Extraterrestrial life[n 1] is life that does not originate from Earth. It is also called alien life, or, if it is a sentient and/or relatively complex individual, an "extraterrestrial" or "alien" (or, to avoid confusion with the legal sense of "alien", a "space alien"). These as-yet-hypothetical life forms range from simple bacteria-like organisms to beings with civilizations far more advanced than humanity. Although many scientists consider extraterrestrial life to be plausible, there is only controversial evidence of its existence.
The science of extraterrestrial life is known as exobiology, the science of astrobiology also considers life on Earth as well, in the context of the Solar System and, also, in the broader astronomical context. Meteorites that have fallen to Earth have sometimes been examined for signs of microscopic extraterrestrial life. Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial intelligence, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction. Over the years, science fiction works, especially Hollywood's involvement, has increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try to get in contact with life in outer space, whereas others argue that it might be dangerous to actively call attention to Earth.
- 1 Background
- 2 Possible basis
- 3 Planetary habitability in the Solar System
- 4 Scientific search
- 5 The Drake equation
- 6 Cultural impact
- 7 See also
- 8 Notes
- 9 References
- 10 Further reading
- 11 External links
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 suggests that there is nothing special about life on Earth. The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the universe was only 10–17 million years old. Life may have emerged independently at many places throughout the universe. Alternatively, life may have formed less frequently, then spread—by meteoroids, for example—between habitable planets in a process called panspermia. In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth. According to these studies, this process may occur outside Earth on several planets and moons of the Solar System and on planets of other stars.
Since the 1950s, scientists have argued the idea that "habitable zones" around stars are the most likely places to find life. Numerous discoveries in these zones since 2007 have generated estimations of frequencies of Earth-like planets —in terms of composition— numbering in the many billions though as of 2013, only a small number of planets have been discovered in these zones. Nonetheless, on November 4, 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.
It is often hypothesized that life forms elsewhere in the universe would, like life on Earth, be based on carbon chemistry and rely on liquid water. Life forms based on ammonia (rather than water) have been suggested, though this solvent appears less suitable than water. It is also conceivable that there are forms of life whose solvent is a liquid hydrocarbon, such as methane, ethane or propane.
About 29 chemical elements are thought to play an active positive role in living organisms on Earth. About 95% of this living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts. The unique characteristics of carbon made it unlikely that any other element could replace carbon, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—to the chemical composition of life on other planets.
Life on Earth requires water as its solvent in which biochemical reactions take place. Sufficient quantities of carbon and the other elements along with water, may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to that of Earth. Terrestrial planets such as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's. The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can provide structural elements for life. Plants derive energy through the conversion of light energy into chemical energy via photosynthesis. Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially oxidized (carbon oxides) states. Nitrogen is needed as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer.
Planetary habitability in the Solar System
Some bodies in the Solar System have been suggested as having the potential for an environment that could host extraterrestrial life, 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.
Some speculate that Mars, may possess niche subsurface environments where microbial life might exist. A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms.
The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth may have been seeded from elsewhere (exogenesis). The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras. In the nineteenth century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834), Kelvin (1871), Hermann von Helmholtz (1879) and, somewhat later, by Svante Arrhenius (1903). Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) are important proponents of the hypothesis who further contended that lifeforms continue to enter Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.
Directed panspermia concerns the deliberate transport of microorganisms in space, sent to Earth to start life here, or sent from Earth to seed new stellar systems with life. The Nobel prize winner Francis Crick, along with Leslie Orgel proposed that seeds of life may have been purposely spread by an advanced extraterrestrial civilization, but considering an early "RNA world" Crick noted later that life may have originated on Earth.
Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and there may still be liquid water beneath the surface. The origin of the potential biosignature of methane observed in Mars atmosphere is unexplained, although abiotic hypotheses have also been proposed. By July 2008, laboratory tests aboard NASA's Phoenix Mars lander identified water in a surface soil sample. Photographs from the Mars Global Surveyor from 2006 showed evidence of recent (i.e. within 10 years) flows of a liquid on Mars's frigid surface. There is evidence that Mars had a warmer and wetter past: dried-up river beds, polar ice caps, volcanos, and minerals that form in the presence of water have all been found. Nevertheless, present conditions on Mars subsurface may support life. Evidence obtained by the Curiosity rover studying Aeolis Palus, Gale Crater in 2013, strongly suggest an ancient freshwater lake that could have been a hospitable environment for microbial life.
Current studies on Mars by the Curiosity and Opportunity rovers are now 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 recently been confirmed by the Herschel Space Observatory to have a thin water vapor atmosphere. Frost on the surface may also have been detected in the form of bright spots. The presence of water on Ceres has led to speculation that life may be possible there.
Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter, however, the intense radiation and other conditions do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely. In contrast, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have indications that heated subsurface oceans of liquid water may exist deep under the crusts of the three outer Galilean moons —Europa, Ganymede, and Callisto. The EJSM/Laplace mission is planned to determine the habitability of these environments.
Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of a liquid water ocean beneath its ice surface. Hydrothermal vents on the bottom of the ocean, if they exist, may warm the ice and could be capable of supporting multicellular microorganisms. It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.
The case for life on Europa was greatly enhanced in 2011 when it was discovered that vast lakes exist within Europa's thick, icy shell. Scientists found that ice shelves surrounding the lakes appear to be collapsing into them, thereby providing a mechanism through which life-forming chemicals created in sunlit areas on Europa's surface could be transferred to its interior.
On December 11, 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 astrobiology mission Europa Clipper may be launched in 2025. Europa's subsurface ocean is considered the best target for the discovery of life.
Titan and Enceladus have been speculated to host possible habitats supportive of life.
Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects. The Cassini–Huygens probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting life—during its 2005 flyby through one of Enceladus's geysers spewing ice and gas. The temperature and density of the plumes indicate a warmer, watery source beneath the surface. No life was detected in the plumes.
Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth. Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface that are consistent with—but do not prove—the hypothesis that organisms there if present, could be consuming hydrogen, acetylene and ethane, and producing methane.
Small Solar System bodies
Small Solar System bodies have also been speculated to host habitats for extremophiles. Fred Hoyle and Chandra Wickramasinghe have proposed that microbial life might exist on comets and asteroids.
The scientific search for extraterrestrial life is being carried out both directly and indirectly.
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, indicates that a non-biological reaction is a more likely hypothesis. In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars.
In February 2005, NASA scientists reported that they may have found some evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claim on methane signatures found in Mars's atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon distanced NASA from the scientists' claims, and Stoker herself backed off from her initial assertions. Though such methane findings are still debated, support among some scientists for the existence of life on Mars seems to be growing.
In November 2011, NASA launched the Mars Science Laboratory that landed the Curiosity rover on Mars. It is designed to assess the past and present habitability on Mars using a variety of scientific instruments. The rover landed on Mars at Gale Crater in August 2012.
The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014 dedicated to the atmospheric characterization of exoplanets in circumstellar habitable zones. Planetary spectroscopic data will be obtained from telescopes like WFIRST and E-ELT.
In August 2011, findings by NASA, based on studies of meteorites found on Earth, suggests 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.
Another biosignature of intelligence, would be to find an abundance of anomalies of heavy elements in the light spectrum of a star, which would take place if the star was used as a repository for nuclear waste material.
Some astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zone of their star. Since 1992, nearly 2000 exoplanets have been discovered (1949 planets in 1233 planetary systems including 487 multiple planetary systems as of 25 August 2015).
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.
The Kepler space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives. 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.
The nearest known exoplanet, if confirmed, would be Alpha Centauri Bb, located 4.37 light-years from Earth in the southern constellation of Centaurus. As of March 2014[update], the least massive planet 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.
The Drake equation
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 civilizations in our galaxy with which radio-communication might be possible (i.e. which are on our current past light cone);
- 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 per star that has planets
- fl = the fraction of planets that could support life that actually develop life at some point
- fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L = the length of time for which such civilizations release detectable signals into space
The Drake equation has proved controversial since several of its factors are extremely uncertain and are entirely based on conjecture. Thus the equation cannot be used to draw firm conclusions of any kind. This has led critics to label the equation a guesstimate, or even meaningless.
Based on observations from the Hubble Space Telescope, there are at least 125 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 we assume that only one out of a billion of these stars have planets supporting life, there would be some 6.25×109 (billion) life-supporting planetary systems in the observable universe.
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.
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 our Sun is just one of many stars, the notion of pluralism was largely mythological and philosophical. With the scientific and Copernican revolutions, and later, during the 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 period
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".
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 convinced that the Solar System are populated by alien life. Other luminaries 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 canal—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. This idea 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.
The science fiction genre, although not so named during the time, develops during the late 19th century. Jules Verne's Around the Moon (1870) features a discussion of the possibility of life on the Moon, but with the conclusion that it is barren. Stories involving extraterrestrials are found in e.g. Garrett P. Serviss's Edison's Conquest of Mars (1898), an unauthorized sequel to The War of the Worlds by H. G. Wells was published in 1897 which stands at the beginning of the popular idea of the "Martian invasion" of Earth prominent in 20th-century pop culture.
Most unidentified flying objects or UFO sightings can be readily explained as sightings of Earth-based aircraft, known astronomical objects, or as hoaxes. Nonetheless, a certain fraction of the public believe that UFOs might actually be of extraterrestrial origin, and, indeed, the notion has had influence on popular culture.
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 an open question.
The failure so far of the SETI program to detect an intelligent radio signal after decades of effort, has at least partially dimmed the prevailing optimism of the beginning of the space age. Notwithstanding, belief in extraterrestrial beings continues to be voiced in pseudoscience, conspiracy theories, and in popular folklore, notably "Area 51" and legends. It has become a pop culture trope given less-than-serious treatment in popular entertainment.
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.
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).
Theoretical physicist Stephen Hawking in 2010 warned 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 has expressed similar concerns.
In November 2011, the White House released an official response to two petitions asking the U.S. government to acknowledge formally that aliens have visited Earth and to disclose any intentional withholding of government interactions with extraterrestrial beings. According to the response, "The U.S. government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race." Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye." The response noted "odds are pretty high" that there may be life on other planets but "the odds of us making contact with any of them—especially any intelligent ones—are extremely small, given the distances involved."
In 2013, the exoplanet Kepler-62f was discovered, along with Kepler-62e and Kepler-62c. A related special issue of the journal Science, published earlier, described the discovery of the exoplanets.
On 17 April 2014, the discovery of the Earth-size exoplanet Kepler-186f, 500 light-years from Earth, was publicly announced; it is the first Earth-size planet to be discovered in the habitable zone and it has been hypothesized that there may be liquid water on its surface.
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".
On 20 July 2015, Stephen Hawking, British physicist, and Yuri Milner, Russian billionaire, 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.
- Searches for extraterrestrial life
- Allen Telescope Array
- Communication with extraterrestrial intelligence
- Nexus for Exoplanet System Science
- Extraterrestrial liquid water
- Planetary protection
- Potential cultural impact of extraterrestrial contact
- 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.
- Davies, Paul (18 November 2013). "Are We Alone in the Universe?". New York Times. Retrieved 20 November 2013.
- Pickrell, John (4 September 2006). "Top 10: Controversial pieces of evidence for extraterrestrial life". New Scientist. Retrieved 2011-02-18.
- Overbye, Dennis (January 6, 2015). "As Ranks of Goldilocks Planets Grow, Astronomers Consider What’s Next". New York Times. Retrieved January 6, 2015.
- BBC News – Scientists in US are urged to seek contact with aliens
- Baum, Seth; Haqq-Misra, Jacob; Domagal-Goldman, Shawn. "Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis", Acta Astronautica, 2011, 68 (11–12):2014–2129, April 22, 2011, accessed August 18, 2011.
- Weaver, Rheyanne. "Ruminations on other worlds". State Press. Retrieved 10 March 2014.
- Brad Steiger, John White, eds. (1986). Other Worlds, Other Universes. Health Research Books. p. 3. ISBN 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 0-465-08198-3.
- Rauchfuss, Horst (2008). Chemical Evolution and the Origin of Life. T. N. Mitchell. Springer. ISBN 3-540-78822-0
- Loeb, Abraham (October 2014). "The Habitable Epoch of the Early Universe". International Journal of Astrobiology 13 (04): 337–339. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. doi:10.1017/S1473550414000196. Retrieved 15 December 2014.
- Loeb, Abraham (2 December 2013). "The Habitable Epoch of the Early Universe" (PDF). Arxiv. arXiv:1312.0613v3. Retrieved 15 December 2014.
- Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life". New York Times. Retrieved 3 December 2014.
- Rampelotto, P.H. (2010). "Panspermia: A Promising Field Of Research" (PDF). Astrobiology Science Conference. Retrieved 3 December 2014.
- 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 0-89526-065-4.
- 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 2011-04-24.
- Torres, Abel Mendez (April 26, 2013). "Ten potentially habitable exoplanets now". Habitable Exoplanets Catalog. University of Puerto Rico. Retrieved April 29, 2013.
- Overbye, Dennis (November 4, 2013). "Far-Off Planets Like the Earth Dot the Galaxy". New York Times. Retrieved November 5, 2013.
- Petigura, Eric A.; Howard, Andrew W.; Marcy, Geoffrey W. (October 31, 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. Retrieved November 5, 2013.
- Khan, Amina (November 4, 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved November 5, 2013.
- 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.
- Jones, Eriita G.; Lineweaver, Charles H. (2010). "To What Extent Does Terrestrial Life "Follow The Water"?". Astrobiology 10 (3): 349–361. Bibcode:2010AsBio..10..349J. doi:10.1089/ast.2009.0428. ISSN 1531-1074.
- Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life, National Research Council; The Limits of Organic Life in Planetary Systems; The National Academies Press, 2007; p 74
- Ultratrace minerals. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al.. Baltimore : Williams & Wilkins, c1999., p. 283-303. Issue Date: 1999 URI: 
- Mix, Lucas John (2009). Life in space: astrobiology for everyone. Harvard University Press. p. 76. ISBN 0-674-03321-3. Retrieved 2011-08-08.
- Norman H. Horowitz. To Utopia and Back: The Search for Life in the Solar System 1986 W.H. Freeman & Co, NY ISBN 0-7167-1765-4 ISBN 0-7167-1766-2
- Pace, Norman R. (January 20, 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.
- 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.
- Dyches, Preston; Chou, Felcia (7 April 2015). "The Solar System and Beyond is Awash in Water". NASA. Retrieved 8 April 2015.
- 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". Astrobiology 11 (2): 157–81. Bibcode:2011AsBio..11..157S. doi:10.1089/ast.2010.0506. PMID 21417945.
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 2013-06-06.
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.
- Jeffrey S. Kargel, Jonathan Z. Kaye, James W. Head, III et al. (2000). "Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life" (PDF). Icarus (Planetary Sciences Group, Brown University) 148 (1): 226–265. Bibcode:2000Icar..148..226K. doi:10.1006/icar.2000.6471.
- Schulze-Makuch, Dirk; and 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.
- Margaret O'Leary (2008) Anaxagoras and the Origin of Panspermia Theory, iUniverse publishing Group, # ISBN 978-0-595-49596-2
- Berzelius (1799–1848), J. J. "Analysis of the Alais meteorite and implications about life in other worlds".
- Thomson (Lord Kelvin), W. (1871). "Inaugural Address to the British Association Edinburgh. "We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space."". Nature 4 (92): 261–278 . Bibcode:1871Natur...4..261.. doi:10.1038/004261a0.
- Darwin's contribution to the development of the Panspermia theory. By Demets R. Astrobiology. 2012 October; 12(10) pages: 946–50. doi: 10.1089/ast.2011.0790
- Arrhenius, S., Worlds in the Making: The Evolution of the Universe. New York, Harper & Row, 1908.
- Fred Hoyle, Chandra Wickramasinghe and John Watson, Viruses from Space and Related Matters, University College Cardiff Press, 1986.
- Crick, F. H.; Orgel, L. E. (1973). "Directed Panspermia". Icarus 19: 341–348. Bibcode:1979JBIS...32..419M. doi:10.1016/0019-1035(73)90110-3+.
- "Anticipating an RNA world. Some past speculations on the origin of life: where are they today?" by L. E. Orgel and F. H. C. Crick in FASEB J. (1993) Volume 7 pages 238–239.
- "Top 10 Places To Find Alien Life : Discovery News". News.discovery.com. 2010-06-08. Retrieved 2012-06-13.
- "Water 'flowed recently' on Mars". BBC News. 2006-12-06. Retrieved 2010-05-02.
- 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. Retrieved 27 April 2012.[dead link]
- Chang, Kenneth (December 9, 2013). "On Mars, an Ancient Lake and Perhaps Life". New York Times. Retrieved December 9, 2013.
- Various (December 9, 2013). "Science – Special Collection – Curiosity Rover on Mars". Science. Retrieved December 9, 2013.
- Grotzinger, John P. (January 24, 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. Retrieved January 24, 2014.
- Various (January 24, 2014). "Special Issue – Table of Contents – Exploring Martian Habitability". Science 343 (6169): 345–452. Retrieved 24 January 2014.
- Various (January 24, 2014). "Special Collection – Curiosity – Exploring Martian Habitability". Science. Retrieved January 24, 2014.
- Grotzinger, J.P. et al. (January 24, 2014). "A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars". Science 343 (6169). Bibcode:2014Sci...343A.386G. doi:10.1126/science.1242777. Retrieved January 24, 2014.
- 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. (2014-01-23). "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.
- Campins, H.; Comfort, C. M. (23 January 2014). "Solar system: Evaporating asteroid". Nature 505 (7484): 487–488. doi:10.1038/505487a. PMID 24451536.
- A'Hearn, Michael F.; Feldman, Paul D. (1992). "Water vaporization on Ceres". Icarus 98 (1): 54–60. Bibcode:1992Icar...98...54A. doi:10.1016/0019-1035(92)90206-M.
- A. Duffy – Cosmos – What on Ceres are those bright spots?
- Rivkin, Andrew (21 July 2015). "Dawn at Ceres: A haze in Occator crater?". The Planetary Society. Retrieved 2015-07-24.
- 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 0-19-958645-4.
- 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.
- 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.
- Jeffrey S. Kargel, Jonathan Z. Kaye, James W. Head, III et al. (2000). "Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life" (PDF). Icarus (Planetary Sciences Group, Brown University) 148 (1): 226–265. Bibcode:2000Icar..148..226K. doi:10.1006/icar.2000.6471.
- Staff (March 12, 2015). "NASA’s Hubble Observations Suggest Underground Ocean on Jupiter's Largest Moon". NASA News. Retrieved 2015-03-15.
- "Jupiter moon Ganymede could have ocean with more water than Earth – NASA". Russia Today (RT). 13 March 2015. Retrieved 2015-03-13.
- Clavin, Whitney (1 May 2014). "Ganymede May Harbor 'Club Sandwich' of Oceans and Ice". NASA (Jet Propulsion Laboratory). Retrieved 2014-05-01.
- 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. Bibcode:2014P&SS...96...62V. doi:10.1016/j.pss.2014.03.011. Retrieved 2014-05-02.
- Staff (1 May 2014). "Video (00:51) – Jupiter's 'Club Sandwich' Moon". NASA. Retrieved 2014-05-02.
- Chang, Kenneth (March 12, 2015). "Suddenly, It Seems, Water Is Everywhere in Solar System". New York Times. Retrieved March 12, 2015.
- Kuskov, O.L.; Kronrod, V.A. (2005). "Internal structure of Europa and Callisto". Icarus 177 (2): 550–369. 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.
- Possibility of Life on Europa. University of Victoria, Department of Physics And Astronomy. Canada
- Friedman, Louis (December 14, 2005). "Projects: Europa Mission Campaign". The Planetary Society. Retrieved 2011-08-08
- Nancy Atkinson (2009). "Europa Capable of Supporting Life, Scientist Says". Universe Today. Retrieved 2011-08-18.
- Phil Plait, "Huge lakes of water may exist under Europa's ice", "Bad Astronomy Blog"
- "SCIENTISTS FIND EVIDENCE FOR "GREAT LAKE" ON EUROPA AND POTENTIAL NEW HABITAT FOR LIFE"
- Cook, Jia-Rui c. (December 11, 2013). "Clay-Like Minerals Found on Icy Crust of Europa". NASA. Retrieved December 11, 2013.
- Wall, Mike (5 March 2014). "NASA hopes to launch ambitious mission to icy Jupiter moon". Space.com. Retrieved 2014-04-15.
- Clark, Stephen (14 March 2014). "Economics, water plumes to drive Europa mission study". Spaceflight Now. Retrieved 2014-04-15.
- 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 (Nature). doi:10.1038/news.2011.337. Retrieved 2011-06-03.
- SPACE.com – Scientists Reconsider Habitability of Saturn's Moon
- SPACE.com – Lakes Found on Saturn's Moon Titan
- "Lakes on Titan, Full-Res: PIA08630". 2006-07-24.
- "What is Consuming Hydrogen and Acetylene on Titan?". NASA/JPL. 2010. Archived from the original on 6 June 2010. Retrieved 2010-06-06.
- Darrell F. Strobel (2010). "Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions". Icarus 208 (2): 878. 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.
- Hoyle, Fred, Evolution from Space, Omni Lecture, Royal Institution, London, 12 January 1982; Evolution from Space (1982) pp. 27–28 ISBN 0-89490-083-8; Evolution from Space: A Theory of Cosmic Creationism (1984) ISBN 0-671-49263-2
- Hoyle, Fred (1985). Living Comets. Cardiff: University College, Cardiff Press.
- Wickramasinghe, Chandra (June 2011). "Viva Panspermia". The Observatory.
- Wesson, P (2010). "Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space". Sp. Sci.Rev. 1–4 156: 239–252. arXiv:1011.0101. Bibcode:2010SSRv..156..239W. doi:10.1007/s11214-010-9671-x.
- Crenson, Matt (6 August 2006). "Experts: Little Evidence of Life on Mars". Associated Press (on discovery.com). Archived from the original on 16 April 2011. Retrieved 8 March 2011.
- McKay DS, Gibson EK, ThomasKeprta KL, Vali H, Romanek CS, Clemett SJ, Chillier XDF, Maechling CR, Zare RN (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.
- McKay DS, Thomas-Keprta KL, Clemett, SJ, Gibson, EK Jr, Spencer L, Wentworth SJ (2009). Hoover, Richard B; Levin, Gilbert V; Rozanov, Alexei Y; Retherford, Kurt D, eds. "Life on Mars: new evidence from martian meteorites". Proc. SPIE. Proceedings of SPIE 7441 (1): 744102. doi:10.1117/12.832317. Retrieved 8 March 2011.
- Webster, Guy (27 February 2014). "NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars". NASA. Retrieved 27 February 2014.
- White, Lauren M.; Gibson, Everett K.; Thomnas-Keprta, Kathie L.; Clemett, Simon J.; McKay, David (19 February 2014). "Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593". Astrobiology 14 (2): 170–181. Bibcode:2014AsBio..14..170W. doi:10.1089/ast.2011.0733. 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 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. (1976-10-01). "The Viking Biological Investigation: Preliminary Results". Science 194 (4260): 99–105. Bibcode:1976Sci...194...99K. doi:10.1126/science.194.4260.99. PMID 17793090. Retrieved 2008-08-15.
- 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. Retrieved 2009-07-20.
- "ExoMars rover". ESA. Retrieved 2014-04-14.
- Berger, Brian (2005). "Exclusive: NASA Researchers Claim Evidence of Present Life on Mars".
- "NASA denies Mars life reports". spacetoday.net. 2005.
- Spotts, Peter N. (2005-02-28). "Sea boosts hope of finding signs of life on Mars". The Christian Science Monitor. Retrieved 2006-12-18.
- Chow, Dennis (22 July 2011). "NASA's Next Mars Rover to Land at Huge Gale Crater". Space.com. Retrieved 2011-07-22.
- Amos, Jonathan (22 July 2011). "Mars rover aims for deep crater". BBC News. Retrieved 2011-07-22.
- Glaser, Linda (January 27, 2015). "Introducing: The Carl Sagan Institute". Retrieved 2015-05-11.
- "Carl Sagan Institute - Research". May 2015. Retrieved 2015-05-11.
- Cofield, Calla (30 March 2015). "Catalog of Earth Microbes Could Help Find Alien Life". Space.com. Retrieved 2015-05-11.
- 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". PNAS. doi:10.1073/pnas.1106493108. Retrieved 2011-08-15.
- Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". NASA. Retrieved 2011-08-10.
- ScienceDaily Staff (9 August 2011). "DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests". ScienceDaily. Retrieved 2011-08-09.
- Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Retrieved 2011-10-26.
- ScienceDaily Staff (26 October 2011). "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". ScienceDaily. Retrieved 2011-10-27.
- 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.
- Than, Ker (August 29, 2012). "Sugar Found In Space". National Geographic. Retrieved August 31, 2012.
- Staff (August 29, 2012). "Sweet! Astronomers spot sugar molecule near star". Associated Press. Retrieved August 31, 2012.
- Jørgensen, J. K.; Favre, C.; Bisschop, S.; Bourke, T.; Dishoeck, E.; Schmalzl, M. (2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA" (PDF). eprint.
- Schenkel, Peter (May 2006). "SETI Requires a Skeptical Reappraisal". Skeptical Inquirer. Retrieved June 28, 2009.
- Moldwin, Mark (November 2004). "Why SETI is science and UFOlogy is not". Skeptical Inquirer.
- "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.
- SPACE.com – Major Discovery: New Planet Could Harbor Water and Life
- Schneider, J. "Interactive Extra-solar Planets Catalog". The Extrasolar Planets Encyclopedia.
- Mike Wall, "NASA's Kepler Observatory to continue hunt for strange new worlds"[dead link], "The Christian Science Monitor"
- "NASA – Kepler". 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: 5. arXiv:1212.2915. Bibcode:2013ApJS..206....5T. doi:10.1088/0067-0049/206/1/5.
- "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.
- 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?". arXiv:1310.2133 [astro-ph.EP].
- Cassan, A. et al. (January 11, 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.
- 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. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110.
- 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. 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.
- Dumusque, X.; Pepe, F.; Lovis, C.; Ségransan, D.; Sahlmann, J.; Benz, W.; Bouchy, F.; Mayor, M. et al. (17 October 2012). "An Earth mass planet orbiting Alpha Centauri B" (PDF). Nature 490 (7423): 207–11. Bibcode:2012Natur.491..207D. doi:10.1038/nature11572. PMID 23075844. Retrieved 17 October 2012.
- "DENIS-P J082303.1-491201 b". Caltech. Retrieved 8 March 2014.
- Sahlmann, J.; Lazorenko, P. F.; Ségransan, D.; Martín, E. L.; Queloz, D.; Mayor, M.; Udry, S. (August 2013). "Astrometric orbit of a low-mass companion to an ultracool dwarf". Harvard University 556: 133. arXiv:1306.3225. Bibcode:2013A&A...556A.133S. doi:10.1051/0004-6361/201321871.
- "Chapter 3 — Philosophy: "Solving the Drake Equation". SETI League. December 2002. Retrieved July 24, 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.
- Aguirre, L. (1 July 2008). "The Drake Equation". Nova ScienceNow. PBS. Retrieved 2010-03-07.
- Jack Cohen and Ian Stewart (2002). Evolving the Alien. John Wiley and Sons, Inc., Hoboken, NJ. Chapter 6, What does a Martian look like?
- 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.
- Who discovered that the Sun was a Star? Stanford Solar Center.
- Michael J. Crowe (1999). The Extraterrestrial Life Debate, 1750–1900. Courier Dover Publications. ISBN 0-486-40675-X.
- Wiker, Benjamin D. (November 4, 2002). "Alien Ideas: Christianity and the Search for Extraterrestrial Life". Crisis Magazine. Archived from the original on February 10, 2003.
- Irwin, Robert (2003). The Arabian Nights: A Companion. Tauris Parke Paperbacks. p. 204 & 209. ISBN 1-86064-983-1.
- Conversations on the Plurality of Worlds— Bernard le Bovier de Fontenelle
- 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". Retrieved 4 October 2014.
- "Rheita.htm". cosmovisions.com.
- Evans, J. E. and Maunder, E. W. (1903) "Experiments as to the Actuality of the 'Canals' observed on Mars", MNRAS, 63 (1903) 488
- Is Mars habitable? A critical examination of Professor Percival Lowell's book "Mars and its canals.", an alternative explanation, by Alfred Russel Wallace, F.R.S., etc. London, Macmillan and co., 1907.
- Chambers, Paul (1999). Life on Mars; The Complete Story. London: Blandford. ISBN 0-7137-2747-0.
- Cross, Anne (2004). "The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers". Qualitiative Sociology 27 (1): 3–34. doi:10.1023/B:QUAS.0000015542.28438.41.
- 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 2015-07-25.
- Amazon.com: Rare Earth: Why Complex Life is Uncommon in the Universe: Books: Peter Ward, Donald Brownlee
- "Hawking warns over alien beings". BBC News. 2010-04-25. Retrieved 2010-05-02.
- Diamond, Jared. "The Third Chimpanzee", Harper Perennial, 2006, Chapter 12.
- Larson, Phil (5 November 2011). "Searching for ET, But No Evidence Yet". White House. Retrieved 2011-11-06.
- Atkinson, Nancy (5 November 2011). "No Alien Visits or UFO Coverups, White House Says". UniverseToday. Retrieved 2011-11-06.
- Staff (May 3, 2013). "Special Issue: Exoplanets". Science. Retrieved May 18, 2013.
- Chang, Kenneth (17 April 2014). "Scientists Find an 'Earth Twin', or Maybe a Cousin". New York Times.
- Borenstein, Seth (of AP News) (13 February 2015). "Should We Call the Cosmos Seeking ET? Or Is That Risky?". New York Times. Retrieved 14 February 2015.
- Ghosh, Pallab (12 February 2015). "Scientist: 'Try to contact aliens'". BBC News. Retrieved 12 February 2015.
- Various (13 February 2015). "Statement – Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI)". University of California, Berkeley. Retrieved 14 February 2015.
- Katz, Gregory (20 July 2015). "Searching for ET: Hawking to look for extraterrestrial life". AP News. Retrieved 20 July 2015.
- 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 0-87451-406-1.
- Cohen, Jack; Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2.
- Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN 0-521-26305-0.
- Crowe, Michael J. (2008). The extraterrestrial life debate Antiquity to 1915: A Source Book. University of Notre Dame Press. ISBN 0-268-02368-9.
- 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 0-521-34326-7.
- Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN 0-521-79912-0.
- Dick, Steven J.; Strick, James E. (2004). The Living Universe: NASA And the Development of Astrobiology. Rutgers. ISBN 0-8135-3447-X.
- 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 0-525-94336-6.
- Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 0-06-018540-6.
- Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book.
- Michaud, Michael (2006). Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials. Berlin: Springer. ISBN 0-387-28598-9.
- Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN 0-465-07315-8.
- 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 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 0-670-03458-4.
- Tumminia, Diana G. (2007). Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact. Syracuse: Syracuse University Press. ISBN 978-0-8156-0858-5.
|Wikimedia Commons has media related to Aliens.|
|Wikiquote has quotations related to: Alien life|
|Wikisource has original works on the topic: Extraterrestrial life|
- "Is it true that there could be intelligent life out there?". physics.org. Retrieved 2 November 2012.
- Minerals and the Origins of Life (Robert Hazen, NASA) (video, 60m, April 2014).
- "Search for Life in the Universe" (NASA) (video, 87m, July 14, 2014).