HAT-P-26b: A LOW-DENSITY NEPTUNE-MASS PLANET TRANSITING A K STAR

We report the discovery of HAT-P-26b, a transiting extrasolar planet orbiting the moderately bright V = 11.744 K1 dwarf star GSC 0320-01027, with a period P = 4.234516 +/- 0.000015 days, transit epoch T-c = 2455304.65122 +/- 0.00035 ( BJD; Barycentric Julian dates throughout the paper are calculated from Coordinated Universal Time (UTC)), and transit duration 0.1023 +/- 0.0010 days. The host star has a mass of 0.82 +/- 0.03 M-circle dot, radius of 0.79(0.04)(+0.10) R-circle dot, effective temperature 5079 +/- 88 K, and metallicity [Fe/H] = -0.04 +/- 0.08. The planetary companion has a mass of 0.059 +/- 0.007 M-J, and radius of 0.565(-0.032)(+0.072) R-J yielding a mean density of 0.40 +/- 0.10 g cm(-3). HAT-P-26b is the fourth Neptune-mass transiting planet discovered to date. It has a mass that is comparable to those of Neptune and Uranus, and slightly smaller than those of the other transiting Super-Neptunes, but a radius that is similar to 65% larger than those of Neptune and Uranus, and also larger than those of the other transiting Super-Neptunes. HAT-P-26b is consistent with theoretical models of an irradiated Neptune-mass planet with a 10 M-circle plus heavy element core that comprises greater than or similar to 50% of its mass with the remainder contained in a significant hydrogen-helium envelope, though the exact composition is uncertain as there are significant differences between various theoretical models at the Neptune-mass regime. The equatorial declination of the star makes it easily accessible to both Northern and Southern ground-based facilities for follow-up observations.