HAT-P-26b: A low-density Neptune-mass planet transiting a K star

Hartman, J.D.; Bakos, G.A.; Kipping, D.M.; Torres, G.; Kovacs, G.; Noyes, R.W.; Latham, D.W.; Howard, A.W.; Fischer, D.A.; Johnson, J.A.; Marcy, G.W.; Isaachson, H.; Quinn, S.N.; Buchhave, L.A.; Beky, B.; Sasselov, D.D.; Stefanik, R.P.; Esquerdo, G.A.; Everett, M.; Perumpilly, G.; Lazar, J.; Papp, I.; Sari, P. (2011) .

• Astrophysical Journal

. 728: A138.



Abstract:

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 Tc =
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_⊙, radius of 0.79_-0.04^+0.10R_⊙, 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 ~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_⊕ heavy element core that comprise ≳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.