Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars

TL;DR

This study reveals that the key factor distinguishing rocky from water-rich small exoplanets orbiting M dwarfs is their density, not their radius, challenging previous assumptions based on size alone.

Contribution

It introduces a density-based separation between rocky and water-rich exoplanets, supported by analysis of all known small transiting planets around M dwarfs.

Findings

Density gap separates rocky and water-rich planets

Formation models with migration explain observed distributions

Bimodal radius distribution is inconsistent with data

Abstract

Exoplanets smaller than Neptune are common around red dwarf stars (M dwarfs), with those that transit their host star constituting the bulk of known temperate worlds amenable for atmospheric characterization. We analyze the masses and radii of all known small transiting planets around M dwarfs, identifying three populations: rocky, water-rich, and gas-rich. Our results are inconsistent with the previously known bimodal radius distribution arising from atmospheric loss of a hydrogen/helium envelope. Instead, we propose that a density gap separates rocky from water-rich exoplanets. Formation models that include orbital migration can explain the observations: Rocky planets form within the snow line, whereas water-rich worlds form outside it and later migrate inward.