2011A&A...527A..73S

We present Gran Telescopio Canarias (GTC) optical transit narrowband photometry of the hot-Jupiter exoplanet XO-2b using the OSIRIS instrument. This unique instrument has the capabilities to deliver high-cadence narrowband photometric lightcurves, allowing us to probe the atmospheric composition of hot Jupiters from the ground. The observations were taken during three transit events that cover four wavelengths at spectral resolutions near 500, necessary for observing atmospheric features, and have near-photon limited sub-mmag precisions. Precision narrowband photometry on a large aperture telescope allows for atmospheric transmission spectral features to be observed for exoplanets around much fainter stars than those of the well studied targets HD 209458b and HD 189733b, providing access to the majority of known transiting planets. For XO-2b, we measure planet-to-star radius contrasts of R_pl/R_*=0.10508±0.00052 at 6792Å, 0.10640±0.00058 at 7582Å, and 0.10686±0.00060 at 7664.9Å, and 0.10362±0.00051 at 8839Å. These measurements reveal significant spectral features at two wavelengths, with an absorption level of 0.067±0.016% at 7664.9Å caused by atmospheric potassium in the line core (a 4.1-σ significance level), and an absorption level of 0.058±0.016% at 7582Å, (a 3.6-σ significance level). When comparing our measurements to hot-Jupiter atmospheric models, we find good agreement with models that are dominated in the optical by alkali metals. This is the first evidence for potassium in an extrasolar planet, an element that has along with sodium long been supposed to be a dominant source of opacity at optical wavelengths for hot Jupiters.