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A mega-Earth is a proposed neologism for a massive terrestrial exoplanet that is at least ten times the mass of Earth. Mega-Earths would be substantially more massive than super-Earths (terrestrial and ocean planets with masses around 5–10 Earths). 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,^[1] though it has since been determined to be a typical volatile-rich planet weighing just under half that mass.^[2]

Summary regarding formation models, massive stars, blanets

Examples

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_E) 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_E 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_E, making it a typical volatile-rich mini-Neptune and not a mega-Earth.^[3]^[2]

K2-56b, also designated BD+20594b, is a much more likely mega-Earth,^[4] with about 16 M_E 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.^[5]

Kepler-145b is one of the most massive planets classified as mega-Earths, with a mass of 37.1 M_E 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_E 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.^[6]

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_E and 64.2 M_E, and radii of about 2.92 R_🜨 and 3.36 R_🜨, respectively.^[7]

PSR J1719−1438 b is the most massive mega-Earth ever known with a mass of about 330 M_E and a radius less than 4 R_🜨. PSR J1719−1438 b is a pulsar planet which is most likely composed largely of crystalline carbon but with a density far greater than diamond.^[8]^[9]

Formation

Massive solid planets seemingly can also exist, though their formation mechanisms and occurrence remain subjects of ongoing research and debate.

The possibility of solid planets up to thousands of M_E forming around massive stars (B-type and O-type stars; 5–120 M_☉) has been suggested in some earlier studies.^[10] 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_E, Jupiter has 318 M_E, and the 13 M_J limit used in the IAU's working definition of an exoplanet equals approximately 4000 M_E.^[10] Assumed as an upper limit,^[10] also may talk about the limit debate.

However, it is important to note that more recent research has questioned the likelihood of massive solid planet formation around very massive stars.^[11] Studies have shown that the ratio of protoplanetary disk mass to stellar mass decreases rapidly for stars with initial masses above 10 M_☉, falling to less than 10⁻⁴.^[11] Furthermore, no protoplanetary disks have been observed around O-type stars to date.

The original suggestion of massive solid planets forming around 5-120 M_☉ stars, presented in earlier literature, lacks substantial supporting evidence or citations to planetary formation theories.^[10] The study in question primarily focused on simulating mass-radius relationships for rocky planets, including hypothetical super-massive solid planets, but did not investigate whether planetary formation theories support the existence of such objects. The authors of that study acknowledged that "Such massive exoplanets are not yet known to exist."^[10]

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

References

^ "Astronomers Find a New Type of Planet: The "Mega-Earth"2014-14". Archived from the original on 2 June 2014.
^ ^a ^b The mass of Kepler-10c revisited: upping the radial velocities game, Leonardo dos Santos, 7 August 2017, Astrobites
^ 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.
^ 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.
^ 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.
^ 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.
^ 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.
^ Hirschler, Ben (25 August 2011). "Astronomers discover planet made of diamond". Reuters. Retrieved 25 August 2011.
^ 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.
^ ^a ^b ^c ^d ^e Cite error: The named reference seager2007 was invoked but never defined (see the help page).
^ ^a ^b A bot will complete this citation soon. Click here to jump the queue arXiv:1103.0556.

Works cited

Futó, P; Gucsik, A (2018). Basic Mineralogical Models for Silicate- and Carbon-Rich Mega-Earths Considering Compositional and Geophysical Constraints (PDF). 49th Lunar and Planetary Science Conference. Retrieved 6 September 2020.^{[unreliable source?]}