Are sub-Neptunes irradiated ocean planets?

Small planets (1-3.9 Rearth) 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 Rearth 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 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.