Most Super-Earths Have Less Than 3% Water

TL;DR

This study models super-Earth interiors and atmospheres to estimate that they contain less than 3% water by mass, suggesting a common origin for water on small terrestrial planets.

Contribution

It provides the first combined interior-atmosphere models for super-Earths to constrain their water content using observational data.

Findings

Water mass fraction in super-Earths is likely less than 3%.

Constraints are consistent with water originating from atmospheric reactions with magma oceans.

Upper limits remain robust under various planetary formation scenarios.

Abstract

Super-Earths are highly irradiated, small planets with bulk densities approximately consistent with Earth. We construct combined interior-atmosphere models of super-Earths that trace the partitioning of water throughout a planet, including an iron-rich core, silicate-rich mantle, and steam atmosphere. We compare these models with exoplanet observations to infer a $1\sigma$ upper limit on total water mass fraction of $\lesssim 3\%$ at the population level. We consider end-member scenarios that may change this value, including the efficiency of mantle outgassing, escape of high mean-molecular weight atmospheres, and increased iron core mass fractions. Although our constraints are agnostic as to the origin of water, we show that our upper limits are consistent with its production via chemical reactions of primordial hydrogen-dominated atmospheres with magma oceans. This mechanism has also been hypothesised to explain Earth's water content, possibly pointing to a unified channel for the origins of water on small terrestrial planets.