Ground-based transit observations of the super-Earth GJ 1214 b ⋆

Context. GJ 1214 b is one of the few known transiting super-Earth-sized exoplanets with a measured mass and radius. It orbits an Mdwarf, only 14.55 pc away, making it a favorable candidate for follow-up studies. However, the composition of GJ 1214 b’s mysterious atmosphere has yet to be fully unveiled. Aims. Our goal is to distinguish between the various proposed atmospheric models to explain the properties of GJ 1214 b: hydrogenrich or hydrogen-He mix, or a heavy molecular weight atmosphere with reflecting high clouds, as latest studies have sugge sted. Methods. Wavelength-dependent planetary radii measurements from the transit depths in the optical/NIR are the best tool to investigate the atmosphere of GJ 1214 b. We present here (i) photometric transit observations with a narrow-band filter centered on 2.14µm and a broad-band I-Bessel filter centered on 0.8665 µm, and (ii) transmission spectroscopy in the H and K atmospheric windows that cover three transits. The obtained photometric and spectrophotometric time series were analyzed with MCMC simulations to measure the planetary radii as a function of wavelength. We determined radii ratios of 0.1173 +0.0022 −0.0024 for I-Bessel and 0.11735 +0.00072 −0.00076 at 2.14µm. Results. Our measurements indicate a flat transmission spectrum, in a greement with last atmospheric models that favor featureless spectra with clouds and high molecular weight compositions.