WASP-39b: a highly inflated Saturn-mass planet orbiting a late G-type star

Faedi, F.; Barros, S. C. C.; Anderson, D. R.; Brown, D. J. A.; Collier Cameron, A.; Pollacco, D.; Boisse, I.; Hébrard, G.; Lendl, M.; Lister, T. A.; Smalley, B.; Street, R. A.; Triaud, A. H. M. J.; Bento, J.; Bouchy, F.; Butters, O. W.; Enoch, B.; Haswell, C. A.; Hellier, C.; Keenan, F. P.; Miller, G. R. M.; Moulds, V.; Moutou, C.; Norton, A. J.; Queloz, D.; Santerne, A.; Simpson, E. K.; Skillen, I.; Smith, A. M. S.; Udry, S.; Watson, C. A.; West, R. G. and Wheatley, P. J. (2011). WASP-39b: a highly inflated Saturn-mass planet orbiting a late G-type star. Astronomy & Astrophysics, 531, article no. A40.

DOI: https://doi.org/10.1051/0004-6361/201116671


We present the discovery of WASP-39b, a highly inflated transiting Saturn-mass planet orbiting a late G-type dwarf star with a period of 4.055259 ± 0.000008 d, Transit Epoch T0 = 2 455 342.9688 ± 0.0002 (HJD), of duration 0.1168 ± 0.0008 d. A combined analysis of the WASP photometry, high-precision follow-up transit photometry, and radial velocities yield a planetary mass of Mpl = 0.28 ± 0.03 MJ and a radius of Rpl = 1.27 ± 0.04 RJ, resulting in a mean density of 0.14 ± 0.02 ρJ. The stellar parameters are mass M = 0.93 ± 0.03 M, radius R = 0.895 ± 0.23 R, and age 9+3-4 Gyr. Only WASP-17b and WASP-31b have lower densities than WASP-39b, although they are slightly more massive and highly irradiated planets. From our spectral analysis, the metallicity of WASP-39 is measured to be [Fe/H] = -0.12 ± 0.1 dex, and we find the planet to have an equilibrium temperature of 1116+33-32 K. Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets.

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