We report on the discovery of HAT-P-10b , the lowest mass ( 0.46 \pm 0.03 M _ { J } ) transiting extrasolar planet ( TEP ) 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 = 3.7224690 \pm 0.0000067 d , transit epoch T _ { c } = 2454729.90631 \pm 0.00030 ( BJD ) and duration 0.1100 \pm 0.0015 d. HAT-P-10b has a radius of 1.05 _ { -0.03 } ^ { +0.05 } R _ { J } yielding a mean density of 0.498 \pm 0.064 g cm ^ { -3 } .
Comparing these observations with recent theoretical models we find that HAT-P-10 is consistent with a \sim 4.5 { Gyr } , coreless , pure hydrogen and helium gas giant planet .
With an equilibrium temperature of T _ { eq } = 1030 _ { -19 } ^ { +26 } K , HAT-P-10b is one of the coldest TEPs .
Curiously , its Safronov number \theta = 0.047 \pm 0.003 falls close to the dividing line between the two suggested TEP populations .