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Super Earth

A broad classification used for extrasolar planets that are larger and more massive than Earth, but not so massive that the belong to the gas giant class. In general, super Earths (often hyphenated as 'super-Earths') can range up to about ten times Earth's diameter and up to about fifteen times its mass.

The term does not necessarily imply that a planet is terrestrial in nature, but merely that it falls within a certain range of mass or size, though for planets with lower, more Earth-like, values for these factors, the planet will likely be mainly composed of rock or ice. At the larger and more massive end of the scale, 'super Earths' are thought more likely to be small gas planets, of a kind often called 'mini-Neptunes' (or occasionally 'gas dwarfs'), smaller versions of the gas giants and ice giants of the Solar System. At the other end of the scale, Earth-like planets that are similar in size and mass to Earth itself, or smaller, are commonly termed 'Earth-mass planets'.

The Solar System contains no planets that might be classified as super Earths, but planets of this kind appear to be relatively common around other stars. The closest example to the Sun is potentially in orbit around its nearest neighbour in space, Proxima Centauri, which appears to have a planet, Proxima c, that has some seven times the mass of Earth in an orbit comparable to that of Mars around the Sun. A similar planet is suspected in a close orbit of Wolf 359, a red dwarf 7.9 light years from the Sun, which another nearby super Earth is to be found in the Lalande 21185 system, where a planet some 2.6 times the mass of Earth follows a very close orbit around its star.

Though no super Earths are to be found in the Solar System, the fact that so many planets of this kind have been identified within just a few light years of the Solar System shows just how common they must be. To date, the total number of super Earths to be discovered approaches 1,600 (that is, about a third of all confirmed exoplanets) and is growing all the time.


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