Probing the Blow-Off Criteria of Hydrogen-Rich "Super-Earths"

TL;DR

This study uses a numerical model to determine the conditions under which hydrogen-rich atmospheres of super-Earths can undergo hydrodynamic blow-off due to stellar XUV radiation, finding most retain their atmospheres over time.

Contribution

It introduces a time-dependent numerical approach to evaluate atmospheric escape criteria for hydrogen-rich super-Earths under stellar XUV heating.

Findings

All studied super-Earths can experience atmospheric expansion to large distances.

Most super-Earths are unlikely to lose their hydrogen atmospheres during their lifetimes.

Mass-loss rates are significantly lower than those of hot Jupiters.

Abstract

The discovery of transiting "super-Earths" with inflated radii and known masses such as Kepler-11b-f, GJ 1214b and 55 Cnc e, indicates that these exoplanets did not lose their nebula-captured hydrogen-rich, degassed or impact-delivered protoatmospheres by atmospheric escape processes. Because hydrodynamic blow-off of atmospheric hydrogen atoms is the most efficient atmospheric escape process we apply a time-dependent numerical algorithm which is able to solve the system of 1-D fluid equations for mass, momentum, and energy conservation to investigate the criteria under which "super-Earths" with hydrogen-dominated upper atmospheres can experience hydrodynamic expansion by heating of the stellar XUV (soft X-rays and extreme ultraviolet) radiation and thermal escape via blow-off. Depending on orbit location, XUV flux, heating efficiency and the planet's mean density our results indicate that the upper atmospheres of all "super-Earths" can expand to large distances, so that besides of Kepler-11c all of them experience atmospheric mass-loss due to Roche lobe overflow. The atmospheric mass-loss of the studied "super-Earths" is one to two orders of magnitude lower compared to that of "hot Jupiters" such as HD 209458b, so that one can expect that these exoplanets cannot lose their hydrogen-envelopes during their remaining lifetimes.