Mass–Radius Relationships for Irradiated Ocean Planets

• Published in: The Astrophysical journal Vol. 914; no. 2; pp. 84 - 97
• Main Authors: Aguichine, Artyom, Mousis, Olivier, Deleuil, Magali, Marcq, Emmanuel
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.06.2021; IOP Publishing; American Astronomical Society
• Subjects: Astrophysics; Atmospheric models; Computational methods; Equations of state; Exoplanet astronomy; Exoplanet structure; Exoplanets; Extrasolar planets; Hydrosphere; Irradiation; Oceans; Planetary atmospheres; Planetary composition; Planetary interior; Planetary theory; Planets; Radiative transfer; Sciences of the Universe; Stellar planets; Venus; Water masses; Computational methods; Exoplanet astronomy; Planetary theory; Exoplanet structure; Planetary interior; Hydrosphere; Exoplanets
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abfa99

Massive and water-rich planets should be ubiquitous in the universe. Many of these worlds are expected to be subject to important irradiation from their host star, and display supercritical water layers surrounded by extended steam atmospheres. Irradiated ocean planets with such inflated hydrospheres have been recently shown to be good candidates for matching the mass–radius distribution of sub-Neptunes. Here we describe a model that computes a realistic structure for water-rich planets by combining an interior model with an updated equation of state (EOS) for water, and an atmospheric model that takes into account radiative transfer. We find that the use of inappropriate EOSs can lead to the overestimation of the planetary radius by up to ∼10%, depending on the planet size and composition. Our model has been applied to the GJ 9827 system as a test case and indicates Earth- or Venus-like interiors for planets b and c, respectively. Planet d could be an irradiated ocean planet with a water mass fraction (WMF) of ∼20% ± 10%. We also provide fits for the mass–radius relationships, allowing one to directly retrieve a wide range of planetary compositions, without the requirement to run the model. Our calculations finally suggest that highly irradiated planets lost their H/He content through atmospheric loss processes, and that the leftover material led to either super-Earths or sub-Neptunes, depending on the WMF.