Editing Habitable zone (section)

===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]]
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| 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>}}
|}