WASP-117b: a 10-day-period Saturn in an eccentric and misaligned orbit

TL;DR

WASP-117b is a newly discovered 10-day period Saturn-sized exoplanet with an eccentric, misaligned orbit around a bright star, making it ideal for atmospheric studies and offering insights into planetary dynamics.

Contribution

This is the first planet with a period beyond 10 days discovered by the WASP survey, with detailed orbital and physical characterization including misalignment.

Findings

WASP-117b has a mass of 0.2755 M_J and a radius of 1.021 R_J.

The planet's orbit is eccentric (e=0.302) and highly misaligned (obliquity ~70°).

The host star's brightness facilitates follow-up atmospheric observations.

Abstract

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 $M_p= 0.2755 \pm 0.0089 \, M_{J}$, a radius of $R_p= 1.021_{-0.065}^{+0.076}\, R_{J}$ 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 ($\rho_p= 0.259_{-0.048}^{+0.054} \, \rho_{J}$) 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.