Confirmation of a retrograde orbit for exoplanet wasp-17b

We present high-precision radial velocity observations of WASP-17 throughout the transit of its close-in giant planet, using theMIKE spectrograph on the 6.5 mMagellan Telescope at Las Campanas Observatory. By modeling the Rossiter-McLaughlin effect, we find the sky-projected spin-orbit angle to be λ = 167.4±11.2 deg. This independently confirms the previous finding thatWASP-17b is on a retrograde orbit, suggesting it underwent migration via amechanism other than just the gravitational interaction between the planet and the disk. Interestingly, our result for λ differs by 45 ± 13 deg from the previously announced value, and we also find that the spectroscopic transit occurs 15 ± 5 minutes earlier than expected, based on the published ephemeris. The discrepancy in the ephemeris highlights the need for contemporaneous spectroscopic and photometric transit observations whenever possible.