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 \mathrm { R } _ { Jup } and mass of 0.30 \mathrm { 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 .