On the Stability of Super-Earth Atmospheres

TL;DR

This paper develops an analytical framework to assess the stability of super-Earth atmospheres around M stars, revealing how atmospheric composition and planetary parameters influence stability and potential habitability.

Contribution

It introduces stability diagrams in parameter space that predict atmospheric stability based on composition, size, and orbit, applied to known exoplanets and Kepler candidates.

Findings

Nitrogen-dominated atmospheres are less stable than hydrogen-dominated ones.

Some super Earths in habitable zones may lack stable atmospheres.

Approximately half of Kepler exoplanet candidates could have stable atmospheres under Earth-like conditions.

Abstract

We investigate the stability of super Earth atmospheres around M stars using a 7-parameter, analytical framework. We construct stability diagrams in the parameter space of exoplanetary radius versus semi-major axis and elucidate the regions in which the atmospheres are stable against the condensation of their major constituents, out of the gas phase, on their permanent nightside hemispheres. We find that super Earth atmospheres which are nitrogen-dominated ("Earth-like") occupy a smaller region of allowed parameter space, compared to hydrogen-dominated atmospheres, because of the dual effects of diminished advection and enhanced radiative cooling. Furthermore, some super Earths which reside within the habitable zones of M stars may not possess stable atmospheres, depending on the mean molecular weight and infrared photospheric pressure of their atmospheres. We apply our stability diagrams to GJ 436b and GJ 1214b, and demonstrate that atmospheric compositions with high mean molecular weights are disfavoured if these exoplanets possess solid surfaces and shallow atmospheres. Finally, we construct stability diagrams tailored to the Kepler dataset, for G and K stars, and predict that about half of the exoplanet candidates are expected to habour stable atmospheres if Earth-like conditions are assumed. We include 55 Cancri e and CoRoT-7b in our stability diagram for G stars.