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 _sun, radius of 1.08 ± 0.04 R _sun, 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.

Based in part on observations obtained at the W. M. Keck Observatory, which is operated by the University of California and the California Institute of Technology.