We report on the discovery of HAT-P-12b , a transiting extrasolar planet orbiting the moderately bright V \approx 12.8 K4 dwarf GSC 03033-00706 , with a period P = 3.2130598 \pm 0.0000021 d , transit epoch T _ { c } = 2454419.19556 \pm 0.00020 ( BJD ) and transit duration 0.0974 \pm 0.0006 d. The host star has a mass of 0.73 \pm 0.02 M _ { \sun } , radius of 0.70 _ { -0.01 } ^ { +0.02 } R _ { \sun } , effective temperature 4650 \pm 60 K and metallicity [ Fe / H ] = -0.29 \pm 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 \pm 0.012 M _ { J } , and radius of 0.959 _ { -0.021 } ^ { +0.029 } R _ { J } yielding a mean density of 0.295 \pm 0.025 g cm ^ { -3 } .
Comparing these observations with recent theoretical models we find that HAT-P-12b is consistent with a \sim 1 - 4.5 Gyr , mildly irradiated , H/He dominated planet with a core mass M _ { C } \lesssim 10 M _ { \earth } .
HAT-P-12b is thus the least massive H/He dominated gas giant planet found to date .
This record was previously held by Saturn .