Extraterrestrial life: Difference between revisions
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2. [[SETI|Listening for signals]] (Image: [[Allen Telescope Array|Allen array]])<br> |
2. [[SETI|Listening for signals]] (Image: [[Allen Telescope Array|Allen array]])<br> |
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3. [[Robotic spacecraft|Robotic exploration]] of the Solar System (Image: [[Curiosity (rover)|''Curiosity'' rover]]).]] |
3. [[Robotic spacecraft|Robotic exploration]] of the Solar System (Image: [[Curiosity (rover)|''Curiosity'' rover]]).]] |
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'''Extraterrestrial life''' (from the [[Latin]] words: ''extra'' ["beyond", or "not of"] and ''[[wikt:terrestris|terrestris]]'' ["of or belonging to Earth"]) is defined as [[life]] that does not originate from [[Earth]]. It is often also referred to as '''alien life''', or simply '''aliens''' (or '''space aliens''', to differentiate from other definitions of [[alien (disambiguation)|alien]] or aliens). These |
'''Extraterrestrial life''' (from the [[Latin]] words: ''extra'' ["beyond", or "not of"] and ''[[wikt:terrestris|terrestris]]'' ["of or belonging to Earth"]) is defined as [[life]] that does not originate from [[Earth]]. It is often also referred to as '''alien life''', or simply '''aliens''' (or '''space aliens''', to differentiate from other definitions of [[alien (disambiguation)|alien]] or aliens). These forms of life range from simple [[bacteria]]-like organisms to beings far more complex than [[human]]s. The possibility that sapient beings might also exist extraterrestrially has been proven upon the recent discovery of the planet [http://elderscrolls.wikia.com/wiki/Nirn Nirn] .<ref name="Astro-20130814">{{cite journal |last=Griffin |first=Dale Warren |title=The Quest for Extraterrestrial Life: What About the Viruses? |url=http://online.liebertpub.com/doi/abs/10.1089/ast.2012.0959 |date=14 August 2013 |journal=[[Astrobiology (journal)]] |volume=13 |issue=8 |pages=774–783 |doi=10.1089/ast.2012.0959 |accessdate=6 September 2013 |bibcode = 2013AsBio..13..774G }}</ref> |
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The development and testing of hypotheses on extraterrestrial life is known as exobiology or [[astrobiology]]; the term astrobiology, however, includes the study of life on Earth viewed in its astronomical context. Scientists at the [[National Institutes of Health]] reported [[Panspermia#Complexity|studies]] that life in the [[universe]] may have begun "{{val|9.7|2.5}} billion years ago", billions of years before the Earth was formed, based on extrapolating the "genetic complexity of organisms" [from "major phylogenetic lineages"] to earlier times.<ref name="arXiv-20130328">{{cite journal|last1=Sharov|first1=Alexei A. |last2=Gordon|first2=Richard |title=Life Before Earth|url=http://arxiv.org/ftp/arxiv/papers/1304/1304.3381.pdf |date=28 March 2013|journal=[[arXiv]]|arxiv=1304.3381v1 |accessdate=16 April 2013 }}</ref><ref name="NIH-20060612">{{cite journal|last=Sharov |first=Alexei A. |title=Genome increase as a clock for the origin and evolution of life|journal=[[Biology Direct]] |volume=1 |pages=1–17 |date=12 June 2006 |issue= |doi=10.1186/1745-6150-1-17|pmc=1526419}}</ref> |
The development and testing of hypotheses on extraterrestrial life is known as exobiology or [[astrobiology]]; the term astrobiology, however, includes the study of life on Earth and Nirn viewed in its astronomical context. Scientists at the [[National Institutes of Health]] reported [[Panspermia#Complexity|studies]] that life in the [[universe]] may have begun "{{val|9.7|2.5}} billion years ago", billions of years before the Earth and Nirn was formed, based on extrapolating the "genetic complexity of organisms" [from "major phylogenetic lineages"] to earlier times.<ref name="arXiv-20130328">{{cite journal|last1=Sharov|first1=Alexei A. |last2=Gordon|first2=Richard |title=Life Before Earth|url=http://arxiv.org/ftp/arxiv/papers/1304/1304.3381.pdf |date=28 March 2013|journal=[[arXiv]]|arxiv=1304.3381v1 |accessdate=16 April 2013 }}</ref><ref name="NIH-20060612">{{cite journal|last=Sharov |first=Alexei A. |title=Genome increase as a clock for the origin and evolution of life|journal=[[Biology Direct]] |volume=1 |pages=1–17 |date=12 June 2006 |issue= |doi=10.1186/1745-6150-1-17|pmc=1526419}}</ref> |
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==Background== |
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Alien life, such as bacteria, has been hypothesized to exist in the [[Solar System]] and throughout the universe. This hypothesis relies on the [[Size of the universe|vast size]] and consistent [[physical law]]s of the [[observable universe]]. According to this argument, made by scientists such as [[Carl Sagan]] and [[Stephen Hawking]], it would be improbable for life ''not'' to exist somewhere other than Earth.<ref>{{cite book | page=3 | title=Other Worlds, Other Universes | editors=Brad Steiger, John White | publisher=Health Research Books | year=1986 | isbn=0-7873-1291-6 | url=http://books.google.com/books?id=vgQj5D524PYC&pg=PA3 }}</ref><ref>{{cite book | first1=David | last1=Filkin | first2=Stephen W. | last2=Hawking | title=Stephen Hawking's universe: the cosmos explained | page=194 | series=Art of Mentoring Series | publisher=Basic Books | year=1998 | isbn=0-465-08198-3 | url=http://books.google.com/books?id=95Kog5v4ZxkC&pg=PA194 }}</ref> This argument is embodied in the [[Copernican principle]], which states that the Earth does not occupy a unique position in the Universe, and the [[mediocrity principle]], which holds that there is nothing special about life on Earth.<ref>{{Cite book | first=Horst | last=Rauchfuss | year=2008 | title=Chemical Evolution and the Origin of Life | publisher=Springer | isbn=3-540-78822-0 | url=http://books.google.com/books?id=aRkvNoDYtvEC&pg=PA300 | others=T. N. Mitchell | postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}</ref> Life may have emerged independently at many places throughout the [[Universe]]. Alternatively life may form less frequently, then spread between [[habitable planet]]s through [[panspermia]] or exogenesis.<ref>{{cite book | first1=Guillermo | last1=Gonzalez | first2=Jay Wesley | last2=Richards | title=The privileged planet: how our place in the cosmos is designed for discovery | pages=343–345 | publisher=Regnery Publishing | year=2004 | isbn=0-89526-065-4 | url=http://books.google.com/books?id=KFdu4CyQ1k0C&pg=PA343 }}</ref> In any case, [[organic compound|complex organic molecules]] necessary for life may have formed in the [[protoplanetary disk]] of [[cosmic dust|dust grains]] surrounding the [[Sun]] before the formation of the Earth based on [[computer simulation|computer model studies]].<ref name="Space-20120329">{{cite web |last=Moskowitz |first=Clara |title=Life's Building Blocks May Have Formed in Dust Around Young Sun |url=http://www.space.com/15089-life-building-blocks-young-sun-dust.html |date=29 March 2012 |publisher=[[Space.com]] |accessdate=30 March 2012 }}</ref> According to these studies, this same process may also occur around other [[stars]] that acquire [[planets]].<ref name="Space-20120329" /> (Also see [[Abiogenesis#Extraterrestrial organic molecules|Extraterrestrial organic molecules]].) |
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Suggested locations at which life might have developed include the planets [[Venus]]<ref>{{cite news|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 | first=Martin | last=Redfern}}</ref> and [[Mars]], [[Jupiter]]'s moon [[Europa (moon)|Europa]],<ref name = "EuropaPlanetary"/> and [[Saturn]]'s moons [[Titan (moon)|Titan]] and [[Enceladus]].<ref>{{cite journal | display-authors=1 | title=TandEM: Titan and Enceladus mission | last1=Coustenis | first1=A. | last2=Atreya | first2=S. K. | last3=Balint | first3=T. | last4=Brown | first4=R. H. | last5=Dougherty | first5=M. K. | last6=Ferri | first6=F. | last7=Fulchignoni | first7=M. | last8=Gautier | first8=D. | last9=Gowen | first9=R. A. | journal=Experimental Astronomy | volume=23 | issue=3 | pages=893–946 |date=March 2009 | doi=10.1007/s10686-008-9103-z | bibcode=2009ExA....23..893C }}</ref> In May 2011, NASA scientists reported that Enceladus "is emerging as the most habitable spot beyond Earth in the Solar System for life as we know it".<ref>{{cite journal |last1=Lovett |first1=Richard A. |title=Enceladus named sweetest spot for alien life |url=http://www.nature.com/news/2011/110531/full/news.2011.337.html |date=31 May 2011 |publisher=[[Nature (journal)|Nature]] |doi=10.1038/news.2011.337 |accessdate=2011-06-03 |journal=Nature }}</ref> |
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Since the 1950s, scientists have promoted the idea that "[[habitable zone]]s" are the most likely places for life to be found. Numerous discoveries in this zone since 2007 have stimulated estimations of frequencies of Earth-like habitats numbering in the many billions<ref name=wethington-2008>{{cite news | url=http://www.universetoday.com/22380/how-many-stars-are-in-the-milky-way/ | title=How Many Stars are in the Milky Way? | work=UniverseToday | date=September 16, 2008 | agency=UniverseToday | accessdate=April 21, 2013 | author=Wethington, Nicholos}}</ref><ref name="ChoiCQ">{{cite web |last1=Choi |first1=Charles Q. |url=http://www.space.com/11188-alien-earths-planets-sun-stars.html |title=New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone |date=21 March 2011 |publisher=[[Space.com]] |accessdate=2011-04-24}}</ref> though as of 2013, only a small number of planets have been discovered in these zones.<ref name=torres-2013-2>{{cite web | url=http://phl.upr.edu/press-releases/tenpotentiallyhabitableexoplanetsnow | title=Ten potentially habitable exoplanets now | publisher=University of Puerto Rico | work=Habitable Exoplanets Catalog | date=April 26, 2013 | accessdate=April 29, 2013 | author=Torres, Abel Mendez}}</ref> Nonetheless, on November 4, 2013, astronomers reported, based on [[Kepler (spacecraft)|''Kepler'' space mission]] data, that there could be as many as 40 billion [[Terrestrial planet|Earth-sized]] [[extrasolar planets|planets]] orbiting in the [[habitable zone]]s of [[Solar analog|Sun-like stars]] and [[red dwarf]]s in the [[Milky Way]],<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis |title=Far-Off Planets Like the Earth Dot the Galaxy |url=http://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=November 4, 2013 |work=[[New York Times]] |accessdate=November 5, 2013 }}</ref><ref name="PNAS-20131031">{{cite journal |last1=Petigura |first1=Eric A. |last2=Howard |first2=Andrew W. |last3=Marcy|first3=Geoffrey W. |title=Prevalence of Earth-size planets orbiting Sun-like stars|url=http://www.pnas.org/content/early/2013/10/31/1319909110 |date=October 31, 2013 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|doi=10.1073/pnas.1319909110 |accessdate=November 5, 2013 }}</ref> 11 billion of which may be orbiting Sun-like stars.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=November 4, 2013 |work=[[Los Angeles Times]] |accessdate=November 5, 2013 }}</ref> The nearest such planet may be 12 light-years away, according to the scientists.<ref name="NYT-20131104" /><ref name="PNAS-20131031" /> Astrobiologists have also considered a "follow the energy" view of potential habitats.<ref name="HoehlerAmend2007">{{cite journal|last1=Hoehler|first1=Tori M.|last2=Amend|first2=Jan P.|last3=Shock|first3=Everett L.|title=A "Follow the Energy" Approach for Astrobiology|journal=Astrobiology|volume=7|issue=6|year=2007|pages=819–823|issn=1531-1074|doi=10.1089/ast.2007.0207|bibcode = 2007AsBio...7..819H }}</ref><ref name="JonesLineweaver2010">{{cite journal|last1=Jones|first1=Eriita G.|last2=Lineweaver|first2=Charles H.|title=To What Extent Does Terrestrial Life "Follow The Water"?|journal=Astrobiology|volume=10|issue=3|year=2010|pages=349–361|issn=1531-1074|doi=10.1089/ast.2009.0428|bibcode = 2010AsBio..10..349J }}</ref> |
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No widely accepted evidence of extraterrestrial life has been found; however, various controversial claims have been made.<ref>{{cite news | first = John Pickrell | title = Top 10: Controversial pieces of evidence for extraterrestrial life | date = 4 September 2006 | url = http://www.newscientist.com/article/dn9943-top-10-controversial-pieces-of-evidence-for-extraterrestrial-life.html | work = New Scientist | accessdate = 2011-02-18}}</ref> Beliefs that some [[unidentified flying object]]s are [[Extraterrestrial hypothesis|of extraterrestrial origin]],<ref>{{cite journal |
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| first=Anne | last=Cross | title=The Flexibility of Scientific Rhetoric: A Case Study of UFO Researchers |
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| journal=Qualitiative Sociology |
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| volume=27 | issue=1 | pages=3–34 | doi=10.1023/B:QUAS.0000015542.28438.41 |
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| year=2004 }}</ref> along with claims of [[alien abduction]],<ref>{{cite book |
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| first=Susan A. | last=Clancy | year=2005 | pages=4–6 |
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| title=Abducted: how people come to believe they were kidnapped by aliens |
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| publisher=Harvard University Press |
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| isbn=067402401X }}</ref> are dismissed by most scientists. Most [[List of UFO sightings|UFO sightings]] are explained either as sightings of Earth-based aircraft or known [[astronomical object]]s, or as [[hoax]]es.<ref>{{cite journal |
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| first=Philippe | last=Ailleris |date=January–February 2011 |
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| title=The lure of local SETI: Fifty years of field experiments |
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| journal=Acta Astronautica | volume=68 | issue=1–2 |
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| pages=2–15 | doi=10.1016/j.actaastro.2009.12.011 }}</ref> |
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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."<ref name="WhiteHouse">{{cite web |last=Larson|first=Phil |title=Searching for ET, But No Evidence Yet|url=https://wwws.whitehouse.gov/petitions#!/response/searching-et-no-evidence-yet|date=5 November 2011 |publisher=[[White House]] |accessdate=2011-11-06}}</ref><ref name="Atkinson">{{cite web |last=Atkinson |first=Nancy |title=No Alien Visits or UFO Coverups, White House Says|url=http://www.universetoday.com/90717/no-alien-visits-or-ufo-coverups-white-house-says/|date=5 November 2011 |publisher=[[UniverseToday]] |accessdate=2011-11-06 }}</ref> Also, according to the response, there is "no credible information to suggest that any evidence is being hidden from the public's eye."<ref name="WhiteHouse"/><ref name="Atkinson"/> The response further noted that efforts, like [[SETI]], the ''Kepler'' space telescope and the [[Mars Science Laboratory|NASA Mars rover]], continue looking for [[life|signs of life]]. 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 [[Extraterrestrial intelligence|intelligent ones]]—are extremely small, given the distances involved."<ref name="WhiteHouse"/><ref name="Atkinson"/> |
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==Possible basis== |
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Several hypotheses have been proposed about the possible basis of alien life from a [[Biochemistry|biochemical]], [[evolution]]ary or [[Morphology (biology)|morphological]] viewpoint. |
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===Biochemistry=== |
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{{Main|Biochemistry|Hypothetical types of biochemistry|Water and life}} |
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All [[life on Earth]] is based upon 26 [[chemical element]]s. However, about 95% of this life is built upon only six of these elements: [[carbon]], [[hydrogen]], [[nitrogen]], [[oxygen]], [[phosphorus]] and [[sulfur]], abbreviated [[CHNOPS]]. These six elements form the basic building blocks of virtually all life on Earth, while most of the remaining elements are found in only trace amounts.<ref>{{cite book | first1=Lucas John | last1=Mix | year=2009 | title=Life in space: astrobiology for everyone | page=76 | publisher=Harvard University Press | isbn=0-674-03321-3 | url=http://books.google.com/books?id=A60E-v4Sr-gC&pg=PA76 | accessdate=2011-08-08 }}</ref> |
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Life on Earth requires [[water]] as the [[solvent]] in which biochemical reactions take place. Sufficient quantities of carbon and the other elements along with water, may enable the formation of living organisms on other planets with a chemical make-up and temperature range similar to that of Earth.<ref>{{cite journal | first=Norman R. | last=Pace | date=January 20, 2001 | title=The universal nature of biochemistry | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=98 | issue=3 | pages=805–808 | doi=10.1073/pnas.98.3.805 | pmid=11158550 | bibcode=2001PNAS...98..805P | pmc=33372}}</ref> [[Terrestrial planet]]s such as Earth are formed in a process that allows for the possibility of having compositions similar to Earth's.<ref>{{cite journal | last1=Bond | first1=Jade C. | last2=O'Brien | first2=David P. | last3=Lauretta | first3=Dante S. | title=The Compositional Diversity of Extrasolar Terrestrial Planets. I. In Situ Simulations | journal=The Astrophysical Journal | volume=715 | issue=2 | pages=1050–1070 |date=June 2010 | doi=10.1088/0004-637X/715/2/1050 | bibcode=2010ApJ...715.1050B |arxiv = 1004.0971 }}</ref> The combination of carbon, hydrogen and oxygen in the chemical form of [[carbohydrate]]s (e.g. [[sugar]]) can be a source of chemical [[energy]] on which life depends, and can 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, as currently recognized, requires carbon in both reduced (methane derivatives) and partially [[oxidized]] ([[carbon oxide]]s) states. Nitrogen is needed 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. |
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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]]. Additionally, the [[hydrogen bond]]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. |
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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]] and [[starches]], such as [[glucose]]. The [[oxidation]] of glucose releases biochemical energy needed to fuel all other biochemical reactions. |
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The ability to form [[organic acid]]s (–COOH) and [[amine]] [[Base (chemistry)|bases]] (–NH<sub>2</sub>) gives rise to the possibility of [[neutralization (chemistry)|neutralization]] dehydrating reactions to build long [[polymer]] [[peptides]] and [[Catalyst|catalytic]] [[protein]]s from [[monomer]] [[amino acid]]s. When combined with [[phosphate]]s, these acids can build the information-storing molecule of inheritance, DNA, and the principal energy transfer molecule of cellular life, [[Adenosine triphosphate|ATP]]. |
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Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would utilize these basic materials. However, other elements and solvents could provide a basis for life. |
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Life forms based in [[ammonia]] (rather than water) have 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> |
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From a chemical perspective, life is fundamentally 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.<ref name="Darlingvariety">{{cite web| url=http://www.daviddarling.info/encyclopedia/E/etlifevar.html| publisher=daviddarling.info| title=extraterrestrial life, variety of}}</ref> Life on the surface of a neutron star, based on nuclear reactions, has also been suggested. However, communicating with such creatures would be difficult because the time scales involved are much faster.<ref name="GO247"> |
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{{cite book |
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| last = Goldsmith |
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| first = Donald |
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| last2 = Owen |
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| first2 = Tobias |
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| title = The Search for Life in the Universe |
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| publisher = [[Addison-Wesley]] |
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| edition = 2 |
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| year = 1992 |
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| pages = 247 |
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| isbn = 0201569493 |
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</ref> |
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Several pre-conceived ideas about the characteristics of life outside Earth have been questioned. For example, a [[NASA]] scientist suggested that the color of [[photosynthesis|photosynthesizing pigments]] of hypothetical life on extrasolar planets might not be green.<ref>{{cite news | first = Rob Gutro | title = NASA Predicts Non-Green Plants on Other Planets | date = 4 November 2007 | publisher = NASA | url = http://www.nasa.gov/centers/goddard/news/topstory/2007/spectrum_plants.html | work = Goddard Space Flight Center | accessdate = 2011-02-18}}</ref> |
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===Evolution and morphology=== |
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In addition to the biochemical basis of extraterrestrial life, many have considered evolution and [[comparative anatomy|morphology]]. [[Science fiction]] has often depicted extraterrestrial life with [[humanoid]] 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 [[Felidae|felines]], [[insect]]s, blobs, etc., have occurred in fictional representations of aliens. |
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A division [[Evolving the Alien|has been suggested]] between universal and [[parochial]] (narrowly restricted) characteristics. Universals are features which are thought to 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. The most fundamental of these is probably bilateral symmetry, but more complex (though still basic) characteristics include [[flight]], [[visual perception|sight]], [[photosynthesis]] and [[Limb (anatomy)|limbs]], all of which are thought to 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]], [[Polarized light|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, by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, [[cicada]]s vibrate their wings or [[Cricket (insect)|crickets]] [[stridulation|stridulate]] their wings, or visually through [[bioluminescence]] or [[chromatophore]]-like structures. |
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Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity. [[Skeleton]]s, 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. The assumption of radical [[Biodiversity|diversity]] amongst putative extraterrestrials is by no means settled. While many exobiologists do stress that the enormously heterogeneous nature of life on Earth foreshadows an even greater variety in [[outer 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 name="Darlingvariety"/> |
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==Planetary habitability in the Solar System== |
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{{See also|Planetary habitability|Natural satellite habitability}} |
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Some bodies in the Solar System have been suggested as having the potential for an environment that could host extraterrestrial life, particularly those with possible subsurface oceans. Though due to the lack of habitable environments beyond Earth, should life be discovered elsewhere in the Solar System, astrobiologists suggest that it will more likely be in the form of [[extremophile]] [[microorganism]]s. |
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The planets Venus and Mars, along with several natural satellites orbiting Jupiter and Saturn, and even comets, are suspected to possess niche environments in which life might exist. A subsurface marine environment on Jupiter's moon Europa might be the most suitable habitat in the Solar System, outside Earth, for [[multicellular organism]]s. |
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[[Panspermia]] suggests that life elsewhere in the Solar System may have a common origin. If extraterrestrial life was found on another body in the [[Solar System]], it could have originated from Earth just as life on Earth may have been seeded from elsewhere ([[Exogenesis (astrobiology)|exogenesis]]). The [[Living Interplanetary Flight Experiment]], developed by the [[Planetary Society]] launched in 2011 was designed to test some aspect of these hypotheses, but it was destroyed along with the carrier [[Fobos-Grunt]] mission.<ref name=UT-11-12-13>Universe Today, [http://www.universetoday.com/91766/russian-space-program-prepares-for-phobos-grunt-re-entry/ "Russian Space Program Prepares for Phobos-Grunt Re-Entry"], David Warmflash, 13 December 2011</ref> |
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The first known mention of the term Panspermia was in the writings of the 5th century BC [[Ancient Greece|Greek]] philosopher [[Anaxagoras]].<ref>Margaret O'Leary (2008) Anaxagoras and the Origin of Panspermia Theory, iUniverse publishing Group, # ISBN 978-0-595-49596-2</ref> In the nineteenth century it was again revived in modern form by several scientists, including [[Jöns Jacob Berzelius]] (1834),<ref>{{Cite journal| last = Berzelius (1799-1848) | first = J. J. | title = Analysis of the Alais meteorite and implications about life in other worlds | postscript = <!--None-->}}</ref> [[William Thomson, 1st Baron Kelvin|Kelvin]] (1871),<ref>{{Cite journal| last = Thomson (Lord Kelvin) | first = W. | title = Inaugural Address to the British Association Edinburgh. "We must regard it as probably to the highest degree that there are countless seed-bearing meteoritic stones moving through space." | journal = Nature | volume = 4 | pages = 261–278 [262] | year = 1871 | postscript = <!--None-->|doi=10.1038/004261a0| issue = 92 |bibcode = 1871Natur...4..261. }}</ref> [[Hermann von Helmholtz]] (1879){{Citation needed|date=September 2012}} and, somewhat later, by [[Svante Arrhenius]] (1903).<ref>Arrhenius, S., ''Worlds in the Making: The Evolution of the Universe''. New York, Harper & Row, 1908.</ref> |
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[[Sir Fred Hoyle]] (1915–2001) and [[Chandra Wickramasinghe]] (born 1939) were important proponents of the hypothesis who further contended that lifeforms continue to enter the Earth's atmosphere, and may be responsible for epidemic outbreaks, new diseases, and the genetic novelty necessary for [[macroevolution]].<ref>Fred Hoyle, Chandra Wickramasinghe and John Watson, ''Viruses from Space and Related Matters'', University College Cardiff Press, 1986.</ref> |
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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. |
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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,<ref name="Crick_Orgel">{{Cite journal| last = Crick | first = F. H. | last2 = Orgel | first2 = L. E.| title = Directed Panspermia | journal = Icarus | volume = 19 | pages = 341–348 | year = 1973| doi = 10.1016/0019-1035(73)90110-3+| bibcode=1979JBIS...32..419M | postscript = <!--None--> }}</ref> but considering an early "[[RNA world]]". Crick noted later that life may have originated on Earth.<ref>"[http://www.fasebj.org/cgi/reprint/7/1/238.pdf Anticipating an RNA world. Some past speculations on the origin of life: where are they today?]" by L. E. Orgel and F. H. C. Crick in ''FASEB J.'' (1993) Volume 7 pages 238-239.</ref> |
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In a virtual presentation on Tuesday, April 7, 2009, Stephen Hawking discussed the possibility of building a human base on another planet and gave reasons why alien life might not be contacting the human race, during his conclusion of the Origins Symposium at Arizona State University. Hawking also talked about what humans may find when venturing into space, such as the possibility of alien life through the theory of panspermia.<ref>{{cite web|last=Weaver|first=Rheyanne|title=Ruminations on other worlds|url=http://www.statepress.com/archive/node/5745|work=State Press|accessdate=10 March 2014}}</ref> |
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===Venus=== |
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Carl Sagan, [[David Grinspoon]], [[Geoffrey A. Landis]] and [[Dirk Schulze-Makuch]] have put forward a hypothesis that microbes could exist in the stable cloud layers {{convert|50|km|0|abbr=on}} above the surface of [[Venus]]; the hypothesis is based on the premises of hospitable climates and chemical disequilibrium.<ref>[http://web.archive.org/web/20070807083309/http://www.astrobio.net/news/article311.html Venusian Cloud Colonies] :: Astrobiology Magazine</ref> |
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<ref>Geoffrey A. Landis [http://web.archive.org/web/20110807004311/http://gltrs.grc.nasa.gov/reports/2003/TM-2003-212310.pdf Astrobiology: The Case for Venus ]</ref> |
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<ref name="Cockell1999"> |
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{{cite journal |
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| doi = 10.1016/S0032-0633(99)00036-7 |
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| last = Cockell |
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| first = C. S. |
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| title = Life on Venus |
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| journal = Planetary and Space Science |
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| volume = 47 |
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| issue = 12 |
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| pages = 1487–1501 |
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|date=December 1999 |
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| bibcode = 1999P&SS...47.1487C |
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}}</ref> |
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===Mars=== |
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{{Main|Life on Mars}} |
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{{See also|Water on Mars}} |
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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. It may also be present as thin films of salty brine in the first centimeter or so of the soil for part of the year in some locations.<ref>{{cite journal | doi = 10.1073/pnas.031581098 | last1 = Lobitz | first1 = B. | last2 = Wood | year = 2001 | first2 = BL | last3 = Averner | first3 = MM | last4 = McKay | first4 = CP | title = Use of spacecraft data to derive regions on Mars where liquid water would be stable | url = | journal=Proc. Natl. Acad. Sci. | volume = 98 | issue = 5| pages = 2132–2137 | pmid = 11226204 | pmc = 30104 |bibcode = 2001PNAS...98.2132L }}</ref><ref>{{cite journal | doi = 10.1029/2000JE001360 | last1 = Haberie | first1 = Robert M.| last2 = McKay | year = 2001 | first2 = Christopher P. | last3 = Schaeffer | first3 = James | last4 = Cabrol | first4 = Nathalie A. | last5 = Grin | first5 = Edmon A. | last6 = Zent | first6 = Aaron P. | last7 = Quinn | first7 = Richard | title = On the possibility of liquid water on present-day Mars | url = | journal=J. Geophysical Research | volume = 106 | pages = 23317–23326 | bibcode=2001JGR...10623317H}}</ref> The origin of the potential biosignature of [[Atmosphere of Mars#Methane|methane]] in Mars atmosphere is unexplained, although abiotic hypotheses have also been proposed.<ref name="news.discovery">{{cite web|url=http://news.discovery.com/space/top-10-places-to-find-alien-life.html |title=Top 10 Places To Find Alien Life : Discovery News |publisher=News.discovery.com |date=2010-06-08 |accessdate=2012-06-13}}</ref> 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 vapours produced by the heating of samples. Photographs from the [[Mars Global Surveyor]] from 2006 showed evidence of recent (i.e. within 10 years) flows of a liquid on Mars's frigid surface.<ref>{{cite news| url=http://news.bbc.co.uk/1/hi/sci/tech/6214834.stm | work=BBC News | title=Water 'flowed recently' on Mars | date=2006-12-06 | accessdate=2010-05-02}}</ref> |
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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. Nonetheless, present conditions on Mars may support life since [[lichen]]s were found to successfully survive [[Life on Mars#Life on Earth under Martian conditions|Martian conditions]] in the Mars Simulation Laboratory (MSL) maintained by the [[German Aerospace Center]] (DLR).<ref name="Skymania-20120426">{{cite web |last=Baldwin |first=Emily |title=Lichen survives harsh Mars environment |url=http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html |date=26 April 2012 |publisher=Skymania News |accessdate=27 April 2012 }}</ref><ref name="EGU-20120426">{{cite web |last1=de Vera |first1=J.-P. |last2=Kohler |first2=Ulrich |title=The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars |url=http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |date=26 April 2012 |publisher=[[European Geosciences Union]] |accessdate=27 April 2012 }}</ref> In June 2012, scientists reported that measuring the ratio of hydrogen and [[methane]] levels on Mars may help determine the likelihood of life on Mars.<ref name="PNAS-20120607">{{cite journal |last1=Oze|first1=Christopher |last2=Jones |first2=Camille |last3=Goldsmith |first3=Jonas I.|last4=Rosenbauer |first4=Robert J. |title=Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces|url=http://www.pnas.org/content/109/25/9750.abstract |date=June 7, 2012|journal=[[PNAS]] |volume=109| issue = 25 |pages=9750–9754 |doi=10.1073/pnas.1205223109|accessdate=June 27, 2012 |bibcode = 2012PNAS..109.9750O |pmid=22679287 |pmc=3382529}}</ref><ref name="Space-20120625">{{cite web|author=Staff |title=Mars Life Could Leave Traces in Red Planet's Air: Study|url=http://www.space.com/16284-mars-life-atmosphere-hydrogen-methane.html|date=June 25, 2012 |publisher=[[Space.com]] |accessdate=June 27, 2012 }}</ref> According to the scientists, "...low H<sub>2</sub>/CH<sub>4</sub> ratios (less than approximately 40) indicate that life is likely present and active."<ref name="PNAS-20120607" /> Other scientists have recently reported methods of detecting hydrogen and methane in [[extraterrestrial atmospheres]].<ref name="Nature-20120627">{{cite journal |last1=Brogi |first1=Matteo |last2=Snellen |first2=Ignas A. G. |last3=de Krok |first3=Remco J. |last4=Albrecht |first4=Simon |last5=Birkby |first5=Jayne |last6=de Mooij |first6=Ernest J. W. |title=The signature of orbital motion from the dayside of the planet t Boötis b |url=http://www.nature.com/nature/journal/v486/n7404/full/nature11161.html?WT.ec_id=NATURE-20120628 |date=June 28, 2012 |journal=[[Nature (journal)|Nature]] |volume=486 |pages=502–504 |doi=10.1038/nature11161 |accessdate=June 28, 2012 |arxiv = 1206.6109 |bibcode = 2012Natur.486..502B |issue=7404}}</ref><ref name="Wired-20120627">{{cite web |last=Mann |first=Adam |title=New View of Exoplanets Will Aid Search for E.T. |url=http://www.wired.com/wiredscience/2012/06/tau-bootis-b/ |date=June 27, 2012 |publisher=[[Wired (magazine)]] |accessdate=June 28, 2012 }}</ref> On December 9, 2013, NASA reported that, based on evidence from ''Curiosity'' studying [[Aeolis Palus]], [[Gale (crater)|Gale Crater]] contained an ancient freshwater [[lake]] that could have been a hospitable environment for [[microbial life]].<ref name="NYT-20131209">{{cite news |last=Chang |first=Kenneth |title=On Mars, an Ancient Lake and Perhaps Life |url=http://www.nytimes.com/2013/12/10/science/space/on-mars-an-ancient-lake-and-perhaps-life.html |date=December 9, 2013 |work=[[New York Times]]|accessdate=December 9, 2013 }}</ref><ref name="SCI-20131209">{{cite journal |authors=Various |title=Science - Special Collection - Curiosity Rover on Mars |url=http://www.sciencemag.org/site/extra/curiosity/ |date=December 9, 2013 |journal=[[Science (journal)|Science]] |accessdate=December 9, 2013 }}</ref> |
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On January 24, 2014, NASA reported that [[Timeline of Mars Science Laboratory#Current status|current studies]] on the planet [[Mars]] by the [[Curiosity (rover)|''Curiosity'']] and [[Opportunity (rover)|''Opportunity'']] [[Mars rover|rovers]] will now be searching for evidence of ancient life, including a [[biosphere]] based on [[autotroph]]ic, [[chemotroph]]ic and/or [[Lithotroph#Chemolithotrophs|chemolithoautotrophic]] [[microorganism]]s, as well as ancient water, including [[Lacustrine plain|fluvio-lacustrine environments]] ([[plain]]s related to ancient [[river]]s or [[lake]]s) that may have been [[Planetary habitability|habitable]].<ref name="SCI-20140124a">{{cite journal |last=Grotzinger |first=John P. |title=Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars |url=http://www.sciencemag.org/content/343/6169/386 |journal=[[Science (journal)|Science]] |date=January 24, 2014 |volume=343 |number=6169 |pages=386–387 |doi=10.1126/science.1249944 |accessdate=January 24, 2014 }}</ref><ref name="SCI-20140124special">{{cite journal |authors=Various |title=Special Issue - Table of Contents - Exploring Martian Habitability |url=http://www.sciencemag.org/content/343/6169.toc#SpecialIssue |date=January 24, 2014|journal=[[Science (journal)|Science]] |volume=343 |number=6169 |pages=345–452 |accessdate=24 January 2014 }}</ref><ref name="SCI-20140124">{{cite journal |authors=Various |title=Special Collection - Curiosity - Exploring Martian Habitability |url=http://www.sciencemag.org/site/extra/curiosity/|date=January 24, 2014 |journal=[[Science (journal)|Science]] |accessdate=January 24, 2014 }}</ref><ref name="SCI-20140124c">{{cite journal |authors=Grotzinger, J.P. et al. |title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |url=http://www.sciencemag.org/content/343/6169/1242777 |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |number=6169 |doi=10.1126/science.1242777 |accessdate=January 24, 2014 }}</ref> The search for evidence of [[Planetary habitability|habitability]], [[taphonomy]] (related to [[fossils]]), and [[organic carbon]] on the planet [[Mars]] is now a primary [[NASA]] objective.<ref name="SCI-20140124a" /> |
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===Ceres=== |
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[[Ceres (dwarf planet)|Ceres]], a dwarf planet, has recently been confirmed by the [[Herschel Space Observatory]] to have water vapor in its atmosphere.<ref name="Kuppers2014">{{cite journal |last1=Küppers |first1=M. |last2=O'Rourke |first2=L. |last3=Bockelée-Morvan |first3=D. |last4=Zakharov |first4=V. |last5=Lee |first5=S. |last6=Von Allmen |first6=P. |last7=Carry |first7=B. |last8=Teyssier |first8=D. |last9=Marston |first9=A. |last10=Müller |first10=T. |last11=Crovisier |first11=J. |last12=Barucci |first12=M. A. |last13=Moreno |first13=R. |title=Localized sources of water vapour on the dwarf planet (1) Ceres |journal=Nature |volume=505 |issue=7484 |date=2014-01-23 |pages=525–527 |issn=0028-0836 |doi=10.1038/nature12918 |pmid=24451541 }}</ref><ref name="Campins2014">{{Cite doi|10.1038/505487a}}</ref> Frost on the surface may also have been detected.<ref name="Ahearn1992">{{cite journal|last1=A'Hearn|first1=Michael F.|author2=Feldman, Paul D.|title=Water vaporization on Ceres|journal=Icarus|volume=98|issue=1|pages=54–60|year=1992|doi=10.1016/0019-1035(92)90206-M|bibcode= 1992Icar...98...54A}}</ref> The presence of water, and the temperatures on Ceres has led to speculation that life may be possible there.<ref>{{cite web|url=http://www.universetoday.com/26587/life-on-ceres-could-the-dwarf-planet-be-the-root-of-panspermia/|title=Life on Ceres: Could the Dwarf Planet be the Root of Panspermia|date=5 March 2009|author=O'Neill, Ian|work=Universe Today|accessdate=30 January 2012}}</ref><ref>{{cite book|author=Catling, David C.|year=2013|title=Astrobiology: A Very Short Introduction|publisher=Oxford University Press|location=Oxford|isbn=0-19-958645-4|page=99}}</ref><ref>[http://www.nbcnews.com/science/there-life-ceres-dwarf-planet-spews-water-vapor-space-2D11970722 Is there life on Ceres? Dwarf planet spews water vapor into space.] (22 January 2014)</ref> The [[Dawn (spacecraft)|Dawn]] space probe is scheduled to enter orbit around Ceres in spring 2015.<ref name="DawnDepartedVesta">{{cite web|date=2012-09-05|url=http://dawn.jpl.nasa.gov/feature_stories/dawn_departs_vesta.asp|title='Dawn has departed the giant asteroid Vesta'|publisher=NASA/JPL}}</ref> |
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===Jupiter=== |
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Carl Sagan and others<ref>{{cite journal |
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| author=Ponnamperuma, Cyril; Molton, Peter |
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| title=The prospect of life on Jupiter |
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| journal=Space Life Sciences | volume=4 | issue=1 | pages=32–44 |
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|date=January 1973 |
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| doi=10.1007/BF02626340 |
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| bibcode=1973SLSci...4...32P |
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}}</ref> in the 1960s and 1970s computed conditions for hypothetical [[amino acid]]-based [[macroscopic]] life in the [[atmosphere of Jupiter]], based on observed conditions of this atmosphere. However, the conditions do not appear to permit the type of encapsulation thought necessary for molecular biochemistry, so life is thought to be unlikely.<ref>{{cite journal |
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| author=Irwin, Louis Neal; Schulze-Makuch, Dirk |
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| title=Assessing the Plausibility of Life on Other Worlds |
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| journal=Astrobiology | volume=1 | issue=2 | pages=143–160 |
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|date=June 2001 |
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| doi=10.1089/153110701753198918 |
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| bibcode=2001AsBio...1..143I | pmid=12467118}}</ref> |
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However, some of Jupiter's moons may have habitats capable of sustaining life. Scientists have suggested that heated subsurface oceans of water may exist deep under the crusts of the three outer [[Galilean moons]]—Europa, [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]]. The [[EJSM/Laplace]] mission is planned to determine the habitability of these environments. However, Europa is seen as the main target for the discovery of life. |
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====Europa==== |
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[[File:PIA01130 Interior of Europa.jpg|thumb|250px|Subsurface oceans such as the one pictured of Europa could possibly harbor life.<ref>[http://orca.phys.uvic.ca/~hsiao/talks/hsiao_europa.pdf Possibility of Life on Europa]</ref>]] |
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Jupiter's moon Europa has been subject to speculation about the existence of life due to the strong possibility of liquid water beneath an ice layer. [[Hydrothermal vent]]s on the bottom of the ocean, if they exist, may warm the ice and could be capable of supporting multicellular [[microorganism]]s.<ref name="EuropaPlanetary">{{Cite journal |
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| first1=Louis | last1=Friedman |
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| date=December 14, 2005 |
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| title=Projects: Europa Mission Campaign |
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| publisher=The Planetary Society |
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| url=http://www.planetary.org/programs/projects/explore_europa/update_12142005.html |
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| accessdate=2011-08-08 |
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| postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}} }}</ref> It is also possible that Europa could support aerobic macrofauna using oxygen created by cosmic rays impacting its surface ice.<ref>{{cite web|title=Europa Capable of Supporting Life, Scientist Says|author= Nancy Atkinson|year=2009|publisher=Universe Today|url=http://www.universetoday.com/2009/10/08/europa-capable-of-supporting-life-scientist-says/|accessdate=2011-08-18}}</ref> |
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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.<ref>Phil Plait, [http://blogs.discovermagazine.com/badastronomy/2011/11/17/huge-lakes-of-water-may-exist-under-europas-ice/ "Huge lakes of water may exist under Europa's ice"], "Bad Astronomy Blog"</ref><ref>[http://www.jsg.utexas.edu/news/2011/11/scientists-find-evidence-for-great-lake-on-europa/ "SCIENTISTS FIND EVIDENCE FOR "GREAT LAKE" ON EUROPA AND POTENTIAL NEW HABITAT FOR LIFE"]</ref> |
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On December 11, 2013, NASA reported the detection of "[[Clay minerals|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with [[organic materials]], on the icy crust of [[Europa (moon)|Europa]], a moon of [[Jupiter]].<ref name="NASA-20131211">{{cite web |last=Cook |first=Jia-Rui c. |title=Clay-Like Minerals Found on Icy Crust of Europa |url=http://www.jpl.nasa.gov/news/news.php?release=2013-362 |date=December 11, 2013 |work=[[NASA]] |accessdate=December 11, 2013 }}</ref> The presence of the minerals may have been the result of a collision with an [[asteroid]] or [[comet]] according to the scientists.<ref name="NASA-20131211" /> |
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===Saturn=== |
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Although astronomers consider Saturn inhospitable to life,{{citation needed|date=December 2013}} its natural satellites Titan and Enceladus have been speculated to possess possible habitats for life. |
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====Titan==== |
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{{Main|Life on Titan}} |
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[[Titan (moon)|Titan]], the largest [[Moons of Saturn|moon of Saturn]], is the only known moon with a significant atmosphere. Data from the [[Cassini–Huygens]] mission refuted the hypothesis of a global [[hydrocarbon]] ocean, but later demonstrated the existence of [[Lakes of Titan|liquid hydrocarbon lake]]s in the polar regions—the first stable bodies of surface liquid discovered outside 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> Analysis of data from the mission has uncovered aspects of atmospheric chemistry near the surface which are consistent with—but do not prove—the hypothesis that [[Life on Titan|organisms there]] are consuming hydrogen, acetylene and ethane, and producing methane.<ref name=methlife>{{cite web|title= What is Consuming Hydrogen and Acetylene on Titan?|publisher=NASA/JPL|year=2010 |accessdate=2010-06-06|url=http://www.jpl.nasa.gov/news/news.cfm?release=2010-190}}</ref><ref>{{cite journal|title=Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions|author=Darrell F. Strobel|journal=Icarus|volume=208|chapter=In press|page=878|year=2010|doi=10.1016/j.icarus.2010.03.003|bibcode=2010Icar..208..878S|issue=2}}</ref><ref name=mckay>{{cite journal|journal = Icarus|volume= 178|issue = 1|pages = 274–276|year= 2005|doi = 10.1016/j.icarus.2005.05.018|title = Possibilities for methanogenic life in liquid methane on the surface of Titan|author = McKay, C. P.; Smith, H. D.|bibcode=2005Icar..178..274M}}</ref> |
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An alternate explanation for the hypothetical existence of microbial life on [[Titan (moon)|Titan]] has already been formally proposed<ref>{{cite news| url=http://news.bbc.co.uk/1/hi/sci/tech/4819370.stm |title=Earth could seed Titan with life |publisher=BBC News |date=March 18, 2006 |accessdate=2007-03-10 | first=Paul | last=Rincon}}</ref><ref>{{cite journal | author = Gladman, Brett; Dones, Luke; Levinson, Harold F.; Burns, Joseph A. | title = Impact Seeding and Reseeding in the Inner Solar System | year = 2005 | journal = Astrobiology | volume = 5 | pages = 483–496 |doi=10.1089/ast.2005.5.483 | pmid = 16078867 | issue = 4 | bibcode=2005AsBio...5..483G}}</ref>—hypothesizing that microorganisms could have left Earth when it suffered a massive [[asteroid]] or [[comet]] [[impact event|impact]] (such as the impact that created [[Chicxulub crater]] only 66 [[mya (unit)|mya]]), and survived a journey through [[space]] to land on Titan. |
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====Enceladus==== |
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[[Enceladus]], a moon of Saturn, has some of the conditions for life including geothermal activity and water vapor as well as possible under-ice oceans heated by tidal effects. The Cassini probe detected carbon, hydrogen, nitrogen and oxygen—all key elements for supporting living organisms—during a fly-by through one of Enceladus's geysers spewing ice and gas in 2005. The temperature and density of the plumes indicate a warmer, watery source beneath the surface. However, no life has been confirmed.<ref name="news.discovery"/> |
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===Small Solar System bodies=== |
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[[Small Solar System body|Small Solar System bodies]] have also been suggested as habitats for extremophiles. [[Fred Hoyle]] has proposed that microbial life might exist on [[comet]]s.<ref>Hoyle, Fred, ''Evolution from Space'', Omni Lecture, Royal Institution, London, 12 January 1982; ''Evolution from Space'' (1982) pp. 27–28 ISBN 0-89490-083-8; ''Evolution from Space: A Theory of Cosmic Creationism'' (1984) ISBN 0-671-49263-2</ref> Live bacteria were [[Reports of Streptococcus mitis on the Moon|found on the camera of the Surveyor 3 probe]] that had stayed on the surface of the Moon for two and a half years.{{citation needed|date=January 2014}} This finding was later considered doubtful as sterile procedures may not have been fully followed. |
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==Scientific search== |
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[[File:Keplerspacecraft-20110215.jpg|thumb|230px|The NASA ''Kepler'' for the search of extrasolar planets.]] |
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The scientific search for extraterrestrial life is being carried out both directly and indirectly. |
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===Direct search=== |
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{{see also|Life#Extraterrestrial life|List of molecules in interstellar space}} |
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Scientists are directly searching for [[biosignature]]s within the [[Solar System]], carrying out studies on the surface of Mars and examining [[Meteoroid|meteors]] which have fallen to Earth. At the moment, no concrete plan exists for exploration of Europa for life. In 2008, a joint mission by NASA and the European Space Agency was announced that would have included studies of Europa.<ref>[http://opfm.jpl.nasa.gov/, "Outer Planet Flagship Mission" ], "Jet Propulsion Laboratory"</ref> However, in 2011 NASA was forced to deprioritize the mission due to a lack of funding, and it is possible that the ESA will take on the mission by itself.<ref>[http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=48661, "New approach for L-class mission candidates"], "ESA"</ref> |
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There is some limited evidence that microbial life might possibly exist (or have existed) on Mars.<ref>[https://web.archive.org/web/20090910024302/http://www.spherix.com/mars.html Spherix Mars Research (archived page)]</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 lander 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 controversial. |
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[[File:ALH84001 structures.jpg|thumb|230px|Electron micrograph of martian meteorite [[ALH84001]] showing structures that some scientists think could be fossilized bacteria-like life forms.]] |
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In February 2005, NASA scientists reported that they may have found some 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| year=2005}}</ref> The two scientists, Carol Stoker and Larry Lemke of NASA's [[Ames Research Center]], based their claim 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 distanced NASA from 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| year=2005}}</ref> |
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Though such methane findings are still very much in debate, support among some scientists for the existence of life on Mars seems to be growing: an informal survey conducted at the conference at which the [[European Space Agency]] presented its findings on methane in Mars' atmosphere, indicated that 75% of the people present agreed that bacteria once lived on Mars. Roughly 25% agreed that bacteria inhabit the planet today.<ref name="Spotts">{{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> |
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In November 2011, NASA launched the [[Mars Science Laboratory]] (MSL) rover which is designed to search for past or present habitability on Mars using a variety of scientific instruments. The MSL landed on Mars at [[Gale (crater)|Gale Crater]] in August 2012.<ref name="Gale Crater">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA's Next Mars Rover To Land At Gale Crater |date=22 July 2011 |publisher=[[NASA JPL]] |url=http://www.jpl.nasa.gov/news/news.cfm?release=2011-222#1 |accessdate=2011-07-22 }}</ref><ref name="Gale Crater2">{{cite web |last1= Chow |first1=Dennis |title=NASA's Next Mars Rover to Land at Huge Gale Crater |url=http://www.space.com/12394-nasa-mars-rover-landing-site-unveiled.html |date=22 July 2011 |publisher=[[Space.com]] |accessdate=2011-07-22 }}</ref><ref name="Gale Crater3">{{cite news |last1=Amos |first1=Jonathan |title=Mars rover aims for deep crater |date=22 July 2011 |url=http://www.bbc.co.uk/news/science-environment-14249524 |work=[[BBC News]] |accessdate = 2011-07-22}}</ref> |
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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. |
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In March 2011, [[Richard B. Hoover]], an astrobiologist with the [[Marshall Space Flight Center]], speculated on the finding of alleged [[microfossil]]s similar to [[cyanobacteria]] in [[CI1 fossils|CI1]] carbonaceous [[meteorites]].<ref name=Tenney>{{cite news |last=Tenney |first=Garrett |title=Exclusive: NASA Scientist Claims Evidence of Alien Life on Meteorite |url=http://www.foxnews.com/scitech/2011/03/05/exclusive-nasa-scientists-claims-evidence-alien-life-meteorite/ |publisher=[[FoxNews]] |date=5 March 2011 |accessdate=2011-03-06 }}</ref><ref name=Hoover>{{cite journal |title=Fossils of Cyanobacteria in CI1 Carbonaceous Meteorites: Implications to Life on Comets, Europa, and Enceladus |last=Hoover |first=Richard B. |journal=Journal of Cosmology |volume=13 |pages=xxx |year=2011 |url=http://journalofcosmology.com/Life100.html |accessdate=2011-03-06 }}</ref> However, NASA formally distanced itself from Hoover's claim.<ref>{{cite news | first = Kerry Sheridan | title = NASA shoots down alien fossil claims | date = 7 March 2011 | url =http://www.abc.net.au/news/stories/2011/03/08/3157645.htm | work = ABC News| accessdate = 2011-03-07}}</ref><ref name=Borenstein>{{cite web|last=Borenstein |first=Seth |title=Scientists skeptical of meteorite alien life claim|url=http://www.starnewsonline.com/article/20110307/APA/1103071081|publisher=Associated Press (on [http://www.starnewsonline.comStarnewsonline.com]) |date=7 March 2011 |accessdate=2011-03-07 }}</ref><ref name="Redfield">{{cite web |last=Redfield |first=Rosemary |title=Is this claim of bacteria in a meteorite any better than the 1996 one?|work=[http://rrresearch.blogspot.com/ RR Research blog] |date=6 March 2011|url=http://rrresearch.blogspot.com/2011/03/is-this-claim-of-bacteria-in-meteorite.html|accessdate=7 March 2011 }}</ref> See [[Hoover paper controversy]] for more details. |
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In August 2011, findings by NASA, based on studies of [[meteorites]] found on Earth, suggests DNA and [[RNA]] components ([[adenine]], [[guanine]] and related [[organic molecules]]), building blocks for life as we know it, may be formed extraterrestrially in [[outer space]].<ref name="Callahan">{{cite web |last1=Callahan |first1=M.P. |last2=Smith |first2=K.E. |last3=Cleaves |first3=H.J. |last4=Ruzica |first4=J. |last5=Stern |first5=J.C. |last6=Glavin |first6=D.P. |last7=House |first7=C.H. |last8=Dworkin |first8=J.P. |date=11 August 2011 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |url=http://www.pnas.org/content/early/2011/08/10/1106493108 |publisher=[[PNAS]] |doi=10.1073/pnas.1106493108 |accessdate=2011-08-15 }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=[[NASA]] |date=8 August 2011 |accessdate=2011-08-10 }}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests |url=http://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |publisher=[[ScienceDaily]] |accessdate=2011-08-09}}</ref> In October 2011, scientists reported that [[cosmic dust]] contains complex [[organic compound|organic]] matter ("amorphous organic solids with a mixed [[aromatic]]-[[aliphatic]] structure") that could be created naturally, and rapidly, by [[stars]].<ref name="Space-20111026">{{cite web |last=Chow |first=Denise |title=Discovery: Cosmic Dust Contains Organic Matter from Stars |url=http://www.space.com/13401-cosmic-star-dust-complex-organic-compounds.html |date=26 October 2011 |publisher=[[Space.com]] |accessdate=2011-10-26 }}</ref><ref name="ScienceDaily-20111026">{{cite web |author=[[ScienceDaily]] Staff |title=Astronomers Discover Complex Organic Matter Exists Throughout the Universe |url=http://www.sciencedaily.com/releases/2011/10/111026143721.htm |date=26 October 2011 |publisher=[[ScienceDaily]] |accessdate=2011-10-27 }}</ref><ref name="Nature-20111026">{{cite journal |last1=Kwok |first1=Sun |last2=Zhang |first2=Yong |title=Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features |date=26 October 2011 |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature10542 |volume=479 |issue=7371 |page=80 |bibcode = 2011Natur.479...80K |pmid=22031328}}</ref> 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."<ref name="Space-20111026"/> |
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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 [[protostar|protostellar]] binary ''IRAS 16293-2422'', which is located 400 light years from Earth.<ref name="NG-20120829">{{cite journal|title=Sugar Found In Space|journal=National Geographic |last=Than |first=Ker |date=August 29, 2012 |url=http://news.nationalgeographic.com/news/2012/08/120829-sugar-space-planets-science-life/ |accessdate=August 31, 2012 }}</ref><ref name="AP-20120829">{{cite web |author=Staff |title=Sweet! Astronomers spot sugar molecule near star |url=http://apnews.excite.com/article/20120829/DA0V31D80.html |date=August 29, 2012 |publisher=[[AP News]] |accessdate=August 31, 2012 }}</ref> 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.<ref>{{cite journal|title=Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA|author=Jørgensen, J. K.|coauthors=Favre, C.; Bisschop, S.; Bourke, T.; Dishoeck, E.; Schmalzl, M.|version=eprint |year=2012|url=http://www.eso.org/public/archives/releases/sciencepapers/eso1234/eso1234a.pdf}}</ref> |
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In September 2012, NASA scientists reported that [[polycyclic aromatic hydrocarbons|polycyclic aromatic hydrocarbons (PAHs)]], subjected to [[Interstellar medium|interstellar medium (ISM)]] conditions, are transformed, through [[hydrogenation]], [[Oxygenate|oxygenation]] and [[hydroxylation]], to more complex [[Organic compound|organics]] - "a step along the path toward [[amino acids]] and [[nucleotides]], the raw materials of [[proteins]] and DNA, respectively".<ref name="Space-20120920">{{cite web |author=Staff |title=NASA Cooks Up Icy Organics to Mimic Life's Origins |url=http://www.space.com/17681-life-building-blocks-nasa-organic-molecules.html |date=September 20, 2012 |publisher=[[Space.com]] |accessdate=September 22, 2012 }}</ref><ref name="AJL-20120901">{{cite journal |last1=Gudipati |first1=Murthy S. |last2=Yang |first2=Rui |title=In-Situ Probing Of Radiation-Induced Processing Of Organics In Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-Of-Flight Mass Spectroscopic Studies |url=http://iopscience.iop.org/2041-8205/756/1/L24 |date=September 1, 2012 |journal=[[The Astrophysical Journal Letters]] |volume=756 |number=1 |doi=10.1088/2041-8205/756/1/L24 |accessdate=September 22, 2012 |bibcode = 2012ApJ...756L..24G }}</ref> Further, as a result of these transformations, the PAHs lose their [[Spectroscopy|spectroscopic signature]] which could be one of the reasons "for the lack of PAH detection in [[interstellar ice]] [[Cosmic dust#Dust grain formation|grains]], particularly the outer regions of cold, dense clouds or the upper molecular layers of [[protoplanetary disks]]."<ref name="Space-20120920" /><ref name="AJL-20120901" /> |
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In January 2013, Chandra Wickramasinghe reported in the [[fringe science]] [[Journal of Cosmology]], of shapes resembling fossil [[diatom]] [[frustule]]s in a new carbonaceous meteorite that landed in the North Central Province of Sri Lanka on 29 December 2012.<ref name="JCosmology-20130110">{{Cite journal |first=N.C. |last=Wickramasinghe |coauthors=Wallis, J.; Wallis, D. H.; Samaranayake, Anil |authorlink=Chandra Wickramasinghe |title=Fossil Diatoms in a New Carbonaceous Meteorite |date=January 10, 2013 |url=http://www.buckingham.ac.uk/wp-content/uploads/2011/09/Polonnaruwa-meteorite.pdf |publisher=[[Journal of Cosmology]] |volume=21 |number=37 |pages=1–14 |accessdate=January 14, 2013 }}</ref> |
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On February 21, 2014, [[NASA]] announced a [http://www.astrochem.org/pahdb/ greatly upgraded database] for tracking [[polycyclic aromatic hydrocarbons]] (PAHs) in the [[universe]]. According to scientists, more than 20% of the [[carbon]] in the universe may be associated with PAHs, possible [[PAH world hypothesis|starting materials]] for the [[Abiogenesis#PAH world hypothesis|formation]] of [[Life#Extraterrestrial life|life]]. PAHs seem to have been formed shortly after the [[Big Bang]], are widespread throughout the universe, and are associated with [[Star formation|new stars]] and [[exoplanets]].<ref name="NASA-20140221">{{cite web |last=Hoover |first=Rachel |title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That |url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |date=February 21, 2014 |work=[[NASA]] |accessdate=February 22, 2014 }}</ref> |
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===Indirect search=== |
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[[File:Terrestrial Planet Finder PIA04499.jpg|thumb|230px|[[Terrestrial Planet Finder]]]] |
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If there is an advanced extraterrestrial society, there is no guarantee that they are transmitting information in the direction of Earth or that this information could be interpreted as such by humans.{{Citation needed|date=February 2012}} The length of time required for a signal to travel across the vastness of space means that any signal detected, or not detected, would come from the distant past.{{citation needed|date=July 2012}} |
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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. This results in radio transmission as the method of choice.{{citation needed|date=July 2012}} |
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Some have hypothesized that very advanced civilizations may create artificial black holes as an energy source or method of waste disposal. Thus, they suggest that the observation of a black hole with a mass of less than 3.5 solar masses, the theoretical lower mass limit for a naturally occurring black hole, would be evidence of an alien civilization.<ref>Ray Villard, [http://news.discovery.com/space/super-civilizations-might-live-off-black-holes-110430.html "Super-Civilizations Might Live Off Black Holes"], "Discovery News"</ref> |
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===Extrasolar planets=== |
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{{Main|Extrasolar planets}} |
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{{See also|List of planetary systems}} |
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Astronomers search for extrasolar planets that may be conducive to life, narrowing the search to terrestrial planets within the habitable zone.<ref name="Earth-likeplanet1">{{cite web |url=http://planet.iap.fr/OB05390.news.html |date=25 January 2006 |title=Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet |work=IAP.fr |postscript=<!--None--> }}</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, since 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]] – and would reveal key information, such as the presence of free [[oxygen]] in a planet's atmosphere: |
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[[File:Glieseupdated.jpg|thumb|230px|Artist's Impression of [[Gliese 581 c]], the first terrestrial extrasolar planet discovered within its star's habitable zone.]] |
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*[[Darwin (ESA)|Darwin]] was a proposed ESA mission designed to find Earth-like planets and analyze their atmosphere. |
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*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. |
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*The [[Terrestrial Planet Finder]] was supposed to have been launched by NASA, but as of 2011, budget cuts have caused it to be delayed indefinitely. |
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*The ''Kepler'' mission, largely replacing the Terrestrial Planet Finder, was launched in March 2009. |
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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> |
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On April 24, 2007, scientists at the [[La Silla Observatory|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 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 581 c 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 the star's traditional habitable zone.<ref name="Hopes Dim for life on distant planet">{{cite news| url=http://www.usatoday.com/tech/science/space/2007-06-18-earthlike-planet-questioned_N.htm | work=USA Today | title=Hopes dim for life on distant planet | date=2007-06-18 | accessdate=2010-05-02 | first=Ker | last=Than}}</ref> |
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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">{{cite news| url=http://news.bbc.co.uk/2/hi/science/nature/6699893.stm | work=BBC News | title=Planet hunters spy distant haul | date=2007-05-29 | accessdate=2010-05-02}}</ref> |
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In May 2011, researchers predicted that Gliese 581 d, not only exists in the "[[Circumstellar habitable zone|Goldilocks zone]]" where water can be present in liquid form, but is big enough to have a stable carbon dioxide atmosphere and "warm enough to have oceans, clouds, and rainfall," according to France's National Centre for Scientific Research.<ref>{{cite web|author=View all 26 comments Leave a comment |url=http://www.newser.com/story/118825/gliese-581d-identified-as-habitable-exoplanet.html |title=Scientists ID 'Habitable' Exoplanet — New model suggests Gliese 581D a likely candidate for life |publisher=Newser.com |date=2011-05-18 |accessdate=2012-06-13}}</ref> |
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In December 2011, NASA confirmed that 600-light-year distant [[Kepler-22b]], at 2.4 times the radius of Earth, is potentially the closest match to Earth in terms of both size and temperature.<ref name="bbc20111205">{{cite news|title=Kepler 22-b: Earth-like planet confirmed|url=http://www.bbc.co.uk/news/science-environment-16040655|publisher=BBC|accessdate=5 December 2011|date=2011-12-05}}</ref><ref name="NASA20111205_2011_373">{{cite news | work=NASA Press Release | title=NASA's Kepler Confirms Its First Planet In Habitable Zone | date=2011-12-05 | url=http://www.jpl.nasa.gov/news/news.cfm?release=2011-373 }}</ref> |
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Since 1992, hundreds of [[planets]] around other stars ("extrasolar planets" or "[[exoplanets]]") in the [[Milky Way|Milky Way Galaxy]] have been discovered. As of {{Extrasolar planet counts|asof|mdy}}, the [[Extrasolar Planets Encyclopaedia]] identified {{Extrasolar planet counts|planet_count}} [[extrasolar planets]] (in {{Extrasolar planet counts|system_count}} [[planetary systems]] and {{Extrasolar planet counts|multiplanetsystem_count}} [[List of multiplanetary systems|multiple planetary systems]]); the extrasolar planets range in size from that of [[terrestrial planet]]s similar to Earth to that of gas giants larger than Jupiter.<ref name="Encyclopaedia">{{cite web|title=Interactive Extra-solar Planets Catalog |work=[[The Extrasolar Planets Encyclopaedia]] |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=10 September 2011 |accessdate=2012-01-30 }}</ref> The number of observed exoplanets is expected to increase greatly in the coming years. Because the ''Kepler'' spacecraft must view three stellar transits by exoplanets before it identifies them as candidate planets, it has so far only been able to identify planets that orbit their star at a relatively quick rate. The mission is expected to continue until at least 2016, in which time many more exoplanet candidates are expected to be found.<ref>Mike Wall, [http://www.csmonitor.com/Science/2012/0405/NASA-s-Kepler-observatory-to-continue-hunt-for-strange-new-worlds-video, "NASA's Kepler Observatory to continue hunt for strange new worlds"], "The Christian Science Monitor"</ref> |
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Despite these successes, the transit method employed by the ''Kepler'' spacecraft requires that planetary orbits be at a small inclination to the line of sight of the observer. Due to this constraint, the probability of detecting a planet of Earth's size and orbital radius around a distant star is just 0.47%. Thus, the number of planets we are currently able to detect is only a small fraction of the total number of planets present within the galaxy.<ref>[http://kepler.nasa.gov/Science/about/characteristicsOfTransits/ "Kepler: A Search for Habitable Planets"], "NASA Ames Research Center"</ref> |
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==The Drake equation== |
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{{Main|Drake equation}} |
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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: |
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* The rate of formation of suitable stars. |
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* The fraction of those stars which are orbited by planets. |
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* The number of Earth-like worlds per [[planetary system]]. |
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* The fraction of planets where intelligent life develops. |
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* The fraction of possible communicative planets. |
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* The "lifetime" of possible communicative civilizations. |
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Criticism of the Drake equation follows mostly from the observation that the terms in the equation are entirely based on conjecture. Thus the equation cannot be used to draw firm conclusions of any kind.<ref>{{cite book |author= [[Jack Cohen (scientist)|Jack Cohen]] and [[Ian Stewart (mathematician)|Ian Stewart]] |title=[[Evolving the Alien]] |publisher=John Wiley and Sons, Inc., Hoboken, NJ |year=2002}} Chapter 6, ''What does a Martian look like?''</ref> Although the Drake equation currently involves speculation about unmeasured parameters, it was not meant to be science, but intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.<ref>{{cite web|url=http://www.planetary.org/explore/topics/seti/seti_history_07.html|first=Amir|last=Alexander|title=The Search for Extraterrestrial Intelligence: A Short History — Part 7: The Birth of the Drake Equation}}</ref> |
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Drake used the equation to estimate that there are approximately 10,000 planets in the [[Milky Way]] galaxy containing intelligent life with the possible capability of communicating with Earth.<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> |
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Based on observations from the [[Hubble Space Telescope]], there are at least 125 billion galaxies in the observable Universe. It is estimated that at least ten percent of all sun-like stars have a system of planets,<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=24–42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi=10.1143/PTPS.158.24 |arxiv = astro-ph/0505003 |bibcode = 2005PThPS.158...24M }}</ref> i.e. there are 6.25×10<sup>18</sup> stars with planets orbiting them in the observable Universe. Even if we assume that only one out of a billion of these stars have planets supporting life, there would be some 6.25×10<sup>9</sup> (billion) life-supporting planetary systems in the observable Universe. |
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The apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for,<ref name='Vinn2014'>{{cite journal |
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| doi = |
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| title = Potential incompatibility of inherited behavior patterns with civilization |
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| year = 2014 |
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| author = Vinn, O |
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| journal = PublishResearch |
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| volume = |
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| pages = 1–3 |
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| url = http://www.publishresearch.com/publication/203 |
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| accessdate = 2014-03-05}}</ref> or contact with, such civilizations is known as the [[Fermi paradox]]. |
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== Cultural impact == |
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===Ancient and medieval ideas=== |
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{{main|Cosmic pluralism}} |
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[[File:Vittore carpaccio, scuola degli albanesi, annunciazione, ca' d'oro.jpg|thumb|Anunciacion by Carpaccio]] |
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In antiquity, it was common to assume a cosmos consisting of "many worlds" inhabited by intelligent, non-human life-forms, but these "worlds" were [[mythology|mythological]] and not informed by the [[heliocentric]] understanding of the [[solar system]], or the understanding of the [[Sun]] as one among countless [[star]]s.<ref>[http://solar-center.stanford.edu/FAQ/Qsunasstar.html ''Who discovered that the Sun was a Star?''] Stanford Solar Center.</ref> An example would be the fourteen [[Loka]] of Hindu cosmology, or the [[Nine Worlds]] of Old Norse mythology, etc. |
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The [[Sun]] and the [[Moon]] often appear as inhabited worlds in such contexts, or as vehicles (chariots or boats, etc.) driven by gods. The Japanese folk tale of ''[[The Tale of the Bamboo Cutter]]'' (10th century) is an example of a princess of the Moon people visiting Earth. |
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[[Buddhist]] and [[Hindu]] beliefs of endlessly repeated cycles of life called Samsara 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 Buddhist and Hindu scriptures, there are numerous universes to facilitate the fulfillment of the separated desires of the numerous living entities. However, the purpose of such creations is to bring back the deluded souls to correct understanding about the purpose of life and achieve Nirvana. Aside from the numerous universes which are material, there is an unlimited number spirit worlds, where the purified living entities live with perfect conception about life and ultimate reality. 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.{{Citation needed|date=July 2011}} |
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The Jewish [[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 humans 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> |
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According to [[Ahmadiyya]] 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> |
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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 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 Kues]] speculated about aliens on the Moon and [[Sun]].<ref>{{cite news| url=http://www.challzine.net/29/29extraterr.html| publisher=Challenger 29| title=The Man Who Invented Extraterrestrials: Nicholas of Cusa | first=Richard | last=Dengrove | date= Winter 2008 | accessdate=2010-08-27}}</ref> [[William Vorilong]] also speculated about the existence of humans on alien worlds, but he came to the conclusion that God, although empowered to create them, would choose to not do so. |
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===Early modern period=== |
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[[File:Giordano Bruno.jpg|thumb|[[Giordano Bruno]], ''De l'Infinito Universo et Mondi, 1584'']] |
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There was a dramatic shift in thinking initiated by the invention of the [[telescope]] and the [[Copernican Revolution|Copernican]] assault on geocentric cosmology. Once it became clear that the 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".<ref>[http://www.positiveatheism.org/hist/brunoiuw0.htm#IUW0III Giordano Bruno: On the Infinite Universe and Worlds (De l'Infinito Universo et Mondi) Introductory Epistle: Argument of the Third Dialogue]</ref> |
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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> |
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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. |
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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). |
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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". [[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. |
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The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated. [[William Herschel]], the discoverer of [[Uranus]], was one of many 18th–19th-century astronomers convinced that the [[Solar System]], 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. |
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===19th century=== |
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Since the 1830s, [[Mormons]] have believed that God has created and will create many Earth-like planets on which humans live.<ref>[http://www.lds.org/scriptures/dc-testament/dc/76.23-24?lang=eng#22 Doctrine and Covenants 76:23-24]</ref> They believe that all of these people are children of God. [[Joseph Smith]], the founder of [[The Church of Jesus Christ of Latter-day Saints]], taught that God revealed this information to [[Moses]] and that the Creation account written by Moses corresponded only to "our" earth.<ref>[http://www.lds.org/scriptures/pgp/moses/1.33?lang=eng#32 Moses 1:33]</ref> There is no official doctrine relating to the location or commonality of these inhabited planets.<ref>[http://www.ldsces.org/content/talks/general/2002-maxwell-our-creators-cosmos__eng.pdf Neal A. Maxwell, ''Our Creator's Cosmos'']</ref> |
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Speculation about life on Mars increased in the late 19th century, following telescopic observation by some observers of apparent [[Martian canal]]s — which were however soon found to be optical illusions. Despite this, in 1895, American astronomer [[Percival Lowell]] published his book ''Mars,'' followed by ''Mars and its Canals'' in 1906, proposing that the canals were the work of a long-gone civilization.<ref>''Is Mars habitable? A critical examination of Professor Percival Lowell's book "Mars and its canals''.", an alternative explanation, by Alfred Russel Wallace, F.R.S., etc.'' London, Macmillan and co., 1907.</ref> This idea led British writer [[H. G. Wells]] to write ''[[The War of the Worlds (novel)|The War of the Worlds]]'' in 1897, telling of an invasion by aliens from Mars who were fleeing the planet's desiccation. |
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[[Spectroscopy|Spectroscopic]] analysis of Mars' 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]].<ref name="chambers">{{Cite book|first=Paul|last=Chambers|title=Life on Mars; The Complete Story|place=London|publisher=Blandford|year=1999|doi=|isbn=0-7137-2747-0}}</ref> |
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By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis. |
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The [[science fiction]] genre, although not so named during the time, develops during the late 19th century. [[Jules Verne]]'s ''[[Around the Moon]]'' (1870) features a discussion of the possibility of life on the Moon, but with the conclusion that it is barren. |
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Stories involving extraterrestrials are found in e.g. [[Garrett P. Serviss]]'s ''[[Edison's Conquest of Mars]]'' (1897). |
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''[[The War of the Worlds]]'' by H. G. Wells was published in 1898 and stands at the beginning of the popular idea of the "Martian invasion" of Earth prominent in 20th-century pop culture. |
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===20th century=== |
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{{see also|Space exploration}} |
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[[File: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.]] A [[The War of the Worlds (radio drama)|radio drama version]] of Wells' novel broadcast in 1938 over the [[CBS Radio Network]] led to outrage because it supposedly suggested to many listeners that an actual alien invasion by [[Martian]]s was in progress. |
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In the wake of the [[Roswell UFO incident]] in 1947, [[UFO conspiracy theory|conspiracy theories]] on the presence of extraterrestrials became a widespread phenomenon in the [[United States]] during the 1940s and the beginning [[Space Age]] during the 1950s, accompanied by a surge of [[Unidentified flying object|UFO]] reports. The term UFO itself was coined in 1952 in the context of the enormous popularity of the concept of "[[flying saucer]]s" in the wake of the [[Kenneth Arnold UFO sighting]] in 1947. The [[Majestic 12]] documents published in 1982 suggest that there was genuine interest in UFO conspiracy theories in the US government during the 1940s. |
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The trend to assume that celestial bodies were populated almost by default was tempered as actual probes visited potential alien abodes in the Solar System beginning in the second half of the 20th century, and by the 1970s belief in UFOs had become part of the fringe beliefs associated with the [[paranormal]], [[New Age]], [[Earth mysteries]], [[Forteana]] etc. A number of [[UFO religion]]s developed during the surge in UFO belief during the 1950s to 1970s period, and some, such as [[Scientology]] (founded 1953) and [[Raëlism]] (founded 1974) remain active into the present. The idea of "paleocontact", supposing that extraterrestrials ("[[ancient astronauts]]") have visited the Earth in the remote past and left traces in ancient cultures, appears in early-20th-century fiction such as ''[[The Call of Cthulhu]]'' (1926) and the idea came to be established as a notable aspect of the [[Ufology]] subculture in the wake of [[Erich von Däniken]]'s ''[[Chariots of the Gods?]]'' (1968). [[Alien abduction]] claims were widespread during the 1960s and 1970s in the United States. |
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On the scientific side, the possibility of extraterrestrial life on the Moon was decisively ruled out by the 1960s, and during the 1970s it became clear that most of the other bodies of the Solar System do not harbour highly developed life, although the question of primitive life on bodies in the Solar System remains an open question. Carl Sagan, [[Bruce C. Murray|Bruce Murray]], and [[Louis Friedman]] founded the U.S. [[Planetary Society]], partly as a vehicle for [[SETI]] studies in 1980, and since the 1990s, systematic search for radio signals attributable to intelligent extraterrestrial life has been ongoing. |
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In the early 1990s, NASA was set to join in on SETI research with a planned targeted search and all-sky survey. However, Senator Richard Bryan of Nevada cut funding for the project, and no comparable search has taken place since.<ref>Ray Villard, [http://news.discovery.com/space/the-search-for-extraterrestrial-civilizations-comes-of-age.html "The Search for Extraterrestrial Civilizations Comes of Age"], "Discovery News"</ref> |
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===Recent history=== |
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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, the belief in extraterrestrial beings continues to be voiced in [[pseudoscience]], conspiracy theories, and in popular [[folklore]], notably "[[Sector Four|Area 51]]" and [[Planets proposed in religion and ufology|legends]]. It has become a pop culture trope given less-than-serious treatment in popular entertainment with e.g. the [[ALF (TV series)|ALF]] TV series (1986–1990), ''[[The X-Files]]'' (1993–2002), etc. |
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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> |
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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 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 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. |
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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/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). |
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The possible existence of primitive (microbial) life outside 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. |
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The [[Catholic Church]] has not made a formal ruling on the existence of extraterrestrials. However, writing in the [[Holy See|Vatican]] newspaper, the astronomer, Father [[José Gabriel Funes]], director of the [[Vatican Observatory]] near Rome, said in 2008 that intelligent beings created by [[God]] could exist in outer space.<ref>{{cite news| url=http://news.bbc.co.uk/2/hi/europe/7399661.stm| publisher=news.bbc.co.uk| title=Vatican says aliens could exist | date=2008-05-13 | accessdate=2010-01-04 | first=David | last=Willey}}</ref><ref>{{cite news| url=http://www.washingtonpost.com/wp-dyn/content/article/2009/11/06/AR2009110601899.html| publisher=The Washington Post, November 8, 2009| title=When E.T. phones the pope | first=Marc | last=Kaufman | date=2009-11-08 | accessdate=2010-05-02}}</ref> |
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In September 2010, it was reported that the U.N. General Assembly had appointed [[Mazlan Othman]] as their official extraterrestrial liaison by the UK paper ''The Sunday Times''.<ref name="Venezia">{{Cite news|last=Venezia|first=Todd|title=UN names official space 'host'|work=[[New York Post]]|date= 2010-09-26|url=http://www.nypost.com/p/news/international/un_names_official_space_host_ltMCKbI4WmIPKteaugcanM |accessdate = 2010-10-17}}</ref> This claim was later refuted.<ref>{{cite news| url=http://www.foxnews.com/story/0,2933,601525,00.html | work=Fox News | title=U.N. Denies Appointing 'First Contact' for Visiting Space Aliens | date=2010-09-27}}</ref> |
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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 [[Christopher Columbus|Columbus]] landed in [[Americas|America]], which didn't turn out well for the [[Indigenous peoples of the Americas|Native Americans]]", he said.<ref>{{cite news| url=http://news.bbc.co.uk/2/hi/uk_news/8642558.stm | work=BBC News | title=Hawking warns over alien beings | date=2010-04-25 | accessdate=2010-05-02}}</ref> [[Jared Diamond]] has expressed similar concerns.<ref>Diamond, Jared. "The Third Chimpanzee", Harper Perennial, 2006, Chapter 12.</ref> Scientists at NASA and [[Penn State University]] published a paper in April 2011 addressing the question "Would contact with extraterrestrials benefit or harm humanity?" The paper describes positive, negative and neutral scenarios.<ref>Baum, Seth; Haqq-Misra, Jacob; Domagal-Goldman, Shawn. [http://arxiv.org/ftp/arxiv/papers/1104/1104.4462.pdf "Would Contact with Extraterrestrials Benefit or Harm Humanity? A Scenario Analysis"], ''Acta Astronautica, 2011, 68 (11-12):2014-2129'', April 22, 2011, accessed August 18, 2011.</ref> |
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In 2011, Richard Hoover, an astrobiologist at the U.S. Space Flight Center in Alabama, claimed that filaments and other structures in rare meteorites appear to be microscopic fossils of extraterrestrial beings that resemble cyanobacteria—a phylum of photosynthetic bacteria.<ref>{{cite news| url=http://www.guardian.co.uk/science/2011/mar/06/nasa-scientist-evidence-extraterrestrial-life | location=London | work=The Guardian | first=Ian | last=Sample | title=Nasa scientist claims evidence of extraterrestrial life | date=2011-03-06}}</ref> |
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On May 9, 2013, a [http://science.edgeboss.net/wmedia/science/sst2013/SP050913.wvx congressional hearing] by two [[U. S. House of Representatives]] [[United States congressional committee|subcommittees]] discussed "[https://science.house.gov/hearing/subcommittee-space-and-subcommittee-research-joint-hearing-exoplanet-discoveries-have-we Exoplanet Discoveries: Have We Found Other Earths?]", prompted by the discovery of [[exoplanet]] [[Kepler-62f]], along with [[Kepler-62e]] and [[Kepler-62c]]. A related [http://www.sciencemag.org/content/340/6132.toc special issue] of the journal [[Science (journal)|Science]], published earlier, described the discovery of the exoplanets.<ref name="SCI-20130503">{{cite journal |authors=Staff |title=Special Issue: Exoplanets |url=http://www.sciencemag.org/site/special/exoplanets/ |date=May 3, 2013 |journal=[[Science (journal)|Science]] |accessdate=May 18, 2013 }}</ref> |
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==Recent Discovery== |
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Due to the success of NASA's latest interstellar mission [http://files.gamebanana.com/img/ico/sprays/52f98ebb265c8.gif Cropollo 18] on March 21st 2014, the human race has officially made contact with intelligent life [namely Elves, Orcs, Reptilian Humanoids and Feline Drug Dealers] on the planet Nirn. United States President Barack Obama issued a statement shortly after arrival: "Earth's resources are dwindling, we need more land to grow crops. Can't even eat without crops." |
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==See also== |
==See also== |
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{{Portal|Astronomy|Space|Astrobiology}} |
{{Portal|Astronomy|Space|Astrobiology}} |
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Revision as of 00:07, 14 April 2014
1. Looking for extrasolar planets (Image: Kepler spacecraft)
2. Listening for signals (Image: Allen array)
3. Robotic exploration of the Solar System (Image: Curiosity rover).
Extraterrestrial life (from the Latin words: extra ["beyond", or "not of"] and terrestris ["of or belonging to Earth"]) is defined as life that does not originate from Earth. It is often also referred to as alien life, or simply aliens (or space aliens, to differentiate from other definitions of alien or aliens). These forms of life range from simple bacteria-like organisms to beings far more complex than humans. The possibility that sapient beings might also exist extraterrestrially has been proven upon the recent discovery of the planet Nirn .[1]
The development and testing of hypotheses on extraterrestrial life is known as exobiology or astrobiology; the term astrobiology, however, includes the study of life on Earth and Nirn viewed in its astronomical context. Scientists at the National Institutes of Health reported studies that life in the universe may have begun "9.7±2.5 billion years ago", billions of years before the Earth and Nirn was formed, based on extrapolating the "genetic complexity of organisms" [from "major phylogenetic lineages"] to earlier times.[2][3]
Recent Discovery
Due to the success of NASA's latest interstellar mission Cropollo 18 on March 21st 2014, the human race has officially made contact with intelligent life [namely Elves, Orcs, Reptilian Humanoids and Feline Drug Dealers] on the planet Nirn. United States President Barack Obama issued a statement shortly after arrival: "Earth's resources are dwindling, we need more land to grow crops. Can't even eat without crops."
See also
2
References
- ^ Griffin, Dale Warren (14 August 2013). "The Quest for Extraterrestrial Life: What About the Viruses?". Astrobiology (journal). 13 (8): 774–783. Bibcode:2013AsBio..13..774G. doi:10.1089/ast.2012.0959. Retrieved 6 September 2013.
- ^ Sharov, Alexei A.; Gordon, Richard (28 March 2013). "Life Before Earth" (PDF). arXiv. arXiv:1304.3381v1. Retrieved 16 April 2013.
- ^ Sharov, Alexei A. (12 June 2006). "Genome increase as a clock for the origin and evolution of life". Biology Direct. 1: 1–17. doi:10.1186/1745-6150-1-17. PMC 1526419.
{{cite journal}}: CS1 maint: unflagged free DOI (link)
Further reading
- Baird, John C. (1987). The Inner Limits of Outer Space: A Psychologist Critiques Our Efforts to Communicate With Extraterrestrial Beings. Hanover: University Press of New England. ISBN 0-87451-406-1.
- Cohen, Jack; Stewart, Ian (2002). Evolving the Alien: The Science of Extraterrestrial Life. Ebury Press. ISBN 0-09-187927-2.
- Crowe, Michael J. (1986). The Extraterrestrial Life Debate, 1750–1900. Cambridge. ISBN 0-521-26305-0.
- Crowe, Michael J. (2008). The extraterrestrial life debate Antiquity to 1915: A Source Book. University of Notre Dame Press. ISBN 0-268-02368-9.
- Dick, Steven J. (1984). Plurality of Worlds: The Extraterrestrial Life Debate from Democratis to Kant. Cambridge.
- Dick, Steven J. (1996). The Biological Universe: The Twentieth Century Extraterrestrial Life Debate and the Limits of Science. Cambridge. ISBN 0-521-34326-7.
- Dick, Steven J. (2001). Life on Other Worlds: The 20th Century Extraterrestrial Life Debate. Cambridge. ISBN 0-521-79912-0.
- Dick, Steven J.; Strick, James E. (2004). The Living Universe: NASA And the Development of Astrobiology. Rutgers. ISBN 0-8135-3447-X.
- Fasan, Ernst (1970). Relations with alien intelligences – the scientific basis of metalaw. Berlin: Berlin Verlag.
- Goldsmith, Donald (1997). The Hunt for Life on Mars. New York: A Dutton Book. ISBN 0-525-94336-6.
- Grinspoon, David (2003). Lonely Planets: The Natural Philosophy of Alien Life. HarperCollins. ISBN 0-06-018540-6.
- Lemnick, Michael T. (1998). Other Worlds: The Search for Life in the Universe. New York: A Touchstone Book.
- Michaud, Michael (2006). Contact with Alien Civilizations – Our Hopes and Fears about Encountering Extraterrestrials. Berlin: Springer. ISBN 0-387-28598-9.
- Pickover, Cliff (2003). The Science of Aliens. New York: Basic Books. ISBN 0-465-07315-8.
- Roth, Christopher F. (2005). Debbora Battaglia (ed.). Ufology as Anthropology: Race, Extraterrestrials, and the Occult. Durham, NC: Duke University Press.
{{cite book}}:|work=ignored (help) - Sagan, Carl; Shklovskii, I. S. (1966). Intelligent Life in the Universe. Random House.
- Sagan, Carl (1973). Communication with Extraterrestrial Intelligence. MIT Press. ISBN 0-262-19106-7.
- Ward, Peter D. (2005). Life as we do not know it-the NASA search for (and synthesis of) alien life. New York: Viking. ISBN 0-670-03458-4.
- Tumminia, Diana G. (2007). Alien Worlds – Social and Religious Dimensions of Extraterrestrial Contact. Syracuse: Syracuse University Press. ISBN 978-0-8156-0858-5.
External links
Wikimedia Commons has media related to Aliens.
Wikiquote has quotations related to Alien life.
- Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox by Robert A. Freitas Jr.
- 'Is There Anybody Out There?' Freeview video by the Vega Science Trust and the BBC/OU.
- PBS: Life Beyond Earth a film by Timothy Ferris
- PBS: Exploring Space – The Quest for Life by Scott Pearson
- Xenopsychology" by Robert A. Freitas Jr.
- "What Aliens Might Look Like" from National Geographic Channel
- Top stars picked in alien search, from BBC News
- Taiwan's Museum of Alien Studies: a new view of the extraterrestrial, from Erenlai
- The short film "Who's Out There? (1975)" NASA is available for free viewing and download at the Internet Archive.
- Tanzella-Nitti, Giuseppe. "Extraterrestrial Life". Retrieved 23 April 2012.
- "Is it true that there could be intelligent life out there?". physics.org. Retrieved 2 November 2012.
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