Mass-radius relationships for irradiated ocean planets

TL;DR

This paper develops a detailed model for water-rich, irradiated ocean planets, highlighting the importance of accurate equations of state and providing mass-radius relationships to infer planetary composition and evolution.

Contribution

It introduces a comprehensive structure model combining interior and atmospheric physics for irradiated ocean planets, with updated water EoS and practical mass-radius fits.

Findings

Incorrect EoS can overestimate planetary radius by up to 10%.

Application to GJ 9827 suggests diverse interior compositions.

Highly irradiated planets likely lose H/He, becoming super-Earths or sub-Neptunes.

Abstract

Massive and water-rich planets should be ubiquitous in the universe. Many of those 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 non appropriate EoSs can lead to the overestimation of the planetary radius by up to $\sim$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 of $\sim$$20\pm10\%$. 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 water mass fraction.