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Mega-Earth

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Size comparison of the former Mega-Earth candidate Kepler-10c, with Earth (left) and Neptune (right)

A mega-Earth or massive solid planet[1][2] is a proposed neologism for a massive terrestrial exoplanet that is at least ten times the mass of Earth (M🜨). Mega-Earths would be substantially more massive than super-Earths (terrestrial and ocean planets with masses around 5–10 M🜨). The term "mega-Earth" was coined in 2014, when Kepler-10c was revealed to be a Neptune-mass planet with a density considerably greater than that of Earth.[3] However, it has since been determined to be a typical volatile-rich planet weighing just under half that mass.[4]

Mega-Earths or comparable objects may exist as remnant cores of evaporated gas giants or white dwarfs,[5][6][7] and may also form around massive stars and supermassive black holes as blanets for the latter.[1][8]

Examples

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Kepler-10c was the first exoplanet to be classified as a mega-Earth. At the time of its discovery, it was believed to have a mass around 17 times that of Earth (M🜨) and a radius around 2.3 times Earth's (R🜨), 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 17 M🜨 estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around 7.4 M🜨, making it a typical volatile-rich mini-Neptune and not a mega-Earth.[9][4]

K2-56b, also designated BD+20594b, is a much more likely mega-Earth,[10] with about 16 M🜨 and 2.2 R🜨. 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.[11]

Kepler-145b is one of the most massive planets classified as mega-Earths, with a mass of 37.1 M🜨 and a radius of 2.65 R🜨, 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 21.3 M🜨 and a radius of about 2.49 R🜨, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.[12]

Kepler-277b and Kepler-277c are a pair of planets orbiting the same star, both thought to be mega-Earths with masses of about 87.4 M🜨 and 64.2 M🜨, and radii of about 2.92 R🜨 and 3.36 R🜨, respectively.[13]

PSR J1719−1438 b may be one of the most massive mega-Earths ever known, with a mass of about 330 M🜨 and a radius less than 4 R🜨, 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.[6][14] 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.[15][16]

Origin

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The discovery of mega-Earths had challenged planetary formation theories.[17] Formation mechanisms and the occurrence of such objects remain subjects of ongoing research and debate.[17]

Around massive stars

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A 2007 study had suggested the possibility of hypothetical solid planets up to thousands of M🜨 forming around massive stars (B-type and O-type stars; 5–120 M☉).[1] 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 17 M🜨, Jupiter has 318 M🜨. The most massive of those objects were assumed to be up to approximately 4,000 M🜨 (or 13 MJ) per the said upper mass limit used in the IAU's working definition of an exoplanet.[1] 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 60 MJ.[18]

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.[1] The 2007 study acknowledged that such massive exoplanets are not yet known to exist.[1] More recent research has shown that the ratio of protoplanetary disk mass to stellar mass decreases rapidly for massive stars with initial masses above 10 M☉, falling to less than 10−4.[19] Furthermore, no protoplanetary disks have been observed around O-type stars to date.[19]

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

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Main article: Blanet

See also

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References

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  1. ^ a b c d e f Seager, S.; Kuchner, M.; Hier-Majumder, C. A.; Militzer, B. (2007). "Mass-Radius Relationships for Solid Exoplanets". The Astrophysical Journal. 669 (2): 1279–1297. arXiv:0707.2895. Bibcode:2007ApJ...669.1279S. doi:10.1086/521346. S2CID 8369390.
  2. ^ "'Mega Earth': first strong evidence of massive solid planets". School of Physics & Astronomy. 5 June 2014. Retrieved 22 August 2025.
  3. ^ "Astronomers Find a New Type of Planet: The "Mega-Earth"2014-14". www.cfa.harvard.edu/. 30 May 2014. Archived from the original on 2 June 2014.
  4. ^ a b The mass of Kepler-10c revisited: upping the radial velocities game, Leonardo dos Santos, 7 August 2017, Astrobites
  5. ^ Hébrard, G.; Lecavelier Des Étangs, A. [fr], Vidal-Madjar, A.; Désert, J.-M.; Ferlet, R. (2003), 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.
  6. ^ a b Hirschler, Ben (25 August 2011). "Astronomers discover planet made of diamond". Reuters. Retrieved 25 August 2011.
  7. ^ Lemonick, Michael (26 August 2011). "Scientists Discover a Diamond as Big as a Planet". Time. Archived from the original on 26 August 2011.
  8. ^ Wada, K.; Tsukamoto, Y.; Kokubo, E. (2021). "Formation of "Blanets" from Dust Grains around the Supermassive Black Holes in Galaxies". The Astrophysical Journal. 909 (1): 96. arXiv:2007.15198. Bibcode:2021ApJ...909...96W. doi:10.3847/1538-4357/abd40a. S2CID 220870610.
  9. ^ Rajpaul, V.; Buchhave, L. A.; Aigrain, S. (2017). "Pinning down the mass of Kepler-10c: The importance of sampling and model comparison". Monthly Notices of the Royal Astronomical Society: Letters. 471: L125 – L130. arXiv:1707.06192. doi:10.1093/mnrasl/slx116.
  10. ^ Futó, P (2017). BD+20594B: A Mega-Earth Detected in the C4 field of the Kepler K2 mission (PDF). 48th Lunar and Planetary Science Conference. Retrieved 6 September 2020.
  11. ^ Espinoza, Néstor; Brahm, Rafael; Jordán, Andrés; Jenkins, James S.; Rojas, Felipe; Jofré, Paula; Mädler, Thomas; Rabus, Markus; Chanamé, Julio; Pantoja, Blake; Soto, Maritza G.; Morzinski, Katie M.; Males, Jared R.; Ward-Duong, Kimberly; Close, Laird M. (2016). "Discovery and Validation of a High-Density Sub-Neptune from the K2 Mission". The Astrophysical Journal. 830 (1): 43. arXiv:1601.07608. Bibcode:2016ApJ...830...43E. doi:10.3847/0004-637X/830/1/43.
  12. ^ Futó, P (2018). Kepler-145b and K2-66b: A Kepler- and a K2-Mega-Earth with Different Compositional Characteristics (PDF). 49th Lunar and Planetary Science Conference. Retrieved 6 September 2020.
  13. ^ Futó, P (2020). Kepler-277 b: A Supermassive Terrestrial Exoplanet in the Kepler-277 Planetary System (PDF). 51st Lunar and Planetary Science Conference. Retrieved 6 September 2020.
  14. ^ Bailes, M.; Bates, S. D.; et al. (25 August 2011). "Transformation of a Star into a Planet in a Millisecond Pulsar Binary". Science. 333 (6050): 1717–1720. arXiv:1108.5201. Bibcode:2011Sci...333.1717B. doi:10.1126/science.1208890. PMID 21868629. S2CID 206535504.
  15. ^ Blanchard, C.; Guillemot, L.; Voisin, G.; Cognard, I.; Theureau, G. (2025). "A census of galactic spider binary millisecond pulsars with the Nançay Radio Telescope". Astronomy & Astrophysics. 698: A239. arXiv:2504.10037. Bibcode:2025A&A...698A.239B. doi:10.1051/0004-6361/202453499.
  16. ^ Bailes, M.; Bates, S. D.; Bhalerao, V.; Bhat, N. D. R.; Burgay, M.; Burke-Spolaor, S.; d'Amico, N.; Johnston, S.; et al. (2011). "Transformation of a Star into a Planet in a Millisecond Pulsar Binary". Science. 333 (6050): 1717–20. arXiv:1108.5201. Bibcode:2011Sci...333.1717B. doi:10.1126/science.1208890. PMID 21868629. S2CID 206535504.
  17. ^ a b "Mega-Earth messes with models". Exoplanet Exploration: Planets Beyond our Solar System. Retrieved 22 August 2025.
  18. ^ Schneider, Jean (July 2016). "Exoplanets versus brown dwarfs: the CoRoT view and the future". The CoRoT Legacy Book. p. 157. arXiv:1604.00917. doi:10.1051/978-2-7598-1876-1.c038. ISBN 978-2-7598-1876-1. S2CID 118434022.
  19. ^ a b Williams, Jonathan P.; Cieza, Lucas A. (2011). "Protoplanetary Disks and Their Evolution". Annual Review of Astronomy and Astrophysics. 49 (1): 67–117. arXiv:1103.0556. Bibcode:2011ARA&A..49...67W. doi:10.1146/annurev-astro-081710-102548.

Further reading

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