Sub-Earth

Comparing the size of Earth, Mars, and exoplanets of Kepler-20 and Kepler-42.

A sub-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 Kepler Orbiting Observatory space telescope, opened the realm of sub-Earths by its discovery of them. 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 <1R_ⴲ, with 135 of them being smaller than 0.8 R_ⴲ.^[1][2]

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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