The Prograde Orbit of Exoplanet TrES-2b

We monitored the Doppler shift of the G0 V 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° relative to the projected stellar equator. With 98% confidence, the orbit is prograde.

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