A search for methane in the atmosphere of GJ 1214b via GTC narrow-band transmission spectrophotometry

We present narrow-band photometric measurements of the exoplanet GJ 1214b using the 10.4m Gran Telescopio Canarias and the Optical System for Imaging and low Resolution Integrated Spectroscopy instrument. Using tuneable filters, we observed a total of five transits, three of which were observed at two wavelengths nearly simultaneously, producing a total of eight individual light curves, six of these probed the possible existence of a methane absorption feature in the 8770-8850Å region at high resolution. We detect no increase in the planet-to-star radius ratio across the methane feature with a change in radius ratio of δR̄ =-0.0007 ± 0.0017 corresponding to a scaleheight (H) change of-0.5 ± 1.2H across the methane feature, assuming a hydrogen-dominated atmosphere. We find that a variety of water and cloudy atmospheric models fit the data well, but find that cloud-free models provide poor fits. These observations support a flat transmission spectrum resulting from the presence of a high-altitude haze or awater-rich atmosphere, in agreement with previous studies. In this study, the observations are pre-dominantly limited by the photometric quality and the limited number of data points (resulting from a long observing cadence), which make the determination of the systematic noise challenging. With tuneable filters capable of highresolution measurements (R≈ 600-750) of narrow absorption features, the interpretation of our results are also limited by the absence of high-resolution methane models below 1 μm.