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[[Image:Soviet Union-1967-Stamp-0.16. Satellite of Extraterrestrial Civilization.jpg|thumb|200px|A 1967 [[Soviet Union]] 16 [[Soviet ruble|kopeks]] [[postage stamp]], with a satellite from an imagined extraterrestrial civilization.]]

'''Extraterrestrial life''' is [[life]] which does not originate from [[Earth]]. It is the subject of [[astrobiology]] and its existence remains hypothetical, because there is no credible evidence of extraterrestrial life which has been generally accepted by the mainstream scientific community. Hypotheses regarding the origin(s) of extraterrestrial life, if it indeed exists, are as follows: one proposes that it may have emerged, independently, from different places in the universe. An alternative hypothesis is [[panspermia]], which holds that life emerges from one location, then spreads between [[habitable planet]]s. These two hypotheses are not [[mutually exclusive]]. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculated forms of extraterrestrial life range from [[Sapience|sapient]] or [[sentient]] beings, to life at the scale of [[Bacterium|bacteria]].

Suggested locations which might have once developed, or presently continue to host life similar to our own, include the planets [[Venus]]<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 [[Mars]], moons of [[Jupiter]] and [[Saturn]] (e.g. [[Europa (moon)|Europa]],<ref name = "EuropaPlanetary"/> [[Enceladus (moon)|Enceladus]] and [[Titan (moon)|Titan]]) and [[Gliese 581 c]] and [[Gliese 581 d|d]], recently discovered to be near Earth-mass [[extrasolar planet]]s apparently located in their star's [[habitable zone]], and with the potential to have liquid water.<ref>{{cite web |url=http://arxiv.org/abs/0705.3758v1 |title=The Habitability of Super-Earths in Gliese 581 |accessdate=2007-12-01 }}</ref>

==Possible basis of extraterrestrial life==
Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.

===Biochemistry===
{{Main|Biochemistry|Alternative biochemistry}}

All [[life on Earth]] requires [[carbon]], [[hydrogen]], [[oxygen]], [[nitrogen]], [[sulfur]] and
[[phosphorus]] as well as numerous other elements in smaller amounts, notably minerals; it also requires [[water]] as the [[solvent]] 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 that of Earth. Because Earth and other planets are made up of "[[star dust]]", i.e. relatively abundant [[chemical element]]s formed from stars which have ended their lives as [[supernova]]e, it is very probable that other planets may have been formed by elements of a similar composition to the Earth's. The combination of carbon and water in the chemical form of [[carbohydrate]]s (e.g. [[sugar]]) can be a source of chemical [[energy]] on which life depends, and can also provide structural elements for life (such as [[ribose]], in the molecules [[DNA]] and [[RNA]], and [[cellulose]] in plants). [[Plant]]s derive energy through the conversion of light energy into chemical energy via [[photosynthesis]]. Life requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon oxides) states. It also requires [[nitrogen]] as a reduced [[ammonia]] derivative in all [[protein]]s, [[sulfur]] as a derivative of [[hydrogen sulfide]] in some necessary proteins, and [[phosphorus]] oxidized to [[phosphate]]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 [[pH]] due to its continued dissociation between [[hydroxide]] and [[hydronium]] [[ionic bond|ions]]. As a result, it can dissolve both positive [[cations|metallic ions]] and negative [[anions|non-metallic ions]] with equal ability. Furthermore, the fact that organic molecules can be either [[hydrophobic]] (repelled by water) or [[hydrophilic]] (soluble in water) creates the ability of organic compounds to orient themselves to form water-enclosing [[biological membrane|membranes]]. 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. Without this quality, the oceans could have frozen solid during the [[Snowball Earth]] episodes. Additionally, the [[Van der Waals force]]s between water molecules give it an ability to store [[latent heat|energy]] with [[evaporation]], which upon [[condensation]] is released. This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life.

Carbon is fundamental to terrestrial life for its immense flexibility in creating [[covalent bond|covalent chemical bonds]] with a variety of non-metallic elements, principally [[nitrogen]], [[oxygen]] and [[hydrogen]]. [[Carbon dioxide]] and water together enable the storage of solar energy in [[sugars]], such as [[glucose]]. The [[oxidation]] of glucose releases biochemical energy needed to fuel all other biochemical reactions.

The ability to form [[organic acid]]s (–COOH) and [[amine]] [[bases]] (–NH<sub>2</sub>) gives rise to the possibility of [[neutralization]] dehydrating reactions to build long [[polymer]] [[peptides]] and [[Catalyst|catalytic]] [[proteins]] from [[monomer]] [[amino acids]], and with [[phosphate]]s to build not only [[DNA]] (the information-storing molecule of inheritance), but also [[Adenosine triphosphate|ATP]] (the principal energy "currency" of cellular life).

Due to their relative abundance and usefulness in sustaining life, many have hypothesized that lifeforms elsewhere in the universe would also utilize these basic materials. However, other elements and solvents could also provide a basis for [[life]]. [[Silicon]] is most often deemed to be the probable alternative to carbon. [[Silicon life]]forms are proposed to have a crystalline [[morphology (materials science)|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 [[ammonia]] (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>

Technically, life is basically a self-replicating reaction, but one which could arise under a great many conditions and with various possible 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 [[Plasma (physics)|plasma]] of a star, though it would be highly unconventional.{{Who|date=December 2008}}

Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned. For example, [[NASA]] scientists believe that the color of photosynthesizing pigments on [[extrasolar planet]]s might not be green.<ref>[http://www.nasa.gov/centers/goddard/news/topstory/2007/spe Plants on Other Planets<!-- Bot generated title -->]</ref>

===Evolution and morphology===


In addition to the biochemical basis of extraterrestrial life, many have also considered [[evolution]] and [[comparative anatomy|morphology]]. Science fiction has often depicted extraterrestrial life with [[humanoid]] and/or [[reptiles|reptilian]] forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs - i.e. fundamentally humanoid. Other subjects, such as [[Cat|felines]] and [[insects]], have also occurred in fictional representations of aliens.

A division [[Evolving the Alien|has been suggested]] between universal and parochial (narrowly restricted) characteristics. Universals are features which have evolved independently more than once on Earth (and thus, presumably, are not too difficult to develop) and are so intrinsically useful that species will inevitably tend towards them. These include [[flight]], [[sight]], [[photosynthesis]] and [[Limb (anatomy)|limbs]], all of which have evolved several times here on Earth. There is a huge variety of [[eye]]s, for example, and many of these have radically different working schematics and different visual foci: the [[visual spectrum]], [[infrared]], [[Polarization|polarity]] and [[Acoustic location|echolocation]]. 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. Intelligent aliens could communicate through gestures, as deaf humans do, or by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings, or crickets rub their legs.

Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity. [[Skeletons]], which are essential to large terrestrial organisms according to the experts of the field of [[Gravitational biology]], are almost assured to be replicated elsewhere in one form or another. Many also conjecture as to some type of egg-laying amongst extraterrestrial creatures, but mammalian [[mammary gland]]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 life on Earth foregrounds an even greater variety in space, others point out that [[convergent evolution]] may dictate substantial similarities between Earth and extraterrestrial 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}}

Beliefs in extraterrestrial life may have been present in ancient [[India]], [[Egypt]], [[Arabia]], [[China]], [[Babylon]], [[Assyria]] and [[Sumer]], although in these societies, [[cosmology]] was often associated with the [[supernatural]], 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 [[Greek language|Greek]] writer [[Thales]] and his student [[Anaximander]] in the 7th and 6th centuries B.C. The [[atomists]] 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 [[geocentric]] universe. Championed by [[Aristotle]] and codified by [[Ptolemy]], it favored the Earth and Earth-life (Aristotle denied that there could be a plurality of worlds) and seemingly rendered extraterrestrial life philosophically untenable. [[Lucian]], in his novels, described inhabitants of the Moon and other celestial bodies as humanoids, but significantly different from humans.

Authors of [[Jewish]] sources also considered extraterrestrial life. The [[Talmud]] states that there are at least 18,000 other worlds, but provides little elaboration on the nature of those worlds, or on whether they are physical or spiritual. Based on this, however, the 18th century exposition "Sefer HaB'rit" posits that extraterrestrial creatures exist, and that some may well possess intelligence. 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>

[[Hindu]] beliefs of endlessly repeated cycles of life have led to descriptions of multiple worlds in existence and their mutual contacts ([[Sanskrit]] word Sampark (सम्पर्क) means 'contact' as in Mahasamparka (महासम्पर्क) = the great contact). According to Hindu scriptures, there are innumerable universes created by God 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. Aside from the innumerable universes which are material, there is also the 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 on loving devotional services to God. The spiritually aspiring saints and devotees, as well as thoughtful men of the material world, have been getting guidance and help from these purified living entities of the 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 [[science]] at those times.

Within [[Islam]], the statement of the [[Qur'an]] "All praise belongs to [[God]], Lord of all the worlds" indicates multiple universal bodies, and maybe even multiple universes, which may indicate extraterrestrial and even extradimensional life. Surat [[Al-Jinn]] also mentioned a statement from a [[Genie|Jinn]] regarding the current status and ability of his group in the heavens.

In the [[Qadiani]] faith, a more direct reference from the Quran is presented by [[Mirza Tahir Ahmad]] as a proof that life on other planets may exist according to the Quran. In his book, ''[[Revelation, Rationality, Knowledge & Truth]]'', 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>[http://www.alislam.org/library/books/revelation/part_4_section_7.html Revelation, Rationality, Knowledge & Truth, by Mirza Tahir Ahmad. Chapter; The Quran and Extraterrestrial Life]</ref>

When [[Christianity]] 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 [[Bishop of Paris]], [[Étienne Tempier]], did overturn Aristotle on one point: God ''could'' have created more than one world (given His omnipotence). Taking a further step, and arguing that aliens actually existed, remained rare. Notably, Cardinal [[Nicholas of Cusa]] speculated about [[Alien moon base|aliens on the moon]] and [[sun]].
[[Image:Giordano Bruno.jpg|thumb|[[Giordano Bruno]], ''De l'Infinito, Universo e Mondi, 1584'']]

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 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 [[Giordano Bruno]], 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 [[Catholic church]] for his heretical ideas. The Church did not declare that his ideas regarding extraterrestrial life were heretical. It charged him with sorcery and heresy concerning the person of Jesus Christ and several other bizarre theories also objected to by Protstants, and in no way connected with his theories of extraterrestrial life. 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 the Earth, in proportion to the [characteristics] of the two spheres".<ref>{{cite web| url=http://cosmovisions.com/Rheita.htm| publisher=cosmovisions.com| title=Rheita.htm}}</ref> Dominican monk [[Tommaso Campanella]] wrote about a Solarian alien race in his ''[[Civitas Solis]]''. The Catholic Church has not made a formal ruling on existence of extraterrestrials.

Such comparisons also appeared in poetry of the era. 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". 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). With the new relative viewpoint that the Copernican revolution had wrought, he suggested "our world's sunne / Becomes a starre elsewhere". [[Bernard le Bovier de Fontenelle|Fontanelle]]'s "Conversations on the Plurality of Worlds" (translated into English in 1686) offered similar excursions on the possibility of extraterrestrial life, expanding, rather than denying, the creative sphere of a Maker.

The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. [[William Herschel]], the discoverer of [[Uranus]], was one of many 18th-19th century astronomers convinced that our Solar System, and perhaps others, would be well-populated by alien life. Other luminaries of the period who championed "cosmic pluralism" included [[Immanuel Kant]] and [[Benjamin Franklin]]. At the height of the [[Age of Enlightenment|Enlightenment]], even the [[Sun]] and [[Moon]] were considered candidates for extraterrestrial inhabitants.

===Extraterrestrials and the modern era===
[[Image:Arecibo message.svg|100px|thumb|The [[Arecibo message]] is a digital message sent to [[Great Globular Cluster in Hercules|globular star cluster M13]], 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 [[Scientific Revolution]] through to 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 [[Venus]] and [[Mars]], 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 ([[Io (moon)|Io]]'s volcanism, [[Europa (moon)|Europa]]'s ocean, [[Titan (moon)|Titan]]'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>[http://controlled-hominization.com An intelligent design : Controlled hominization in cosmic apartheid<!-- Bot generated title -->]</ref> the failure of the [[SETI]] program to detect anything resembling an intelligent radio signal after four decades of effort has at least partially dimmed the prevailing optimism of the beginning of the space age. Notwithstanding, the unproven belief in extraterrestrial beings is voiced (not as a hypothesis) in pseudoscience, conspiracy theories in popular folklore like about '[[Area 51#UFO and other conspiracy theories concerning Area 51|Area 51]]' and [[Planets proposed in religion and ufology|legends]]. Emboldened critics{{Who|date=October 2008}} view the search for extraterrestrials as unscientific, despite the fact that the [[SETI]] program is not the result of a continuous, dedicated search, but instead utilizes what resources and manpower it can, when it can. Furthermore, the SETI program only searches a limited range of frequencies at any one time.<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=[[January 17]], [[2003]]}}</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 galaxy appears increasingly dubious to many scientists. Still, in the words of SETI's [[Frank Drake]], "All we know for sure is that the sky is not littered with powerful microwave transmitters".<ref>[http://www.faqs.org/docs/air/taseti.html SETI: Search For Extra-Terrestrial Intelligence]</ref> Drake has also 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 [[Planetary habitability|habitability criteria]] on other worlds, and to confirm that at least [[extrasolar planet|other planets]] are plentiful, though aliens remain a question mark. The [[Wow! signal]], from SETI, remains a speculative debate.

In 2000, [[geologist]] and [[paleontologist]] [[Peter Ward (paleontologist)|Peter Ward]] and [[astrobiology|astrobiologist]] Donald Brownlee published a book entitled ''Rare Earth: Why Complex Life is Uncommon in the Universe''.<ref>[http://www.amazon.com/Rare-Earth-Complex-Uncommon-Universe/dp/0387987010 Amazon.com: Rare Earth: Why Complex Life is Uncommon in the Universe: Books: Peter Ward,Donald Brownlee<!-- Bot generated title -->]</ref> In it, they discussed the [[Rare Earth hypothesis]], in which they claim that [[Earth]]-like life is rare in the [[universe]], while [[bacteria|microbial]] life is common. Ward and Brownlee are open to the idea of evolution on other planets which is not based on essential Earth-like characteristics (such as DNA and carbon).

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 Mars. However, the conclusions that should be drawn from such evidence remain in debate.

==Scientific search for extraterrestrial life==
[[Image:Keplerpacecraft.019e.jpg|thumb|230px|The planned [[NASA]] [[Kepler mission]] for the search of [[extrasolar planets]].]]
The scientific search for extraterrestrial life is being carried out in two different ways: directly and indirectly.

===Direct search===
Scientists are directly searching for evidence of unicellular life within the [[solar system]], carrying out studies on the surface of Mars and examining [[meteors]] which have fallen to Earth. A mission is also proposed to [[Europa (moon)|Europa]], one of [[Jupiter (planet)|Jupiter]]'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>[http://www.spherix.com/mars.html Spherix: Makers of Naturlose (tagatose), a natural, low-calorie sugar made from whey that may be useful as a treatment for Type 2 diabetes<!-- Bot generated title -->]</ref> An experiment on the [[Viking program|Viking]] 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. Independently, in 1996, structures resembling [[nanobacterium|nanobacteria]] were reportedly discovered in a meteorite, [[ALH84001]], thought to be formed of [[martian meteorite|rock ejected from Mars]]. This report is also controversial, and scientific debate continues.

[[File:ALH84001 structures.jpg|thumb|230px|Electron micrograph of martian meteorite [[ALH84001]] showing structures that some scientists think could be fossilized bacteria-like lifeforms]]
In February 2005, [[NASA]] scientists reported that they had found strong evidence of present life on [[Mars]].<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, [[Carol Stoker]] and [[Larry Lemke]] of NASA's [[Ames Research Center]], based their claims on methane signatures found in Mars' 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 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 at which the [[European Space Agency]] presented its findings, 75 percent of the scientists in attendance were reported to believe that life once existed on Mars, and 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 = [[2005-02-28]]| url = http://www.csmonitor.com/2005/0228/p02s02-usgn.html| accessdate = 2006-12-18 }}</ref>

The [[Gaia hypothesis]] stipulates that any planet with a robust population of life will have an atmosphere not in chemical equilibrium, 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 to their star are necessary before this can be used to analyze [[extrasolar planet]]s.

===Indirect search===
[[Image:Terrestrial Planet Finder PIA04499.jpg|thumb|230px|[[Terrestrial Planet Finder]] - A planned Infrared interferometer for finding Earth-like [[extrasolar planets]] (as of 2007 , it has not received the funding from NASA which it needs — that funding is going towards the [[Kepler mission]]).]]
It is theorized that any technological society in space will be transmitting information. Projects such as [[SETI]] are conducting an astronomical search for radio activity which would confirm the presence of intelligent life. A related suggestion is that aliens might broadcast pulsed and continuous [[laser]] 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. While other communication techniques, including laser transmission and interstellar spaceflight, have been discussed seriously and may well be feasible, the measure of effectiveness is the amount of information communicated per unit cost, resulting with radio as the method of choice.

===Extrasolar planets===
Astronomers also search for [[extrasolar planet]]s that they believe would be conducive to life, such as [[Gliese 581 c]] and [[OGLE-2005-BLG-390Lb]], 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">[http://www.space.com/scienceastronomy/070424_hab_exoplanet.html SPACE.com - Major Discovery: New Planet Could Harbor Water and Life<!-- Bot generated title -->]</ref> Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for a 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 [[Atomic absorption spectroscopy|spectrography]] which would reveal key information, such as the presence of free [[oxygen]] in a planet's atmosphere:
[[Image:Glieseupdated.jpg|thumb|230px|Artist's Impression of [[Gliese 581 c]], the first [[extrasolar planet]] discovered within its star's [[habitable zone]].]]
*[[Darwin (ESA)|Darwin]] is an ESA mission designed to find Earth-like planets and analyze their atmosphere.
*The [[COROT]] mission, initiated by the [[French Space Agency]], was launched in 2006, and is currently looking for extrasolar planets; it is the first of its kind.
*The [[Terrestrial Planet Finder]] was supposed to have been launched by [[NASA]], but as of 2009, budget cuts have caused it to be delayed indefinitely.
*The [[Kepler Mission]], largely replacing the Terrestrial Planet Finder, is to be launched in March 2009.

It has been argued that [[Alpha Centauri]], the closest star system to Earth, may contain planets which could be capable of sustaining life.<ref>[http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1997AJ....113.1445W 1997AJ 113.1445W Page 1445<!-- Bot generated title -->]</ref>

On [[April 24]] [[2007]], scientists at the European Southern Observatory in La Silla, Chile said they had found the first Earth-like planet. The planet, known as [[Gliese 581 c]], orbits within the [[habitable zone]] of its star [[Gliese 581]], a [[red dwarf]] star which is a scant 20.5 [[light years]] (194 trillion km) from the Earth. It was initially thought that this planet could contain liquid water, but 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 that 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 [[Gliese 581 d]], which lies just outside of the star's traditional habitable zone.<ref name="Hopes Dim for life on distant planet">[http://www.usatoday.com/tech/science/space/2007-06-18-earthlike-planet-questioned_N.htm Hopes dim for life on distant planet - USATODAY.com<!-- Bot generated title -->]</ref>

On [[May 29]] [[2007]], the Associated Press released a report stating that scientists identified twenty-eight new extra-solar planetary bodies. One of these newly-discovered planets is said to have many similarities to Neptune.<ref name="Planet Hunters Spy Distant Haul">[http://news.bbc.co.uk/2/hi/science/nature/6699893.stm BBC NEWS | Science/Nature | Planet hunters spy distant haul<!-- Bot generated title -->]</ref>

To date, 313 extrasolar planets have been discovered (with 29 multi-planet systems), and new discoveries occur monthly.<ref name="Extrasolar Planet Encyclopedia">[http://exoplanet.eu/catalog.php Extrasolar Planets Encyclopedia]</ref>

==Drake equation==
{{mainarticle|Drake equation}}
In 1961, [[University of California, Santa Cruz]] [[astronomer]] and astrophysicist Dr. [[Frank Drake]] devised the [[Drake equation]]. This controversial equation multiplied estimates of the following terms together:

*The rate of formation of suitable stars.

*The fraction of those stars which contain planets.

*The number of [[Earth]]-like worlds per planetary system.

*The fraction of planets where intelligent life develops.

*The fraction of possible communicative planets.

*The “lifetime” of possible communicative civilizations.

Drake used the equation to estimate that there are approximately 10,000 planets containing intelligent life, with the possible capability of communicating with [[Earth]] in the [[Milky Way]] 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>

Based on observations from the Hubble Space Telescope, there are at least 125 billion galaxies in the universe. It is estimated that at least ten percent of all sun-like stars have a system of planets<ref name="marcyprogth05">{{cite journal | author=Marcy, G.; Butler, R.; Fischer, D.; et.al. | title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities | journal=Progress of Theoretical Physics Supplement | year=2005 | volume=158 | issue= | pages=24 &ndash; 42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi=10.1143/PTPS.158.24 }}</ref>, there are 6.25<sup>*</sup>10<sup>18</sup> stars with planets orbiting them in the universe. If even a billionth of these stars have planets supporting life, there are some 6.25 billion life-supporting solar systems in the universe.

Gene Roddenberry used a form of the Drake Equation in an episode of ''Star Trek: the Next Generation''; however, he didn't know the actual equation, so he invented a similar work, incorporating what he remembered of the real Drake Equation. Drake himself viewed the episode in question, and then gently pointed out that Roddenberry's version was mathematically incorrect.

==Extraterrestrial life in the Solar System==
[[Image:Habitable zone-en.svg|thumb|250px|This [[planetary habitability]] chart shows where life might exist on extrasolar planets, based on our own [[Solar System]] and life on [[Earth]].]]
[[Image:PIA01130 Interior of Europa.jpg|thumb|250px|[[Europa (moon)|Europa]], due to the ocean under its icy crust, might host some form of [[bacteria|microbial life.]]<ref>[http://people.msoe.edu/~tritt/sf/europa.life.html Possibility of Life on Europa<!-- Bot generated title -->]</ref><ref name = "EuropaPlanetary"/>]]
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 ten 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.

* [[Mars]] - [[Life on Mars]] has been long speculated. Liquid water is widely thought to have existed on Mars in the past, and there may still be liquid water beneath the surface. [[Methane]] was found in the atmosphere of Mars. By July 2008, laboratory tests aboard NASA's Phoenix Mars Lander had identified water in a soil sample. The lander's robotic arm delivered the sample to an instrument which identifies vapors produced by the heating of samples. Recent photographs from the [[Mars Global Surveyor]] show evidence of recent (i.e. within 10 years) flows of a liquid on the Red Planet's frigid surface.<ref>[http://news.bbc.co.uk/1/hi/sci/tech/6214834.stm BBC NEWS | Science/Nature | Water 'flowed recently' on Mars<!-- Bot generated title -->]</ref>
* [[Mercury (planet)|Mercury]] - The [[MESSENGER]] expedition to Mercury has discovered that a large amount of water exists in its exosphere.
* [[Europa (moon)|Europa]] - Europa may contain liquid water beneath its thick ice layer. It is possible that vents on the bottom of the ocean warm the ice, so liquid could exist beneath the ice layer, perhaps capable of supporting [[microbes]] and simple [[plants]].<ref name="EuropaPlanetary">http://www.planetary.org/programs/projects/explore_europa/update_12142005.html"</ref>
* [[Jupiter]] - [[Carl Sagan]] and others in the 1960s and 70s computed conditions for hypothetical [[amino acid]]-based macroscopic life in Jupiter's atmosphere, based on observed conditions of this atmosphere. These investigations inspired some [[Jovian (fiction)|science fiction stories]].
* [[Ganymede (moon)|Ganymede]] - Possible underground ocean (see [[Europa (moon)|Europa]]).
* [[Callisto (moon)|Callisto]] - Possible underground ocean (see [[Europa (moon)|Europa]]).
* [[Saturn]] - Possible floating creatures (see [[Jupiter]]).
* [[Enceladus (moon)|Enceladus]] - Geothermal activity, water vapor. Possible under-ice oceans heated by tidal effects.
* [[Titan (moon)|Titan]] ([[Saturn]]'s largest moon) - The only known moon with a significant atmosphere was recently visited by the [[Huygens probe]]. Latest discoveries indicate there is no global or widespread ocean, but that small and/or seasonal liquid [[hydrocarbon]] [[lake]]s are present on the surface (the first liquid lakes discovered outside of Earth).<ref>[http://www.space.com/scienceastronomy/050913_titan_life.html SPACE.com - Scientists Reconsider Habitability of Saturn's Moon<!-- Bot generated title -->]</ref><ref>[http://www.space.com/scienceastronomy/060728_titan_lake.html SPACE.com - Lakes Found on Saturn's Moon Titan<!-- Bot generated title -->]</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=2006-07-24}}</ref>
* [[Venus]] - Recently, scientists have speculated on the existence of microbes in the stable cloud layers 50 km above the surface, evidenced by hospitable climates and chemical disequilibrium.<ref>[http://www.astrobio.net/news/article311.html Venusian Cloud Colonies :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth science evolution distribution Origin of life in universe terrestrial<!-- Bot generated title -->]</ref>
Numerous other bodies have been suggested as potential hosts for [[bacteria|microbial life]]. [[Fred Hoyle]] 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 (i.e. animals and 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
*[[ALH84001]] - ''a [[Mars meteorite]] with disputed seemingly microbial formations''
*[[Ceres (dwarf planet)]]
*[[Gliese 581 c]]
*[[Gliese 581 d]]
*[[Wow! signal]]

;Searches for extraterrestrial life
*[[Fred_Hoyle#Rejection_of_chemical_evolution|Rejection of Chemical Evolution]]
*[[Communication with Extraterrestrial Intelligence]]
*[[Darwin (ESA)|Darwin]] - ''an [[ESA]] mission designed to find Earth-like planets and analyze their atmosphere for signs of life''.
*[[SETI|SETI (Search for Extra-Terrestrial Intelligence)]]
*[[Allen Telescope Array]]

;Subjects
<div style="-moz-column-count:2; column-count:2;">
*[[Astrobiology]]
*[[Cryptobiology]]
*[[Exobiology]]
*[[Extraterrestrials in fiction]]
*[[Habitable zone]]
*[[Planetary habitability]]
*[[Rare Earth hypothesis]]
*[[Scientific skepticism]]
*[[Extraterrestrial liquid water]]
</div>

;Theories
<div style="-moz-column-count:2; column-count:2;">
*[[Zecharia Sitchin]]
*[[Aurelia and Blue Moon]]
*[[Back-contamination]]
*[[Drake equation]]
*[[Expedition (book)|''Expedition'']]
*[[Fermi paradox]]
*[[Panspermia]]
*[[Sentience Quotient]]
*[[Snaiad]]
*[[Kardashev scale]]
</div>

==References==
{{Reflist|2}}

==Further reading==
{{Refbegin}}
* {{cite book
| author = [[John C. Baird|Baird, John C.]]
| year = 1987
| title = [[The Inner Limits of Outer Space]]: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings
| location = Hanover
| publisher = University Press of New England
| isbn = 0-87451-406-1
}}
* {{cite book
| author = [[Jack Cohen (scientist)|Cohen, Jack]] and [[Ian Stewart (mathematician)|Ian Stewart]]
| year =2002
| title=[[Evolving the Alien|Evolving the Alien: The Science of Extraterrestrial Life]]
| publisher = Ebury Press
| 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 = Crowe | first = Michael J.
| title = The extraterrestrial life debate Antiquity to 1915: A Source Book
| publisher = University of Notre Dame Press
| year = 2008
| isbn = 0-268-02368-9
}}
* {{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 = [[Donald Goldsmith|Goldsmith, Donald]]
| year = 1997
| title = [[The Hunt for Life on Mars]]
| location = New York
| publisher = A Dutton Book
| isbn = 0-525-94336-6
}}
* {{cite book
| author = [[David Grinspoon (scientist)|Grinspoon, David]]
| year =2003
| title=[[Lonely Planets|Lonely Planets: The Natural Philosophy of Alien Life]]
| publisher = HarperCollins
| isbn = 0-06-018540-6
}}
* {{cite book
| author = [[Michael T. Lemnick|Lemnick, Michael T.]]
| year = 1998
| title = Other Worlds: The Search for Life in the Universe
| location = New York
| publisher = A Touchstone Book
}}
* {{cite book
| author = [[Cliff Pickover|Pickover, Cliff]]
| year = 2003
| title = [[The Science of Aliens]]
| location = New York
| publisher = Basic Books
| 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 = [[Carl Sagan|Sagan, Carl]] and [[Iosif Samuilovich Shklovskii|I.S. Shklovskii]]
| title = [[Intelligent Life in the Universe]]
| publisher = Random House
| year = 1966
}}
* {{cite book
| last = Sagan | first = Carl
| title = [[Communication with Extraterrestrial Intelligence]]
| publisher = MIT Press
| year = 1973
}}
{{Refend}}

==External links==
*[http://www.vega.org.uk/video/programme/9 'Is There Anybody Out There?'] Freeview video by the Vega Science Trust and the BBC/OU.
*[http://www.pbs.org/lifebeyondearth/index.html ''PBS: Life Beyond Earth'' a film by Timothy Ferris]
*[http://www.pbs.org/exploringspace/ ''PBS: Exploring Space - The Quest for Life'' by Scott Pearson]
*[http://www.rfreitas.com/Astro/Xenopsychology.htm ''Xenopsychology'' by Robert A. Freitas Jr.]
*[http://news.nationalgeographic.com/news/2005/05/0520_050520_tv_aliens.html "What Aliens Might Look Like"] from [[National Geographic]]
*[http://news.bbc.co.uk/1/hi/sci/tech/4728228.stm Top stars picked in alien search]

{{Nature nav}}

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