A Distinct Population of Small Planets: Sub-Earths

Abstract

Abstract The sizes of small planets are known to be bimodal, with a gap separating planets that have lost their primordial atmospheres (super-Earths) and the ones that retain them (mini-Neptunes). Here, we report evidence for another distinct population at smaller sizes. By focusing on planets orbiting around GK dwarfs inward of 16 days and correcting for observational completeness, we find that the number of super-Earths peaks around 1.4 Earth radii and disappears shortly below this size. Instead, a new population of planets (sub-Earths) appears to dominate at sizes below ∼1 Earth radius, with an occurrence that increases with decreasing size. This pattern is also observed in ultra-short-period planets. The end of super-Earths supports earlier claims that super-Earths and mini-Neptunes, planets that likely form in gaseous protoplanetary disks, have a narrow mass distribution. Sub-Earths, in contrast, can be described by a power-law mass distribution and may be explained by the theory of terrestrial planet formation. We therefore speculate that they are formed well after the gaseous disks have dissipated. The extension of these sub-Earths toward longer orbital periods, currently invisible, may be the true terrestrial analogs. This strongly motivates new searches.