OBSERVATIONAL EVIDENCE FOR A METAL-RICH ATMOSPHERE ON THE SUPER-EARTH GJ1214b

We report observations of two consecutive transits of the warm super-Earth exoplanet GJ 1214b at 3.6 and 4.5 mu m 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 parameters 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(-0.0009)(+0.0008) and 0.1163(-0.0008)(+0.0010) at 3.6 and 4.5 mu m, respectively. Combining these data with the previously reported MEarth Observatory measurements in the red optical 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 Very Large Telescope (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.