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Extraterrestrial life
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{{other uses|Astrobiology}}{{short description|Lifeform that does not originate from Earth}}{{multiple image|perrow = 2|total_width=300
width1=864 |height1=591width2=408 |height2=500width3=2500 |height3=1406| image4 = | footer = Some major international efforts to search for extraterrestrial life. Clockwise from top left: }}Extraterrestrial lifeWhere "extraterrestrial" is derived from the Latin extra ("beyond", "not of") and (wikt:terrestris|terrestris) ("of Earth", "belonging to Earth"). is life that occurs outside of Earth which did not originate on Earth. Such hypothetical life might range from simple prokaryotes (or comparable life forms) to beings with civilizations far more advanced than humanity.NEWS, Davies, Paul Davies, Paul, Are We Alone in the Universe?,weblink 18 November 2013, The New York Times, 20 November 2013, NEWS, John, Pickrell, Top 10: Controversial pieces of evidence for extraterrestrial life, 4 September 2006,weblink New Scientist, 18 February 2011, The Drake equation speculates about the existence of intelligent life elsewhere in the universe. The science of extraterrestrial life in all its forms is known as exobiology.Since the mid-20th century, active ongoing research has taken place to look for signs of extraterrestrial life. This encompasses a search for current and historic extraterrestrial life, and a narrower search for extraterrestrial intelligent life. Depending on the category of search, methods range from the analysis of telescope and specimen dataNEWS, Overbye, Dennis, Dennis Overbye, As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next,weblink 6 January 2015, The New York Times, 6 January 2015, to radios used to detect and send communication signals.The concept of extraterrestrial life, and particularly extraterrestrial intelligence, has had a major cultural impact, chiefly in works of science fiction. Over the years, science fiction has introduced a number of theoretical ideas, each having a wide range of possibilities. Many have piqued public interest in the possibilities of extraterrestrial life. One particular concern is the wisdom of attempting communication with extraterrestrial intelligence. Some encourage aggressive methods to make contact with intelligent extraterrestrial life. Others argue to do so may give away the location of earth, making an invasion possible in the future.NEWS,weblink Scientists in US are urged to seek contact with aliens, BBC News, Pallab, Ghosh, 12 February 2015, JOURNAL, Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis, Acta Astronautica, Seth, Baum, Jacob, Haqq-Misra, Shawn, Domagal-Goldman, 68, 11, 2114–2129, June 2011, 10.1016/j.actaastro.2010.10.012, 2011AcAau..68.2114B, 1104.4462,

General

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,WEB, Weaver, Rheyanne, Ruminations on other worlds,weblink State Press, 10 March 2014, yes,weblink" title="web.archive.org/web/20131024035014weblink">weblink 24 October 2013, as well as well-regarded thinkers such as Winston Churchill,JOURNAL, Livio, Mario, Winston Churchill's essay on alien life found,weblink 15 February 2017, Nature (journal), Nature, 542, 7641, 289–291, 10.1038/542289a, 28202987, 18 February 2017, 2017Natur.542..289L, NEWS, De Freytas-Tamura, Kimiko, Winston Churchill Wrote of Alien Life in a Lost Essay,weblink 15 February 2017, The New York Times, 18 February 2017, it would be improbable for life not to exist somewhere other than Earth.BOOK,weblink Other Worlds, Other Universes, Health Research Books, Brad, Steiger, John, White, 3, 1986, 978-0-7873-1291-6, BOOK, David, Filkin, Stephen W., Hawking, Stephen Hawking's universe: the cosmos explained, 194, Art of Mentoring Series, Basic Books, 1998, 978-0-465-08198-1,weblink 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.BOOK,weblink Chemical Evolution and the Origin of Life, Springer, Horst, Rauchfuss, trans. Terence N. Mitchell, 2008, 978-3-540-78822-5, 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.JOURNAL, Loeb, Abraham, Abraham Loeb, The Habitable Epoch of the Early Universe,weblink October 2014, International Journal of Astrobiology, 13, 4, 337–339, 10.1017/S1473550414000196, 15 December 2014, 1312.0613, 2014IJAsB..13..337L, 10.1.1.748.4820, NEWS, Dreifus, Claudia, Claudia Dreifus, Much-Discussed Views That Go Way Back – Avi Loeb Ponders the Early Universe, Nature and Life,weblink 2 December 2014, The New York Times, 3 December 2014, 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.CONFERENCE,weblink Panspermia: A Promising Field Of Research, Astrobiology Science Conference 2010: Evolution and Life: Surviving Catastrophes and Extremes on Earth and Beyond. 20–26 April 2010. League City, Texas., P. H., Rampelotto, April 2010, 2010LPICo1538.5224R, BOOK, Guillermo, Gonzalez, Jay Wesley, Richards, The privileged planet: how our place in the cosmos is designed for discovery, 343–345, Regnery Publishing, 2004, 978-0-89526-065-9,weblink In any case, complex organic molecules may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of Earth.WEB, Moskowitz, Clara, Life's Building Blocks May Have Formed in Dust Around Young Sun,weblink 29 March 2012, Space.com, 30 March 2012, 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.WEB, Choi, Charles Q.,weblink New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone, 21 March 2011, Space.com, 24 April 2011, {{as of|2013}}, only a few planets have been discovered in these zones.WEB,weblink Ten potentially habitable exoplanets now, University of Puerto Rico, Habitable Exoplanets Catalog, 26 April 2013, 29 April 2013, Torres, Abel Mendez, 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,NEWS, Overbye, Dennis, Far-Off Planets Like the Earth Dot the Galaxy,weblink 4 November 2013, The New York Times, 5 November 2013, JOURNAL, Petigura, Eric A., Howard, Andrew W., Marcy, Geoffrey W., Prevalence of Earth-size planets orbiting Sun-like stars,weblink 31 October 2013, Proceedings of the National Academy of Sciences of the United States of America, 10.1073/pnas.1319909110, 5 November 2013, 1311.6806, 2013PNAS..11019273P, 110, 48, 19273–19278, 24191033, 3845182, 11 billion of which may be orbiting Sun-like stars.NEWS, Khan, Amina, Milky Way may host billions of Earth-size planets,weblink 4 November 2013, Los Angeles Times, 5 November 2013, 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.JOURNAL, Hoehler, Tori M., Amend, Jan P., Shock, Everett L., A "Follow the Energy" Approach for Astrobiology, Astrobiology, 7, 6, 2007, 819–823, 1531-1074, 10.1089/ast.2007.0207, 2007AsBio...7..819H, 18069913, JOURNAL, Jones, Eriita G., Lineweaver, Charles H., To What Extent Does Terrestrial Life "Follow The Water"?, Astrobiology, 10, 3, 2010, 349–361, 1531-1074, 10.1089/ast.2009.0428, 20446874, 2010AsBio..10..349J,weblink 10.1.1.309.9959,

Evolution

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."WEB,weblink Aliens may be more like us than we think, University of Oxford, 31 October 2017, 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.JOURNAL, Evolutionary exobiology: Towards the qualitative assessment of biological potential on exoplanets, International Journal of Astrobiology, David S., Stevenson, Sean, Large, 25 October 2017, 10.1017/S1473550417000349, 1–5,

Biochemical basis

Life on Earth requires water as a solvent in which biochemical reactions take place. Sufficient quantities of carbon and other elements, along with water, might enable the formation of living organisms on terrestrial planets with a chemical make-up and temperature range similar to that of Earth.JOURNAL, Bond, Jade C., O'Brien, David P., Lauretta, Dante S., The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations, The Astrophysical Journal, 715, 2, 1050–1070, June 2010, 10.1088/0004-637X/715/2/1050, 2010ApJ...715.1050B, 1004.0971, JOURNAL, Norman R., Pace, 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, 10.1073/pnas.98.3.805, 11158550, 2001PNAS...98..805P, 33372, 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.BOOK,weblink 6.2.2: Nonpolar Solvents, The Limits of Organic Life in Planetary Systems, The National Academies Press, National Research Council, 74, 2007, 10.17226/11919, 978-0-309-10484-5, About 29 chemical elements play an active role in living organisms on Earth.BOOK,weblink Ultratrace Minerals, Modern Nutrition in Health and Disease, Williams & Wilkins, Forrest H., Nielsen, Maurice E., Shils, Moshe, Shike, 283–303, 9th, 1999, 978-0-683-30769-6, About 95% of living matter is built upon only six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. These six elements form the basic building blocks of virtually all life on Earth, whereas most of the remaining elements are found only in trace amounts.BOOK, Lucas John, Mix, 2009, Life in space: astrobiology for everyone, 76, Harvard University Press, 978-0-674-03321-4,weblink 8 August 2011, The unique characteristics of carbon make it unlikely that it could be replaced, even on another planet, to generate the biochemistry necessary for life. The carbon atom has the unique ability to make four strong chemical bonds with other atoms, including other carbon atoms. These covalent bonds have a direction in space, so that carbon atoms can form the skeletons of complex 3-dimensional structures with definite architectures such as nucleic acids and proteins. Carbon forms more compounds than all other elements combined. The great versatility of the carbon atom makes it the element most likely to provide the bases—even exotic ones—for the chemical composition of life on other planets.BOOK, To Utopia and Back: The Search for Life in the Solar System, W. H. Freeman & Co., Norman H., Horowitz, 1986, 978-0-7167-1765-2,

Planetary habitability in the Solar System

{{See also|Planetary habitability|Habitability of natural satellites|Exobiology}}Some bodies in the Solar System have the potential for an environment in which extraterrestrial life can exist, particularly those with possible subsurface oceans.WEB, Dyches, Preston, Chou, Felcia, The Solar System and Beyond is Awash in Water,weblink 7 April 2015, NASA, 8 April 2015, Should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of extremophile microorganisms. According to NASA's 2015 Astrobiology Strategy, "Life on other worlds is most likely to include microbes, and any complex living system elsewhere is likely to have arisen from and be founded upon microbial life. Important insights on the limits of microbial life can be gleaned from studies of microbes on modern Earth, as well as their ubiquity and ancestral characteristics."WEB,weblink NASA Astrobiology Strategy 2015, NASA, Lindsay, Hays, 65, 2015, 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.WEB,weblink Life Thrives Within the Earth’s Crust, The Scientist Magazine®, en, 2019-04-02, 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.”WEB,weblink Life Deep Underground Is Twice the Volume of the Oceans: Study, The Scientist Magazine®, en, 2019-04-02, Mars may have niche subsurface environments where microbial life might exist.JOURNAL, 10.1089/ast.2010.0506, There is general consensus that extant microbial life on Mars would probably exist (if at all) in the subsurface and at low abundance., Preservation of Martian Organic and Environmental Records: Final Report of the Mars Biosignature Working Group, 2011, Summons, Roger E., Amend, Jan P., Bish, David, Buick, Roger, Cody, George D., Des Marais, David J., Dromart, Gilles, Eigenbrode, Jennifer L., Knoll, Andrew H., Sumner, Dawn Y., 8, Astrobiology, 11, 2, 157–81, 21417945, 2011AsBio..11..157S,weblink JOURNAL, 10.1038/ngeo1706, Groundwater activity on Mars and implications for a deep biosphere, 2013, Michalski, Joseph R., Cuadros, Javier, Niles, Paul B., Parnell, John, Deanne Rogers, A., Wright, Shawn P., Nature Geoscience, 6, 2, 133–8, 2013NatGe...6..133M, WEB,weblink Habitability and Biology: What are the Properties of Life?, 6 June 2013, Phoenix Mars Mission, The University of Arizona, If any life exists on Mars today, scientists believe it is most likely to be in pockets of liquid water beneath the Martian surface., A subsurface marine environment on Jupiter's moon Europa might be the most likely habitat in the Solar System, outside Earth, for extremophile microorganisms.WEB,weblink Possibility of Life on Europa, Tritt, Charles S., 10 August 2007, Milwaukee School of Engineering, 2002, yes,weblink" title="web.archive.org/web/20070609150109weblink">weblink 9 June 2007, JOURNAL, Europa's Crust and Ocean: Origin, Composition, and the Prospects for Life, Icarus, Jeffrey S., Kargel, Jonathan Z., Kaye, James W., Head, Giles M., Marion, Roger, Sassen, James K., Crowley, Olga Prieto, Ballesteros, Steven A., Grant, David L., Hogenboom, 5, 148, 1, 226–265, November 2000, 10.1006/icar.2000.6471, 2000Icar..148..226K, WEB, Alternative Energy Sources Could Support Life on Europa,weblink Schulze-Makuch, Dirk, Irwin, Louis N., Departments of Geological and Biological Sciences, University of Texas at El Paso, 2001, 21 December 2007, PDF,weblink" title="web.archive.org/web/20060703033956weblink">weblink 3 July 2006, The panspermia hypothesis proposes that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the Solar System, it could have originated from Earth just as life on Earth could have been seeded from elsewhere (exogenesis). The first known mention of the term 'panspermia' was in the writings of the 5th century BC Greek philosopher Anaxagoras.BOOK, Anaxagoras and the Origin of Panspermia Theory, iUniverse, Margaret R., O'Leary, 2008, 978-0-595-49596-2, In the 19th century it was again revived in modern form by several scientists, including Jöns Jacob Berzelius (1834),JOURNAL, Analysis of the Alais meteorite and implications about life in other worlds, Liebigs Annalen, Annalen der Chemie und Pharmacie, Jöns Jacob, Berzelius, Jöns Jacob Berzelius, 10, 134–135, 1834, Kelvin (1871),JOURNAL, The British Association Meeting at Edinburgh, Nature, William, Thomson, William Thomson, 1st Baron Kelvin, 4, 92, 261–278, August 1871, 10.1038/004261a0, 1871Natur...4..261., We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space., 2070380, Hermann von Helmholtz (1879)JOURNAL, Darwin's Contribution to the Development of the Panspermia Theory, Astrobiology, René, Demets, 12, 10, 946–950, October 2012, 10.1089/ast.2011.0790, 23078643, 2012AsBio..12..946D, and, somewhat later, by Svante Arrhenius (1903).BOOK,weblink Worlds in the Making: The Evolution of the Universe, Harper & Brothers, Svante, Arrhenius, trans. H. Borns, March 1908, 1935295, Sir Fred Hoyle (1915–2001) and Chandra Wickramasinghe (born 1939) are important proponents of the hypothesis who further contended that life forms continue to enter Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for macroevolution.BOOK,weblink Viruses from Space and Related Matters, University College Cardiff Press, Fred, Hoyle, Chandra, Wickramasinghe, John, Watson, 1986, 978-0-906449-93-6, 1986vfsr.book.....H, 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,JOURNAL, Crick, F. H., Orgel, L. E., Directed Panspermia, Icarus, 19, 3, 341–348, 1973, 10.1016/0019-1035(73)90110-3, 1973Icar...19..341C, but considering an early "RNA world" Crick noted later that life may have originated on Earth.JOURNAL,weblink Anticipating an RNA world. Some past speculations on the origin of life: Where are they today?, FASEB Journal, L. E., Orgel, F. H., Crick, 7, 1, 238–239, January 1993, 7678564,

Venus

In the early 20th century, Venus was often thought to be similar to Earth in terms of habitability, but observations since the beginning of the Space Age have revealed that Venus's surface is inhospitable to Earth-like life. However, between an altitude of 50 and 65 kilometers, the pressure and temperature are Earth-like, and it has been speculated that thermoacidophilic extremophile microorganisms might exist in the acidic upper layers of the Venusian atmosphere.NEWS,weblink Acidic clouds of Venus could harbour life, New Scientist, Stuart, Clark, 26 September 2003, 30 December 2015, Redfern, Martin (25 May 2004). "Venus clouds 'might harbour life'". BBC News. Retrieved 30 December 2015.JOURNAL, Constraints on a potential aerial biosphere on Venus: I. Cosmic rays, Icarus, September 2015, Dartnell, Lewis R., Nordheim, Tom Andre, Patel, Manish R., Mason, Jonathon P., 257, 396–405, 10.1016/j.icarus.2015.05.006,weblink 20 August 2015, etal, 2015Icar..257..396D, WEB,weblink Did the Early Venus Harbor Life? (Weekend Feature), 2 June 2012, The Daily Galaxy, 22 May 2016, Furthermore, Venus likely had liquid water on its surface for at least a few million years after its formation.WEB,weblink Was Venus once a habitable planet?, 24 June 2010, European Space Agency, 22 May 2016, WEB,weblink Was Venus once a waterworld?, 24 June 2010, Nancy, Atkinson, Universe Today, 22 May 2016, WEB,weblink Was Venus Alive? 'The Signs are Probably There', 26 August 2004, Henry, Bortman, Space.com, 22 May 2016,

Mars

Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and now can occasionally be found as low-volume liquid brines in shallow Martian soil.JOURNAL, 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, 10.1038/ngeo2546, 8, 11, 829–832, 2015NatGe...8..829O, The origin of the potential biosignature of methane observed in Mars' atmosphere is unexplained, although hypotheses not involving life have also been proposed.WEB,weblink Top 10 Places To Find Alien Life : Discovery News, News.discovery.com, 8 June 2010, 13 June 2012, There is evidence that Mars had a warmer and wetter past: dried-up river beds, polar ice caps, volcanoes, and minerals that form in the presence of water have all been found. Nevertheless, present conditions on Mars' subsurface may support life.WEB, Baldwin, Emily, Lichen survives harsh Mars environment,weblink 26 April 2012, Skymania News, 27 April 2012, WEB, de Vera, J.-P., Kohler, Ulrich, The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars,weblink 26 April 2012, European Geosciences Union, 27 April 2012, yes,weblink 8 June 2012, 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.NEWS, Chang, Kenneth, On Mars, an Ancient Lake and Perhaps Life,weblink 9 December 2013, The New York Times, 9 December 2013, JOURNAL, Science – Special Collection – Curiosity Rover on Mars,weblink 9 December 2013, Science (journal), Science, 9 December 2013, Current studies on Mars by the Curiosity and Opportunity 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.JOURNAL, Grotzinger, John P., Introduction to Special Issue – Habitability, Taphonomy, and the Search for Organic Carbon on Mars,weblink Science (journal), Science, 24 January 2014, 343, 6169, 386–387, 10.1126/science.1249944, 24 January 2014, 2014Sci...343..386G, 24458635, JOURNAL, Special Issue – Table of Contents – Exploring Martian Habitability,weblink 24 January 2014, Science (journal), Science, 343, 6169, 345–452, 24 January 2014, JOURNAL, Special Collection – Curiosity – Exploring Martian Habitability,weblink 24 January 2014, Science (journal), Science, 24 January 2014, JOURNAL, Grotzinger, J. P., etal, A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars,weblink 24 January 2014, Science (journal), Science, 343, 6169, 10.1126/science.1242777, 24 January 2014, 2014Sci...343A.386G, 24324272, 1242777, 10.1.1.455.3973, The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on Mars is now a primary NASA objective.

Ceres

Ceres, the only dwarf planet in the asteroid belt, has a thin water-vapor atmosphere.JOURNAL, 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., Localized sources of water vapour on the dwarf planet (1) Ceres, Nature, 505, 7484, 23 January 2014, 525–527, 0028-0836, 10.1038/nature12918, 24451541, 2014Natur.505..525K, JOURNAL, Campins, H., Comfort, C. M., 10.1038/505487a, Solar system: Evaporating asteroid, Nature, 505, 7484, 487–488, 23 January 2014, 24451536, 2014Natur.505..487C, Frost on the surface may also have been detected in the form of bright spots.JOURNAL, A'Hearn, Michael F., Feldman, Paul D., Water vaporization on Ceres, Icarus, 98, 1, 54–60, 1992, 10.1016/0019-1035(92)90206-M, 1992Icar...98...54A, JOURNAL,weblink What on Ceres are those bright spots?, Cosmos (magazine), Cosmos, Alan, Duffy, 15 June 2015, NEWS, Rivkin, Andrew,weblink Dawn at Ceres: A haze in Occator crater?, The Planetary Society, 21 July 2015, 24 July 2015, The presence of water on Ceres has led to speculation that life may be possible there.WEB,weblink Life on Ceres: Could the Dwarf Planet be the Root of Panspermia, 5 March 2009, Ian, O'Neill, Universe Today, 30 January 2012, BOOK, Catling, David C., 2013, Astrobiology: A Very Short Introduction, Oxford University Press, Oxford, 978-0-19-958645-5, 99, NEWS,weblink Alan, Boyle, Is There Life on Ceres? Dwarf Planet Spews Water Vapor, NBC, 22 January 2014, 10 February 2015,

Jupiter system

Jupiter

Carl Sagan and others in the 1960s and 1970s computed conditions for hypothetical microorganisms living in the atmosphere of Jupiter.JOURNAL, Ponnamperuma, Cyril, Molton, Peter, The prospect of life on Jupiter, Space Life Sciences, 4, 1, 32–44, January 1973, 10.1007/BF02626340, 4197410, 1973SLSci...4...32P, The intense radiation and other conditions, however, do not appear to permit encapsulation and molecular biochemistry, so life there is thought unlikely.JOURNAL, Irwin, Louis Neal, Schulze-Makuch, Dirk, Assessing the Plausibility of Life on Other Worlds, Astrobiology, 1, 2, 143–160, June 2001, 10.1089/153110701753198918, 2001AsBio...1..143I, 12467118, 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,WEB, Dyches, Preston, Brown, Dwayne, NASA Research Reveals Europa's Mystery Dark Material Could Be Sea Salt,weblink 12 May 2015, NASA, 12 May 2015, Ganymede,NEWS,weblink NASA's Hubble Observations Suggest Underground Ocean on Jupiter's Largest Moon, NASA News, 12 March 2015, 15 March 2015, NEWS,weblink Jupiter moon Ganymede could have ocean with more water than Earth – NASA, Russia Today (RT), 13 March 2015, 13 March 2015, NEWS, Clavin, Whitney,weblink Ganymede May Harbor 'Club Sandwich' of Oceans and Ice, NASA, Jet Propulsion Laboratory, 1 May 2014, 1 May 2014, JOURNAL, Ganymede's internal structure including thermodynamics of magnesium sulfate oceans in contact with ice, Planetary and Space Science, 12 April 2014, Vance, Steve, Bouffard, Mathieu, Choukroun, Mathieu, Sotina, Christophe, 10.1016/j.pss.2014.03.011,weblink 2 May 2014, 2014P&SS...96...62V, 96, 62–70, WEB, Video (00:51) – Jupiter's 'Club Sandwich' Moon,weblink 1 May 2014, NASA, 2 May 2014, and Callisto.NEWS, Chang, Kenneth, Suddenly, It Seems, Water Is Everywhere in Solar System,weblink 12 March 2015, The New York Times, 12 March 2015, JOURNAL, Kuskov, O. L., Kronrod, V. A., Internal structure of Europa and Callisto, 2005, 177, 2, 550–569, 10.1016/j.icarus.2005.04.014, 2005Icar..177..550K, Icarus, JOURNAL, Showman, Adam P., Malhotra, Renu, The Galilean Satellites, 1999, Science, 286, 77–84, 10.1126/science.286.5437.77,weblink PDF, 10506564, 5437, The EJSM/Laplace mission is planned to determine the habitability of these environments.

Europa

(File:PIA01130 Interior of Europa.jpg|thumb|Internal structure of Europa. The blue is a subsurface ocean. Such subsurface oceans could possibly harbor life.WEB,weblink Possibility of Life on Europa, University of Victoria, Eric, Hsiao, 2004, )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 water and could be capable of supporting nutrients and energy to microorganisms.Europa may be home to alien life. Melissa Hogenboom, BBC News. 26 March 2015. It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.WEB, Europa Capable of Supporting Life, Scientist Says, Nancy, Atkinson, 2009, Universe Today,weblink 18 August 2011, 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.WEB,weblink Huge lakes of water may exist under Europa's ice, Bad Astronomy Blog, Discover (magazine), Discover, Phil, Plait, 17 November 2011, WEB,weblink Scientists Find Evidence for "Great Lake" on Europa and Potential New Habitat for Life, The University of Texas at Austin, 16 November 2011, 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.WEB, Cook, Jia-Rui C., Clay-Like Minerals Found on Icy Crust of Europa,weblink 11 December 2013, NASA, 11 December 2013, The presence of the minerals may have been the result of a collision with an asteroid or comet according to the scientists. The Europa Clipper, which would assess the habitability of Europa, is planned for launch in 2025.NEWS, Wall, Mike,weblink NASA hopes to launch ambitious mission to icy Jupiter moon, Space.com, 5 March 2014, 15 April 2014, NEWS, Clark, Stephen,weblink Economics, water plumes to drive Europa mission study, Spaceflight Now, 14 March 2014, 15 April 2014, Europa's subsurface ocean is considered the best target for the discovery of life.

Saturn system

Like Jupiter, Saturn isn't likely to have life. However, Titan and Enceladus have been speculated to have possible habitats supportive of life.

Enceladus

Enceladus, a moon of Saturn, has some of the conditions for life, including geothermal activity and water vapor, as well as possible under-ice oceans heated by tidal effects.JOURNAL, 1, TandEM: Titan and Enceladus mission, Coustenis, A., Atreya, S. K., Balint, T., Brown, R. H., Dougherty, M. K., Ferri, F., Fulchignoni, M., Gautier, D., Gowen, R. A., Experimental Astronomy, 23, 3, 893–946, March 2009, 10.1007/s10686-008-9103-z, 2009ExA....23..893C, JOURNAL, Lovett, Richard A., Enceladus named sweetest spot for alien life,weblink 31 May 2011, 10.1038/news.2011.337, 3 June 2011, Nature, 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.

Titan

Titan, the largest moon of Saturn, is the only known moon in the Solar System with a significant atmosphere. Data from the Cassini–Huygens mission refuted the hypothesis of a global hydrocarbon ocean, but later demonstrated the existence of liquid hydrocarbon lakes in the polar regions—the first stable bodies of surface liquid discovered outside Earth.NEWS,weblink Scientists Reconsider Habitability of Saturn's Moon, Space.com, Ker, Than, 13 September 2005, NEWS,weblink Lakes Found on Saturn's Moon Titan, Space.com, Robert Roy, Britt, 28 July 2006, WEB,weblink Lakes on Titan, Full-Res: PIA08630, 24 July 2006, yes,weblink" title="web.archive.org/web/20060929000841weblink">weblink 29 September 2006, dmy-all, 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.WEB, What is Consuming Hydrogen and Acetylene on Titan?, NASA/JPL, 2010, 6 June 2010,weblink yes,weblink" title="web.archive.org/web/20110629185640weblink">weblink 29 June 2011, JOURNAL, Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions, Darrell F., Strobel, Icarus, 208, 878–886, 2010, 10.1016/j.icarus.2010.03.003, 2010Icar..208..878S, 2, JOURNAL, Icarus, 178, 1, 274–276, 2005, 10.1016/j.icarus.2005.05.018, Possibilities for methanogenic life in liquid methane on the surface of Titan, C. P., McKay, H. D., Smith, 2005Icar..178..274M,

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.BOOK, Evolution from Space (The Omni Lecture) and Other Papers on the Origin of Life, Enslow, Fred, Hoyle, Fred Hoyle, 1982, 27–28, 978-0-89490-083-9, BOOK, Evolution from Space: A Theory of Cosmic Creationism, Simon & Schuster, Fred, Hoyle, Chandra, Wickramasinghe, 1984, 978-0-671-49263-2, BOOK, Hoyle, Fred, Living Comets, 1985, University College, Cardiff Press, Cardiff, NEWS, Wickramasinghe, Chandra, Viva Panspermia, The Observatory, June 2011, JOURNAL, Wesson, P, Panspermia, Past and Present: Astrophysical and Biophysical Conditions for the Dissemination of Life in Space, Sp. Sci.Rev., 2010, 156, 1–4, 1–4, 239–252, 10.1007/s11214-010-9671-x, 1011.0101, 2010SSRv..156..239W,

Other bodies

Models of heat retention and heating via radioactive decay in smaller icy Solar System bodies suggest that Rhea, Titania, Oberon, Triton, Pluto, Eris, Sedna, and Orcus may have oceans underneath solid icy crusts approximately 100 km thick.JOURNAL, 10.1016/j.icarus.2006.06.005, 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 (journal), Icarus, 185, 1, 258–273,weblink 2006Icar..185..258H, {{sfnRef, Hussmann Sohl et al., 2006, }} Of particular interest in these cases is the fact that the models indicate that the liquid layers are in direct contact with the rocky core, which allows efficient mixing of minerals and salts into the water. This is in contrast with the oceans that may be inside larger icy satellites like Ganymede, Callisto, or Titan, where layers of high-pressure phases of ice are thought to underlie the liquid water layer.Hydrogen sulfide has been proposed as a hypothetical solvent for life and is quite plentiful on Jupiter's moon Io, and may be in liquid form a short distance below the surface.WEB,weblink The Chance for Life on Io, 25 May 2013, Choi, Charles Q., 2010-06-10,

Scientific search

The scientific search for extraterrestrial life is being carried out both directly and indirectly. {{As of|2017|09}}, 3,667 exoplanets in 2,747 systems have been identified, and other planets and moons in our own solar system hold the potential for hosting primitive life such as microorganisms.

Direct search

File:NASA-WhatBiosignaturesDoesLifeProduce-20180625.jpg|thumb|upright|Lifeforms produce a variety of biosignatures that may be detectable by telescopes.WEB,weblink NASA Asks: Will We Know Life When We See It?, NASA, Cofield, Calla, Chou, Felicia, 25 June 2018, 26 June 2018, NEWS,weblink UCR Team Among Scientists Developing Guidebook for Finding Life Beyond Earth, UCR Today, University of California, RiversideUniversity of California, RiversideScientists 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.WEB, Experts: Little Evidence of Life on Mars,weblink Crenson, Matt, Associated Press, 6 August 2006, 8 March 2011,weblink" title="web.archive.org/web/20110416094930weblink">weblink 16 April 2011, yes, JOURNAL, Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001, Science, David S., McKay, Everett K., Jr., Gibson, Kathie L., Thomas-Keprta, Hojatollah, Vali, Christopher S., Romanek, Simon J., Clemett, Xavier D. F., Chillier, Claude R., Maechling, Richard N., Zare, 5, 273, 5277, 924–930, August 1996, 10.1126/science.273.5277.924, 1996Sci...273..924M, 8688069, WEB, Webster, Guy, NASA Scientists Find Evidence of Water in Meteorite, Reviving Debate Over Life on Mars,weblink 27 February 2014, NASA, 27 February 2014, WEB, Gannon, Megan, Mars Meteorite with Odd 'Tunnels' & 'Spheres' Revives Debate Over Ancient Martian Life,weblink 28 February 2014, Space.com, 28 February 2014, 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.BOOK, Paul, Chambers, Life on Mars; The Complete Story, London, Blandford, 1999, 978-0-7137-2747-0, Lack of corroborating evidence from other experiments on the same samples suggests that a non-biological reaction is a more likely hypothesis.JOURNAL, The Viking Biological Investigation: Preliminary Results, Science, 1 October 1976, Harold P., Klein, 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., 194, 4260, 99–105, 10.1126/science.194.4260.99,weblink 17793090, 1976Sci...194...99K, 15 August 2008, JOURNAL, A Concept for NASA's Mars 2016 Astrobiology Field Laboratory, Astrobiology, August 2007, Beegle, Luther W., Wilson, Michael G., 7, 4, 17723090, 545–577, 10.1089/ast.2007.0153, 2007AsBio...7..545B, Abilleira, Fernando, Jordan, James F., Wilson, Gregory R., WEB,weblink ExoMars rover, ESA, 14 April 2014, In 1996, a controversial report stated that structures resembling nanobacteria were discovered in a meteorite, ALH84001, formed of rock ejected from Mars.File:ALH84001 structures.jpg|thumb|left|Electron micrograph of Martian meteorite ALH84001ALH84001In February 2005, NASA scientists reported that they may have found some evidence of present life on Mars.NEWS,weblink Exclusive: NASA Researchers Claim Evidence of Present Life on Mars, Berger, Brian, 2005, 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.NEWS,weblink NASA denies Mars life reports, spacetoday.net, 2005, Though such methane findings are still debated, support among some scientists for the existence of life on Mars exists.NEWS, Spotts, Peter N., Sea boosts hope of finding signs of life on Mars, The Christian Science Monitor, 28 February 2005,weblink 18 December 2006, 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.WEB, Chow, Dennis, NASA's Next Mars Rover to Land at Huge Gale Crater,weblink 22 July 2011, Space.com, 22 July 2011, NEWS, Amos, Jonathan, Mars rover aims for deep crater, 22 July 2011,weblink BBC News, 22 July 2011, The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets. To that effect, the Carl Sagan Institute was founded in 2014 and is dedicated to the atmospheric characterization of exoplanets in circumstellar habitable zones.WEB,weblink Introducing: The Carl Sagan Institute, Glaser, Linda, 27 January 2015, 11 May 2015, yes,weblink" title="web.archive.org/web/20150227012302weblink">weblink 27 February 2015, WEB,weblink Carl Sagan Institute – Research, May 2015, 11 May 2015, Planetary spectroscopic data will be obtained from telescopes like WFIRST and ELT.NEWS, Cofield, Calla,weblink Catalog of Earth Microbes Could Help Find Alien Life, Space.com, 30 March 2015, 11 May 2015, 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.JOURNAL, 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,weblink 10.1073/pnas.1106493108, 15 August 2011, 108, 34, Proceedings of the National Academy of Sciences, 13995–13998, 2011PNAS..10813995C, 21836052, 3161613, WEB, Steigerwald, John, NASA Researchers: DNA Building Blocks Can Be Made in Space,weblink NASA, 8 August 2011, 10 August 2011, WEB, DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests,weblink 9 August 2011, ScienceDaily, 9 August 2011, 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.WEB, Chow, Denise, Discovery: Cosmic Dust Contains Organic Matter from Stars,weblink 26 October 2011, Space.com, 26 October 2011, WEB, Astronomers Discover Complex Organic Matter Exists Throughout the Universe,weblink 26 October 2011, ScienceDaily, 27 October 2011, JOURNAL, Kwok, Sun, Zhang, Yong, Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features, 26 October 2011, Nature (journal), Nature, 10.1038/nature10542, 479, 7371, 80–3, 2011Natur.479...80K, 22031328, 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.JOURNAL, Sugar Found In Space, National Geographic, Than, Ker, 29 August 2012,weblink 31 August 2012, NEWS, Sweet! Astronomers spot sugar molecule near star,weblink 29 August 2012, Associated Press, 31 August 2012, 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.JOURNAL,weblink Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA, The Astrophysical Journal Letters, Jes K., Jørgensen, Cécile, Favre, Suzanne E., Bisschop, Tyler L., Bourke, Ewine F., van Dishoeck, Markus, Schmalzl, 757, 1, L4, September 2012, 10.1088/2041-8205/757/1/L4, 2012ApJ...757L...4J, 1208.5498,

Indirect search

Projects such as SETI are monitoring the galaxy for electromagnetic interstellar communications from civilizations on other worlds.WEB, SETI Requires a Skeptical Reappraisal, Schenkel, Peter, Skeptical Inquirer, May 2006, 28 June 2009,weblink WEB, Why SETI is science and UFOlogy is not, Moldwin, Mark, Skeptical Inquirer, November 2004,weblink yes,weblink" title="web.archive.org/web/20090313001822weblink">weblink 13 March 2009, 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.WEB,weblink The Columbus Optical SETI Observatory, The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum, The presence of heavy elements in a star's light-spectrum is another potential biosignature; such elements would (in theory) be found if the star was being used as an incinerator/repository for nuclear waste products.JOURNAL, Whitmire, Daniel P., Wright, David P., Nuclear waste spectrum as evidence of technological extraterrestrial civilizations, Icarus, April 1980, 42, 1, 149–156, 10.1016/0019-1035(80)90253-5, 1980Icar...42..149W,

Extrasolar planets

{{See also|List of planetary systems}}File:Glieseupdated.jpg|thumb|Artist's Impression of Gliese 581 c, the first terrestrial extrasolar planet discovered within its star's habitable zone.]]File:NASA-KeplerSpaceTelescope-ArtistConcept-20141027.jpg|thumb|Artist's impression of the Kepler telescope in space.]]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.WEB,weblink 25 January 2006, Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet, IAP.fr, NEWS,weblink Major Discovery: New Planet Could Harbor Water and Life, Space.com, Ker, Than, 24 April 2007, Since 1992 over two thousand exoplanets have been discovered ({{Extrasolar planet counts|planet_count}} planets in {{Extrasolar planet counts|system_count}} planetary systems including {{Extrasolar planet counts|multiplanetsystem_count}} multiple planetary systems as of {{Extrasolar planet counts|asof}}).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.WEB, Interactive Extra-solar Planets Catalog, The Extrasolar Planets Encyclopaedia,weblink Schneider, Jean, 10 September 2011, 30 January 2012, The number of observed exoplanets is expected to increase greatly in the coming years.NEWS,weblink NASA Extends Planet-Hunting Kepler Mission Through 2016, Space.com, Mike, Wall, 4 April 2012, The Kepler space telescope has also detected a few thousandWEB, NASA – Kepler,weblink 4 November 2013, yes,weblink" title="web.archive.org/web/20131105082102weblink">weblink 5 November 2013, WEB, Harrington, J. D., Johnson, M., 4 November 2013, NASA Kepler Results Usher in a New Era of Astronomy,weblink candidate planets,JOURNAL, 10.1088/0067-0049/206/1/5, 1212.2915, Detection of Potential Transit Signals in the First 12 Quarters of Kepler Mission Data, The Astrophysical Journal Supplement Series, 206, 1, 5, 2013, 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., 2013ApJS..206....5T, PRESS_RELEASE,weblink My God, it's full of planets! They should have sent a poet., Planetary Habitability Laboratory, University of Puerto Rico at Arecibo, 3 January 2012, of which about 11% may be false positives.ARXIV, 1310.2133, Santerne, A., Díaz, R. F., Almenara, J.-M., Lethuillier, A., Deleuil, M., Moutou, C., Astrophysical false positives in exoplanet transit surveys: Why do we need bright stars?, astro-ph.EP, 2013, There is at least one planet on average per star.JOURNAL, 1, Cassan, A., Kubas, D., Beaulieu, J. -P., Dominik, M., Horne, K., Greenhill, J., Wambsganss, J., Menzies, J., Williams, A., Jørgensen, 10.1038/nature10684, U. G., Udalski, A., Bennett, D. P., Albrow, M. D., Batista, V., Brillant, S., Caldwell, J. A. R., Cole, A., Coutures, C., Cook, K. H., Dieters, S., Prester, D. D., Donatowicz, J., Fouqué, P., Hill, K., Kains, N., Kane, S., Marquette, J. -B., Martin, R., Pollard, K. R., Sahu, K. C., One or more bound planets per Milky Way star from microlensing observations, Nature, 481, 7380, 167–169, 11 January 2012, 22237108, 2012Natur.481..167C, 1202.0903, About 1 in 5 Sun-like starsFor 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 have an "Earth-sized"For the purpose of this 1 in 5 statistic, Earth-sized means 1–2 Earth radii planet in the habitable zone,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). with the nearest expected to be within 12 light-years distance from Earth.WEB, Sanders, R., 4 November 2013, Astronomers answer key question: How common are habitable planets?,weblink newscenter.berkeley.edu, JOURNAL, 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, 1311.6806, 2013PNAS..11019273P, 10.1073/pnas.1319909110, 24191033, 3845182, Assuming 200 billion stars in the Milky Way,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. 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.JOURNAL, Strigari, L. E., Barnabè, M., Marshall, P. J., Blandford, R. D., Nomads of the Galaxy, 2012, 423, 2, 1856–1865, Monthly Notices of the Royal Astronomical Society, 1201.2687, 2012MNRAS.423.1856S, 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.The nearest known exoplanet is Proxima Centauri b, located {{convert|4.2|ly|pc|lk=on}} from Earth in the southern constellation of Centaurus.NEWS,weblink One Star Over, a Planet That Might Be Another Earth, The New York Times, Kenneth, Chang, 24 August 2016, 4 September 2016, {{asof|March 2014}}, 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,WEB, DENIS-P J082303.1-491201 b,weblink Caltech, 8 March 2014, JOURNAL, Sahlmann, J., Lazorenko, P. F., Ségransan, D., Martín, E. L., Queloz, D., Mayor, M., Udry, S., Astrometric orbit of a low-mass companion to an ultracool dwarf, 556, 133, 2013A&A...556A.133S, August 2013, 1306.3225, Astronomy & Astrophysics, 10.1051/0004-6361/201321871, 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.WEB,weblink Future Evidence for Extraterrestrial Life Might Come from Dying Stars, Harvard-Smithsonian Center for Astrophysics, David A., Aguilar, Christine, Pulliam, 25 February 2013, 9 June 2017, Release 2013-06,

Terrestrial analysis

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.NEWS, Borenstein, Seth, Hints of life on what was thought to be desolate early Earth,weblink 19 October 2015, Excite, Yonkers, NY, Mindspark Interactive Network, Associated Press, yes,weblink" title="web.archive.org/web/20151023200248weblink">weblink 23 October 2015, 8 October 2018, JOURNAL, Bell, Elizabeth A., Boehnike, Patrick, Harrison, T. Mark, Mao, Wendy L., 3, 19 October 2015, Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon,weblink PDF, Proc. Natl. Acad. Sci. U.S.A., 10.1073/pnas.1517557112, 1091-6490, 20 October 2015, 26483481, 4664351, 112, 47, 14518–21, 2015PNAS..11214518B, Early edition, published online before print. According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."

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).WEB,weblink Chapter 3 — Philosophy: "Solving the Drake Equation, 24 July 2015, December 2002, SETI League, 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.JOURNAL, W(h)ither the Drake equation?, Burchell, M. J., International Journal of Astrobiology, 5, 3, 243–250, 2006,weblink 10.1017/S1473550406003107, 2006IJAsB...5..243B, The Drake equation is:
N = R_{ast} cdot f_p cdot n_e cdot f_{ell} cdot f_i cdot f_c cdot L
where:
N = the number of Milky Way galaxy civilizations already capable of communicating across interplanetary space
and
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:10{,}000 = 5 cdot 0.5 cdot 2 cdot 1 cdot 0.2 cdot 1 cdot 10{,}000WEB, Aguirre, L., 1 July 2008, The Drake Equation,weblink Nova ScienceNow, PBS, 7 March 2010, The Drake equation has proved controversial since several of its factors are uncertain and based on conjecture, not allowing conclusions to be made.BOOK, Chapter 6: What does a Martian look like?, Evolving the Alien: The Science of Extraterrestrial Life, John Wiley and Sons, Hoboken, NJ, Jack, Cohen, Jack Cohen (scientist), Ian, Stewart, Ian Stewart (mathematician), 2002, 978-0-09-187927-3, Evolving the Alien: The Science of Extraterrestrial Life, 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.WEB,weblink How many galaxies are there in the universe?, Temming, M., Sky & Telescope, 18 July 2014, 17 December 2015, It is estimated that at least ten percent of all Sun-like stars have a system of planets,JOURNAL, G., Marcy, R., Butler, D., Fischer, etal, Observed Properties of Exoplanets: Masses, Orbits and Metallicities, Progress of Theoretical Physics Supplement, 2005, 158, 24–42,weblink 10.1143/PTPS.158.24, astro-ph/0505003, 2005PThPS.158...24M, yes,weblink" title="web.archive.org/web/20081002085400weblink">weblink 2 October 2008, i.e. there are {{val|6.25|e=18}} stars with planets orbiting them in the observable universe. Even if it is assumed that only one out of a billion of these stars has planets supporting life, there would be some 6.25 billion life-supporting planetary systems in the observable universe.A 2013 study based on results from the Kepler spacecraft estimated that the Milky Way contains at least as many planets as it does stars, resulting in 100–400 billion exoplanets.JOURNAL, Characterizing the Cool KOIs. IV. Kepler-32 as a Prototype for the Formation of Compact Planetary Systems throughout the Galaxy, The Astrophysical Journal, Jonathan J., Swift, John Asher, Johnson, Timothy D., Morton, Justin R., Crepp, Benjamin T., Montet, Daniel C., Fabrycky, Philip S., Muirhead, 5, 764, 1, 105, January 2013, 10.1088/0004-637X/764/1/105, 2013ApJ...764..105S, 1301.0023, NEWS,weblink 100 Billion Alien Planets Fill Our Milky Way Galaxy: Study, Space.com, 2 January 2013, 10 March 2016, yes,weblink" title="web.archive.org/web/20130103060601weblink">weblink 3 January 2013, Also based on Kepler data, scientists estimate that at least one in six stars has an Earth-sized planet.EPISODE,weblink Alien Planets Revealed, Nova (TV series), Nova, 41, 10, 50:56, 8 January 2014, 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.NEWS, Overbye, Dennis, Dennis Overbye, The Flip Side of Optimism About Life on Other Planets,weblink 3 August 2015, The New York Times, 29 October 2015,

Cultural impact

Cosmic pluralism

File:Shri Simandhar Swami.jpg|thumb|200px|The statue of Simandhara, an enlightened man in JainJain 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,WEB,weblink Who discovered that the Sun was a Star?, Stanford Solar Center, the notion of pluralism was largely mythological and philosophical.BOOK,weblink The Extraterrestrial Life Debate, 1750–1900, Michael J., Crowe, Courier Dover Publications, 1999, 978-0-486-40675-6, WEB, Benjamin D., Wiker, 4 November 2002, Alien Ideas: Christianity and the Search for Extraterrestrial Life,weblink Crisis Magazine,weblink" title="web.archive.org/web/20030210140752weblink">weblink 10 February 2003, BOOK, The Arabian Nights: A Companion, Robert, Irwin, I.B. Tauris, Tauris Parke Paperbacks, 2003, 978-1-86064-983-7, 204 & 209, Medieval Muslim writers like Fakhr al-Din al-Razi and Muhammad al-Baqir supported cosmic pluralism on the basis of the Qur'an.David A. Weintraub (2014). "Islam," Religions and Extraterrestrial Life (pp 161-168). Springer International Publishing.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.BOOK, Conversations on the Plurality of Worlds, University of California Press, Bernard le Bovier, de Fontenelle, trans. H. A. Hargreaves, 1990, 978-0-520-91058-4, Pluralism was also championed by philosophers such as John Locke, Giordano Bruno 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.WEB,weblink Flammarion, (Nicolas) Camille (1842–1925), The Internet Encyclopedia of Science, 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".WEB,weblink Giordano Bruno: On the Infinite Universe and Worlds (De l'Infinito Universo et Mondi) Introductory Epistle: Argument of the Third Dialogue, 4 October 2014, yes,weblink" title="web.archive.org/web/20141013120648weblink">weblink 13 October 2014, 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".WEB,weblink cosmovisions.com, Rheita.htm, 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 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.

19th century

(File:Lowell Mars channels.jpg|thumb|Artificial Martian channels, depicted by Percival Lowell)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.JOURNAL, Experiments as to the actuality of the "Canals" observed on Mars, Monthly Notices of the Royal Astronomical Society, J. E., Evans, E. W., Maunder, Edward Walter Maunder, 63, 8, 488–499, June 1903, 10.1093/mnras/63.8.488, 1903MNRAS..63..488E, 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.BOOK,weblink Is Mars Habitable? A Critical Examination of Professor Lowell's Book "Mars and Its Canals," With an Alternative Explanation, Macmillan, London, Alfred Russel, Wallace, 1907, 8257449, 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.BOOK, Paul, Chambers, Life on Mars; The Complete Story, London, Blandford, 1999, 978-0-7137-2747-0, 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, developed 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 toThe 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.

20th century

{{see also|Space exploration}}File:Arecibo message.svg|thumb|100px|The Arecibo message is a digital message sent to Messier 13Messier 13Most unidentified flying objects or UFO sightingsJOURNAL, Anne, Cross, The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers, Qualitative Sociology, 27, 1, 3–34, 10.1023/B:QUAS.0000015542.28438.41, 2004, can be readily explained as sightings of Earth-based aircraft, known astronomical objects, or as hoaxes.JOURNAL, Philippe, Ailleris, January–February 2011, The lure of local SETI: Fifty years of field experiments, Acta Astronautica, 68, 1–2, 2–15, 10.1016/j.actaastro.2009.12.011, 2011AcAau..68....2A, 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 open.

Recent history

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".WEB,weblink LECTURE 4: MODERN THOUGHTS ON EXTRATERRESTRIAL LIFE, The University of Antarctica, 25 July 2015, 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.BOOK, Rare Earth: Why Complex Life is Uncommon in the Universe, Copernicus, Peter, Ward, Donald, Brownlee, 2000, 2000rewc.book.....W, 978-0-387-98701-9, 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.NEWS,weblink BBC News, Hawking warns over alien beings, 25 April 2010, 2 May 2010, Jared Diamond had earlier expressed similar concerns.BOOK, Chapter 12, The Third Chimpanzee: The Evolution and Future of the Human Animal, Harper Perennial, Jared M., Diamond, 2006, 978-0-06-084550-6, 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."WEB, Larson, Phil, Searching for ET, But No Evidence Yet,weblink 5 November 2011, White House, 6 November 2011, yes,weblink 24 November 2011, WEB, Atkinson, Nancy, No Alien Visits or UFO Coverups, White House Says,weblink 5 November 2011, UniverseToday, 6 November 2011, 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.JOURNAL, Special Issue: Exoplanets,weblink Science (journal), Science, 3 May 2013, 18 May 2013, On 17 April 2014, the discovery of the Earth-size exoplanet Kepler-186f, 500 light-years from Earth, was publicly announced;NEWS, Chang, Kenneth, Scientists Find an 'Earth Twin', or Maybe a Cousin,weblink 17 April 2014, The New York Times, 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;NEWS,weblink Should We Call the Cosmos Seeking ET? Or Is That Risky?, The New York Times, Associated Press, Seth, Borenstein, 13 February 2015,weblink" title="web.archive.org/web/20150214152940weblink">weblink 14 February 2015, yes, NEWS, Ghosh, Pallab, Scientist: 'Try to contact aliens',weblink 12 February 2015, BBC News, 12 February 2015, one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".WEB,weblink Regarding Messaging To Extraterrestrial Intelligence (METI) / Active Searches For Extraterrestrial Intelligence (Active SETI), University of California, Berkeley, 13 February 2015, 14 February 2015, On 20 July 2015, British physicist Stephen 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.NEWS,weblink Searching for ET: Hawking to look for extraterrestrial life, Excite!, Associated Press, Katz, Gregory, 20 July 2015, 20 July 2015,

See also

{hide}Columns-list|colwidth=30em|
Searches for extraterrestrial life
  • {{annotated link|Allen Telescope Array{edih}
  • {{annotated link|Astrobiology}}
  • {{annotated link|Communication with extraterrestrial intelligence}}
  • {{annotated link|Nexus for Exoplanet System Science}}
  • {{annotated link|Search for extraterrestrial intelligence|abbreviation=SETI}}


Subjects
  • {{annotated link|Extraterrestrial liquid water}}
  • {{annotated link|Planetary protection}}
  • {{annotated link|Potential cultural impact of extraterrestrial contact}}


Hypotheses
  • {{annotated link|Extraterrestrial (TV program)}}
  • {{annotated link|Fermi paradox}}
  • {{annotated link|Hypothetical types of biochemistry}}
  • {{annotated link|Kardashev scale}}
  • {{annotated link|Metalaw}}
  • {{annotated link|Rare Earth hypothesis}}
  • {{annotated link|Zoo hypothesis}}
}}

Notes

{{reflist|group="n"}}{{reflist|group=lower-alpha}}

References

{{reflist|30em}}

Further reading

{{Commons category|Aliens}}{{Wikisource portal|Extraterrestrial life}}
  • BOOK, 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, 978-0-87451-406-3,
  • BOOK, Cohen, Jack, Jack Cohen (scientist), Stewart, Ian, Ian Stewart (mathematician), 2002, Evolving the Alien: The Science of Extraterrestrial Life, Ebury Press, 978-0-09-187927-3, Evolving the Alien,
  • BOOK, Crowe, Michael J., The Extraterrestrial Life Debate, 1750–1900, Cambridge, 1986, 978-0-521-26305-4,
  • BOOK, Crowe, Michael J., The extraterrestrial life debate Antiquity to 1915: A Source Book, University of Notre Dame Press, 2008, 978-0-268-02368-3,
  • BOOK, Dick, Steven J., Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant, Cambridge, 1984,
  • BOOK, Dick, Steven J., The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science, Cambridge, 1996, 978-0-521-34326-8,
  • BOOK, Dick, Steven J., Life on Other Worlds: The 20th Century Extraterrestrial Life Debate, Cambridge, 2001, 978-0-521-79912-6,
  • BOOK, Dick, Steven J., James E., Strick, The Living Universe: NASA And the Development of Astrobiology, Rutgers, 2004, 978-0-8135-3447-3,
  • BOOK, Fasan, Ernst, Relations with alien intelligences â€“ the scientific basis of metalaw, Berlin Verlag, Berlin, 1970,
  • BOOK, Goldsmith, Donald, 1997, The Hunt for Life on Mars, New York, A Dutton Book, 978-0-525-94336-5,
  • BOOK, Grinspoon, David, 2003, Lonely Planets: The Natural Philosophy of Alien Life, HarperCollins, 978-0-06-018540-4,
  • BOOK, Lemnick, Michael T., 1998, Other Worlds: The Search for Life in the Universe, New York, A Touchstone Book,
  • BOOK, Michaud, Michael, Contact with Alien Civilizations â€“ Our Hopes and Fears about Encountering Extraterrestrials., Springer, Berlin, 2006, 978-0-387-28598-6,
  • BOOK, Pickover, Cliff, Cliff Pickover, 2003, The Science of Aliens, New York, Basic Books, 978-0-465-07315-3, The Science of Aliens,
  • BOOK, Roth, Christopher F., 2005, Ufology as Anthropology: Race, Extraterrestrials, and the Occult, E.T. Culture: Anthropology in Outerspaces, Debbora Battaglia, Durham, NC, Duke University Press,
  • BOOK, Sagan, Carl, Carl Sagan, I. S., Shklovskii, I.S. Shklovskii, Intelligent Life in the Universe, Random House, 1966,
  • BOOK, Sagan, Carl, Communication with Extraterrestrial Intelligence, MIT Press, 1973, 978-0-262-19106-7, Communication with Extraterrestrial Intelligence,
  • BOOK, Peter D., Ward, Life as we do not know it-the NASA search for (and synthesis of) alien life, Viking, New York, 2005, 978-0-670-03458-1,
  • BOOK, Tumminia, Diana G., Alien Worlds â€“ Social and Religious Dimensions of Extraterrestrial Contact, Syracuse University Press, Syracuse, 2007, 978-0-8156-0858-5,
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