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

TL;DR

This paper reports the discovery and detailed characterization of WASP-39b, a highly inflated Saturn-mass exoplanet orbiting a late G-type star, with implications for understanding planetary inflation and composition.

Contribution

The study provides the first detailed analysis of WASP-39b, including its mass, radius, density, and host star properties, highlighting its unique low density among Saturn-mass planets.

Findings

WASP-39b has a mass of 0.28 Jupiter masses and a radius of 1.27 Jupiter radii.

The planet's density is exceptionally low at 0.14 times that of Jupiter.

The host star's metallicity is slightly sub-solar, at -0.12 dex.

Abstract

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 $4.055259 \pm 0.000008$\,d, Transit Epoch T$_{0}=2455342.9688\pm0.0002$\,(HJD), of duration $0.1168 \pm 0.0008$\,d. A combined analysis of the WASP photometry, high-precision follow-up transit photometry, and radial velocities yield a planetary mass of $\mpl=0.28\pm0.03\,\mj$ and a radius of $\rpl=1.27\pm0.04\,\rj$, resulting in a mean density of $0.14 \pm 0.02\,\rhoj$. The stellar parameters are mass $\mstar = 0.93 \pm 0.03\,\msun$, radius $\rstar = 0.895\pm 0.23\,\rsun$, 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 \feh\,$= -0.12\pm0.1$\,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.