Habitable zone

File:Maunder Edward Walter.jpg, British astronomer, who introduced the concept of habitable zones]]

An estimate of the range of distances from the Sun allowing the existence of liquid water appears in Newton's Principia (Book III, Section 1, corol. 4).{{cite book |last1=Newton |first1=Isaac |author-link=Isaac Newton |title=Philosophiae Naturalis Principia Mathematica |date=1729 |page=739 |edition=3rd |url=https://www.17centurymaths.com/contents/newton/book3s1.pdf#page=18 |archive-url=https://web.archive.org/web/20231113064651/http://17centurymaths.com/contents/newton/book3s1.pdf#page=18 |archive-date=13 November 2023 |chapter=Book III - Section I - Proposition VIII - Corol. 4 |url-status=live }} The philosopher Louis Claude de Saint-Martin speculated in his 1802 work Man: His True Nature and Ministry, "... we may presume, that, being susceptible of vegetation, it [the Earth] has been placed, in the series of planets, in the rank which was necessary, and at exactly the right distance from the sun, to accomplish its secondary object of vegetation; and from this we might infer that the other planets are either too near or too remote from the sun, to vegetate."{{cite book |last1= de Saint-Martin |first1= Louis Claude |author-link= Louis Claude de Saint-Martin |title= Man: His True Nature and Ministry |date=1802 |page=78}}

Possibly the earliest use of the term habitable zone was in 1913,{{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=978-1108471541}} by Edward Maunder in his book "Are The Planets Inhabited?".{{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 }} 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.{{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}}{{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}} 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.{{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}} Su-Shu Huang, an American astrophysicist argued in 1960 that circumstellar habitable zones, and by extension extraterrestrial life, would be uncommon in multiple star systems, given the gravitational instabilities of those systems.{{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}}{{cite book |url=https://books.google.com/books?id=D0UrAAAAYAAJ |title=Extraterrestrial life: An Anthology and Bibliography |publisher=National Academy of Sciences |author=Huang, Su-Shu |date=1966 |location=Washington, D. C. |pages=87–93 |others=National Research Council (U.S.). Study Group on Biology and the Exploration of Mars|bibcode=1966elab.book.....S }}{{cite journal |title=Life-Supporting Regions in the Vicinity of Binary Systems |author=Huang, Su-Shu |journal=Publications of the Astronomical Society of the Pacific |date=April 1960 |volume=72 |issue=425 |pages=106–114 |bibcode=1960PASP...72..106H |doi=10.1086/127489|doi-access=free }}

The concept of habitable zones was further developed in 1964 by Stephen H. Dole in his book Habitable Planets for Man, in which he discussed the concept of the circumstellar habitable zone as well as various other determinants of planetary habitability, eventually estimating the number of habitable planets in the Milky Way to be about 600  million. At the same time, science-fiction author Isaac Asimov introduced the concept of a circumstellar habitable zone to the general public through his various explorations of space colonization.{{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]}} 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.{{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/ |url-status=dead }} In 1993, astronomer James Kasting introduced the term "circumstellar habitable zone" to refer more precisely to the region then (and still) known as the habitable zone. Kasting was the first to present a detailed model for the habitable zone for exoplanets.{{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 }}

An update to the habitable zone concept came in 2000 when astronomers Peter Ward and Donald Brownlee introduced the idea of the "galactic habitable zone", which they later developed with Guillermo Gonzalez.{{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 }} 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.

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. 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.{{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 |url-status=dead |archive-date=May 2, 2013 |archive-url=https://web.archive.org/web/20130502064349/http://www.astrobio.net/exclusive/5364/the-habitable-edge-of-exomoons }}

It has been noted that the current term of 'circumstellar habitable zone' poses confusion as the name suggests that planets within this region will possess a habitable environment.{{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|pages=0042|doi=10.1038/s41550-017-0042|arxiv=1708.01363|bibcode=2017NatAs...1E..42T|s2cid=118952886}} However, surface conditions are dependent on a host of different individual properties of that planet.{{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."}} This misunderstanding is reflected in excited reports of 'habitable planets'.{{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}}{{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}}[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. Since it is completely unknown whether conditions on these distant HZ worlds could host life, different terminology is needed.{{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}}{{Cite web|url=https://www.numerama.com/sciences/562381-exoplanetes-faut-il-en-finir-avec-la-zone-dhabitabilite.html| title=Exoplanètes: faut-il en finir avec la "zone d'habitabilité"? - Sciences| last=Ruher| first=Hugo| date=2019-10-20| website=Numerama| language=fr-FR|access-date=2019-10-21}}

File:Triple_point_diagram_indicating_planets_within_Solar_System_habitable_zone.png

Whether a body is in the circumstellar habitable zone of its host star is dependent on the radius of the planet's orbit (for natural satellites, the host planet's orbit), the mass of the body itself, and the radiative flux of the host star. Given the large spread in the masses of planets within a circumstellar habitable zone, coupled with the discovery of super-Earth planets that can sustain thicker atmospheres and stronger magnetic fields than Earth, circumstellar habitable zones are now split into two separate regions—a "conservative habitable zone" in which lower-mass planets like Earth can remain habitable, complemented by a larger "extended habitable zone" in which a planet like Venus, with stronger greenhouse effects, can have the right temperature for liquid water to exist at the surface.{{cite journal| title=Runaway and moist greenhouse atmospheres and the evolution of Earth and Venus |journal=Icarus |date=June 1988|author=Kasting, James F.|volume = 74|issue = 3|pages = 472–494| doi=10.1016/0019-1035(88)90116-9|pmid = 11538226|bibcode=1988Icar...74..472K |url = https://zenodo.org/record/1253896}}

{{seealso|Water on terrestrial planets of the Solar System}}

File: Estimated extent of the Solar Systems habitable zone.png of Venus, whereas an extended HZ, extending to the orbit of the dwarf planet Ceres, is indicated by a light-green band.]]

File:Terrestrial planet size comp 2024.pngs with partial or full orbits within the Extended Habitable Zone from left to right: Mercury, Venus, Earth & 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 range from 0.38 to 10.0 astronomical units,{{cite web| url=http://depts.washington.edu/naivpl/sites/default/files/hz.shtml| title=Stellar habitable zone calculator| publisher=University of Washington| access-date=17 December 2015}} though arriving at these estimates has been challenging for a variety of reasons. Numerous planetary mass objects orbit within, or close to, this range and as such receive sufficient sunlight to raise temperatures above the freezing point of water. However, their atmospheric conditions vary substantially.

The aphelion of Venus, for example, touches the inner edge of the zone in most estimates and, while atmospheric pressure at the surface is sufficient for liquid water, a strong greenhouse effect raises surface temperatures to {{convert|462|C|F}} at which water can only exist as vapor.{{cite web|url=http://burro.cwru.edu/stu/advanced/venus.html |title=Venus |publisher=Case Western Reserve University |date=13 September 2006 |access-date=2011-12-21 |archive-url=https://web.archive.org/web/20120426064658/http://burro.cwru.edu/stu/advanced/venus.html |archive-date=2012-04-26 }} The entire orbits of the Moon,{{cite web |url=http://www.space.com/18067-moon-atmosphere.html |title=Atmosphere of the Moon |publisher=TechMediaNetwork |work=Space.com |access-date=April 23, 2013 |author=Sharp, Tim}} Mars,{{Cite book|first=Alexander A.|last=Bolonkin|date=2009|title=Artificial Environments on Mars|publisher=Springer |place=Berlin Heidelberg|pages=599–625|isbn=978-3-642-03629-3}} and numerous asteroids also lie within various estimates of the habitable zone. Only at Mars' lowest elevations (less than 30% of the planet's surface) is atmospheric pressure and temperature sufficient for water to, if present, exist in liquid form for short periods.{{cite journal|last1=Haberle|first1=Robert M.|last2=McKay|first2=Christopher P.|last3=Schaeffer|first3=James|last4=Cabrol|first4=Nathalie A.|last5=Grin|first5=Edmon A.|last6=Zent|first6=Aaron P.|last7=Quinn|first7=Richard|title=On the possibility of liquid water on present-day Mars|journal=Journal of Geophysical Research|volume=106|issue=E10|year=2001|pages=23317|issn=0148-0227|doi=10.1029/2000JE001360|bibcode = 2001JGR...10623317H |doi-access=free}} At Hellas Basin, for example, atmospheric pressures can reach 1,115 Pa and temperatures above zero Celsius (about the triple point for water) for 70 days in the Martian year. Despite indirect evidence in the form of seasonal flows on warm Martian slopes,{{cite magazine |last=Mann |first=Adam |title=Strange Dark Streaks on Mars Get More and More Mysterious|url=https://www.wired.com/wiredscience/2014/02/flowing-lineae-water-mars/ |date=February 18, 2014 |magazine=Wired |access-date=February 18, 2014 }}{{cite web|url=https://www.voanews.com/a/nasa-finds-possible-signs-of-flowing-water-on-mars-126807133/143341.html|title=NASA Finds Possible Signs of Flowing Water on Mars|date=3 August 2011 |publisher=Voice of America|access-date=August 5, 2011|url-status=live|archive-url=https://web.archive.org/web/20110917071451/http://www.voanews.com/english/news/science-technology/NASA-Finds-Possible-Signs-of-Flowing-Water-on-Mars-126807133.html|archive-date=September 17, 2011}}{{cite web|url=http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html|title=Is Mars Weeping Salty Tears?|publisher=news.sciencemag.org|access-date=August 5, 2011|archive-url=https://web.archive.org/web/20110814065220/http://news.sciencemag.org/sciencenow/2011/08/is-mars-weeping-salty-tears.html|archive-date=August 14, 2011}}{{cite web |last1=Webster |first1=Guy |last2=Brown |first2=Dwayne |title=NASA Mars Spacecraft Reveals a More Dynamic Red Planet |url=http://www.jpl.nasa.gov/news/news.php?release=2013-361&1#1 |date=December 10, 2013 |work=NASA |access-date=December 10, 2013 }} no confirmation has been made of the presence of liquid water at the surface. While other objects orbit partly within this zone, including comets, Ceres{{cite journal|last=A'Hearn|first=Michael F.|author2=Feldman, Paul D.|title=Water vaporization on Ceres|journal=Icarus|volume=98|issue=1|pages=54–60|date=1992|doi=10.1016/0019-1035(92)90206-M|bibcode= 1992Icar...98...54A}} is the only one of planetary mass.

Despite this, studies indicate the strong possibility of past liquid water on the surface of Venus,{{cite journal|last1=Salvador|first1=A.|last2=Massol|first2=H.|last3=Davaille|first3=A.|last4=Marcq|first4=E.|last5=Sarda|first5=P.|last6=Chassefière|first6=E.|title=The relative influence of H2 O and CO2 on the primitive surface conditions and evolution of rocky planets|journal=Journal of Geophysical Research: Planets|volume=122|issue=7|year=2017|pages=1458–1486|issn=2169-9097|doi=10.1002/2017JE005286|bibcode=2017JGRE..122.1458S|s2cid=135136696 |url=https://hal-insu.archives-ouvertes.fr/insu-01540209/file/2017JE005286.pdf}} the Moon,{{cite web |url=https://news.wsu.edu/2018/07/23/possibility-of-moon-life/ |title=Mysteries from the moon's past |date=23 July 2018 |publisher=Washington State University |access-date=22 August 2020}}{{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}} Mars,{{cite web |url=http://www.space.com/scienceastronomy/flashback-water-on-mars-announced-10-years-ago-100622.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2000| access-date=December 19, 2010}}{{cite web|url=https://www.space.com/8642-flashback-water-mars-announced-10-years.html |title=Flashback: Water on Mars Announced 10 Years Ago| publisher=SPACE.com| date=June 22, 2010| access-date=May 13, 2018}}{{cite web| url=https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| title=Science@NASA, The Case of the Missing Mars Water| access-date=March 7, 2009| archive-url=https://web.archive.org/web/20090327234049/https://science.nasa.gov/headlines/y2001/ast05jan_1.htm| archive-date=March 27, 2009}} Vesta{{cite journal|last1=Scully|first1=Jennifer E.C.|last2=Russell|first2=Christopher T.|last3=Yin|first3=An|last4=Jaumann|first4=Ralf|last5=Carey|first5=Elizabeth|last6=Castillo-Rogez|first6=Julie|last7=McSween|first7=Harry Y.|last8=Raymond|first8=Carol A.|last9=Reddy|first9=Vishnu|last10=Le Corre|first10=Lucille|title=Geomorphological evidence for transient water flow on Vesta|journal=Earth and Planetary Science Letters|volume=411|year=2015|pages=151–163|issn=0012-821X|doi=10.1016/j.epsl.2014.12.004|bibcode=2015E&PSL.411..151S}} and Ceres,{{cite journal|last1=Raponi|first1=Andrea|last2=De Sanctis|first2=Maria Cristina|last3=Frigeri|first3=Alessandro|last4=Ammannito|first4=Eleonora|last5=Ciarniello|first5=Mauro|last6=Formisano|first6=Michelangelo|last7=Combe|first7=Jean-Philippe|last8=Magni|first8=Gianfranco|last9=Tosi|first9=Federico|last10=Carrozzo|first10=Filippo Giacomo|last11=Fonte|first11=Sergio|last12=Giardino|first12=Marco|last13=Joy|first13=Steven P.|last14=Polanskey|first14=Carol A.|last15=Rayman|first15=Marc D.|last16=Capaccioni|first16=Fabrizio|last17=Capria|first17=Maria Teresa|last18=Longobardo|first18=Andrea|last19=Palomba|first19=Ernesto|last20=Zambon|first20=Francesca|last21=Raymond|first21=Carol A.|last22=Russell|first22=Christopher T.|title=Variations in the amount of water ice on Ceres' surface suggest a seasonal water cycle|journal=Science Advances|volume=4|issue=3|year=2018|pages=eaao3757|issn=2375-2548|doi=10.1126/sciadv.aao3757|pmid=29546238|pmc=5851659|bibcode=2018SciA....4.3757R }}[https://photojournal.jpl.nasa.gov/catalog/PIA21471 NASA.gov] PIA21471: Landslides on Ceres suggesting a more common phenomenon than previously thought. Since sustainable liquid water is thought to be essential to support complex life, most estimates, therefore, are inferred from the effect that a repositioned orbit would have on the habitability of Earth or Venus as their surface gravity allows sufficient atmosphere to be retained for several billion years.

According to the extended habitable zone concept, planetary-mass objects with atmospheres capable of inducing sufficient radiative forcing could possess liquid water farther out from the Sun. Such objects could include those whose atmospheres contain a high component of greenhouse gas and terrestrial planets much more massive than Earth (super-Earth class planets), that have retained atmospheres with surface pressures of up to 100 kbar. There are no examples of such objects in the Solar System to study; not enough is known about the nature of atmospheres of these kinds of extrasolar objects, and their position in the habitable zone cannot determine the net temperature effect of such atmospheres including induced albedo, anti-greenhouse or other possible heat sources.

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.

{{see also|Habitability of red dwarf systems|Habitability of K-type main-sequence star systems}}

File:Orbit of 82 G. Eridani d.png 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. For example, according to Kopparapu's habitable zone estimate, although the Solar System has a circumstellar habitable zone centered at 1.34 AU from the Sun, a star with 0.25 times the luminosity of the Sun would have a habitable zone centered at $\backslash sqrt\{0.25\}$, or 0.5, the distance from the star, corresponding to a distance of 0.67 AU. Various complicating factors, though, including the individual characteristics of stars themselves, mean that extrasolar extrapolation of the HZ concept is more complex.

File:Circling Two Suns.ogv

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.{{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}} If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU.{{cite journal|doi=10.1126/science.278.5341.1273| title=Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation| date=1997| last1=Forget| first1=F.| journal=Science| volume=278|issue=5341|pages=1273–6| pmid=9360920| last2=Pierrehumbert|first2=RT|bibcode = 1997Sci...278.1273F| citeseerx=10.1.1.41.621}}{{cite journal| doi=10.1006/icar.2000.6380| title=Influence of Carbon Dioxide Clouds on Early Martian Climate| date=2000 |last1=Mischna| first1=M| journal=Icarus| volume=145 |issue=2| pages=546–54| pmid=11543507| last2=Kasting| first2=JF| last3=Pavlov| first3=A|last4=Freedman| first4=R| bibcode = 2000Icar..145..546M }}

With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions.{{cite press release| url=http://www.spitzer.caltech.edu/news/863-feature06-31-Planets-Prefer-Safe-Neighborhoods |title=Planets Prefer Safe Neighborhoods |publisher=Spitzer.caltech.edu |access-date=April 22, 2013 |author=Vu, Linda |agency=NASA/Caltech}} Studying ultraviolet emissions, Andrea Buccino found that only 40% of stars studied (including the Sun) had overlapping liquid water and ultraviolet habitable zones.{{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}} 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.{{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 }}

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. Astronomy professor Eric Agol argues that even white dwarfs may support a relatively brief habitable zone through planetary migration.{{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 }} At the same time, others have written in similar support of semi-stable, temporary habitable zones around brown dwarfs. 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.{{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 }}

File:Magnetosphere rendition.jpg, 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,{{cite book |last1=Carroll |first1=Bradley W. |last2=Ostlie |first2=Dale A. |edition=2nd |date=2007 |title=An Introduction to Modern Astrophysics}} 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.{{cite web

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|title=Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere

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}} 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.{{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}} 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.

In red dwarf systems, gigantic stellar flares which could double a star's brightness in minutes{{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] and huge starspots which can cover 20% of the star's surface area,{{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}} have the potential to strip an otherwise habitable planet of its atmosphere and water. As with more massive stars, though, stellar evolution changes their nature and energy flux,{{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=}} so by about 1.2  billion years of age, red dwarfs generally become sufficiently constant to allow for the development of life.{{cite journal| last=Alpert |first=Mark |title=Red Star Rising |journal=Scientific American |volume=293 |issue=5 |pages=28 |date=November 7, 2005 |pmid=16318021 |doi=10.1038/scientificamerican1105-28 |bibcode=2005SciAm.293e..28A }}{{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 }}

Once a star has evolved sufficiently to become a red giant, its circumstellar habitable zone will change dramatically from its main-sequence size.{{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|date=27 July 2009|access-date=8 April 2016|url-status=dead|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/}} For example, the Sun is expected to engulf the previously habitable Earth as a red giant.{{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}} 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.{{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 }} At such stage, Saturn's moon Titan would likely be habitable in Earth's temperature sense.{{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 }} 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. 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.{{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}} Moreover, as Ramirez and Kaltenegger (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 |pages=6 |journal=The Astrophysical Journal|bibcode=2016ApJ...823....6R|s2cid=119225201 |doi-access=free }} 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. 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.

File:Tharsis and Valles Marineris - Mars Orbiter Mission (30055660701).png

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.[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{{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 }}

Image:BlueMarble-2001-2002.jpg accounting for 97.3% of the water distribution on Earth.]]

{{see also|Planetary habitability|Habitability of natural satellites}}

A planet cannot have a hydrosphere—a key ingredient for the formation of carbon-based life—unless there is a source for water within its stellar system. The origin of water on Earth is still not completely understood; possible sources include the result of impacts with icy bodies, outgassing, mineralization, leakage from hydrous minerals from the lithosphere, and photolysis.{{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 }}{{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 }} 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{{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}} such as GJ 1214 b{{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

}}{{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 }} or Kepler-22b may be.{{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}}

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.{{cite journal|last1=Robinson|first1=Tyler D.|last2=Catling|first2=David C.| title=An Analytic Radiative-Convective Model for Planetary Atmospheres| journal=The Astrophysical Journal| volume=757|issue=1|date=2012|pages=104|doi=10.1088/0004-637X/757/1/104|arxiv = 1209.1833 |bibcode = 2012ApJ...757..104R |s2cid=54997095}} Atmospheres are thought to be maintained through similar processes along with biogeochemical cycles and the mitigation of atmospheric escape.{{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 }} 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. Below an atmospheric pressure of about 15 millibars, it was found that habitability could not be maintained because even a small shift in pressure or temperature could render water unable to form as a liquid.{{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}}

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), a study 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{{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 }} had eliminated the CO₂-H₂O concept entirely, arguing that young planets could accrete many tens to hundreds of bars of hydrogen from the protoplanetary disc, providing enough of a greenhouse effect to extend the solar system outer edge to 10 AU. In this case, though, the hydrogen is not continuously replenished by volcanism and is lost within millions to tens of millions of years.

In the case of planets orbiting in the HZs of red dwarf stars, the extremely close distances to the stars cause tidal locking, an important factor in habitability. For a tidally locked planet, the sidereal day is as long as the orbital period, causing one side to permanently face the host star and the other side to face away. In the past, such tidal locking was thought to cause extreme heat on the star-facing side and bitter cold on the opposite side, making many red dwarf planets uninhabitable; however, three-dimensional climate models in 2013 showed that the side of a red dwarf planet facing the host star could have extensive cloud cover, increasing its bond albedo and reducing significantly temperature differences between the two sides.{{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 }}

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. 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, but must remain within the Hill radius of the planet so that they are not pulled out of the orbit of their host planet.{{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 }} 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.

File:Eccentric Habitable Zones.jpg

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}}{{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}} and {{convert|151|C|K|order=flip}}.{{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}} 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.{{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 }}

{{see also|List of potentially habitable exoplanets}}

A 2015 review concluded that the exoplanets Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable.{{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}} 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}}.{{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}}

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_e, the fraction of stars with planets in the HZ, at 0.48, meaning that there may be roughly 95–180 billion habitable planets in the Milky Way.{{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}} However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.{{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 |url-status=dead }}

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500 million habitable planets in the Milky Way.{{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 }} 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.{{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}}{{Cite journal | last1 = Catanzarite | first1 = J. | last2 = Shao | first2 = M. | doi = 10.1088/0004-637X/738/2/151 | title = The Occurrence Rate of Earth Analog Planets Orbiting Sun-Like Stars | journal = The Astrophysical Journal | volume = 738 | issue = 2 | pages = 151 | year = 2011 |arxiv = 1103.1443 |bibcode = 2011ApJ...738..151C | s2cid = 119290692 }}

{{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.{{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 }}

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.{{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}} 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.{{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 }}

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.{{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 }} 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{{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 }} and has a low orbital eccentricity, comparable to that of Mars in the Solar System.{{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 }} Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years,{{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 }} though it is unclear whether such satellites could form in the first place.{{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 }}

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.{{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 }} The following year, 55 Cancri f was discovered within the HZ of its host star 55 Cancri A.{{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}}{{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 }} Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.{{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}}

Though, in theory, such giant planets could possess moons, the technology did not exist to detect moons around them, and no extrasolar moons had been discovered. Planets within the zone with the potential for solid surfaces were therefore of much higher interest.

{{Category see also|Super-Earths in the habitable zone}}

File:Gliese 581 - 2010.jpg

The 2007 discovery of Gliese 581c, the first super-Earth in the circumstellar habitable zone, created significant interest in the system by the scientific community, although the planet was later found to have extreme surface conditions that may resemble Venus.{{cite news |url=http://www.space.com/scienceastronomy/070424_exoplanet_side.html |title=Planet Hunters Edge Closer to Their Holy Grail |last=Than |first=Ker |date=2007-02-24 |publisher=space.com |access-date=2007-04-29}} Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, but only for a short time. As of 2015, the planet has no newer disconfirmations. Gliese 581 g, yet another planet thought to have been discovered in the circumstellar habitable zone of the system, was considered to be more habitable than both Gliese 581 c and d. However, its existence was also disconfirmed in 2014,{{cite journal|last1=Robertson |first1=Paul |author2-link=Suvrath Mahadevan |last2=Mahadevan |first2=Suvrath |last3=Endl |first3=Michael |last4=Roy |first4=Arpita |title=Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581 |journal=Science |date=3 July 2014 |doi=10.1126/science.1253253 |pmid=24993348 |arxiv=1407.1049 |bibcode=2014Sci...345..440R |volume=345 |issue=6195 |pages=440–444|citeseerx=10.1.1.767.2071 |s2cid=206556796 }} and astronomers are divided about its existence.

File:Kepler-22 diagram.jpg

Discovered in August 2011, HD 85512 b was initially speculated to be habitable,{{cite web| url=http://www.maxisciences.com/plan%E8te-habitable/des-chercheurs-decouvrent-une-planete-potentiellement-habitable_art16635.html| title=Researchers find potentially habitable planet| publisher=maxisciences.com| language=fr| access-date=2011-08-31| date=2011-08-30| archive-date=2019-04-13| archive-url=https://web.archive.org/web/20190413203316/https://www.maxisciences.com/planete-habitable/des-chercheurs-decouvrent-une-planete-potentiellement-habitable_art16635.html| url-status=dead}} but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.

Kepler-22 b, discovered in December 2011 by the Kepler space probe,{{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}} 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.{{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". Gliese 667 Cc, discovered in 2011 but announced in 2012,{{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 }} 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{{cite web |author= |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 }} 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.{{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 |url-status=dead }}{{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}}{{cite web| url=http://www.spacedaily.com/reports/A_Hot_Potential_Habitable_Exoplanet_around_Gliese_163_999.html |title=A Hot Potential Habitable Exoplanet around Gliese 163 |publisher=Spacedaily.com |access-date=2013-02-10}} HD 40307 g, a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of HD 40307.{{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 }} 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.{{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}} Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone;{{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 }} however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study.{{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}} It is now considered as uninhabitable.

File:Kepler186f-ComparisonGraphic-20140417 improved.jpg (17 April 2014)]]

File:Kepler-452b System.jpg

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.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]" 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.{{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}} 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.{{cite web|url=http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html|title=Stellar Luminosity Calculator|publisher=University of Nebraska-Lincoln astronomy education group}}{{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}}{{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}}

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.{{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 }}

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.{{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 }}{{cite journal|doi=10.1088/0004-637X/768/2/101| title=A Super-Earth-Sized Planet Orbiting in or Near the Habitable Zone Around a Sun-Like Star| date=2013|last1=Barclay|first1=Thomas|last2=Burke|first2=Christopher J.|last3=Howell|first3=Steve B.|last4=Rowe|first4=Jason F.|last5=Huber|first5=Daniel|last6=Isaacson|first6=Howard|last7=Jenkins|first7=Jon M.|last8=Kolbl|first8=Rea|last9=Marcy|first9=Geoffrey W. |journal=The Astrophysical Journal| volume=768|issue=2|pages=101|arxiv = 1304.4941 |bibcode = 2013ApJ...768..101B | s2cid=51490784}}{{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 }} 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.{{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 |url-status=dead }}{{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 }}

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{{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 }}{{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 }}{{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 }} 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.{{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 }} However, further analysis concluded that this claim was an artefact of stellar rotation and activity.{{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 | pages=230

| arxiv=2103.02709 | doi=10.3847/1538-3881/abec89

| bibcode=2021AJ....161..230B | s2cid=232110395 | doi-access=free }}

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. 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.{{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}}

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.{{cite journal| last1=Jenkins|first1=Jon M.|last2=Twicken|first2=Joseph D.|last3=Batalha|first3=Natalie M.|last4=Caldwell|first4=Douglas A.|last5=Cochran|first5=William D.|last6=Endl|first6=Michael|last7=Latham|first7=David W.|last8=Esquerdo|first8=Gilbert A.|last9=Seader|first9=Shawn|last10=Bieryla|first10=Allyson|last11=Petigura|first11=Erik|last12=Ciardi|first12=David R.|last13=Marcy|first13=Geoffrey W.|last14=Isaacson|first14=Howard|last15=Huber|first15=Daniel|last16=Rowe|first16=Jason F.|last17=Torres|first17=Guillermo|last18=Bryson|first18=Stephen T.|last19=Buchhave|first19=Lars|last20=Ramirez|first20=Ivan|last21=Wolfgang|first21=Angie|last22=Li|first22=Jie|last23=Campbell|first23=Jennifer R.|last24=Tenenbaum|first24=Peter|last25=Sanderfer|first25=Dwight|last26=Henze|first26=Christopher E.|last27=Catanzarite|first27=Joseph H.|last28=Gilliland|first28=Ronald L.|last29=Borucki|first29=William J.| title=Discovery and Validation of Kepler-452b: A 1.6 R_🜨 Super Earth Exoplanet in the Habitable Zone of a G2 Star| journal=The Astronomical Journal| date=23 July 2015| volume=150|issue=2|page=56|issn=1538-3881|doi=10.1088/0004-6256/150/2/56|arxiv = 1507.06723 |bibcode = 2015AJ....150...56J |s2cid=26447864}}{{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}}

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.{{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}} 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.{{cite journal|last1=Dressing|first1=Courtney D.|last2=Vanderburg|first2=Andrew|last3=Schlieder|first3=Joshua E.|last4=Crossfield|first4=Ian J. M.|last5=Knutson|first5=Heather A.|last6=Newton|first6=Elisabeth R.|last7=Ciardi|first7=David R.|last8=Fulton|first8=Benjamin J.|last9=Gonzales|first9=Erica J.|last10=Howard|first10=Andrew W.|last11=Isaacson|first11=Howard|last12=Livingston|first12=John|last13=Petigura|first13=Erik A.|last14=Sinukoff|first14=Evan|last15=Everett|first15=Mark|last16=Horch|first16=Elliott|last17=Howell|first17=Steve B.|title=Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7|journal=The Astronomical Journal|volume=154|issue=5|year=2017|pages=207|issn=1538-3881|doi=10.3847/1538-3881/aa89f2|arxiv=1703.07416|bibcode=2017AJ....154..207D|s2cid=13419148|url=https://authors.library.caltech.edu/78341/2/Dressing_2017_AJ_154_207.pdf |doi-access=free }}

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.{{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}} 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.{{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}}

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.{{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}}

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.{{Cite web|title=K2-155 d|url=https://exoplanets.nasa.gov/newworldsatlas/6173/|publisher=Exoplanet Exploration|year=2018}}{{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}}{{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}}

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.{{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}}

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.{{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 }}

Image:Liquid lakes on titan.jpg 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.{{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 |url-status = dead }}

Outside the HZ, tidal heating and radioactive decay are two possible heat sources that could contribute to the existence of liquid water. 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.

With some theorising that life on Earth may have actually originated in stable, subsurface habitats,{{Citation

|title = Miners deep underground in northern Ontario find the oldest water ever known

|url = https://nationalpost.com/news/canada/worlds-oldest-water-bubbling-into-northern-ontario-mine

|date = 2013

|author = Munro, Margaret

|work = National Post

|access-date = 2013-10-06

}}{{cite journal

|title = The Origin of Life II: How did it begin?

|journal = Science Progress

|issn = 2047-7163

|volume = 84

|issue = 1

|pages = 17–29

|date = February 2001

|doi = 10.3184/003685001783239096

|doi-access = free

|pmid = 11382135

|pmc = 10367499

|last = Davies

|first = Paul

}} it has been suggested that it may be common for wet subsurface extraterrestrial habitats such as these to 'teem with life'.{{Citation

|title = Life Underground

|url = http://www.psrd.hawaii.edu/Dec96/PSRD-LifeUnderground.pdf

|date = 1996

|author = Taylor, Geoffrey

|journal = Planetary Science Research Discoveries

|page = 4

|bibcode = 1996psrd.reptE...4T

|access-date = 2013-10-06

}} On Earth itself, living organisms may be found more than {{convert|6|km|mi|abbr=on}} below the surface.{{Citation

|title = Deep underground, worms and "zombie microbes" rule

|url = https://www.reuters.com/article/us-life-idUSBRE9230WM20130304

|author = Doyle, Alister

|access-date = 2013-10-06

|work=Reuters

|date=4 March 2013

}}

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. 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. In addition, testing of a number of organisms has found some are capable of surviving in extra-HZ conditions.

{{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

}}

File:Deinococcus_radiodurans.jpg bacteria like Deinococcus radiodurans, are capable of surviving conditions outside the habitable zone]]

The Rare Earth hypothesis argues that complex and intelligent life is uncommon and that the HZ is one of many critical factors. According to Ward & Brownlee (2004) and others, not only is a HZ orbit and surface water a primary requirement to sustain life but a requirement to support the secondary conditions required for multicellular life to emerge and evolve. The secondary habitability factors are both geological (the role of surface water in sustaining necessary plate tectonics){{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}} and biochemical (the role of radiant energy in supporting photosynthesis for necessary atmospheric oxygenation).{{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}} But others, such as Ian Stewart and Jack Cohen in their 2002 book Evolving the Alien argue that complex intelligent life may arise outside the HZ.{{cite book |title=Evolving the Alien |publisher=Ebury Press |author1=Stewart, Ian |author2=Cohen, Jack |date=2002 |isbn=978-0-09-187927-3}} Intelligent life outside the HZ may have evolved in subsurface environments, from alternative biochemistries or even from nuclear reactions.

{{cite book

|last1 = Goldsmith

|first1 = Donald

|last2 = Owen

|first2 = Tobias

|title = The Search for Life in the Universe

|publisher = Addison-Wesley

|edition = 2

|date = 1992

|page = 247

|isbn = 978-0-201-56949-0

}}

[[File:SEM_image_of_Milnesium_tardigradum_in_active_state_-_journal.pone.0045682.g001-2_(white_background).png|

thumb|Milnesium tardigradum, one of the few animals on Earth capable of surviving outside the habitable zone]]

On Earth, several complex multicellular life forms (or eukaryotes) have been identified with the potential to survive conditions that might exist outside the conservative habitable zone. Geothermal energy sustains ancient circumvent ecosystems, supporting large complex life forms such as Riftia pachyptila.{{cite book|author=Vaclav Smil|title=The Earth's Biosphere: Evolution, Dynamics, and Change|url=https://books.google.com/books?id=8ntHWPMUgpMC|year=2003|publisher=MIT Press|isbn=978-0-262-69298-4|page=166}} Similar environments may be found in oceans pressurised beneath solid crusts, such as those of Europa and Enceladus, outside of the habitable zone.{{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 }} Numerous microorganisms have been tested in simulated conditions and in low Earth orbit, including eukaryotes. An animal example is the Milnesium tardigradum, which can withstand extreme temperatures well above the boiling point of water and the cold vacuum of outer space.{{cite journal|author1=Guidetti, R. |author2=Jönsson, K.I.|date=2002|title=Long-term anhydrobiotic survival in semi-terrestrial micrometazoans|journal=Journal of Zoology|volume=257|pages=181–187|doi=10.1017/S095283690200078X|issue=2|citeseerx=10.1.1.630.9839}} A desert moss, Syntrichia caninervis is one of few plants believed capable of surviving on Mars.{{cite journal|display-authors=3|last1=Li|first1=X.|last2=Bai|first2=W.|last3=Yang|first3=Q.|last4=Yin|first4=B.|last5=Zhang|first5=Z.|last6=Zhao|first6=B.|last7=Kuang|first7=T.|last8=Zhang|first8=Y.|last9=Zhang|first9=D.|date=2024|title=The extremotolerant desert moss Syntrichia caninervis is a promising pioneer plant for colonizing extraterrestrial environments|journal=The Innovation|volume=5|issue=4|pages=1–9|doi=10.1016/j.xinn.2024.100657|doi-access=free|pmid=39071942 |pmc=11282406|bibcode=2024Innov...500657L }}

• {{cite news|last=Davis|first=Nicola|url=https://www.theguardian.com/science/article/2024/jun/30/scientists-find-desert-moss-that-can-survive-on-mars|title=Scientists find desert moss 'that can survive on Mars'|date=30 June 2024|work=The Guardian|access-date=2024-07-07|archive-date=2024-07-08|archive-url=https://web.archive.org/web/20240708154553/https://www.theguardian.com/science/article/2024/jun/30/scientists-find-desert-moss-that-can-survive-on-mars|url-status=live}} In addition, the lichens Rhizocarpon geographicum and Rusavskia elegans have been found to survive in an environment where the atmospheric pressure is far too low for surface liquid water and where the radiant energy is also much lower than that which most plants require to photosynthesize.{{cite web |last=Baldwin |first=Emily |title=Lichen survives harsh Mars environment |url=http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html |date=26 April 2012 |publisher=Skymania News |access-date=27 April 2012 |archive-url=https://web.archive.org/web/20120528145425/http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html/ |archive-date=28 May 2012 }}{{cite web |last1=de Vera |first1=J.-P. |last2=Kohler |first2=Ulrich |title=The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars |url=http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |date=26 April 2012 |publisher=European Geosciences Union |access-date=27 April 2012 |archive-url=https://web.archive.org/web/20120504224706/http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |archive-date=4 May 2012 }}{{cite journal|last1=Onofri|first1=Silvano|last2=de Vera|first2=Jean-Pierre|last3=Zucconi|first3=Laura|last4=Selbmann|first4=Laura|last5=Scalzi|first5=Giuliano|last6=Venkateswaran|first6=Kasthuri J.|last7=Rabbow|first7=Elke|last8=de la Torre|first8=Rosa|last9=Horneck|first9=Gerda|title=Survival of Antarctic Cryptoendolithic Fungi in Simulated Martian Conditions On Board the International Space Station|journal=Astrobiology|volume=15|issue=12|year=2015|pages=1052–1059|issn=1531-1074|doi=10.1089/ast.2015.1324|bibcode = 2015AsBio..15.1052O|pmid=26684504}} The fungi Cryomyces antarcticus and Cryomyces minteri are also able to survive and reproduce in Mars-like conditions.

Species, including humans, known to possess animal cognition require large amounts of energy,{{cite journal|last1=Isler|first1=K.|last2=van Schaik|first2=C. P|title=Metabolic costs of brain size evolution|journal=Biology Letters|volume=2|issue=4|year=2006|pages=557–560|issn=1744-9561|doi=10.1098/rsbl.2006.0538|pmid=17148287|pmc=1834002}} and have adapted to specific conditions, including an abundance of atmospheric oxygen and the availability of large quantities of chemical energy synthesized from radiant energy. If humans are to colonize other planets, true Earth analogs in the HZ are most likely to provide the closest natural habitat; this concept was the basis of Stephen H. Dole's 1964 study. With suitable temperature, gravity, atmospheric pressure and the presence of water, the necessity of spacesuits or space habitat analogs on the surface may be eliminated, and complex Earth life can thrive.

Planets in the HZ remain of paramount interest to researchers looking for intelligent life elsewhere in the universe.{{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}} The Drake equation, sometimes used to estimate the number of intelligent civilizations in our galaxy, contains the factor or parameter {{mvar|n_e}}, 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. A 1971 NASA report by Drake and Bernard Oliver proposed the "water hole", based on the spectral absorption lines of the hydrogen and hydroxyl components of water, as a good, obvious band for communication with extraterrestrial intelligence{{cite web |title=Project Cyclops: A design study of a system for detecting extraterrestrial intelligent life |publisher=NASA |date=1971 |access-date=June 28, 2009 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730010095_1973010095.pdf}}{{cite book|author=Joseph A. Angelo|title=Life in the Universe|url=https://books.google.com/books?id=I5gHntgLLvIC&pg=PA163|access-date=26 June 2013|date=2007|publisher=Infobase Publishing|isbn=978-1-4381-0892-6|page=163}} that has since been widely adopted by astronomers involved in the search for extraterrestrial intelligence. According to Jill Tarter, Margaret Turnbull and many others, HZ candidates are the priority targets to narrow waterhole searches{{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}}{{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|pages=94|doi=10.1088/0004-637X/767/1/94|arxiv = 1302.0845 |bibcode = 2013ApJ...767...94S |s2cid=119302350}} and the Allen Telescope Array now extends Project Phoenix to such candidates.{{Cite web

|title = HabStars: Speeding Up In the Zone

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|date = 2011

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}}

Because the HZ is considered the most likely habitat for intelligent life, METI efforts have also been focused on systems likely to have planets there. The 2001 Teen Age Message and 2003 Cosmic Call 2, for example, were sent to the 47 Ursae Majoris system, known to contain three Jupiter-mass planets and possibly with a terrestrial planet in the HZ.{{cite conference |url=http://www.cplire.ru/rus/ra%26sr/VAK-2004.html |title=Transmission and reasonable signal searches in the Universe |script-title=ru:Передача и поиски разумных сигналов во Вселенной |access-date=2013-06-30 |author=Zaitsev, A. L. |book-title=Horizons of the Universe |date=June 2004 |conference=Plenary presentation at the National Astronomical Conference WAC-2004 "Horizons of the Universe", Moscow, Moscow State University, June 7, 2004 |location=Moscow |language=ru |archive-date=2019-05-30 |archive-url=https://web.archive.org/web/20190530145209/https://www.plover.com/misc/Dumas-Dutil/messages.pdf |url-status=dead }}{{Cite magazine |author=David Grinspoon |date=July 13, 2012 |orig-date=December 12, 2007 |title=Who Speaks for Earth? |magazine=Seed |url=http://seedmagazine.com/content/article/who_speaks_for_earth/ |archive-url=https://web.archive.org/web/20120713064523/http://seedmagazine.com/content/article/who_speaks_for_earth/ |url-status=dead |archive-date=2012-07-13|access-date=2021-06-24}}

{{cite journal

|author1=P. C. Gregory |author2=D. A. Fischer |date=2010

|title=A Bayesian periodogram finds evidence for three planets in 47 Ursae Majoris

|journal=Monthly Notices of the Royal Astronomical Society

|volume=403 |issue=2 |pages=731–747

|doi=10.1111/j.1365-2966.2009.16233.x

|doi-access=free |bibcode=2010MNRAS.403..731G

|arxiv = 1003.5549 |s2cid=16722873 }}

{{cite journal

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|bibcode=2005ApJ...622.1091J

|doi=10.1086/428108

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|last3=Sleep

|first3=P. Nick |s2cid=119089227

}} The Teen Age Message was also directed to the 55 Cancri system, which has a gas giant in its HZ. A Message from Earth in 2008,{{cite news|url = https://www.telegraph.co.uk/news/newstopics/howaboutthat/3166709/Messages-from-Earth-sent-to-distant-planet-by-Bebo.html|title = Messages from Earth sent to distant planet by Bebo |last = Moore |first = Matthew|date = October 9, 2008|publisher = .telegraph.co.uk|access-date = 2008-10-09| archive-url = https://web.archive.org/web/20081011142445/http://www.telegraph.co.uk/news/newstopics/howaboutthat/3166709/Messages-from-Earth-sent-to-distant-planet-by-Bebo.html|archive-date = 11 October 2008 | url-status = live |location=London}} and Hello From Earth in 2009, were directed to the Gliese 581 system, containing three planets in the HZ—Gliese 581 c, d, and the unconfirmed g.

{{Clear}}

{{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

{{Reflist|refs=

{{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 | pages=131 |arxiv=1301.6674 |bibcode=2013ApJ...765..131K |doi = 10.1088/0004-637X/765/2/131 |s2cid=76651902 }}

}}

{{Wiktionary|habitable zone}}

{{Commons category|Habitable zone}}

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• {{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 }}

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