Editing Habitable zone (section)

====Other considerations====
[[Image:BlueMarble-2001-2002.jpg|thumb|Earth's hydrosphere. Water covers 71% of Earth's surface, with the [[global ocean]] accounting for 97.3% of the [[water distribution on Earth]].]]
{{see also|Planetary habitability|Habitability of natural satellites}}
A planet cannot have a [[hydrosphere]]—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The [[origin of water on Earth]] is still not completely understood; possible sources include the result of impacts with icy bodies, [[outgassing]], [[mineralization (geology)|mineralization]], leakage from [[hydrous]] minerals from the [[lithosphere]], and [[photolysis]].<ref name="source_mrk1">{{cite journal |title= Origin of water in the terrestrial planets |last1= Drake |first1= Michael J. |s2cid= 12808812 |journal=Meteoritics & Planetary Science |date=April 2005 |volume=40 |issue= 4 |pages= 519–527 |doi= 10.1111/j.1945-5100.2005.tb00960.x |bibcode= 2005M&PS...40..519D|doi-access= free }}</ref><ref name="source_mrk2">{{cite conference |url= http://journals.cambridge.org/action/displayFulltext?type=6&fid=415222&jid=IAU&volumeId=1&issueId=S229&aid=414784&bodyId=&membershipNumber=&societyETOCSession=&fulltextType=RA&fileId=S1743921305006861 |title= Origin of water in the terrestrial planets |display-authors=1 |last1= Drake |first1= Michael J. |last2= Humberto |first2= Campins |conference = 229th Symposium of the International Astronomical Union |date=August 2005 |location = Búzios, Rio de Janeiro, Brazil |publisher= Cambridge University Press |volume=1 |issue= 4 |pages= 381–394 |doi= 10.1017/S1743921305006861 |bibcode= 2006IAUS..229..381D |book-title= Asteroids, Comets, and Meteors (IAU S229) |isbn= 978-0-521-85200-5|doi-access= free }}</ref> For an extrasolar system, an icy body from beyond the [[frost line (astrophysics)|frost line]] could migrate into the habitable zone of its star, creating an [[ocean planet]] with seas hundreds of kilometers deep<ref name=kuchner-2003>{{Cite journal|arxiv=astro-ph/0303186|title=Volatile-rich Earth-Mass Planets in the Habitable Zone|first=Marc|last=Kuchner|journal=Astrophysical Journal|date=2003|volume=596|issue=1|pages=L105–L108|doi=10.1086/378397|bibcode=2003ApJ...596L.105K|s2cid=15999168}}</ref> such as [[GJ 1214 b]]<ref name="disco-charbonneau">{{cite journal |last1=Charbonneau |first1=David
|author2=Zachory K. Berta
|author3=Jonathan Irwin
|author4=Christopher J. Burke
|author5=Philip Nutzman
|author6=Lars A. Buchhave
|author7=Christophe Lovis
|author8=Xavier Bonfils
|author9=David W. Latham
|author10=Stéphane Udry
|author11=Ruth A. Murray-Clay
|author12=Matthew J. Holman
|author13=Emilio E. Falco
|author14=Joshua N. Winn
|author15=Didier Queloz
|author16=Francesco Pepe
|author17=Michel Mayor
|author18=Xavier Delfosse
|author19=Thierry Forveille
|display-authors=8
|date=2009 |title=A super-Earth transiting a nearby low-mass star |journal=Nature |volume=462 |issue=17 December 2009 |pages=891–894 |doi=10.1038/nature08679 |pmid=20016595 |bibcode=2009Natur.462..891C|arxiv = 0912.3229 |s2cid=4360404
}}</ref><ref name="planetmodels">{{cite journal |last1= Kuchner |first1= Seager |first2=M.|last2=Hier-Majumder | first3=C. A.|last3=Militzer |date=2007 |title=Mass–radius relationships for solid exoplanets |journal=The Astrophysical Journal |volume=669 |issue= 2|pages=1279–1297 |doi=10.1086/521346 |bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 |s2cid= 8369390 }}</ref> or [[Kepler-22b]] may be.<ref name=vastag-2011>{{cite news |url=https://www.washingtonpost.com/national/health-science/newest-alien-planet-is-just-the-right-temperature-for-life/2011/12/05/gIQAPk1vWO_story.html |title=Newest alien planet is just the right temperature for life |newspaper=The Washington Post |date=December 5, 2011 |access-date=April 27, 2013 |author=Vastag, Brian}}</ref>

Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorized to outgassing, impact degassing, and ingassing.<ref name="RobinsonCatling2012">{{cite journal|last1=Robinson|first1=Tyler D.|last2=Catling|first2=David C.| title=An Analytic Radiative-Convective Model for Planetary Atmospheres| journal=The Astrophysical Journal| volume=757|issue=1|date=2012|pages=104|doi=10.1088/0004-637X/757/1/104|arxiv = 1209.1833 |bibcode = 2012ApJ...757..104R |s2cid=54997095}}</ref> Atmospheres are thought to be maintained through similar processes along with [[biogeochemical cycle]]s and the mitigation of [[atmospheric escape]].<ref name="Shizgal, 1996">{{cite journal |last1=Shizgal |first1=B. D. |last2=Arkos |first2=G. G. |s2cid=7852371 |date=1996 |title=Nonthermal escape of the atmospheres of Venus, Earth, and Mars |journal=[[Reviews of Geophysics]] |volume=34 |issue=4 |pages=483–505 |doi=10.1029/96RG02213 |bibcode = 1996RvGeo..34..483S }}</ref> In a 2013 study led by Italian astronomer [[Giovanni Vladilo]], it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure.<ref name=vladilo-2013 /> Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained<ref name=vladilo-2013 /> because even a small shift in pressure or temperature could render water unable to form as a liquid.<ref name=chaplin-2013>{{cite web |url=http://www.lsbu.ac.uk/water/phase.html |title=Water Phase Diagram |publisher=London South Bank University |work=Ices |date=April 8, 2013 |access-date=April 27, 2013 |author=Chaplin, Martin}}</ref>

Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth),<ref name=kasting-1993 /> a study<ref name="rk-2017"/> led by Ramses Ramirez and co-author [[Lisa Kaltenegger]] has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in the Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos<ref name=rayeric-2011>{{cite journal |title=Hydrogen Greenhouse Planets Beyond the Habitable Zone |last1=Pierrehumbert |first1=Raymond |date=2011 |arxiv=1105.0021|last2=Gaidos |first2=Eric |doi=10.1088/2041-8205/734/1/L13 |volume=734 |issue=1 |pages=L13 |journal=The Astrophysical Journal Letters|bibcode=2011ApJ...734L..13P|s2cid=7404376 }}</ref> had eliminated the CO<sub>2</sub>-H<sub>2</sub>O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens of millions of years.

In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause [[tidal locking]], an important factor in habitability. For a tidally locked planet, the [[sidereal day]] is as long as the [[orbital period]], causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its [[bond albedo]] and reducing significantly temperature differences between the two sides.<ref name=yang-2013>{{Cite journal | last1 = Yang | first1 = J. | last2 = Cowan | first2 = N. B. | last3 = Abbot | first3 = D. S. | doi = 10.1088/2041-8205/771/2/L45 | title = Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets | journal = The Astrophysical Journal | volume = 771 | issue = 2 | pages = L45 | year = 2013| arxiv = 1307.0515| bibcode = 2013ApJ...771L..45Y| s2cid = 14119086 }}</ref>

Planetary mass [[exomoon|natural satellites]] have the potential to be habitable as well. However, these bodies need to fulfill additional parameters, in particular being located within the circumplanetary habitable zones of their host planets.<ref name=hadhazy-2013 /> More specifically, moons need to be far enough from their host giant planets that they are not transformed by tidal heating into volcanic worlds like [[Io (moon)|Io]],<ref name=hadhazy-2013 /> but must remain within the [[Hill radius]] of the planet so that they are not pulled out of the orbit of their host planet.<ref name="HamiltonBurns92">{{cite journal |author1=D.P. Hamilton |author2=J.A. Burns | title= Orbital stability zones about asteroids. II – The destabilizing effects of eccentric orbits and solar radiation| journal= Icarus| date= 1992| volume= 96 |issue= 1| pages= 43–64| bibcode= 1992Icar...96...43H |doi= 10.1016/0019-1035(92)90005-R|url=http://www.astro.umd.edu/~hamilton/research/reprints/HamBurns91.pdf|citeseerx=10.1.1.488.4329 }}</ref> Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability.<ref name=hadhazy-2013 />
[[File:Eccentric Habitable Zones.jpg|thumb|Artist's concept of a planet on an eccentric orbit that passes through the HZ for only part of its orbit]]
A planetary object that orbits a star with high [[orbital eccentricity]] may spend only some of its year in the HZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation ([[cryptobiosis]]) and/or [[hyperthermophile|hyperthermostability]]. [[Tardigrades]], for example, can survive in a dehydrated state temperature between {{convert|-273|C|K|order=flip}}<ref>{{cite journal|author=Becquerel P.|date=1950| title=La suspension de la vie au dessous de 1/20 K absolu par demagnetization adiabatique de l'alun de fer dans le vide les plus eléve| journal=C. R. Acad. Sci. Paris| volume=231| pages=261–263| language=fr}}</ref> and {{convert|151|C|K|order=flip}}.<ref name="survival">{{cite book| last=Horikawa|first=Daiki D.|chapter=Survival of Tardigrades in Extreme Environments: A Model Animal for Astrobiology |series=Cellular Origin, Life in Extreme Habitats and Astrobiology | title=Anoxia Evidence for Eukaryote Survival and Paleontological Strategies.| date=2012|volume=21 |publisher=Springer Netherlands| isbn=978-94-007-1895-1|pages=205–217| edition=21|editor=Alexander V. Altenbach, Joan M. Bernhard & Joseph Seckbach|doi=10.1007/978-94-007-1896-8_12}}</ref> Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the [[apastron]] where the planet is coolest and become active on approach to the [[periastron]] when the planet is sufficiently warm.<ref name=kane-2012>{{cite journal |title=The Habitable Zone and Extreme Planetary Orbits |author1=Kane, Stephen R. |author2=Gelino, Dawn M. |journal=Astrobiology |date=2012 |volume=12 |pages=940–945 |doi=10.1089/ast.2011.0798 |arxiv=1205.2429 |issue=10 |pmid=23035897|bibcode = 2012AsBio..12..940K |s2cid=10551100 }}</ref>