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

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 = 2455342.9688 ± 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 M_pl = 0.28 ± 0.03 M_J and a radius of R_pl = 1.27 ± 0.04 R_J, resulting in a mean density of 0.14 ± 0.02 ρ_J. 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 K . Both values strengthen the observed empirical correlation between these parameters and the planetary radius for the known transiting Saturn-mass planets.