Sub-Earth (also known as subterrestrial planet (STEP)) is a classification of planets less massive than Earth and Venus. In the Solar System, this category includes Mercury and Mars. Sub-Earths are the most difficult planet 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 pulsarPSR B1257+12. Kepler opened the realm of sub-Earths by discovering them. On January 10, 2012 Kepler discovered the first three sub-Earths around an ordinary star Kepler-42. As of April 2013, Kepler has confirmed seven sub-Earth exoplanets as well as 36 candidates.
Sub-Earths commonly lack substantial atmospheres because of their low gravity and weak magnetic fields, allowing stellar radiation to wear away the atmospheres. Due to their small sizes, and unless there's significant tidal forces when orbiting close to the parent star, sub-Earths also have short periods of geologic activity.