Editing Habitable zone (section)

==Extrasolar discoveries==
{{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.<ref name=centauridreams>{{cite web|url=http://www.centauri-dreams.org/?p=32470|title=A Review of the Best Habitable Planet Candidates|author1=Paul Gilster |author2=Andrew LePage |date=2015-01-30|publisher=Centauri Dreams, Tau Zero Foundation|access-date=2015-07-24}}</ref> These are at a distance of 990, 490 and 1,120 [[light-years]] away, respectively. Of these, Kepler-186f is closest in size to Earth with 1.2 times Earth's radius, and it is located towards the outer edge of the habitable zone around its [[red dwarf]] star. Among [[List of nearest terrestrial exoplanet candidates|nearest terrestrial exoplanet candidates]], [[Tau Ceti e]] is 11.9 light-years away. It is in the inner edge of its planetary system's habitable zone, giving it an estimated average surface temperature of {{convert|68|C}}.<ref>{{cite book| title=The Mystery of the Seven Spheres: How Homo sapiens will Conquer Space| author=Giovanni F. Bignami| publisher=Springer| year=2015| isbn=978-3-319-17004-6|url=https://books.google.com/books?id=crvpCQAAQBAJ&pg=PA110|page=110}}</ref>

Studies that have attempted to estimate the number of terrestrial planets within the circumstellar habitable zone tend to reflect the availability of scientific data. A 2013 study by Ravi Kumar Kopparapu put ''η<sub>e</sub>'', the fraction of stars with planets in the HZ, at 0.48,<ref name="kopparapu-2013" /> meaning that there may be roughly 95–180&nbsp;billion habitable planets in the Milky Way.<ref name=wethington-2008>{{cite news |url=http://www.universetoday.com/22380/how-many-stars-are-in-the-milky-way/ |title=How Many Stars are in the Milky Way? |work=Universe Today |date=September 16, 2008 |access-date=April 21, 2013 |author=Wethington, Nicholos}}</ref> However, this is merely a statistical prediction; only a small fraction of these possible planets have yet been discovered.<ref name=torres-2013-2>{{cite web |url=http://phl.upr.edu/press-releases/tenpotentiallyhabitableexoplanetsnow |title=Ten potentially habitable exoplanets now |publisher=University of Puerto Rico |work=Habitable Exoplanets Catalog |date=April 26, 2013 |access-date=April 29, 2013 |author=Torres, Abel Mendez |archive-date=October 21, 2019 |archive-url=https://web.archive.org/web/20191021202042/http://phl.upr.edu/press-releases/tenpotentiallyhabitableexoplanetsnow |url-status=dead }}</ref>

Previous studies have been more conservative. In 2011, Seth Borenstein concluded that there are roughly 500&nbsp;million habitable planets in the Milky Way.<ref name="BorensteinS">{{cite news |last1=Borenstein |first1=Seth |title=Cosmic census finds crowd of planets in our galaxy |agency=Associated Press |date=19 February 2011 |url=http://apnews.excite.com/article/20110219/D9LG45NO0.html |access-date=24 April 2011 |archive-url=https://web.archive.org/web/20110927053134/http://apnews.excite.com/article/20110219/D9LG45NO0.html |archive-date=27 September 2011 }}</ref> NASA's [[Jet Propulsion Laboratory]] 2011 study, based on observations from the [[Kepler (spacecraft)|Kepler]] mission, raised the number somewhat, estimating that about "1.4 to 2.7 percent" of all stars of spectral class [[F-type main-sequence star|F]], [[G-type main-sequence star|G]], and [[orange dwarf|K]] are expected to have planets in their HZs.<ref name="ChoiCQ">{{cite web |last1=Choi |first1=Charles Q.|url=http://www.space.com/11188-alien-earths-planets-sun-stars.html |title=New Estimate for Alien Earths: 2 Billion in Our Galaxy Alone |date=21 March 2011 |publisher=Space.com |access-date=2011-04-24}}</ref><ref name=shao-2011>{{Cite journal | last1 = Catanzarite | first1 = J. | last2 = Shao | first2 = M. | doi = 10.1088/0004-637X/738/2/151 | title = The Occurrence Rate of Earth Analog Planets Orbiting Sun-Like Stars | journal = The Astrophysical Journal | volume = 738 | issue = 2 | pages = 151 | year = 2011 |arxiv = 1103.1443 |bibcode = 2011ApJ...738..151C | s2cid = 119290692 }}</ref>

===Early findings===
{{Category see also|Giant planets in the habitable zone}}
The first discoveries of extrasolar planets in the HZ occurred just a few years after the first extrasolar planets were discovered. However, these early detections were all gas giant-sized, and many were in eccentric orbits. Despite this, studies indicate the possibility of large, Earth-like moons around these planets supporting liquid water.<ref>{{cite journal|author1=Williams, D. |author2=Pollard, D. | title=Earth-like worlds on eccentric orbits: excursions beyond the habitable zone| journal=International Journal of Astrobiology| volume=1|issue=1|pages=61–69|date=2002| doi=10.1017/S1473550402001064|bibcode = 2002IJAsB...1...61W |s2cid=37593615 }}</ref>
One of the first discoveries was [[70 Virginis b]], a gas giant initially nicknamed "Goldilocks" due to it being neither "too hot" nor "too cold". Later study revealed temperatures analogous to Venus, ruling out any potential for liquid water.<ref name="Extrasolar.net">{{cite web |url = http://www.extrasolar.net/planettour.asp?PlanetID=22 |title = 70 Virginis b |work = Extrasolar Planet Guide |publisher = Extrasolar.net |access-date = 2009-04-02 |archive-url=https://web.archive.org/web/20120619015814/http://www.extrasolar.net/planettour.asp?PlanetID=22 |archive-date=2012-06-19}}</ref> [[16 Cygni Bb]], also discovered in 1996, has an extremely eccentric orbit that spends only part of its time in the HZ, such an orbit would causes extreme [[season]]al effects. In spite of this, simulations have suggested that a sufficiently large companion could support surface water year-round.<ref>{{cite journal|author1=Williams, D. |author2=Pollard, D. | title=Earth-like worlds on eccentric orbits: excursions beyond the habitable zone| journal=International Journal of Astrobiology| volume=1|issue=1|pages=61–69|date=2002|doi=10.1017/S1473550402001064|bibcode = 2002IJAsB...1...61W |s2cid=37593615 }}</ref>

[[Gliese 876 b]], discovered in 1998, and [[Gliese 876 c]], discovered in 2001, are both gas giants discovered in the habitable zone around [[Gliese 876]] that may also have large moons.<ref name="Sudarsky2003">{{cite journal |title=Theoretical Spectra and Atmospheres of Extrasolar Giant Planets |last1=Sudarsky |first1=David |last2=Burrows |first2=Adam |last3=Hubeny |first3=Ivan |display-authors=1 |journal=The Astrophysical Journal |volume=588 |issue=2 |pages=1121–1148 |date=2003 |doi=10.1086/374331 |bibcode=2003ApJ...588.1121S |arxiv=astro-ph/0210216 |s2cid=16004653 }}</ref> Another gas giant, [[Upsilon Andromedae d]] was discovered in 1999 orbiting Upsilon Andromidae's habitable zone.

Announced on April 4, 2001, [[HD 28185 b]] is a gas giant found to orbit entirely within its star's circumstellar habitable zone<ref>{{Cite journal | doi = 10.1086/506557| last1 = Jones | first1 = B. W. | last2 = Sleep | first2 = P. N. | last3 = Underwood | first3 = D. R. | title = Habitability of Known Exoplanetary Systems Based on Measured Stellar Properties | journal = The Astrophysical Journal | volume = 649 | issue = 2 | pages = 1010–1019 | year = 2006 | bibcode=2006ApJ...649.1010J|arxiv = astro-ph/0603200 | s2cid = 119078585 }}</ref> and has a low orbital eccentricity, comparable to that of Mars in the Solar System.<ref>{{Cite journal | last1 = Butler | first1 = R. P. | last2 = Wright | first2 = J. T. | last3 = Marcy | first3 = G. W. | last4 = Fischer | first4 = D. A. | last5 = Vogt | first5 = S. S. | last6 = Tinney | first6 = C. G. | last7 = Jones | first7 = H. R. A. | last8 = Carter | first8 = B. D. | last9 = Johnson | first9 = J. A. | last10 = McCarthy | first10 = C. | last11 = Penny | first11 = A. J. | title = Catalog of Nearby Exoplanets | doi = 10.1086/504701 | journal = The Astrophysical Journal | volume = 646 | issue = 1 | pages = 505–522 | year = 2006 |arxiv = astro-ph/0607493 |bibcode = 2006ApJ...646..505B | s2cid = 119067572 }}</ref> Tidal interactions suggest it could harbor habitable Earth-mass satellites in orbit around it for many billions of years,<ref>{{Cite journal | doi = 10.1086/341477 | last1 = Barnes | first1 = J. W. | last2 = O'Brien | first2 = D. P. | title = Stability of Satellites around Close-in Extrasolar Giant Planets | journal = The Astrophysical Journal | volume = 575 | issue = 2 | pages = 1087–1093 | year = 2002 | bibcode=2002ApJ...575.1087B|arxiv = astro-ph/0205035 | s2cid = 14508244 }}</ref> though it is unclear whether such satellites could form in the first place.<ref>{{Cite journal | last1 = Canup | first1 = R. M. | author-link = Robin Canup | last2 = Ward | first2 = W. R. | doi = 10.1038/nature04860 | title = A common mass scaling for satellite systems of gaseous planets | journal = Nature | volume = 441 | issue = 7095 | pages = 834–839 | year = 2006 | pmid =  16778883|bibcode = 2006Natur.441..834C | s2cid = 4327454 }}</ref>

[[HD 69830 d]], a gas giant with 17 times the mass of Earth, was found in 2006 orbiting within the circumstellar habitable zone of [[HD 69830]], 41 light years away from Earth.<ref name=lovis-2006>{{cite journal |author=Lovis| title=An extrasolar planetary system with three Neptune-mass planets |journal=[[Nature (journal)|Nature]] |volume=441 |date=2006 |pages=305–309| doi=10.1038/nature04828 |pmid=16710412 |last2=Mayor |first2=M |last3=Pepe |first3=F |last4=Alibert |first4=Y |last5=Benz |first5=W |last6=Bouchy |first6=F |last7=Correia |first7=AC |last8=Laskar |first8=J |last9=Mordasini |first9=C |issue=7091 |arxiv = astro-ph/0703024 |bibcode = 2006Natur.441..305L | s2cid=4343578 |display-authors=1 }}</ref> The following year, [[55 Cancri f]] was discovered within the HZ of its host star [[55 Cancri|55 Cancri A]].<ref name="ScienceDaily">{{cite web |url = https://www.sciencedaily.com/releases/2007/11/071106133058.htm |title = Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri |publisher = Sciencedaily.com |date = November 6, 2007 |access-date = 2008-09-14| archive-url = https://web.archive.org/web/20080926142319/https://www.sciencedaily.com/releases/2007/11/071106133058.htm| archive-date = 26 September 2008 | url-status = live}}</ref><ref name="Fischer2008">{{Cite journal |title=Five Planets Orbiting 55 Cancri |last1=Fischer |first1=Debra A. |last2=Marcy |first2=Geoffrey W. |last3=Butler |first3=R. Paul |last4=Vogt |first4=Steven S. |last5=Laughlin |first5=Greg |last6=Henry |first6=Gregory W. |last7=Abouav |first7=David |last8=Peek |first8=Kathryn M. G. |last9=Wright |first9=Jason T. |display-authors=1 |journal=The Astrophysical Journal |date=2008 |volume=675 |issue=1 |pages=790–801 |arxiv=0712.3917 |bibcode=2008ApJ...675..790F |doi=10.1086/525512 |s2cid=55779685 }}</ref> Hypothetical satellites with sufficient mass and composition are thought to be able to support liquid water at their surfaces.<ref name="guardian">{{cite news| url=https://www.theguardian.com/science/2007/nov/07/spaceexploration |title=Could this be Earth's near twin? Introducing planet 55 Cancri f |newspaper=[[The Guardian]] |author=Ian Sample |date= 7 November 2007|access-date=17 October 2008 |location=London| archive-url= https://web.archive.org/web/20081002080911/http://www.guardian.co.uk/science/2007/nov/07/spaceexploration| archive-date= 2 October 2008 | url-status= live}}</ref>

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.

===Habitable super-Earths===
{{Category see also|Super-Earths in the habitable zone}}
[[File:Gliese 581 - 2010.jpg|thumb|The habitable zone of Gliese 581 compared with the Solar System's habitable zone]]

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.<ref>{{cite news |url=http://www.space.com/scienceastronomy/070424_exoplanet_side.html |title=Planet Hunters Edge Closer to Their Holy Grail |last=Than |first=Ker |date=2007-02-24 |publisher=space.com |access-date=2007-04-29}}</ref> Gliese 581 d, another planet in the same system and thought to be a better candidate for habitability, was also announced in 2007. Its existence was later disconfirmed in 2014, 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,<ref name="SCI-20140703">{{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 (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 }}</ref> and astronomers are divided about its existence.

[[File:Kepler-22 diagram.jpg|thumb|left|A diagram comparing size (artist's impression) and orbital position of planet Kepler-22b within Sun-like star Kepler 22's habitable zone and that of Earth in the Solar System]]

Discovered in August 2011, [[HD 85512 b]] was initially speculated to be habitable,<ref name=maxisciences>{{cite web| url=http://www.maxisciences.com/plan%E8te-habitable/des-chercheurs-decouvrent-une-planete-potentiellement-habitable_art16635.html| title=Researchers find potentially habitable planet| publisher=maxisciences.com| language=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}}</ref> but the new circumstellar habitable zone criteria devised by Kopparapu et al. in 2013 place the planet outside the circumstellar habitable zone.<ref name=torres-2013-2 />

[[Kepler-22 b]], discovered in December 2011 by the Kepler space probe,<ref name="bbc20111205">{{cite news| url=https://www.bbc.co.uk/news/science-environment-16040655 |title=Kepler 22-b: Earth-like planet confirmed |publisher=BBC |date=December 5, 2011 |access-date=May 2, 2013}}</ref> is the first [[transit method|transiting]] exoplanet discovered around a [[solar analog|Sun-like star]]. With a radius 2.4 times that of Earth, Kepler-22b has been predicted by some to be an ocean planet.<ref name="Caleb Scharf Blog">{{cite web |url=http://blogs.scientificamerican.com/life-unbounded/2011/12/08/cant-always-tell-an-exoplanet-by-its-size |title=You Can't Always Tell an Exoplanet by Its Size |date=2011-12-08 |last=Scharf |first=Caleb A. |magazine=Scientific American |access-date=2012-09-20 }}: "If it [Kepler-22b] had a similar composition to Earth, then we're looking at a world in excess of about 40 Earth masses".</ref> [[Gliese 667 Cc]], discovered in 2011 but announced in 2012,<ref name=arxiv12020446>{{Cite journal |first1=Guillem |last1=Anglada-Escude |first2=Pamela |last2=Arriagada |first3=Steven |last3=Vogt |first4=Eugenio J. |last4=Rivera |first5=R. Paul |last5=Butler |first6=Jeffrey D. |last6=Crane |first7=Stephen A. |last7=Shectman |first8=Ian B. |last8=Thompson |first9=Dante |last9=Minniti |title=A planetary system around the nearby M dwarf GJ 667C with at least one super-Earth in its habitable zone |date=2012 |arxiv=1202.0446 |doi=10.1088/2041-8205/751/1/L16 |volume=751 |issue=1 |journal=The Astrophysical Journal |page=L16|bibcode = 2012ApJ...751L..16A |s2cid=16531923 }}</ref> is a super-Earth orbiting in the circumstellar habitable zone of [[Gliese 667 C]]. It is one of the most Earth-like planets known.

[[Gliese 163 c]], discovered in September 2012 in orbit around the red dwarf [[Gliese 163]]<ref name="Simbad-20120920">{{cite web |author=<!--Staff writer--> |title=LHS 188 – High proper-motion Star |url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=HIP+19394 |date=September 20, 2012 |publisher=[[Centre de données astronomiques de Strasbourg]] (Strasbourg astronomical Data Center) |access-date=September 20, 2012 }}</ref> is located 49 [[light year]]s from Earth. The planet has 6.9 Earth masses and 1.8–2.4 Earth radii, and with its close orbit receives 40 percent more stellar radiation than Earth, leading to surface temperatures of about {{formatnum:60}}° [[celsius|C]].<ref name="PHL-20120829">{{cite web |last=Méndez |first=Abel |title=A Hot Potential Habitable Exoplanet around Gliese 163 |url=http://phl.upr.edu/press-releases/ahotpotentialhabitableexoplanetaroundgliese163 |date=August 29, 2012 |publisher=[[University of Puerto Rico at Arecibo]] (Planetary Habitability Laboratory) |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 }}</ref><ref name="Space-20120920">{{cite web |last=Redd |title=Newfound Alien Planet a Top Contender to Host Life |url=http://www.space.com/17684-alien-planet-gliese-163c-extraterrestrial-life.html |date=September 20, 2012 |publisher=Space.com |access-date=September 20, 2012}}</ref><ref>{{cite web| url=http://www.spacedaily.com/reports/A_Hot_Potential_Habitable_Exoplanet_around_Gliese_163_999.html |title=A Hot Potential Habitable Exoplanet around Gliese 163 |publisher=Spacedaily.com |access-date=2013-02-10}}</ref> [[HD 40307 g]], a candidate planet tentatively discovered in November 2012, is in the circumstellar habitable zone of [[HD 40307]].<ref name="hd40307g_tuomi12">{{cite journal |doi=10.1051/0004-6361/201220268 |last1=Tuomi |first1=Mikko |last2=Anglada-Escudé |first2=Guillem |last3=Gerlach |first3=Enrico |last4=Jones |first4=Hugh R. A. |last5=Reiners |first5=Ansgar |last6=Rivera |first6=Eugenio J. |last7=Vogt |first7=Steven S. |last8=Butler |first8=R. Paul |title=Habitable-zone super-Earth candidate in a six-planet system around the K2.5V star HD 40307 |journal=Astronomy & Astrophysics |date=17 December 2012 |volume=549 |pages=A48 |arxiv=1211.1617 |bibcode=2013A&A...549A..48T |s2cid=7424216 }}</ref> In December 2012, [[Tau Ceti e]] and [[Tau Ceti f]] were found in the circumstellar habitable zone of [[Tau Ceti]], a Sun-like star 12 light years away.<ref name=aron-2012>{{cite web |url=https://www.newscientist.com/article/dn23021-nearby-tau-ceti-may-host-two-planets-suited-to-life.html |title=Nearby Tau Ceti may host two planets suited to life |publisher=Reed Business Information |work=New Scientist |date=December 19, 2012 |access-date=April 1, 2013 |author=Aron, Jacob}}</ref> Although more massive than Earth, they are among the least massive planets found to date orbiting in the habitable zone;<ref name="tuomi-2013">{{Cite journal | last1 = Tuomi | first1 = M. | last2 = Jones | first2 = H. R. A. | last3 = Jenkins | first3 = J. S. | last4 = Tinney | first4 = C. G. | last5 = Butler | first5 = R. P. | last6 = Vogt | first6 = S. S. | last7 = Barnes | first7 = J. R. | last8 = Wittenmyer | first8 = R. A. | last9 = o'Toole | first9 = S. | last10 = Horner | first10 = J. | last11 = Bailey | first11 = J. | last12 = Carter | first12 = B. D. | last13 = Wright | first13 = D. J. | last14 = Salter | first14 = G. S. | last15 = Pinfield | first15 = D. | title = Signals embedded in the radial velocity noise | doi = 10.1051/0004-6361/201220509 | journal = Astronomy & Astrophysics | volume = 551 | pages = A79 | year = 2013 |arxiv = 1212.4277 |bibcode = 2013A&A...551A..79T | s2cid = 2390534 }}</ref> however, Tau Ceti f, like HD 85512 b, did not fit the new circumstellar habitable zone criteria established by the 2013 Kopparapu study.<ref name=mendes-2013-3>{{cite web |url=http://phl.upr.edu/projects/habitable-exoplanets-catalog |title=The Habitable Exoplanets Catalog |publisher=University of Puerto Rico |date=May 1, 2013 |access-date=May 1, 2013 |author=Torres, Abel Mendez}}</ref> It is now considered as uninhabitable.

===Near Earth-sized planets and Solar analogs===
[[File:Kepler186f-ComparisonGraphic-20140417 improved.jpg|thumb|right|Comparison of the HZ position of Earth-radius planet Kepler-186f and the [[Solar System]] (17 April 2014)]]
[[File:Kepler-452b System.jpg|thumb|right|While larger than Kepler 186f, Kepler-452b's orbit and star are more similar to Earth's.]]

Recent discoveries have uncovered planets that are thought to be similar in size or mass to Earth. "Earth-sized" ranges are typically defined by mass. The lower range used in many definitions of the super-Earth class is 1.9 Earth masses; likewise, sub-Earths range up to the size of Venus (~0.815 Earth masses). An upper limit of 1.5 Earth radii is also considered, given that above {{Earth radius|1.5|link=y}} the average planet density rapidly decreases with increasing radius, indicating these planets have a significant fraction of volatiles by volume overlying a rocky core.<ref>Lauren M. Weiss, and Geoffrey W. Marcy. "[https://arxiv.org/abs/1312.0936 The mass-radius relation for 65 exoplanets smaller than 4 Earth radii]"</ref> A genuinely Earth-like planet – an [[Earth analog]] or "Earth twin" – would need to meet many conditions beyond size and mass; such properties are not observable using current technology.

A [[solar analog]] (or "solar twin") is a star that resembles the Sun. No solar twin with an exact match as that of the Sun has been found. However, some stars are nearly identical to the Sun and are considered solar twins. An exact solar twin would be a G2V star with a 5,778 K temperature, be 4.6&nbsp; billion years old, with the correct [[metallicity]] and a 0.1% [[solar luminosity]] variation.<ref>{{cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2013/08jan_sunclimate/|title=Solar Variability and Terrestrial Climate  |date=2013-01-08|publisher=NASA Science}}</ref> Stars with an age of 4.6&nbsp;billion years are at the most stable state. Proper metallicity and size are also critical to low luminosity variation.<ref>{{cite web|url=http://astro.unl.edu/classaction/animations/stellarprops/stellarlum.html|title=Stellar Luminosity Calculator|publisher=University of Nebraska-Lincoln astronomy education group}}</ref><ref>{{Cite book|url=http://www.nap.edu/catalog/13519/the-effects-of-solar-variability-on-earths-climate-a-workshop|title=The Effects of Solar Variability on Earth's Climate: A Workshop Report|first=National Research|last=Council|date=18 September 2012|doi=10.17226/13519|isbn=978-0-309-26564-5}}</ref><ref>{{cite web|url=http://scienceblogs.com/startswithabang/2013/06/05/most-of-earths-twins-arent-identical-or-even-close/|title=Most of Earth's twins aren't identical, or even close!|author=Ethan|date=June 5, 2013|publisher=ScienceBlogs.com}}</ref>

Using data collected by NASA's [[Kepler space telescope]] and the [[W. M. Keck Observatory]], scientists have estimated that 22% of solar-type stars in the Milky Way galaxy have Earth-sized planets in their habitable zone.<ref name="NOAA 2017">{{cite web |url=https://oceanservice.noaa.gov/facts/et-oceans.html |title=Are there oceans on other planets? |work=National Oceanic and Atmospheric Administration |date=6 July 2017 |access-date=2017-10-03 }}</ref>

On 7 January 2013, astronomers from the Kepler team announced the discovery of [[Kepler-69c]] (formerly ''KOI-172.02''), an Earth-size [[exoplanet]] candidate (1.7 times the radius of Earth) orbiting [[Kepler-69]], a star similar to the Sun, in the HZ and expected to offer habitable conditions.<ref name="Space-20130109">{{cite web |last=Moskowitz |first=Clara |title=Most Earth-Like Alien Planet Possibly Found |url=http://www.space.com/19201-most-earth-like-alien-planet.html |date=January 9, 2013 |publisher=Space.com |access-date=January 9, 2013 }}</ref><ref name="arXiv-20130417">{{cite journal|doi=10.1088/0004-637X/768/2/101| title=A Super-Earth-Sized Planet Orbiting in or Near the Habitable Zone Around a Sun-Like Star| date=2013|last1=Barclay|first1=Thomas|last2=Burke|first2=Christopher J.|last3=Howell|first3=Steve B.|last4=Rowe|first4=Jason F.|last5=Huber|first5=Daniel|last6=Isaacson|first6=Howard|last7=Jenkins|first7=Jon M.|last8=Kolbl|first8=Rea|last9=Marcy|first9=Geoffrey W. |journal=The Astrophysical Journal| volume=768|issue=2|pages=101|arxiv = 1304.4941 |bibcode = 2013ApJ...768..101B | s2cid=51490784}}</ref><ref name="NASA-20130418" /><ref name="NYT-20130418">{{cite news |last=Overbye |first=Dennis |title=Two Promising Places to Live, 1,200 Light-Years from Earth| url=https://www.nytimes.com/2013/04/19/science/space/2-new-planets-are-most-earth-like-yet-scientists-say.html| date=18 April 2013 |newspaper=The New York Times |access-date=18 April 2013 }}</ref> The discovery of two planets orbiting in the habitable zone of [[Kepler-62]], by the Kepler team was announced on April 19, 2013. The planets, named [[Kepler-62e]] and [[Kepler-62f]], are likely solid planets with sizes 1.6 and 1.4 times the radius of Earth, respectively.<ref name="NASA-20130418">{{cite web |last1=Johnson |first1=Michele |last2=Harrington |first2=J.D. |title=NASA's Kepler Discovers Its Smallest 'Habitable Zone' Planets to Date |url=http://www.nasa.gov/mission_pages/kepler/news/kepler-62-kepler-69.html |date=18 April 2013 |work=[[NASA]] |access-date=18 April 2013 |archive-date=8 May 2020 |archive-url=https://web.archive.org/web/20200508010029/https://www.nasa.gov/mission_pages/kepler/news/kepler-62-kepler-69.html |url-status=dead }}</ref><ref name="NYT-20130418"/><ref name="Borucki-2013">{{Cite journal |last=Borucki |first=William J. |author-link=William J. Borucki |title=Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone |journal=Science Express| date=18 April 2013 |doi=10.1126/science.1234702 |volume=340 |issue=6132 |pages=587–90|arxiv = 1304.7387 |bibcode = 2013Sci...340..587B |display-authors=etal |pmid=23599262|hdl=1721.1/89668 |s2cid=21029755 }}</ref>

With a radius estimated at 1.1 Earth, [[Kepler-186f]], discovery announced in April 2014, is the closest yet size to Earth of an exoplanet confirmed by the transit method<ref name="NYT-20140417">{{cite news |last=Chang |first=Kenneth |title=Scientists Find an 'Earth Twin,' or Maybe a Cousin |url=https://www.nytimes.com/2014/04/18/science/space/scientists-find-an-earth-twin-or-maybe-a-cousin.html |date=17 April 2014 |work=The New York Times |access-date=17 April 2014 }}</ref><ref name="AP-20140417">{{cite news |last=Chang |first=Alicia |title=Astronomers spot most Earth-like planet yet |url=http://apnews.excite.com/article/20140417/DAD832V81.html |date=17 April 2014 |work=[[AP News]] |access-date=17 April 2014 }}</ref><ref name="BBC-20140417">{{cite news |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |title='Most Earth-like planet yet' spotted by Kepler |url=https://www.bbc.co.uk/news/science-environment-27054366 |date=17 April 2014 |work=[[BBC News]] |access-date=17 April 2014 }}</ref> though its mass remains unknown and its parent star is not a Solar analog.

[[Kapteyn b]], discovered in June 2014, was thought to is a possible rocky world of about 4.8 Earth masses and about 1.5 Earth radii orbiting the habitable zone of the red subdwarf [[Kapteyn's Star]], 12.8 light-years away.<ref name="SP-20140603">{{cite web |last=Wall |first=Mike |title=Found! Oldest Known Alien Planet That Might Support Life |url=http://www.space.com/26115-oldest-habitable-alien-planet-kapteyn-b.html |date=3 June 2014 |work=[[Space.com]] |access-date=10 January 2015 }}</ref> However, further analysis concluded that this claim was an artefact of stellar rotation and activity.<ref name="BortleFausey2021">{{citation | postscript=.
 | title=A Gaussian Process Regression Reveals No Evidence for Planets Orbiting Kapteyn's Star
 | last1=Bortle | first1=Anna | last2=Fausey | first2=Hallie
 | last3=Ji | first3=Jinbiao | last4=Dodson-Robinson | first4=Sarah
 | last5=Ramirez Delgado | first5=Victor | last6=Gizis | first6=John
 | display-authors=1 | journal=The Astronomical Journal
 | volume=161 | issue=5 | year=2021 | pages=230
 | arxiv=2103.02709 | doi=10.3847/1538-3881/abec89
 | bibcode=2021AJ....161..230B | s2cid=232110395 | doi-access=free }}</ref>

On 6 January 2015, NASA announced the 1000th confirmed [[exoplanet]] discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their related [[star]]s: two of the three, [[Kepler-438b]] and [[Kepler-442b]], are near-Earth-size and likely [[Terrestrial planet|rocky]]; the third, [[Kepler-440b]], is a [[super-Earth]].<ref name="NASA-20150106" /> However, [[Kepler-438b]] is found to be a subject of powerful flares, so it is now considered uninhabitable. 16 January, [[K2-3d]] a planet of 1.5 Earth radii was found orbiting within the habitable zone of [[K2-3]], receiving 1.4 times the intensity of visible light as Earth.<ref>{{cite news |first=Mari N.|last=Jensen |url=https://www.sciencedaily.com/releases/2015/01/150116093052.htm |title=Three nearly Earth-size planets found orbiting nearby star: One in 'Goldilocks' zone |work=[[Science Daily]] |date=16 January 2015 |access-date=25 July 2015}}</ref>

[[Kepler-452b]], announced on 23 July 2015 is 50% bigger than Earth, likely rocky and takes approximately 385 Earth days to orbit the habitable zone of its [[G-type main-sequence star|G-class]] (solar analog) star [[Kepler-452]].<ref name=Jenkins2015>{{cite journal| last1=Jenkins|first1=Jon M.|last2=Twicken|first2=Joseph D.|last3=Batalha|first3=Natalie M.|last4=Caldwell|first4=Douglas A.|last5=Cochran|first5=William D.|last6=Endl|first6=Michael|last7=Latham|first7=David W.|last8=Esquerdo|first8=Gilbert A.|last9=Seader|first9=Shawn|last10=Bieryla|first10=Allyson|last11=Petigura|first11=Erik|last12=Ciardi|first12=David R.|last13=Marcy|first13=Geoffrey W.|last14=Isaacson|first14=Howard|last15=Huber|first15=Daniel|last16=Rowe|first16=Jason F.|last17=Torres|first17=Guillermo|last18=Bryson|first18=Stephen T.|last19=Buchhave|first19=Lars|last20=Ramirez|first20=Ivan|last21=Wolfgang|first21=Angie|last22=Li|first22=Jie|last23=Campbell|first23=Jennifer R.|last24=Tenenbaum|first24=Peter|last25=Sanderfer|first25=Dwight|last26=Henze|first26=Christopher E.|last27=Catanzarite|first27=Joseph H.|last28=Gilliland|first28=Ronald L.|last29=Borucki|first29=William J.| title=Discovery and Validation of Kepler-452b: A 1.6 R<sub>🜨</sub> Super Earth Exoplanet in the Habitable Zone of a G2 Star| journal=The Astronomical Journal| date=23 July 2015| volume=150|issue=2|page=56|issn=1538-3881|doi=10.1088/0004-6256/150/2/56|arxiv = 1507.06723 |bibcode = 2015AJ....150...56J |s2cid=26447864}}</ref><ref name="bno">{{cite web |url=http://bnonews.com/news/index.php/news/id961 |title=NASA telescope discovers Earth-like planet in star's habitable zone |date=23 July 2015 |work=[[BNO News]] |access-date=23 July 2015}}</ref>

The discovery of a system of three tidally locked planets orbiting the habitable zone of an ultracool dwarf star, [[TRAPPIST-1]], was announced in May 2016.<ref>{{cite web|url=http://www.eso.org/public/news/eso1615/|title=Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star|publisher=European Southern Observatory|date=2 May 2016}}</ref> The discovery is considered significant because it dramatically increases the possibility of smaller, cooler, more numerous and closer stars possessing habitable planets.

Two potentially habitable planets, discovered by the K2 mission in July 2016 orbiting around the M dwarf [[K2-72]] around 227 light years from the Sun: [[K2-72c]] and [[K2-72e]] are both of similar size to Earth and receive similar amounts of stellar radiation.<ref name="DressingVanderburg2017">{{cite journal|last1=Dressing|first1=Courtney D.|last2=Vanderburg|first2=Andrew|last3=Schlieder|first3=Joshua E.|last4=Crossfield|first4=Ian J. M.|last5=Knutson|first5=Heather A.|last6=Newton|first6=Elisabeth R.|last7=Ciardi|first7=David R.|last8=Fulton|first8=Benjamin J.|last9=Gonzales|first9=Erica J.|last10=Howard|first10=Andrew W.|last11=Isaacson|first11=Howard|last12=Livingston|first12=John|last13=Petigura|first13=Erik A.|last14=Sinukoff|first14=Evan|last15=Everett|first15=Mark|last16=Horch|first16=Elliott|last17=Howell|first17=Steve B.|title=Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7|journal=The Astronomical Journal|volume=154|issue=5|year=2017|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 }}</ref>

Announced on the 20 April 2017, [[LHS 1140b]] is a super-dense [[super-Earth]] 39 light years away, 6.6 times Earth's mass and 1.4 times radius, its star 15% the mass of the Sun but with much less observable stellar flare activity than most M dwarfs.<ref>{{cite journal|doi=10.1038/nature22055|pmid=28426003|title=A temperate rocky super-Earth transiting a nearby cool star|journal=Nature|volume=544|issue=7650|pages=333–336|year=2017|last1=Dittmann|first1=Jason A.|last2=Irwin|first2=Jonathan M.|last3=Charbonneau|first3=David|last4=Bonfils|first4=Xavier|last5=Astudillo-Defru|first5=Nicola|last6=Haywood|first6=Raphaëlle D.|last7=Berta-Thompson|first7=Zachory K.|last8=Newton|first8=Elisabeth R.|last9=Rodriguez|first9=Joseph E.|last10=Winters|first10=Jennifer G.|last11=Tan|first11=Thiam-Guan|last12=Almenara|first12=Jose-Manuel|last13=Bouchy|first13=François|last14=Delfosse|first14=Xavier|last15=Forveille|first15=Thierry|last16=Lovis|first16=Christophe|last17=Murgas|first17=Felipe|last18=Pepe|first18=Francesco|last19=Santos|first19=Nuno C.|last20=Udry|first20=Stephane|last21=Wünsche|first21=Anaël|last22=Esquerdo|first22=Gilbert A.|last23=Latham|first23=David W.|last24=Dressing|first24=Courtney D.|arxiv = 1704.05556 |bibcode = 2017Natur.544..333D |s2cid=2718408}}</ref> The planet is one of few observable by both transit and radial velocity that's mass is confirmed with an atmosphere may be studied.

Discovered by radial velocity in June 2017, with approximately three times the mass of Earth, [[Luyten b]] orbits within the habitable zone of [[Luyten's Star]] just 12.2 light-years away.<ref>{{Cite magazine | url=https://www.wired.co.uk/article/sonar-sending-music-into-space-habitable-exoplanet | title=Astronomers are beaming techno into space for aliens to decode| magazine=Wired UK| date=2017-11-16| last1=Bradley| first1=Sian}}</ref>

At 11 light-years away, the second closest planet, [[Ross 128 b]], was announced in November 2017 following a decade's radial velocity study of relatively "quiet" red dwarf star Ross 128. At 1.35 times Earth's mass, is it roughly Earth-sized and likely rocky in composition.<ref>{{cite web | url=https://www.space.com/38782-possibly-earth-like-alien-planet-ross-128b.html | title=In Earth's Backyard: Newfound Alien Planet May be Good Bet for Life| website=[[Space.com]]| date=15 November 2017}}</ref>

Discovered in March 2018, [[K2-155d]] is about 1.64 times the radius of Earth, is likely rocky and orbits in the habitable zone of its [[red dwarf]] star 203 light years away.<ref name="Exoplanet Exploration">{{Cite web|title=K2-155 d|url=https://exoplanets.nasa.gov/newworldsatlas/6173/|publisher=Exoplanet Exploration|year=2018}}</ref><ref name=CNET>{{cite web|last1=Mack|first1=Eric|title=A super-Earth around a red star could be wet and wild|url=https://www.cnet.com/news/super-earth-exoplanet-k2-155d-found-could-be-habitable-nasa/|website=[[CNET]]|date=March 13, 2018}}</ref><ref name=ExtremeTech>{{Cite web|last1=Whitwam|first1=Ryan|title=Kepler Spots Potentially Habitable Super-Earth Orbiting Nearby Star|url=https://www.extremetech.com/extreme/265576-kepler-spots-potentially-habitable-super-earth-orbiting-nearby-star|publisher=[[ExtremeTech]]|date=March 14, 2018}}</ref>

One of the earliest discoveries by the [[Transiting Exoplanet Survey Satellite]] (TESS) announced on July 31, 2019, is a Super-Earth planet [[GJ 357 d]] orbiting the outer edge of a red dwarf 31 light years away.<ref name="LuquePallé2019">{{cite journal|last1=Luque|first1=R.|last2=Pallé|first2=E.|last3=Kossakowski|first3=D.|last4=Dreizler|first4=S.|last5=Kemmer|first5=J.|last6=Espinoza|first6=N.|title=Planetary system around the nearby M dwarf GJ 357 including a transiting, hot, Earth-sized planet optimal for atmospheric characterization|journal=Astronomy & Astrophysics|volume=628|pages=A39|year=2019|issn=0004-6361|doi=10.1051/0004-6361/201935801|arxiv=1904.12818|bibcode=2019A&A...628A..39L|doi-access=free}}</ref>

[[K2-18b]] is an exoplanet 124 light-years away, orbiting in the habitable zone of the [[K2-18]], a red dwarf. This planet is significant for water vapor found in its atmosphere; this was announced on September 17, 2019.

In September 2020, astronomers identified 24 [[superhabitable planet]] (planets better than Earth) contenders, from among more than 4000 confirmed [[exoplanet]]s at present, based on [[Astrophysics|astrophysical parameters]], as well as the [[Life|natural history]] of [[Earliest known life forms|known life forms]] on the [[Earth]].<ref name="AB-20200918">{{cite journal |last1=Schulze-Makuch |first1=Dirk |last2=Heller |first2=Rene |last3=Guinan |first3=Edward |title=In Search for a Planet Better than Earth: Top Contenders for a Superhabitable World |date=18 September 2020 |journal=[[Astrobiology (journal)|Astrobiology]] |volume=20 |issue=12 |pages=1394–1404 |doi=10.1089/ast.2019.2161 |pmid=32955925 |pmc=7757576 |bibcode=2020AsBio..20.1394S |doi-access=free }}</ref>

{| class="wikitable" style="margin:0.5em auto; width:600px;"
! Notable [[exoplanets]] – [[Kepler space telescope]]
|-
| style="font-size:88%" | [[File:PIA19827-Kepler-SmallPlanets-HabitableZone-20150723.jpg|600px]]
{{center|1=Confirmed small exoplanets in habitable zones.<br />([[Kepler-62e]], [[Kepler-62f]], [[Kepler-186f]], [[Kepler-296e]], [[Kepler-296f]], [[Kepler-438b]], [[Kepler-440b]], [[Kepler-442b]])<br />(Kepler Space Telescope; January 6, 2015).<ref name="NASA-20150106">{{cite web |last1=Clavin |first1=Whitney |last2=Chou |first2=Felicia |last3=Johnson |first3=Michele |title=NASA's Kepler Marks 1,000th Exoplanet Discovery, Uncovers More Small Worlds in Habitable Zones |url=http://www.jpl.nasa.gov/news/news.php?release=2015-003 |date=6 January 2015 |work=[[NASA]] |access-date=6 January 2015 }}</ref>}}
|}