially habitable exoplanet is Proxima Centauri b. This means it would take more than 18,264 years to get there if a vessel could consistently travel as fast as the Juno spacecraft (250,000 kilometers per hour or 150,000 miles per hour). To put that time-scale into perspective, that would be over 608 human generations. Strong irradiation by UV radiation and X-rays from Proxima Centauri constitutes a challenge to habitability.

NASA considers exoplanet Kepler-452b and its star to be the closest analog to our planet and Sun so far. NASA called it “Earth 2.0” for a reason. It’s a “Goldilocks planet,” meaning it sits in the habitable zone of its star, where the temperatures are not too hot or cold for liquid water to form. Though it’s 60% larger than Earth in diameter, Kepler-452b is thought to be rocky and within the habitable zone of a G-type star similar to ours. The parent star of Kepler-452b is 10 percent bigger than the sun. The exoplanet Kepler-452b orbits in the habitable zone and shares many similarities with our earth, but the planet is about 1,800 light-years (550 pc) away from the Solar System. At the speed of Juno spacecraft (250,000 kilometers per hour or 150,000 miles per hour), it would take more than 7.5 million years to get there.

In other words, Humans are not the right lifeform to do interstellar flights. It is not feasible to send humans or even probes to search for biosignatures outside of the Solar System. The only way to search for biosignatures outside of the Solar System is by observing exoplanets with telescopes.

Note:

Light sail propelled spacecraft can travel at 10 to 20% speed of light. You can reach exoplanet Proxima Centauri b in 20 to 40 years. However, the maximum acceleration you get from the Sun with a solar sail drops with the square of the distance from the Sun. So they are not much use when you are very far from the Sun. Another way to propel spacecraft by light sail would be extremely strong light pulses with lasers from ground-based laser array.

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Answers to the research questions:

[1. Does extraterrestrial life (simple or complex) exist?

Yes, extraterrestrial life exists no matter whether you find it or not.

[2. Is extraterrestrial life carbon-based and has liquid water as its universal solvent (earth-like)?

Yes, extraterrestrial life would use carbon compounds for basic structural and metabolic functions, liquid water as a solvent, and DNA/ RNA to define and control their form, although they may have different biochemical peculiarities and different lifeforms.

[3. Is extraterrestrial life intelligent?

No, most extraterrestrial life would be primitive and microbial.

Rarely and exceptionally, there could be intelligent thinking species like us but it would be thousands of light years away. Neither we nor them would be having lifeforms to do interstellar travel of thousands to millions of years. We would never physically meet any intelligent alien ever no matter the technological advances. Even if we harness nuclear fusion energy to propel spacecraft and even if we travel at speed of light, it will still take thousands of years to reach alien civilization. Realistically it is unlikely that we will ever harness nuclear fusion in a spacecraft and it is impossible to travel at speed of light. In other words, it is impossible for humans to physically meet any intelligent alien ever and vice versa. Scientist cannot use the word impossible but I am using it looking at the vastness of universe, the speed limit of speed of light and very limited lifespan of humans. Since aliens are based on the same carbon, the same water and the same/similar DNA/RNA, their lifespans will be similar to us. For interstellar travel, we need intelligent lifeforms having lifespan of thousands to million years. That is impossible.

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Dr Rajiv Desai. MD.

January 29, 2024

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Dr Rajiv Desai

Extraterrestrial Life (Life Beyond Earth)

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