{{short description|Supermassive terrestrial planet}} [[File:TOI-1853 b size comparison.png|upright=1.5|thumb|Size comparison of Earth and Neptune with the mega-Earth TOI-1853 b (center), a very dense Neptune-sized exoplanet believed to be nearly entirely made of solid rock, metal, and water.<ref name="Naponiello2023"/>]]
A '''mega-Earth''' or '''massive solid planet'''<ref name="seager2007"/><ref name=sch>{{cite web |url=https://www.ph.ed.ac.uk/news/mega-earth-05-06-14 |title='Mega Earth': first strong evidence of massive solid planets |work=School of Physics & Astronomy |date=5 June 2014 |access-date=August 22, 2025}}</ref> is type of terrestrial exoplanet that is very massive and dense — more massive than super-Earths. The term "mega-Earth" was coined in 2014,<ref name="Kepler-10c"/> though it remained an informal category until a quantitative definition was proposed for it in 2026.<ref name="Kroft2026"/> Based on the measured radii and densities of known exoplanets {{asof|March 2026|lc=y}}, mega-Earths appear to be a distinct category of exoplanets that are between 2.1 and 5.0 Earth radii ({{Earth radius}}) and have densities higher than Earth's ({{val|5.5|u=g/cm3}}).<ref name="Kroft2026"/>
Based on their observed high densities, mega-Earths are inferred to be largely made of solid material,<ref name="Kroft2026"/> such as rock, metal, and ice.<ref name="Naponiello2023"/> If the mega-Earth is rich in volatiles like water, it may harbor a supercritical ocean under a thin atmosphere of hydrogen, helium, and other gases.<ref name="Naponiello2023"/> The existence of mega-Earths challenges conventional theories for planetary formation, as massive planets should accrete large amounts of gas in addition to solid material from the host star's protoplanetary disk.<ref name="Naponiello2023"/> It has been hypothesized that mega-Earths may either be remnant cores of evaporated gas giants or white dwarfs<ref name="hebrard"/><ref name="Reuters"/><ref name="time-diamond"/> or products of consecutive collisions between super-Earths.<ref name="Naponiello2023"/> Mega-Earths as evaporated gas giants have been theorized to occur frequently around luminous massive stars and supermassive black holes.<ref name="seager2007"/><ref name="Wada2020" />
==Examples== [[File:Mega-Earth radius-density plot 2026.png|thumb|upright=1.5|Scatter plot of measured radii and densities of known exoplanets {{asof|March 2026|lc=y}}. Mega-Earths (red points) have radii smaller than that of Neptune ({{val|2.1|-|5.0|ul=Earth radius}}) and have densities higher than that of Earth (>{{val|5.5|u=g/cm3}}).]]
Kepler-10c was the first exoplanet to be classified as a mega-Earth.<ref name="Kepler-10c"/> At the time of its discovery, it was believed to have a mass around 17 times that of Earth ({{Earth mass|sym=y|link=yes}}) and a radius around 2.3 times Earth's ({{Earth radius|sym=y|link=yes}}), giving it a high density that implied a mainly rocky composition. However, several follow-up radial velocity studies produced different results for Kepler-10c's mass, all much below the original {{Earth mass|sym=y|17}} estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around {{Earth mass|sym=y|7.4}}, making it a typical volatile-rich mini-Neptune and not a mega-Earth.<ref name="Kepler-10c-new"/><ref name="upping"/>
K2-56b, also designated BD+20°594b, is a much more likely mega-Earth,<ref>{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2017/pdf/1078.pdf |title=BD+20594B: A Mega-Earth Detected in the C4 field of the Kepler K2 mission |first=P |last=Futó |year=2017 |conference=48th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2017/ |language=en |access-date=6 September 2020}}</ref> with about {{Earth mass|sym=y|16}} and {{Earth radius|sym=y|2.2}}. At the time of its discovery in 2016, it had the highest chance of being rocky for a planet its size, with a posterior probability that it is dense enough to be terrestrial at about 0.43. For comparison, at the time the corresponding probability for Kepler-10c was calculated as 0.1, and as 0.002 for Kepler-131b.<ref name="Espinoza2016"/>
{{ill|Kepler-145b|fr|Kepler-145 b|preserve=yes}} is one of the most massive planets classified as mega-Earths, with a mass of {{Earth mass|sym=y|37.1}} and a radius of {{Earth radius|sym=y|2.65}}, so large that it could belong to a sub-category of mega-Earths known as "supermassive terrestrial planets" (SMTP). It likely has an Earth-like composition of rock and iron without any volatiles. A similar mega-Earth, K2-66b, has a mass of about {{Earth mass|sym=y|21.3}} and a radius of about {{Earth radius|sym=y|2.49}}, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.<ref name="Futo2018"/>
Kepler-277b and Kepler-277c are a pair of planets orbiting the same star, both thought to be mega-Earths with masses of about {{Earth mass|sym=y|87.4}} and {{Earth mass|sym=y|64.2}}, and radii of about {{Earth radius|sym=y|2.92}} and {{Earth radius|sym=y|3.36}}, respectively.<ref name="Futo2020"/>
PSR J1719−1438 b may be one of the most massive mega-Earths ever known, with a mass of about {{Earth mass|sym=y|330}} and a radius less than {{Earth radius|sym=y|4}}, slightly more massive but smaller than Jupiter. It is a pulsar planet which is most likely composed largely of crystalline carbon but with a density far greater than diamond.<ref name="Reuters" /><ref name=":0"/> However, as it is a likely remnant core of a former white dwarf companion of PSR J1719−1438, it is instead considered an ultra-low-mass carbon white dwarf or object per some definitions.<ref name="Blanchard2025"/><ref name=":0"/>
A 2026 study defined a mega-Earth to be a planet with a radius between 2.1 and 5 Earth radii and a density greater than {{val|5.5|u=g/cm3}}, and listed 13 "confirmed" examples with well-measured radii and densities: K2-263 b, HD 88986 b, HD 207897 b, Kepler-538 b, HIP 97166 b, TOI-2093 c, GJ 143b, TOI-815 c, K2-292 b, GJ 523b, TOI-332 b, TOI-1853 b, and Kepler-411 b.<ref name="Kroft2026"/> The 2.1 radius limit is chosen to be above the small planet radius gap that separates super-Earths from sub-Neptunes.<ref name="Kroft2026"/>
==Origin== {{Missing information|date=March 2026|planetary collisions}} The discovery of mega-Earths had challenged planetary formation theories.<ref name=NASA_models/> Formation mechanisms and the occurrence of such objects remain subjects of ongoing research and debate.<ref name=NASA_models>{{cite web |url=https://exoplanets.nasa.gov/news/160/mega-earth-messes-with-models/ |title=Mega-Earth messes with models |work=Exoplanet Exploration: Planets Beyond our Solar System |access-date=2025-08-22|url-status=dead|archive-url=https://web.archive.org/web/20240423114952/https://exoplanets.nasa.gov/news/160/mega-earth-messes-with-models/|archive-date=2024-04-23}}</ref>
===Around massive stars=== A 2007 study had suggested the possibility of hypothetical solid planets up to {{Earth mass|sym=y|thousands of}} forming around massive stars (B-type and O-type stars; {{solar mass|5–120|link=y}}).<ref name="seager2007"/> The hypothesis proposed that the protoplanetary disk around such stars would contain enough heavy elements, and that high UV radiation and strong winds could photoevaporate the gas in the disk, leaving just the heavy elements. For comparison, Neptune's mass equals {{Earth mass|sym=y|17}}, Jupiter has {{Earth mass|sym=y|318}}. The most massive of those objects were assumed to be up to approximately {{Earth mass|sym=y|4,000}} (or {{Jupiter mass|13}}) per the said upper mass limit used in the IAU's working definition of an exoplanet.<ref name="seager2007"/> However, this limit has been debated due to no precise physical significance, with many exoplanet catalogs including objects with heavier masses, such as up to {{jupiter mass|60}}.<ref>{{cite book |last=Schneider |first=Jean |arxiv=1604.00917 |chapter=Exoplanets versus brown dwarfs: the CoRoT view and the future |title=The CoRoT Legacy Book |date=July 2016 |page=157 |doi=10.1051/978-2-7598-1876-1.c038 |isbn=978-2-7598-1876-1|s2cid=118434022 }}</ref>
Despite the suggestion of the possibility of massive solid planets, it lacks supporting evidence for planetary formation theories and was primarily based on simulating mass-radius relationships for rocky planets, without investigating whether planetary formation theories support the existence of such objects.<ref name="seager2007"/> The 2007 study acknowledged that such massive exoplanets are not yet known to exist.<ref name="seager2007"/> More recent research has shown that the ratio of protoplanetary disk mass to stellar mass decreases rapidly for massive stars with initial masses above {{solar mass|10}}, falling to less than {{val|e=-4}}.<ref name=jonathan2011>{{cite journal|last1=Williams |first1=Jonathan P. |last2=Cieza |first2=Lucas A. |title=Protoplanetary Disks and Their Evolution |journal=Annual Review of Astronomy and Astrophysics |date=2011 |volume=49 |issue=1 |pages=67–117 |doi=10.1146/annurev-astro-081710-102548 |arxiv=1103.0556 |bibcode=2011ARA&A..49...67W }}</ref> Furthermore, no protoplanetary disks have been observed around O-type stars to date.<ref name=jonathan2011/>
Given these considerations, the formation and existence of massive solid planets around massive stars remain speculative and require further research and observational evidence.
===Around supermassive black holes=== {{Main|Blanet}}
==See also== * Super-Neptune
==References== <references>
<ref name="Kepler-10c">{{cite web|url=https://www.cfa.harvard.edu/news/2014-14|title=Astronomers Find a New Type of Planet: The "Mega-Earth"2014-14|work=www.cfa.harvard.edu/ |date=30 May 2014 |archive-url=https://web.archive.org/web/20140602204141/https://www.cfa.harvard.edu/news/2014-14|archive-date=2 June 2014}}</ref>
<ref name="upping">[https://astrobites.org/2017/08/07/the-mass-of-kepler-10c-revisited-upping-the-radial-velocities-game/ The mass of Kepler-10c revisited: upping the radial velocities game], Leonardo dos Santos, 7 August 2017, Astrobites</ref>
<ref name="Kepler-10c-new">{{cite journal | arxiv=1707.06192 | doi=10.1093/mnrasl/slx116 | doi-access=free | title=Pinning down the mass of Kepler-10c: The importance of sampling and model comparison | date=2017 | last1=Rajpaul | first1=V. | last2=Buchhave | first2=L. A. | last3=Aigrain | first3=S. | journal=Monthly Notices of the Royal Astronomical Society: Letters | volume=471 | pages=L125–L130 }}</ref>
<ref name="seager2007">{{Cite journal | doi = 10.1086/521346| title = Mass-Radius Relationships for Solid Exoplanets| journal = The Astrophysical Journal| volume = 669| issue = 2| pages = 1279–1297| year = 2007| last1 = Seager | first1 = S.| last2 = Kuchner | first2 = M.| last3 = Hier-Majumder | first3 = C. A.| last4 = Militzer | first4 = B. | bibcode=2007ApJ...669.1279S|arxiv = 0707.2895 | s2cid = 8369390}}</ref>
<ref name="hebrard">Hébrard, G.; {{ill|lt=Lecavelier Des Étangs, A.|Alain Lecavelier des Étangs|fr}}, Vidal-Madjar, A.; Désert, J.-M.; Ferlet, R. (2003), [https://arxiv.org/abs/astro-ph/0312384 ''Evaporation Rate of Hot Jupiters and Formation of chthonian Planets''], Extrasolar Planets: Today and Tomorrow, ASP Conference Proceedings, Vol. 321, held 30 June – 4 July 2003, Institut d'astrophysique de Paris, France. Edited by Jean-Philippe Beaulieu, Alain Lecavelier des Étangs and Caroline Terquem.</ref>
<ref name=":0">{{cite journal |last1=Bailes |first1=M. |last2=Bates |first2=S. D. |last3=Bhalerao |first3=V. |last4=Bhat |first4=N. D. R. |last5=Burgay |first5=M. |last6=Burke-Spolaor |first6=S. |last7=d'Amico |first7=N. |last8=Johnston |first8=S. |last9=Keith |first9=M. J. |last10=Kramer |first10=M. |last11=Kulkarni |first11=S. R. |display-authors=2 |date=August 25, 2011 |title=Transformation of a Star into a Planet in a Millisecond Pulsar Binary |url=https://www.science.org/doi/10.1126/science.1208890 |journal=Science |volume=333 |issue=6050 |pages=1717–1720 |arxiv=1108.5201 |bibcode=2011Sci...333.1717B |doi=10.1126/science.1208890 |pmid=21868629 |last12=Levin |first12=L. |last13=Lyne |first13=A. G. |last14=Milia |first14=S. |last15=Possenti |first15=A. |last16=Spitler |first16=L. |last17=Stappers |first17=B. |last18=Van Straten |first18=W. |s2cid=206535504}}</ref>
<ref name="time-diamond">{{cite magazine|magazine=Time|url=http://www.time.com/time/health/article/0,8599,2090471,00.html|archive-url=https://web.archive.org/web/20110826125732/http://www.time.com/time/health/article/0,8599,2090471,00.html|url-status=dead|archive-date=26 August 2011|title=Scientists Discover a Diamond as Big as a Planet|first=Michael|last=Lemonick|date=26 August 2011}}</ref>
<ref name="Reuters">{{cite news |last=Hirschler |first=Ben |date=August 25, 2011 |title=Astronomers discover planet made of diamond |work=Reuters |url=https://www.reuters.com/article/us-planet-diamond-idUSTRE77O69A20110825 |access-date=August 25, 2011}}</ref>
<ref name="Espinoza2016">{{cite journal | arxiv=1601.07608 | doi=10.3847/0004-637X/830/1/43 | doi-access=free | title=Discovery and Validation of a High-Density Sub-Neptune from the K2 Mission | date=2016 | last1=Espinoza | first1=Néstor | last2=Brahm | first2=Rafael | last3=Jordán | first3=Andrés | last4=Jenkins | first4=James S. | last5=Rojas | first5=Felipe | last6=Jofré | first6=Paula | last7=Mädler | first7=Thomas | last8=Rabus | first8=Markus | last9=Chanamé | first9=Julio | last10=Pantoja | first10=Blake | last11=Soto | first11=Maritza G. | last12=Morzinski | first12=Katie M. | last13=Males | first13=Jared R. | last14=Ward-Duong | first14=Kimberly | last15=Close | first15=Laird M. | journal=The Astrophysical Journal | volume=830 | issue=1 | page=43 | bibcode=2016ApJ...830...43E }}</ref>
<ref name="Futo2018">{{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2018/pdf/1224.pdf |title=Kepler-145b and K2-66b: A Kepler- and a K2-Mega-Earth with Different Compositional Characteristics |first=P |last=Futó |year=2018 |conference=49th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2018/ |language=en |access-date=6 September 2020}}</ref>
<ref name="Futo2020">{{cite conference |url=http://www.hou.usra.edu/meetings/lpsc2020/pdf/1055.pdf |title=Kepler-277 b: A Supermassive Terrestrial Exoplanet in the Kepler-277 Planetary System |first=P |last=Futó |year=2020 |conference=51st Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2020/ |language=en |access-date=6 September 2020}}</ref>
<ref name="Wada2020">{{cite journal |last1= Wada|first1= K.|last2= Tsukamoto|first2= Y.|last3= Kokubo|first3= E.|title= Formation of "Blanets" from Dust Grains around the Supermassive Black Holes in Galaxies|journal= The Astrophysical Journal|year= 2021|volume= 909|issue= 1|page= 96|doi= 10.3847/1538-4357/abd40a|arxiv=2007.15198|bibcode= 2021ApJ...909...96W|s2cid= 220870610|doi-access= free}}</ref>
<ref name="Naponiello2023">{{cite journal |display-authors = etal |first1 = Luca |last1 = Naponiello |first2 = Luigi |last2 = Mancini |first3 = Alessandro |last3 = Sozzetti |first4 = Aldo S. |last4 = Bonomo |first5 = Alessandro |last5 = Morbidelli |first6 = Jingyao |last6 = Dou |title = A super-massive Neptune-sized planet |journal = Nature |date = 2023-08-30 |volume = 622 |issue = 7982 |pages = 255–260 |doi = 10.1038/s41586-023-06499-2 |arxiv = 2309.01464 |bibcode = 2023Natur.622..255N}}</ref>
<ref name="Blanchard2025">{{cite journal|last1=Blanchard |first1=C. |last2=Guillemot |first2=L. |last3=Voisin |first3=G. |last4=Cognard |first4=I. |last5=Theureau |first5=G. |title=A census of galactic spider binary millisecond pulsars with the Nançay Radio Telescope |journal=Astronomy & Astrophysics |date=2025 |volume=698 |pages=A239 |doi=10.1051/0004-6361/202453499 |arxiv=2504.10037 |bibcode=2025A&A...698A.239B }}</ref>
<ref name="Kroft2026">{{cite journal |display-authors = etal |first1 = Maxwell A. |last1 = Kroft |first2 = Thomas G. |last2 = Beatty |first3 = Joseph M. |last3 = Salzer |first4 = Claire |last4 = Zwicker |first5 = Juliette |last5 = Becker |first6 = Jingyao |last6 = Dou |title = GJ 523b is a Massive, 170 Myr-old Mega-Earth, Likely on a Polar Orbit |journal = Astronomical Journal |date = 2026-03-25 |volume = forthcoming |issue = |pages = |doi-access = |doi = |arxiv = 2603.24682 |bibcode = }}</ref>
</references>
==Further reading== * {{cite conference |url=https://www.hou.usra.edu/meetings/lpsc2018/pdf/1453.pdf |title=Basic Mineralogical Models for Silicate- and Carbon-Rich Mega-Earths Considering Compositional and Geophysical Constraints |first1=P |last1=Futó |first2=A |last2=Gucsik |year=2018 |conference=49th Lunar and Planetary Science Conference |conference-url=https://www.hou.usra.edu/meetings/lpsc2018/ |language=en |access-date=6 September 2020}}{{rs?|date=July 2021}}
==External links== *[https://www.nationalgeographic.com/adventure/article/140603-massive-rocky-planet-mega-earth-astronomy-science Astronomers Find "Mega-Earth," Most Massive Rocky Planet Yet], BY MARCUS WOO FOR NATIONAL GEOGRAPHIC, JUNE 5, 2014 *[https://www.newscientist.com/article/dn25663-impossibly-heavy-planet-is-the-first-mega-earth/ Impossibly heavy planet is the first 'mega-Earth'], New Scientist, 2 June 2014, By Jacob Aron *[https://www.washingtonpost.com/national/health-science/kepler-space-telescope-spies-a-mega-earth/2014/06/02/b73cf006-e8e7-11e3-afc6-a1dd9407abcf_story.html Kepler space telescope spies a ‘Mega-Earth’], Washington Post, June 2 2014 *[https://www.space.com/26085-godzilla-mega-earth-kepler-10c-aas224.html 'Godzilla of Earths': Alien Planet 17 Times Heavier Than Our World Discovered], By Miriam Kramer June 02, 2014
{{exoplanet}}
{{Use dmy dates|date=June 2020}}
Category:Mega-Earths * Category:Types of planet Category:Giant planets