A lower mass for the exoplanet WASP-21b

Abstract

We present high-precision transit observations of the exoplanet WASP-21b, obtained with the Rapid Imager to Search for Exoplanets instrument mounted on the 2.0-m Liverpool Telescope. A transit model is fitted, coupled with a Markov chain Monte Carlo routine, to derive accurate system parameters. The two new high-precision transits allow us to estimate the stellar density directly from the light curve. Our analysis suggests that WASP-21 is evolving off the main sequence which led to a previous overestimation of the stellar density. Using isochrone interpolation, we find a stellar mass of 0.86 ± 0.04M⊙, which is significantly lower than previously reported (1.01 ± 0.03M⊙). Consequently, we find a lower planetary mass of 0.27 ± 0.01MJup. A lower inclination (874 ± 03) is also found for the system than previously reported, resulting in a slightly larger stellar (R*= 1.10 ± 0.03R⊙) and planetary radius (Rp= 1.14 ± 0.04RJup). The planet radius suggests a hydrogen/helium composition with no core which strengthens the correlation between planetary density and host star metallicity. A new ephemeris is determined for the system, i.e. T0= 2455084.51974 ± 0.00020 (HJD) and P= 4.3225060 ± 0.0000031d. We found no transit timing variations in WASP-21b. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.