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| ] 16 ] stamp, with a satellite from an imagined extraterrestrial civilization.]] | |||
| i like poo | |||
| '''Extraterrestrial life''' is ] originating outside of the ]. It is the subject of ], and its existence remains hypothetical. There is no credible evidence of extraterrestrial life that has been widely accepted by the ]. | |||
| There are several hypotheses regarding the origin of extraterrestrial life. One proposes that its emergence occurred independently, in different places in the universe. An alternative hypothesis is ], which holds that life emerging in one location then spreads between ]s. These two hypotheses are not ]. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculative forms of extraterrestrial life range from ] beings to life at the scale of ]. | |||
| Suggested locations that might have once developed or continue to host life include the planets ]<ref>{{cite web|url=http://news.bbc.co.uk/2/hi/science/nature/3746583.stm|title=Venus clouds 'might harbour life'|publisher=BBC News|accessdate=2007-12-05|date=2004-05-25}}</ref> and ], natural satellites of ] and ] (e.g. ],<ref name="EuropaPlanetary">http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"</ref> ] and ]). ] and ], recently discovered to be near Earth-mass ] apparently located in their star's ], and having the potential to have liquid water.<ref>{{cite web |url=http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.3758v1.pdf |title=The Habitability of Super-Earths in Gliese 581 |accessdate=2007-12-01 }}</ref> | |||
| ==Possible basis of extraterrestrial life== | |||
| ===Biochemistry=== | |||
| {{Main|Biochemistry|Alternative biochemistry}} | |||
| All ] is made up of the principal elements, ], ], ], ], and ]; it also requires ] as the ] in which biochemical reactions take place. Sufficient quantities of carbon and the other major life-forming elements along with water may enable the formation of living organisms on other planets with a chemical make up and average temperature similar to Earth. Because Earth and other planets are made up of "star dust", relatively abundant ]s formed from stars which have ended their life as ], it is very probable that other planets may have been formed by elements of a similar composition as Earth. The combination of carbon and water in the chemical form of ]s (e.g., ]), can be a source of chemical ] on which life depends, and also provide structural elements for life (such as ], in the molecules ] and ] and ] in plants).]s derive energy through the conversion of light energy into chemical energy via ]. Life requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon oxides) states. It also requires ] as a reduced ] derivative in all ]s, ] as a derivative of ] in some necessary proteins, and ] oxidized to ]s in genetic material and in energy transfer. Adequate water as a solvent supplies adequate oxygen as constituents of biochemical substances. | |||
| Pure water is useful because it has a neutral ], due to its continued dissociation between ] and ] ]. As a result, it can dissolve both positive ] and negative ] with equal ability. Furthermore, the fact that organic molecules can be either ] (repelled by water) or ] (soluble in water) creates the ability of organic compounds to orient themselves to form water-enclosing ]. The fact that solid water (ice) is less dense than liquid water also means that ice floats, thereby preventing Earth's oceans from slowly freezing solid. Additionally, the ]s between water molecules give it an ability to store ] with ], which upon ] is released. This helps moderate climate, cooling the tropics and warming the poles, helping to maintain a thermodynamic stability needed for life. | |||
| Carbon is fundamental to terrestrial life for its immense flexibility in creating ] with a variety of non-metallic elements, principally ], ] and ]. ] and water together enable the storage of solar energy in ], such as ]. The ] of glucose releases biochemical energy needed to fuel all other biochemical reactions. | |||
| The ability to form ]s (–COOH) and ] ] (–NH<sub>2</sub>) gives it the possibility of ] dehydrating reactions to build long ] ] and ] ] from ] ], and with ]s to build not only ], the information storing molecule of inheritance, but also ] (ATP) the principal energy "currency" of cellular life. | |||
| Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would also utilize these basic materials. However, other elements and solvents could also provide a basis for ]. ] is most often deemed to be the probable alternative to carbon. ]forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as on planets which are very close to their star. Life forms based in ] rather than water have also been suggested, though this solution appears less optimal than water.<ref>{{cite web| url=http://www.daviddarling.info/encyclopedia/A/ammonialife.html| publisher=daviddarling.info| title=Ammonia based life}}</ref> | |||
| Indeed, technically life is little more than any self-replicating reaction, which could arise in a great many conditions and with various ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive. Suggestions have even been made that self-replicating reactions of some sort could occur within the ] of a star, though it would be highly unconventional. | |||
| Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned. For example, ] scientists believe that the color of photosynthesizing pigments on ]s could be non-green.<ref></ref> | |||
| ===Evolution and morphology=== | |||
| In addition to the biochemical basis of extraterrestrial life, many have also considered ] and ]. Science fiction has often depicted extraterrestrial life with ] and/or ] forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs—i.e., this depiction is fundamentally humanoid. Other subjects such as ] and ] have also occurred in fictional representations of aliens. | |||
| A division has ] between universal and parochial (narrowly restricted) characteristics. Universals are features which have evolved independently more than once on Earth (and thus presumably are not difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. These include ], ], ] and ], all of which have evolved several times here on Earth. There is a huge variety of ]s, for example, and many of these have radically different working schematics and different visual foci: the ], ], ] and ]. Parochials, however, are essentially arbitrary evolutionary forms. These often have little inherent utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. A classic example of a parochial is the curious and often fatal conjunction of the feeding and breathing passages found within many animals, although it is possible this conjunction allowed for the ] of human ].{{Fact|date=February 2007}} | |||
| Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity. ], which are essential to large terrestrial organisms according to the experts of the field of ], are almost assuredly to be replicated elsewhere in one form or another. Many also conjecture as to some type of egg laying amongst extraterrestrial creatures but mammalian ]s might be a singular case. | |||
| The assumption of radical diversity amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of Earth life foregrounds even greater variety in space, others point out that ] may dictate substantial similarities between Earth and off-Earth life. These two schools of thought are called "divergionism" and "convergionism", respectively.<ref>{{cite web| url=http://www.daviddarling.info/encyclopedia/E/etlifevar.html| publisher=daviddarling.info| title=Variety of extraterrestrial life}}</ref> | |||
| ==Beliefs in extraterrestrial life== | |||
| ===Ancient and early modern ideas=== | |||
| {{seealso|Cosmic pluralism}} | |||
| Belief in extraterrestrial life may have been present in ancient ], ], ], ], ], ] and ], although in these societies, ] was fundamentally ] and the notion of alien life is difficult to distinguish from that of gods, demons, and such. The first important Western thinkers to argue systematically for a universe full of other planets and, therefore, possible extraterrestrial life were the ancient ] writer ] and his student ] in the 7th and 6th centuries B.C. The ] of Greece took up the idea, arguing that an infinite universe ought to have an infinity of populated worlds. Ancient Greek cosmology worked against the idea of extraterrestrial life in one critical respect, however: the ] universe. Championed by ] and codified by ], it favored the Earth and Earth-life (Aristotle denied there could be a plurality of worlds) and seemingly rendered extraterrestrial life philosophically untenable. ] in his novels described inhabitants of the Moon and other celestial bodies as humanoids, but with significant differences from humans. | |||
| ], ''De l'Infinito, Universo e Mondi, 1584'' Photo courtesy of P.C.]] | |||
| Authors of ] sources also considered extraterrestrial life. The ] states that there are at least 18,000 other worlds, but provides little elaboration on the nature of the worlds and on whether they are physical or spiritual. Based on this, however, the 18th century exposition "Sefer HaB'rit" posits that extraterrestrial creatures exist but that they have no ] (and are thus equivalent to animal life). It adds that human beings should not expect creatures from another world to resemble earthly life, any more than sea creatures resemble land animals.<ref>{{cite web| url=http://ohr.edu/ask_db/ask_main.php/318/Q1/| publisher=ohr.edu| title=Star Struck, a letter to a Rabbi}}</ref><ref>{{cite web| url=http://www.torah.org/features/secondlook/extraterrestrial.html| publisher=torah.org| title=Extraterrestrial life| first=Rabbi Aryeh| last=Kaplan}}</ref> | |||
| ] beliefs of endlessly repeated cycles of life have led to descriptions of multiple worlds in existence and their mutual contacts ( ] word Sampark (समपर्क) means 'contact' as in ] (महसमपर्क) = the great contact). According to Hindu scriptures there are innumerable universes created by the Supreme Personality of Godhead to facilitate the fulfillment of the separated desires of innumerable living entities. However, the purpose of such creations is to bring back the deluded souls to correct understanding about the purpose of life. Apart from the innumerable universes which are material, there is also the existence of unlimited spiritual world, where the purified living entities live with perfect conception about life and ultimate reality. The life of these purified beings is centered around loving devotional services to Supreme Personality of Godhead. The spiritually aspiring saints and devotees as well as thoughtful men of material world have been getting guidance and help from these purified living entities of spiritual world from time immemorial. However, the relevance of such descriptions has to be evaluated in the context of a correct understanding of geography and ] at those times. | |||
| Within ], the statement of the ] "All praise belongs to ], Lord of all the worlds" indicates multiple universal bodies and maybe even multiple universes that may indicate extraterrestrial and even extradimensional life. Surat ] also mentioned a statement from a ] regarding the current status and ability of his group in the heavens. A more direct reference from Quran is presented by ] as a proof that life on other planets may exist according to Quran. In his book, ], he quotes verse 42:29 "And among His Signs is the creation of the heavens and the earth, and of whatever living creatures (da'bbah) He has spread forth in both..."; according to this verse there is life in heavens. According to the same verse "And He has the power to gather them together (jam-'i-him) when He will so please"; indicates the bringing together the life on Earth and the life elsewhere in the universe. The verse does not specify the time or the place of this meeting but rather states that this event will most certainly come to pass whenever God so desires. It should be pointed out that the Arabic term Jam-i-him used to express the gathering event can imply either a physical encounter or a contact through communication.<ref>Revelation, Rationality, Knowledge & Truth, by Mirza Tahir Ahmad. Chapter; The Quran and Extraterrestrial Life</ref> | |||
| When ] spread throughout the West, the Ptolemaic system became very widely accepted, and although the Church never issued any formal pronouncement on the question of alien life<ref>{{cite web| url=http://www.crisismagazine.com/november2002/feature7.htm| publisher=crisismagazine.com| title=Christianity and the Search for Extraterrestrial Life| first=Benjamin D.| last=Wiker}}</ref> at least tacitly the idea was aberrant. In 1277 the ], ], did overturn Aristotle on one point: God ''could'' have created more than one world (given His omnipotence) yet we know by revelation He only made one. Taking a further step and arguing that aliens actually existed remained rare. Notably, Cardinal ] speculated about aliens on the ] and ]. | |||
| There was a dramatic shift in thinking initiated by the invention of the ] and the ] assault on geocentric cosmology. Once it became clear that the Earth was merely one planet amongst countless bodies in the universe the extraterrestrial idea moved towards the scientific mainstream. God's omnipotence, it could be argued, not only allowed for other worlds and other life, on some level it necessitated them. The best known early-modern proponent of such ideas was ], who argued in the 16th century for an infinite universe in which every star is surrounded by its own solar system; he was eventually burned at the stake by the ] for his heretical ideas. The Catholic church under John Paul II apologized for this. In the early 17th century the Czech astronomer ] mused that "if Jupiter has…inhabitants…they must be larger and more beautiful than the inhabitants of the Earth, in proportion to the of the two spheres."<ref>{{cite web| url=http://cosmovisions.com/Rheita.htm| publisher=cosmovisions.com| title=Rheita.htm}}</ref> Dominican monk ] wrote about a Solarian alien race in his '']''. | |||
| Such comparisons also appeared in poetry of the era. In "The Creation: a Philosophical Poem in Seven Books" (1712) Sir ] observed: "We may pronounce each orb sustains a race / Of living things adapted to the place". The didactic poet ] took up the classical theme of the Greek ] in "Democritus Platonissans, or an Essay Upon the Infinity of Worlds" (1647). With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere." ]'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. ], the discoverer of ], was one of many 18th-19th century astronomers convinced that our Solar System, and perhaps others, would be well populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included ] and ]. At the height of the ] even the ] and ] were considered candidates for extraterrestrial inhabitants. | |||
| ===Extraterrestrials and the modern era=== | |||
| ] is a digital message sent to ], and is a well-known symbol of human attempts to contact extraterrestrials.]] | |||
| This enthusiasm toward the possibility of alien life continued well into the 20th century. Indeed, the roughly three centuries from the ] through the beginning of the modern era of solar system probes were essentially the zenith for belief in extraterrestrials in the West. Many astronomers and other secular thinkers, at least some religious thinkers, and much of the general public were largely satisfied that aliens were a reality. This trend was finally tempered as actual probes visited potential alien abodes in the solar system. The moon was decisively ruled out as a possibility while ] and ], long the two main candidates for extraterrestrials, showed no obvious evidence of current life. The other large moons of our system which have been visited appear similarly lifeless, though the interesting geothermic forces observed (]'s volcanism, ]'s ocean, ]'s thick atmosphere) have underscored how broad the range of potentially habitable environments may be. Although the hypothesis of a deliberate cosmic silence of advanced extraterrestrials is also a possibility,<ref></ref> the failure of the ] program to detect anything resembling an intelligent radio signal after four decades of effort has partially dimmed the optimism that prevailed at the beginning of the space age. Emboldened critics view the search for extraterrestrials as unscientific, despite the fact the ] program is not the result of a continuous, dedicated search but instead utilizes what resources and manpower it can, when it can.<ref>{{cite web| url=http://www.crichton-official.com/speeches/speeches_quote04.html| publisher=crichton-official.com| title=Aliens Cause Global Warming| first=Michael| last=Crichton| date=], ]}}</ref> | |||
| Thus, the three decades preceding the turn of the second millennium saw a crossroads reached in beliefs in alien life. The prospect of ubiquitous, intelligent, space-faring civilizations in our solar system appears increasingly dubious to many scientists. Still, in the words of SETI's ], "All we know for sure is that the sky is not littered with powerful microwave transmitters."<ref></ref> Drake has also noted that it is entirely possible 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 ] on other worlds and to confirm that, at least, ] are plentiful though aliens remain a question mark. | |||
| In 2000, ] and ] ] and ] Donald Brownlee published a book entitled ''Rare Earth: Why Complex Life is Uncommon in the Universe''.<ref></ref> In it, they discussed the ], in which they claim that ]-like life is rare in the ], while ] life is common in the universe. | |||
| The possible existence of primitive (microbial) life outside of Earth is much less controversial to mainstream scientists although at present no direct evidence of such life has been found. Indirect evidence has been offered for the current existence of primitive life on the planet Mars. However, the conclusions that should be drawn from such evidence remain in debate. | |||
| ==Scientific search for extraterrestrial life== | |||
| The scientific search for extraterrestrial life is being carried out in two different ways, directly and indirectly. | |||
| ===Direct search=== | |||
| ] ] for the search of ].]] | |||
| Scientists are directly searching for evidence of unicellular life within the ], carrying out studies on the surface of Mars and examining ] that have fallen to Earth. A mission is also proposed to ], one of ]'s moons with a possible liquid water layer under its surface, which might contain life. | |||
| There is some limited evidence that microbial life might possibly exist or have existed on Mars.<ref></ref> An experiment on the ] Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Recently, ] have been allegedly discovered in Viking data. The interpretation is controversial. Independently in 1996 structures resembling ] were reportedly discovered in a meteorite, ], thought to be formed of ]. This report is also controversial and scientific debate continues (See ]). | |||
| In February 2005, ] scientists reported that they had found strong evidence of present life on ].<ref>{{cite news| url=http://www.space.com/scienceastronomy/mars_life_050216.html| title= Exclusive: NASA Researchers Claim Evidence of Present Life on Mars| last= Berger| first= Brian| date=2005}}</ref> The two scientists, ] and ] of NASA's ], based their claims on methane signatures found in Mars' atmosphere that resemble the methane production of some forms of primitive life on Earth, as well as their own study of primitive life near the ] in ]. NASA officials soon denied the scientists' claims, and Stoker herself backed off from her initial assertions.<ref>{{cite news| url=http://www.spacetoday.net/Summary/2804| title= NASA denies Mars life reports| publisher=spacetoday.net| date=2005}}</ref> | |||
| Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey conducted at the conference in which the European Space Agency presented its findings, 75 percent of the scientists in attendance reported to believe that life once existed on Mars; 25 percent reported a belief that life currently exists there.<ref>{{cite news| last = Spotts|first = Peter N.| title = Sea boosts hope of finding signs of life on Mars| publisher = The Christian Science Monitor| date = ]| url = http://www.csmonitor.com/2005/0228/p02s02-usgn.html| accessdate = 2006-12-18 }}</ref> | |||
| The ] stipulates that any planet with a robust population of life will have an atmosphere that is not in chemical equilibrium, which is relatively easy to determine from a distance by ]. However, significant advances in the ability to find and resolve light from smaller rocky worlds near to their star are necessary before this can be used to analyze ]s | |||
| ===Indirect search=== | |||
| ] - A planned Infrared interferometer for finding Earth-like ] (as of 2007 , it has not received the funding from NASA it needs — that funding is going towards the ]).]] | |||
| It is theorised that any technological society in space will be transmitting information. Projects such as ] are conducting an astronomical search for radio activity that would confirm the presence of intelligent life. A related suggestion is that aliens might broadcast pulsed and continuous ] signals in the optical as well as infrared spectrum;<ref>{{cite web| url=http://www.coseti.org/| publisher=The Columbus Optical SETI Observatory| title=The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum}}</ref> laser signals have the advantage of not "smearing" in the interstellar medium and may prove more conducive to communication between the stars. And while other communication techniques including laser transmission and interstellar spaceflight have been discussed seriously and may not be infeasible, the measure of effectiveness is the amount of information communicated per unit cost, resulting with the radio as method of choice. | |||
| ===Extrasolar planets=== | |||
| Astronomers also search for ]s that they believe would be conducive to life, such as ] and ], which have been found to have Earth-like qualities.<ref name="Earth-likeplanet1">{{cite journal | first = | last = |title = http://planet.iap.fr/OB05390.news.html | year = }}</ref><ref name="GlieseSpace"></ref> Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through ] which would reveal key information such as the presence of free ] in a planet's atmosphere): | |||
| ], the first ] discovered within its star's ].]] | |||
| *] is an ESA mission designed to find Earth-like planets, and analyse their atmosphere. | |||
| *The ] mission, initiated by the ], was launched in 2006 and is currently looking for extrasolar planets -- it is the first of its kind | |||
| *The ] was supposed to be launched by ], but as of 2007 , budget cuts have caused it to be delayed indefinitely | |||
| *The ], largely replacing the Terrestrial Planet Finder, to be launched in November 2008 | |||
| It has been argued that ], the closest star system to Earth, may contain planets which could be capable of sustaining life.<ref></ref> | |||
| On ], ], scientists at the European Southern Observatory in La Silla, Chile said they had found the first Earth-like planet. The planet, known as ], orbits within the ] of its star ], a ] star which is a scant 20.5 ] (194 trillion km) from Earth. It was initially thought that this planet could contain liquid water. However, recent computer simulations of the climate on Gliese 581c by Werner Von Bloh and his team at Germany's Institute for Climate Impact Research suggest carbon dioxide and methane in the atmosphere would create a runaway greenhouse effect. This would warm the planet well above the boiling point of water (100 degrees Celsius/212 degrees Fahrenheit), thus dimming the hopes of finding life. As a result of greenhouse models, scientists are now turning their attention to ], which lies just outside of the star's traditional habitable zone.<ref name="Hopes Dim for life on distant planet"></ref> | |||
| On ], ], the Associated Press released a report stating that scientists have identified twenty-eight exo-solar planetary bodies. One of these newly discovered planets is said to have many similarities with Neptune.<ref name="Planet Hunters Spy Distant Haul"></ref> | |||
| ==Drake equation== | |||
| {{mainarticle|Drake equation}} | |||
| In 1961, ] ] and astrophysicist Dr. ] devised the ], which mathematically simplifies the rate of formation of suitable stars, the fraction of those stars which contain planets, the number of ]-like worlds per planetary system, the fraction of planets where intelligent life develops, and the fraction of possible communicative planets, and the "lifetime" of possible communicative civilizations which scientifically stated there are an estimated 10,000 planets containing intelligent life with the possible capability of communicating with ] ''in the ] galaxy''.<ref>{{cite web|url= http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970924.html|title= The Drake Equation|accessdate= 2008-02-05|last= Boyd|first= Padi|work= Imagine the Universe|publisher= NASA|quote= Frank Drake's own current estimate puts the number of communicating civilizations in the galaxy at 10,000}}</ref> | |||
| ==Extraterrestrial life in the Solar System== | |||
| ] chart shows where life might exist on extrasolar planets based on our own ] and life on ].]] | |||
| ], due to the ocean under its icy crust, might host some form of ]<ref></ref><ref name="EuropaPlanetary">http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"</ref>]] | |||
| Many bodies in the Solar System have been suggested as being capable of containing conventional organic life. The most commonly suggested ones are listed below; of these, five of the nine are moons, and are thought to have large bodies of underground liquid (streams), where life may have evolved in a similar fashion to deep sea vents. | |||
| * ] - ] 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. ] was found in the atmosphere of Mars. Recent photographs from ] show evidence of recent (within 10 years) flows of a liquid on the Red Planet's frigid surface.<ref></ref> | |||
| * ] - Europa may contain liquid water beneath its {{convert|100|mi|km|sing=on}} thick ice layer. Vents on the bottom of the ocean warm the ice so that {{convert|60|mi|km}} of liquid could exist beneath the ice layer, perhaps capable of supporting ] and simple ].<ref name="EuropaPlanetary">http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"</ref> | |||
| * ] - Possible supporter of ], as hypothesized by ] for gas giants in general. This point of view is somewhat controversial because these creatures would not be water-based, but ]-based.<ref></ref> | |||
| * ] - Possible underground ocean (see ]). | |||
| * ] - Possible underground ocean (see ]). | |||
| * ] - Possible floating creatures (see ]). | |||
| * ] - Geothermal activity, watervapour. Possible underice oceans heated by tidal effects. | |||
| * ] (]'s largest moon) - The only known moon with a significant atmosphere was recently visited by the ]. Latest discoveries indicate that there is no global or widespread ocean, but small and/or seasonal liquid ] ]s are present on the surface (the first liquid lakes discovered outside of Earth).<ref></ref><ref></ref><ref>{{cite web| url=http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2214| title=Lakes on Titan, Full-Res: PIA08630| date=July 24, 2006}}</ref> | |||
| * ] - Recently, scientists have speculated the existence of microbes in the stable cloud layers 50 km above the surface, evidenced by hospitable climates and chemical disequilibrium.<ref></ref> | |||
| Numerous other bodies have been suggested as potential hosts for ]. ] has proposed that life might exist on comets, as some Earth microbes managed to survive on a lunar probe for many years. However, it is considered highly unlikely that complex multicellular organisms of the conventional chemistry of terrestrial life (animals, plants) could exist under these living conditions. | |||
| ==See also== | |||
| <!-- Do not link to science fiction or conspiracy theory related alien articles. This article is for the scientific possibility of life not of earthly origin./--> | |||
| ;Events and objects | |||
| *] - ''a ] with disputed microbial formations'' | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| ;Searches for extraterrestrial life | |||
| *] | |||
| *] - ''an ] mission designed to find Earth-like planets and analyze their atmosphere for signs of life''. | |||
| *] | |||
| *] | |||
| ;Subjects | |||
| <div style="-moz-column-count:2; column-count:2;"> | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| </div> | |||
| ;Theories | |||
| <div style="-moz-column-count:2; column-count:2;"> | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| *] | |||
| </div> | |||
| ==References== | |||
| {{Reflist|2}} | |||
| ==Further reading== | |||
| {{Refbegin}} | |||
| * {{cite book | |||
| | author = ] | |||
| | year = 1987 | |||
| | title = ]: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings | |||
| | location = Hanover | |||
| | publisher = University Press of New England | |||
| | id = ISBN 0-87451-406-1 | |||
| }} | |||
| * {{cite book | |||
| | author = ] and ] | |||
| | year =2002 | |||
| | title=] | |||
| | publisher = Ebury Press | |||
| | id = ISBN 0-09-187927-2 | |||
| }} | |||
| * {{cite book | |||
| | last = Crowe | first = Michael J. | |||
| | title = The Extraterrestrial Life Debate, 1750--1900 | |||
| | publisher = Cambridge | |||
| | year = 1986 | |||
| }} | |||
| * {{cite book | |||
| | last = Dick | first = Steven J. | |||
| | title = Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant | |||
| | publisher = Cambridge | |||
| | year = 1984 | |||
| }} | |||
| * {{cite book | |||
| | last = Dick | first = Steven J. | |||
| | title = The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science | |||
| | publisher = Cambridge | |||
| | year = 1996 | |||
| }} | |||
| * {{cite book | |||
| | last = Dick | first = Steven J. | |||
| | title = Life on Other Worlds: The 20th Century Extraterrestrial Life Debate | |||
| | publisher = Cambridge | |||
| | year = 2001 | |||
| }} | |||
| * {{cite book | |||
| | author = Dick, Steven J. and James E. Strick | |||
| | title = The Living Universe: Nasa And the Development of Astrobiology | |||
| | publisher = Rutgers | |||
| | year = 2004 | |||
| }} | |||
| * {{cite book | |||
| | author = ] | |||
| | year = 1997 | |||
| | title = ] | |||
| | location = New York | |||
| | publisher = A Dutton Book | |||
| | id = ISBN 0-525-94336-6 | |||
| }} | |||
| * {{cite book | |||
| | author = ] | |||
| | year =2003 | |||
| | title=] | |||
| | publisher = HarperCollins | |||
| | id = ISBN 0-06-018540-6 | |||
| }} | |||
| * {{cite book | |||
| | author = ] | |||
| | year = 1998 | |||
| | title = Other Worlds: The Search for Life in the Universe | |||
| | location = New York | |||
| | publisher = A Touchstone Book | |||
| }} | |||
| * {{cite book | |||
| | author = ] | |||
| | year = 2003 | |||
| | title = ] | |||
| | location = New York | |||
| | publisher = Basic Books | |||
| | id = ISBN 0-465-07315-8 | |||
| }} | |||
| *Roth, Christopher F. (2005) "Ufology as Anthropology: Race, Extraterrestrials, and the Occult." In ''E.T. Culture: Anthropology in Outerspaces,'' ed. by Debbora Battaglia. Durham, N.C.: Duke University Press. | |||
| * {{cite book | |||
| | author = ] and ] | |||
| | title = ] | |||
| | publisher = Random House | |||
| | year = 1966 | |||
| }} | |||
| * {{cite book | |||
| | last = Sagan | first = Carl | |||
| | title = ] | |||
| | publisher = MIT Press | |||
| | year = 1973 | |||
| }} | |||
| {{Refend}} | |||
| ==External links== | |||
| *A Demand for Congressional hearings on UFOs and related technology "Video" | |||
| *UFOs Filmed ] ]. | |||
| * Freeview video by the Vega Science Trust and the BBC/OU. | |||
| * | |||
| * | |||
| * | |||
| * | |||
| * from ] | |||
| *: Didactic and other poetry concerning other inhabited worlds, well-known and obscure poets, 17th-18th centuries. 19th century | |||
| * | |||
| *, a realistic world-design exercise by ] | |||
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Revision as of 11:00, 21 April 2008
"Green people" redirects here. For green people in fantasy fiction, see Goblinoid.i like poo