Exoplanets & Exomoons

Density, Not Radius, Separates Rocky And Water-rich Small Planets Orbiting M Dwarf Stars

By Keith Cowing
September 8, 2022
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Density, Not Radius, Separates Rocky And Water-rich Small Planets Orbiting M Dwarf Stars
Mass-radius (A) and mass-density (B) diagrams for small transiting planets around M dwarfs (STPMs). Numerical values are provided in Data S1 and include nine planets with revised masses and radii (Table S8). Error bars indicate 1σ uncertainties of individual measurements. In both panels, two theoretical composition models (14) are plotted: an Earth-like composition (32.5% iron mass fraction and 67.5% silicates, green curve) and a planet consisting of 50% water-dominated ices and 50% silicates (blue curve). In (A), planets are colour-coded by their equilibrium temperature Teq. In both panels, light grey points are planets with mass or radius determinations worse than our thresholds of 25% and 8%, respectively, so are not included in the subsequent analysis. In (B), densities are normalised by the Earth-like model and planets are colour-coded according to their characteristic bulk densities: rocky planets (brown), water worlds (light blue), and puffy sub-Neptunes (dark blue). The vertical dashed lines mark the 2M⊕ lower limit for water worlds (light blue) and 6M⊕ lower limit for puffy sub-Neptunes (dark blue). For reference, Earth is shown with a green ⊕ symbol

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.

R. Luque, E. Pallé

Comments: Submitted 30 July 2021; published 9 September 2022. Supplementary Materials are available at this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2209.03871 [astro-ph.EP]
(or arXiv:2209.03871v1 [astro-ph.EP] for this version)
Journal reference: Science 377, 6611 (2022)
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Submission history
From: Rafael Luque
[v1] Thu, 8 Sep 2022 15:19:08 UTC (13,772 KB)
Full paper: https://arxiv.org/abs/2209.03871

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