Confirmation of a Retrograde Orbit for Exoplanet WASP-17b

TL;DR

This study confirms that exoplanet WASP-17b orbits in a retrograde direction using high-precision radial velocity measurements, revealing insights into its migration history and emphasizing the importance of synchronized observations.

Contribution

First independent confirmation of WASP-17b's retrograde orbit through detailed Rossiter-McLaughlin effect modeling, with updated measurements and discussion on transit timing discrepancies.

Findings

WASP-17b has a sky-projected spin-orbit angle of 167.4 ± 11.2 degrees.

The spectroscopic transit occurs 15 ± 5 minutes earlier than predicted.

The measured lambda differs from previous estimates by 45 ± 13 degrees.

Abstract

We present high-precision radial velocity observations of WASP-17 throughout the transit of its close-in giant planet, using the MIKE spectrograph on the 6.5m Magellan Telescope at Las Campanas Observatory. By modeling the Rossiter-McLaughlin effect, we find the sky-projected spin-orbit angle to be lambda = 167.4 \pm 11.2 deg. This independently confirms the previous finding that WASP-17b is on a retrograde orbit, suggesting it underwent migration via a mechanism other than just the gravitational interaction between the planet and the disk. Interestingly, our result for lambda differs by 45 \pm 13 deg from the previously announced value, and we also find that the spectroscopic transit occurs 15 \pm 5 min 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.