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

TL;DR

This paper introduces a simple, quantitative method to estimate the minimum atmospheric height of small exoplanets using only their mass and radius, aiding in the classification of their atmospheric properties.

Contribution

The authors propose a novel metric, R_MAH, to quickly assess whether small exoplanets likely have extended atmospheres based on mass-radius data, without requiring detailed atmospheric modeling.

Findings

R_MAH can be calculated from mass and radius alone.

The metric helps prioritize planets for atmospheric follow-up.

It provides a statistical confidence level for the presence of an extended atmosphere.

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 analog 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' (R_MAH) 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 R_MAH>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.