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Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations

Abstract : Small planets (~1–3.9 ${R}_{\oplus }$) constitute more than half of the inventory of the 4000-plus exoplanets discovered so far. Smaller planets are sufficiently dense to be rocky, but those with radii larger than ~1.6 ${R}_{\oplus }$ are thought to display in many cases hydrogen/helium gaseous envelopes up to ~30% of the planetary mass. These low-mass planets are highly irradiated and the question of their origin, evolution, and possible links remains open. Here we show that close-in ocean planets affected by the greenhouse effect display hydrospheres in supercritical state, which generate inflated atmospheres without invoking the presence of large hydrogen/helium gaseous envelopes. We present a new set of mass–radius relationships for ocean planets with different compositions and different equilibrium temperatures, which are found to be well adapted to low-density sub-Neptune planets. Our model suggests that super-Earths and water-rich sub-Neptunes could belong to the same family of planets, i.e., hydrogen/helium-free planets, with differences between their interiors simply resulting from the variation in the water content.
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Olivier Mousis, Magali Deleuil, Artyom Aguichine, Emmanuel Marcq, Joseph Naar, et al.. Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations. The Astrophysical journal letters, Bristol : IOP Publishing, 2020, 896 (2), pp.L22. ⟨10.3847/2041-8213/ab9530⟩. ⟨insu-02876788⟩

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