The Flat Transmission Spectrum of the Super-Earth GJ1214b from Wide Field Camera 3 on the Hubble Space Telescope

TL;DR

This study presents the first WFC3 transmission spectrum of GJ1214b, revealing a flat spectrum that suggests a high mean molecular weight atmosphere or high-altitude clouds, challenging cloud-free, low-density atmospheric models.

Contribution

First WFC3 observations of a transiting exoplanet atmosphere showing a flat transmission spectrum, indicating a dense or cloudy atmosphere for GJ1214b.

Findings

Transmission spectrum is flat between 1.1 and 1.7 microns.

Inconsistent with a cloud-free solar composition atmosphere at 8.2 sigma.

Suggests a high mean molecular weight atmosphere or optically thick high-altitude clouds.

Abstract

Capitalizing on the observational advantage offered by its tiny M dwarf host, we present HST/WFC3 grism measurements of the transmission spectrum of the super-Earth exoplanet GJ1214b. These are the first published WFC3 observations of a transiting exoplanet atmosphere. After correcting for a ramp-like instrumental systematic, we achieve nearly photon-limited precision in these observations, finding the transmission spectrum of GJ1214b to be flat between 1.1 and 1.7 microns. Inconsistent with a cloud-free solar composition atmosphere at 8.2 sigma, the measured achromatic transit depth most likely implies a large mean molecular weight for GJ1214b's outer envelope. A dense atmosphere rules out bulk compositions for GJ1214b that explain its large radius by the presence of a very low density gas layer surrounding the planet. High-altitude clouds can alternatively explain the flat transmission spectrum, but they would need to be optically thick up to 10 mbar or consist of particles with a range of sizes approaching 1 micron in diameter.