Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars
arXiv:2209.03871 · doi:10.1126/science.abl7164
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.
arXiv commentSubmitted 30 July 2021; published 9 September 2022. Supplementary Materials are available at https://www.science.org/doi/10.1126/science.abl7164
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