Editing Exoplanet (section)

=== Habitable zone ===
{{main|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|A diagram depicting [[habitable zone]] boundaries across [[Stellar classification|star type]] with September 2024 data. [[Earth]] is plotted alongside 42 potentially rocky exoplanets within the habitable zone.]]
The habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on the surface of a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances for different stars. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: [[desert planet]]s (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.<ref>{{Cite web|last=Choi |first=Charles Q. |date=1 September 2011 |website=Astrobiology Magazine |title=Alien Life More Likely on 'Dune' Planets|url=http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%25E2%2580%2598dune%25E2%2580%2599-planets|archive-url=https://web.archive.org/web/20131202223111/http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%25E2%2580%2598dune%25E2%2580%2599-planets |archive-date=2 December 2013 }}</ref><ref>{{Cite journal | last1 = Abe | first1 = Y. | last2 = Abe-Ouchi | first2 = A. | last3 = Sleep | first3 = N. H. | last4 = Zahnle | first4 = K. J. | title = Habitable Zone Limits for Dry Planets | doi = 10.1089/ast.2010.0545 | journal = Astrobiology | volume = 11 | issue = 5 | pages = 443–460 | year = 2011 | pmid =  21707386|bibcode = 2011AsBio..11..443A }}</ref> Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance.<ref>{{Cite journal | doi = 10.1126/science.1232226|pmid=23641111 | title = Exoplanet Habitability| journal = Science| volume = 340| issue = 6132| pages = 577–581| year = 2013| last1 = Seager | first1 = S.|bibcode=2013Sci...340..577S|citeseerx=10.1.1.402.2983 |s2cid=206546351 }}</ref> Planets with larger mass have wider habitable zones because gravity reduces the water cloud column depth which reduces the greenhouse effect of water vapor, thus moving the inner edge of the habitable zone closer to the star.<ref>{{cite journal| arxiv=1404.5292| bibcode = 2014ApJ...787L..29K |doi = 10.1088/2041-8205/787/2/L29 | volume=787| issue = 2 |title=Habitable Zones around Main-sequence Stars: Dependence on Planetary Mass| journal=The Astrophysical Journal|pages=L29|year = 2014 |last1 = Kopparapu |first1 = Ravi Kumar |last2 = Ramirez |first2 = Ramses M. |last3 = Schottelkotte |first3 = James |last4 = Kasting |first4 = James F. |last5 = Domagal-Goldman |first5 = Shawn |last6 = Eymet |first6 = Vincent | s2cid = 118588898 }}</ref>

Planetary [[#Rotation and axial tilt|rotation rate]] is one of the major factors determining the [[#Atmospheric circulation|circulation of the atmosphere]] and hence the pattern of clouds: slowly rotating planets create thick clouds that [[albedo|reflect]] more and so can be habitable much closer to their star. Earth with its current atmosphere would be habitable in Venus's orbit, if it had Venus's slow rotation. If Venus lost its water ocean due to a [[runaway greenhouse effect#Venus|runaway greenhouse effect]], it is likely to have had a higher rotation rate in the past. Alternatively, Venus never had an ocean because water vapor was lost to space during its formation <ref>{{Cite journal | doi = 10.1038/nature12163|pmid=23719462| title = Emergence of two types of terrestrial planet on solidification of magma ocean| journal = Nature| volume = 497| issue = 7451| pages = 607–610| year = 2013| last1 = Hamano | first1 = K. | last2 = Abe | first2 = Y. | last3 = Genda | first3 = H. |bibcode=2013Natur.497..607H|s2cid=4416458}}</ref> and could have had its slow rotation throughout its history.<ref>{{Cite journal | doi = 10.1088/2041-8205/787/1/L2 | arxiv = 1404.4992 | url = http://home.uchicago.edu/~junyang28/Papers/Yang-et-al-Rotation_Rate.pdf | title = Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate | journal = The Astrophysical Journal | volume = 787 | issue = 1 | pages = L2 | year = 2014 | last1 = Yang | first1 = J. | last2 = Boué | first2 = G. L. | last3 = Fabrycky | first3 = D. C. | last4 = Abbot | first4 = D. S. | bibcode = 2014ApJ...787L...2Y | s2cid = 56145598 | access-date = 2016-07-28 | archive-url = https://web.archive.org/web/20160412161026/http://home.uchicago.edu/~junyang28/Papers/Yang-et-al-Rotation_Rate.pdf | archive-date = 2016-04-12 | url-status = dead }}</ref>

[[Tidal locking|Tidally locked planets]] (a.k.a. "eyeball" planets<ref>{{cite web| url=http://planetplanet.net/2014/10/07/real-life-sci-fi-world-2-the-hot-eyeball-planet/| title=Real-life Sci-Fi World #2: the Hot Eyeball planet|work=planetplanet| date=2014-10-07}}</ref>) can be habitable closer to their star than previously thought due to the effect of clouds: at high stellar flux, strong convection produces thick water clouds near the substellar point that greatly increase the planetary albedo and reduce surface temperatures.<ref>{{cite journal| arxiv=1307.0515|bibcode = 2013ApJ...771L..45Y |doi = 10.1088/2041-8205/771/2/L45 | volume=771|issue = 2 | journal=The Astrophysical Journal| pages=L45|year = 2013 |last1 = Yang |first1 = Jun |title = Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets |last2 = Cowan |first2 = Nicolas B. |last3 = Abbot |first3 = Dorian S. |s2cid = 14119086 }}</ref>

Planets in the habitable zones of stars with [[Metallicity|low metallicity]] are more habitable for complex life on land than high metallicity stars because the stellar spectrum of high metallicity stars is less likely to cause the formation of ozone thus enabling more ultraviolet rays to reach the planet's surface.<ref name="SA-20230419">{{cite news |last=Starr |first=Michelle |title=Scientists Think They've Narrowed Down The Star Systems Most Likely to Host Life |url=https://www.sciencealert.com/scientists-think-theyve-narrowed-down-the-star-systems-most-likely-to-host-life |date=19 April 2023 |work=[[ScienceAlert]] |accessdate=19 April 2023 }}</ref><ref name="NC-20230418">{{cite journal |author=Shapiro, Anna V. |display-authors=et al. |title=Metal-rich stars are less suitable for the evolution of life on their planets |date=18 April 2023 |journal=[[Nature Communications]] |volume=14 |issue=1893 |page=1893 |doi=10.1038/s41467-023-37195-4 |pmid=37072387 |pmc=10113254 |bibcode=2023NatCo..14.1893S }}</ref>

Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes,<ref>{{Cite journal | doi = 10.1016/j.palaeo.2004.10.018| title = Expanding frontiers in deep subsurface microbiology| journal = Palaeogeography, Palaeoclimatology, Palaeoecology| volume = 219| issue = 1–2| pages = 131–155| year = 2005| last1 = Amend | first1 = J. P. | last2 = Teske | first2 = A. | bibcode = 2005PPP...219..131A}}</ref> and the temperature increases with depth, so the subsurface can be conducive for microbial life when the surface is frozen and if this is considered, the habitable zone extends much further from the star,<ref>{{Cite news|date=2014-01-07|title=Further away planets 'can support life' say researchers|language=en-GB|work=BBC News|url=https://www.bbc.com/news/uk-scotland-north-east-orkney-shetland-25639306|access-date=2023-02-12}}</ref> even [[rogue planet]]s could have liquid water at sufficient depths underground.<ref>{{Cite journal | doi = 10.1088/2041-8205/735/2/L27| arxiv=1102.1108| url=https://www.researchgate.net/publication/48202561| title = The Steppenwolf: A Proposal for a Habitable Planet in Interstellar Space| journal = The Astrophysical Journal| volume = 735| issue = 2| pages = L27| year = 2011| last1 = Abbot | first1 = D. S.| last2 = Switzer | first2 = E. R.|bibcode=2011ApJ...735L..27A| s2cid=73631942}}</ref> In an earlier era of the [[universe]] the temperature of the [[cosmic microwave background]] would have allowed any rocky planets that existed to have liquid water on their surface regardless of their distance from a star.<ref>{{Cite journal | doi = 10.1017/S1473550414000196| title = The habitable epoch of the early Universe| journal = International Journal of Astrobiology| volume = 13| issue = 4| pages = 337–339| year = 2014| last1 = Loeb | first1 = A. |arxiv = 1312.0613 |bibcode = 2014IJAsB..13..337L | citeseerx = 10.1.1.748.4820| s2cid = 2777386}}</ref> Jupiter-like planets might not be habitable, but they could have [[Habitability of natural satellites|habitable moons]].<ref>{{Cite web|first=Andy |last=Ridgway |date=29 July 2015 |title=Home, sweet exomoon: The new frontier in the search for ET|url=https://www.newscientist.com/article/mg22730320-300-home-sweet-exomoon-the-new-frontier-in-the-search-for-et/|access-date=2023-02-12|website=New Scientist|language=en-US}}</ref>