Evolution of steam worlds: energetic aspects

TL;DR

This paper develops a comprehensive model for water-rich sub-Neptune planets, revealing that pure water envelopes lead to smaller, slower-evolving radii and highlighting the importance of detailed volatile-rich planet modeling for interpreting observations.

Contribution

It introduces a novel combined atmosphere and interior evolution model for water-rich sub-Neptunes, providing new insights into their radius evolution and internal states.

Findings

Water-rich planets have smaller radii than previous models predicted.

Radius evolution of water-rich planets is significantly slower (~10%).

Deep interior water can transition from plasma to superionic ice, affecting evolution.

Abstract

Sub-Neptunes occupy an intriguing region of planetary mass-radius space, where theoretical models of interior structure predict that they could be water-rich, where water is in steam and supercritical state. Such planets are expected to evolve according to the same principles as canonical H$_2$-He rich planets, but models that assume a water-dominated atmosphere consistent with the interior have not been developed yet. Here, we present a state of the art structure and evolution model for water-rich sub-Neptunes. Our set-up combines an existing atmosphere model that controls the heat loss from the planet, and an interior model that acts as the reservoir of energy. We compute evolutionary tracks of planetary radius over time. We find that planets with pure water envelopes have smaller radii than predicted by previous models, and the change in radius is much slower (within $\sim$10\%). We also find that water in the deep interior is colder than previously suggested, and can transition from plasma state to superionic ice, which can have additional implications for their evolution. We provide a grid of evolutionary tracks that can be used to infer the bulk water content of sub-Neptunes. We compare the bulk water content inferred by this model and other models available in the literature, and find statistically significant differences between models when the uncertainty on measured mass and radius are both smaller than 10\%. This study shows the importance of pursuing efforts in the modeling of volatile-rich planets, and how to connect them to observations.