It wasn’t the Martians’ fault their planet died. If they existed – once – Martians were likely microbes, living in a world much like our own, warmed by an atmosphere and crisscrossed by waterways. But Mars began to lose that atmosphere, perhaps because its gravity wasn’t strong enough to hold onto it after an asteroid impact, or perhaps it was gradually blown away by solar winds. The cause is still mysterious, but the ending is clear: Mars’s liquid water dried up or froze into ice caps, leaving life without its most precious resource. Any Martians would have been victims of a planet-wide natural disaster they could neither foresee nor prevent.
For Chris McKay, a planetary scientist at NASA’s Ames Research Center in California, the moral implications are clear: we should help our neighbours. Earthlings might not have been able to intervene when Martians were dying en masse (we were just microbes ourselves), but now, billions of years later, we could make it up to them. We’ve already figured out an effective way to warm up a planet: pump greenhouse gases into its atmosphere. McKay imagines a not-too-distant future in which we park machinery on Mars that converts carbon and fluorine in the Martian soil into insulating chlorofluorocarbons, and spews them into the planet’s puny atmosphere like a protein shake designed to bulk it up. ‘On Earth, we would call it pollution. On Mars, it’s called medicine,’ McKay told me in an interview. On his calculation, Mars would be warm enough to support water and microbial life within 100 years.
The practice of making a dead world habitable is called terraforming. In science fiction, Earthlings terraform other planets in order to occupy them, usually after trashing Earth. Think of the TV show Firefly (2002), where humans use terraforming technologies to settle the galaxy, pioneer-style. This is not what McKay has in mind. When it comes to Mars, he says, ‘it’s a question of restoration rather than creation’. It’s a distinction that makes the project not only possible, but also ethical: ‘If there were Martians, and they’re still viable, then in my view they own the planet.’
On Earth, scientists have managed to revive bacteria that has been frozen in ice sheets or entombed in salt crystals for millions of years. So it’s possible that extinct Martians aren’t extinct at all. Warm up Mars, McKay reasons, and the red planet might just spring back to life. But that won’t happen without Earth’s intervention. As McKay put it to me: ‘We should say: “We can help you. We’ll bring back the water, we’ll make it warm again, and you can flourish.”’
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McKay’s terraforming scenario raises the question of what our moral obligations are to any alien life we might meet. NASA scientists have stated publicly that we are likely to find life elsewhere in the Universe in 10-20 years, if not sooner. The first signs could come from Curiosity, the rover currently combing Mars for organic compounds, or from a mission to Europa, the moon of Jupiter that might host teeming ecosystems in its ice-covered, planet-wide sea. It might equally come from an exoplanet atmosphere, whose spectrum carries a chemical signature (such as abundant oxygen) that could have been created only by life on its surface. Whatever it is, we’re going to see it soon.
We’ve rehearsed this moment in popular culture many times over. The way we tell it – from Star Trek to Avatar – it will be the story of a technologically advanced civilisation encountering a less advanced one and bending it to its will; humans can play either role. Such narratives tend to draw on a grossly simplified history, a reworking of human-human meetings between Old World and New. Of course, these encounters – and the conflicts that followed – were never as one-sided as we like to claim today; just try telling the Spanish conquistador Hernán Cortés, gazing at the web of artificial islands that formed the lake city of Tenochtitlán (now Mexico City), that the Aztecs were technologically unsophisticated. A meeting between civilisations from different planets would be just as nuanced (and messy), and just as easy for the conquerors (who might not be us) to rewrite after the fact. Historical encounters have many lessons to teach us about how (not) to treat ‘the other’ – on Earth and off. It’s just that, when it comes to the discovery of alien life, that’s not what’s going to happen.
There are two forms the discovery of alien life could realistically take, neither of them a culture clash between civilisations. The first is finding a ‘biosignature’ of, say, oxygen, in the atmosphere of an expolanet, created by life on the exoplanet’s surface. This kind of long-distance discovery of alien life, which astronomers are already scanning for, is the most likely contact scenario, since it doesn’t require us going anywhere, or even sending a robot. But its consequences will be purely theoretical. At long last we’ll know we’re not alone, but that’s about it. We won’t be able to establish contact, much less meet our counterparts – for a very long time, if ever. We’d reboot scientific, philosophical and religious debates about how we fit into a biologically rich universe, and complicate our intellectual and moral stances in previously unimaginable ways. But any ethical questions would concern only us and our place in the Universe.
‘first contact’ will not be a back-and-forth between equals, but like the discovery of a natural resource
If, on the other hand, we discover microbial or otherwise non-sentient life within our own solar system – logistics will be on our side. We’d be able to visit within a reasonable period of time (as far as space travel goes), and I hope we’d want to. If the life we find resembles plants, their complexity will wow us. Most likely we’ll find simple single-celled microbes or maybe – maybe – something like sponges or tubeworms. In terms of encounter, we’d be making all the decisions about how to proceed.
None of this eliminates the possibility that alien life might discover us. But if NASA’s current timeline holds water, another civilisation has only a few more decades to get here before we claim the mantle of ‘discoverer’ rather than ‘discovered’. With every passing day, it grows more likely that ‘first contact’ will not take the form of an intellectual or moral back-and-forth between equals. It will be more like the discovery of a natural resource, and one we might be able to exploit. It won’t be an encounter, or even a conquest. It will be a gold rush.
This makes defining an ethics of contact necessary now, before we have to put it into practice. The aliens we find could stretch our definitions of life to the absolute limit. We won’t see ourselves in them. We will struggle to understand their reality (who among us feels true empathy for a tubeworm latched to a rock near a hydrothermal vent in the deep ocean?) On Earth, humans long ago became the global force that decides these strange creatures’ fates, despite the fact that we barely think about them and, in many cases, only recently discovered their existence. The same will be true for any nearby planet. We are about to export the best and worst of the Anthropocene to the rest of our solar system, so we better figure out what our responsibilities will be when we get there.
Philosophers and scientists at this year’s meeting of the American Association for the Advancement of Science (AAAS), in San Jose, California, were tasked with pondering the societal questions bound up in astrobiology. The topics on the table were as diverse as the emerging field. The astronomer Chris Impey of the University of Arizona discussed the coming boom in commercial space travel, connecting the companies’ missions with the ‘Manifest Destiny’ arguments used by American settlers in the 19th century. Arsev Umur Aydinoglu, a social scientist from the Middle East Technical University in Turkey, talked about how scientists in an interdisciplinary field such as astrobiology find ways to collaborate in the notoriously siloed and bureaucratic behemoth that is NASA. Synthetic biology and artificial intelligence came up a lot as possible parallels for understanding life with a different history to ours.
But it was Sara Waller, a philosopher from Montana State University, who dived most deeply into questions about discovery, ownership and exploitation, which would play out in any off-world tripping. She invited the audience to consider the way we decide who owns territory: does it go to the discoverers, or the political or commercial entity they represent? To ‘productive’ settlers, who, as the 17th-century philosopher John Locke believed, mix their labour with the land? Or to the species who ‘needs’ it most? And how do we decide what ‘need’ even means? After we’ve laid claim to territory in a capitalist system, can we ever be trusted to preserve it?
The ethics of encountering non-sentient alien life in our solar systems boils down to a core dilemma, says Waller. ‘Is it about conservation and preservation? Or is it about our needs, wants, and desires?’ On Earth, natural-resource grabs have a history of bringing out the worst in us as a species. Consider the example of gold. Conquistadores exterminated entire societies in their hunt for gold, when they weren’t enslaving people to mine it. Prospectors in California blasted away mountains with water canons to access it, permanently altering the geology of the state. Today, small-scale gold miners in South America rip apart rainforests and pollute rivers with mercury, trying to sop up the last specks of gold to sell to a market still in upswing.
There’s plenty of reason to believe other planets will be chock-full of resources we’d like to exploit, even if the life forms are microbial – perhaps especially if they’re microbial.
Think of everything we use Earth microbes for: creating and preserving food, treating disease, and processing waste, to name a few. All of that could be enhanced by exploiting a whole new tree of life. Imagine new antibiotics to which Earth bacteria could never evolve resistance, or microbes that excreted a renewable fuel that burns hotter than oil. Or just all the weird cheeses we could make! Synthetic biologists would see their toolkit multiply exponentially the more trees of alien life we discover.
could we bring ourselves to care about ruining another planet, especially when no sentient beings are objecting?
Perhaps the scariest lesson of resource grabs is that we rarely label our actions as evil or even problematic while we’re engaged in them. That realisation comes later, once we find ourselves living in degraded environments. Before then, it’s an ecological and economic free-for-all. Already, as Impey pointed out to the AAAS panel, private companies are engaged in a space race of sorts. For now, the viable ones operate with the blessing of NASA, catering directly to its (governmental) needs. But if capitalism becomes the driving force behind space travel – whether through luxury vacations to the Moon, safari tours of Europa, mining asteroids for precious minerals, or turning alien worlds into microbial gardens we harvest for ourselves – the balance struck between preservation and exploitation, unless strictly defined and powerfully enforced, will be at risk of shifting in line with companies’ profit margins. Given the chance, today’s nascent space industry could become the next oil industry, raking in the cash by destroying environments with society’s tacit approval.
On Earth, it’s in our interest as a species to stave off ecological meltdown – and still we refuse to put the brakes on our consumption of fossil fuels. It’s hard to believe that we could bring ourselves to care about ruining the environment of another planet, especially when no sentient beings are objecting and we’re reaping rewards back on Earth.
But maybe conservation won’t be our ethical choice when it comes to alien worlds. Let’s revisit those resistance-proof antibiotics. Could we really leave that possibility on the table, condemning members of our own species to suffer and die in order to preserve an alien ecosystem? If alien life is non-sentient, we might think our allegiances should lie foremost with our fellow Earthlings. It’s not necessarily unethical to give Earthling needs extra weight in our moral calculus. But now is the time to discuss under what conditions we’d be willing to exploit alien life for our own ends. If we go in blind, we risk leaving a solar system of altered or destroyed ecosystems in our wake, with little to show for it back home.
The way Montana State’s Sara Waller sees it, there is a middle ground between fanatical preservation and free-for-all exploitation. We might still study how the resources of alien worlds could be used back home, but the driving force would be peer review rather than profit. This is similar to McKay’s dream of a flourishing Mars. ‘Making a home for humans is not really the objective of terraforming Mars,’ he explains. ‘Making a home for life, so that we humans can study it, is what terraforming Mars is about.’
Martian life could appear superficially similar to Earth life, taking forms we might recognise, such as amoebas or bacteria or even something like those teddy-bear tardigrades. But its origin and evolution would be entirely different. It might accomplish many of the same tasks and be recognisable as members of the same category (computers; living things), but its programming would be entirely different. The Martians might have different chemical bases in their DNA, or run off RNA alone. Maybe their amino acids will be mirror images of ours. Finally we’d have something to compare ourselves to, and who’s to say we won’t decide the other way has some advantages?
From a scientific perspective, passing up the opportunity to study a completely new biology would be irresponsible – perhaps even unconscionable. But the question remains: can we be trusted to control ourselves?
Happily, we do have one example of a land grab made good here on Earth: Antarctica. The Antarctic Treaty System, first signed in 1959 and still in effect, allows nations to establish as many scientific bases as they want on the continent but prohibits them from laying claim to the land or its resources. (Some nations, including the UK and Argentina, claimed Antarctic territory before the treaty went into effect. The treaty neither recognises nor disputes those claims, and no new claims are permitted.) Military activities are prohibited, a provision that allowed both the US and the Soviet Union to maintain scientific research stations there for a large part of the Cold War. Among the few non-scientists who get to visit the continent are grant-funded artists, tasked with documenting its glory, hardship and reality.
Antarctica is often compared to an alien world, and its strange and extreme life forms will no doubt inform how and where we look for life on other planets. So much astrobiology research is performed in Antarctica that it makes both practical and poetic sense to base our interactions with alien environments on our approach to that continent. We’re on our way; international rules prohibiting the introduction of invasive species in Antarctica already guide the precautions scientists take to eliminate any hitchhiking Earth microbes on space rovers and probes. As we look toward exploring alien environments on other planets, Antarctica should be our guide.
The Antarctic Treaty, impressive as it is as an example of cooperation and compromise, gets a huge assist from the continent itself: Antarctica is difficult to get to, and almost impossible to live on. There’s not a lot to want there. Its main attraction either as a research location or tourist destination (such as it is) is its extremity. It’s conceivable that Europa or even a rehabilitated Mars would be the same: inaccessible, inhospitable, interesting only to a self-selecting group of scientists and auxiliary weirdos drawn to the adventure and isolation of it all, as in Werner Herzog’s beautiful documentary about Antarctica, Encounters at the End of the World (2007), funded by one of those artist grants. (One hopes those will exist for other planets, too.) But if alien worlds are full of things we desire, the ideal of Antarctica might get quickly left behind.
Earthlings have no vested interest in the status quo on Mars, and no one else seems to either – so let’s play
Still, the Antarctic Treaty ought to be our starting point for international discussion of the ethics of alien contact. Even if Mars, Europa or other biologically rich worlds are designated as scientific preserves, open to heavily vetted research and little else, it is impossible to know where that science will take us, or how it will affect the territories in question. Science might also be used as a mask for more nefarious purposes. The environmental protection provisions of the Antarctic Treaty will be up for review in 2048, and China and Argentina are already strategically positioning themselves to take advantage of an open Antarctica. If the treaty isn’t renewed, we could see mining and fishing operations devastate the continent. And even when we follow the rules, we can’t always control the outcome. The treaty’s best regulations haven’t prevented the human-assisted arrival of introduced species such as grasses, many of which are quickly colonising the habitable portion of the continent.
Of course, science is unpredictable, by design. Let’s return to the example of terraforming Mars one final time. Once we set the process in motion, we have no way of knowing what the outcome will be. Ancient Martians might be awakened from their slumber, or new life could evolve. Maybe we’ve already introduced microbes on one of our rovers, despite our best efforts, and, given the chance, they’ll overrun the world like those grasses in Antarctica. Maybe nothing at all will happen, and Mars will remain as lifeless as it is today. Any of those outcomes is worthy of study, argues Chris McKay. Earthlings have no vested interest in the status quo on Mars, and no one else seems to either – so let’s play. When it comes to experiments, barrelling into the unknown with few ideas and no assurances is kind of the point.
In some ways, the discovery of alien life is a singularity, a point in our history after which everything will be so transformed that we won’t even recognise the future. But we can be sure of one thing: we’ll still be human, for better and for worse. We’ll still be selfish and short-sighted, yet capable of great change. We’ll reflect on our actions in the moment, which doesn’t rule out our regretting them later. We’ll do the best that we can, and we’ll change our minds along the way. We’ll be the same explorers and experimenters we’ve always been, and we’ll shape the solar system in our image. It remains to be seen if we’ll like what we see.
In the end, the only questions that matter are ones we’ve been wrestling with since we became human: when must my needs give way to another’s? How can we make life better for ourselves and for others? And how do we correct course once we realise we’ve made something worse? Discussing how we’ll answer those questions in relation to alien life is our greatest chance to wrestle with who we want to be as a species. What we decide – and what we do – will illuminate who we really are.
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is a science writer. She is the Latin America correspondent for Science, and her work has also appeared in Wired and Slate. She lives in Mexico City.
For other uses, see Astrobiology.
Extraterrestrial life,[n 1] also called alien life (or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien"), is life that occurs outside of Earth and that probably did not originate from Earth. These hypothetical life forms may range from simple prokaryotes to beings with civilizations far more advanced than humanity. The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as exobiology.
Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial life. This encompasses a search for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Depending on the category of search, methods range from the analysis of telescope and specimen data to radios used to detect and send communication signals.
The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, chiefly in works of science fiction. Over the years, science fiction communicated scientific ideas, imagined a wide range of possibilities, and influenced public interest in and perspectives of extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try for contact with intelligent extraterrestrial life. Others—citing the tendency of technologically advanced human societies to enslave or wipe out less advanced societies—argue that it may be dangerous to actively call attention to Earth.
Alien life, such as microorganisms, has been hypothesized to exist in the Solar System and throughout the universe. This hypothesis relies on the vast size and consistent physical laws of the observable universe. According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, as well as well-regarded thinkers such as Winston Churchill, it would be improbable for life not to exist somewhere other than Earth. This argument is embodied in the Copernican principle, which states that Earth does not occupy a unique position in the Universe, and the mediocrity principle, which states that there is nothing special about life on Earth. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old. Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth. According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.
Since the 1950s, scientists have proposed that "habitable zones" around stars are the most likely places to find life. Numerous discoveries in such zones since 2007 have generated numerical estimates of Earth-like planets —in terms of composition—of many billions. As of 2013, only a few planets have been discovered in these zones. Nonetheless, on 4 November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sizedplanets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars. The nearest such planet may be 12 light-years away, according to the scientists. Astrobiologists have also considered a "follow the energy" view of potential habitats.
A study published in 2017 suggests that due to how complexity evolved in species on Earth, the level of predictability for alien evolution elsewhere would make them look similar to life on our planet. One of the study authors, Sam Levin, notes "Like humans, we predict that they are made-up of a hierarchy of entities, which all cooperate to produce an alien. At each level of the organism there will be mechanisms in place to eliminate conflict, maintain cooperation, and keep the organism functioning. We can even offer some examples of what these mechanisms will be." There is also research in assessing the capacity of life for developing intelligence. It has been suggested that this capacity arises with the number of potential niches a planet contains, and that the complexity of life itself is reflected in the information density of planetary environments, which in turn can be computed from its niches.
Main articles: Biochemistry, Hypothetical types of biochemistry, and Water and life
Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth. More generally, life based on ammonia (rather than water) has been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.
About 29 chemical elements play an active positive role in living organisms on Earth. About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts. The unique characteristics of carbon make it unlikely that it could be replaced, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets.
Planetary habitability in the Solar System
See also: Planetary habitability, Habitability of natural satellites, and Exobiology
Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can exist, particularly those with possible subsurface oceans. Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophilemicroorganisms. According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life. Important insights on the limits of microbial life can be gleaned from studies of microbes on modern Earth, as well as their ubiquity and ancestral characteristics."
Mars may have niche subsurface environments where microbial life might exist. A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophilemicroorganisms.
The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth could have been seeded from elsewhere (exogenesis). The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras. In the 19th century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834),Kelvin (1871),Hermann von Helmholtz (1879) and, somewhat later, by Svante Arrhenius (1903).Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) are important proponents of the hypothesis who further contended that life forms continue to enter Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.
Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization, but considering an early "RNA world" Crick noted later that life may have originated on Earth.
Main article: Life on Venus
In the early 20th century, Venus was often thought to be similar to Earth in terms of habitability, but observations since the beginning of the Space Age have revealed that Venus's surface is inhospitable to Earth-like life. However, between an altitude of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it has been speculated that thermoacidophilic extremophilemicroorganisms might exist in the acidic upper layers of the Venusian atmosphere. Furthermore, Venus likely had liquid water on its surface for at least a few million years after its formation.
Main article: Life on Mars
Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil. The origin of the potential biosignature of methane observed in Mars' atmosphere is unexplained, although hypotheses not involving life have also been proposed.
There is evidence that Mars had a warmer and wetter past: dried-up river beds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. Nevertheless, present conditions on Mars' subsurface may support life. Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013 strongly suggests an ancient freshwater lake that could have been a hospitable environment for microbial life.
Current studies on Mars by the Curiosity and Opportunityrovers are searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophicmicroorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable. The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.
Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere. Frost on the surface may also have been detected in the form of bright spots. The presence of water on Ceres has led to speculation that life may be possible there.
Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter. The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely. In contrast, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons—Europa,Ganymede, and Callisto. The EJSM/Laplace mission is planned to determine the habitability of these environments.
Main article: Life on Europa
Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of a liquid water ocean beneath its ice surface.Hydrothermal vents on the bottom of the ocean, if they exist, may warm the ice and could be capable of supporting multicellular microorganisms. It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.
The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior.
On 11 December 2013, NASA reported the detection of "clay-like minerals" (specifically, phyllosilicates), often associated with organic materials, on the icy crust of Europa. The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists. The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2025. Europa's subsurface ocean is considered the best target for the discovery of life.
Titan and Enceladus have been speculated to have possible habitats supportive of life.
Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects. The Cassini–Huygens probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The temperature and density of the plumes indicate a warmer, watery source beneath the surface.
Main article: Life on Titan
Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth. Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there if present, could be consuming hydrogen, acetylene and ethane, and producing methane.
Small Solar System bodies
Small Solar System bodies have also been speculated to host habitats for extremophiles. Fred Hoyle and Chandra Wickramasinghe have proposed that microbial life might exist on comets and asteroids.
Models of heat retention and heating via radioactive decay in smaller icy Solar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick. Of particular interest in these cases is the fact that the models indicate that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.
Hydrogen sulfide has been proposed as a hypothetical solvent for life and is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface.
The scientific search for extraterrestrial life is being carried out both directly and indirectly. As of September 2017[update], 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in our own solar system hold the potential for hosting primitive life such as microorganisms.
Scientists search for biosignatures within the Solar System by studying planetary surfaces and examining meteorites. Some claim to have identified evidence that microbial life has existed on Mars. An experiment on the two Viking Mars landers reported gas emissions from heated Martian soil samples that some scientists argue are consistent with the presence of living microorganisms. Lack of corroborating evidence from other experiments on the same samples, suggests that a non-biological reaction is a more likely hypothesis. In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars.
In February 2005, NASA scientists reported that they may have found some evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions. Though such methane findings are still debated, support among some scientists for the existence of life on Mars exists.
In November 2011, NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012.
The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, 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 their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014 and is dedicated to the atmospheric characterization of exoplanets in circumstellar habitable zones. Planetary spectroscopic data will be obtained from telescopes like WFIRST and ELT.
In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules), building blocks for life as we know it, may be formed extraterrestrially in outer space. In October 2011, scientists reported that cosmic dust contains complex organic matter ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars. One of the scientists suggested that these compounds may have been related to the development of life on Earth and said that, "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
In August 2012, and in a world first, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Glycolaldehyde is needed to form ribonucleic acid, or RNA, which is similar in function to DNA. This finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.
Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds. If there is an advanced extraterrestrial civilization, there is no guarantee that it is transmitting radio communications in the direction of Earth or that this information could be interpreted as such by humans. The length of time required for a signal to travel across the vastness of space means that any signal detected would come from the distant past.
The presence of heavy elements in a star's light-spectrum is another potential biosignature; such elements would (in theory) be found if the star was being used as an incinerator/repository for nuclear waste products.
Main article: Extrasolar planets
See also: List of planetary systems
Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zone of their star. Since 1992 over two thousand exoplanets have been discovered (3,743 planets in 2,796 planetary systems including 625 multiple planetary systems as of 8 March 2018). The extrasolar planets so far discovered range in size from that of terrestrial planets