GJ 1214b is one of the few known transiting super-Earth-sized exoplanets with a measured mass and radius. It orbits an M-dwarf, only 14.55 pc away, making it a favorable candidate for follow-up studies. However, the composition of GJ 1214bs mysteriou
s atmosphere has yet to be fully unveiled. Our goal is to distinguish between the various proposed atmospheric models to explain the properties of GJ 1214b: hydrogen-rich or hydrogen-He mix, or a heavy molecular weight atmosphere with reflecting high clouds, as latest studies have suggested. Wavelength-dependent planetary radii measurements from the transit depths in the optical/NIR are the best tool to investigate the atmosphere of GJ 1214b. We present here (i) photometric transit observations with a narrow-band filter centered on 2.14 microns and a broad-band I-Bessel filter centered on 0.8665 microns, 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 for I-Bessel and 0.11735 at 2.14 microns. Our measurements indicate a flat transmission spectrum, in agreement with last atmospheric models that favor featureless spectra with clouds and high molecular weight compositions.
We present an investigation of the transmission spectrum of the 6.5 M_earth planet GJ1214b based on new ground-based observations of transits of the planet in the optical and near-infrared, and on previously published data. Observations with the VLT+
FORS and Magellan+MMIRS using the technique of multi-object spectroscopy with wide slits yielded new measurements of the planets transmission spectrum from 0.61 to 0.85 micron, and in the J, H, and K atmospheric windows. We also present a new measurement based on narrow-band photometry centered at 2.09 micron with the VLT+HAWKI. We combined these data with results from a re-analysis of previously published FORS data from 0.78 to 1.00 micron using an improved data reduction algorithm, and previously reported values based on Spitzer data at 3.6 and 4.5 micron. All of the data are consistent with a featureless transmission spectrum for the planet. Our K-band data are inconsistent with the detection of spectral features at these wavelengths reported by Croll and collaborators at the level of 4.1 sigma. The planets atmosphere must either have at least 70% water by mass or optically thick high-altitude clouds or haze to be consistent with the data.
