The Prograde Orbit of Exoplanet TrES-2b

TL;DR

This study measures the Rossiter-McLaughlin effect during TrES-2b's transit, revealing a prograde orbit with a slight tilt, despite the star's slow rotation, confirming the planet's orbital direction.

Contribution

First measurement of the Rossiter-McLaughlin effect for TrES-2b, demonstrating a prograde orbit with a small tilt relative to the stellar equator.

Findings

Orbit is prograde with 98% confidence.

The stellar rotation axis is nearly aligned with the planet's orbital plane.

The star's slow rotation still allows detection of the effect.

Abstract

We monitored the Doppler shift of the G0V star TrES-2 throughout a transit of its giant planet. The anomalous Doppler shift due to stellar rotation (the Rossiter-McLaughlin effect) is discernible in the data, with a signal-to-noise ratio of 2.9, even though the star is a slow rotator. By modeling this effect we find that the planet's trajectory across the face of the star is tilted by -9 +/- 12 degrees relative to the projected stellar equator. With 98% confidence, the orbit is prograde.