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 10.4 m 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 8770–8850 A 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 ΔR = -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 a water-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 high-resolution 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.