A mega-Earth is a proposed neologism for a massive terrestrial

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

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 Kepler-10c is an exoplanet orbiting the G-type star Kepler-10, located around 608 light-years away in Draco. Its discovery was announced by Kepler in May 2011, although it had been seen as a planetary candidate since January 2011, when Kepler-1 ...

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 () 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 A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made ...

and not a mega-Earth.The mass of Kepler-10c revisited: upping the radial velocities game
Leonardo dos Santos, 7 August 2017, astrobites K2-56b, also designated

BD+20594b BD+2 0594b (also known as K2-56b) is a massive exoplanet discovered by the Kepler spacecraft in collaboration with the HARPS spectrometer at La Silla in Chile. Naming BD+20 594b indicates that the planet circles a star found in the Bonner Durchm ...

, is a much more likely mega-Earth, 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. 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 K2-66b is a confirmed mega-Earth orbiting the subgiant K2-66, about form Earth in the direction of Aquarius. It is an extremely hot and dense planet heavier than Neptune, but with only about half its radius. Planet properties Mass, radius, ...

, 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.

Kepler-277b Kepler-277b (also known by its Kepler Objects of Interest designation KOI-1215.01) is the second most massive and third-largest rocky planet ever discovered, with a mass close to that of Saturn. Discovered in 2014 by the Kepler Space Telescope, Kep ...

and

Kepler-277c Kepler-277c (also known by its Kepler Objects of Interest designation KOI-1215.02) is the third most massive and second-largest rocky planet ever discovered, with a mass about 64 times that of Earth. Discovered in 2014 by the Kepler Space Telescope ...

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.

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