Where are the Water Worlds? Identifying the Exo-water-worlds Using Models of Planet Formation and Atmospheric Evolution

TL;DR

This study combines planet formation and atmospheric evolution models to identify potential water-rich exoplanets, suggesting a significant fraction of bare planets could be water worlds, especially at longer orbital periods.

Contribution

It introduces a novel approach integrating formation and atmospheric models to estimate the prevalence of water worlds among observed exoplanets.

Findings

Over 10-20% of bare planets may be water-ice-rich.

Most water worlds are predicted to orbit beyond 10 days.

Longer orbital periods are better targets for detecting water worlds.

Abstract

Planet formation models suggest that the small exoplanets that migrate from beyond the snowline of the protoplanetary disk likely contain water-ice-rich cores ($\sim 50\%$ by mass), also known as the water worlds. While the observed radius valley of the Kepler planets is well explained with the atmospheric dichotomy of the rocky planets, precise measurements of mass and radius of the transiting planets hint at the existence of these water worlds. However, observations cannot confirm the core compositions of those planets owing to the degeneracy between the density of a bare water-ice-rich planet and the bulk density of a rocky planet with a thin atmosphere. We combine different formation models from the Genesis library with atmospheric escape models, such as photo-evaporation and impact stripping, to simulate planetary systems consistent with the observed radius valley. We then explore the possibility of water worlds being present in the currently observed sample by comparing them with the simulated planets in the mass-radius-orbital period space. We find that the migration models suggest $\gtrsim 10\%$ and $\gtrsim 20\%$ of the bare planets, i.e. planets without primordial H/He atmospheres, to be water-ice-rich around G- and M-type host stars respectively, consistent with the mass-radius distributions of the observed planets. However, most of the water worlds are predicted to be outside a period of 10 days. A unique identification of water worlds through radial velocity and transmission spectroscopy is likely to be more successful when targeting such planets with longer orbital periods.