Probing the blow-off criteria of hydrogen-rich 'super-Earths'

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 1D 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 soft X-rays and extreme ultraviolet (XUV) 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 except for 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.