Editing Exoplanet (section)

== Habitability ==
{{see also|Astrobiology|Circumstellar habitable zone|Planetary habitability}}
As more planets are discovered, the field of [[exoplanetology]] continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of [[Astrobiology|life on planets]] beyond the [[Solar System]].<ref name="Ollivier2014">{{cite journal |title=Planetary Environments and Origins of Life: How to reinvent the study of Origins of Life on the Earth and Life in the |journal=BIO Web of Conferences 2 |date=2014|last1= Ollivier |first1=Marc |last2=Maurel |first2=Marie-Christine |doi=10.1051/bioconf/20140200001|volume=2 |pages=00001|doi-access=free }}</ref> At cosmic distances, [[life]] can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).<ref name="Ollivier2014"/> For example, molecular oxygen ({{O2}}) in the [[atmosphere of Earth]] is a result of [[photosynthesis]] by living plants and many kinds of microorganisms, so it can be used as an [[Biomarker|indication of life]] on exoplanets, although small amounts of oxygen could also be produced by non-biological means.<ref name='NAOJ2014'>{{cite news |url=http://astrobiology.com/2015/09/oxygen-is-not-definitive-evidence-of-life-on-extrasolar-planets.html |title=Oxygen Is Not Definitive Evidence of Life on Extrasolar Planets |work=NAOJ|publisher=Astrobiology Web |date=10 September 2015 |access-date=11 September 2015}}</ref> Furthermore, a potentially habitable planet must orbit a stable [[star]] at a distance within which [[planetary-mass object]]s with sufficient [[atmospheric pressure]] can support [[liquid water]] at their surfaces.<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>

=== 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>

=== Ice ages and snowball states ===
{{See also|Ice age|Snowball Earth}}
The outer edge of the habitable zone is where planets are completely frozen, but planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling, then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops [[Solution weathering|carbon dioxide weathering]], resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a [[greenhouse effect]] which thaws the planet again. Planets with a large [[axial tilt]]<ref>{{cite journal| arxiv=1401.5323|bibcode = 2015P&SS..105...43L|title = Habitability of Earth-like planets with high obliquity and eccentric orbits: Results from a general circulation model |journal = Planetary and Space Science|last1 = Linsenmeier |first1 = Manuel |last2 = Pascale |first2 = Salvatore |last3 = Lucarini |first3 = Valerio |year = 2014 |doi=10.1016/j.pss.2014.11.003 |volume=105 |pages=43–59|s2cid = 119202437}}</ref> are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt.<ref>{{Cite web|last=Kelley |first=Peter |date=15 April 2014 |title=Astronomers: 'Tilt-a-worlds' could harbor life|url=https://www.washington.edu/news/2014/04/15/astronomers-tilt-a-worlds-could-harbor-life/|access-date=2023-02-12|website=UW News|language=en}}</ref><ref>{{Cite journal | doi = 10.1089/ast.2013.1129| pmid = 24611714| title = Effects of Extreme Obliquity Variations on the Habitability of Exoplanets| journal = Astrobiology| volume = 14| issue = 4| pages = 277–291| year = 2014| last1 = Armstrong | first1 = J. C. | last2 = Barnes | first2 = R.| last3 = Domagal-Goldman | first3 = S.| last4 = Breiner | first4 = J.| last5 = Quinn | first5 = T. R. | last6 = Meadows | first6 = V. S. | bibcode=2014AsBio..14..277A|arxiv = 1404.3686 | pmc=3995117}}</ref> Paradoxically, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.<ref>{{Cite web|last=Kelley |first=Peter |date=18 July 2013 |title=A warmer planetary haven around cool stars, as ice warms rather than cools|url=https://www.washington.edu/news/2013/07/18/a-warmer-planetary-haven-around-cool-stars-as-ice-warms-rather-than-cools/|access-date=2023-02-12|website=UW News|language=en}}</ref><ref>{{Cite journal | doi = 10.1088/2041-8205/785/1/L9| title = Spectrum-Driven Planetary Deglaciation Due to Increases in Stellar Luminosity| journal = The Astrophysical Journal| volume = 785| issue = 1| pages = L9| year = 2014| last1 = Shields | first1 = A. L. | last2 = Bitz | first2 = C. M. |author-link2=Cecilia Bitz| last3 = Meadows | first3 = V. S. | last4 = Joshi | first4 = M. M. | last5 = Robinson | first5 = T. D. |arxiv = 1403.3695 |bibcode = 2014ApJ...785L...9S | s2cid = 118544889}}</ref>

=== Tidal heating ===
If a planet has an eccentric orbit, then [[tidal heating]] can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also [[tidal circularization|circularize]] orbits over time, so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits.<ref>{{Cite journal | doi = 10.1089/ast.2012.0851| pmid = 23537135| title = Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating| journal = Astrobiology| volume = 13| issue = 3| pages = 225–250| year = 2013| last1 = Barnes | first1 = R. | last2 = Mullins | first2 = K. | last3 = Goldblatt | first3 = C. | last4 = Meadows | first4 = V. S. | last5 = Kasting | first5 = J. F. | last6 = Heller | first6 = R. | bibcode=2013AsBio..13..225B|arxiv = 1203.5104 | pmc=3612283}}</ref> Eccentric planets further out than the habitable zone would still have frozen surfaces, but the tidal heating could create a subsurface ocean similar to [[Europa (moon)|Europa]]'s.<ref name="Superhabitable">{{Cite journal | doi = 10.1089/ast.2013.1088| pmid = 24380533| title = Superhabitable Worlds| journal = Astrobiology| volume = 14| issue = 1| pages = 50–66| year = 2014| last1 = Heller | first1 = R. | last2 = Armstrong | first2 = J. | bibcode=2014AsBio..14...50H|arxiv = 1401.2392 | s2cid = 1824897}}</ref> In some planetary systems, such as in the [[Upsilon Andromedae]] system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.<ref>{{Cite journal | doi = 10.1111/j.1365-2966.2008.13868.x| title = Tidal heating of terrestrial extrasolar planets and implications for their habitability| journal = Monthly Notices of the Royal Astronomical Society| volume = 391| issue = 1| pages = 237–245| year = 2008| last1 = Jackson | first1 = B. | last2 = Barnes | first2 = R. | last3 = Greenberg | first3 = R. | doi-access = free| bibcode = 2008MNRAS.391..237J|arxiv = 0808.2770 | s2cid = 19930771}}</ref>

=== Potentially habitable planets ===
{{See also|List of potentially habitable exoplanets|List of nearest terrestrial exoplanet candidates}}

A review in 2015 identified exoplanets [[Kepler-62f]], [[Kepler-186f]] and [[Kepler-442b]] as the best candidates for being potentially habitable.<ref name="centauridreams">{{cite web |author=Gilster |first1=Paul |last2=LePage |first2=Andrew |date=2015-01-30 |title=A Review of the Best Habitable Planet Candidates |url=http://www.centauri-dreams.org/?p=32470 |access-date=2015-07-24 |publisher=Centauri Dreams, Tau Zero Foundation}}</ref> These are at a distance of 1200, 490 and 1,120 [[light-years]] away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its [[red dwarf]] star.

When looking at the nearest terrestrial exoplanet candidates, [[Proxima Centauri b]] is about 4.2 light-years away. Its equilibrium temperature is estimated to be {{convert|-39|C|K|abbr=}}.<ref>{{cite book |author=Bignami |first=Giovanni F. |url=https://books.google.com/books?id=crvpCQAAQBAJ&pg=PA110 |title=The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space |publisher=Springer |year=2015 |isbn=978-3-319-17004-6 |page=110}}</ref>

==== Earth-size planets ====
{{See also|Earth analog}}
* In November 2013, it was estimated that 22±8% of Sun-like<ref group="lower-alpha" name="footnoteA"/> stars in the Milky Way galaxy may have an Earth-sized<ref group="lower-alpha" name="footnoteB"/> planet in the habitable<ref group=lower-alpha name=footnoteC/> zone.<ref name="ucb1in5" /><ref name="earthsunhzprev" /> Assuming 200 billion stars in the Milky Way,<ref group="lower-alpha" name="footnoteD"/> that would be 11 billion potentially habitable Earths, rising to 40 billion if [[red dwarf]]s are included.<ref name="LATimes-20131104"/>
* [[Kepler-186f]], a 1.2-Earth-radius planet in the habitable zone of a [[red dwarf]], was reported in April 2014.
*Proxima Centauri b, a planet in the habitable zone of [[Proxima Centauri]], the nearest known star to the solar system with an estimated minimum mass of 1.27 times the mass of the Earth.
* In February 2013, researchers speculated that up to 6% of small red dwarfs may have Earth-size planets. This suggests that the closest one to the Solar System could be 13 light-years away. The estimated distance increases to 21 light-years when a 95% [[confidence interval]] is used.<ref name="howell-2013">{{cite news | url=http://www.space.com/19667-closest-alien-earth-exoplanets.html | title=Closest 'Alien Earth' May Be 13 Light-Years Away | work=Space.com | date=6 February 2013 | agency=TechMediaNetwork | access-date=7 February 2013 | author=Howell, Elizabeth}}</ref> In March 2013, a revised estimate gave an occurrence rate of 50% for Earth-size planets in the habitable zone of red dwarfs.<ref name="Habitable Exoplanet Re-estimate">{{cite journal| last=Kopparapu |first=Ravi Kumar |title=A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around Kepler M-dwarfs |journal=[[The Astrophysical Journal Letters]] |date=March 2013 |arxiv=1303.2649 |bibcode=2013ApJ...767L...8K|volume=767|issue=1 |pages=L8| doi=10.1088/2041-8205/767/1/L8|s2cid=119103101 }}</ref>
* At 1.63 times Earth's radius [[Kepler-452b]] is the first discovered near-Earth-size planet in the [[Circumstellar habitable zone|"habitable zone"]] around a [[G star|G2-type]] [[Sun-like]] star (July 2015).<ref>{{cite web| title = NASA's Kepler Mission Discovers Bigger, Older Cousin to Earth| url = http://www.nasa.gov/press-release/nasa-kepler-mission-discovers-bigger-older-cousin-to-earth |access-date = 2015-07-23| date = 2015-07-23 }}</ref>
{| class="wikitable" style="margin:0.5em auto; width:450px; align=right"
! [[Exoplanet|Notable Exoplanets]] – [[Kepler (spacecraft)|Kepler Space Telescope]] 
|-
| style="font-size:88%" | [[File:PIA19827-Kepler-SmallPlanets-HabitableZone-20150723.jpg|600px]]
{{center|Comparison of small planets found by ''[[Kepler (spacecraft)|Kepler]]'' in the [[habitable zone]] of their host stars.}}
|}