HAT-P-12b: A low-density sub-saturn mass planet transiting a metal-poor K dwarf

We report on the discovery of HAT-P-12b, a transiting extrasolar planet orbiting the moderately bright V 12.8 K4 dwarf GSC 03033 - 00706, with a period P = 3.2130598±0.0000021d, transit epoch T_c = 2454419.19556± 0.00020 (BJD), and transit duration 0.0974±0.0006d. The host star has a mass of 0.73 0.02M _⊙, radius of 0.70 ^+0.02 _-0.01R _⊙, effective temperature 4650 60K, and metallicity [Fe/H] = -0.29 0.05. We find a slight correlation between the observed spectral line bisector spans and the radial velocity, so we consider, and rule out, various blend configurations including a blend with a background eclipsing binary, and hierarchical triple systems where the eclipsing body is a star or a planet. We conclude that a model consisting of a single star with a transiting planet best fits the observations, and show that a likely explanation for the apparent correlation is contamination from scattered moonlight. Based on this model, the planetary companion has a mass of 0.211±0.012M _J and radius of 0.959^+0.029 _-0.021R _J yielding a mean density of 0.295±0.025g cm^-3. Comparing these observations with recent theoretical models, we find that HAT-P-12b is consistent with a ∼1-4.5 Gyr, mildly irradiated, H/He-dominated planet with a core mass M_C ≲ 10 M _⊕. HAT-P-12b is thus the least massive H/He-dominated gas giant planet found to date. This record was previously held by Saturn.