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

J. D. Hartman, G. Á Bakos, D. M. Kipping, G. Torres, G. Kovács, R. W. Noyes, D. W. Latham, A. W. Howard, D. A. Fischer, J. A. Johnson, G. W. Marcy, H. Isaacson, S. N. Quinn, L. A. Buchhave, B. Béky, D. D. Sasselov, R. P. Stefanik, G. A. Esquerdo, M. Everett, G. PerumpillyJ. Lázár, I. Papp, P. Sári

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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 Mȯ, radius of 0.79+010-0.04R Ȯ, effective temperature 5079 ±88 K, and metallicity [Fe/H] = -0.04 ±0.08. The planetary companion has a mass of 0.059 ± 0.007 MJ, and radius of 0.565+0072 -0.032 Rj yielding a mean density of 0.40 ± 0.10 gcm -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.

Original languageEnglish (US)
JournalAstrophysical Journal
Issue number2
StatePublished - Feb 20 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planetary systems
  • Stars: individual (HAT-P-26, GSC 0320-01027)
  • Techniques: photometric
  • Techniques: spectroscopic

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    Hartman, J. D., Bakos, G. Á., Kipping, D. M., Torres, G., Kovács, G., Noyes, R. W., Latham, D. W., Howard, A. W., Fischer, D. A., Johnson, J. A., Marcy, G. W., Isaacson, H., Quinn, S. N., Buchhave, L. A., Béky, B., Sasselov, D. D., Stefanik, R. P., Esquerdo, G. A., Everett, M., ... Sári, P. (2011). HAT-P-26b: A low-density Neptune-mass planet transiting a K star. Astrophysical Journal, 728(2). https://doi.org/10.1088/0004-637X/728/2/138