terrestrial planet sub-Earth The first sub-Earths were discovered by the Kepler Orbiting Observatory space telescope on 2012-01-10, orbiting the 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 Earth radii. In addition, there are over 310 planet candidates with an estimated radius of less than 1 Earth radius, with 135 of them being smaller than 0.8 Earth radii. A terrestrial planet which has a mass smaller than Earth's, such that 1) it lacks a magnetic field strong enough to shield its atmosphere from erosion by stellar wind and 2) it has comparatively short periods of geological activity unless it is subject to gravitational tidal forces from a much more massive object. envoAstro