A ground-based transmission spectrum of the super-Earth exoplanet GJ1214b

TL;DR

This study presents ground-based transmission spectrum measurements of super-Earth GJ1214b, constraining its atmospheric composition by ruling out cloud-free hydrogen atmospheres and suggesting possible water vapor or cloudy atmospheres.

Contribution

First ground-based transmission spectrum of GJ1214b that constrains atmospheric composition and rules out cloud-free hydrogen atmospheres.

Findings

No features detected in the spectrum, ruling out cloud-free hydrogen atmospheres.

Hydrogen-dominated atmospheres must have clouds or hazes at certain pressures.

A dense water vapor atmosphere remains consistent with the data.

Abstract

In contrast to planets with masses similar to that of Jupiter and higher, the bulk compositions of planets in the so-called super-Earth regime cannot be uniquely determined from a mass and radius measurement alone. For these planets, there is a degeneracy between the mass and composition of the interior and a possible atmosphere in theoretical models. The recently discovered transiting super-Earth GJ1214b is one example of this problem. Three distinct models for the planet that are consistent with its mass and radius have been suggested, and breaking the degeneracy between these models requires obtaining constraints on the planet's atmospheric composition. Here we report a ground-based measurement of the transmission spectrum of GJ1214b between 780 and 1000 nm. The lack of features in this spectrum rules out cloud-free atmospheres composed primarily of hydrogen at 4.9 sigma confidence. If the planet's atmosphere is hydrogen-dominated, then it must contain clouds or hazes that are optically thick at the observed wavelengths at pressures less than 200 mbar. Alternatively, the featureless transmission spectrum is also consistent with the presence of a dense water vapor atmosphere.