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Revision 915792438 of "Circumstellar habitable zone" on enwiki{{redirect|Goldilocks Zone|the more general principle|Goldilocks principle}}
{{redirect|Habitable zone|the galactic zone|Galactic habitable zone}}
{{short description|Zone around a star with strong possibilities for stable liquid water on a suitable planet}}
[[File:Diagram of different habitable zone regions by Chester Harman.jpg|thumb|300px|A diagram depicting the habitable zone boundaries around stars, and how the boundaries are affected by [[Stellar classification|star type]]. This new plot includes [[Solar System]] planets ([[Venus]], [[Earth]], and [[Mars]]) as well as especially significant [[exoplanet]]s such as [[TRAPPIST-1d]], [[Kepler-186f]], and our nearest neighbor [[Proxima Centauri b]].]]
In [[astronomy]] and [[astrobiology]], the '''circumstellar habitable zone''' ('''CHZ'''), or simply the '''habitable zone''', 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 |url=http://iopscience.iop.org/2041-8205/767/1/L8 |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}}</ref><ref name="SCI-20130503">{{cite journal |last1=Cruz |first1=Maria |last2=Coontz |first2=Robert |title=Exoplanets - Introduction to Special Issue |url=http://www.sciencemag.org/content/340/6132/565 |journal=[[Science (journal)|Science]] |volume=340 |page=565 |doi=10.1126/science.340.6132.565 |pmid=23641107 |accessdate=18 May 2013 |issue=6132 |date=2013}}</ref> The bounds of the CHZ 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 [[planetary habitability|nature of the CHZ]] and the objects within it may be instrumental in determining the scope and distribution of Earth-like [[extraterrestrial life]] and [[extraterrestrial intelligence|intelligence]].
The habitable zone is also called the '''Goldilocks zone''', a metaphor of the children's [[fairy tale]] of "[[Goldilocks and the Three Bears]]", in which a little girl chooses from sets of three items, ignoring 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 in 1953,<ref name=huggett-1995>{{cite book |url=https://books.google.com/books/about/Geoecology.html?id=VyQjwI9UkVIC |title=Geoecology: An Evolutionary Approach |publisher=Routledge, Chapman & Hall |author=Huggett, Richard J. |date=1995 |page=10 |isbn=978-0-415-08689-9}}</ref> many stars have been confirmed to possess a CHZ planet, including some systems that consist of multiple CHZ planets.<ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |authorlink=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]] |accessdate=January 6, 2015}}</ref> Most such planets, being [[super-Earth]]s or [[gas giant]]s, are more massive than Earth, because such [[extrasolar planet|planets]] are [[sampling bias|easier to detect]]. On November 4, 2013, astronomers reported, based on [[Kepler (spacecraft)|''Kepler'']] data, that there could be as many as 40 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 |accessdate=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|url=http://www.pnas.org/content/early/2013/10/31/1319909110 |date=October 31, 2013|journal=[[Proceedings of the National Academy of Sciences of the United States of America]]|doi=10.1073/pnas.1319909110 |accessdate=November 5, 2013 |arxiv = 1311.6806 |bibcode = 2013PNAS..11019273P |volume=110 |issue=48|pages=19273–19278 |pmid=24191033 |pmc=3845182}}</ref> 11 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]] |accessdate=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 |last=Anglada-Escudé |first=Guillem |display-authors=etal |title=A terrestrial planet candidate in a temperate orbit around Proxima Centauri |year=2016 |journal=Nature |volume=536 |issue=7617 |pages=437–440 |doi=10.1038/nature19106 |pmid=27558064 |arxiv=1609.03449 |bibcode=2016Natur.536..437A}}</ref> The CHZ is also of particular interest to the emerging field of [[habitability of natural satellites]], because planetary-mass [[exomoon|moons]] in the CHZ 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 |accessdate=9 May 2013}}</ref>
In subsequent decades, the CHZ concept began to be challenged as a primary criterion for life, so the concept is still evolving.<ref name="Review 2009"/> 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 biospheres, 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 within 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}}</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 |accessdate=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}}</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 |accessdate=24 June 2013}}</ref> Liquid water can also exist at a wider range of temperatures and pressures as a [[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 |accessdate=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...6E8156S|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 |accessdate=April 22, 2013 |author=Villard, Ray |agency=Discovery Communications LLC}}</ref>
==History==
An estimate of the range of distances from the Sun allowing the existence of liquid water appears in [[Isaac Newton|Newton's]] ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'' (Book III, Section 1, corol. 4).<ref>[http://www.17centurymaths.com/contents/newton/book3s1.pdf 3rd Edition (1728), trans Bruce, I]</ref> The concept of a circumstellar habitable zone was first introduced in 1953 by [[Hubertus Strughold]], who in his treatise ''The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars'', coined the term "ecosphere" and referred to various "zones" in which life could emerge.<ref name=huggett-1995 /><ref name=strughold-1953>{{cite book |url=https://books.google.com/books/about/The_green_and_red_planet.html?id=zNbPAAAAMAAJ |title=The Green and Red Planet: A Physiological Study of the Possibility of Life on Mars |publisher=University of New Mexico Press |author=Strughold, Hubertus |date=1953}}</ref> In the same year, [[Harlow Shapley]] wrote "Liquid Water Belt", which described the same theory in further scientific detail. Both works stressed the importance of liquid water to life.<ref name="Kasting2010">{{cite book|author=Kasting, James|title=How to Find a Habitable Planet|url=https://books.google.com/books?id=xPqEeB-SRvUC|accessdate=4 May 2013|date=2010|publisher=Princeton University Press|isbn=978-0-691-13805-3|page=127}}</ref> [[Su-Shu Huang]], an American astrophysicist, first introduced the term "habitable zone" in 1959 to refer to the area around a star where liquid water could exist on a sufficiently large body, and was the first to introduce it in the context of planetary habitability and extraterrestrial life.<ref name=kasting-1993>{{cite journal |url= |title=Habitable Zones around Main Sequence Stars |author1=Kasting, James F. |author2=Whitmire, Daniel P. |author3=Reynolds, Ray T. |journal=Icarus |date=January 1993 |volume=101 |issue=1 |pages=108–118 |doi=10.1006/icar.1993.1010 |bibcode=1993Icar..101..108K |pmid=11536936}}</ref><ref name=huang-1966>{{cite book |url=https://books.google.com/books?id=D0UrAAAAYAAJ |title=Extraterrestrial life: An Anthology and Bibliography |publisher=National Academy of Sciences |author=Huang, Su-Shu |date=1966 |location=Washington, D. C. |pages=87–93 |others=National Research Council (U.S.). Study Group on Biology and the Exploration of Mars}}</ref> A major early contributor to habitable zone theory, Huang argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in [[multiple star system]]s, given the gravitational instabilities of those systems.<ref name=huang-1960>{{cite journal |title=Life-Supporting Regions in the Vicinity of Binary Systems |author=Huang, Su-Shu |journal=Publications of the Astronomical Society of the Pacific |date=April 1960 |volume=72 |issue=425 |pages=106–114 |bibcode=1960PASP...72..106H |doi=10.1086/127489}}</ref>
The theory of habitable zones was further developed in 1964 by [[Stephen H. Dole]] in his book ''Habitable Planets for Man'', in which he discussed the concept of circumstellar habitable zone as well as various other determinants of planetary habitability, eventually guestimating the number of habitable planets in the Milky Way to be about 600 million.<ref name="dole-1964"/> At the same time, science-fiction author [[Isaac Asimov]] introduced the concept of a circumstellar habitable zone to the general public through his various explorations of [[space colonization]].<ref name=gilster-2004>{{cite book |url=https://books.google.com/books/about/Centauri_Dreams.html?id=L4fffd3SivkC |title=Centauri Dreams: Imagining and Planning Interstellar Exploration |publisher=Springer |author=Gilster, Paul |date=2004 |isbn=978-0-387-00436-5 |page=40}}</ref> The term "[[Goldilocks principle|Goldilocks zone]]" emerged in the 1970s, referencing specifically a region around a star whose temperature is "just right" for water to be present in the liquid phase.<ref name=nasa-2003>{{cite press release |url=https://science.nasa.gov/science-news/science-at-nasa/2003/02oct_goldilocks/ |title=The Goldilocks Zone |publisher=NASA |date=October 2, 2003 |accessdate=April 22, 2013}}</ref> In 1993, astronomer [[James Kasting]] introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone.<ref name=kasting-1993 /> Kasting was the first to present a detailed model for the habitable zone for exoplanets.<ref name="F. Kasting, D. P 1993"/><ref name="Seager 2013">{{cite journal |title=Exoplanet Habitability |journal=Science |year=2013 |last=Seager |first=Sara |volume=340 |issue=577 |pages=577–581 |doi=10.1126/science.1232226 |pmid=23641111 |bibcode=2013Sci...340..577S }}</ref>
An update to habitable zone theory came in 2000, when astronomers [[Peter Ward (paleontologist)|Peter Ward]] and [[Donald Brownlee]] introduced the idea of the "[[galactic habitable zone]]", which they later developed with [[Guillermo Gonzalez (astronomer)|Guillermo Gonzalez]].<ref name="Rare Earth" /><ref name=gonzalez-2001>{{cite journal |title=The Galactic Habitable Zone I. Galactic Chemical Evolution |author1=Gonzalez, Guillermo |author2=Brownlee, Donald |author3=Ward, Peter |journal=Icarus |date=July 2001 |volume=152 |issue=1 |pages=185–200 |doi=10.1006/icar.2001.6617 |arxiv=astro-ph/0103165|bibcode = 2001Icar..152..185G }}</ref> The galactic habitable zone, defined as the region where life is most likely to emerge in a galaxy, encompasses those regions close enough to a [[galactic center]] that stars there are enriched with [[metallicity|heavier elements]], but not so close that star systems, planetary orbits, and the emergence of life would be frequently disrupted by the intense radiation and enormous gravitational forces commonly found at galactic centers.<ref name="Rare Earth"/>
Subsequently, some astrobiologists propose that the concept be extended to other solvents, including dihydrogen, sulfuric acid, dinitrogen, formamide, and methane, among others, which would support hypothetical life forms that use an [[alternative biochemistry]].<ref name=villard-2011 /> In 2013, further developments in habitable zone theory were made with the proposal of a circum''planetary'' habitable zone, also known as the "habitable edge", to encompass the region around a planet where the orbits of natural satellites would not be disrupted, and at the same time tidal heating from the planet would not cause liquid water to boil away.<ref name=hadhazy-2013>{{cite news |url=http://www.astrobio.net/exclusive/5364/the-habitable-edge-of-exomoons |title=The 'Habitable Edge' of Exomoons |work=Astrobiology Magazine |date=April 3, 2013 |agency=NASA |accessdate=April 22, 2013 |author=Hadhazy, Adam}}</ref>
==Determination==
[[File:Triple_point_diagram_indicating_planets_within_Solar_System_habitable_zone.png|thumb|Thermodynamic properties of water depicting the conditions at the surface of the terrestrial planets: Mars is near the triple point, Earth in the liquid; and Venus near the critical point.]]
[[File:Estimated extent of the Solar Systems habitable zone.png|thumb|The range of published estimates for the extent of the Sun's CHZ. The conservative CHZ<ref name=dole-1964 /> is indicated by a dark-green band crossing the inner edge of the [[aphelion]] of [[Venus]], whereas an extended CHZ,<ref name=fogg-1992 /> extending to the orbit of the [[dwarf planet]] [[Ceres (dwarf planet)|Ceres]], is indicated by a light-green band.]]
Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets which can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth or Venus can remain habitable, complemented by a larger "extended habitable zone" in which super-Earth planets, with stronger [[greenhouse effect]]s, can have the right temperature for liquid water to exist at the surface.<ref name=redd-2011>{{cite news |url=http://www.astrobio.net/exclusive/4174/greenhouse-effect-could-extend-habitable-zone |title=Greenhouse Effect Could Extend Habitable Zone |work=Astrobiology Magazine |date=25 August 2011 |agency=NASA |accessdate=25 June 2013 |author=Redd, Nola Taylor}}</ref>
The inner edge of the HZ is the distance where a [[runaway greenhouse effect]] vaporizes the whole water reservoir and,<ref name="Review 2009">{{cite journal |title=What makes a planet habitable? |journal=The Astronomy and Astrophysics Review |year=2009 |last=Lammer |first=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 |format=PDF |accessdate=2016-05-03 |bibcode=2009A&ARv..17..181L |display-authors=etal |deadurl=yes |archiveurl=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 |archivedate=2016-06-02 |df= }}</ref> as a second effect, induces the photodissociation of water vapor and the loss of hydrogen to space. The outer edge of the HZ is the distance from the star where adding more carbon dioxide to the atmosphere fails to keep the surface of the planet above the freezing point.<ref name="Review 2009"/>
===Solar System estimates===
Estimates for the habitable zone within the Solar System range from 0.38 to 10.0 [[astronomical units]],<ref name=zsom-2013 /><ref name= rayeric-2011 /><ref name= rk-2017 /><ref>{{cite web| url=http://depts.washington.edu/naivpl/sites/default/files/hz.shtml| title=Stellar habitable zone calculator| publisher=[[University of Washington]]| accessdate=17 December 2015}}</ref> though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However their atmospheric conditions vary substantially. The aphelion of Venus, for example, touches the inner edge of the zone and while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to {{convert|462|C|F}} at which water can only exist as vapour.<ref name=venus-2006>{{cite web|url=http://burro.cwru.edu/stu/advanced/venus.html |title=Venus |publisher=Case Western Reserve University |date=13 September 2006 |accessdate=2011-12-21 |deadurl=yes |archiveurl=https://web.archive.org/web/20120426064658/http://burro.cwru.edu/stu/advanced/venus.html |archivedate=2012-04-26 |df= }}</ref> The entire orbits of the [[Moon]],<ref name=sharp>{{cite web |url=http://www.space.com/18067-moon-atmosphere.html |title=Atmosphere of the Moon |publisher=TechMediaNetwork |work=Space.com |accessdate=April 23, 2013 |author=Sharp, Tim}}</ref> [[Mars]],<ref name="bolonkin09">{{Cite book|first=Alexander A.|last=Bolonkin|date=2009|title=Artificial Environments on Mars|publisher=Springer |place=Berlin Heidelberg|pages=599–625|isbn=978-3-642-03629-3}}</ref> and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods.<ref name="HaberleMcKay2001">{{cite journal|last1=Haberle|first1=Robert M.|last2=McKay|first2=Christopher P.|last3=Schaeffer|first3=James|last4=Cabrol|first4=Nathalie A.|last5=Grin|first5=Edmon A.|last6=Zent|first6=Aaron P.|last7=Quinn|first7=Richard|title=On the possibility of liquid water on present-day Mars|journal=Journal of Geophysical Research|volume=106|issue=E10|year=2001|pages=23317|issn=0148-0227|doi=10.1029/2000JE001360|bibcode = 2001JGR...10623317H }}</ref> At [[Hellas Basin]], for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year.<ref name = "HaberleMcKay2001"/> Despite indirect evidence in the form of [[seasonal flows on warm Martian slopes]],<ref name="WRD-20140218">{{cite web |last=Mann |first=Adam |title=Strange Dark Streaks on Mars Get More and More Mysterious|url=https://www.wired.com/wiredscience/2014/02/flowing-lineae-water-mars/ |date=February 18, 2014 |work=[[Wired (magazine)|Wired]] |accessdate=February 18, 2014 }}</ref><ref name=voanews>{{cite web|url=http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html|title=NASA Finds Possible Signs of Flowing Water on Mars|publisher=voanews.com|accessdate=August 5, 2011}}</ref><ref name=mag>{{cite web|url=http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html|title=Is Mars Weeping Salty Tears?|publisher=news.sciencemag.org|accessdate=August 5, 2011|deadurl=yes|archiveurl=https://web.archive.org/web/20110814065220/http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html|archivedate=August 14, 2011|df=}}</ref><ref name="NASA-20131210">{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA Mars Spacecraft Reveals a More Dynamic Red Planet |url=http://www.jpl.nasa.gov/news/news.php?release=2013-361&1#1 |date=December 10, 2013 |work=[[NASA]] |accessdate=December 10, 2013 }}</ref> no confirmation has been made of the presence of liquid water there. While other objects orbit partly within this zone, including comets, [[Ceres (dwarf planet)|Ceres]]<ref name=ahearn-1992>{{cite journal|last=A'Hearn|first=Michael F.|author2=Feldman, Paul D.|title=Water vaporization on Ceres|journal=Icarus|volume=98|issue=1|pages=54–60|date=1992|doi=10.1016/0019-1035(92)90206-M|bibcode= 1992Icar...98...54A}}</ref> is the only one of planetary mass. A combination of low mass and an inability to mitigate evaporation and atmosphere loss against the [[solar wind]] make it impossible for these bodies to sustain liquid water on their surface. Despite this, studies are strongly suggestive of past liquid water on the surface of Venus,<ref name="SalvadorMassol2017">{{cite journal|last1=Salvador|first1=A.|last2=Massol|first2=H.|last3=Davaille|first3=A.|last4=Marcq|first4=E.|last5=Sarda|first5=P.|last6=Chassefière|first6=E.|title=The relative influence of H2 O and CO2 on the primitive surface conditions and evolution of rocky planets|journal=Journal of Geophysical Research: Planets|volume=122|issue=7|year=2017|pages=1458–1486|issn=2169-9097|doi=10.1002/2017JE005286|bibcode=2017JGRE..122.1458S}}</ref> Mars,<ref>{{cite web |url=http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2000| accessdate=December 19, 2010}}</ref><ref name='Willson 2018'>{{cite web|url=https://www.space.com/8642-flashback-water-mars-announced-10-years.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2010| accessdate=May 13, 2018}}</ref><ref>{{cite web| url=https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| title=Science@NASA, The Case of the Missing Mars Water| accessdate=March 7, 2009| deadurl=yes| archiveurl=https://web.archive.org/web/20090327234049/https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| archivedate=March 27, 2009| df=}}</ref> [[4 Vesta|Vesta]]<ref name="ScullyRussell2015">{{cite journal|last1=Scully|first1=Jennifer E.C.|last2=Russell|first2=Christopher T.|last3=Yin|first3=An|last4=Jaumann|first4=Ralf|last5=Carey|first5=Elizabeth|last6=Castillo-Rogez|first6=Julie|last7=McSween|first7=Harry Y.|last8=Raymond|first8=Carol A.|last9=Reddy|first9=Vishnu|last10=Le Corre|first10=Lucille|title=Geomorphological evidence for transient water flow on Vesta|journal=Earth and Planetary Science Letters|volume=411|year=2015|pages=151–163|issn=0012-821X|doi=10.1016/j.epsl.2014.12.004|bibcode=2015E&PSL.411..151S}}</ref> and Ceres,<ref name="RaponiDe Sanctis2018">{{cite journal|last1=Raponi|first1=Andrea|last2=De Sanctis|first2=Maria Cristina|last3=Frigeri|first3=Alessandro|last4=Ammannito|first4=Eleonora|last5=Ciarniello|first5=Mauro|last6=Formisano|first6=Michelangelo|last7=Combe|first7=Jean-Philippe|last8=Magni|first8=Gianfranco|last9=Tosi|first9=Federico|last10=Carrozzo|first10=Filippo Giacomo|last11=Fonte|first11=Sergio|last12=Giardino|first12=Marco|last13=Joy|first13=Steven P.|last14=Polanskey|first14=Carol A.|last15=Rayman|first15=Marc D.|last16=Capaccioni|first16=Fabrizio|last17=Capria|first17=Maria Teresa|last18=Longobardo|first18=Andrea|last19=Palomba|first19=Ernesto|last20=Zambon|first20=Francesca|last21=Raymond|first21=Carol A.|last22=Russell|first22=Christopher T.|title=Variations in the amount of water ice on Ceres' surface suggest a seasonal water cycle|journal=Science Advances|volume=4|issue=3|year=2018|pages=eaao3757|issn=2375-2548|doi=10.1126/sciadv.aao3757|pmid=29546238|pmc=5851659|bibcode=2018SciA....4O3757R}}</ref><ref>https://photojournal.jpl.nasa.gov/catalog/PIA21471 PIA21471: Landslides on Ceres</ref> suggesting a more common phenomena than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.
According to extended habitable zone theory, planetary mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth ([[super-Earth]] class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources cannot be determined by their position in the habitable zone.
For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone are: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU.
{| class="wikitable sortable"
|- style="text-align:center; align:center; background:#90b0f0;"
|+ Estimates of the circumstellar habitable zone boundaries of the Solar System
! Inner edge ([[Astronomical unit|AU]]) !! Outer edge (AU) !! Year !! Notes
|-
| 0.725 || 1.24 || Dole 1964<ref name=dole-1964 /> || Used optically thin atmospheres and fixed albedos. Places the aphelion of Venus just inside the zone.
|-
| || 1.385–1.398 || Budyko 1969<ref name=budyko-1969>{{Cite journal | last1 = Budyko | first1 = M. I. | title = The effect of solar radiation variations on the climate of the Earth | doi = 10.1111/j.2153-3490.1969.tb00466.x | journal = Tellus | volume = 21 | issue = 5 | pages = 611–619 | year = 1969 | pmid = | pmc = | bibcode = 1969TellA..21..611B| citeseerx = 10.1.1.696.824 }}</ref> || Based on studies of ice albedo feedback models to determine the point at which Earth would experience global glaciation. This estimate was supported in studies by Sellers 1969<ref>{{cite journal |title=A Global Climatic Model Based on the Energy Balance of the Earth-Atmosphere System |author=Sellers, William D. |journal=Journal of Applied Meteorology |date=June 1969 |volume=8 |issue=3 |pages=392–400 |doi=10.1175/1520-0450(1969)008<0392:AGCMBO>2.0.CO;2 |doi-access=free |bibcode=1969JApMe...8..392S}}</ref> and North 1975.<ref>{{cite journal
|last1 = North
|first1 = Gerald R.
|date=November 1975
|title = Theory of Energy-Balance Climate Models
|journal = Journal of the Atmospheric Sciences
|volume = 32
|issue = 11
|pages = 2033–2043
|doi = 10.1175/1520-0469(1975)032<2033:TOEBCM>2.0.CO;2
|doi-access=free
|bibcode = 1975JAtS...32.2033N
}}</ref>
|-
| 0.88–0.912 || || Rasool and De Bergh 1970<ref name=rasool-1970>{{cite journal | pages = 1037–1039 | issue = 5250 | volume = 226 | date = Jun 1970 | doi = 10.1038/2261037a0 | pmid = 16057644 | issn = 0028-0836 | journal = Nature | first2 = C. | first1 = I. | title = The Runaway Greenhouse and the Accumulation of CO<sub>2</sub> in the Venus Atmosphere | url = http://pubs.giss.nasa.gov/docs/1970/1970_Rasool_DeBergh_1.pdf | last1 = Rasool | format = | last2 = De Bergh | bibcode = 1970Natur.226.1037R }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>|| Based on studies of Venus's atmosphere, Rasool and De Bergh concluded that this is the minimum distance at which Earth would have formed stable oceans.
|-
| 0.95 || 1.01 || Hart et al. 1979<ref name=hart-1979>{{Cite journal | last1 = Hart | first1 = M. H. | doi = 10.1016/0019-1035(79)90141-6 | title = Habitable zones about main sequence stars | journal = Icarus | volume = 37 | issue = 1 | pages = 351–357 | year = 1979 | pmid = | pmc = |bibcode = 1979Icar...37..351H }}</ref> || Based on computer modelling and simulations of the evolution of Earth's atmospheric composition and surface temperature. This estimate has often been cited by subsequent publications.
|-
| || 3.0 || Fogg 1992<ref name=fogg-1992>{{cite journal |title=An Estimate of the Prevalence of Biocompatible and Habitable Planets |author=Fogg, M. J. |journal=Journal of the British Interplanetary Society |date=1992 |volume=45 |pages=3–12 |bibcode=1992JBIS...45....3F |pmid=11539465 |issue=1}}</ref>|| Used the [[carbon cycle]] to estimate the outer edge of the circumstellar habitable zone.
|-
| 0.95 || 1.37 || Kasting et al. 1993<ref name=kasting-1993 />|| Founded the most common working definition of the habitable zone used today. Assumes that CO<sub>2</sub> and H<sub>2</sub>O are the key greenhouse gases as they are for the Earth. Argued that the habitable zone is wide because of the [[carbonate-silicate cycle]]. Noted the cooling effect of cloud albedo. Table shows conservative limits. Optimistic limits were 0.84–1.67 AU.
|-
| || 2.0 || Spiegel et al. 2010<ref name="speigel-2010">{{Cite journal | last1 = Spiegel | first1 = D. S. | last2 = Raymond | first2 = S. N. | last3 = Dressing | first3 = C. D. | last4 = Scharf | first4 = C. A. | last5 = Mitchell | first5 = J. L. | title = Generalized Milankovitch Cycles and Long-Term Climatic Habitability | doi = 10.1088/0004-637X/721/2/1308 | journal = The Astrophysical Journal | volume = 721 | issue = 2 | pages = 1308–1318 | year = 2010 | pmid = | pmc = |arxiv = 1002.4877 |bibcode = 2010ApJ...721.1308S }}</ref> || Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.
|-
| 0.75 || || Abe et al. 2011<ref name="abe-2011">{{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| pmc = |bibcode = 2011AsBio..11..443A }}</ref> || Found that land-dominated "desert planets" with water at the poles could exist closer to the Sun than watery planets like Earth.
|-
| ||10 || Pierrehumbert and Gaidos 2011<ref name= rayeric-2011 /> ||Terrestrial planets that accrete tens-to-thousands of bars of primordial hydrogen from the protoplanetary disc may be habitable at distances that extend as far out as 10 AU in our solar system.
|-
| 0.77–0.87 || 1.02–1.18 || Vladilo et al. 2013<ref name=vladilo-2013>{{cite journal |url=http://iopscience.iop.org/0004-637X/767/1/65/ |title=The habitable zone of Earth-like planets with different levels of atmospheric pressure |author1=Vladilo, Giovanni |author2=Murante, Giuseppe |author3=Silva, Laura |author4=Provenzale, Antonello |author5=Ferri, Gaia |author6=Ragazzini, Gregorio |journal=The Astrophysical Journal |date=March 2013 |volume=767 |issue=1 |pages=65–? |doi=10.1088/0004-637X/767/1/65 |arxiv=1302.4566|bibcode = 2013ApJ...767...65V }}</ref> || Inner edge of circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 [[millibar]].
|-
| 0.99 || 1.70 || Kopparapu et al. 2013<ref name=kopparapu-2013 /><ref name="Kopparapu2013b" /> || Revised estimates of the Kasting et al. (1993) formulation using updated moist greenhouse and water loss algorithms. According to this measure Earth is at the inner edge of the HZ and close to, but just outside, the moist greenhouse limit. As with Kasting et al. (1993), this applies to an Earth-like planet where the "water loss" (moist greenhouse) limit, at the inner edge of the habitable zone, is where the temperature has reached around 60 Celsius and is high enough, right up into the troposphere, that the atmosphere has become fully saturated with water vapour. Once the stratosphere becomes wet, water vapour photolysis releases hydrogen into space. At this point cloud feedback cooling does not increase significantly with further warming. The "maximum greenhouse" limit, at the outer edge, is where a {{CO2}} dominated atmosphere, of around 8 bars, has produced the maximum amount of greenhouse warming, and further increases in {{CO2}} will not create enough warming to prevent {{CO2}} catastrophically freezing out of the atmosphere. Optimistic limits were 0.97–1.70 AU. This definition does not take into account possible radiative warming by {{CO2}} clouds.
|-
| 0.38 || || Zsom et al. 2013<br /><ref name=zsom-2013>{{cite journal |title=Towards the Minimum Inner Edge Distance of the Habitable Zone |last=Zsom |first=Andras |date=2013 |arxiv=1304.3714 |last2=Seager |first2=Sara |last3=De Wit |first3=Julien |doi=10.1088/0004-637X/778/2/109 |volume=778 |issue=2 |journal=The Astrophysical Journal |page=109 |bibcode=2013ApJ...778..109Z}}</ref> || Estimate based on various possible combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere.
|-
| 0.95 || || Leconte et al. 2013<ref name= leconte-2013>{{cite journal |title=Increased insolation threshold for runaway greenhouse processes on Earth like planets |last=Leconte |first=Jeremy |date=2013 |arxiv=1312.3337 |last2=Forget |first2=Francois |last3=Charnay |first3=Benjamin |last4=Wordsworth |first4=Robin |last5=Pottier |first5=Alizee |doi=10.1038/nature12827 |pmid=24336285 |volume=504 |issue=7479 |pages=268–71 |journal=Nature |bibcode=2013Natur.504..268L}}</ref> ||Using 3-D models, these authors computed an inner edge of 0.95 AU for our solar system.
|-
| 0.95 ||2.4 || Ramirez and Kaltenegger 2017<br /><ref name=rk-2017>{{cite journal |title=A Volcanic Hydrogen Habitable Zone |last=Ramirez |first=Ramses |date=2017 |arxiv=1702.08618|last2=Kaltenegger |first2=Lisa |doi=10.3847/2041-8213/aa60c8 |volume=837 |issue=1 |pages=L4 |journal=The Astrophysical Journal Letters|bibcode=2017ApJ...837L...4R}}</ref> || An expansion of the classical carbon dioxide-water vapor habitable zone <ref name=kasting-1993 /> assuming a volcanic hydrogen atmospheric concentration of 50%.
|}
===Extrasolar extrapolation===
{{see also|Habitability of red dwarf systems|Habitability of orange dwarf systems}}
Astronomers use stellar flux and the [[inverse-square law]] to extrapolate circumstellar habitable zone models created for the Solar System to other stars. For example, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun,<ref name="kopparapu-2013" /> a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at <math>\sqrt{0.25}</math>, or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the CHZ concept is more complex.
====Spectral types and star-system characteristics====
[[File:Circling Two Suns.ogv|thumb|300px|A video explaining the significance of the 2011 discovery of a planet in the circumbinary habitable zone of Kepler-47.]]
Some scientists argue that the concept of a circumstellar habitable zone is actually limited to stars in certain types of systems or of certain [[spectral type]]s. Binary systems, for example, have circumstellar habitable zones that differ from those of single-star planetary systems, in addition to the orbital stability concerns inherent with a three-body configuration.<ref>{{cite journal| arxiv=1303.6645| title=S-Type and P-Type Habitability in Stellar Binary Systems: A Comprehensive Approach. I. Method and Applications| date=2013 |last=Cuntz |first=Manfred | doi=10.1088/0004-637X/780/1/14 | volume=780 | issue=1| journal=The Astrophysical Journal | page=14 | bibcode=2014ApJ...780...14C}}</ref> If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU.<ref name="forget-1997">{{cite journal|doi=10.1126/science.278.5341.1273| title=Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation| date=1997| last1=Forget| first1=F.| journal=Science| volume=278|issue=5341|pages=1273–6| pmid=9360920| last2=Pierrehumbert|first2=RT|bibcode = 1997Sci...278.1273F| citeseerx=10.1.1.41.621}}</ref><ref name="mischna-2000">{{cite journal| doi=10.1006/icar.2000.6380| title=Influence of Carbon Dioxide Clouds on Early Martian Climate| date=2000 |last1=Mischna| first1=M| journal=Icarus| volume=145 |issue=2| pages=546–54| pmid=11543507| last2=Kasting| first2=JF| last3=Pavlov| first3=A|last4=Freedman| first4=R| bibcode = 2000Icar..145..546M }}</ref>
With regard to spectral types, [[Zoltán Balog (astronomer)|Zoltán Balog]] proposes that [[O-type stars]] cannot form planets due to the [[photoevaporation]] caused by their strong [[ultraviolet]] emissions.<ref>{{cite press release| url=http://www.spitzer.caltech.edu/news/863-feature06-31-Planets-Prefer-Safe-Neighborhoods |title=Planets Prefer Safe Neighborhoods |publisher=Spitzer.caltech.edu |accessdate=April 22, 2013 |author=Vu, Linda |agency=NASA/Caltech}}</ref> Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.<ref name="BuccinoLemarchand2006">{{cite journal| last1=Buccino|first1=Andrea P.|last2=Lemarchand|first2=Guillermo A.|last3=Mauas|first3=Pablo J.D.| title=Ultraviolet radiation constraints around the circumstellar habitable zones| journal=Icarus| volume=183| issue=2|date=2006|pages=491–503|doi=10.1016/j.icarus.2006.03.007|arxiv = astro-ph/0512291 |bibcode = 2006Icar..183..491B |citeseerx=10.1.1.337.8642}}</ref> Stars smaller than the Sun, on the other hand, have distinct impediments to habitability. For example, Michael Hart proposed that only main-sequence stars of [[spectral class]] [[K-type main-sequence star|K0]] or brighter could offer habitable zones, an idea which has evolved in modern times into the concept of a [[tidal locking]] radius for [[red dwarf]]s. Within this radius, which is coincidental with the red-dwarf habitable zone, it has been suggested that the volcanism caused by tidal heating could cause a "tidal Venus" planet with high temperatures and no hospitable environment to life.<ref name=barnes-2013>{{cite journal |title=Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary |journal=Astrobiology |date=March 2013 |volume=13 |issue=3 |pages=279–291 |doi=10.1089/ast.2012.0867 |arxiv=1203.5104 |last1=Barnes |first1=Rory |last2=Heller |first2=René |pmid=23537137 |pmc=3612282|bibcode = 2013AsBio..13..279B }}</ref>
Others maintain that circumstellar habitable zones are more common, and that it is indeed possible for water to exist on planets orbiting cooler stars. Climate modelling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces in spite of tidal locking.<ref name=yang-2013 /> Astronomy professor {{ill|Eric Agol|de}} argues that even [[white dwarf]]s may support a relatively brief habitable zone through planetary migration.<ref>{{cite journal |url=http://iopscience.iop.org/2041-8205/731/2/L31/ |title=Transit Surveys for Earths in the Habitable Zones of White Dwarfs |author=Agol, Eric |journal=The Astrophysical Journal Letters |date=April 2011 |volume=731 |issue=2 |pages=1–5 |doi=10.1088/2041-8205/731/2/L31 |arxiv=1103.2791|bibcode = 2011ApJ...731L..31A }}</ref> At the same time, others have written in similar support of semi-stable, temporary habitable zones around [[brown dwarf]]s.<ref name=barnes-2013 /> Also, a habitable zone in the outer parts of stellar systems may exist during the pre-main-sequence phase of stellar evolution, especially around M-dwarfs, potentially lasting for billion-year timescales.<ref name=rk-2014>{{cite journal |title=Habitable Zones of Pre-Main-Sequence Stars |last=Ramirez |first=Ramses |date=2014 |arxiv=1412.1764|last2=Kaltenegger |first2=Lisa |doi=10.1088/2041-8205/797/2/L25 |volume=797 |issue=2 |pages=L25 |journal=The Astrophysical Journal Letters|bibcode=2014ApJ...797L..25R}}</ref>
====Stellar evolution====
[[File:Magnetosphere rendition.jpg|thumb|left|Natural shielding against space weather, such as the magnetosphere depicted in this artistic rendition, may be required for planets to sustain surface water for prolonged periods.]]
Circumstellar habitable zones change over time with stellar evolution. For example, hot O-type stars, which may remain on the [[main sequence]] for fewer than 10 million years,<ref name="carroll">{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |edition=2nd |date=2007 |title=An Introduction to Modern Astrophysics}}</ref> would have rapidly changing habitable zones not conducive to the development of life. Red dwarf stars, on the other hand, which can live for hundreds of billions of years on the main sequence, would have planets with ample time for life to develop and evolve.<ref name="richmond">{{cite web
|last=Richmond |first=Michael |date=November 10, 2004
|url=http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html
|title=Late stages of evolution for low-mass stars
|publisher=Rochester Institute of Technology
|accessdate=2007-09-19 }}</ref><ref name="guo-2009">{{Cite journal | last1 = Guo | first1 = J. | last2 = Zhang | first2 = F. | last3 = Chen | first3 = X. | last4 = Han | first4 = Z. | title = Probability distribution of terrestrial planets in habitable zones around host stars | doi = 10.1007/s10509-009-0081-z | journal = Astrophysics and Space Science | volume = 323 | issue = 4 | pages = 367–373 | year = 2009 | pmid = | pmc = |arxiv = 1003.1368 |bibcode = 2009Ap&SS.323..367G }}</ref> Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out; our Sun, for example, was 75% as bright in the [[Archean|Archaean]] as it is now,<ref name="Kasting-1968">{{Cite journal
|last=Kasting |first=J.F.
|last2=Ackerman |first2=T.P.
|title=Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere
|journal=Science
|volume=234 |issue=4782 |pages=1383–1385
|date=1986
|doi=10.1126/science.11539665
|pmid=11539665
|ref=harv
|url=https://zenodo.org/record/1230890/files/article.pdf
}}</ref> and in the future, continued increases in energy output will put Earth outside the Sun's habitable zone, even before it reaches the [[red giant]] phase.<ref name=franck-2002>{{cite conference |url=http://www.pik-potsdam.de/PLACES/publications/datenfiles/ASP_269.pdf |title=Habitable Zones and the Number of Gaia's Sisters |publisher=Astronomical Society of the Pacific |accessdate=April 26, 2013 |author1=Franck, S. |author2=von Bloh, W. |author3=Bounama, C. |author4=Steffen, M. |author5=Schönberner, D. |author6=Schellnhuber, H.-J. |editor1=Montesinos, Benjamin |editor2=Giménez, Alvaro |editor3=Guinan, Edward F. |booktitle=ASP Conference Series |date=2002 |conference=The Evolving Sun and its Influence on Planetary Environments |pages=261–272 |bibcode=2002ASPC..269..261F |isbn=1-58381-109-5}}</ref> In order to deal with this increase in luminosity, the concept of a ''continuously habitable zone'' has been introduced. As the name suggests, the continuously habitable zone is a region around a star in which planetary-mass bodies can sustain liquid water for a given period of time. Like the general circumstellar habitable zone, the continuously habitable zone of a star is divided into a conservative and extended region.<ref name=franck-2002 />
In red dwarf systems, gigantic [[stellar flare]]s which could double a star's brightness in minutes<ref>{{cite web |first=Ken| last=Croswell| url=https://www.newscientist.com/article/mg16922754.200-red-willing-and-able.html |title=Red, willing and able |accessdate=August 5, 2007|date=January 27, 2001 |format= [http://www.kencroswell.com/reddwarflife.html Full reprint] |magazine=[[New Scientist]]}}</ref> and huge [[starspot]]s which can cover 20% of the star's surface area,<ref name=alekseev-2002>{{Cite journal | last1 = Alekseev | first1 = I. Y.| last2 = Kozlova | first2 = O. V.| title = Starspots and active regions on the emission red dwarf star LQ Hydrae| journal = Astronomy and Astrophysics| volume = 396| pages = 203–211| year = 2002| bibcode = 2002A&A...396..203A| doi = 10.1051/0004-6361:20021424 | pmid =| pmc = }}</ref> have the potential to strip an otherwise habitable planet of its atmosphere and water.<ref name=alpert-2005 /> As with more massive stars, though, stellar evolution changes their nature and energy flux,<ref name=west-2006>{{cite journal| url=http://earthsky.org/space/fewer-flares-starspots-for-older-dwarf-stars |title=Andrew West: 'Fewer flares, starspots for older dwarf stars' |journal=EarthSky |date=December 19, 2006 |accessdate=April 27, 2013 |author=Research Corporation}}</ref> so by about 1.2 billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.<ref name=alpert-2005>{{cite web| last=Alpert |first=Mark |url=http://www.sciam.com/article.cfm?id=red-star-rising |title=Red Star Rising |magazine=Scientific American |date=November 7, 2005 |accessdate=January 19, 2013}}</ref><ref>{{cite web |title=AstronomyCast episode 40: American Astronomical Society Meeting, May 2007 |work=Universe Today |last1=Cain |first1=Fraser |last2=Gay |first2=Pamela |authorlink2=Pamela L. Gay |url=http://media-c02m01.libsyn.com/podcasts/c50d001e8872db18d96cd44a73adccdc/46762eec/astronomycast/AstroCast-070611.mp3 |archive-url=https://wayback.archive-it.org/all/20070926102556/http://media-c02m01.libsyn.com/podcasts/c50d001e8872db18d96cd44a73adccdc/46762eec/astronomycast/AstroCast-070611.mp3 |dead-url=yes |archive-date=2007-09-26 |date=2007 |accessdate=2007-06-17 |df= }}</ref>
Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.<ref>{{cite web| url=http://www.astrobio.net/topic/solar-system/sun/living-in-a-dying-solar-system-part-1/ |title=Living in a Dying Solar System, Part 1| publisher=Astrobiology| language=English|author=Ray Villard|date=27 July 2009|accessdate=8 April 2016}}</ref> For example, the Sun is expected to engulf the previously-habitable Earth as a red giant.<ref name=christensen-2005>{{cite news |url=http://www.space.com/920-red-giants-planets-live.html |title=Red Giants and Planets to Live On |work=Space.com |date=April 1, 2005 |agency=TechMediaNetwork |accessdate=April 27, 2013 |author=Christensen, Bill}}</ref><ref name=rk-2016 /> However, once a red giant star reaches the [[horizontal branch]], it achieves a new equilibrium and can sustain a new circumstellar habitable zone, which in the case of the Sun would range from 7 to 22 AU.<ref name=lopez-2005>{{Cite journal | last1 = Lopez | first1 = B. | last2 = Schneider | first2 = J. | last3 = Danchi | first3 = W. C. | doi = 10.1086/430416 | title = Can Life Develop in the Expanded Habitable Zones around Red Giant Stars? | journal = The Astrophysical Journal | volume = 627 | issue = 2 | pages = 974–985 | year = 2005 | pmid = | pmc = |arxiv = astro-ph/0503520 |bibcode = 2005ApJ...627..974L }}</ref> At such stage, Saturn's moon [[Titan (moon)|Titan]] would likely be habitable in Earth's temperature sense.<ref name="LorenzLunine1997">{{cite journal| last1=Lorenz|first1=Ralph D.|last2=Lunine|first2=Jonathan I.|last3=McKay|first3=Christopher P.|title=Titan under a red giant sun: A new kind of "habitable" moon| journal=Geophysical Research Letters|volume=24|issue=22|date=1997|pages=2905–2908|issn=0094-8276|doi=10.1029/97GL52843|bibcode=1997GeoRL..24.2905L|pmid=11542268|citeseerx=10.1.1.683.8827}}</ref> Given that this new equilibrium lasts for about 1 [[Byr|Gyr]], and because life on Earth emerged by 0.7 Gyr from the formation of the Solar System at latest, life could conceivably develop on planetary mass objects in the habitable zone of red giants.<ref name=lopez-2005 /> However, around such a helium-burning star, important life processes like [[photosynthesis]] could only happen around planets where the atmosphere has carbon dioxide, as by the time a solar-mass star becomes a red giant, planetary-mass bodies would have already absorbed much of their free carbon dioxide.<ref name=voisey-2011>{{cite news |url=http://www.universetoday.com/83248/plausibility-check-habitable-planet-around-red-giants/ |title=Plausibility Check – Habitable Planets around Red Giants |work=Universe Today |date=February 23, 2011 |accessdate=April 27, 2013 |author=Voisey, Jon}}</ref> Moreover, as Ramirez and Kaltenegger (2016)<ref name=rk-2016>{{cite journal |title=Habitable Zones of Post-Main Sequence Stars|last=Ramirez |first=Ramses |date=2016 |arxiv=1605.04924v1|last2=Kaltenegger |first2=Lisa |doi=10.3847/0004-637X/823/1/6 |volume=823 |issue=1 |pages=6 |journal=The Astrophysical Journal|bibcode=2016ApJ...823....6R}}</ref> showed, intense stellar winds would completely remove the atmospheres of such smaller planetary bodies, rendering them uninhabitable anyway. Thus, Titan would not be habitable even after the Sun becomes a red giant.<ref name=rk-2016/> Nevertheless, life need not originate during this stage of stellar evolution for it to be detected. Once the star becomes a red giant, and the habitable zone extends outward, the icy surface would melt, forming a temporary atmosphere that can be searched for signs of life that may have been thriving before the start of the red giant stage.<ref name=rk-2016/>
====Desert planets====
A planet's atmospheric conditions influence its 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>[http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%E2%80%98dune%E2%80%99-planets Alien Life More Likely on 'Dune' Planets] {{webarchive |url=https://web.archive.org/web/20131202223111/http://www.astrobio.net/exclusive/4188/alien-life-more-likely-on-%E2%80%98dune%E2%80%99-planets |date=December 2, 2013 }}, 09/01/11, Charles Q. Choi, ''Astrobiology Magazine''</ref><ref>[http://online.liebertpub.com/doi/abs/10.1089/ast.2010.0545 Habitable Zone Limits for Dry Planets], Yutaka Abe, Ayako Abe-Ouchi, Norman H. Sleep, and Kevin J. Zahnle. ''Astrobiology''. June 2011, 11(5): 443–460. {{DOI|10.1089/ast.2010.0545}}</ref>
====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|Natural satellite habitability}}
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. |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}}</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. |authorlink1= |last2= Humberto |first2= Campins |authorlink2= |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 |booktitle= Asteroids, Comets, and Meteors (IAU S229) |isbn= 978-0-521-85200-5}}</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}}</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 |url=http://www.nature.com/nature/journal/v462/n7275/full/nature08679.html |doi=10.1038/nature08679 |accessdate=2009-12-15 |pmid=20016595 |bibcode=2009Natur.462..891C|arxiv = 0912.3229 }}</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 |url=http://www.iop.org/EJ/abstract/0004-637X/669/2/1279/ |doi=10.1086/521346 |bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 }}</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 |work=The Washington Post |date=December 5, 2011 |accessdate=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 theorised 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 }}</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 |last=Shizgal |first=B. D. |last2=Arkos |first2=G. G. |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 |accessdate=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 our 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 |last=Pierrehumbert |first=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}}</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 CHZs 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}}</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 still remain within the [[Hill radius]] of the planet so that they are not pulled out of 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 of 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 CHZ 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 CHZ 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 temperatures 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=French}}</ref> and {{convert|151|C|K|order=flip}}.<ref name="survival">{{cite book| last=Horikawa|first=Daiki D.| title=Anoxia Evidence for Eukaryote Survival and Paleontological Strategies.| date=2012|publisher=Springer Netherlands| isbn=978-94-007-1895-1|pages=205–217| url=http://www.springerlink.com/content/wp400661m4236045/abstract/| edition=21|editor=Alexander V. Altenbach, Joan M. Bernhard & Joseph Seckbach|accessdate=21 January 2012}}</ref> Life on a planetary object orbiting outside CHZ 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 }}</ref>
==Extrasolar discoveries==
{{see also|List of potentially habitable exoplanets}}
Among [[exoplanets]], a review in 2015 came to the conclusion that [[Kepler-62f]], [[Kepler-186f]] and [[Kepler-442b]] were likely the best candidates for being potentially habitable.<ref name=centauridreams>{{cite web|url=http://www.centauri-dreams.org/?p=32470|title=A Review of the Best Habitable Planet Candidates|author1=Paul Gilster |author2=Andrew LePage |date=2015-01-30|publisher=Centauri Dreams, Tau Zero Foundation|accessdate=2015-07-24}}</ref> These are at a distance of 1200, 490 and 1,120 [[light-years]] away, respectively. Of these, Kepler-186f is similar in size to Earth with a 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its [[red dwarf]] star. Among [[List of nearest terrestrial exoplanet candidates|nearest terrestrial exoplanet candidates]], [[Tau Ceti e]] is 11.9 light-years away. It is in the inner edge of its solar system's habitable zone, giving it an estimated average surface temperature of {{convert|68|C}}.<ref>{{cite book| title=The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space| author=Giovanni F. Bignami| publisher=Springer| year=2015| isbn=978-3-319-17004-6|url=https://books.google.com/books?id=crvpCQAAQBAJ&pg=PA110|page=110}}</ref>
Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put ''η<sub>e</sub>'', the fraction of stars with planets in the CHZ, at 0.48,<ref name="kopparapu-2013" /> meaning that there may be roughly 95–180 billion habitable planets in the Milky Way.<ref name=wethington-2008>{{cite news |url=http://www.universetoday.com/22380/how-many-stars-are-in-the-milky-way/ |title=How Many Stars are in the Milky Way? |work=Universe Today |date=September 16, 2008 |accessdate=April 21, 2013 |author=Wethington, Nicholos}}</ref> However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.<ref name=torres-2013-2>{{cite web |url=http://phl.upr.edu/press-releases/tenpotentiallyhabitableexoplanetsnow |title=Ten potentially habitable exoplanets now |publisher=University of Puerto Rico |work=Habitable Exoplanets Catalog |date=April 26, 2013 |accessdate=April 29, 2013 |author=Torres, Abel Mendez}}</ref>
Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way.<ref name="BorensteinS">{{cite news |last1=Borenstein |first1=Seth |title=Cosmic census finds crowd of planets in our galaxy |agency=Associated Press |date=19 February 2011 |url=http://apnews.excite.com/article/20110219/D9LG45NO0.html |accessdate=24 April 2011 |archive-url=https://web.archive.org/web/20110927053134/http://apnews.excite.com/article/20110219/D9LG45NO0.html |archive-date=27 September 2011 |dead-url=yes }}</ref> NASA's [[Jet Propulsion Laboratory]] 2011 study, based on observations from the ''[[Kepler (spacecraft)|Kepler]]'' mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class [[F-type main-sequence star|F]], [[G-type main-sequence star|G]], and [[orange dwarf|K]] are expected to have planets in their CHZs.<ref name="ChoiCQ">{{cite web |last1=Choi |first1=Charles Q.|url=http://www.space.com/11188-alien-earths-planets-sun-stars.html |title=New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone |date=21 March 2011 |publisher=Space.com |accessdate=2011-04-24}}</ref><ref name=shao-2011>{{Cite journal | last1 = Catanzarite | first1 = J. | last2 = Shao | first2 = M. | doi = 10.1088/0004-637X/738/2/151 | title = The Occurrence Rate of Earth Analog Planets Orbiting Sun-Like Stars | journal = The Astrophysical Journal | volume = 738 | issue = 2 | pages = 151 | year = 2011 | pmid = | pmc = |arxiv = 1103.1443 |bibcode = 2011ApJ...738..151C }}</ref>
===Early findings===
{{Category see also|Giant planets in the habitable zone}}
The first discoveries of extrasolar planets in the CHZ occurred just a few years after the first extrasolar planets were discovered. However these early detections were all gas giant sized, and many in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water.<ref>{{cite journal| url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=105145|author1=Williams, D. |author2=Pollard, D. | title=Earth-like worlds on eccentric orbits: excursions beyond the habitable zone| journal=International Journal of Astrobiology| volume=1|issue=1|pages=61–69|date=2002| doi=10.1017/S1473550402001064|bibcode = 2002IJAsB...1...61W }}</ref>
One of the first discoveries was [[70 Virginis b]], a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold." Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water.<ref name="Extrasolar.net">{{cite web |url = http://www.extrasolar.net/planettour.asp?PlanetID=22 |title = 70 Virginis b |work = Extrasolar Planet Guide |publisher = Extrasolar.net |accessdate = 2009-04-02 |archiveurl=https://web.archive.org/web/20120619015814/http://www.extrasolar.net/planettour.asp?PlanetID=22 |archivedate=2012-06-19}}</ref> [[16 Cygni Bb]], also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the CHZ, such an orbit would causes extreme [[season]]al effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.<ref>{{cite journal| url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=105145 |author1=Williams, D. |author2=Pollard, D. | title=Earth-like worlds on eccentric orbits: excursions beyond the habitable zone| journal=International Journal of Astrobiology| volume=1|issue=1|pages=61–69|date=2002|doi=10.1017/S1473550402001064|bibcode = 2002IJAsB...1...61W }}</ref>
[[Gliese 876 b]], discovered in 1998, and [[Gliese 876 c]], discovered in 2001, are both gas giants discovered in the habitable zone around [[Gliese 876]] that may also have large moons.<ref name="Sudarsky2003">{{cite journal |title=Theoretical Spectra and Atmospheres of Extrasolar Giant Planets |url=http://iopscience.iop.org/0004-637X/588/2/1121/fulltext |last1=Sudarsky |first1=David |last2=Burrows |first2=Adam |last3=Hubeny |first3=Ivan |display-authors=1 |journal=The Astrophysical Journal |volume=588 |issue=2 |pages=1121–1148 |date=2003 |doi=10.1086/374331 |bibcode=2003ApJ...588.1121S |arxiv=astro-ph/0210216 }}</ref> Another gas giant, [[Upsilon Andromedae d]] was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.
Announced on April 4, 2001, [[HD 28185 b]] is a gas giant found to orbit entirely within its star's circumstellar habitable zone<ref>{{Cite journal | doi = 10.1086/506557| last1 = Jones | first1 = B. W. | last2 = Sleep | first2 = P. N. | last3 = Underwood | first3 = D. R. | title = Habitability of Known Exoplanetary Systems Based on Measured Stellar Properties | journal = The Astrophysical Journal | volume = 649 | issue = 2 | pages = 1010–1019 | year = 2006 | bibcode=2006ApJ...649.1010J|arxiv = astro-ph/0603200 }}</ref> and has a low orbital eccentricity, comparable to that of Mars in the Solar System.<ref>{{Cite journal | last1 = Butler | first1 = R. P. | last2 = Wright | first2 = J. T. | last3 = Marcy | first3 = G. W. | last4 = Fischer | first4 = D. A. | last5 = Vogt | first5 = S. S. | last6 = Tinney | first6 = C. G. | last7 = Jones | first7 = H. R. A. | last8 = Carter | first8 = B. D. | last9 = Johnson | first9 = J. A. | last10 = McCarthy | first10 = C. | last11 = Penny | first11 = A. J. | title = Catalog of Nearby Exoplanets | doi = 10.1086/504701 | journal = The Astrophysical Journal | volume = 646 | issue = 1 | pages = 505–522 | year = 2006 | pmid = | pmc = |arxiv = astro-ph/0607493 |bibcode = 2006ApJ...646..505B }}</ref> Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years,<ref>{{Cite journal | doi = 10.1086/341477 | last1 = Barnes | first1 = J. W. | last2 = O'Brien | first2 = D. P. | title = Stability of Satellites around Close‐in Extrasolar Giant Planets | journal = The Astrophysical Journal | volume = 575 | issue = 2 | pages = 1087–1093 | year = 2002 | bibcode=2002ApJ...575.1087B|arxiv = astro-ph/0205035 }}</ref> though it is unclear whether such satellites could form in the first place.<ref>{{Cite journal | last1 = Canup | first1 = R. M. | last2 = Ward | first2 = W. R. | doi = 10.1038/nature04860 | title = A common mass scaling for satellite systems of gaseous planets | journal = Nature | volume = 441 | issue = 7095 | pages = 834–839 | year = 2006 | pmid = 16778883| pmc = |bibcode = 2006Natur.441..834C }}</ref>
[[HD 69830 d]], a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of [[HD 69830]], 41 light years away from Earth.<ref name=lovis-2006>{{cite journal |url=http://www.nature.com/nature/journal/v441/n7091/abs/nature04828.html |author=Lovis| title=An extrasolar planetary system with three Neptune-mass planets |journal=[[Nature (journal)|Nature]] |volume=441 |date=2006 |pages=305–309| doi=10.1038/nature04828 |pmid=16710412 |last2=Mayor |first2=M |last3=Pepe |first3=F |last4=Alibert |first4=Y |last5=Benz |first5=W |last6=Bouchy |first6=F |last7=Correia |first7=AC |last8=Laskar |first8=J |last9=Mordasini |first9=C |issue=7091 |arxiv = astro-ph/0703024 |bibcode = 2006Natur.441..305L |display-authors=1 }}</ref> The following year, [[55 Cancri f]] was discovered within the CHZ of its host star [[55 Cancri|55 Cancri A]].<ref name="ScienceDaily">{{cite web |url = https://www.sciencedaily.com/releases/2007/11/071106133058.htm |title = Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri |publisher = Sciencedaily.com |date = November 6, 2007 |accessdate = 2008-09-14| archiveurl = https://web.archive.org/web/20080926142319/https://www.sciencedaily.com/releases/2007/11/071106133058.htm| archivedate = 26 September 2008 <!--DASHBot-->| deadurl = no}}</ref><ref name="Fischer2008">{{Cite journal |title=Five Planets Orbiting 55 Cancri |url=http://iopscience.iop.org/0004-637X/675/1/790/fulltext/ |last1=Fischer |first1=Debra A. |last2=Marcy |first2=Geoffrey W. |last3=Butler |first3=R. Paul |last4=Vogt |first4=Steven S. |last5=Laughlin |first5=Greg |last6=Henry |first6=Gregory W. |last7=Abouav |first7=David |last8=Peek |first8=Kathryn M. G. |last9=Wright |first9=Jason T. |display-authors=1 |journal=The Astrophysical Journal |date=2008 |volume=675 |issue=1 |pages=790–801 |arxiv=0712.3917 |bibcode=2008ApJ...675..790F |doi=10.1086/525512 }}</ref> Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.<ref name="guardian">{{cite news| url=https://www.theguardian.com/science/2007/nov/07/spaceexploration |title=Could this be Earth's near twin? Introducing planet 55 Cancri f |newspaper=[[The Guardian]] |author=Ian Sample, science correspondent |date= 7 November 2007|accessdate=17 October 2008 |location=London| archiveurl= https://web.archive.org/web/20081002080911/http://www.guardian.co.uk/science/2007/nov/07/spaceexploration| archivedate= 2 October 2008 <!--DASHBot-->| deadurl= no}}</ref>
Though in theory such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been detected. Planets within the zone with the potential for solid surfaces were therefore of much greater interest.
===Habitable super-Earths===
{{Category see also|Super-Earths in the habitable zone}}
[[File:Gliese 581 - 2010.jpg|thumb|The [[habitable zone]] of Gliese 581 compared with our Solar System's habitable zone.]]
The 2007 discovery of [[Gliese 581 c]], the first [[super-Earth]] in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus.<ref>{{cite news |url=http://www.space.com/scienceastronomy/070424_exoplanet_side.html |title=Planet Hunters Edge Closer to Their Holy Grail |last=Than |first=Ker |date=2007-02-24 |publisher=space.com |accessdate=2007-04-29}}</ref> Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014. [[Gliese 581 g]], yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014.<ref name="SCI-20140703">{{cite journal|last=Robertson |first=Paul |last2=Mahadevan |first2=Suvrath |last3=Endl |first3=Michael |last4=Roy |first4=Arpita |title=Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581 |journal=[[Science (journal)|Science]] |date=3 July 2014 |doi=10.1126/science.1253253 |pmid=24993348 |arxiv=1407.1049 |bibcode=2014Sci...345..440R |volume=345 |issue=6195 |pages=440–444|citeseerx=10.1.1.767.2071 }}</ref>
[[File:Kepler-22 diagram.jpg|thumb|left|A diagram comparing size (artist's impression) and orbital position of planet Kepler-22b within Sun-like star Kepler 22's habitable zone and that of Earth in the Solar System]]
Discovered in August 2011, [[HD 85512 b]] was initially speculated to be habitable,<ref name=maxisciences>{{cite web| url=http://www.maxisciences.com/plan%E8te-habitable/des-chercheurs-decouvrent-une-planete-potentiellement-habitable_art16635.html| title=Researchers find potentially habitable planet| publisher=maxisciences.com| language=French| accessdate=2011-08-31| date=2011-08-30}}</ref> but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.<ref name=torres-2013-2 />
[[Kepler-22 b]], discovered in December 2011 by the ''Kepler'' space probe,<ref name="bbc20111205">{{cite news| url=https://www.bbc.co.uk/news/science-environment-16040655 |title=Kepler 22-b: Earth-like planet confirmed |publisher=BBC |date=December 5, 2011 |accessdate=May 2, 2013}}</ref> is the first [[transit method|transiting]] exoplanet discovered around a [[solar analog|Sun-like star]]. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet.<ref name="Caleb Scharf Blog">{{cite web |url=http://blogs.scientificamerican.com/life-unbounded/2011/12/08/cant-always-tell-an-exoplanet-by-its-size |title=You Can't Always Tell an Exoplanet by Its Size |date=2011-12-08 |last=Scharf |first=Caleb A. |magazine=Scientific American |accessdate=2012-09-20 }}: "If it [Kepler-22b] had a similar composition to Earth, then we're looking at a world in excess of about 40 Earth masses".</ref>
[[Gliese 667 Cc]], discovered in 2011 but announced in 2012,<ref name=arxiv12020446>{{Cite journal |first1=Guillem |last1=Anglada-Escude |first2=Pamela |last2=Arriagada |first3=Steven |last3=Vogt |first4=Eugenio J. |last4=Rivera |first5=R. Paul |last5=Butler |first6=Jeffrey D. |last6=Crane |first7=Stephen A. |last7=Shectman |first8=Ian B. |last8=Thompson |first9=Dante |last9=Minniti |title=A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone |date=2012 |arxiv=1202.0446 |doi=10.1088/2041-8205/751/1/L16 |volume=751 |issue=1 |journal=The Astrophysical Journal |page=L16|bibcode = 2012ApJ...751L..16A }}</ref> is a super-Earth orbiting in the circumstellar habitable zone of [[Gliese 667 C]].
[[Gliese 163 c]], discovered in September 2012 in orbit around the red dwarf [[Gliese 163]]<ref name="Simbad-20120920">{{cite web |author=Staff |title=LHS 188 – High proper-motion Star |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HIP+19394 |date=September 20, 2012 |publisher=[[Centre de données astronomiques de Strasbourg]] (Strasbourg astronomical Data Center) |accessdate=September 20, 2012 }}</ref> is located 49 [[light year]]s from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about {{formatnum:60}}° [[celsius|C]].<ref name="PHL-20120829">{{cite web |last=Méndez |first=Abel |title=A Hot Potential Habitable Exoplanet around Gliese 163 |url=http://phl.upr.edu/press-releases/ahotpotentialhabitableexoplanetaroundgliese163 |date=August 29, 2012 |publisher=[[University of Puerto Rico at Arecibo]] (Planetary Habitability Laboratory) |accessdate=September 20, 2012 }}</ref><ref name="Space-20120920">{{cite web |last=Redd |title=Newfound Alien Planet a Top Contender to Host Life |url=http://www.space.com/17684-alien-planet-gliese-163c-extraterrestrial-life.html |date=September 20, 2012 |publisher=Space.com |accessdate=September 20, 2012}}</ref><ref>{{cite web| url=http://www.spacedaily.com/reports/A_Hot_Potential_Habitable_Exoplanet_around_Gliese_163_999.html |title=A Hot Potential Habitable Exoplanet around Gliese 163 |publisher=Spacedaily.com |accessdate=2013-02-10}}</ref> [[HD 40307 g]], a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of [[HD 40307]].<ref name="hd40307g_tuomi12">{{cite journal |authorlink= |title= Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307 |author1=Tuomi, Mikko|author2= Anglada-Escude, Guillem |author3= Gerlach, Enrico |author4= Jones, Hugh R. R. |author5= Reiners, Ansgar |author6= Rivera, Eugenio J. |author7= Vogt, Steven S. |author8= Butler, Paul |journal=Astronomy and Astrophysics |arxiv=1211.1617|doi = 10.1051/0004-6361/201220268 |date= 2012 |volume= 549 |pages= A48 |bibcode = 2013A&A...549A..48T }}</ref> In December 2012, [[Tau Ceti e]] and [[Tau Ceti f]] were found in the circumstellar habitable zone of [[Tau Ceti]], a Sun-like star 12 light years away.<ref name=aron-2012>{{cite web |url=https://www.newscientist.com/article/dn23021-nearby-tau-ceti-may-host-two-planets-suited-to-life.html |title=Nearby Tau Ceti may host two planets suited to life |publisher=Reed Business Information |work=New Scientist |date=December 19, 2012 |accessdate=April 1, 2013 |author=Aron, Jacob}}</ref> Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone;<ref name="tuomi-2013">{{Cite journal | last1 = Tuomi | first1 = M. | last2 = Jones | first2 = H. R. A. | last3 = Jenkins | first3 = J. S. | last4 = Tinney | first4 = C. G. | last5 = Butler | first5 = R. P. | last6 = Vogt | first6 = S. S. | last7 = Barnes | first7 = J. R. | last8 = Wittenmyer | first8 = R. A. | last9 = o'Toole | first9 = S. | last10 = Horner | first10 = J. | last11 = Bailey | first11 = J. | last12 = Carter | first12 = B. D. | last13 = Wright | first13 = D. J. | last14 = Salter | first14 = G. S. | last15 = Pinfield | first15 = D. | title = Signals embedded in the radial velocity noise | doi = 10.1051/0004-6361/201220509 | journal = Astronomy & Astrophysics | volume = 551 | pages = A79 | year = 2013 | pmid = | pmc = |arxiv = 1212.4277 |bibcode = 2013A&A...551A..79T }}</ref> however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study.<ref name=mendes-2013-3>{{cite web |url=http://phl.upr.edu/projects/habitable-exoplanets-catalog |title=The Habitable Exoplanets Catalog |publisher=University of Puerto Rico |work=Habitable Exoplanets Catalog |date=May 1, 2013 |accessdate=May 1, 2013 |author=Torres, Abel Mendez}}</ref>
===Near Earth-sized planets and Solar analogs===
[[File:Kepler186f-ComparisonGraphic-20140417 improved.jpg|thumb|right|Comparison of the CHZ position of Earth-radius planet Kepler-186f and the [[Solar System]] (17 April 2014)]]
[[File:Kepler-452b System.jpg|thumb|right|While larger than Kepler 186f, Kepler-452b's orbit and star are more similar to Earth's.]]
Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above {{Earth radius|1.5|link=y}} the average planet density rapidly decreases with increasing radius, indicating these planets have a large fraction of volatiles by volume overlying a rocky core.<ref>Lauren M. Weiss, and Geoffrey W. Marcy. "[https://arxiv.org/abs/1312.0936 The mass-radius relation for 65 exoplanets smaller than 4 Earth radii]"</ref> A truly Earth-like planet, an [[Earth analog]] or "Earth twin", would need to meet many conditions beyond size and mass; such properties are not observable using current technology.
A [[solar analog]] (or "solar twin") is a star that resembles the Sun. To date no solar twin with an exact match as that of the Sun has been found, however, there are some stars that are nearly identical to the Sun, and are such considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6 billion years old, with the correct metallicity and a 0.1% [[solar luminosity]] variation.<ref>{{cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/|title=Solar Variability and Terrestrial Climate |date=2013-01-08|publisher=NASA Science}}</ref> Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity and size are also very important to low luminosity variation.<ref>{{cite web|url=http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html|title=Stellar Luminosity Calculator|publisher=University of Nebraska-Lincoln astronomy education group}}</ref><ref>{{Cite book|url=http://www.nap.edu/catalog/13519/the-effects-of-solar-variability-on-earths-climate-a-workshop|title=The Effects of Solar Variability on Earth's Climate: A Workshop Report|first=National Research|last=Council|date=18 September 2012|publisher=|doi=10.17226/13519|isbn=978-0-309-26564-5}}</ref><ref>[http://scienceblogs.com/startswithabang/2013/06/05/most-of-earths-twins-arent-identical-or-even-close/ Most of Earth's twins aren't identical, or even close!], By Ethan. June 5, 2013.</ref>
Using data collected by NASA's [[Kepler (spacecraft)|''Kepler'' Space observatory]] and the [[W. M. Keck Observatory]], scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their [[habitable zone]].<ref name="NOAA 2017">{{cite web |url=https://oceanservice.noaa.gov/facts/et-oceans.html |title=Are there oceans on other planets? |work=National Oceanic and Atmospheric Administration |date=6 July 2017 |accessdate=2017-10-03 }}</ref>
On 7 January 2013, astronomers from the ''Kepler'' team announced the discovery of [[Kepler-69c]] (formerly ''KOI-172.02''), an Earth-size [[exoplanet]] candidate (1.7 times the radius of Earth) orbiting [[Kepler-69]], a star similar to our Sun, in the CHZ and expected to offer habitable conditions.<ref name="Space-20130109">{{cite web |last=Moskowitz |first=Clara |title=Most Earth-Like Alien Planet Possibly Found |url=http://www.space.com/19201-most-earth-like-alien-planet.html |date=January 9, 2013 |publisher=Space.com |accessdate=January 9, 2013 }}</ref><ref name="arXiv-20130417">{{cite journal|doi=10.1088/0004-637X/768/2/101| title=A Super-Earth-Sized Planet Orbiting in or Near the Habitable Zone Around a Sun-Like Star| date=2013|last1=Barclay|first1=Thomas|last2=Burke|first2=Christopher J.|last3=Howell|first3=Steve B.|last4=Rowe|first4=Jason F.|last5=Huber|first5=Daniel|last6=Isaacson|first6=Howard|last7=Jenkins|first7=Jon M.|last8=Kolbl|first8=Rea|last9=Marcy|first9=Geoffrey W. |journal=The Astrophysical Journal| volume=768|issue=2|pages=101|arxiv = 1304.4941 |bibcode = 2013ApJ...768..101B }}</ref><ref name="NASA-20130418" /><ref name="NYT-20130418">{{cite news |last=Overbye |first=Dennis |title=Two Promising Places to Live, 1,200 Light-Years from Earth| url=https://www.nytimes.com/2013/04/19/science/space/2-new-planets-are-most-earth-like-yet-scientists-say.html| date=18 April 2013 |newspaper=The New York Times |accessdate=18 April 2013 }}</ref> The discovery of two planets orbiting in the habitable zone of [[Kepler-62]], by the Kepler team was announced on April 19, 2013. The planets, named [[Kepler-62e]] and [[Kepler-62f]], are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.<ref name="NASA-20130418">{{cite web |last1=Johnson |first1=Michele |last2=Harrington |first2=J.D. |title=NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-62-kepler-69.html |date=18 April 2013 |work=[[NASA]] |accessdate=18 April 2013 }}</ref><ref name="NYT-20130418"/><ref name="Borucki-2013">{{Cite journal |last=Borucki |first=William J. |authorlink=William J. Borucki |title=Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone |journal=Science Express| date=18 April 2013 |doi=10.1126/science.1234702 |accessdate=18 April 2013| url=http://www.sciencemag.org/content/early/2013/04/17/science.1234702 |volume=340 |issue=6132 |pages=587–90|arxiv = 1304.7387 |bibcode = 2013Sci...340..587B |display-authors=etal |pmid=23599262|hdl=1721.1/89668 }}</ref>
With a radius estimated at 1.1 Earth, [[Kepler-186f]], discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method<ref name="NYT-20140417">{{cite news |last=Chang |first=Kenneth |title=Scientists Find an 'Earth Twin,' or Maybe a Cousin |url=https://www.nytimes.com/2014/04/18/science/space/scientists-find-an-earth-twin-or-maybe-a-cousin.html |date=17 April 2014 |work=The New York Times |accessdate=17 April 2014 }}</ref><ref name="AP-20140417">{{cite news |last=Chang |first=Alicia |title=Astronomers spot most Earth-like planet yet |url=http://apnews.excite.com/article/20140417/DAD832V81.html |date=17 April 2014 |work=[[AP News]] |accessdate=17 April 2014 }}</ref><ref name="BBC-20140417">{{cite news |last=Morelle |first=Rebecca |title='Most Earth-like planet yet' spotted by Kepler |url=https://www.bbc.co.uk/news/science-environment-27054366 |date=17 April 2014 |work=[[BBC News]] |accessdate=17 April 2014 }}</ref> though its mass remains unknown and its parent star is not a Solar analog.
[[Kapteyn b]], discovered in June 2014 is a possible rocky world of about 4.8 Earth masses and about 1.5 earth radii was found orbiting the habitable zone of the red subdwarf [[Kapteyn's Star]], 12.8 light-years away.<ref name="SP-20140603">{{cite web |last=Wall |first=Mike |title=Found! Oldest Known Alien Planet That Might Support Life |url=http://www.space.com/26115-oldest-habitable-alien-planet-kapteyn-b.html |date=3 June 2014 |work=[[Space.com]] |accessdate=10 January 2015 }}</ref>
On 6 January 2015, NASA announced the 1000th confirmed [[exoplanet]] discovered by the ''Kepler'' Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related [[star]]s: two of the three, [[Kepler-438b]] and [[Kepler-442b]], are near-Earth-size and likely [[Terrestrial planet|rocky]]; the third, [[Kepler-440b]], is a [[super-Earth]].<ref name="NASA-20150106" /> Announced 16 January, [[K2-3d]] a planet of 1.5 Earth radii was found orbiting within the habitable zone of [[K2-3]], receiving 1.4 times the intensity of visible light as Earth.<ref>{{cite news |first=Mari N.|last=Jensen |url=https://www.sciencedaily.com/releases/2015/01/150116093052.htm |title=Three nearly Earth-size planets found orbiting nearby star: One in 'Goldilocks' zone |work=[[Science Daily]] |date=16 January 2015 |accessdate=25 July 2015}}</ref>
[[Kepler-452b]], announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its [[G-type main-sequence star|G-class]] (solar analog) star [[Kepler-452]].<ref name=Jenkins2015>{{cite journal| last1=Jenkins|first1=Jon M.|last2=Twicken|first2=Joseph D.|last3=Batalha|first3=Natalie M.|last4=Caldwell|first4=Douglas A.|last5=Cochran|first5=William D.|last6=Endl|first6=Michael|last7=Latham|first7=David W.|last8=Esquerdo|first8=Gilbert A.|last9=Seader|first9=Shawn|last10=Bieryla|first10=Allyson|last11=Petigura|first11=Erik|last12=Ciardi|first12=David R.|last13=Marcy|first13=Geoffrey W.|last14=Isaacson|first14=Howard|last15=Huber|first15=Daniel|last16=Rowe|first16=Jason F.|last17=Torres|first17=Guillermo|last18=Bryson|first18=Stephen T.|last19=Buchhave|first19=Lars|last20=Ramirez|first20=Ivan|last21=Wolfgang|first21=Angie|last22=Li|first22=Jie|last23=Campbell|first23=Jennifer R.|last24=Tenenbaum|first24=Peter|last25=Sanderfer|first25=Dwight|last26=Henze|first26=Christopher E.|last27=Catanzarite|first27=Joseph H.|last28=Gilliland|first28=Ronald L.|last29=Borucki|first29=William J.| title=Discovery and Validation of Kepler-452b: A 1.6 R⨁ Super Earth Exoplanet in the Habitable Zone of a G2 Star| journal=The Astronomical Journal| date=23 July 2015| volume=150|issue=2|page=56|issn=1538-3881|doi=10.1088/0004-6256/150/2/56|url=http://iopscience.iop.org/1538-3881/150/2/56/article|accessdate=24 July 2015|arxiv = 1507.06723 |bibcode = 2015AJ....150...56J }}</ref><ref name="bno">{{cite web |url=http://bnonews.com/news/index.php/news/id961 |title=NASA telescope discovers Earth-like planet in star's habitable zone |date=23 July 2015 |work=[[BNO News]] |accessdate=23 July 2015}}</ref>
The discovery of a system of three tidally-locked planets orbiting the habitable zone of an ultracool dwarf star, [[TRAPPIST-1]], was announced in May 2016.<ref>{{cite web|url=http://www.eso.org/public/news/eso1615/|title=Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star|publisher=European Southern Observatory|date=2 May 2016}}</ref> The discovery is considered significant because it greatly increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.
Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf [[K2-72]] around 227 light year from the Sun: [[K2-72c]] and [[K2-72e]] are both of similar size to Earth and receive similar amounts of stellar radiation.<ref name="DressingVanderburg2017">{{cite journal|last1=Dressing|first1=Courtney D.|last2=Vanderburg|first2=Andrew|last3=Schlieder|first3=Joshua E.|last4=Crossfield|first4=Ian J. M.|last5=Knutson|first5=Heather A.|last6=Newton|first6=Elisabeth R.|last7=Ciardi|first7=David R.|last8=Fulton|first8=Benjamin J.|last9=Gonzales|first9=Erica J.|last10=Howard|first10=Andrew W.|last11=Isaacson|first11=Howard|last12=Livingston|first12=John|last13=Petigura|first13=Erik A.|last14=Sinukoff|first14=Evan|last15=Everett|first15=Mark|last16=Horch|first16=Elliott|last17=Howell|first17=Steve B.|title=Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7|journal=The Astronomical Journal|volume=154|issue=5|year=2017|pages=207|issn=1538-3881|doi=10.3847/1538-3881/aa89f2|arxiv=1703.07416|bibcode=2017AJ....154..207D|url=https://authors.library.caltech.edu/78341/2/Dressing_2017_AJ_154_207.pdf}}</ref>
Announced on the 20 April 2017, [[LHS 1140b]] is a super-dense [[super-Earth]] 39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs.<ref>{{cite journal|doi=10.1038/nature22055|pmid=28426003|title=A temperate rocky super-Earth transiting a nearby cool star|journal=Nature|volume=544|issue=7650|pages=333–336|year=2017|last1=Dittmann|first1=Jason A.|last2=Irwin|first2=Jonathan M.|last3=Charbonneau|first3=David|last4=Bonfils|first4=Xavier|last5=Astudillo-Defru|first5=Nicola|last6=Haywood|first6=Raphaëlle D.|last7=Berta-Thompson|first7=Zachory K.|last8=Newton|first8=Elisabeth R.|last9=Rodriguez|first9=Joseph E.|last10=Winters|first10=Jennifer G.|last11=Tan|first11=Thiam-Guan|last12=Almenara|first12=Jose-Manuel|last13=Bouchy|first13=François|last14=Delfosse|first14=Xavier|last15=Forveille|first15=Thierry|last16=Lovis|first16=Christophe|last17=Murgas|first17=Felipe|last18=Pepe|first18=Francesco|last19=Santos|first19=Nuno C.|last20=Udry|first20=Stephane|last21=Wünsche|first21=Anaël|last22=Esquerdo|first22=Gilbert A.|last23=Latham|first23=David W.|last24=Dressing|first24=Courtney D.|arxiv = 1704.05556 |bibcode = 2017Natur.544..333D }}</ref> The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied.
Discovered by radial velocity in June 2017, with approximately 3 times the mass of Earth, [[Luyten b]] orbits within the habitable zone of [[Luyten's Star]] just 12.2 light years away.<ref>{{Cite journal | url=https://www.wired.co.uk/article/sonar-sending-music-into-space-habitable-exoplanet | title=Astronomers are beaming techno into space for aliens to decode| journal=Wired UK| date=2017-11-16}}</ref>
At 11 light-years away, a second closest planet, [[Ross 128 b]], was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 Earth's mass is it roughly Earth sized and likely rocky in composition.<ref>{{cite web | url=https://www.space.com/38782-possibly-earth-like-alien-planet-ross-128b.html | title=In Earth's Backyard: Newfound Alien Planet May be Good Bet for Life}}</ref>
Discovered in March 2018, [[K2-155d]] is about 1.64 time the radius of Earth, is likely rocky and orbits in the habitable zone of its [[red dwarf]] star 203 light years away.<ref name="Exoplanet Exploration">{{Cite web|title=K2-155 d|url=https://exoplanets.nasa.gov/newworldsatlas/6173/|publisher=Exoplanet Exploration|year=2018}}</ref><ref name=CNET>{{cite web|last1=Mack|first1=Eric|title=A super-Earth around a red star could be wet and wild|url=https://www.cnet.com/news/super-earth-exoplanet-k2-155d-found-could-be-habitable-nasa/|website=[[CNET]]|date=March 13, 2018}}</ref><ref name=ExtremeTech>{{Cite web|last1=Whitwam|first1=Ryan|title=Kepler Spots Potentially Habitable Super-Earth Orbiting Nearby Star|url=https://www.extremetech.com/extreme/265576-kepler-spots-potentially-habitable-super-earth-orbiting-nearby-star|publisher=[[ExtremeTech]]|date=March 14, 2018}}</ref>
One of the earliest discoveries by the [[Transiting Exoplanet Survey Satellite]] (TESS) announced July 31, 2019 is a Super Earth planet [[GJ 357 d]] orbiting the outer edge of a red dwarf 31 light years away.<ref name="LuquePallé2019">{{cite journal|last1=Luque|first1=R.|last2=Pallé|first2=E.|last3=Kossakowski|first3=D.|last4=Dreizler|first4=S.|last5=Kemmer|first5=J.|last6=Espinoza|first6=N.|title=Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization|journal=Astronomy & Astrophysics|year=2019|issn=0004-6361|doi=10.1051/0004-6361/201935801}}</ref>
{| class="wikitable" style="margin:0.5em auto; width:600px;"
! Notable [[exoplanets]] – [[Kepler (spacecraft)|Kepler Space Telescope]]
|-
| style="font-size:88%" | [[File:PIA19827-Kepler-SmallPlanets-HabitableZone-20150723.jpg|600px]]
<center>Confirmed small exoplanets in [[habitable zone]]s.<br>([[Kepler-62e]], [[Kepler-62f]], [[Kepler-186f]], [[Kepler-296e]], [[Kepler-296f]], [[Kepler-438b]], [[Kepler-440b]], [[Kepler-442b]])<br>(Kepler Space Telescope; January 6, 2015).<ref name="NASA-20150106">{{cite web |last1=Clavin |first1=Whitney |last2=Chou |first2=Felicia |last3=Johnson |first3=Michele |title=NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones |url=http://www.jpl.nasa.gov/news/news.php?release=2015-003 |date=6 January 2015 |work=[[NASA]] |accessdate=6 January 2015 }}</ref></center>
|}
==Habitability outside the CHZ==
[[Image:Liquid lakes on titan.jpg|thumb|upright|The discovery of hydrocarbon lakes on Saturn's moon Titan has begun to call into question the [[carbon chauvinism]] that underpins CHZ theory.]]
Liquid-water environments have been found to exist in the absence of atmospheric pressure, and at temperatures outside the CHZ temperature range. For example, [[Saturn]]'s moons [[Titan (moon)|Titan]] and [[Enceladus]] and [[Jupiter]]'s moons [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]], all of which are outside the habitable zone, may hold large volumes of liquid water in [[subsurface ocean]]s.<ref>{{cite web |url = http://phl.upr.edu/library/media/liquidwaterinthesolarsystem |title = Liquid Water in the Solar System |accessdate = 2013-12-15 |last = Torres |first = Abel |date = 2012-06-12}}</ref>
Outside the CHZ, [[tidal heating]] and [[radioactive decay]] are two possible heat sources that could contribute to the existence of liquid water.<ref name="Cowen2008"/><ref name="Bryner, Jeanna"/> Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on [[rogue planet]]s as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.<ref name="physcisarxivlab-2011"/>
With some theorising that life on Earth may have actually originated in stable, subsurface habitats,<ref name=Munro2013>{{Citation
|title = Miners deep underground in northern Ontario find the oldest water ever known
|url = http://news.nationalpost.com/2013/05/15/worlds-oldest-water-bubbling-into-northern-ontario-mine/
|date = 2013
|author = Munro, Margaret
|work = National Post
|accessdate = 2013-10-06
}}</ref><ref name=Davies2013>{{Citation
|title = The Origin of Life II: How did it begin?
|url = http://cosmos.asu.edu/sites/default/files/publication_files/originsoflife_ii.pdf
|date = 2013
|author = Davies, Paul
|accessdate = 2013-10-06
}}{{dead link|date=September 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.<ref name=Taylor1996>{{Citation
|title = Life Underground
|url = http://www.psrd.hawaii.edu/Dec96/PSRD-LifeUnderground.pdf
|date = 1996
|author = Taylor, Geoffrey
|journal = Planetary Science Research Discoveries
|accessdate = 2013-10-06
}}</ref> Indeed, on Earth itself living organisms may be found more than 6 kilometres below the surface.<ref name=Doyle2013>{{Citation
|title = Deep underground, worms and "zombie microbes" rule
|url = https://www.reuters.com/article/2013/03/04/us-life-idUSBRE9230WM20130304
|author = Doyle, Alister
|accessdate = 2013-10-06
|newspaper=Reuters
|date=4 March 2013
}}</ref>
Another possibility is that outside the CHZ organisms may use [[alternative biochemistry|alternative biochemistries]] that do not require water at all. Astrobiologist [[Christopher McKay]], has suggested that [[methane]] ({{chem|C|H|4}}) may be a solvent conducive to the development of "cryolife", with the Sun's "methane habitable zone" being centered on {{convert|1610000000|km|mi AU|sigfig=2|abbr=on}} from the star.<ref name=villard-2011 /> This distance is coincident with the location of Titan, whose lakes and rain of methane make it an ideal location to find McKay's proposed cryolife.<ref name=villard-2011 /> In addition, [[list of microorganisms tested in outer space|testing of a number of organisms]] has found some are capable of surviving in extra-CHZ conditions.<ref>
{{cite journal
|last1=Nicholson |first1=W. L.
|last2=Moeller |first2=R.
|last3=Horneck |first3=G.
|author4=PROTECT Team
|date=2012
|title=Transcriptomic Responses of Germinating Bacillus subtilis Spores Exposed to 1.5 Years of Space and Simulated Martian Conditions on the EXPOSE-E Experiment PROTECT
|journal=Astrobiology
|volume=12 |issue=5 |pages=469–86
|bibcode=2012AsBio..12..469N
|doi=10.1089/ast.2011.0748
|pmid=22680693
}}</ref>
==Significance for complex and intelligent life==
The [[Rare Earth hypothesis]] argues that complex and intelligent life is uncommon and that the CHZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a CHZ 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 |pages= |isbn=978-0-387-95289-5|oclc= |doi= |accessdate=}}</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 |date=2011 |pages=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 CHZ.<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 CHZ may have evolved in subsurface environments, from alternative biochemistries<ref name=cohen-2002 /> or even from nuclear reactions.<ref name="GO247">
{{cite book
|last = Goldsmith
|first = Donald
|last2 = Owen
|first2 = Tobias
|authorlink =
|title = The Search for Life in the Universe
|publisher = [[Addison-Wesley]]
|series =
|volume =
|edition = 2
|date = 1992
|location =
|page = 247
|url =
|doi =
|id =
|isbn = 978-0-201-56949-0
|mr =
|zbl =
|jfm = }}</ref>
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 circumvental 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 |url=|bibcode = 1987AdSpR...7..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> In addition, the plants ''[[Rhizocarpon geographicum]]'' and ''[[Xanthoria 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 |accessdate=27 April 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]] |accessdate=27 April 2012 |deadurl=yes |archiveurl=https://web.archive.org/web/20120504224706/http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |archivedate=4 May 2012 |df= }}</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 CHZ 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]] analogues on the surface may be eliminated and complex Earth life can thrive.<ref name=dole-1964 />
Planets in the CHZ 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 |accessdate=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 CHZ 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 |accessdate=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|accessdate=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, CHZ 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 }}</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 |authorlink1=Andrew Siemion |title=A 1.1 to 1.9 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 }}</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
|accessdate = 2013-06-26
|postscript = <!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}
}}</ref>
Because the CHZ 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 the 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 CHZ.<ref name="cplire.ru">{{cite conference |url=http://www.cplire.ru/rus/ra&sr/VAK-2004.html|title=Transmission and reasonable signal searches in the Universe |script-title=ru:Передача и поиски разумных сигналов во Вселенной |accessdate=2013-06-30 |author=Zaitsev, A. L. |booktitle=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=Russian}}</ref><ref>{{cite web|last=Grinspoon |first=David|url=http://seedmagazine.com/content/article/who_speaks_for_earth/ |title=Who Speaks for Earth? |publisher=Seedmagazine.com |date=12 December 2007|accessdate=2012-08-21}}</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
|bibcode=2010MNRAS.403..731G
|arxiv = 1003.5549 }}</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 }}</ref> The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its CHZ.<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|accessdate = 2008-10-09| archiveurl = 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|archivedate = 11 October 2008 <!--DASHBot-->| deadurl = no |location=London}}</ref> and [[Hello From Earth]] in 2009, were directed to the Gliese 581 system, containing three planets in the CHZ—Gliese 581 c, d, and the unconfirmed g.
{{Clear}}
==See also==
{{Portal|Astronomy|Space}}
{{div col}}
*{{annotated link|Hypothetical types of biochemistry}}
*{{annotated link|Earth analog}}
*{{annotated link|Earth Similarity Index}}
*{{annotated link|Extraterrestrial liquid water}}
*{{annotated link|Extraterrestrial life}}
*{{annotated link|Galactic habitable zone}}
*''[[Star Trek]]'s'' [[Class M planet]] classification
*{{annotated link|Habitability of natural satellites}}
*{{annotated link|Planetary habitability}}
*{{annotated link|Rare Earth hypothesis}}
*{{annotated link|Venus zone}}
{{div col end}}
==References==
{{Reflist|30em|refs=
<ref name="Kopparapu2013b">{{cite journal |url=https://iopscience.iop.org/article/10.1088/0004-637X/765/2/131/meta|title=Habitable Zones Around Main-Sequence Stars: New Estimates |first=Ravi Kumar |last=Kopparapu |first2=Ramses |last2=Ramirez |first3=James F. |last3=Kasting |first4=Vincent |last4=Eymet |first5=Tyler D. | last5=Robinson |first6=Suvrath | last6=Mahadevan |first7=Ryan C. | last7=Terrien |first8=Shawn | last8=Domagal-Goldman |first9=Victoria | last9=Meadows |first10=Rohit | last10=Deshpande |display-authors=1 |date=2013|arxiv=1301.6674 |bibcode=2013ApJ...765..131K|doi = 10.1088/0004-637X/765/2/131 | volume=765 | journal=The Astrophysical Journal | pages=131}}</ref>
}}
==External links==
{{Wiktionary|habitable zone}}
{{Commons category|Habitable zone}}
{{div col}}
*{{cite web |url=http://astro.unl.edu/naap/habitablezones/animations/stellarHabitableZone.html |title=Circumstellar Habitable Zone Simulator |publisher=Astronomy Education at the University of Nebraska-Lincoln}}
*{{cite web |url=http://phl.upr.edu/projects/habitable-exoplanets-catalog |title=The Habitable Exoplanets Catalog |publisher=PHL/University of Puerto Rico at Arecibo}}
*{{cite web |url=http://www.hzgallery.org/ |title=The Habitable Zone Gallery}}
*{{cite web |url=http://www.solstation.com/habitable.htm |title=Stars and Habitable Planets |publisher=SolStation |deadurl=yes |archiveurl=https://web.archive.org/web/20110628175616/http://www.solstation.com/habitable.htm |archivedate=2011-06-28 |df= }}
*{{cite journal |title=On the Galactic Habitable Zone|author1=Nikos Prantzos|doi=10.1007/s11214-007-9236-9|date=2006 |journal=Space Science Reviews |volume=135 |issue=1–4|pages=313–322|arxiv=astro-ph/0612316|bibcode = 2008SSRv..135..313P }}
*[http://btc.montana.edu/ceres/astrobiology/files/HabitableZone.htm Interstellar Real Estate: Location, Location, Location – Defining the Habitable Zone]
*{{cite web |url=http://www.planetarybiology.com/exoexplorer_planets/| title=Exoplanets in relation to host star's current habitable zone |work=www.planetarybiology.com}}
*{{cite web |url=http://www.planetarybiology.com/exoexplorer/|title=exoExplorer: a free Windows application for visualizing exoplanet environments in 3D |work=www.planetarybiology.com}}
*{{cite web |url=https://www.newscientist.com/article/mg20026831.600-why-the-universe-may-be-teeming-with-aliens.html?full=true |title=Why the universe may be teeming with aliens |date=November 19, 2009 |last=Shiga |first=David |magazine=New Scientist}}
*{{cite web |url=http://newworlds.colorado.edu/info/documents/NewWorldsObserver2004.pdf |title=The New Worlds Observer: a mission for high-resolution spectroscopy of extra-solar terrestrial planets |author=Simmons |work=New Worlds|display-authors=etal}}
*{{cite journal |url=https://link.springer.com/content/pdf/10.1007%2Fs10686-008-9121-x.pdf |title=Darwin – an experimental astronomy mission to search for extrasolar planets |date=2009 |volume=23 |issue=1 |pages=435–461 |doi=10.1007/s10686-008-9121-x |bibcode=2009ExA....23..435C |last2=Herbst |first2=Tom |last3=Léger |first3=Alain |last4=Absil |first4=O. |last5=Beichman |first5=Charles |last6=Benz |first6=Willy |last7=Brack |first7=Andre |last8=Chazelas |first8=Bruno |last9=Chelli |first9=Alain |journal=Experimental Astronomy |last1=Cockell |first1=Charles S.}}
*{{cite web |url=http://www.universetoday.com/2009/03/19/jwst-will-provide-capability-to-search-for-biomarkers-on-earth-like-worlds/ |title=JWST Will Provide Capability to Search for Biomarkers on Earth-like Worlds |work=Universe Today |last=Atkinson |first=Nancy |date=March 19, 2009}}
{{div col end}}
{{Exoplanet}}
{{Extraterrestrial life}}
{{Astrobiology}}
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[[Category:Planetary habitability]]
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