A simple, quantitative method to infer the minimum atmospheric height of small exoplanets

Abstract

Amongst the many hundreds of transiting planet candidates discovered by the Kepler mission, one finds a large number of candidates with sizes between that of the Earth and Neptune. The composition of these worlds is not immediately obvious with no Solar system analogue to draw upon and there exists some ambiguity as to whether a given candidate is a rocky super-Earth or a gas-enveloped mini-Neptune. The potential scientific value and observability of the atmospheres of these two classes of worlds varies significantly, and given the sheer number of candidates in this size range, there is evidently a need for a quick, simple metric to rank whether the planets have an extended atmosphere or not. In this work, we propose a way to calculate the 'minimum atmospheric height'(RMAH) using only a planet's radius and mass as inputs. We assume and exploit the boundary condition that the bulk composition of a solid/liquid super-Earth cannot be composed of a material lighter than that of water. Mass- radius loci above a pure-water composition planet correspond to RMAH < 0. The statistical confidence of a planet maintaining an extended atmosphere can be therefore easily calculated to provide a simple ranking of target planets for follow-up observations. We also discuss how this metric can be useful in the interpretation of the spectra of observed planetary atmospheres. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.