Bulk Composition of GJ 1214b and other sub-Neptune exoplanets

TL;DR

This study uses internal structure and evolutionary models to constrain the composition of GJ 1214b, revealing it contains at most 7% hydrogen and helium by mass, and discusses implications for other sub-Neptune exoplanets.

Contribution

It provides the first detailed modeling-based estimate of the H/He content in GJ 1214b and compares it with other low-mass exoplanets, highlighting the sensitivity of radius to H/He.

Findings

GJ 1214b's H/He envelope is at most 7% by mass.

Most low-mass exoplanets have less than 10% H/He.

Water composition alone can fit GJ 1214b's data, but heavier materials are likely present.

Abstract

GJ1214b stands out among the detected low-mass exoplanets, because it is, so far, the only one amenable to transmission spectroscopy. Up to date there is no consensus about the composition of its envelope although most studies suggest a high molecular weight atmosphere. In particular, it is unclear if hydrogen and helium are present or if the atmosphere is water dominated. Here, we present results on the composition of the envelope obtained by using an internal structure and evolutionary model to fit the mass and radius data. By examining all possible mixtures of water and H/He, with the corresponding opacities, we find that the bulk amount of H/He of GJ1214b is at most 7% by mass. In general, we find the radius of warm sub-Neptunes to be most sensitive to the amount of H/He. We note that all (Kepler-11b,c,d,f, Kepler-18b, Kepler-20b, 55Cnc-e, Kepler-36c and Kepler-68b) but two (Kepler-11e and Kepler-30b) of the discovered low-mass planets so far have less than 10% H/He. In fact, Kepler-11e and Kepler-30b have 10-18% and 5-15% bulk H/He. Conversely, little can be determined about the H2O or rocky content of sub-Neptune planets. We find that although a 100% water composition fits the data for GJ1214b, based on formation constraints the presence of heavier refractory material on this planet is expected, and hence, so is a component lighter than water required. A robust determination by transmission spectroscopy of the composition of the upper atmosphere of GJ1214b will help determine the extent of compositional segregation between the atmosphere and envelope.