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

F. Bouchy, L. Hebb, I. Skillen, A. Collier Cameron, B. Smalley, S. Udry, D.~R. Anderson, I. Boisse, B. Enoch, C.~A. Haswell, G. Hébrard, C. Hellier, Y. Joshi, S.~R. Kane, P.~F.~L. Maxted, M. Mayor, C. Moutou, F. Pepe, D. Pollacco, D. QuelozD. Ségransan, E.~K. Simpson, A.~M.~S. Smith, H.~C. Stempels, R. Street, A.~H.~M.~J. Triaud, R.~G. West, P.~J. Wheatley

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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 $$_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. Based on observations made with the SuperWASP-North camera hosted by the Isaac Newton Group on La Palma, the FIES spectrograph on the Nordic Optical Telescope, the CORALIE spectrograph on the 1.2-m Euler Swiss telescope on La Silla Observatory, the SOPHIE spectrograph on the 1.93-m telescope on Haute Provence Observatory and the HARPS spectrograph on the 3.6-m ESO telescope at La Silla Observatory under programs 081.C-0388, 082.C-0040, 084.C-0185.Tables of photometric data are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/519/A98
Original languageEnglish
Pages (from-to)A98
JournalAstronomy and Astrophysics
Publication statusPublished - 1 Sept 2010


  • planetary systems, techniques: photometric, techniques: radial velocities


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