Helium Atmospheres on Warm Neptune- and Sub-Neptune-Sized Exoplanets and Applications to GJ 436 b

TL;DR

This paper proposes that warm Neptune- and sub-Neptune-sized exoplanets can develop helium-dominated atmospheres through hydrodynamic escape, which significantly alters their spectral features and explains observations of GJ 436 b.

Contribution

It introduces a model for atmospheric evolution leading to helium atmospheres on small exoplanets, with implications for spectral signatures and planetary history.

Findings

Helium atmospheres can form from hydrogen-rich primordial atmospheres via hydrodynamic escape.

Helium atmospheres exhibit distinct spectral features, especially in thermal emission and transmission spectra.

The model explains the observed spectrum of GJ 436 b and predicts detectable signatures of helium atmospheres.

Abstract

Warm Neptune- and sub-Neptune-sized exoplanets in orbits smaller than Mercury's are thought to have experienced extensive atmospheric evolution. Here we propose that a potential outcome of this atmospheric evolution is the formation of helium-dominated atmospheres. The hydrodynamic escape rates of Neptune- and sub-Neptune-sized exoplanets are comparable to the diffusion-limited escape rate of hydrogen, and therefore the escape is heavily affected by diffusive separation between hydrogen and helium. A helium atmosphere can thus be formed -- from a primordial hydrogen-helium atmosphere -- via atmospheric hydrodynamic escape from the planet. The helium atmosphere has very different abundances of major carbon and oxygen species from those of a hydrogen atmosphere, leading to distinctive transmission and thermal emission spectral features. In particular, the hypothesis of a helium-dominated atmosphere can explain the thermal emission spectrum of GJ 436 b, a warm Neptune-sized exoplanet, while also consistent with the transmission spectrum. This model atmosphere contains trace amounts of hydrogen, carbon, and oxygen, with the predominance of CO over CH4 as the main form of carbon. With our atmospheric evolution model, we find that if the mass of the initial atmosphere envelope is 1E-3 planetary mass, hydrodynamic escape can reduce the hydrogen abundance in the atmosphere by several orders of magnitude in ~10 billion years. Observations of exoplanet transits may thus detect signatures of helium atmospheres and probe the evolutionary history of small exoplanets.