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
JournalAstrophysical Journal
Issue number2
Publication statusPublished - Feb 20 2011



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

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

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