Mega-Earth


A mega-Earth is a massive terrestrial exoplanet that is at least ten times the mass of Earth. Mega-Earths are substantially more massive than super-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. However, it has since been determined to be a typical volatile-rich planet.

Examples

Kepler-10c was the first exoplanet to be classified as a Mega-Earth. At the time of its discovery, it was believed to be around 17 and 2.3, 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 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, 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 and 2.2. 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 Kepler-10c had a Procky of 0.1, and Kepler-131b has a Procky of 0.002.
Kepler-145b is one of the most massive planets classified as Mega-Earths, with a mass of 37.1 and a radius of 2.65, so large that it could belong to a sub-category of Mega-Earths known as Supermassive Terrestrial Planets. It likely has an Earth-like composition of rock and iron without any volatiles. A similar Mega-Earth, K2-66b, is about 21.3 times the mass and 2.49 times the radius of Earth, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere.