Editing Habitable zone (section)

==Significance for complex and intelligent life==
[[File:Deinococcus_radiodurans.jpg|thumb|[[Polyextremophile]] bacteria like [[Deinococcus radiodurans]], are capable of surviving conditions outside the habitable zone]]
The [[Rare Earth hypothesis]] argues that complex and intelligent life is uncommon and that the HZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a HZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for [[multicellular life]] to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics)<ref name="Rare Earth">{{cite book |author1=Brownlee, Donald |author2=Ward, Peter |title=Rare Earth: Why Complex Life Is Uncommon in the Universe |publisher=Copernicus |location=New York |date=2004 |isbn=978-0-387-95289-5}}</ref> and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation).<ref name="DeckerHolde2011">{{cite book |last1=Decker |first1=Heinz |last2=Holde |first2=Kensal E. |chapter=Oxygen and the Exploration of the Universe |title=Oxygen and the Evolution of Life |url=https://archive.org/details/oxygenevolutionl00deck |url-access=limited |date=2011 |pages=[https://archive.org/details/oxygenevolutionl00deck/page/n162 157]–168 |doi=10.1007/978-3-642-13179-0_9 |isbn=978-3-642-13178-3}}</ref> But others, such as [[Ian Stewart (mathematician)|Ian Stewart]] and [[Jack Cohen (scientist)|Jack Cohen]] in their 2002 book ''[[Evolving the Alien]]'' argue that complex intelligent life may arise outside the HZ.<ref name=cohen-2002>{{cite book |title=Evolving the Alien |publisher=Ebury Press |author1=Stewart, Ian |author2=Cohen, Jack |date=2002 |isbn=978-0-09-187927-3}}</ref> Intelligent life outside the HZ may have evolved in subsurface environments, from alternative biochemistries<ref name=cohen-2002 /> or even from nuclear reactions.<ref name="GO247">
{{cite book
 |last1 = Goldsmith
 |first1 = Donald
 |last2 = Owen
 |first2 = Tobias
 |title = The Search for Life in the Universe
 |publisher = [[Addison-Wesley]]
 |edition = 2
 |date = 1992
 |page = 247
 |isbn = 978-0-201-56949-0
 }}</ref>

[[File:SEM_image_of_Milnesium_tardigradum_in_active_state_-_journal.pone.0045682.g001-2_(white_background).png|
thumb|Milnesium tardigradum, one of the few animals on Earth capable of surviving outside the habitable zone]]
On Earth, several complex multicellular life forms (or [[eukaryote]]s) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvent ecosystems, supporting large complex life forms such as ''[[Riftia pachyptila]]''.<ref name="Smil2003">{{cite book|author=Vaclav Smil|title=The Earth's Biosphere: Evolution, Dynamics, and Change|url=https://books.google.com/books?id=8ntHWPMUgpMC|year=2003|publisher=MIT Press|isbn=978-0-262-69298-4|page=166}}</ref> Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone.<ref>{{cite journal |author=Reynolds, R.T. |author2=McKay, C.P. |author3=Kasting, J.F. |title=Europa, Tidally Heated Oceans, and Habitable Zones Around Giant Planets |journal=Advances in Space Research |volume=7 |issue=5 |pages=125–132 |date=1987 |doi=10.1016/0273-1177(87)90364-4 |pmid=11538217 |bibcode = 1987AdSpR...7e.125R }}</ref> [[List of microorganisms tested in outer space|Numerous microorganisms have been tested]] in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the ''[[Milnesium tardigradum]]'', which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space.<ref>{{cite journal|author1=Guidetti, R.  |author2=Jönsson, K.I.|date=2002|title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans|journal=[[Journal of Zoology]]|volume=257|pages=181–187|doi=10.1017/S095283690200078X|issue=2|citeseerx=10.1.1.630.9839}}</ref> A desert moss, ''[[Syntrichia caninervis]]'' is one of few plants believed capable of surviving on Mars.<ref name=":1">{{cite journal|display-authors=3|last1=Li|first1=X.|last2=Bai|first2=W.|last3=Yang|first3=Q.|last4=Yin|first4=B.|last5=Zhang|first5=Z.|last6=Zhao|first6=B.|last7=Kuang|first7=T.|last8=Zhang|first8=Y.|last9=Zhang|first9=D.|date=2024|title=The extremotolerant desert moss Syntrichia caninervis is a promising pioneer plant for colonizing extraterrestrial environments|journal=The Innovation|volume=5|issue=4|pages=1–9|doi=10.1016/j.xinn.2024.100657|doi-access=free|pmid=39071942 |pmc=11282406|bibcode=2024Innov...500657L }}
*{{cite news|last=Davis|first=Nicola|url=https://www.theguardian.com/science/article/2024/jun/30/scientists-find-desert-moss-that-can-survive-on-mars|title=Scientists find desert moss 'that can survive on Mars'|date=30 June 2024|work=The Guardian|access-date=2024-07-07|archive-date=2024-07-08|archive-url=https://web.archive.org/web/20240708154553/https://www.theguardian.com/science/article/2024/jun/30/scientists-find-desert-moss-that-can-survive-on-mars|url-status=live}}</ref> In addition, the lichens ''[[Rhizocarpon geographicum]]'' and ''[[Rusavskia elegans]]'' have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize.<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 |access-date=27 April 2012 |archive-url=https://web.archive.org/web/20120528145425/http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html/ |archive-date=28 May 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]] |access-date=27 April 2012 |archive-url=https://web.archive.org/web/20120504224706/http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |archive-date=4 May 2012 }}</ref><ref name="Onofride Vera2015">{{cite journal|last1=Onofri|first1=Silvano|last2=de Vera|first2=Jean-Pierre|last3=Zucconi|first3=Laura|last4=Selbmann|first4=Laura|last5=Scalzi|first5=Giuliano|last6=Venkateswaran|first6=Kasthuri J.|last7=Rabbow|first7=Elke|last8=de la Torre|first8=Rosa|last9=Horneck|first9=Gerda|title=Survival of Antarctic Cryptoendolithic Fungi in Simulated Martian Conditions On Board the International Space Station|journal=Astrobiology|volume=15|issue=12|year=2015|pages=1052–1059|issn=1531-1074|doi=10.1089/ast.2015.1324|bibcode = 2015AsBio..15.1052O|pmid=26684504}}</ref> The fungi ''[[Cryomyces antarcticus]]'' and ''[[Cryomyces minteri]]'' are also able to survive and reproduce in Mars-like conditions.<ref name="Onofride Vera2015"/>

Species, including [[human]]s, known to possess [[animal cognition]] require large amounts of energy,<ref name="Islervan Schaik2006">{{cite journal|last1=Isler|first1=K.|last2=van Schaik|first2=C. P|title=Metabolic costs of brain size evolution|journal=Biology Letters|volume=2|issue=4|year=2006|pages=557–560|issn=1744-9561|doi=10.1098/rsbl.2006.0538|pmid=17148287|pmc=1834002}}</ref> and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true [[Earth analog]]s in the HZ are most likely to provide the closest natural habitat; this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of [[spacesuit]]s or [[space habitat]] analogs on the surface may be eliminated, and complex Earth life can thrive.<ref name=dole-1964 />

Planets in the HZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe.<ref>{{cite news |url= https://www.npr.org/templates/story/story.php?storyId=130215192| title= 'Goldilocks' Planet's Temperature Just Right For Life|author= Palca, Joe|date=September 29, 2010 |work=NPR |publisher=NPR |access-date=April 5, 2011}}</ref> The [[Drake equation]], sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter {{mvar|n<sub>e</sub>}}, which is the average number of planetary-mass objects orbiting within the HZ of each star. A low value lends support to the Rare Earth hypothesis, which posits that intelligent life is a rarity in the Universe, whereas a high value provides evidence for the [[Copernican principle|Copernican]] [[mediocrity principle]], the view that habitability—and therefore life—is common throughout the Universe.<ref name="Rare Earth" /> A 1971 NASA report by Drake and [[Bernard M. Oliver|Bernard Oliver]] proposed the "[[Water hole (radio)|water hole]]", based on the spectral [[absorption lines]] of the [[hydrogen]] and [[hydroxyl]] components of water, as a good, obvious band for communication with extraterrestrial intelligence<ref>{{cite web |title=Project Cyclops: A design study of a system for detecting extraterrestrial intelligent life |publisher=NASA |date=1971 |access-date=June 28, 2009 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730010095_1973010095.pdf}}</ref><ref name="Angelo2007">{{cite book|author=Joseph A. Angelo|title=Life in the Universe|url=https://books.google.com/books?id=I5gHntgLLvIC&pg=PA163|access-date=26 June 2013|date=2007|publisher=Infobase Publishing|isbn=978-1-4381-0892-6|page=163}}</ref> that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to [[Jill Tarter]], [[Margaret Turnbull]] and many others, HZ candidates are the priority targets to narrow waterhole searches<ref name="TurnbullTarter2003">{{cite journal|last1=Turnbull|first1=Margaret C.|last2=Tarter|first2=Jill C.|title=Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems|journal=The Astrophysical Journal Supplement Series|volume=145|issue=1|date=2003|pages=181–198|doi=10.1086/345779|arxiv = astro-ph/0210675 |bibcode = 2003ApJS..145..181T |s2cid=14734094}}</ref><ref name="SiemionDemorest2013">{{cite journal|last1=Siemion|first1=Andrew P. V.|last2=Demorest|first2=Paul|last3=Korpela|first3=Eric|last4=Maddalena|first4=Ron J.|last5=Werthimer|first5=Dan|last6=Cobb|first6=Jeff|last7=Howard|first7=Andrew W.|last8=Langston|first8=Glen|last9=Lebofsky|first9=Matt |author-link1=Andrew Siemion |title=A 1.1 to 1.9&nbsp;GHz SETI Survey of the ''Kepler'' Field: I. A Search for Narrow-band Emission from Select Targets|journal=The Astrophysical Journal|volume=767|issue=1|date=2013|pages=94|doi=10.1088/0004-637X/767/1/94|arxiv = 1302.0845 |bibcode = 2013ApJ...767...94S |s2cid=119302350}}</ref> and the [[Allen Telescope Array]] now extends [[Project Phoenix (SETI)|Project Phoenix]] to such candidates.<ref name=Wall2011>{{Cite web
 |title = HabStars: Speeding Up In the Zone
 |url = http://www.space.com/13832-seti-ata-search-kepler-planet-candidates.html
 |date = 2011
 |author = Wall, Mike
 |website = [[Space.com]]
 |access-date = 2013-06-26
 }}</ref>

Because the HZ is considered the most likely habitat for intelligent life, [[active SETI|METI]] efforts have also been focused on systems likely to have planets there. The 2001 [[Teen Age Message]] and 2003 [[Cosmic Call|Cosmic Call 2]], for example, were sent to the [[47 Ursae Majoris]] system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the HZ.<ref name="cplire.ru">{{cite conference |url=http://www.cplire.ru/rus/ra%26sr/VAK-2004.html |title=Transmission and reasonable signal searches in the Universe |script-title=ru:Передача и поиски разумных сигналов во Вселенной |access-date=2013-06-30 |author=Zaitsev, A. L. |book-title=Horizons of the Universe |date=June 2004 |conference=Plenary presentation at the National Astronomical Conference WAC-2004 "Horizons of the Universe", Moscow, Moscow State University, June 7, 2004 |location=Moscow |language=ru |archive-date=2019-05-30 |archive-url=https://web.archive.org/web/20190530145209/https://www.plover.com/misc/Dumas-Dutil/messages.pdf |url-status=dead }}</ref><ref>{{Cite magazine |author=David Grinspoon |date=July 13, 2012 |orig-date=December 12, 2007 |title=Who Speaks for Earth? |magazine=Seed |url=http://seedmagazine.com/content/article/who_speaks_for_earth/ |archive-url=https://web.archive.org/web/20120713064523/http://seedmagazine.com/content/article/who_speaks_for_earth/ |url-status=dead |archive-date=2012-07-13|access-date=2021-06-24}}</ref><ref name="bayesian">
{{cite journal
 |author1=P. C. Gregory |author2=D. A. Fischer |date=2010
 |title=A Bayesian periodogram finds evidence for three planets in 47 Ursae Majoris
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=403 |issue=2 |pages=731–747
 |doi=10.1111/j.1365-2966.2009.16233.x
|doi-access=free |bibcode=2010MNRAS.403..731G
|arxiv = 1003.5549 |s2cid=16722873 }}</ref><ref>
{{cite journal
 |author=B. Jones
 |date=2005
 |title=Prospects for Habitable "Earths" in Known Exoplanetary Systems
 |journal=[[Astrophysical Journal]]
 |volume=622 |issue=2 |pages=1091–1101
 |bibcode=2005ApJ...622.1091J
 |doi=10.1086/428108
|arxiv = astro-ph/0503178
 |display-authors=2
 |last2=Underwood
 |first2=David R.
 |last3=Sleep
 |first3=P. Nick |s2cid=119089227
 }}</ref> The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its HZ.<ref name="ScienceDaily" /> A Message from Earth in 2008,<ref name="moore">{{cite news|url = https://www.telegraph.co.uk/news/newstopics/howaboutthat/3166709/Messages-from-Earth-sent-to-distant-planet-by-Bebo.html|title = Messages from Earth sent to distant planet by Bebo |last = Moore |first = Matthew|date = October 9, 2008|publisher = .telegraph.co.uk|access-date = 2008-10-09| archive-url = https://web.archive.org/web/20081011142445/http://www.telegraph.co.uk/news/newstopics/howaboutthat/3166709/Messages-from-Earth-sent-to-distant-planet-by-Bebo.html|archive-date = 11 October 2008 | url-status = live |location=London}}</ref> and [[Hello From Earth]] in 2009, were directed to the Gliese 581 system, containing three planets in the HZ—Gliese 581 c, d, and the unconfirmed g.
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