2009ApJ...696.1950B

We report on the discovery of HAT-P-10b, one of the lowest mass (0.487 ±0.018 M_J) transiting extrasolar planets (TEPs) discovered to date by transit searches. HAT-P-10b orbits the moderately bright V = 11.89 K dwarf GSC 02340-01714, with a period P = 3.7224747±0.0000065 days, transit epoch T_c= 2454759.68683±0.00016 (BJD), and duration 0.1090±0.0008 days. HAT-P-10b has a radius of 1.005^+0.032_–0.027R_J yielding a mean density of 0.594±0.052 g/cm3. Comparing these observations with recent theoretical models we find that HAT-P-10b is consistent with a ∼4.5 Gyr, almost pure hydrogen and helium gas giant planet with a 10 M_⊕core. With an equilibrium temperature of T_eq= 1020±17 K, HAT-P-10b is one of the coldest TEPs. Curiously, its Safronov number θ = 0.053±0.002 falls close to the dividing line between the two suggested TEP populations.