HAT-P-15b: A 10.9 Day Extrasolar Planet Transiting a Solar-type Star

We report the discovery of HAT-P-15b, a transiting extrasolar planet in the "period valley," a relatively sparsely populated period regime of the known extrasolar planets. The host star, GSC 2883-01687, is a G5 dwarf with V = 12.16. It has a mass of 1.01 ± 0.04 M_☉, radius of 1.08 ± 0.04 R_☉, effective temperature 5568 ± 90 K, and metallicity [Fe/H] = +0.22 ± 0.08. The planetary companion orbits the star with a period P = 10.863502 ± 0.000027 days, transit epoch T_c = 2454638.56019 ± 0.00048 (BJD), and transit duration 0.2285 ± 0.0015 days. It has a mass of 1.946 ± 0.066 M_J and radius of 1.072 ± 0.043 R_J yielding a mean density of 1.96 ± 0.22 g cm^(-3). At an age of 6.8^(+2.5))(–1.6) Gyr, the planet is H/He-dominated and theoretical models require about 2% (10 M_⊕) worth of heavy elements to reproduce its measured radius. With an estimated equilibrium temperature of ~820 K during transit, and ~1000 K at occultation, HAT-P-15b is a potential candidate to study moderately cool planetary atmospheres by transmission and occultation spectroscopy.