We report the discovery of WASP-117b , the first planet with a period beyond 10 days found by the WASP survey .
The planet has a mass of M _ { p } = 0.2755 \pm 0.0089 \mbox { $ { M } _ { J } $ } , a radius of R _ { p } = 1.021 _ { -0.065 } ^ { +0.076 } \mbox { $ { R } _ { J } $ } and is in an eccentric ( e = 0.302 \pm 0.023 ) , 10.02165 \pm 0.00055 d orbit around a main-sequence F9 star .
The host star ’ s brightness ( V=10.15 mag ) makes WASP-117 a good target for follow-up observations , and with a periastron planetary equilibrium temperature of T _ { eq } = 1225 _ { -39 } ^ { +36 } K and a low planetary mean density ( \rho _ { p } = 0.259 _ { -0.048 } ^ { +0.054 } \mbox { $ \rho _ { J } $ } ) it is one of the best targets for transmission spectroscopy among planets with periods around 10 days .
From a measurement of the Rossiter-McLaughlin effect , we infer a projected angle between the planetary orbit and stellar spin axes of \beta = -44 \pm 11 deg , and we further derive an orbital obliquity of \psi = 69.6 ^ { +4.7 } _ { -4.1 } deg .
Owing to the large orbital separation , tidal forces causing orbital circularization and realignment of the planetary orbit with the stellar plane are weak , having had little impact on the planetary orbit over the system lifetime .
WASP-117b joins a small sample of transiting giant planets with well characterized orbits at periods above 8 days .