Mega-Earth Explained

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.

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 and a radius around 2.3 times Earth's, 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 estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around, making it a typical volatile-rich mini-Neptune and not a mega-Earth.[2] [3]

K2-56b, also designated BD+20594b, is a much more likely mega-Earth,[4] with about and . 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 and a radius of, 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 and a radius of about, 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 and, and radii of about and, respectively.[7]

PSR J1719−1438 b is the most massive mega-Earth ever known with a mass of about and a radius less than . 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]

See also

References

  1. Web site: Astronomers Find a New Type of Planet: The "Mega-Earth"2014-14. https://web.archive.org/web/20140602204141/https://www.cfa.harvard.edu/news/2014-14. 2 June 2014.
  2. 1707.06192 . 10.1093/mnrasl/slx116 . free . Pinning down the mass of Kepler-10c: The importance of sampling and model comparison . 2017 . Rajpaul . V. . Buchhave . L. A. . Aigrain . S. . Monthly Notices of the Royal Astronomical Society: Letters . 471 . L125–L130 .
  3. 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
  4. BD+20594B: A Mega-Earth Detected in the C4 field of the Kepler K2 mission . P . Futó . 2017 . 48th Lunar and Planetary Science Conference . https://www.hou.usra.edu/meetings/lpsc2017/ . en . 6 September 2020.
  5. 1601.07608 . 10.3847/0004-637X/830/1/43 . free . Discovery and Validation of a High-Density Sub-Neptune from the K2 Mission . 2016 . 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. . The Astrophysical Journal . 830 . 1 . 43 . 2016ApJ...830...43E .
  6. Kepler-145b and K2-66b: A Kepler- and a K2-Mega-Earth with Different Compositional Characteristics . P . Futó . 2018 . 49th Lunar and Planetary Science Conference . https://www.hou.usra.edu/meetings/lpsc2018/ . en . 6 September 2020.
  7. Kepler-277 b: A Supermassive Terrestrial Exoplanet in the Kepler-277 Planetary System . P . Futó . 2020 . 51st Lunar and Planetary Science Conference . https://www.hou.usra.edu/meetings/lpsc2020/ . en . 6 September 2020.
  8. News: Hirschler . Ben . August 25, 2011 . Astronomers discover planet made of diamond . . August 25, 2011.
  9. Bailes . M. . Bates . S. D. . Bhalerao . V. . Bhat . N. D. R. . Burgay . M. . Burke-Spolaor . S. . d'Amico . N. . Johnston . S. . Keith . M. J. . Kramer . M. . Kulkarni . S. R. . 2 . August 25, 2011 . Transformation of a Star into a Planet in a Millisecond Pulsar Binary . Science . 333 . 6050 . 1717–1720 . 1108.5201 . 2011Sci...333.1717B . 10.1126/science.1208890 . 21868629 . Levin . L. . Lyne . A. G. . Milia . S. . Possenti . A. . Spitler . L. . Stappers . B. . Van Straten . W. . 206535504.

Works cited

Further reading

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