Water-rich planets should be ubiquitous in the universe, and could represent a notable fraction of the sub-Neptune population. Among the detected exoplanets that have been characterized as sub-Neptunes, many are subject to important irradiation from their host star. As a consequence, hydrospheres of such planets are not in condensed phase, but are rather in supercritical state, with steam atmospheres on top. Such irradiated ocean planets (IOP) are good candidates to explain the distribution of masses and radii in the sub-Neptune category of exoplanets.
Here, we present a state-of-the-art coupled interior-atmosphere model [1] that computes the structure of water-rich planets that have high equilibrium temperatures. The interior model model accounts for several refractory layers (iron core and rocky mantle), and on top of them an hydrosphere with an updated equation of state (EOS) that remains valid even when water is in plasma state. The atmosphere model connects the top of the interior model with the host star by solving the equations of radiative transfer.
Our model produces a new set of mass-radius relationships that can be used to characterize exoplanets by computing possible water mass fractions (WMF). We do so for the GJ 9827 system as a test case, and find that planets b and c are compatible with Earth- and Venus-like interiors, respectively. Planet d could be an irradiated ocean planet with a water mass fraction of ?20 ± 10%.
Due to their high irradiation temperatures, sub-Neptunes are expected to be subject to strong atmospheric escape. We also investigate how this argument can help breaking the degeneracy between H2-He dominated envelopes and water worlds.
[1] Aguichine, A., Mousis, O., Deleuil, M., et al. 2021, ApJ, 914, 84A.