Optical to near-infrared transit observations of super-Earth GJ 1214b: water-world or mini-Neptune?

GJ 1214b, the 6.55 Earth-mass transiting planet recently discovered by the MEarth team, has a mean density of similar to 35% of that of the Earth. It is thought that this planet is either a mini-Neptune, consisting of a rocky core with a thick, hydrogen-rich atmosphere, or a planet with a composition dominated by water. The authors have obtained observations of the transit of GJ 1214b in the r- and I-band with the Isaac Newton Telescope, in the g-, r-, i- and z-bands with the 2.2 m MPI/ESO telescope, in the Ks-band with the Nordic Optical Telescope, and in the Kc-band with the William Herschel Telescope. They do not detect clearly significant variations in the planet-to-star size ratio as function of wavelength. Although the ratio at the shortest measured wavelength, in g-band, is 2 sigma larger than in the other bands. The uncertainties in the Ks and Kc bands are large, due to systematic features in the light curves.