WASP-21b: a hot-Saturn exoplanet transiting a thick disc star

TL;DR

WASP-21b is a low-density Saturn-mass exoplanet orbiting a thick disc star, discovered through combined photometric and spectroscopic observations, highlighting correlations between planetary density and host star metallicity.

Contribution

This study reports the discovery and detailed characterization of WASP-21b, a Saturn-like exoplanet orbiting a thick disc star, and explores its implications for planetary formation and host star properties.

Findings

WASP-21b has a radius of 1.07 R_Jup and mass of 0.30 M_Jup.

The planet's density is approximately 0.24 times that of Jupiter.

WASP-21b is likely the first transiting planet in the thick disc population.

Abstract

We report the discovery of WASP-21b, a new transiting exoplanet discovered by the Wide Angle Search for Planets (WASP) Consortium and established and characterized with the FIES, SOPHIE, CORALIE and HARPS fiber-fed echelle spectrographs. A 4.3-d period, 1.1% transit depth and 3.4-h duration are derived for WASP-21b using SuperWASP-North and high precision photometric observations at the Liverpool Telescope. Simultaneous fitting to the photometric and radial velocity data with a Markov Chain Monte Carlo procedure leads to a planet in the mass regime of Saturn. With a radius of 1.07 R_Jup and mass of 0.30 M_Jup, WASP-21b has a density close to 0.24 rho_Jup corresponding to the distribution peak at low density of transiting gaseous giant planets. With a host star metallicity [Fe/H] of -0.46, WASP-21b strengthens the correlation between planetary density and host star metallicity for the five known Saturn-like transiting planets. Furthermore there are clear indications that WASP-21b is the first transiting planet belonging to the thick disc.