extraterrestrials

Extraterrestrial life, colloquially referred to as alien life, is life that may occur outside Earth and which did not originate on Earth. No extraterrestrial life has yet been conclusively detected, although efforts are underway. Such life might range from simple forms like prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than humankind. The Drake equation speculates about the existence of sapient life elsewhere in the universe. The science of extraterrestrial life is known as astrobiology.

Speculation about the possibility of inhabited "worlds" outside the planet Earth dates back to antiquity. Multiple early Christian writers discussed the idea of a "plurality of worlds" as proposed by earlier thinkers such as Democritus; Augustine references Epicurus's idea of innumerable worlds "throughout the boundless immensity of space" (originally expressed in his Letter to Herodotus) in ''The City of God''. In his first century poem De rerum natura (Book 2:1048–1076), the Epicurean philosopher Lucretius predicted that we would find innumerable exoplanets with life-forms similar to, and different from, the ones on Earth, and even other races of man.

Pre-modern writers typically assumed that extraterrestrial "worlds" would be inhabited by living beings. William Vorilong, in the 15th century, acknowledged the possibility that Christ could have visited extraterrestrial worlds to redeem their inhabitants. Nicholas of Cusa wrote in 1440 that the Earth was "a brilliant star" like other celestial objects visible in space, which would appear similar to the Sun from an exterior perspective due to a layer of "fiery brightness" in the outer layer of the atmosphere. He theorized that all extraterrestrial bodies could be inhabited by men, plants, and animals, including the Sun. Descartes wrote that there was no means to prove that the stars were not inhabited by "intelligent creatures," but their existence was a matter of speculation. The writings of these thinkers show that interest in extraterrestrial life existed throughout history, but it is only recently that humans have had any means of investigating it.

Since the mid-20th century, active research has taken place to look for signs of extraterrestrial life, encompassing searches 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, especially extraterrestrials in fiction. Over the years, science fiction has communicated scientific ideas, imagined a wide range of possibilities, and influenced public interest in and perspectives on extraterrestrial life. One shared space is the debate over the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to try to contact 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.

Characteristics

Extraterrestrial 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 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, as it arose on Earth roughly 4.2 billion years ago through chemical processes. 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, astronomers have proposed that " habitable zones" around stars are the most likely places for life to exist. Numerous discoveries of such zones since 2007 have generated numerical estimates of many billions of planets with Earth-like compositions. , only a few planets had 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-sized planets 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.

Life on Earth is quite ubiquitous 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 adaptative. However, the origin of life is unrelated to its ease of adaptation, and may have stricter requirements. A planet or moon may not have any life on it, even if it was habitable.

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.

Biochemical basis

The first basic requirement for life is an environment with non-equilibrium thermodynamics, which means that the thermodynamic equilibrium must be broken by a source of energy. The traditional sources of energy in the cosmos are the stars, such as for life on Earth, which depends on the energy of the sun. However, there are other alternative energy sources, such as volcanos, plate tectonics, and hydrothermal vents. There are ecosystems on Earth in deep areas of the ocean that do not receive sunlight, and take energy from black smokers instead. Magnetic fields and radioactivity have also been proposed as sources of energy, although they would be less efficient ones.

Life on Earth requires water in a liquid state as a solvent in which biochemical reactions take place. It is highly unlikely that an abiogenesis process can start within a gaseous or solid medium: the atom speeds, either too fast or too slow, make it difficult for specific ones to meet and start chemical reactions. A liquid medium also allows the transport of nutrients and substances required for metabolism. 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. Life based on ammonia rather than water has been suggested as an alternative, 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.

Another unknown aspect of potential extraterrestrial life would be the chemical elements that would compose it. Life on Earth is largely composed of carbon, but there could be other hypothetical types of biochemistry. A potential replacement for carbon should be able to create complex molecules, store information required for evolution, and be freely available in the medium. To create DNA, RNA, or a close analog, such an element should be able to bind its atoms with many others, creating complex and stable molecules. It should be able to create at least three covalent bonds; two for making long strings and at least a third to add new links and allow for diverse information. Only nine elements meet this requirement: boron, nitrogen, phosphorus, arsenic, antimony (three bonds), carbon, silicon, germanium and tin (four bonds). As for abundance, carbon, nitrogen, and silicon are the most abundant ones in the universe, far more than the others. On Earth's crust the most abundant of those elements is silicon, in the Hydrosphere it's carbon and in the atmosphere, it's carbon and nitrogen. Silicon, however, has disadvantages over carbon. The molecules formed with silicon atoms are less stable, and more vulnerable to acids, oxygen, and light. An ecosystem of silicon-based lifeforms would require very low temperatures, high atmospheric pressure, an atmosphere devoid of oxygen, and a solvent other than water. The low temperatures required would add an extra problem, the difficulty to kickstart a process of abiogenesis to create life in the first place.

Even if extraterrestrial life is based on carbon and uses water as a solvent, like Earth life, it may still have a radically different biochemistry. Life on Earth started with a RNA world and later evolved to its current form, where some of the RNA tasks were transferred to the DNA and proteins. Extraterrestrial life may still be stuck on the RNA world, or evolve into other configurations. It is unclear if our biochemistry is the most efficient one that could be generated, or which elements would follow a similar pattern. However, it is likely that, even if cells had a different composition to those from Earth, they would still have a cell membrane. Life on Earth jumped from prokaryotes to eukaryotes and from unicellular organisms to multicellular organisms through evolution. So far no alternative process to achieve such a result has been conceived, even if hypothetical. Evolution requires life to be divided into individual organisms, and no alternative organization has been satisfactorily proposed either. At the basic level, membranes define the limit of a cell, between it and its environment, while remaining partially open to exchange energy and resources with it.

The evolution from simple cells to eukaryotes, and from them to multicellular lifeforms, is not guaranteed. The Cambrian explosion took place thousands of millions of years after the origin of life, and its causes are not fully known yet. On the other hand, the jump to multicellularity took place several times, which suggests that it could be a case of convergent evolution, and so likely to take place on other planets as well. Palaeontologist Simon Conway Morris considers that convergent evolution would lead to kingdoms similar to our plants and animals, and that many features are likely to develop in alien animals as well, such as bilateral symmetry, limbs, digestive systems and heads with sensory organs. The planetary context would also have an influence: a planet with higher gravity would have smaller animals, and other types of stars can lead to non-green photosynthesizers. The amount of energy available would also affect biodiversity, as an ecosystem sustained by black smokers or hydrothermal vents would have less energy available than those sustained by a star's light and heat, and so its lifeforms would not grow beyond a certain complexity.

Planetary habitability in the Solar System

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 extremophile microorganisms. 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." Researchers found a stunning array of subterranean organisms, mostly microbial, deep underground and estimate that approximately 70 percent of the total number of Earth's bacteria and archaea organisms live within the Earth's crust. Rick Colwell, a member of the Deep Carbon Observatory team from Oregon State University, told the BBC: "I think it’s probably reasonable to assume that the subsurface of other planets and their moons are habitable, especially since we’ve seen here on Earth that organisms can function far away from sunlight using the energy provided directly from the rocks deep underground".

Mars may have niche subsurface environments where microbial life exists. A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms.

The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life were 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. 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.

Mercury

The spacecraft ''MESSENGER'' found evidence of water ice on Mercury.
There may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have been habitable, and perhaps that life forms, albeit likely primitive microorganisms, may have existed on the planet.

Venus

In the early 20th century, Venus was considered to be similar to Earth for habitability, but observations since the beginning of the Space Age revealed that the Venusian surface temperature is around , making it inhospitable for Earth-like life. Likewise, the atmosphere of Venus is almost completely carbon dioxide, which can be toxic to Earth-like life. Between the altitudes of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it may accommodate thermoacidophilic extremophile microorganisms 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. The putative detection of an absorption line of phosphine in Venus's atmosphere, with no known pathway for abiotic production, led to speculation in September 2020 that there could be extant life currently present in the atmosphere. Later research attributed the spectroscopic signal that was interpreted as phosphine to sulfur dioxide, or found that in fact there was no absorption line.

The Moon

3.5 to 4 billion years ago, the Moon could have had a magnetic field, an atmosphere, and liquid water sufficient to sustain life on its surface. Warm and pressurized regions in the Moon's interior might still contain liquid water.

As of 2021, no native lunar life has been found, including any signs of life in the samples of Moon rocks and soil.

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 the atmosphere of Mars is unexplained, although hypotheses not involving life have been proposed.

There is evidence that Mars had a warmer and wetter past: Dried-up riverbeds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. 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. Furthermore, present conditions on the subsurface of Mars may support life.

Current studies on Mars by the ''Curiosity and Perseverance'' rovers are searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, 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

Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere. The vapor could have been produced by ice volcanoes or by ice near the surface sublimating (transforming from solid to gas). Nevertheless, the presence of water on Ceres had led to speculation that life may be possible there. It is one of the few places in the Solar System where scientists would like to search for possible signs of life. Although the dwarf planet might not have living things today, there could be signs it harbored life in the past.

Jupiter system

Jupiter

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 was planned to determine the habitability of these environments; however, due to lack of funding, the program was not continued. Similar missions, like ESA's JUICE and NASA's Europa Clipper are currently in development and are slated for launch in 2023 and 2024, respectively.

Europa

Jupiter's moon Europa has been the subject of 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 water and could be capable of supplying nutrients and energy to 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 2024. Europa's subsurface ocean is considered the best target for the discovery of life.

Saturn system

Like Jupiter, Saturn is not likely to host life. However, its moons Titan and Enceladus have been speculated to have possible habitats supportive of life.

Enceladus

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. Of the bodies on which life is possible, living organisms could most easily enter the other bodies of the Solar System from Enceladus.

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. NASA's Dragonfly mission is slated to land on Titan in the mid-2030s with a VTOL-capable rotorcraft with a launch date set for 2027.

Other bodies

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.

Scientific search

The scientific search for extraterrestrial life is being carried out both directly and indirectly. , 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in the Solar System hold the potential for hosting primitive life such as microorganisms. As of 8 February 2021, an updated status of studies considering the possible detection of lifeforms on Venus (via phosphine) and Mars (via methane) was reported.

Direct search

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 they may have found some evidence of extraterrestrial life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center