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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, though it has since been determined to be a typical volatile-rich planet weighing just under half that mass.

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.

K2-56b, also designated BD+20594b, is a much more likely mega-Earth, 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.

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.

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.

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.

See also

• Sub-Neptune
• Super-Neptune

References

1. "Astronomers Find a New Type of Planet: The "Mega-Earth"2014-14". Archived from the original on 2 June 2014.
2. ^ The mass of Kepler-10c revisited: upping the radial velocities game, Leonardo dos Santos, 7 August 2017, Astrobites
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. Hirschler, Ben (25 August 2011). "Astronomers discover planet made of diamond". Reuters. Retrieved 25 August 2011.
9. 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.

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.

Further reading

• Astronomers Find "Mega-Earth," Most Massive Rocky Planet Yet, BY MARCUS WOO FOR NATIONAL GEOGRAPHIC, JUNE 5, 2014
• Impossibly heavy planet is the first 'mega-Earth', New Scientist, 2 June 2014, By Jacob Aron
• Kepler space telescope spies a ‘Mega-Earth’, Washington Post, June 2 2014
• 'Godzilla of Earths': Alien Planet 17 Times Heavier Than Our World Discovered, By Miriam Kramer June 02, 2014

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