{{Short description|Orbits where planets may have liquid surface water}} {{redirect|Goldilocks zone|the more general principle|Goldilocks principle}} {{About|the circumstellar zone|the galactic zone|Galactic habitable zone}} [[File:Diagram of habitable zone rocky exoplanets, from NASA Exoplanet Archive and Gaia DR3 data.png|alt=A diagram depicting habitable zone boundaries across star type. The y-axis is stellar temperature, with the Sun (5772 Kelvin) at the top. The x-axis is the percentage of starlight that reaches the planet, ranging from 25% of Earth's starlight to 150% of Earth's starlight on the inner edge of the habitable zone. The image plots 42 exoplanets, most of which orbit red dwarfs. The coldest planets around red dwarfs are depicted as icy "eyeball" planets due to tidal locking, while most of the other planets around red dwarfs are purple, due to speculations about purple photosynthesizing creatures. Habitable one planets around yellow stars are depicted as green or blue. Earth is plotted near the top, with only Kepler-452 b close to its position.|thumb|440x440px|A diagram depicting habitable zone boundaries across star type with December 2025 data.<ref>{{Cite journal |last1=Bohl |first1=A. |last2=Lawrence |first2=L. |last3=Lowry |first3=G. |last4=Kaltenegger |first4=L. |date=March 19, 2026 |title=Probing the limits of habitability: a catalogue of rocky exoplanets in the habitable zone |url=https://academic.oup.com/mnras/article/547/3/stag028/8526432 |journal=Monthly Notices of the Royal Astronomical Society |volume=547 |issue=3 |article-number=stag028 |doi=10.1093/mnras/stag028 |doi-access=free |via=Oxford Academic|arxiv=2501.14054 }}</ref> Earth is plotted alongside 45 exoplanets with radii less than 2 times that of Earth or masses less than 5 times that of Earth, making them potentially rocky worlds in the habitable zone.]] {{Life in the Universe}} In astronomy and astrobiology, the '''habitable zone''' ('''HZ'''), also known as the '''circumstellar habitable zone''' ('''CHZ'''), or as the '''Goldilocks zone,''' is the range of orbits around a star within which a planetary surface could potentially support liquid water. Liquid water is considered by many scientists as necessary for a planet to be habitable. The range depends upon the brightness of the star interacting with a planet's atmosphere. At the inner edge the star's light, trapped by greenhouse gases in a planet's atmosphere, boils off the planet's water. At the outer edge light from the star is insufficient even with help from atmospheric gas and the planet's water freezes.<ref name="Rare Earth"/>{{rp|16}}<ref name="SCI-20130503">{{cite journal |last1=Cruz |first1=Maria |last2=Coontz |first2=Robert |title=Exoplanets - Introduction to Special Issue |journal=Science |volume=340 |page=565 |doi=10.1126/science.340.6132.565 |pmid=23641107 |issue=6132 |date=2013|doi-access=free }}</ref> Many other factors are added in various habitable zone models. The habitable zone has become a key tool in the search for habitable planets because discoveries of exoplanets yield approximate orbital radii.
The alternative name, Goldilocks zone, is a metaphor, allusion and antonomasia of the children's fairy tale of "Goldilocks and the Three Bears", in which a little girl chooses from sets of three items, rejecting the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is "just right".
Since the concept was first presented many stars have been confirmed to possess an HZ planet, including some systems that consist of multiple HZ planets.<ref name="NYT-20150106-DB">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=As Ranks of Goldilocks Planets Grow, Astronomers Consider What's Next |url=https://www.nytimes.com/2015/01/07/science/space/as-ranks-of-goldilocks-planets-grow-astronomers-consider-whats-next.html |date=January 6, 2015 |work=The New York Times |access-date=January 6, 2015}}</ref> Most such planets, being either super-Earths or gas giants, are more massive than Earth, because massive planets are easier to detect.<ref>{{cite journal |url=https://www.researchgate.net/publication/2227681 |title=Probability of Detecting a Planetary Companion during a Microlensing Event |doi=10.1086/320562 | arxiv=astro-ph/0101316 |date=January 2021|last1=Peale |first1=S. J. |journal=The Astrophysical Journal |volume=552 |issue=2 |pages=889–911 |s2cid=17080374 }}</ref> On November 4, 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way.<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis|title=Far-Off Planets Like the Earth Dot the Galaxy|url=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=November 4, 2013 |work=The New York Times |access-date=November 5, 2013}}</ref><ref name="PNAS-20131031">{{cite journal|last1=Petigura |first1=Eric A.|last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W.|title=Prevalence of Earth-size planets orbiting Sun-like stars|date=October 31, 2013|journal=Proceedings of the National Academy of Sciences of the United States of America|doi=10.1073/pnas.1319909110 |arxiv = 1311.6806 |bibcode = 2013PNAS..11019273P |volume=110 |issue=48|pages=19273–19278 |pmid=24191033 |pmc=3845182|doi-access=free}}</ref> About 11 billion of these may be orbiting Sun-like stars.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story|date=November 4, 2013 |work=Los Angeles Times |access-date=November 5, 2013}}</ref> Proxima Centauri b, located about 4.2 light-years (1.3 parsecs) 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 | bibcode = 2016Natur.536..437A | title = A terrestrial planet candidate in a temperate orbit around Proxima Centauri | journal = Nature | volume = 536 | issue = 7617 | pages = 437–440 | last1 = Anglada-Escudé | first1 = Guillem | last2 = Amado | first2 = Pedro J. | last3 = Barnes | first3 = John | last4 = Berdiñas | first4 = Zaira M. | last5 = Butler | first5 = R. Paul | last6 = Coleman | first6 = Gavin A. L. | last7 = de la Cueva | first7 = Ignacio | last8 = Dreizler | first8 = Stefan | last9 = Endl | first9 = Michael | last10 = Giesers | first10 = Benjamin | last11 = Jeffers | first11 = Sandra V. | last12 = Jenkins | first12 = James S. | last13 = Jones | first13 = Hugh R. A. | last14 = Kiraga | first14 = Marcin | last15 = Kürster | first15 = Martin | last16 = López-González | first16 = María J. | last17 = Marvin | first17 = Christopher J. | last18 = Morales | first18 = Nicolás | last19 = Morin | first19 = Julien | last20 = Nelson | first20 = Richard P. | last21 = Ortiz | first21 = José L. | last22 = Ofir | first22 = Aviv | last23 = Paardekooper | first23 = Sijme-Jan | last24 = Reiners | first24 = Ansgar | last25 = Rodríguez | first25 = Eloy | last26 = Rodríguez-López | first26 = Cristina | last27 = Sarmiento | first27 = Luis F. | last28 = Strachan | first28 = John P. | last29 = Tsapras | first29 = Yiannis | last30 = Tuomi | first30 = Mikko | first31=Mathias |last31=Zechmeister | display-authors = 3 | year = 2016 | arxiv = 1609.03449 | doi = 10.1038/nature19106 | pmid = 27558064 | s2cid = 4451513 | url=https://www.nature.com/articles/nature19106 }}</ref> The HZ is also of particular interest to the emerging field of habitability of natural satellites because planetary mass moons in the HZ might outnumber planets.<ref name="Schirber 2009-10-26">{{cite web |last=Schirber |first=Michael |title=Detecting Life-Friendly Moons |url=http://www.astrobio.net/exclusive/3291/detecting-life-friendly-moons |date=26 Oct 2009 |work=Astrobiology Magazine |agency=NASA |access-date=9 May 2013 |archive-url=https://web.archive.org/web/20091029111850/http://www.astrobio.net/exclusive/3291/detecting-life-friendly-moons |archive-date=29 October 2009 }}</ref>
The classical habitable zone concept was defined only for planetary surfaces where habitat depends on stellar energy. Deep biospheres are known on Earth, but they would not be detectable on exoplanets.<ref name="Review 2009">{{cite journal |title=What makes a planet habitable? |journal=The Astronomy and Astrophysics Review |year=2009 |last1=Lammer |first1=H. |last2=Bredehöft |first2=J. H. |last3=Coustenis |first3=A. |last4=Khodachenko |first4=M. L. |volume=17 |issue=2 |pages=181–249 |doi=10.1007/s00159-009-0019-z |url=http://veilnebula.jorgejohnson.me/uploads/3/5/8/7/3587678/lammer_et_al_2009_astron_astro_rev-4.pdf |access-date=2016-05-03 |bibcode=2009A&ARv..17..181L |s2cid=123220355 |display-authors=etal |archive-url=https://web.archive.org/web/20160602235333/http://veilnebula.jorgejohnson.me/uploads/3/5/8/7/3587678/lammer_et_al_2009_astron_astro_rev-4.pdf |archive-date=2016-06-02 }}</ref> Other circumstellar zones, where non-water solvents favorable to hypothetical life based on alternative biochemistries could exist in liquid form at the surface, have been proposed.<ref name=villard-2011>{{cite news |url=http://news.discovery.com/space/planetary-habitable-zones-defined-by-alien-biochemistry-111118.html |title=Alien Life May Live in Various Habitable Zones: Discovery News |publisher=News.discovery.com |date=November 18, 2011 |access-date=April 22, 2013 |author=Villard, Ray |agency=Discovery Communications LLC |archive-date=May 1, 2012 |archive-url=https://web.archive.org/web/20120501222124/http://news.discovery.com/space/planetary-habitable-zones-defined-by-alien-biochemistry-111118.html }}</ref>
==History== [[File:Maunder Edward Walter.jpg|thumb|Edward Walter Maunder, British astronomer, who introduced the concept of habitable zones]]
The concept of ''habitable zone'' arose in geography in the late 19th century. Alexander Winchell discussed planetary habitability in 1883, using a definition closer to what is now called a habitable zone for complex life.<ref name=Lingam-2021>{{Cite journal |last=Lingam |first=Manasvi |date=October 2021 |title=A brief history of the term 'habitable zone' in the 19th century |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/brief-history-of-the-term-habitable-zone-in-the-19th-century/73FAD17A3E1F7B58CDDE64A79D941CF7 |journal=International Journal of Astrobiology |language=en |volume=20 |issue=5 |pages=332–336 |doi=10.1017/S1473550421000203 |bibcode=2021IJAsB..20..332L |issn=1473-5504|url-access=subscription }}</ref> Possibly the earliest use of the term ''habitable zone'' was in 1913,<ref>{{cite book |last=Lorenz |first=Ralph |date=2019|title=Exploring Planetary Climate: A History of Scientific Discovery on Earth, Mars, Venus, and Titan|publisher=Cambridge University Press |page= 53 |isbn=978-1-108-47154-1}}</ref> by Edward Maunder in his book "Are The Planets Inhabited?".<ref>{{cite journal | journal=Research Notes of the American Astronomical Society |title=Maunder's Work on Planetary Habitability in 1913: Early Use of the term "Habitable Zone" and a "Drake Equation" Calculation|year=2020 |last=Lorenz |first=Ralph |volume=4 |issue=6 |page=79|doi=10.3847/2515-5172/ab9831|bibcode=2020RNAAS...4...79L|s2cid=219930646 |doi-access=free }}</ref> Hubertus Strughold's 1953 treatise ''The Green and the Red Planet: A Physiological Study of the Possibility of Life on Mars'' used the term "ecosphere" and referred to various "zones" in which life could emerge.<ref name=huggett-1995>{{cite book |url=https://archive.org/details/geoecologyevolut0000hugg |url-access=registration |title=Geoecology: An Evolutionary Approach |publisher=Routledge, Chapman & Hall |author=Huggett, Richard J. |date=1995 |page=[https://archive.org/details/geoecologyevolut0000hugg/page/10 10] |isbn=978-0-415-08689-9}}</ref><ref name=strughold-1953>{{cite book |url=https://books.google.com/books?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 concept 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|access-date=4 May 2013|date=2010|publisher=Princeton University Press|isbn=978-0-691-13805-3|page=127}}</ref> Su-Shu Huang, an American astrophysicist developed the modern concept of circumstellar habitable zones in a series of papers in the 1950s and 60s.<ref name="huang-1966">{{Cite journal |last=Huang |first=Su-Shu |date=1959 |title=Occurrence of Life in the Universe |url=https://www.jstor.org/stable/27827376 |journal=American Scientist |volume=47 |issue=3 |pages=397–402 |jstor=27827376 |bibcode=1959AmSci..47..397H |issn=0003-0996}}</ref><ref name=Lingam-2021/>
The concept of habitability was further developed in 1964 by Stephen H. Dole in a RAND Corporation study, ''Habitable Planets for Man'', in which he discussed the determinants of planetary habitability for humans, eventually estimating the number of habitable planets in the Milky Way to be about 600 million and about 50 such planets within {{val|100|u=ly}}.<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="Cockell-review">{{Cite journal |last1=Cockell |first1=C.S. |last2=Bush |first2=T. |last3=Bryce |first3=C. |last4=Direito |first4=S. |last5=Fox-Powell |first5=M. |last6=Harrison |first6=J.P. |last7=Lammer |first7=H. |last8=Landenmark |first8=H. |last9=Martin-Torres |first9=J. |last10=Nicholson |first10=N. |last11=Noack |first11=L. |last12=O'Malley-James |first12=J. |last13=Payler |first13=S.J. |last14=Rushby |first14=A. |last15=Samuels |first15=T. |date=January 2016 |title=Habitability: A Review |url=https://journals.sagepub.com/doi/full/10.1089/ast.2015.1295 |journal=Astrobiology |language=en |volume=16 |issue=1 |pages=89–117 |doi=10.1089/ast.2015.1295 |pmid=26741054 |bibcode=2016AsBio..16...89C |issn=1531-1074|url-access=subscription }}</ref> Dole subsequently co-authored a less technical version of the study in a book with science-fiction author Isaac Asimov which appeared in print at the height of the space race.<ref>{{Cite report |url=https://www.rand.org/pubs/commercial_books/CB183-1.html |title=Planets for Man |last1=Dole |first1=Stephen H. |last2=Asimov |first2=Isaac |date=October 9, 2007 |language=en}}</ref><ref name=gilster-2004>{{cite book |url=https://archive.org/details/springer_10.1007-978-1-4757-3894-0 |title=Centauri Dreams: Imagining and Planning Interstellar Exploration |publisher=Springer |author=Gilster, Paul |date=2004 |isbn=978-0-387-00436-5 |page=[https://archive.org/details/springer_10.1007-978-1-4757-3894-0/page/n51 40]}}</ref> The term "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 |access-date=April 22, 2013 |archive-date=August 29, 2011 |archive-url=https://web.archive.org/web/20110829081900/http://science.nasa.gov/science-news/science-at-nasa/2003/02oct_goldilocks/ }}</ref>
In 1993 James Kasting and colleagues developed a model for habitable zones across stars with different brightness using a simple climate model.<ref name=kasting-1993>{{cite journal |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>{{Cite journal |last1=Winn |first1=Joshua N. |last2=Fabrycky |first2=Daniel C. |date=August 18, 2015 |title=The Occurrence and Architecture of Exoplanetary Systems |url=https://www.annualreviews.org/content/journals/10.1146/annurev-astro-082214-122246 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=53 |issue= |pages=409–447 |doi=10.1146/annurev-astro-082214-122246 |arxiv=1410.4199 |bibcode=2015ARA&A..53..409W |issn=0066-4146}}</ref> Refinements of the climate model and extension to smaller stars led to new zone range estimates and the introduction of "conservative" and "optimistic" habitable zone concepts.<ref name=kopparapu-2013/><ref name="Hill-catalog">{{Cite journal |last1=Hill |first1=Michelle L. |last2=Bott |first2=Kimberly |last3=Dalba |first3=Paul A. |last4=Fetherolf |first4=Tara |last5=Kane |first5=Stephen R. |last6=Kopparapu |first6=Ravi |last7=Li |first7=Zhexing |last8=Ostberg |first8=Colby |date=January 5, 2023 |title=A Catalog of Habitable Zone Exoplanets |journal=The Astronomical Journal |volume=165 |issue=2 |pages=34 |doi=10.3847/1538-3881/aca1c0 |doi-access=free |bibcode=2023AJ....165...34H |issn=0004-6256}}</ref><ref name="Kane-2014">{{Cite journal |last=Kane |first=Stephen R. |date=February 4, 2014 |title=Habitable Zone Dependence on Stellar Parameter Uncertainties |journal=The Astrophysical Journal |volume=782 |issue=2 |pages=111 |doi=10.1088/0004-637X/782/2/111 |arxiv=1401.3349 |bibcode=2014ApJ...782..111K |issn=0004-637X}}</ref>
Practical application of the habitable zone concept arrived with improvements in telescope technology.<ref name="Kane-2014"/> In 2009, the Kepler space telescope was launched specifically to detect exoplanets in habitable zones. By the end of the mission the data suggested that at least 20% and perhaps as many as 50% of the stars visible at night have Earth-sized planets in their habitable zone.<ref>{{Cite journal |last1=Borucki |first1=William J. |last2=Agol |first2=Eric |last3=Fressin |first3=Francois |last4=Kaltenegger |first4=Lisa |last5=Rowe |first5=Jason |last6=Isaacson |first6=Howard |last7=Fischer |first7=Debra |last8=Batalha |first8=Natalie |last9=Lissauer |first9=Jack J. |last10=Marcy |first10=Geoffrey W. |last11=Fabrycky |first11=Daniel |last12=Désert |first12=Jean-Michel |last13=Bryson |first13=Stephen T. |last14=Barclay |first14=Thomas |last15=Bastien |first15=Fabienne |date=May 3, 2013 |title=Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone |url=https://www.science.org/doi/10.1126/science.1234702 |journal=Science |volume=340 |issue=6132 |pages=587–590 |doi=10.1126/science.1234702 |pmid=23599262 |arxiv=1304.7387 |bibcode=2013Sci...340..587B }}</ref><ref>{{Cite web |date=May 23, 2023 |title=Kepler / K2 - NASA Science |url=https://science.nasa.gov/mission/kepler/ |access-date=2026-04-02 |language=en-US}}</ref> In 2018, the Transiting Exoplanet Survey Satellite continued the search.<ref>{{Cite web |last=TSSC |title=NASA - TESS Science Support Center |url=https://heasarc.gsfc.nasa.gov/docs/tess/pages/what-is-tess.html |access-date=2026-04-02 |website=TESS |language=en}}</ref>
=== Extensions of the concept ===
In 1999, Guillermo Gonzalez and other noted that metals have important impact on planet formation and the development of life.<ref name="Rare Earth" />{{rp|43}} Together with paleontologist Peter Ward and fellow astronomer Donald Brownlee Gonzalez introduced the idea of the "galactic habitable zone".<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 |s2cid=18179704 }}</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 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"/>{{rp|28}}
The discovery of exoplanets unlike any in the Solar System led to suggestions that life might emerge even in conditions outside of currently defined habitable zones.<ref name="Seager 2013">{{cite journal |title=Exoplanet Habitability |journal=Science |year=2013 |last=Seager |first=Sara |s2cid=206546351 |volume=340 |issue=577 |pages=577–581 |doi=10.1126/science.1232226 |pmid=23641111 |bibcode=2013Sci...340..577S }}</ref> 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 concepts 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 |access-date=April 22, 2013 |author=Hadhazy, Adam |archive-date=May 2, 2013 |archive-url=https://web.archive.org/web/20130502064349/http://www.astrobio.net/exclusive/5364/the-habitable-edge-of-exomoons }}</ref>
==<span class="anchor" id="Optimistic habitable zone"></span><span class="anchor" id="Conservative habitable zone"></span> Definition== 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. A 'circumstellar habitable zone' is the region around a star which allows planets with liquid water. Whether a planet 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) and the radiative flux of the host star. These values are then used in climate models together with absorption coefficients for {{chem|H|2|0}} and {{chem|C|O|2}} to calculate the edges of the zone. Two different cutoffs may be applied. For the '''conservative habitable zone''' the inner edge represents water loss as the planet overheats and the outer edge is the maximum greenhouse effect limit where the atmospheric absorption is inadequate to keep water from freezing. The '''optimistic habitable zone''' (OHZ) expands the range to allow for changes in the host star luminosity with age. Based on analysis of possible water on Venus and Mars in the past, the inner edge of the OHZ is called the "recent Venus" edge and the outer edge is called "early Mars". Different climate models give different predictions for the size of the zone.<ref name=kopparapu-2013>{{cite journal |title=A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs |author=Kopparapu, Ravi Kumar |journal=The Astrophysical Journal Letters |date=2013 |volume=767 |issue=1 |doi=10.1088/2041-8205/767/1/L8 |arxiv=1303.2649 |pages=L8|bibcode = 2013ApJ...767L...8K|s2cid=119103101 }}</ref>
=== Habitable planets === The habitable zone concept is central to the search for habitable planets. Thousands of exoplanets have been discovered but detailed study of these remote objects is very difficult. Each discovery includes an estimate of the planet's orbital radius and this radius can be compared to the edges of the habital zone.<ref name=vladilo-2013/>
A planet may be in a "habitable zone" and still be uninhabitable. While conventional habitable zones are defined by the possibility of water, whether a given planet in a habitable zone has water depends on surface conditions that are dependent on many of different individual properties of that planet beyond those considered in the definition of a habitable zone.<ref name='Tasker Nature 2017'>{{Cite journal|last1=Tasker|first1=Elizabeth|last2=Tan|first2=Joshua|last3=Heng|first3=Kevin|last4=Kane|first4=Stephen|last5=Spiegel|first5=David|last6=Brasser|first6=Ramon|last7=Casey|first7=Andrew|last8=Desch|first8=Steven|last9=Dorn|first9=Caroline|last10=Hernlund|first10=John|last11=Houser|first11=Christine|date=2017-02-02|title=The language of exoplanet ranking metrics needs to change|journal=Nature Astronomy|language=en|volume=1|issue=2|page=0042|doi=10.1038/s41550-017-0042|arxiv=1708.01363|bibcode=2017NatAs...1E..42T|s2cid=118952886}}</ref><ref name='Patel MIT 2019'>{{cite web|url=https://www.technologyreview.com/s/614449/no-one-agrees-what-it-means-for-a-planet-to-be-habitable/|title=No one agrees what it means for a planet to be "habitable"|author=Neel V. Patel|publisher=MIT Technology Review|date=2 October 2019|quote="surface conditions are dependent on a host of different individual properties of that planet, such as internal and geological processes, magnetic field evolution, climate, atmospheric escape, rotational effects, tidal forces, orbits, star formation and evolution, unusual conditions like binary star systems, and gravitational perturbations from passing bodies."}}</ref> This misunderstanding is reflected in excited reports of 'habitable planets'.<ref name="Tan-2017">{{Cite web|url=http://theconversation.com/until-we-get-better-tools-excited-reports-of-habitable-planets-need-to-come-back-down-to-earth-72425|title=Until we get better tools, excited reports of 'habitable planets' need to come back down to Earth| last=Tan| first=Joshua| website=The Conversation |date=8 February 2017 |language=en| access-date=2019-10-21}}</ref><ref name='Sci News Oct2019'>{{Cite web| url=https://www.sciencenews.org/article/why-just-being-habitable-zone-doesnt-make-exoplanets-livable| title=Why just being in the habitable zone doesn't make exoplanets livable| date=2019-10-04|website=Science News| language=en-US| access-date=2019-10-21}}</ref><ref>[https://blogs.scientificamerican.com/observations/no-the-exoplanet-k2-18b-is-not-habitable/ No, the Exoplanet K2-18b Is Not Habitable. News outlets that said otherwise are just crying wolf—but they're not the only ones at fault.] Laura Kreidberg, ''Scientific American''. 23 September 2019.</ref> Some astronomers have suggested that the term "habitability" should not even be used.<ref name='Tasker Sci Am 2019'>{{Cite web| url=https://blogs.scientificamerican.com/observations/lets-lose-the-term-habitable-zone-for-exoplanets/| title=Let's Lose the Term "Habitable Zone" for Exoplanets| last=Tasker|first=Elizabeth| website=Scientific American Blog Network| language=en| access-date=2019-10-21}}</ref> Ultimately only technology for remote determination of signs of life will lead to discovery of habitable planets.<ref name='Patel MIT 2019'/><ref name='Sci News Oct2019'/>
Many additional criteria must be met for a habitable zone for complex life, but again a planet in such a zone may still not support complex life. Planets capable of supporting human life would face even more stringent requirements. These include surface temperatures between {{val|0|-|30|u=degC}} and light in the visible range with brightness {{val|0.02|-|30}} lumens/cm<sup>2</sup>, gravity below about {{val|1.5|u=G}}, an oxygen atmosphere without toxic gases, other life forms essential for food, tolerable wind velocity, low levels of dust, radioactivity, meteor impacts, earthquakes and lightning.<ref name="dole-1964"/>{{rp|22}}
===Solar System estimates=== {{seealso|Water on terrestrial planets of the Solar System}} [[File: Estimated extent of the Solar Systems habitable zone.png|thumb|The range of published estimates for the extent of the Sun's HZ. The conservative HZ<ref name=dole-1964 /> is indicated by a dark-green band crossing the inner edge of the aphelion of Venus, whereas an extended HZ,<ref name=fogg-1992 /> extending to the orbit of the dwarf planet Ceres, is indicated by a light-green band.]] [[File:Terrestrial planet size comp 2024.png|thumb|Solar System's Planetary-mass objects with partial or full orbits within the Extended Habitable Zone from left to right: Mercury, Venus, Earth and Moon, Mars, and Ceres. While many possess surface water in solid state, only Earth has liquid water on the surface. This is mainly due to a combination of low mass and an inability to mitigate evaporation and atmosphere loss against the solar wind.]] Estimates for the habitable zone within the Solar System vary widely. Zones based on the Sun's effective temperature ({{val|5778|u=K}}) and the redistribution of heat via an atmosphere lead a zone between {{val|0.84|u=AU}} and {{val|1.7|u=AU}} (the Earth is defined to be at {{val|1|u=AU}}, an astronomical unit). These limits include Earth and Mars but not Venus.<ref>{{Cite journal |last1=Kane |first1=S. R. |last2=Gelino |first2=D. M. |title=Sage Journals: Discover world-class research |url=https://journals.sagepub.com/action/cookieAbsent |access-date=2026-04-01 |journal=Astrobiology |date=2012 |volume=12 |issue=10 |pages=940–945 |language=en |doi=10.1089/ast.2011.0798 |pmid=23035897 |arxiv=1205.2429 |bibcode=2012AsBio..12..940K }}</ref> The inner edge of the zone could be small as {{val|0.38|u=AU}} for particularly favorable atmospheric and planet surface reflectivity properties.<ref name=zsom-2013 />
For reference, the average distance from the Sun of some major bodies within the various estimates of the habitable zone is: Mercury, 0.39 AU; Venus, 0.72 AU; Earth, 1.00 AU; Mars, 1.52 AU; Vesta, 2.36 AU; Ceres and Pallas, 2.77 AU; Jupiter, 5.20 AU; Saturn, 9.58 AU. In the most conservative estimates, only Earth lies within the zone; in the most permissive estimates, even Saturn at perihelion, or Mercury at aphelion, might be included.
{| class="wikitable sortable" |- style="text-align:center; align:center; background:#90b0f0;" |+ Estimates of the circumstellar habitable zone boundaries of the Solar System ! Inner edge (AU) !! The outer edge (AU) !! Year !! Notes |- | 0.7 || 1.24 || 1946, Dole, Asimov || Broad, qualitative estimate.<ref name=kane-2012/> |- | 0.958 || 1.004 || 1979, Hart<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 |bibcode = 1979Icar...37..351H }}</ref>|| Based on computer modeling and simulations of the evolution of Earth's atmospheric composition and surface temperature. A conservative estimate.<ref name=kane-2012/> |- | 0.95 || 2.0 || 1992, Fogg<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>|| Claims that the carbon cycle stabilizes the climate and expands the outer edge of the circumstellar habitable zone. |- | 0.95 || 1.37 || 1993, Kasting et al.<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 || 2010, Spiegel et al.<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 |arxiv = 1002.4877 |bibcode = 2010ApJ...721.1308S | s2cid = 15899053 }}</ref>|| Proposed that seasonal liquid water is possible to this limit when combining high obliquity and orbital eccentricity.{{Failed verification|date=April 2026|reason=not an HZ paper per se}} |- | 0.75 || || 2011, Abe et al.<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|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 || 2011, Pierrehumbert and Gaidos<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 the Solar System. |- | 0.77–0.87 || 1.02–1.18 || 2013, Vladilo et al.<ref name=vladilo-2013>{{cite journal |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 |s2cid=49553651 }}</ref>|| Inner edge of the circumstellar habitable zone is closer and outer edge is farther for higher atmospheric pressures; determined minimum atmospheric pressure required to be 15 mbar. |- | 0.99 || 1.67 || 2013, Kopparapu et al.<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 vapor. Once the stratosphere becomes wet, water vapor 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 0.97–1.67 AU. This definition does not take into account possible radiative warming by {{CO2}} clouds. |- | 0.38 || || 2013, Zsom et al.<br /><ref name=zsom-2013>{{cite journal |title=Towards the Minimum Inner Edge Distance of the Habitable Zone |last1=Zsom |first1=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|s2cid=27805994 }}</ref>|| Pushed the inner edge to the lowest values using various combinations of atmospheric composition, pressure and relative humidity of the planet's atmosphere. |- | 0.95 || || 2013, Leconte et al.<ref name= leconte-2013>{{cite journal |title=Increased insolation threshold for runaway greenhouse processes on Earth-like planets |last1=Leconte |first1=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|s2cid=2115695 }}</ref>||Using 3-D models, these authors computed an inner edge of 0.95 AU for the Solar System. |- | 0.95 ||2.4 || 2017, Ramirez and Kaltenegger<br /><ref name=rk-2017>{{cite journal |title=A Volcanic Hydrogen Habitable Zone |last1=Ramirez |first1=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|s2cid=119333468 |doi-access=free }}</ref>|| An expansion of the classical carbon dioxide-water vapor habitable zone<ref name=kasting-1993 /> assuming a volcanic hydrogen atmospheric concentration of 50%. |- | 0.93–0.91|| || 2019, Gomez-Leal et al.<br /><ref name=gomez-2019>{{cite journal |title=Climate sensitivity to ozone and its relevance on the habitability of Earth-like planets|last1=Gomez-Leal |first1=Illeana |date=2019 |arxiv=1901.02897|last2=Kaltenegger |first2=Lisa |last3=Lucarini |first3=Valerio |last4=Lunkeit |first4=Frank |doi=10.1016/j.icarus.2018.11.019 |volume=321 |pages=608–618 |journal=Icarus|bibcode=2019Icar..321..608G|s2cid=119209241 }}</ref>|| Estimation of the moist greenhouse threshold by measuring the water mixing ratio in the lower stratosphere, the surface temperature, and the climate sensitivity on an Earth analog with and without ozone, using a global climate model (GCM). It shows the correlation of a water mixing ratio value of 7 g/kg, a surface temperature of about 320 K, and a peak of climate sensitivity in both cases. |- ! 0.99 !! 1.004 !! !! The tightest bounded estimate from above |- ! 0.38 !! 10 !! !! The most relaxed estimate from above |- |}
== Effect of climate model ==
[[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.]]
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, leakage from hydrous minerals from the lithosphere, and photolysis.<ref name="source_mrk1">{{cite journal |title= Origin of water in the terrestrial planets |last1= Drake |first1= Michael J. |s2cid= 12808812 |journal=Meteoritics & Planetary Science |date=April 2005 |volume=40 |issue= 4 |pages= 519–527 |doi= 10.1111/j.1945-5100.2005.tb00960.x |bibcode= 2005M&PS...40..519D|doi-access= free }}</ref><ref name="source_mrk2">{{cite conference |url= http://journals.cambridge.org/action/displayFulltext?type=6&fid=415222&jid=IAU&volumeId=1&issueId=S229&aid=414784&bodyId=&membershipNumber=&societyETOCSession=&fulltextType=RA&fileId=S1743921305006861 |title= Origin of water in the terrestrial planets |display-authors=1 |last1= Drake |first1= Michael J. |last2= Humberto |first2= Campins |conference = 229th Symposium of the International Astronomical Union |date=August 2005 |location = Búzios, Rio de Janeiro, Brazil |publisher= Cambridge University Press |volume=1 |issue= 4 |pages= 381–394 |doi= 10.1017/S1743921305006861 |bibcode= 2006IAUS..229..381D |book-title= Asteroids, Comets, and Meteors (IAU S229) |isbn= 978-0-521-85200-5|doi-access= free }}</ref> For an extrasolar system, an icy body from beyond the frost line could migrate into the habitable zone of its star, creating an ocean planet with seas hundreds of kilometers deep<ref name=kuchner-2003>{{Cite journal|arxiv=astro-ph/0303186|title=Volatile-rich Earth-Mass Planets in the Habitable Zone|first=Marc|last=Kuchner|journal=Astrophysical Journal|date=2003|volume=596|issue=1|pages=L105–L108|doi=10.1086/378397|bibcode=2003ApJ...596L.105K|s2cid=15999168}}</ref> such as GJ 1214 b<ref name="disco-charbonneau">{{cite journal |last1=Charbonneau |first1=David |author2=Zachory K. Berta |author3=Jonathan Irwin |author4=Christopher J. Burke |author5=Philip Nutzman |author6=Lars A. Buchhave |author7=Christophe Lovis |author8=Xavier Bonfils |author9=David W. Latham |author10=Stéphane Udry |author11=Ruth A. Murray-Clay |author12=Matthew J. Holman |author13=Emilio E. Falco |author14=Joshua N. Winn |author15=Didier Queloz |author16=Francesco Pepe |author17=Michel Mayor |author18=Xavier Delfosse |author19=Thierry Forveille |display-authors=8 |date=2009 |title=A super-Earth transiting a nearby low-mass star |journal=Nature |volume=462 |issue=17 December 2009 |pages=891–894 |doi=10.1038/nature08679 |pmid=20016595 |bibcode=2009Natur.462..891C|arxiv = 0912.3229 |s2cid=4360404 }}</ref><ref name="planetmodels">{{cite journal |last1= Kuchner |first1= Seager |first2=M.|last2=Hier-Majumder | first3=C. A.|last3=Militzer |date=2007 |title=Mass–radius relationships for solid exoplanets |journal=The Astrophysical Journal |volume=669 |issue= 2|pages=1279–1297 |doi=10.1086/521346 |bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 |s2cid= 8369390 }}</ref> or Kepler-22b may be.<ref name=vastag-2011>{{cite news |url=https://www.washingtonpost.com/national/health-science/newest-alien-planet-is-just-the-right-temperature-for-life/2011/12/05/gIQAPk1vWO_story.html |title=Newest alien planet is just the right temperature for life |newspaper=The Washington Post |date=December 5, 2011 |access-date=April 27, 2013 |author=Vastag, Brian}}</ref>
Maintenance of liquid surface water also requires a sufficiently thick atmosphere. Possible origins of terrestrial atmospheres are currently theorized to outgassing, impact degassing, and ingassing.<ref name="RobinsonCatling2012">{{cite journal|last1=Robinson|first1=Tyler D.|last2=Catling|first2=David C.| title=An Analytic Radiative-Convective Model for Planetary Atmospheres| journal=The Astrophysical Journal| volume=757|issue=1|date=2012|page=104|doi=10.1088/0004-637X/757/1/104|arxiv = 1209.1833 |bibcode = 2012ApJ...757..104R |s2cid=54997095}}</ref> Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.<ref name="Shizgal, 1996">{{cite journal |last1=Shizgal |first1=B. D. |last2=Arkos |first2=G. G. |s2cid=7852371 |date=1996 |title=Nonthermal escape of the atmospheres of Venus, Earth, and Mars |journal=Reviews of Geophysics |volume=34 |issue=4 |pages=483–505 |doi=10.1029/96RG02213 |bibcode = 1996RvGeo..34..483S }}</ref> In a 2013 study led by Italian astronomer Giovanni Vladilo, it was shown that the size of the circumstellar habitable zone increased with greater atmospheric pressure.<ref name=vladilo-2013 /> Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained<ref name=vladilo-2013 /> because even a small shift in pressure or temperature could render water unable to form as a liquid.<ref name=chaplin-2013>{{cite web |url=http://www.lsbu.ac.uk/water/phase.html |title=Water Phase Diagram |publisher=London South Bank University |work=Ices |date=April 8, 2013 |access-date=April 27, 2013 |author=Chaplin, Martin |archive-date=March 3, 2016 |archive-url=https://web.archive.org/web/20160303173403/http://www.lsbu.ac.uk/water/phase.html }}</ref>
Although traditional definitions of the habitable zone assume that carbon dioxide and water vapor are the most important greenhouse gases (as they are on the Earth),<ref name=kasting-1993 /> a study<ref name="rk-2017"/> led by Ramses Ramirez and co-author Lisa Kaltenegger has shown that the size of the habitable zone is greatly increased if prodigious volcanic outgassing of hydrogen is also included along with the carbon dioxide and water vapor. The outer edge in the Solar System would extend out as far as 2.4 AU in that case. Similar increases in the size of the habitable zone were computed for other stellar systems. An earlier study by Ray Pierrehumbert and Eric Gaidos<ref name=rayeric-2011>{{cite journal |title=Hydrogen Greenhouse Planets Beyond the Habitable Zone |last1=Pierrehumbert |first1=Raymond |date=2011 |arxiv=1105.0021|last2=Gaidos |first2=Eric |doi=10.1088/2041-8205/734/1/L13 |volume=734 |issue=1 |pages=L13 |journal=The Astrophysical Journal Letters|bibcode=2011ApJ...734L..13P|s2cid=7404376 }}</ref> had eliminated the CO<sub>2</sub>-H<sub>2</sub>O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the outer edge of the Solar System 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.
===Desert planets=== thumb|Dry desert planets like Mars may be more common in the habitable zone than wet 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 planets (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>{{cite journal | doi = 10.1089/ast.2010.0545 | pmid=21707386 | volume=11 | title=Habitable zone limits for dry planets | year=2011 | journal=Astrobiology | pages=443–60 | last1 = Abe | first1 = Y | last2 = Abe-Ouchi | first2 = A | last3 = Sleep | first3 = NH | last4 = Zahnle | first4 = KJ| issue=5 | bibcode=2011AsBio..11..443A }}</ref>
==Effect of stellar luminosity== {{see also|Habitability of red dwarf systems|Habitability of K-type main-sequence star systems}} [[File:Orbit of 82 G. Eridani d.png|thumb|The orbit of 82 G. Eridani d which passes through predicted conservative and optimistic habitable zones of its Sun-like G-type parent star.]] Astronomers use stellar flux and the inverse-square law to extrapolate circumstellar habitable zone models created for the Solar System to other stars.<ref name=kasting-1993/> The radial edges of the habitable zones for host stars with different luminosities and spectral types are given by <math display="block">r_\mathrm{edge} = \sqrt\left( \frac{L/L_\odot}{S_\mathrm{edge}(T_\mathrm{eff})} \right)</math> where <math>L/L_\odot</math> is the ratio of the measured or inferred luminosity of the star to the luminosity of the Sun and <math>T_\mathrm{eff}</math> is the effective stellar temperature which depends on the spectral type. Each edge defines a criterion on the surface of the planet and climate simulation are run to determine the stellar flux, <math>S_\mathrm{edge}(T_\mathrm{eff})</math>, that will produce that criterion (inverse climate modeling). For each zone edge criteria, the stellar flux result is fit to a fourth order polynomial in <math>T_\mathrm{eff}</math>. The effective temperature for a given stellar type, say 7200K for an F0 star, substituted into the polynomial for the "runaway greenhouse" edge gives the stellar flux for runaway greenhouse results, and that is combined with the particular F0 stars luminosity to give radius of the inner edge of the conservative habitable zone.<ref name=kopparapu-2013/>
=== Spectral types and star-system characteristics === 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 types. 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| s2cid=118610856}}</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|s2cid=2241081}}</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 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 dwarfs. 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 for 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 modeling from 2013 supports the idea that red dwarf stars can support planets with relatively constant temperatures over their surfaces despite tidal locking.<ref name=yang-2013 /> Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration.<ref>{{cite journal |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=L31 |doi=10.1088/2041-8205/731/2/L31 |arxiv=1103.2791|bibcode = 2011ApJ...731L..31A |s2cid=118739494 }}</ref> At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs.<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 |last1=Ramirez |first1=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|s2cid=119276912 }}</ref>
==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 |access-date=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 |arxiv = 1003.1368 |bibcode = 2009Ap&SS.323..367G | s2cid = 118500534 }}</ref> Even while stars are on the main sequence, though, their energy output steadily increases, pushing their habitable zones farther out;<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 |s2cid=119089227 }}</ref> the Sun, for example, was 75% as bright in the Archaean as it is now,<ref name="Kasting-1968">{{Cite journal |last1=Kasting |first1=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 |bibcode=1986Sci...234.1383K |url=https://zenodo.org/record/1230890 }}</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 |access-date=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. |book-title=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. 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 flares 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 |url-access=subscription |title=Red, willing and able |access-date=August 5, 2007|date=January 27, 2001 |magazine=New Scientist}} [http://www.kencroswell.com/reddwarflife.html Full reprint]</ref> and huge starspots 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 | doi-access = free}}</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 |access-date=April 27, 2013 |author=<!--staff writer-->}}</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 journal| last=Alpert |first=Mark |title=Red Star Rising |journal=Scientific American |volume=293 |issue=5 |page=28 |date=November 7, 2005 |pmid=16318021 |doi=10.1038/scientificamerican1105-28 |bibcode=2005SciAm.293e..28A }}</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 |author-link2=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 |archive-date=2007-09-26 |date=2007 |access-date=2007-06-17 }}</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=en|author=Ray Villard|work=Astrobiology Magazine |date=27 July 2009|access-date=8 April 2016|archive-date=24 April 2016|archive-url=https://web.archive.org/web/20160424143742/http://www.astrobio.net/topic/solar-system/sun/living-in-a-dying-solar-system-part-1/}}</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 |access-date=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 |arxiv = astro-ph/0503520 |bibcode = 2005ApJ...627..974L | s2cid = 17075384 }}</ref> At such stage, Saturn's 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|s2cid=14172341 }}</ref> Given that this new equilibrium lasts for about 1 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 |access-date=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|last1=Ramirez |first1=Ramses |date=2016 |arxiv=1605.04924|last2=Kaltenegger |first2=Lisa |doi=10.3847/0004-637X/823/1/6 |volume=823 |issue=1 |page=6 |journal=The Astrophysical Journal|bibcode=2016ApJ...823....6R|s2cid=119225201 |doi-access=free }}</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/>
==Exoplanets in habitable zones==
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 HZ, 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 |access-date=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 |access-date=April 29, 2013 |author=Torres, Abel Mendez |archive-date=October 21, 2019 |archive-url=https://web.archive.org/web/20191021202042/http://phl.upr.edu/press-releases/tenpotentiallyhabitableexoplanetsnow }}</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 |access-date=24 April 2011 |archive-url=https://web.archive.org/web/20110927053134/http://apnews.excite.com/article/20110219/D9LG45NO0.html |archive-date=27 September 2011 }}</ref> NASA's Jet Propulsion Laboratory 2011 study, based on observations from the Kepler mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class F, G, and K are expected to have planets in their HZs.<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 |access-date=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 | page = 151 | year = 2011 |arxiv = 1103.1443 |bibcode = 2011ApJ...738..151C | s2cid = 119290692 }}</ref>
== Habitable planets == {{see also|List of potentially habitable exoplanets|Planetary habitability}} A 2015 review concluded that the exoplanets 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|access-date=2015-07-24}}</ref> These are at a distance of 990, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is closest in size to Earth with 1.2 times Earth's radius, and it is located towards the outer edge of the habitable zone around its red dwarf star. Among nearest terrestrial exoplanet candidates, Tau Ceti e is 11.9 light-years away. It is in the inner edge of its planetary 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>
===Early findings=== {{Category see also|Giant planets in the habitable zone}} The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water.<ref>{{cite journal|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 |s2cid=37593615 }}</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 |access-date = 2009-04-02 |archive-url=https://web.archive.org/web/20120619015814/http://www.extrasolar.net/planettour.asp?PlanetID=22 |archive-date=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 HZ, such an orbit would causes extreme seasonal effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.<ref>{{cite journal|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 |s2cid=37593615 }}</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 |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 |s2cid=16004653 }}</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 | s2cid = 119078585 }}</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 |arxiv = astro-ph/0607493 |bibcode = 2006ApJ...646..505B | s2cid = 119067572 }}</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 | s2cid = 14508244 }}</ref> though it is unclear whether such satellites could form in the first place.<ref>{{Cite journal | last1 = Canup | first1 = R. M. | author-link = Robin Canup | 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|bibcode = 2006Natur.441..834C | s2cid = 4327454 }}</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 |author=Lovis| title=An extrasolar planetary system with three Neptune-mass planets |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 | s2cid=4343578 |display-authors=1 }}</ref> The following year, 55 Cancri f was discovered within the HZ of its host star 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 |access-date = 2008-09-14| archive-url = https://web.archive.org/web/20080926142319/https://www.sciencedaily.com/releases/2007/11/071106133058.htm| archive-date = 26 September 2008 | url-status = live}}</ref><ref name="Fischer2008">{{Cite journal |title=Five Planets Orbiting 55 Cancri |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 |s2cid=55779685 }}</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 |date= 7 November 2007|access-date=17 October 2008 |location=London| archive-url= https://web.archive.org/web/20081002080911/http://www.guardian.co.uk/science/2007/nov/07/spaceexploration| archive-date= 2 October 2008 | url-status= live}}</ref>
===Super-Earths in a habitable zone=== {{Category see also|Super-Earths in the habitable zone}} thumb|The habitable zone of Gliese 581 compared with the Solar System's habitable zone
The Kepler space telescope discovered that most Sun-like stars have close-in planets in the size range between Earth and Neptune. These planets have been called super-Earths but this term has been characterized as deceptive.<ref>{{Cite journal |last1=Moore |first1=William B. |last2=Lenardic |first2=A. |last3=Jellinek |first3=A. M. |last4=Johnson |first4=C. L. |last5=Goldblatt |first5=C. |last6=Lorenz |first6=R. D. |date=February 2017 |title=How habitable zones and super-Earths lead us astray |url=https://www.nature.com/articles/s41550-017-0043 |journal=Nature Astronomy |language=en |volume=1 |issue=2 |article-number=0043 |doi=10.1038/s41550-017-0043 |bibcode=2017NatAs...1E..43M |issn=2397-3366|url-access=subscription }}</ref> Subsequent study has shown they have hydrogen gas atmospheres and they fall into two categories: the larger ones have retain atmospheres but the smaller ones are stipped cores. Models that simulate this bimodal distribution predict that stellar radiation perhaps combined with heat from planetary cooling drive off the atmosphere causing some of these exoplanets to appear smaller.<ref>{{Cite journal |last1=Bean |first1=Jacob L. |last2=Raymond |first2=Sean N. |last3=Owen |first3=James E. |date=January 2021 |title=The Nature and Origins of Sub-Neptune Size Planets |journal=Journal of Geophysical Research: Planets |language=en |volume=126 |issue=1 |article-number=e2020JE006639 |doi=10.1029/2020JE006639 |issn=2169-9097 |pmc=7900964 |pmid=33680689 |arxiv=2010.11867 |bibcode=2021JGRE..12606639B }}</ref> In general, these exoplanets are not in the habitable zone. Moreover, the processes which strip the hydrogen atmosphere from this type of exoplanet is unlikely to operate on planets in the habitable zone. Thus among this type of exoplanet, those orbital periods closer to the habitable zone more closely resemble Neptune.<ref>{{Cite journal |last1=Bergsten |first1=Galen J. |last2=Pascucci |first2=Ilaria |last3=Mulders |first3=Gijs D. |last4=Fernandes |first4=Rachel B. |last5=Koskinen |first5=Tommi T. |date=October 2022 |title=The Demographics of Kepler's Earths and Super-Earths into the Habitable Zone |journal=The Astronomical Journal |volume=164 |issue=5 |pages=190 |doi=10.3847/1538-3881/ac8fea |doi-access=free |arxiv=2209.04047 |bibcode=2022AJ....164..190B |issn=0004-6256}}</ref>
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
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 |access-date=May 2, 2013}}</ref> is the first transiting exoplanet discovered around a 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 |access-date=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 |s2cid=16531923 }}</ref> is a super-Earth orbiting in the circumstellar habitable zone of Gliese 667 C. It is one of the most Earth-like planets known.
Gliese 163 c, discovered in September 2012 in orbit around the red dwarf Gliese 163<ref name="Simbad-20120920">{{cite web |author=<!--Staff writer--> |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) |access-date=September 20, 2012 }}</ref> is located 49 light years 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}}° 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) |access-date=September 20, 2012 |archive-date=October 21, 2019 |archive-url=https://web.archive.org/web/20191021202448/http://phl.upr.edu/press-releases/ahotpotentialhabitableexoplanetaroundgliese163 }}</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 |access-date=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 |date=5 September 2012 |publisher=Spacedaily.com |access-date=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 |doi=10.1051/0004-6361/201220268 |last1=Tuomi |first1=Mikko |last2=Anglada-Escudé |first2=Guillem |last3=Gerlach |first3=Enrico |last4=Jones |first4=Hugh R. A. |last5=Reiners |first5=Ansgar |last6=Rivera |first6=Eugenio J. |last7=Vogt |first7=Steven S. |last8=Butler |first8=R. Paul |title=Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307 |journal=Astronomy & Astrophysics |date=17 December 2012 |volume=549 |pages=A48 |arxiv=1211.1617 |bibcode=2013A&A...549A..48T |s2cid=7424216 }}</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 |access-date=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 |arxiv = 1212.4277 |bibcode = 2013A&A...551A..79T | s2cid = 2390534 }}</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 |date=May 1, 2013 |access-date=May 1, 2013 |author=Torres, Abel Mendez}}</ref> It is now considered as uninhabitable.
===Near Earth-sized planets and Solar analogs=== [[File:Kepler186f-ComparisonGraphic-20140417 improved.jpg|thumb|right|Comparison of the HZ position of Earth-radius planet Kepler-186f and the Solar System (17 April 2014)]] 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 significant 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 genuinely 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. No solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are 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 critical 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|doi=10.17226/13519|isbn=978-0-309-26564-5}}</ref><ref>{{cite web|url=http://scienceblogs.com/startswithabang/2013/06/05/most-of-earths-twins-arent-identical-or-even-close/|title=Most of Earth's twins aren't identical, or even close!|author=Ethan|date=June 5, 2013|publisher=ScienceBlogs.com}}</ref>
Using data collected by NASA's Kepler space telescope 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 |access-date=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 the Sun, in the HZ 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 |access-date=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|page=101|arxiv = 1304.4941 |bibcode = 2013ApJ...768..101B | s2cid=51490784}}</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 |access-date=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 |access-date=18 April 2013 |archive-date=8 May 2020 |archive-url=https://web.archive.org/web/20200508010029/https://www.nasa.gov/mission_pages/kepler/news/kepler-62-kepler-69.html }}</ref><ref name="NYT-20130418"/><ref name="Borucki-2013">{{Cite journal |last=Borucki |first=William J. |author-link=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 |volume=340 |issue=6132 |pages=587–90|arxiv = 1304.7387 |bibcode = 2013Sci...340..587B |display-authors=etal |pmid=23599262|hdl=1721.1/89668 |s2cid=21029755 }}</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 |access-date=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 |access-date=17 April 2014 |archive-date=15 March 2019 |archive-url=https://web.archive.org/web/20190315091857/http://apnews.excite.com/article/20140417/DAD832V81.html }}</ref><ref name="BBC-20140417">{{cite news |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |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 |access-date=17 April 2014 }}</ref> though its mass remains unknown and its parent star is not a Solar analog.
Kapteyn b, discovered in June 2014, was thought to is a possible rocky world of about 4.8 Earth masses and about 1.5 Earth radii 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 |access-date=10 January 2015 }}</ref> However, further analysis concluded that this claim was an artefact of stellar rotation and activity.<ref name="BortleFausey2021">{{citation | postscript=. | title=A Gaussian Process Regression Reveals No Evidence for Planets Orbiting Kapteyn's Star | last1=Bortle | first1=Anna | last2=Fausey | first2=Hallie | last3=Ji | first3=Jinbiao | last4=Dodson-Robinson | first4=Sarah | last5=Ramirez Delgado | first5=Victor | last6=Gizis | first6=John | display-authors=1 | journal=The Astronomical Journal | volume=161 | issue=5 | year=2021 | page=230 | arxiv=2103.02709 | doi=10.3847/1538-3881/abec89 | bibcode=2021AJ....161..230B | s2cid=232110395 | doi-access=free }}</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 stars: two of the three, Kepler-438b and Kepler-442b, are near-Earth-size and likely rocky; the third, Kepler-440b, is a super-Earth.<ref name="NASA-20150106" /> However, Kepler-438b is found to be a subject of powerful flares, so it is now considered uninhabitable. 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 |access-date=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-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<sub>🜨</sub> 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|arxiv = 1507.06723 |bibcode = 2015AJ....150...56J |s2cid=26447864}}</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 |access-date=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 dramatically 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 years 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|page=207|issn=1538-3881|doi=10.3847/1538-3881/aa89f2|arxiv=1703.07416|bibcode=2017AJ....154..207D|s2cid=13419148|url=https://authors.library.caltech.edu/78341/2/Dressing_2017_AJ_154_207.pdf |doi-access=free }}</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 |s2cid=2718408}}</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 three times the mass of Earth, Luyten b orbits within the habitable zone of Luyten's Star just 12.2 light-years away.<ref>{{Cite magazine | 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| magazine=Wired UK| date=2017-11-16| last1=Bradley| first1=Sian}}</ref>
At 11 light-years away, the 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 times 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| website=Space.com| date=15 November 2017}}</ref>
Discovered in March 2018, K2-155d is about 1.64 times 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 on 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|volume=628|pages=A39|year=2019|issn=0004-6361|doi=10.1051/0004-6361/201935801|arxiv=1904.12818|bibcode=2019A&A...628A..39L|doi-access=free}}</ref>
K2-18b is an exoplanet 124 light-years away, orbiting in the habitable zone of the K2-18, a red dwarf. This planet is significant for water vapor found in its atmosphere; this was announced on September 17, 2019.
In September 2020, astronomers identified 24 superhabitable planet (planets better than Earth) contenders, from among more than 4000 confirmed exoplanets at present, based on astrophysical parameters, as well as the natural history of known life forms on the Earth.<ref name="AB-20200918">{{cite journal |last1=Schulze-Makuch |first1=Dirk |last2=Heller |first2=Rene |last3=Guinan |first3=Edward |title=In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World |date=18 September 2020 |journal=Astrobiology |volume=20 |issue=12 |pages=1394–1404 |doi=10.1089/ast.2019.2161 |pmid=32955925 |pmc=7757576 |bibcode=2020AsBio..20.1394S |doi-access=free }}</ref>
{| class="wikitable" style="margin:0.5em auto; width:600px;" ! Notable exoplanets – Kepler space telescope |- | style="font-size:88%" | 600px {{center|1=Confirmed small exoplanets in habitable zones.<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 |access-date=6 January 2015 }}</ref>}} |}
In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between the two sides.<ref name=yang-2013>{{Cite journal | last1 = Yang | first1 = J. | last2 = Cowan | first2 = N. B. | last3 = Abbot | first3 = D. S. | doi = 10.1088/2041-8205/771/2/L45 | title = Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets | journal = The Astrophysical Journal | volume = 771 | issue = 2 | pages = L45 | year = 2013| arxiv = 1307.0515| bibcode = 2013ApJ...771L..45Y| s2cid = 14119086 }}</ref> === Moons === {{see also|Habitability of natural satellites}} Planetary mass 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,<ref name=hadhazy-2013 /> but must remain within the Hill radius of the planet so that they are not pulled out of the orbit of their host planet.<ref name="HamiltonBurns92">{{cite journal |author1=D.P. Hamilton |author2=J.A. Burns | title= Orbital stability zones about asteroids. II – The destabilizing effects of eccentric orbits and solar radiation| journal= Icarus| date= 1992| volume= 96 |issue= 1| pages= 43–64| bibcode= 1992Icar...96...43H |doi= 10.1016/0019-1035(92)90005-R|url=http://www.astro.umd.edu/~hamilton/research/reprints/HamBurns91.pdf|citeseerx=10.1.1.488.4329 }}</ref> Red dwarfs that have masses less than 20% of that of the Sun cannot have habitable moons around giant planets, as the small size of the circumstellar habitable zone would put a habitable moon so close to the star that it would be stripped from its host planet. In such a system, a moon close enough to its host planet to maintain its orbit would have tidal heating so intense as to eliminate any prospects of habitability.<ref name=hadhazy-2013 /> thumb|Artist's concept of a planet on an eccentric orbit that passes through the HZ for only part of its orbit A planetary object that orbits a star with high orbital eccentricity may spend only some of its year in the HZ and experience a large variation in temperature and atmospheric pressure. This would result in dramatic seasonal phase shifts where liquid water may exist only intermittently. It is possible that subsurface habitats could be insulated from such changes and that extremophiles on or near the surface might survive through adaptions such as hibernation (cryptobiosis) and/or hyperthermostability. Tardigrades, for example, can survive in a dehydrated state temperature between {{convert|-273|C|K|order=flip}}<ref>{{cite journal|author=Becquerel P.|date=1950| title=La suspension de la vie au dessous de 1/20 K absolu par demagnetization adiabatique de l'alun de fer dans le vide les plus eléve| journal=C. R. Acad. Sci. Paris| volume=231| pages=261–263| language=fr}}</ref> and {{convert|151|C|K|order=flip}}.<ref name="survival">{{cite book| last=Horikawa|first=Daiki D.|chapter=Survival of Tardigrades in Extreme Environments: A Model Animal for Astrobiology |series=Cellular Origin, Life in Extreme Habitats and Astrobiology | title=Anoxia Evidence for Eukaryote Survival and Paleontological Strategies.| date=2012|volume=21 |publisher=Springer Netherlands| isbn=978-94-007-1895-1|pages=205–217| edition=21|editor=Alexander V. Altenbach, Joan M. Bernhard & Joseph Seckbach|doi=10.1007/978-94-007-1896-8_12}}</ref> Life on a planetary object orbiting outside HZ might hibernate on the cold side as the planet approaches the apastron where the planet is coolest and become active on approach to the periastron when the planet is sufficiently warm.<ref name=kane-2012>{{cite journal |title=The Habitable Zone and Extreme Planetary Orbits |author1=Kane, Stephen R. |author2=Gelino, Dawn M. |journal=Astrobiology |date=2012 |volume=12 |pages=940–945 |doi=10.1089/ast.2011.0798 |arxiv=1205.2429 |issue=10 |pmid=23035897|bibcode = 2012AsBio..12..940K |s2cid=10551100 }}</ref>
==Alternative habitable zones== [[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 HZ concept.]] Liquid-water environments have been found to exist in the absence of atmospheric pressure and at temperatures outside the HZ temperature range. For example, Saturn's moons Titan and Enceladus and Jupiter's moons Europa and Ganymede, all of which are outside the habitable zone, may hold large volumes of liquid water in subsurface oceans.<ref>{{cite web |url = http://phl.upr.edu/library/media/liquidwaterinthesolarsystem |title = Liquid Water in the Solar System |access-date = 2013-12-15 |last = Torres |first = Abel |date = 2012-06-12 |archive-date = 2013-11-18 |archive-url = https://web.archive.org/web/20131118034218/http://phl.upr.edu/library/media/liquidwaterinthesolarsystem }}</ref>
Tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water.<ref name="Cowen2008">{{cite news |first=Ron |last=Cowen |title=A Shifty Moon |url=http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon |work=Science News |date=2008-06-07 |access-date=2013-04-22 |archive-date=2011-11-04 |archive-url=https://web.archive.org/web/20111104175610/http://www.sciencenews.org/view/generic/id/32135/title/A_shifty_moon }}</ref><ref name="Bryner, Jeanna">{{cite news|url=http://www.space.com/scienceastronomy/090624-enceladus-ocean.html |title=Ocean Hidden Inside Saturn's Moon |work=Space.com |date=24 June 2009 |author=Bryner, Jeanna |access-date=22 April 2013 |agency=TechMediaNetwork}}</ref> A giant planet creates "tidally-heated habitable zone" with heat that might maintain underground water. For example, Europa is considered be in such a 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 |pmid=11538217 |bibcode = 1987AdSpR...7e.125R }}</ref> Abbot and Switzer (2011) put forward the possibility that subsurface water could exist on rogue planets as a result of radioactive decay-based heating and insulation by a thick surface layer of ice.<ref name=physcisarxivlab-2011>{{cite news |url=http://www.technologyreview.com/view/422659/rogue-planets-could-harbor-life-in-interstellar-space-say-astrobiologists/ |title=Rogue Planets Could Harbor Life in Interstellar Space, Say Astrobiologists |work=MIT Technology Review |date=9 February 2011 |agency=MIT Technology Review |access-date=24 June 2013 |archive-date=7 October 2015 |archive-url=https://web.archive.org/web/20151007173209/http://www.technologyreview.com/view/422659/rogue-planets-could-harbor-life-in-interstellar-space-say-astrobiologists/ }}</ref>
Another possibility is that outside the HZ organisms may use 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, testing of a number of organisms has found some are capable of surviving in extra-HZ conditions.<ref> {{cite journal |last1=Nicholson |first1=W. L. |last2=Moeller |first2=R. |last3=Horneck |first3=G. |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== [[File:Deinococcus_radiodurans.jpg|thumb|Polyextremophile bacteria like Deinococcus radiodurans, are capable of surviving conditions outside the habitable zone]] Conventional habitable zones only require liquid water, a minimum requirement for conventional microbial life. Complex aerobic life is likely to have many additional requirements, including limitations on the concentrations of {{chem|C|O|2}} and {{chem|C|O}} in the atmosphere. This has led to the concept of a habitable zone for complex life.<ref>{{Cite journal |last1=Schwieterman |first1=Edward W. |last2=Reinhard |first2=Christopher T. |last3=Olson |first3=Stephanie L. |last4=Harman |first4=Chester E. |last5=Lyons |first5=Timothy W. |date=June 10, 2019 |title=A Limited Habitable Zone for Complex Life |journal=The Astrophysical Journal |volume=878 |issue=1 |pages=19 |doi=10.3847/1538-4357/ab1d52 |doi-access=free |arxiv=1902.04720 |bibcode=2019ApJ...878...19S |issn=0004-637X}}</ref> The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the HZ is one of many critical factors. The secondary habitability factors required for multicellular life 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 |isbn=978-0-387-95289-5}}</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 |url=https://archive.org/details/oxygenevolutionl00deck |url-access=limited |date=2011 |pages=[https://archive.org/details/oxygenevolutionl00deck/page/n162 157]–168 |doi=10.1007/978-3-642-13179-0_9 |isbn=978-3-642-13178-3}}</ref>{{rp|158}}
Researchers looking for intelligent life elsewhere in the universe begin with planets in the habitable zone.<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 |access-date=April 5, 2011}}</ref> SETI uses habitable zone analysis to select target for their Project Phoenix project.<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 |s2cid=14734094}}</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 |author-link1=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|page=94|doi=10.1088/0004-637X/767/1/94|arxiv = 1302.0845 |bibcode = 2013ApJ...767...94S |s2cid=119302350}}</ref> and the Allen Telescope Array now extends 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 |website = Space.com |access-date = 2013-06-26 }}</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 HZ 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 mediocrity principle, the view that habitability—and therefore life—is common throughout the Universe.<ref name="Rare Earth" />
== See also == {{Portal|Biology|Astronomy|Stars|Spaceflight|Outer space|Solar System|Science}} * Habitability of binary star systems * Habitability of F-type main-sequence star systems * Habitability of K-type main-sequence star systems * Habitability of neutron star systems * Habitability of red dwarf systems * Habitability of yellow dwarf systems * Habitable zone for complex life
==References== {{-}}{{Reflist|refs= <ref name="Kopparapu2013b">{{Cite journal |last1=Kopparapu |first1=Ravi Kumar |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=10 March 2013 |title=Habitable Zones Around Main-Sequence Stars: New Estimates |volume=765 |issue = 2 |journal=The Astrophysical Journal | page=131 |arxiv=1301.6674 |bibcode=2013ApJ...765..131K |doi = 10.1088/0004-637X/765/2/131 |s2cid=76651902 }}</ref>
}}
== Further reading == * {{Cite journal|doi=10.1089/ast.2018.1844|title=Was There an Early Habitability Window for Earth's Moon?|year=2018|last1=Schulze-Makuch|first1=Dirk|last2=Crawford|first2=Ian A.|journal=Astrobiology|volume=18|issue=8|pages=985–988|pmid=30035616|pmc=6225594|bibcode=2018AsBio..18..985S}} * {{cite journal|last1=Salvador|first1=A.|last2=Massol|first2=H.|last3=Davaille|first3=A.|author3-link=Anne Davaille|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|s2cid=135136696 |url=https://hal-insu.archives-ouvertes.fr/insu-01540209/file/2017JE005286.pdf}} Impactors can alter atmospheres and thus habitable zone.
==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 |archive-url=https://web.archive.org/web/20110628175616/http://www.solstation.com/habitable.htm |archive-date=2011-06-28}} * {{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 |s2cid=119441813}} * {{cite web|url=http://btc.montana.edu/ceres/astrobiology/files/HabitableZone.htm|title=Interstellar Real Estate: Location, Location, Location – Defining the Habitable Zone|publisher=University of Montana}} * {{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. |s2cid=32204693 |doi-access=free}} * {{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 |access-date=February 6, 2011 |archive-url=https://web.archive.org/web/20090327205016/http://www.universetoday.com/2009/03/19/jwst-will-provide-capability-to-search-for-biomarkers-on-earth-like-worlds/ |archive-date=March 27, 2009 }} {{div col end}}
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