Editing Terrestrial planet (section)

==Extrasolar terrestrial planets==
{{See also|Super-Earth|Mega-Earth|List of nearest terrestrial exoplanet candidates}}

<!---As of 2014,--->Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable.<ref>{{cite book | last=Haswell | first=Carole A. |authorlink=Carole Ann Haswell| title=Transiting Exoplanets | publisher=Cambridge University Press | publication-place=Cambridge | date=2010-07-29 | isbn=978-0-521-13938-0 | page=}}</ref><ref>{{cite book | last=Perryman | first=Michael | title=The Exoplanet Handbook | publisher=Cambridge University Press | publication-place=Cambridge New York | date=2011-05-26 | isbn=978-0-521-76559-6 | page=}}</ref><ref>{{cite book | last=Seager | first=Sara | title=Exoplanets | publisher=University of Arizona Press | publication-place=Tucson | date=2011-01-15 | isbn=978-0-8165-2945-2}}</ref> But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these are [[super-Earth]]s, i.e. planets with masses between Earth's and Neptune's; super-Earths may be [[gas planet]]s or terrestrial, depending on their mass and other parameters.

During the early 1990s, the first extrasolar planets were discovered orbiting the [[pulsar]] [[PSR B1257+12]], with masses of 0.02, 4.3, and 3.9 times that of Earth, by [[Methods of detecting exoplanets#Pulsar timing|pulsar timing]].

When [[51 Pegasi b]], the first planet found around a star still undergoing [[Stellar nucleosynthesis|fusion]], was discovered, many astronomers assumed it to be a gigantic terrestrial,{{citation needed|date=March 2014}} because it was assumed no gas giant could exist as close to its star (0.052&nbsp;AU) as 51&nbsp;Pegasi&nbsp;b did. It was later found to be a gas giant.

In 2005, the first planets orbiting a main-sequence star and which showed signs of being terrestrial planets were found: [[Gliese 876 d]] and [[OGLE-2005-BLG-390Lb]]. Gliese 876&nbsp;d orbits the red dwarf [[Gliese 876]], 15 [[light year]]s from Earth, and has a mass seven to nine&nbsp;times that of Earth and an orbital period of just two Earth days. OGLE-2005-BLG-390Lb has about 5.5&nbsp;times the mass of Earth and orbits a star about 21,000 light-years away in the constellation Scorpius.
From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting within the [[Gliese 581 planetary system]]. The smallest, [[Gliese 581e]], is only about 1.9 Earth masses,<ref name="ESO0904">{{cite web |date=21 April 2009 |title=Lightest exoplanet yet discovered |publisher=ESO (ESO 15/09 – Science Release) |url=http://www.eso.org/public/outreach/press-rel/pr-2009/pr-15-09.html |access-date=15 July 2009 |url-status=live |archive-url=https://web.archive.org/web/20090705084859/http://www.eso.org/public/outreach/press-rel/pr-2009/pr-15-09.html |archive-date=5 July 2009  }}</ref> but orbits very close to the star.<ref name="mayor">{{cite journal |doi=10.1051/0004-6361/200912172 |bibcode=2009A&A...507..487M |journal=[[Astronomy and Astrophysics]] |volume=507 |issue=1 |year=2009 |pages=487–494 |last1=Mayor |first1=Michel |last2=Bonfils |first2=Xavier |last3=Forveille |first3=Thierry |last4=Delfosse |first4=Xavier |last5=Udry |first5=Stéphane |last6=Bertaux |first6=Jean-Loup |last7=Beust |first7=Hervé |last8=Bouchy |first8=François |last9=Lovis |first9=Christophe |last10=Pepe |first10=Francesco |last11=Perrier |first11=Christian |last12=Queloz |first12=Didier |last13=Santos |first13=Nuno C. |title=The HARPS search for southern extra-solar planets, XVIII. An Earth-mass planet in the GJ 581 planetary system |arxiv=0906.2780 |display-authors=3 |url=http://obswww.unige.ch/~udry/Gl581_preprint.pdf |s2cid=2983930 |url-status=dead |archive-url=https://web.archive.org/web/20090521052641/http://obswww.unige.ch/~udry/Gl581_preprint.pdf |archive-date=21 May 2009 }}</ref> Two others, [[Gliese 581c]] and the disputed [[Gliese 581d|Gliese&nbsp;581d]], are more-massive super-Earths orbiting in or close to the habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures.

Another possibly terrestrial planet, [[HD 85512 b]], was discovered in 2011; it has at least 3.6 times the mass of Earth.<ref>{{cite news | first=Rachel | last=Kaufman | url=http://news.nationalgeographic.com/news/2011/08/110830-new-planet-found-most-earthlike-life-clouds-water-space-science/ | title=New Planet May Be Among Most Earthlike – Weather Permitting, Alien world could host liquid water if it has 50 percent cloud cover, study says | publisher=National Geographic News | date=30 August 2011 | access-date=5 September 2011 | url-status=dead | archive-url=https://web.archive.org/web/20110923180724/http://news.nationalgeographic.com/news/2011/08/110830-new-planet-found-most-earthlike-life-clouds-water-space-science | archive-date=23 September 2011 | df=dmy-all }}</ref>
The radius and composition of all these planets are unknown.
[[File:Size of Kepler Planet Candidates.jpg|thumb|upright=1.3|Sizes of [[Kepler space telescope|Kepler planet candidates]] based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013 ([[NASA]])]]

The first confirmed terrestrial [[exoplanet]], [[Kepler-10b]], was found in 2011 by the [[Kepler space telescope]], specifically designed to discover Earth-size planets around other stars using the [[Astronomical transit|transit]] method.<ref>{{cite news|url=https://www.bbc.com/news/science-environment-17454005|title=Thousand-year wait for Titan rain|first=Paul|last=Rincon|work=BBC News|date=22 March 2012|url-status=live|archive-url=https://web.archive.org/web/20171225082214/http://www.bbc.com/news/science-environment-17454005|archive-date=25 December 2017}}</ref>

In the same year, the [[Kepler space telescope|Kepler space telescope mission team]] released a [[Kepler space telescope#Extrasolar planets detected|list of 1235 extrasolar planet candidates]], including six that are "Earth-size" or "super-Earth-size" (i.e. they have a radius less than twice that of the Earth)<ref>Namely: KOI 326.01 [Rp=0.85], KOI 701.03 [Rp=1.73], KOI 268.01 [Rp=1.75], KOI 1026.01 [Rp=1.77], KOI 854.01 [Rp=1.91], KOI 70.03 [Rp=1.96] – Table 6). A more recent study found that one of these candidates (KOI&nbsp;326.01) is in fact much larger and hotter than first reported. {{cite web |last=Grant |first=Andrew |title=Exclusive: "Most Earth-Like" Exoplanet Gets Major Demotion—It Isn't Habitable |url=http://blogs.discovermagazine.com/80beats/2011/03/08/exclusive-most-earth-like-exoplanet-gets-major-demotion%e2%80%94it-isnt-habitable/ |publisher=[[Discover Magazine]] |work=[blogs.discovermagazine.com/80beats 80beats] |date=8 March 2011 |access-date=9 March 2011 |url-status=dead |archive-url=https://web.archive.org/web/20110309132609/http://blogs.discovermagazine.com/80beats/2011/03/08/exclusive-most-earth-like-exoplanet-gets-major-demotion%e2%80%94it-isnt-habitable/ |archive-date=9 March 2011  }}</ref> and in the [[habitable zone]] of their star.<ref name=borucki>{{Cite journal|arxiv=1102.0541|last1=Borucki|first1=William J|title=Characteristics of planetary candidates observed by Kepler, II: Analysis of the first four months of data|journal=The Astrophysical Journal|volume=736|issue=1|pages=19|display-authors=etal|year=2011|doi=10.1088/0004-637X/736/1/19|bibcode=2011ApJ...736...19B|s2cid=15233153}}</ref>
Since then, Kepler has [[List of exoplanets discovered using the Kepler spacecraft|discovered hundreds of planets]] ranging from Moon-sized to super-Earths, with many more candidates in this size range (see image).

In 2016, statistical modeling of the relationship between a planet's mass and radius using a broken power law appeared to suggest that the transition point between rocky, terrestrial worlds and mini-Neptunes without a defined surface was in fact very close to Earth and Venus's, suggesting that rocky worlds much larger than our own are in fact quite rare.<ref name=ChenKipping>{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |doi= 10.3847/1538-4357/834/1/17|arxiv=1603.08614 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}</ref> This resulted in some advocating for the retirement of the term "super-earth" as being scientifically misleading.<ref>{{cite web |url=https://www.forbes.com/sites/startswithabang/2021/06/30/its-time-to-retire-the-super-earth-the-most-unsupported-idea-in-exoplanets/ |title=It's Time To Retire The Super-Earth, The Most Unsupported Idea In Exoplanets |last=Siegel |first=Ethan |date=30 June 2021 |website=Forbes |access-date=27 July 2021}}</ref> Since 2016 the catalog of known exoplanets has increased significantly, and there have been several published refinements of the mass-radius model. As of 2024, the expected transition point between rocky and intermediate-mass planets sits at roughly 4.4 earth masses, and roughly 1.6 earth radii.<ref name="muelleretal">{{cite journal|last1=Müller|first1=Simon|last2=Baron|first2=Jana|last3=Helled|first3=Ravit|last4=Bouchy|first4=François|last5=Parc|first5=Léna|date=June 2024|title=The mass–radius relation of exoplanets revisited|journal=Astronomy & Astrophysics|volume=686|pages=A296 |doi=10.1051/0004-6361/202348690|arxiv=2311.12593|bibcode=2024A&A...686A.296M }}</ref>

In September 2020, astronomers using [[Gravitational microlensing|microlensing techniques]] reported the [[Microlensing Observations in Astrophysics|detection]], for the first time, of an Earth-mass [[rogue planet]] (named [[OGLE-2016-BLG-1928]]) unbounded by any star, and free-floating in the [[Milky Way|Milky Way galaxy]].<ref name="UT-20201001">{{cite news |last=Gough |first=Evan |title=A Rogue Earth-Mass Planet Has Been Discovered Freely Floating in the Milky Way Without a Star |url=https://www.universetoday.com/148097/a-rogue-earth-mass-planet-has-been-discovered-freely-floating-in-the-milky-way-without-a-star/ |date=1 October 2020 |work=[[Universe Today]] |access-date=2 October 2020 }}</ref><ref name="AR-20200929">{{cite journal |author=Mroz, Przemek|display-authors=et al.|title=A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event  |journal=The Astrophysical Journal|date=29 September 2020 |volume=903|issue=1|pages=L11|doi=10.3847/2041-8213/abbfad|arxiv=2009.12377v1|bibcode=2020ApJ...903L..11M|s2cid=221971000 |doi-access=free }}</ref><ref name="SA-20201019a">{{cite news |last=Redd |first=Nola Taylor |title=Rogue Rocky Planet Found Adrift in the Milky Way - The diminutive world and others like it could help astronomers probe the mysteries of planet formation |url=https://www.scientificamerican.com/article/rogue-rocky-planet-found-adrift-in-the-milky-way/
|date=19 October 2020 |work=[[Scientific American]] |access-date=19 October 2020 }}</ref>

===List of terrestrial exoplanets===
{{see also|List of nearest terrestrial exoplanet candidates}}

The following exoplanets have a density of at least {{val|5 |u=g/cm3}} and a mass below Neptune's and are thus very likely terrestrial:

[[Kepler-10b]], [[Kepler-20b]], [[Kepler-36b]], [[Kepler-48b|Kepler-48d]], [[Kepler 68c]], [[Kepler-78b]], [[Kepler-89b]], [[Kepler-93b]], [[Kepler-97b]], [[Kepler-99b]], [[Kepler-100b]], [[Kepler-101c]], [[Kepler-102b]], [[Kepler-102d]], [[Kepler-113b]], [[Kepler-131b]], [[Kepler-131c]], [[Kepler-138c]], [[Kepler-406b]], [[Kepler-406c]], [[Kepler-409b]].

===Frequency===
In 2013, astronomers reported, based on [[Kepler (spacecraft)|Kepler space mission]] data, that there could be as many as 40 billion Earth- and super-Earth-sized [[extrasolar planets|planets]] orbiting in the [[habitable zone]]s of [[Solar analog|Sun-like stars]] and [[red dwarf]]s within the [[Milky Way]].<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis |title=Far-Off Planets Like the Earth Dot the Galaxy |url=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=4 November 2013 |work=[[The New York Times]] |access-date=5 November 2013 |url-status=live |archive-url=https://web.archive.org/web/20131105023653/http://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |archive-date=5 November 2013  }}</ref><ref name="PNAS-20131031">{{cite journal |last1=Petigura |first1=Eric A. |last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W. |title=Prevalence of Earth-size planets orbiting Sun-like stars |date=31 October 2013 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |doi=10.1073/pnas.1319909110 |arxiv=1311.6806 |bibcode=2013PNAS..11019273P |volume=110 |issue=48 |pages=19273–19278 |pmid=24191033 |pmc=3845182 |doi-access=free }}</ref><ref name="Space-20130107">{{cite web |author=Staff |title=17 Billion Earth-Size Alien Planets Inhabit Milky Way |url=http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html |date=7 January 2013 |publisher=[[Space.com]] |access-date=8 January 2013 |url-status=live |archive-url=https://web.archive.org/web/20141006095334/http://www.space.com/19157-billions-earth-size-alien-planets-aas221.html |archive-date=6 October 2014  }}</ref> Eleven billion of these estimated planets may be orbiting Sun-like stars.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=https://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=4 November 2013 |work=[[Los Angeles Times]] |access-date=5 November 2013 |url-status=live |archive-url=https://web.archive.org/web/20131106030558/http://www.latimes.com/science/la-sci-earth-like-planets-20131105%2C0%2C2673237.story |archive-date=6 November 2013  }}</ref> The nearest such planet may be 12 light-years away, according to the scientists.<ref name="NYT-20131104" /><ref name="PNAS-20131031" /> However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (see [[Kepler-138]]d).<ref>{{cite web|url=http://www.cfa.harvard.edu/news/2014-01|title=Newfound Planet is Earth-mass But Gassy|date=3 January 2014|website=harvard.edu|access-date=2 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171028063815/https://www.cfa.harvard.edu/news/2014-01|archive-date=28 October 2017}}</ref>

Estimates show that about 80% of potentially habitable worlds are covered by land, and about 20% are ocean planets. Planets with rations more like those of Earth, which was 30% land and 70% ocean, only make up 1% of these worlds.<ref>[https://www.space.com/habitable-rocky-planets-dominated-by-land 'Pale blue dots' like Earth may be rare among habitable worlds]</ref>