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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- Planets and satellites: dynamical evolution and stability
- Planets and satellites: individual (WASP-17b)
- Stars: individual (WASP-17)
- Techniques: radial velocities