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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2020
- DOI:
- 10.3847/2041-8213/ab9530
- arXiv:
- arXiv:2002.05243
- Bibcode:
- 2020ApJ...896L..22M
- Keywords:
-
- Exoplanets;
- Hydrosphere;
- Planetary interior;
- Planetary theory;
- Exoplanet astronomy;
- Exoplanet structure;
- 498;
- 770;
- 1248;
- 1258;
- 486;
- 495;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- Accepted for publication in The Astrophysical Journal Letters