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

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.