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Sub-Earth

Planet smaller than Earth

Asub-Earth is a planet "substantially less massive" than Earth and Venus.^[1] In the Solar System, this category includes Mercury and Mars. Sub-Earth exoplanets are among the most difficult type to detect because their small sizes and masses produce the weakest signal. Despite the difficulty, one of the first exoplanets found was a sub-Earth around a millisecond pulsar PSR B1257+12. The smallest known is WD 1145+017 b with a size of 0.15 Earth radii, or somewhat smaller than Pluto. However, WD 1145+017 b is not massive enough to qualify as a sub-Earth classical planet and is instead defined as a minor, or dwarf, planet.^[2] It is orbiting within a thick cloud of dust and gas as chunks of itself continually break off to then spiral in towards the star, and within around 5,000 years it will have more-or-less disintegrated.^[3]

The Kepler space telescope opened up a new realm of sub-Earth discoveries. On January 10, 2012, Kepler discovered the first three sub-Earths around an ordinary star, Kepler-42. As of June 2014, Kepler has 45 confirmed planets that are smaller than Earth, with 17 of them being smaller than 0.8 R_ⴲ. In addition, there are over 310 planet candidates with an estimated radius of <1 R_ⴲ, with 135 of them being smaller than 0.8 R_ⴲ.^[1][4]

There is suspected to be a sub-Earth orbiting Proxima Centauri, the closest star to the Sun.^[5] The mass of Proxima d is believed to be between that of Mars and Venus.^[6]

Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away their atmospheres.^[1] Due to their small sizes, and unless there are significant tidal forces when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.

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