2013MNRAS.431.1669T

We observed nine primary transits of the super-Earth exoplanet GJ 1214b in several optical photometric bands from 2012 March to August, with the goal of constraining the short-wavelength slope of the spectrum of GJ 1214b. Our observations were conducted on the Kuiper 1.55m telescope in Arizona and the STELLA-I robotic 1.2 m telescope in Tenerife, Spain. From the derived light curves we extracted transit depths in R (0.65µm), V (0.55µm) and g' (0.475µm) bands. Most previous observations of this exoplanet suggest a flat spectrum varying little with wavelength from the near-infrared to the optical, corresponding to a low-scale height, high-molecular-weight atmosphere. However, a handful of observations around Ks band ( ∼ 2.15µm) and g band ( ∼ 0.46µm) are inconsistent with this scenario and suggest a variation on a hydrogen- or water-dominated atmosphere that also contains a haze layer of small particles. In particular, the g-band observations of de Mooij et al., consistent with Rayleigh scattering, limit the potential atmosphere compositions of GJ 1214b due to the increasing slope at optical wavelengths. We find that our results overlap within errors the short-wavelength observations of de Mooij et al., but are also consistent with a spectral slope of zero in GJ 1214b in the optical wavelength region. Our observations thus allow for a larger suite of possible atmosphere compositions, including those with a high molecular weight and/or hazes.