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

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.

arXiv:1111.5621 [pdf, other]

Zachory K. Berta, David Charbonneau, Jean-Michel Desert, Eliza Miller-Ricci Kempton, Peter R. McCullough, Christopher J. Burke, Jonathan J. Fortney, Jonathan Irwin, Philip Nutzman, Derek Homeier

Comments: 17 pages, 12 figures, accepted for publication in ApJ

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)