WASP-117b: a longer-period planet in an eccentric, misaligned orbit

WASP-117b is an important milestone for the WASP project, being our first confirmed planet with an orbital period longer than 10 days. WASP is strongly biased towards short-period planets since these transit much more often and so are easy to detect, but by accumulating data from multiple seasons we can extend the coverage out to longer periods.

The planet WASP-117b is in a highly eccentric orbit that is misaligned with the spin of the host star. This is in keeping with the suggestion that shorter-period planets are forced into aligned, circular orbits by tidal dissipation. Since tidal dissipation weakens markedly as the orbit widens, we expect the longer-period planets to have un-damped orbits.

Since WASP-117b is a Saturn-mass planet, and given the long, eccentric orbit, it took much more follow-up study than usual in order to tie down its characteristics. Tracing out the radial-velocity orbit took 70 observations with the 1.2-m Euler telescope and CORALIE spectrograph, plus 53 more with the ESO 3.6-m telescope and HARPS spectrograph. As usual for WASP-South planets, EulerCAM and TRAPPIST produced the high-quality transit lightcurves.

Since WASP-117b orbits a relatively bright star, at V = 10.1, it will be an important object for studying the population of longer-period exoplanets,