A Distinct Population of Small Planets: Sub-Earths

TL;DR

This paper identifies a new population of small planets called sub-Earths, which are smaller than Earth and dominate at sizes below 1 Earth radius, suggesting a different formation process from super-Earths.

Contribution

The study provides evidence for a distinct sub-Earth population at sizes below 1 Earth radius, expanding understanding of small planet demographics and formation.

Findings

Sub-Earths peak below 1 Earth radius and increase in occurrence with decreasing size.

Super-Earths peak around 1.4 Earth radii and sharply decline below this size.

Sub-Earths likely formed after gaseous disk dissipation, indicating a different formation history.

Abstract

The sizes of small planets have been known to be bi-modal, with a gap separating planets that have lost their primordial atmospheres (super-Earths), and the ones that retain them (mini-Neptunes). Here, we report evidences for another distinct population at smaller sizes. By focussing on planets orbiting around GK-dwarfs inward of 16 days, and correcting for observational completeness, we find that the number of super-Earths peak around 1.4 Earth radii and disappear shortly below this size. Instead, a new population of planets (sub-Earths) appear to dominate at sizes below ~ 1 Earth radius, with an occurrence that rises 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 proto-planetary disks, have a narrow mass distribution. The 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 towards longer orbital periods, currently invisible, may be the true terrestrial analogues. This strongly motivates new searches.