Helium Enhanced Planets Along the Upper Edge of the Radius Valley

TL;DR

This paper models how atmospheric escape can lead to helium-enriched atmospheres in sub-Neptunes, offering a new way to test planet evolution theories through transmission spectra.

Contribution

It introduces a model accounting for diffusive separation of hydrogen and helium during atmospheric escape, revealing helium enhancement in certain exoplanets over billions of years.

Findings

Planets with 1.6-2.5 Earth radii can become helium-rich due to atmospheric escape.

Helium mass fractions can exceed 40% after billions of years of evolution.

Helium enhancement can be observationally detected via transmission spectra.

Abstract

The low mean densities of sub-Neptunes imply that they formed within a few million years and accreted primordial envelopes. Because these planets receive a total X-ray and extreme ultra-violet flux that is comparable to the gravitational binding energy of their envelopes, their primordial hydrogen-helium atmospheres are susceptible to mass loss. Models of photoevaporating sub-Neptunes have so far assumed that envelope compositions remain constant over time. However, preferential loss of atmospheric hydrogen has the potential to change their compositions. Here, by modeling the thermal and compositional evolution of sub-Neptunes undergoing atmospheric escape with diffusive separation between hydrogen and helium, we show that planets with radii between 1.6 and 2.5 that of Earth can become helium-enhanced from billions of years of photoevaporation, obtaining helium mass fractions in excess of 40%. Atmospheric helium enhancement can be detected through transmission spectra, providing a novel observational test for whether atmospheric escape creates the radius valley.