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Published February 20, 2011 | Published
Journal Article Open

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_⊙ , radius of 0.79^(+0.10)_(–0.04) R_⊙ , 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^(+0.072)_(–0.032) 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 comprises ≳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.

Additional Information

© 2011 American Astronomical Society. Received 2010 October 4; accepted 2010 December 14; published 2011 January 31. Based in part on observations obtained at the W. M. Keck Observatory, which is operated by the University of California and the California Institute of Technology. Keck time has been granted by NASA (N018Hr and N167Hr). HATNet operations have been funded by NASA grants NNG04GN74G, NNX08AF23G, and SAO IR&D. Work of G. Á .B. and J.A.J. were supported by the Postdoctoral Fellowship of the NSF Astronomy and Astrophysics Program (AST-0702843 and AST-0702821, respectively). G.T. acknowledges partial support from NASA grant NNX09AF59G. We acknowledge partial support also from the Kepler Mission under NASA Cooperative Agreement NCC2-1390 (PI: D.W.L.). G.K. thanks the Hungarian Scientific Research Foundation (OTKA) for support through grant K-81373. This research has made use of Keck telescope time granted through NASA (N018Hr and N167Hr).

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