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 in all its forms is known as astrobiology, the multidisciplinary field that investigates the deterministic conditions and contingent events with which life arises, distributes, and evolves in the universe.
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.
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 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 disparitas conjecture states that unicellular life is common in the galaxy but that multicellular life is rare in comparision.
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. As of 2013[update], 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. The number of inhabited worlds may increase if the Goldilocks Edge concept is incorporated, wherein habitable areas in otherwise uninhabitable worlds could exist. 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.
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. In order 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 SystemEdit
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, albeit the extremophile paradox, stating that while extremophiles live in extreme environments, they cannot emerge there, puts some restrictions on this view. 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.
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.
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 467 °C (873 °F), 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.
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.
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, 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.
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.
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.
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, 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.
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.
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 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.
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, 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 stars 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 were being used as an incinerator/repository for nuclear waste products.
Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zones of their stars. Since 1992, over four thousand exoplanets have been discovered (5,157 planets in 3,804 planetary systems including 833 multiple planetary systems as of 1 September 2022). The extrasolar planets so far discovered range in size from that of terrestrial planets similar to Earth's size to that of gas giants larger than Jupiter. The number of observed exoplanets is expected to increase greatly in the coming years.
There is at least one planet on average per star. About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone,[c] with the nearest expected to be within 12 light-years distance from Earth. Assuming 200 billion stars in the Milky Way,[d] that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included. The rogue planets in the Milky Way possibly number in the trillions.
As of March 2014[update], the least massive exoplanet known is PSR B1257+12 A, which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is DENIS-P J082303.1-491201 b, about 29 times the mass of Jupiter, although according to most definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.
One sign that a planet probably already contains life is the presence of an atmosphere with significant amounts of oxygen, since that gas is highly reactive and generally would not last long without constant replenishment. This replenishment occurs on Earth through photosynthetic organisms. One way to analyze the atmosphere of an exoplanet is through spectrography when it transits its star, though this might only be feasible with dim stars like white dwarfs.
The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."
In 1961, University of California, Santa Cruz, astronomer and astrophysicist Frank Drake devised the Drake equation as a way to stimulate scientific dialogue at a meeting on the search for extraterrestrial intelligence (SETI). The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation is best understood not as an equation in the strictly mathematical sense, but to summarize all the various concepts which scientists must contemplate when considering the question of life elsewhere. The Drake equation is:
- N = the number of Milky Way galaxy civilizations already capable of communicating across interplanetary space
- R* = the average rate of star formation in our galaxy
- fp = the fraction of those stars that have planets
- ne = the average number of planets that can potentially support life
- fl = the fraction of planets that actually support life
- fi = the fraction of planets with life that evolves to become intelligent life (civilizations)
- fc = the fraction of civilizations that develop a technology to broadcast detectable signs of their existence into space
- L = the length of time over which such civilizations broadcast detectable signals into space
Drake's proposed estimates are as follows, but numbers on the right side of the equation are agreed as speculative and open to substitution:
The Drake equation has proved controversial since several of its factors are uncertain and based on conjecture, not allowing conclusions to be made. This has led critics to label the equation a guesstimate, or even meaningless.
Based on observations from the Hubble Space Telescope, there are between 125 and 250 billion galaxies in the observable universe. It is estimated that at least ten percent of all Sun-like stars have a system of planets, i.e. there are 6.25×1018 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. Also based on Kepler data, scientists estimate that at least one in six stars has an Earth-sized planet.
The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for such civilizations is known as the Fermi paradox.
History and cultural impactEdit
Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the philosophical belief in numerous "worlds" in addition to Earth, which might harbor extraterrestrial life. Before the development of the heliocentric theory and a recognition that the Sun is just one of many stars, the notion of pluralism was largely mythological and philosophical. The earliest recorded assertion of extraterrestrial human life is found in ancient scriptures of Jainism. There are multiple "worlds" mentioned in Jain scriptures that support human life. These include Bharat Kshetra, Mahavideh Kshetra, Airavat Kshetra, Hari kshetra, etc. Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic pluralism on the basis of the Qur'an.
With the scientific and Copernican revolutions, and later, during the Enlightenment, cosmic pluralism became a mainstream notion, supported by the likes of Bernard le Bovier de Fontenelle in his 1686 work Entretiens sur la pluralité des mondes. Pluralism was also championed by philosophers such as John Locke and astronomers such as William Herschel. The astronomer Camille Flammarion promoted the notion of cosmic pluralism in his 1862 book La pluralité des mondes habités. None of these notions of pluralism were based on any specific observation or scientific information.
Early modern periodEdit
There was a dramatic shift in thinking initiated by the invention of the telescope and the Copernican assault on geocentric cosmology. Once it became clear that Earth was merely one planet amongst countless bodies in the universe, the theory of extraterrestrial life started to become a topic in the scientific community. The best known early-modern proponent of such ideas was the Italian philosopher Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own planetary system. Bruno wrote that other worlds "have no less virtue nor a nature different to that of our earth" and, like Earth, "contain animals and inhabitants". Bruno's belief in the plurality of worlds was one of the charges leveled against him by the Venetian Holy Inquisition, which trialed and executed him.
In the early 17th century, the Czech astronomer Anton Maria Schyrleus of Rheita mused that "if Jupiter has (...) inhabitants (...) they must be larger and more beautiful than the inhabitants of Earth, in proportion to the [characteristics] of the two spheres".
In Baroque literature such as The Other World: The Societies and Governments of the Moon by Cyrano de Bergerac, extraterrestrial societies are presented as humoristic or ironic parodies of earthly society. The didactic poet Henry More took up the classical theme of the Greek Democritus in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). In "The Creation: a Philosophical Poem in Seven Books" (1712), Sir Richard Blackmore observed: "We may pronounce each orb sustains a race / Of living things adapted to the place". With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere". Fontanelle's "Conversations on the Plurality of Worlds" (translated into English in 1686) offered similar excursions on the possibility of extraterrestrial life, expanding, rather than denying, the creative sphere of a Maker.
The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. William Herschel, the discoverer of Uranus, was one of many 18th–19th-century astronomers who believed that the Solar System is populated by alien life. Other scholars of the period who championed "cosmic pluralism" included Immanuel Kant and Benjamin Franklin. At the height of the Enlightenment, even the Sun and Moon were considered candidates for extraterrestrial inhabitants.
Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent Martian canals—which soon, however, turned out to be optical illusions. Despite this, in 1895, American astronomer Percival Lowell published his book Mars, followed by Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilization. The idea of life on Mars led British writer H. G. Wells to write the novel The War of the Worlds in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation.
Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer William Wallace Campbell showed that neither water nor oxygen was present in the Martian atmosphere. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis.
As a consequence of the belief in the spontaneous generation there was little thought about the conditions of each celestial body: it was simply assumed that life would thrive anywhere. This theory was disproved by Louis Pasteur in the 19th century. Popular belief in thriving alien civilizations elsewhere in the solar system still remained strong until Mariner 4 and Mariner 9 provided close images of Mars, which debunked forever the idea of the existence of Martians and decreased the previous expectations of finding alien life in general. The end of the spontaneous generation belief forced to investigate the origin of life. Although abiogenesis is the more accepted theory, a number of authors reclaimed the term "panspermia" and proposed that life was brought to Earth from elsewhere. Some of those authors are Jöns Jacob Berzelius (1834), Kelvin (1871), Hermann von Helmholtz (1879) and, somewhat later, by Svante Arrhenius (1903).
The science fiction genre, although not so named during the time, developed during the late 19th century. The expansion of the genre of extraterrestrials in fiction influenced the popular perception over the real-life topic, making people eager to jump to conclusions about the discovery of aliens. Science marched at a slower pace, some discoveries fueled expectations and others dashed excessive hopes. For example, with the advent of telescopes, most structures seen on the Moon or Mars were immediately attributed to Selenites or Martians, and later ones (such as more powerful telescopes) revealed that all such discoveries were natural features. A famous case is the Cydonia region of Mars, first imagined by the Viking 1 orbiter. The low-resolution photos showed a rock formation that resembled a human face, but later spacecraft took photos in higher detail that showed that there was nothing special about the site.
The search and study of extraterrestrial life became a science of its own, Astrobiology. Also known as exobiology, this discipline is studied by the NASA, the ESA, the INAF, and others. Astrobiology studies life from Earth as well, but with a cosmic perspective. For example, abiogenesis is of interest to astrobiology, not because of the origin of life on Earth, but for the chances of a similar process taking place in other celestial bodies. Many aspects of life, from its definition to its chemistry, are analyzed as either likely to be similar in all forms of life across the cosmos or only native to Earth. Astrobiology, however, remains constrained by the current lack of extraterrestrial lifeforms to study, as all life on Earth comes from the same ancestor, and it is hard to infer general characteristics from a group with a single example to analyze.
The 20th century came with great technological advances, speculations about future hypothetical technologies, and an increased basic knowledge of science by the general population thanks to science divulgation through the mass media. The public interest in extraterrestrial life and the lack of discoveries by mainstream science led to the emergence of pseudosciences that provided affirmative, if questionable, answers to the existence of aliens. Ufology claims that many unidentified flying objects (UFOs) would be spaceships from alien species, and ancient astronauts hypothesis claim that aliens would have visited Earth in antiquity and prehistoric times but people would have failed to understand it by then. Most UFOs or UFO sightings can be readily explained as sightings of Earth-based aircraft (including top-secret aircraft), known astronomical objects or weather phenomenons, or as hoaxes.
The possibility of extraterrestrial life on the Moon was ruled out in the 1960s, and during the 1970s it became clear that most of the other bodies of the Solar System do not harbor highly developed life, although the question of primitive life on bodies in the Solar System remains open.
Many scientists are optimistic about the chances of finding alien life. 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 noted that it is entirely possible that 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. The Wow! signal, detected in 1977 by a SETI project, remains a subject of speculative debate.
On the other hand, other scientists are pessimistic. Jacques Monod wrote that "Man knows at last that he is alone in the indifferent immensity of the universe, whence which he has emerged by chance". 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, whereas microbial life is common. Ward and Brownlee are open to the idea of evolution on other planets that is not based on essential Earth-like characteristics such as DNA and carbon.
As for the possible risks, theoretical physicist Stephen Hawking warned in 2010 that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans", he said. Jared Diamond had earlier expressed similar concerns. On 20 July 2015, Hawking and Russian billionaire Yuri Milner, along with the SETI Institute, announced a well-funded effort, called the Breakthrough Initiatives, to expand efforts to search for extraterrestrial life. The group contracted the services of the 100-meter Robert C. Byrd Green Bank Telescope in West Virginia in the United States and the 64-meter Parkes Telescope in New South Wales, Australia. On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake and David Brin) at a convention of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea; one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".
The 1967 Outer Space Treaty and the 1979 Moon Agreement define rules of planetary protection against potentially hazardous extraterrestrial life. COSPAR also provides guidelines for planetary protection. A committee of the United Nations Office for Outer Space Affairs had in 1977 discussed for a year strategies for interacting with extraterrestrial life or intelligence. The discussion ended without any conclusions. As of 2010, the UN doesn't have response mechanisms for the case of an extraterrestrial contact.
One of the NASA divisions is the Office of Safety and Mission Assurance (OSMA), also known as the Planetary Protection Office. A part of its mission is to “rigorously preclude backward contamination of Earth by extraterrestrial life.”
In 2020, Dmitry Rogozin, the head of the Russian space agency, said the search for extraterrestrial life is one of the main goals of deep space research. He also acknowledged the possibility of existence of primitive life on other planets of the Solar System.
In 2016, the Chinese Government released a white paper detailing its space program. According to the document, one of the research objectives of the program is the search for extraterrestrial life. It is also one of the objectives of the Chinese Five-hundred-meter Aperture Spherical Telescope (FAST) program.
The French space agency has an office for the study of “non-identified aero spatial phenomena”. The agency is maintaining a publicly accessible database of such phenomena, with over 1600 detailed entries. According to the head of the office, the vast majority of entries have a mundane explanation; but for 25% of entries, their extraterrestrial origin can neither be confirmed nor denied.
In 2020, chairman of the Israel Space Agency Isaac Ben-Israel stated that the probability of detecting life in outer space is "quite large". But he disagrees with his former colleague Haim Eshed who stated that there are contacts between an advanced alien civilization and some of Earth's governments.
- For the purpose of this 1 in 5 statistic, "Sun-like" means G-type star. Data for Sun-like stars wasn't available so this statistic is an extrapolation from data about K-type stars
- For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii
- For the purpose of this 1 in 5 statistic, "habitable zone" means the region with 0.25 to 4 times Earth's stellar flux (corresponding to 0.5–2 AU for the Sun).
- About 1/4 of stars are GK Sun-like stars. The number of stars in the galaxy is not accurately known, but assuming 200 billion stars in total, the Milky Way would have about 50 billion Sun-like (GK) stars, of which about 1 in 5 (22%) or 11 billion would be Earth-sized in the habitable zone. Including red dwarfs would increase this to 40 billion.
- Frank, Adam (31 December 2020). "A new frontier is opening in the search for extraterrestrial life - The reason we haven't found life elsewhere in the universe is simple: We haven't really looked until now". The Washington Post. Retrieved 1 January 2021.
- Davies, Paul (18 November 2013). "Are We Alone in the Universe?". The New York Times. Archived from the original on 1 January 2022. Retrieved 20 November 2013.
- Pickrell, John (4 September 2006). "Top 10: Controversial pieces of evidence for extraterrestrial life". New Scientist. Retrieved 18 February 2011.
- von Hegner, Ian (2021). "The indeterminacy bottleneck: Implications for habitable worlds". Acta Biotheoretica. 70 (1).
- Crowe, Michael J. (2008). The extraterrestrial life debate, antiquity to 1915: a source book/edited by Michael J. Crowe. University of Notre Dame. p. 14-16.
- Crowe, Michael J. (2008). The extraterrestrial life debate, antiquity to 1915: a source book/edited by Michael J. Crowe. University of Notre Dame. p. 26-27.
- Nicholas of Cusa. (1954). Of Learned Ignorance. Translated by Germain Heron. Routledge. p. 111-118.
- Crowe, Michael J. (2008). The extraterrestrial life debate, antiquity to 1915: a source book/edited by Michael J. Crowe. University of Notre Dame. p. 67.
- Overbye, Dennis (6 January 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next". The New York Times. Archived from the original on 1 January 2022. Retrieved 6 January 2015.
- Ghosh, Pallab (12 February 2015). "Scientists in US are urged to seek contact with aliens". BBC News.
- Baum, Seth; Haqq-Misra, Jacob; Domagal-Goldman, Shawn (June 2011). "Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis". Acta Astronautica. 68 (11): 2114–2129. arXiv:1104.4462. Bibcode:2011AcAau..68.2114B. doi:10.1016/j.actaastro.2010.10.012. S2CID 16889489.
- "How many alien civilizations are out there? A new galactic survey holds a clue". National Geographic. 2 November 2020.
- Weaver, Rheyanne. "Ruminations on other worlds". State Press. Archived from the original on 24 October 2013. Retrieved 10 March 2014.
- Steiger, Brad; White, John, eds. (1986). Other Worlds, Other Universes. Health Research Books. p. 3. ISBN 978-0-7873-1291-6.
- Filkin, David; Hawking, Stephen W. (1998). Stephen Hawking's universe: the cosmos explained. Art of Mentoring Series. Basic Books. p. 194. ISBN 978-0-465-08198-1.
- Rauchfuss, Horst (2008). Chemical Evolution and the Origin of Life. trans. Terence N. Mitchell. Springer. ISBN 978-3-540-78822-5.
- von Hegner, Ian. "Intelligent characteristics of potential microbial life during the LHB". HAL archives-ouvertes.fr | CCSD, August 2022.
- Loeb, Abraham (October 2014). "The Habitable Epoch of the Early Universe". International Journal of Astrobiology. 13 (4): 337–339. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. CiteSeerX 10.1.1.748.4820. doi:10.1017/S1473550414000196. S2CID 2777386.
- Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life". The New York Times. Archived from the original on 1 January 2022. Retrieved 3 December 2014.
- Rampelotto, P. H. (April 2010). Panspermia: A Promising Field of Research (PDF). Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond. 20–26 April 2010. League City, Texas. Bibcode:2010LPICo1538.5224R.
- Gonzalez, Guillermo; Richards, Jay Wesley (2004). The privileged planet: how our place in the cosmos is designed for discovery. Regnery Publishing. pp. 343–345. ISBN 978-0-89526-065-9.
- Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Retrieved 30 March 2012.
- Choi, Charles Q. (21 March 2011). "New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone". Space.com. Retrieved 24 April 2011.
- Torres, Abel Mendez (26 April 2013). "Ten potentially habitable exoplanets now". Habitable Exoplanets Catalog. University of Puerto Rico. Archived from the original on 21 October 2019. Retrieved 29 April 2013.
- Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times. Archived from the original on 1 January 2022. Retrieved 5 November 2013.
- Petigura, Eric A.; Howard, Andrew W.; Marcy, Geoffrey W. (31 October 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033.
- Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
- von Hegner, Ian (2020). "A limbus mundi elucidation of habitability: the Goldilocks Edge". International Journal of Astrobiology. 19 (4): 320–329. arXiv:1910.13336. Bibcode:2020IJAsB..19..320V. doi:10.1017/S1473550420000075. S2CID 204956992.
- Hoehler, Tori M.; Amend, Jan P.; Shock, Everett L. (2007). "A "Follow the Energy" Approach for Astrobiology". Astrobiology. 7 (6): 819–823. Bibcode:2007AsBio...7..819H. doi:10.1089/ast.2007.0207. ISSN 1531-1074. PMID 18069913.
- Jones, Eriita G.; Lineweaver, Charles H. (2010). "To What Extent Does Terrestrial Life "Follow The Water"?" (PDF). Astrobiology. 10 (3): 349–361. Bibcode:2010AsBio..10..349J. CiteSeerX 10.1.1.309.9959. doi:10.1089/ast.2009.0428. hdl:1885/8711. ISSN 1531-1074. PMID 20446874.
- Aguilera Mochon, pp. 9-10
- "Aliens may be more like us than we think". University of Oxford. 31 October 2017.
- Stevenson, David S.; Large, Sean (25 October 2017). "Evolutionary exobiology: Towards the qualitative assessment of biological potential on exoplanets". International Journal of Astrobiology. 18 (3): 204–208. doi:10.1017/S1473550417000349. S2CID 125275411.
- Aguilera Mochón, p. 42
- Aguilera Mochón, p. 58
- Aguilera Mochón, p. 51
- Bond, Jade C.; O'Brien, David P.; Lauretta, Dante S. (June 2010). "The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations". The Astrophysical Journal. 715 (2): 1050–1070. arXiv:1004.0971. Bibcode:2010ApJ...715.1050B. doi:10.1088/0004-637X/715/2/1050. S2CID 118481496.
- Pace, Norman R. (20 January 2001). "The universal nature of biochemistry". Proceedings of the National Academy of Sciences of the United States of America. 98 (3): 805–808. Bibcode:2001PNAS...98..805P. doi:10.1073/pnas.98.3.805. PMC 33372. PMID 11158550.
- National Research Council (2007). "6.2.2: Nonpolar Solvents". The Limits of Organic Life in Planetary Systems. The National Academies Press. p. 74. doi:10.17226/11919. ISBN 978-0-309-10484-5.
- Aguilera Mochón, pp. 43-49
- Aguilera Mochón, pp. 58-59
- Aguilera Mochón, pp. 42-43
- Aguilera Mochón, pp. 61-66
- Dyches, Preston; Chou, Felcia (7 April 2015). "The Solar System and Beyond is Awash in Water". NASA. Retrieved 8 April 2015.
- von Hegner, Ian (2022). "Extreme exoworlds and the extremophile paradox". Astrobiology. 22 (8).
- Hays, Lindsay, ed. (2015). "NASA Astrobiology Strategy 2015" (PDF). NASA. p. 65. Archived from the original (PDF) on 22 December 2016. Retrieved 12 October 2017.
- Offord, Catherine (30 September 2018). "Life Thrives Within the Earth's Crust". The Scientist Magazine. Retrieved 2 April 2019.
- Wilke, Carolyn (11 December 2018). "Life Deep Underground Is Twice the Volume of the Oceans: Study". The Scientist Magazine. Retrieved 2 April 2019.
- Summons, Roger E.; Amend, Jan P.; Bish, David; Buick, Roger; Cody, George D.; Des Marais, David J.; Dromart, Gilles; Eigenbrode, Jennifer L.; et al. (2011). "Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group" (PDF). Astrobiology. 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. hdl:1721.1/66519. PMID 21417945. S2CID 9963677.
There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance.
- Michalski, Joseph R.; Cuadros, Javier; Niles, Paul B.; Parnell, John; Deanne Rogers, A.; Wright, Shawn P. (2013). "Groundwater activity on Mars and implications for a deep biosphere". Nature Geoscience. 6 (2): 133–8. Bibcode:2013NatGe...6..133M. doi:10.1038/ngeo1706.
- "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Retrieved 6 June 2013.
If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface.
- Tritt, Charles S. (2002). "Possibility of Life on Europa". Milwaukee School of Engineering. Archived from the original on 9 June 2007. Retrieved 10 August 2007.
- Kargel, Jeffrey S.; Kaye, Jonathan Z.; Head, James W.; Marion, Giles M.; Sassen, Roger; et al. (November 2000). "Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life". Icarus. 148 (1): 226–265. Bibcode:2000Icar..148..226K. doi:10.1006/icar.2000.6471.
- Schulze-Makuch, Dirk; Irwin, Louis N. (2001). "Alternative Energy Sources Could Support Life on Europa" (PDF). Departments of Geological and Biological Sciences, University of Texas at El Paso. Archived from the original (PDF) on 3 July 2006. Retrieved 21 December 2007.
- Reuell, Peter (8 July 2019). "Harvard study suggests asteroids might play key role in spreading life". Harvard Gazette. Retrieved 29 September 2019.
- Crick, F. H.; Orgel, L. E. (1973). "Directed Panspermia". Icarus. 19 (3): 341–348. Bibcode:1973Icar...19..341C. doi:10.1016/0019-1035(73)90110-3.
- Orgel, L. E.; Crick, F. H. (January 1993). "Anticipating an RNA world. Some past speculations on the origin of life: Where are they today?". FASEB Journal. 7 (1): 238–239. doi:10.1096/fasebj.7.1.7678564. PMID 7678564. S2CID 11314345.
- Hall, Shannon (24 March 2020). "Life on the Planet Mercury? 'It's Not Completely Nuts' - A new explanation for the rocky world's jumbled landscape opens a possibility that it could have had ingredients for habitability". The New York Times. Retrieved 26 March 2020.
- Roddriquez, J. Alexis P.; et al. (16 March 2020). "The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System". Scientific Reports. 10 (4737): 4737. Bibcode:2020NatSR..10.4737R. doi:10.1038/s41598-020-59885-5. PMC 7075900. PMID 32179758.
- Redd, Nola Taylor (17 November 2012). "How Hot is Venus?". Space.com. Retrieved 28 January 2020.
- Clark, Stuart (26 September 2003). "Acidic clouds of Venus could harbour life". New Scientist. Retrieved 30 December 2015.
- Redfern, Martin (25 May 2004). "Venus clouds 'might harbour life'". BBC News. Retrieved 30 December 2015.
- Dartnell, Lewis R.; Nordheim, Tom Andre; Patel, Manish R.; Mason, Jonathon P.; et al. (September 2015). "Constraints on a potential aerial biosphere on Venus: I. Cosmic rays". Icarus. 257: 396–405. Bibcode:2015Icar..257..396D. doi:10.1016/j.icarus.2015.05.006.
- "Did the Early Venus Harbor Life? (Weekend Feature)". The Daily Galaxy. 2 June 2012. Archived from the original on 28 October 2017. Retrieved 22 May 2016.
- "Was Venus once a habitable planet?". European Space Agency. 24 June 2010. Retrieved 22 May 2016.
- Atkinson, Nancy (24 June 2010). "Was Venus once a waterworld?". Universe Today. Retrieved 22 May 2016.
- Bortman, Henry (26 August 2004). "Was Venus Alive? 'The Signs are Probably There'". Space.com. Retrieved 22 May 2016.
- Drake, Nadia (14 September 2020). "Possible sign of life on Venus stirs up heated debate". National Geographic. Archived from the original on 14 September 2020. Retrieved 14 September 2020.
- Greaves, J.S.; Richards, A.M.S.; Bains, W.; et al. (2020). "Phosphine gas in the cloud decks of Venus". Nature Astronomy. 5 (7): 655–664. arXiv:2009.06593. Bibcode:2020NatAs.tmp..178G. doi:10.1038/s41550-020-1174-4. S2CID 221655755. Archived from the original on 14 September 2020. Retrieved 14 September 2020.
- Lincowski, Andrew P.; Meadows, Victoria S.; Crisp, David; Akins, Alex B.; Schwieterman, Edward W.; Arney, Giada N.; Wong, Michael L.; Steffes, Paul G.; Parenteau, M. Niki; Domagal-Goldman, Shawn (2021). "Claimed Detection of PH3 in the Clouds of Venus is Consistent with Mesospheric SO2" (PDF). The Astrophysical Journal. 908 (2): L44. arXiv:2101.09837. Bibcode:2021ApJ...908L..44L. doi:10.3847/2041-8213/abde47. S2CID 231699227. Archived (PDF) from the original on 28 January 2021.
- Abigail Beall (21 October 2020). "More doubts cast on potential signs of life on Venus". New Scientist.
- Snellen, I.A.G.; Guzman-Ramirez, L.; Hogerheijde, M.R.; Hygate, A.P.S.; van der Tak, F.F.S. (December 2020). "Re-analysis of the 267 GHz ALMA observations of Venus". Astronomy & Astrophysics. 644: L2. doi:10.1051/0004-6361/202039717. S2CID 224803085.
- "Mysteries from the moon's past". Washington State University. 23 July 2018. Retrieved 22 August 2020.
- Schulze-Makuch, Dirk; Crawford, Ian A. (2018). "Was There an Early Habitability Window for Earth's Moon?". Astrobiology. 18 (8): 985–988. Bibcode:2018AsBio..18..985S. doi:10.1089/ast.2018.1844. PMC 6225594. PMID 30035616.
- "Could Life Exist Deep Underground on Mars?". Center for Astrophysics (Harvard & Smithsonian). 23 September 2020. Archived from the original on 28 January 2021. Retrieved 28 January 2021.
- Smith, Kimberly; Anderson, James (15 July 2019). "NASA Searches for Life from the Moon in Recently Rediscovered Historic Footage". nasa.gov. NASA. Retrieved 26 March 2021.
- Ojha, L.; Wilhelm, M. B.; Murchie, S. L.; McEwen, A. S.; Wray, J. J.; Hanley, J.; Massé, M.; Chojnacki, M. (2015). "Spectral evidence for hydrated salts in recurring slope lineae on Mars". Nature Geoscience. 8 (11): 829–832. Bibcode:2015NatGe...8..829O. doi:10.1038/ngeo2546.
- "Top 10 Places To Find Alien Life : Discovery News". News.discovery.com. 8 June 2010. Archived from the original on 22 October 2012. Retrieved 13 June 2012.
- Chang, Kenneth (9 December 2013). "On Mars, an Ancient Lake and Perhaps Life". The New York Times. Archived from the original on 1 January 2022. Retrieved 9 December 2013.
- "Science – Special Collection – Curiosity Rover on Mars". Science. 9 December 2013. Retrieved 9 December 2013.
- Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News. Retrieved 27 April 2012.
- de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF). European Geosciences Union. Archived from the original (PDF) on 4 May 2012. Retrieved 27 April 2012.
- Grotzinger, John P. (24 January 2014). "Introduction to Special Issue – Habitability, Taphonomy, and the Search for Organic Carbon on Mars". Science. 343 (6169): 386–387. Bibcode:2014Sci...343..386G. doi:10.1126/science.1249944. PMID 24458635.
- "Special Issue – Table of Contents – Exploring Martian Habitability". Science. 343 (6169): 345–452. 24 January 2014. Retrieved 24 January 2014.
- "Special Collection – Curiosity – Exploring Martian Habitability". Science. 24 January 2014. Retrieved 24 January 2014.
- Grotzinger, J. P.; et al. (24 January 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science. 343 (6169): 1242777. Bibcode:2014Sci...343A.386G. CiteSeerX 10.1.1.455.3973. doi:10.1126/science.1242777. PMID 24324272. S2CID 52836398.
- Küppers, M.; O'Rourke, L.; Bockelée-Morvan, D.; Zakharov, V.; Lee, S.; Von Allmen, P.; Carry, B.; Teyssier, D.; Marston, A.; Müller, T.; Crovisier, J.; Barucci, M. A.; Moreno, R. (23 January 2014). "Localized sources of water vapour on the dwarf planet (1) Ceres". Nature. 505 (7484): 525–527. Bibcode:2014Natur.505..525K. doi:10.1038/nature12918. ISSN 0028-0836. PMID 24451541. S2CID 4448395.
- Campins, H.; Comfort, C. M. (23 January 2014). "Solar system: Evaporating asteroid". Nature. 505 (7484): 487–488. Bibcode:2014Natur.505..487C. doi:10.1038/505487a. PMID 24451536. S2CID 4396841.
- "In Depth | Ceres". NASA Solar System Exploration. Retrieved 29 January 2020.
- O'Neill, Ian (5 March 2009). "Life on Ceres: Could the Dwarf Planet be the Root of Panspermia". Universe Today. Retrieved 30 January 2012.
- Catling, David C. (2013). Astrobiology: A Very Short Introduction. Oxford: Oxford University Press. p. 99. ISBN 978-0-19-958645-5.
- Boyle, Alan (22 January 2014). "Is There Life on Ceres? Dwarf Planet Spews Water Vapor". NBC. Retrieved 10 February 2015.
- Ponnamperuma, Cyril; Molton, Peter (January 1973). "The prospect of life on Jupiter". Space Life Sciences. 4 (1): 32–44. Bibcode:1973SLSci...4...32P. doi:10.1007/BF02626340. PMID 4197410. S2CID 12491394.
- Irwin, Louis Neal; Schulze-Makuch, Dirk (June 2001). "Assessing the Plausibility of Life on Other Worlds". Astrobiology. 1 (2): 143–160. Bibcode:2001AsBio...1..143I. doi:10.1089/153110701753198918. PMID 12467118.
- Dyches, Preston; Brown, Dwayne (12 May 2015). "NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt". NASA. Retrieved 12 May 2015.
- "NASA's Hubble Observations Suggest Underground Ocean on Jupiter's Largest Moon". NASA News. 12 March 2015. Retrieved 15 March 2015.
- Clavin, Whitney (1 May 2014). "Ganymede May Harbor 'Club Sandwich' of Oceans and Ice". NASA. Jet Propulsion Laboratory. Retrieved 1 May 2014.
- Vance, Steve; Bouffard, Mathieu; Choukroun, Mathieu; Sotina, Christophe (12 April 2014). "Ganymede's internal structure including thermodynamics of magnesium sulfate oceans in contact with ice". Planetary and Space Science. 96: 62–70. Bibcode:2014P&SS...96...62V. doi:10.1016/j.pss.2014.03.011.
- "Video (00:51) – Jupiter's 'Club Sandwich' Moon". NASA. 1 May 2014. Archived from the original on 28 October 2021. Retrieved 2 May 2014.
- Chang, Kenneth (12 March 2015). "Suddenly, It Seems, Water Is Everywhere in Solar System". The New York Times. Archived from the original on 1 January 2022. Retrieved 12 March 2015.
- Kuskov, O. L.; Kronrod, V. A. (2005). "Internal structure of Europa and Callisto". Icarus. 177 (2): 550–569. Bibcode:2005Icar..177..550K. doi:10.1016/j.icarus.2005.04.014.
- Showman, Adam P.; Malhotra, Renu (1999). "The Galilean Satellites" (PDF). Science. 286 (5437): 77–84. doi:10.1126/science.286.5437.77. PMID 10506564.
- Hsiao, Eric (2004). "Possibility of Life on Europa" (PDF). University of Victoria. Archived from the original (PDF) on 11 January 2014. Retrieved 13 May 2013.
- Europa may be home to alien life. Melissa Hogenboom, BBC News. 26 March 2015.
- Atkinson, Nancy (2009). "Europa Capable of Supporting Life, Scientist Says". Universe Today. Retrieved 18 August 2011.
- Plait, Phil (17 November 2011). "Huge lakes of water may exist under Europa's ice". Discover. Bad Astronomy Blog.
- "Scientists Find Evidence for "Great Lake" on Europa and Potential New Habitat for Life". The University of Texas at Austin. 16 November 2011.
- Cook, Jia-Rui C. (11 December 2013). "Clay-Like Minerals Found on Icy Crust of Europa". NASA. Retrieved 11 December 2013.
- Wall, Mike (5 March 2014). "NASA hopes to launch ambitious mission to icy Jupiter moon". Space.com. Retrieved 15 April 2014.
- Clark, Stephen (14 March 2014). "Economics, water plumes to drive Europa mission study". Spaceflight Now. Retrieved 15 April 2014.
- Than, Ker (13 September 2005). "Scientists Reconsider Habitability of Saturn's Moon". Space.com.
- Britt, Robert Roy (28 July 2006). "Lakes Found on Saturn's Moon Titan". Space.com.
- "Lakes on Titan, Full-Res: PIA08630". 24 July 2006. Archived from the original on 29 September 2006.
- Coustenis, A.; et al. (March 2009). "TandEM: Titan and Enceladus mission". Experimental Astronomy. 23 (3): 893–946. Bibcode:2009ExA....23..893C. doi:10.1007/s10686-008-9103-z.
- Lovett, Richard A. (31 May 2011). "Enceladus named sweetest spot for alien life". Nature. doi:10.1038/news.2011.337. Retrieved 3 June 2011.
- Czechowski, L (2018). "Enceladus as a place of origin of life in the Solar System". Geological Quarterly. 61 (1). doi:10.7306/gq.1401.
- "What is Consuming Hydrogen and Acetylene on Titan?". NASA/JPL. 2010. Archived from the original on 29 June 2011. Retrieved 6 June 2010.
- Strobel, Darrell F. (2010). "Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions". Icarus. 208 (2): 878–886. Bibcode:2010Icar..208..878S. doi:10.1016/j.icarus.2010.03.003.
- McKay, C. P.; Smith, H. D. (2005). "Possibilities for methanogenic life in liquid methane on the surface of Titan". Icarus. 178 (1): 274–276. Bibcode:2005Icar..178..274M. doi:10.1016/j.icarus.2005.05.018.
- Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.
- Choi, Charles Q. (10 June 2010). "The Chance for Life on Io". Astrobiology Magazine. Retrieved 25 May 2013.
- Chang, Kenneth; Stirone, Shannon (8 February 2021). "Life on Venus? The Picture Gets Cloudier - Despite doubts from many scientists, a team of researchers who said they had detected an unusual gas in the planet's atmosphere were still confident of their findings". The New York Times. Retrieved 8 February 2021.
- Cofield, Calla; Chou, Felicia (25 June 2018). "NASA Asks: Will We Know Life When We See It?". NASA. Retrieved 26 June 2018.
- Nightingale, Sarah (25 June 2018). "UCR Team Among Scientists Developing Guidebook for Finding Life Beyond Earth". UCR Today. University of California, Riverside. Retrieved 26 June 2018.
- Crenson, Matt (6 August 2006). "Experts: Little Evidence of Life on Mars". Associated Press. Archived from the original on 16 April 2011. Retrieved 8 March 2011.
- McKay, David S.; Gibson, Everett K., Jr.; Thomas-Keprta, Kathie L.; Vali, Hojatollah; Romanek, Christopher S.; et al. (August 1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science. 273 (5277): 924–930. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069. S2CID 40690489.
- Webster, Guy (27 February 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA. Retrieved 27 February 2014.
- Gannon, Megan (28 February 2014). "Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life". Space.com. Retrieved 28 February 2014.
- Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 978-0-7137-2747-0.
- Klein, Harold P.; Levin, Gilbert V.; Levin, Gilbert V.; Oyama, Vance I.; Lederberg, Joshua; Rich, Alexander; Hubbard, Jerry S.; Hobby, George L.; Straat, Patricia A.; Berdahl, Bonnie J.; Carle, Glenn C.; Brown, Frederick S.; Johnson, Richard D. (1 October 1976). "The Viking Biological Investigation: Preliminary Results". Science. 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090. S2CID 24957458.
- Beegle, Luther W.; Wilson, Michael G.; Abilleira, Fernando; Jordan, James F.; Wilson, Gregory R. (August 2007). "A Concept for NASA's Mars 2016 Astrobiology Field Laboratory". Astrobiology. 7 (4): 545–577. Bibcode:2007AsBio...7..545B. doi:10.1089/ast.2007.0153. PMID 17723090.
- "ExoMars rover". ESA. Archived from the original on 19 October 2012. Retrieved 14 April 2014.
- Berger, Brian (16 February 2005). "Exclusive: NASA Researchers Claim Evidence of Present Life on Mars". Space.com.
- "NASA denies Mars life reports". spacetoday.net. 19 February 2005.
- Spotts, Peter N. (28 February 2005). "Sea boosts hope of finding signs of life on Mars". The Christian Science Monitor. Retrieved 18 December 2006.
- Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com. Retrieved 22 July 2011.
- Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News. Retrieved 22 July 2011.
- Glaser, Linda (27 January 2015). "Introducing: The Carl Sagan Institute". Archived from the original on 27 February 2015. Retrieved 11 May 2015.
- "Carl Sagan Institute – Research". May 2015. Retrieved 11 May 2015.
- Cofield, Calla (30 March 2015). "Catalog of Earth Microbes Could Help Find Alien Life". Space.com. Retrieved 11 May 2015.
- Callahan, M.P.; Smith, K.E.; Cleaves, H.J.; Ruzica, J.; Stern, J.C.; Glavin, D.P.; House, C.H.; Dworkin, J.P. (11 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". Proceedings of the National Academy of Sciences. 108 (34): 13995–13998. Bibcode:2011PNAS..10813995C. doi:10.1073/pnas.1106493108. PMC 3161613. PMID 21836052.
- Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". NASA. Retrieved 10 August 2011.
- "DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests". ScienceDaily. 9 August 2011. Retrieved 9 August 2011.
- Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Retrieved 26 October 2011.
- "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". ScienceDaily. 26 October 2011. Retrieved 27 October 2011.
- Kwok, Sun; Zhang, Yong (26 October 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature. 479 (7371): 80–3. Bibcode:2011Natur.479...80K. doi:10.1038/nature10542. PMID 22031328. S2CID 4419859.
- Than, Ker (29 August 2012). "Sugar Found in Space". National Geographic. Retrieved 31 August 2012.
- "Sweet! Astronomers spot sugar molecule near star". Associated Press. 29 August 2012. Retrieved 31 August 2012.
- Jørgensen, Jes K.; Favre, Cécile; Bisschop, Suzanne E.; Bourke, Tyler L.; van Dishoeck, Ewine F.; Schmalzl, Markus (September 2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA" (PDF). The Astrophysical Journal Letters. 757 (1). L4. arXiv:1208.5498. Bibcode:2012ApJ...757L...4J. doi:10.1088/2041-8205/757/1/L4. S2CID 14205612.
- Schenkel, Peter (May–June 2006). "SETI Requires a Skeptical Reappraisal". Skeptical Inquirer. Retrieved 28 June 2009.
- Moldwin, Mark (November 2004). "Why SETI is science and UFOlogy is not". Skeptical Inquirer. Archived from the original on 13 March 2009.
- "The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum". The Columbus Optical SETI Observatory.
- Whitmire, Daniel P.; Wright, David P. (April 1980). "Nuclear waste spectrum as evidence of technological extraterrestrial civilizations". Icarus. 42 (1): 149–156. Bibcode:1980Icar...42..149W. doi:10.1016/0019-1035(80)90253-5.
- "Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet". IAP.fr. 25 January 2006.
- Than, Ker (24 April 2007). "Major Discovery: New Planet Could Harbor Water and Life". Space.com.
- Schneider, Jean (10 September 2011). "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopaedia. Retrieved 30 January 2012.
- Wall, Mike (4 April 2012). "NASA Extends Planet-Hunting Kepler Mission Through 2016". Space.com.
- "NASA – Kepler". Archived from the original on 5 November 2013. Retrieved 4 November 2013.
- Harrington, J. D.; Johnson, M. (4 November 2013). "NASA Kepler Results Usher in a New Era of Astronomy".
- Tenenbaum, P.; Jenkins, J. M.; Seader, S.; Burke, C. J.; Christiansen, J. L.; Rowe, J. F.; Caldwell, D. A.; Clarke, B. D.; Li, J.; Quintana, E. V.; Smith, J. C.; Thompson, S. E.; Twicken, J. D.; Borucki, W. J.; Batalha, N. M.; Cote, M. T.; Haas, M. R.; Hunter, R. C.; Sanderfer, D. T.; Girouard, F. R.; Hall, J. R.; Ibrahim, K.; Klaus, T. C.; McCauliff, S. D.; Middour, C. K.; Sabale, A.; Uddin, A. K.; Wohler, B.; Barclay, T.; Still, M. (2013). "Detection of Potential Transit Signals in the First 12 Quarters of Kepler Mission Data". The Astrophysical Journal Supplement Series. 206 (1): 5. arXiv:1212.2915. Bibcode:2013ApJS..206....5T. doi:10.1088/0067-0049/206/1/5. S2CID 250885680.
- "My God, it's full of planets! They should have sent a poet" (Press release). Planetary Habitability Laboratory, University of Puerto Rico at Arecibo. 3 January 2012. Archived from the original on 25 July 2015. Retrieved 25 July 2015.
- Santerne, A.; Díaz, R. F.; Almenara, J.-M.; Lethuillier, A.; Deleuil, M.; Moutou, C. (2013). "Astrophysical false positives in exoplanet transit surveys: Why do we need bright stars?". Sf2A-2013: Proceedings of the Annual Meeting of the French Society of Astronomy and Astrophysics: 555. arXiv:1310.2133. Bibcode:2013sf2a.conf..555S.
- Cassan, A.; et al. (11 January 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. S2CID 2614136.
- Sanders, R. (4 November 2013). "Astronomers answer key question: How common are habitable planets?". newscenter.berkeley.edu.
- Petigura, E. A.; Howard, A. W.; Marcy, G. W. (2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033.
- Strigari, L. E.; Barnabè, M.; Marshall, P. J.; Blandford, R. D. (2012). "Nomads of the Galaxy". Monthly Notices of the Royal Astronomical Society. 423 (2): 1856–1865. arXiv:1201.2687. Bibcode:2012MNRAS.423.1856S. doi:10.1111/j.1365-2966.2012.21009.x. S2CID 119185094. estimates 700 objects >10−6 solar masses (roughly the mass of Mars) per main-sequence star between 0.08 and 1 Solar mass, of which there are billions in the Milky Way.
- Chang, Kenneth (24 August 2016). "One Star Over, a Planet That Might Be Another Earth". The New York Times. Archived from the original on 1 January 2022. Retrieved 4 September 2016.
- "DENIS-P J082303.1-491201 b". Caltech. Retrieved 8 March 2014.
- Sahlmann, J.; Lazorenko, P. F.; Ségransan, D.; Martín, Eduardo L.; Queloz, D.; Mayor, M.; Udry, S. (August 2013). "Astrometric orbit of a low-mass companion to an ultracool dwarf". Astronomy & Astrophysics. 556: 133. arXiv:1306.3225. Bibcode:2013A&A...556A.133S. doi:10.1051/0004-6361/201321871. S2CID 119193690.
- Aguilar, David A.; Pulliam, Christine (25 February 2013). "Future Evidence for Extraterrestrial Life Might Come from Dying Stars". Harvard-Smithsonian Center for Astrophysics. Release 2013-06. Retrieved 9 June 2017.
- Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Archived from the original on 23 October 2015. Retrieved 8 October 2018.
- Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark; et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon" (PDF). Proc. Natl. Acad. Sci. U.S.A. 112 (47): 14518–21. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. ISSN 1091-6490. PMC 4664351. PMID 26483481. Retrieved 20 October 2015. Early edition, published online before print.
- "Chapter 3 – Philosophy: "Solving the Drake Equation". SETI League. December 2002. Retrieved 24 July 2015.
- Burchell, M. J. (2006). "W(h)ither the Drake equation?". International Journal of Astrobiology. 5 (3): 243–250. Bibcode:2006IJAsB...5..243B. doi:10.1017/S1473550406003107. S2CID 121060763.
- Aguirre, L. (1 July 2008). "The Drake Equation". Nova ScienceNow. PBS. Retrieved 7 March 2010.
- Cohen, Jack; Stewart, Ian (2002). "Chapter 6: What does a Martian look like?". Evolving the Alien: The Science of Extraterrestrial Life. Hoboken, NJ: John Wiley and Sons. ISBN 978-0-09-187927-3.
- Temming, M. (18 July 2014). "How many galaxies are there in the universe?". Sky & Telescope. Retrieved 17 December 2015.
- Marcy, G.; Butler, R.; Fischer, D.; et al. (2005). "Observed Properties of Exoplanets: Masses, Orbits and Metallicities". Progress of Theoretical Physics Supplement. 158: 24–42. arXiv:astro-ph/0505003. Bibcode:2005PThPS.158...24M. doi:10.1143/PTPS.158.24. S2CID 16349463. Archived from the original on 2 October 2008.
- Swift, Jonathan J.; Johnson, John Asher; Morton, Timothy D.; Crepp, Justin R.; Montet, Benjamin T.; et al. (January 2013). "Characterizing the Cool KOIs. IV. Kepler-32 as a Prototype for the Formation of Compact Planetary Systems throughout the Galaxy". The Astrophysical Journal. 764 (1). 105. arXiv:1301.0023. Bibcode:2013ApJ...764..105S. doi:10.1088/0004-637X/764/1/105. S2CID 43750666.
- "100 Billion Alien Planets Fill Our Milky Way Galaxy: Study". Space.com. 2 January 2013. Archived from the original on 3 January 2013. Retrieved 10 March 2016.
- "Alien Planets Revealed". Nova. Season 41. Episode 10. 8 January 2014. Event occurs at 50:56.
- Overbye, Dennis (3 August 2015). "The Flip Side of Optimism About Life on Other Planets". The New York Times. Archived from the original on 1 January 2022. Retrieved 29 October 2015.
- "Who discovered that the Sun was a Star?". Stanford Solar Center.
- Mukundchandra G. Raval (2016). Meru: The Center of our Earth. Notion Press. ISBN 978-1-945400-10-0.
- Crowe, Michael J. (1999). The Extraterrestrial Life Debate, 1750–1900. Courier Dover Publications. ISBN 978-0-486-40675-6.
- Wiker, Benjamin D. (4 November 2002). "Alien Ideas: Christianity and the Search for Extraterrestrial Life". Crisis Magazine. Archived from the original on 10 February 2003.
- Irwin, Robert (2003). The Arabian Nights: A Companion. Tauris Parke Paperbacks. p. 204 & 209. ISBN 978-1-86064-983-7.
- David A. Weintraub (2014). "Islam," Religions and Extraterrestrial Life (pp 161–168). Springer International Publishing.
- J. William Schopf (2002). Life's Origin: The Beginnings of Biological Evolution. University of California Press. ISBN 9780520233911. Retrieved 6 August 2022.
- de Fontenelle, Bernard le Bovier (1990). Conversations on the Plurality of Worlds. trans. H. A. Hargreaves. University of California Press. ISBN 978-0-520-91058-4.
- "Flammarion, (Nicolas) Camille (1842–1925)". The Internet Encyclopedia of Science.
- "Giordano Bruno: On the Infinite Universe and Worlds (De l'Infinito Universo et Mondi) Introductory Epistle: Argument of the Third Dialogue". Archived from the original on 13 October 2014. Retrieved 4 October 2014.
- Aguilera Mochon, p. 8
- "Rheita.htm". cosmovisions.com.
- Evans, J. E.; Maunder, E. W. (June 1903). "Experiments as to the actuality of the "Canals" observed on Mars". Monthly Notices of the Royal Astronomical Society. 63 (8): 488–499. Bibcode:1903MNRAS..63..488E. doi:10.1093/mnras/63.8.488.
- Wallace, Alfred Russel (1907). Is Mars Habitable? A Critical Examination of Professor Lowell's Book "Mars and Its Canals," With an Alternative Explanation. London: Macmillan. OCLC 8257449.
- Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 978-0-7137-2747-0.
- Aguilera Mochon, pp. 8-9
- Berzelius, Jöns Jacob (1834). "Analysis of the Alais meteorite and implications about life in other worlds". Annalen der Chemie und Pharmacie. 10: 134–135.
- Thomson, William (August 1871). "The British Association Meeting at Edinburgh". Nature. 4 (92): 261–278. Bibcode:1871Natur...4..261.. doi:10.1038/004261a0. PMC 2070380.
We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space.
- Demets, René (October 2012). "Darwin's Contribution to the Development of the Panspermia Theory". Astrobiology. 12 (10): 946–950. Bibcode:2012AsBio..12..946D. doi:10.1089/ast.2011.0790. PMID 23078643.
- Arrhenius, Svante (March 1908). Worlds in the Making: The Evolution of the Universe. trans. H. Borns. Harper & Brothers. OCLC 1935295.
- Nola Taylor Tillman (20 August 2012). "The Face on Mars: Fact & Fiction". Space.com. Retrieved 18 September 2022.
- Aguilera Mochon, pp. 10-11
- "Life's Working Definition: Does It Work?". NASA. 2002. Retrieved 17 January 2022.
- Aguilera Mochon, p. 10
- Cross, Anne (2004). "The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers". Qualitative Sociology. 27 (1): 3–34. doi:10.1023/B:QUAS.0000015542.28438.41. S2CID 144197172.
- Ailleris, Philippe (January–February 2011). "The lure of local SETI: Fifty years of field experiments". Acta Astronautica. 68 (1–2): 2–15. Bibcode:2011AcAau..68....2A. doi:10.1016/j.actaastro.2009.12.011.
- "LECTURE 4: MODERN THOUGHTS ON EXTRATERRESTRIAL LIFE". The University of Antarctica. Retrieved 25 July 2015.
- Wood, Lisa (3 July 2010). "WOW!". Ohio History Connection Collections Blog. Retrieved 2 July 2016.
- Paul Davies (1 September 2016). "The Cosmos Might Be Mostly Devoid of Life". Scientific American. Retrieved 8 July 2022.
- Ward, Peter; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus. Bibcode:2000rewc.book.....W. ISBN 978-0-387-98701-9.
- "Hawking warns over alien beings". BBC News. 25 April 2010. Retrieved 2 May 2010.
- Diamond, Jared M. (2006). "Chapter 12". The Third Chimpanzee: The Evolution and Future of the Human Animal. Harper Perennial. ISBN 978-0-06-084550-6.
- Katz, Gregory (20 July 2015). "Searching for ET: Hawking to look for extraterrestrial life". Excite!. Associated Press. Retrieved 20 July 2015.
- Borenstein, Seth (13 February 2015). "Should We Call the Cosmos Seeking ET? Or Is That Risky?". The New York Times. Associated Press. Archived from the original on 14 February 2015.
- Ghosh, Pallab (12 February 2015). "Scientist: 'Try to contact aliens'". BBC News. Retrieved 12 February 2015.
- "Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI)". University of California, Berkeley. 13 February 2015. Retrieved 14 February 2015.
- Matignon, Louis (29 May 2019). "THE FRENCH ANTI-UFO MUNICIPAL LAW OF 1954". Spacelegalissues.com. Retrieved 26 March 2021.
- "Press Conference by Director of Office for Outer Space Affairs | Meetings Coverage and Press Releases". www.un.org.
- "Coronavirus Could Preview What Will Happen When Alien Life Reaches Earth". Time.
- "Рогозин допустил существование жизни на Марсе и других планетах Солнечной системы". ТАСС.
- Wheeler, Michelle (14 July 2017). "Is China The Next Space Superpower?". Particle.
- "China Focus: Earth's largest radio telescope to search for "new worlds" outside solar system". Archived from the original on 11 July 2019.
- "France opens up its UFO files". New Scientist. 22 March 2007.
- Bockman, Chris (4 November 2014). "Why the French state has a team of UFO hunters". BBC News.
- Jeffay, Nathan (10 December 2020). "Israeli space chief says aliens may well exist, but they haven't met humans". The Times of Israel.
- Aguilera Mochón, Juan Antonio (2016). La vida no terrestre [The non-terrestrial life] (in Spanish). RBA. ISBN 978-84-473-8665-9.
- Baird, John C. (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 978-0-87451-406-3.
- Cohen, Jack; Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 978-0-09-187927-3.
- Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN 978-0-521-26305-4.
- Crowe, Michael J. (2008). The extraterrestrial life debate Antiquity to 1915: A Source Book. University of Notre Dame Press. ISBN 978-0-268-02368-3.
- Dick, Steven J. (1984). Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant. Cambridge.
- Dick, Steven J. (1996). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science. Cambridge. ISBN 978-0-521-34326-8.
- Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN 978-0-521-79912-6.
- Dick, Steven J.; Strick, James E. (2004). The Living Universe: NASA And the Development of Astrobiology. Rutgers. ISBN 978-0-8135-3447-3.
- Fasan, Ernst (1970). Relations with alien intelligences – the scientific basis of metalaw. Berlin: Berlin Verlag.
- Goldsmith, Donald (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN 978-0-525-94336-5.
- Gribbin, John, "Alone in the Milky Way: Why we are probably the only intelligent life in the galaxy", Scientific American, vol. 319, no. 3 (September 2018), pp. 94–99.
- Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 978-0-06-018540-4.
- Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book. Bibcode:1998owsl.book.....L.
- Michaud, Michael (2006). Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials. Berlin: Springer. ISBN 978-0-387-28598-6.
- Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN 978-0-465-07315-3.
- Roth, Christopher F. (2005). Debbora Battaglia (ed.). Ufology as Anthropology: Race, Extraterrestrials, and the Occult. E.T. Culture: Anthropology in Outerspaces. Durham, NC: Duke University Press.
- Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House.
- Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. ISBN 978-0-262-19106-7.
- Ward, Peter D. (2005). Life as we do not know it-the NASA search for (and synthesis of) alien life. New York: Viking. ISBN 978-0-670-03458-1.
- Tumminia, Diana G. (2007). Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact. Syracuse: Syracuse University Press. ISBN 978-0-8156-0858-5.