  We present Gran Telescopio Canarias (GTC) optical transit narrow-band
photometry of the hot-Jupiter exoplanet XO-2b using the OSIRIS instrument. This
unique instrument has the capabilities to deliver high cadence narrow-band
photometric lightcurves, allowing us to probe the atmospheric composition of
hot Jupiters from the ground. The observations were taken during three transit
events which cover four wavelengths at spectral resolutions near 500, necessary
for observing atmospheric features, and have near-photon limited sub-mmag
precisions. Precision narrow-band 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
HD209458b and HD189733b, providing access to the majority of known transiting
planets. For XO-2b, we measure planet-to-star radius contrasts of
R_pl/R_star=0.10508+/-0.00052 at 6792 Ang, 0.10640+/-0.00058 at 7582 Ang, and
0.10686+/-0.00060 at 7664.9 Ang, and 0.10362+/-0.00051 at 8839 Ang. These
measurements reveal significant spectral features at two wavelengths, with an
absorption level of 0.067+/-0.016% at 7664.9 Ang due to atmospheric potassium
in the line core (a 4.1-sigma significance level), and an absorption level of
0.058+/-0.016% at 7582 Ang, (a 3.6-sigma significance level). When comparing
our measurements to hot-Jupiter atmospheric models, we find good agreement with
models which are dominated in the optical by alkali metals. This is the first
evidence for potassium in an extrasolar planet, an element that has long been
theorized along with sodium to be a dominant source of opacity at optical
wavelengths for hot Jupiters.
