Editing Habitable zone (section)

{{Short description|Orbits where planets may have liquid surface water}}
{{redirect|Goldilocks zone|the more general principle|Goldilocks principle}}
{{About|the circumstellar zone|the galactic zone|Galactic habitable zone}}
[[File:Diagram of habitable zone rocky exoplanets, from 2024 NASA Exoplanet Archive and Gaia DR3 data.png|alt=A diagram depicting habitable zone boundaries across star type. The y-axis is stellar temperature, with the Sun (5772 Kelvin) at the top. The x-axis is the percentage of starlight that reaches the planet, ranging from 25% of Earth's starlight to 150% of Earth's starlight on the inner edge of the habitable zone. The image plots 42 exoplanets, most of which orbit red dwarfs. The coldest planets around red dwarfs are depicted as icy "eyeball" planets due to tidal locking, while most of the other planets around red dwarfs are purple, due to speculations about purple photosynthesizing creatures. Habitable one planets around yellow stars are depicted as green or blue. Earth is plotted near the top, with only Kepler-452 b close to its position.|thumb|440x440px|A diagram depicting habitable zone boundaries across [[Stellar classification|star type]] with September 2024 data, based on previous habitable zone diagrams.<ref>{{Cite web |last=Astrobites |date=2022-05-03 |title=Which Habitable Zone Planets Are the Best Candidates for Detecting Life? |url=https://aasnova.org/2022/05/03/which-habitable-zone-planets-are-the-best-candidates-for-detecting-life/ |access-date=2025-01-09 |website=AAS Nova |language=en-US}}</ref> [[Earth]] is plotted alongside 42 [[exoplanet]]s with radii less than 2 times that of Earth or masses less than 5 times that of Earth, making them potentially rocky worlds in the habitable zone.]]
{{Life in the Universe}}
In [[astronomy]] and [[astrobiology]], the '''habitable zone''' ('''HZ'''), or more precisely the '''circumstellar habitable zone''' ('''CHZ'''), is the range of [[orbit]]s around a [[star]] within which a [[planetary surface]] can support [[liquid water]] given sufficient [[atmospheric pressure]].<ref>Su-Shu Huang, American Scientist 47, 3, pp. 397–402 (1959)</ref><ref name=dole-1964>{{cite book |url=https://www.rand.org/pubs/commercial_books/CB179-1.html |title=Habitable Planets for Man |publisher=Blaisdell Publishing Company |last=Dole |first=Stephen H. |date=1964 |page=103}}</ref><ref name="F. Kasting, D. P 1993">J. F. Kasting, D. P. Whitmire, R. T. Reynolds, Icarus 101, 108 (1993).</ref><ref name=kopparapu-2013>{{cite journal |title=A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs |author=Kopparapu, Ravi Kumar |journal=The Astrophysical Journal Letters |date=2013 |volume=767 |issue=1 |doi=10.1088/2041-8205/767/1/L8 |arxiv=1303.2649 |pages=L8|bibcode = 2013ApJ...767L...8K|s2cid=119103101 }}</ref><ref name="SCI-20130503">{{cite journal |last1=Cruz |first1=Maria |last2=Coontz |first2=Robert |title=Exoplanets - Introduction to Special Issue |journal=[[Science (journal)|Science]] |volume=340 |page=565 |doi=10.1126/science.340.6132.565 |pmid=23641107 |issue=6132 |date=2013|doi-access=free }}</ref> The bounds of the HZ are based on [[Earth]]'s position in the [[Solar System]] and the amount of [[radiant energy]] it receives from the [[Sun]]. Due to the importance of liquid water to Earth's [[biosphere]], the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like [[extraterrestrial life]] and [[extraterrestrial intelligence|intelligence]]. As such, it is considered by many to be a major factor of [[planetary habitability]], and the most likely place to find [[extraterrestrial liquid water]] and [[biosignature]]s elsewhere in the universe.

The habitable zone is also called the '''Goldilocks zone''', a [[metaphor]], [[allusion]] and [[antonomasia]] of the children's [[fairy tale]] of "[[Goldilocks and the Three Bears]]", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".

Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets.<ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next |url=https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |date=January 6, 2015 |work=[[The New York Times]] |access-date=January 6, 2015}}</ref> Most such planets, being either [[super-Earth]]s or [[gas giant]]s, are more massive than Earth, because massive planets are [[sampling bias|easier to detect]].<ref>{{cite journal |url=https://www.researchgate.net/publication/2227681 |title=Probability of Detecting a Planetary Companion during a Microlensing Event  |doi=10.1086/320562 | arxiv=astro-ph/0101316 |date=January 2021|last1=Peale |first1=S. J. |journal=The Astrophysical Journal |volume=552 |issue=2 |pages=889–911 |s2cid=17080374 }}</ref> On November 4, 2013, astronomers reported, based on [[Kepler space telescope]] data, that there could be as many as 40&nbsp;billion [[terrestrial planet|Earth-sized]] [[exoplanet|planets]] orbiting in the habitable zones 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=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=November 4, 2013 |work=The New York Times |access-date=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|date=October 31, 2013|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|doi=10.1073/pnas.1319909110 |arxiv = 1311.6806 |bibcode = 2013PNAS..11019273P |volume=110 |issue=48|pages=19273–19278 |pmid=24191033 |pmc=3845182|doi-access=free}}</ref> About 11&nbsp;billion of these 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]] |access-date=November 5, 2013}}</ref> [[Proxima Centauri b]], located about 4.2 [[light-year]]s (1.3 [[parsec]]s) from Earth in the constellation of [[Centaurus]], is the nearest known exoplanet, and is orbiting in the habitable zone of its star.<ref name="Planet-Alpha-Centauri">{{cite journal
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}}</ref> The HZ is also of particular interest to the emerging field of [[habitability of natural satellites]] because planetary mass [[exomoon|moons]] in the HZ might outnumber planets.<ref name="Schirber 2009-10-26">{{cite web |last=Schirber |first=Michael |title=Detecting Life-Friendly Moons |url=http://www.astrobio.net/exclusive/3291/detecting-life-friendly-moons |date=26 Oct 2009 |work=Astrobiology Magazine |agency=NASA |access-date=9 May 2013 |archive-url=https://web.archive.org/web/20091029111850/http://www.astrobio.net/exclusive/3291/detecting-life-friendly-moons |archive-date=29 October 2009  |url-status=dead}}</ref>

In subsequent decades, the HZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.<ref name="Review 2009">{{cite journal |title=What makes a planet habitable? |journal=The Astronomy and Astrophysics Review |year=2009 |last1=Lammer |first1=H. |last2=Bredehöft |first2=J. H. |last3=Coustenis |first3=A. |last4=Khodachenko |first4=M. L. |volume=17 |issue=2 |pages=181–249 |doi=10.1007/s00159-009-0019-z |url=http://veilnebula.jorgejohnson.me/uploads/3/5/8/7/3587678/lammer_et_al_2009_astron_astro_rev-4.pdf |access-date=2016-05-03 |bibcode=2009A&ARv..17..181L |s2cid=123220355 |display-authors=etal |archive-url=https://web.archive.org/web/20160602235333/http://veilnebula.jorgejohnson.me/uploads/3/5/8/7/3587678/lammer_et_al_2009_astron_astro_rev-4.pdf |archive-date=2016-06-02 }}</ref>  Since the discovery of evidence for extraterrestrial liquid water, substantial quantities of it are now thought to occur outside the circumstellar habitable zone. The concept of [[deep biosphere]]s, like Earth's, that exist independently of stellar energy, are now generally accepted in astrobiology given the large amount of liquid water known to exist in [[lithosphere]]s and [[asthenosphere]]s of the Solar System.<ref name="EdwardsBecker2012">{{cite journal|last1=Edwards|first1=Katrina J.|last2=Becker|first2=Keir|last3=Colwell|first3=Frederick|title=The Deep, Dark Energy Biosphere: Intraterrestrial Life on Earth|journal=Annual Review of Earth and Planetary Sciences|volume=40|issue=1|year=2012|pages=551–568|issn=0084-6597|doi=10.1146/annurev-earth-042711-105500|bibcode = 2012AREPS..40..551E}}</ref> Sustained by other energy sources, such as [[tidal heating]]<ref name="Cowen2008">{{cite news |first=Ron |last=Cowen |title=A Shifty Moon |url=http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon |work=Science News |date=2008-06-07 |access-date=2013-04-22 |archive-date=2011-11-04 |archive-url=https://web.archive.org/web/20111104175610/http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon |url-status=dead }}</ref><ref name="Bryner, Jeanna">{{cite news|url=http://www.space.com/scienceastronomy/090624-enceladus-ocean.html |title=Ocean Hidden Inside Saturn's Moon |work=Space.com |date=24 June 2009 |author=Bryner, Jeanna |access-date=22 April 2013 |agency=TechMediaNetwork}}</ref> or [[radioactive decay]]<ref name="AbbotSwitzer2011">{{cite journal| last1=Abbot|first1=D. S.|last2=Switzer|first2=E. R.| title=The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space| journal=The Astrophysical Journal| volume=735| issue=2| date=2011| pages=L27| doi=10.1088/2041-8205/735/2/L27|arxiv = 1102.1108 |bibcode = 2011ApJ...735L..27A|s2cid=73631942}}</ref> or pressurized by non-atmospheric means, liquid water may be found even on [[rogue planet]]s, or their moons.<ref name=physcisarxivlab-2011>{{cite news |url=http://www.technologyreview.com/view/422659/rogue-planets-could-harbor-life-in-interstellar-space-say-astrobiologists/ |title=Rogue Planets Could Harbor Life in Interstellar Space, Say Astrobiologists |work=MIT Technology Review |date=9 February 2011 |agency=MIT Technology Review |access-date=24 June 2013 |archive-date=7 October 2015 |archive-url=https://web.archive.org/web/20151007173209/http://www.technologyreview.com/view/422659/rogue-planets-could-harbor-life-in-interstellar-space-say-astrobiologists/ |url-status=dead }}</ref> Liquid water can also exist at a wider range of temperatures and pressures as a [[Solution (chemistry)|solution]], for example with sodium chlorides in [[seawater]] on Earth, chlorides and sulphates on [[Seasonal flows on warm Martian slopes|equatorial Mars]],<ref name="Wall-Brines 2015">{{cite news |last=Wall |first=Mike |url=http://www.space.com/30673-water-flows-on-mars-discovery.html?adbid=10153086098981466&adbpl=fb&adbpr=17610706465 |title=Salty Water Flows on Mars Today, Boosting Odds for Life |work=Space.com |date=28 September 2015 |access-date=2015-09-28}}</ref> or ammoniates,<ref name="SunClark2015">{{cite journal|last1=Sun|first1=Jiming|last2=Clark|first2=Bryan K.|last3=Torquato|first3=Salvatore|last4=Car|first4=Roberto|title=The phase diagram of high-pressure superionic ice|journal=Nature Communications|volume=6|year=2015|pages=8156|issn=2041-1723|doi=10.1038/ncomms9156|bibcode = 2015NatCo...6.8156S|pmid=26315260|pmc=4560814}}</ref> due to its different [[colligative properties]]. In addition, other circumstellar zones, where non-water [[solvent]]s favorable to hypothetical life based on [[hypothetical types of biochemistry|alternative biochemistries]] could exist in liquid form at the surface, have been proposed.<ref name=villard-2011>{{cite news|url=http://news.discovery.com/space/planetary-habitable-zones-defined-by-alien-biochemistry-111118.html |title=Alien Life May Live in Various Habitable Zones: Discovery News |publisher=News.discovery.com |date=November 18, 2011 |access-date=April 22, 2013 |author=Villard, Ray |agency=Discovery Communications LLC}}</ref>