A mega-Earth is a proposed

neologism In linguistics, a neologism (; also known as a coinage) is any newly formed word, term, or phrase that has achieved popular or institutional recognition and is becoming accepted into mainstream language. Most definitively, a word can be considered ...

for a massive terrestrial

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

that is at least ten times the mass of

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

. Mega-Earths would be substantially more massive than

super-Earth A super-Earth is a type of exoplanet with a mass higher than Earth, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17.1 times Earth's, respectively. The term "super-Earth" refers only to t ...

s (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 Extrasolar planet, exoplanet orbiting the Stellar classification#Class G, G-type star Kepler-10, located around 608 light-years away in Draco (constellation), Draco. Its discovery was announced by the Kepler space telescope team ...

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 o ...

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+ 20''°'' 594b (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 ...

, 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 from Earth in the direction of Aquarius (constellation), Aquarius. It is an extremely hot and dense planet heavier than Neptune, but with only about half its radius. Planet pr ...

, 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, Ke ...

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.

PSR J1719−1438 b PSR J1719−1438 b is an extrasolar planet that was discovered on August 25, 2011, in orbit around PSR J1719−1438, a millisecond pulsar. The pulsar planet is most likely composed largely of crystalline carbon but with a density far greater tha ...

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 Pulsar planets are planets that are orbiting pulsars. The first such planets to be discovered were around a millisecond pulsar in 1992 and were the first extrasolar planets to be confirmed as discovered. Pulsars are extremely precise clocks and ...

which is most likely composed largely of crystalline carbon but with a

density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...

far greater than

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...

Examples

See also

References

Works cited

Further reading