Observational evidence for a metal rich atmosphere on the super-Earth GJ1214b

TL;DR

This study uses infrared observations from Spitzer, combined with optical data, to analyze GJ1214b's atmosphere, providing evidence against a cloud-free, hydrogen-rich atmosphere and suggesting a metal-rich composition.

Contribution

It presents the first infrared transit measurements of GJ1214b at 3.6 and 4.5 microns, constraining its transmission spectrum and ruling out a cloud-free, solar composition atmosphere with high confidence.

Findings

GJ1214b's transmission spectrum is flat across optical to infrared wavelengths.

A cloud-free, hydrogen-dominated atmosphere is ruled out at 4.5 sigma.

The atmosphere likely contains more than 30% metals, consistent with a metal-rich composition.

Abstract

We report observations of two consecutive transits of the warm super-Earth exoplanet GJ1214b at 3.6 and 4.5 microns with the Infrared Array Camera instrument on-board the Spitzer Space Telescope. The two transit light curves allow for the determination of the transit parameters for this system. We find these paremeters to be consistent with the previously determined values and no evidence for transit timing variations. The main investigation consists of measuring the transit depths in each bandpass to constrain the planet's transmission spectrum. Fixing the system scale and impact parameters, we measure R_p/R_star=0.1176 (+0.0008/-0.0009) and 0.1163 (+0.0010/-0.0008) at 3.6 and 4.5 microns, respectively. Combining these data with the previously reported MEarth Observatory measurements in the red optical yields constraints on the GJ1214b's transmission spectrum and allows us to rule-out a cloud-free, solar composition (i.e., hydrogen-dominated) atmosphere at 4.5 sigma confidence. This independently confirms a recent finding that was based on a measurement of the planet's transmission spectrum using the VLT. The Spitzer, MEarth, and VLT observations together yield a remarkably flat transmission spectrum over the large wavelength domain spanned by the data. Consequently, cloud-free atmospheric models require more than 30% metals (assumed to be in the form of H2O by volume to be consistent with all the observations.