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

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. The radii of small transiting exoplanets have a bimodal distribution, usually interpreted as planets with and without a thick atmosphere. Luque and Pallé demonstrate that the planets’ densities (which require measurements of both mass and radius) provide a cleaner separation between the populations than radii alone, at least for exoplanets orbiting red dwarf stars (see the Perspective by Teske). They found that one group is consistent with a purely rocky composition, whereas another group matches a model of 50% rock and 50% water. This split between rocky and water-rich planets could reflect where they formed in their planetary systems, before orbital migration. —KTS The density of exoplanets orbiting red dwarf stars separates into rocky and water-rich populations.