A possible spin-orbit misalignment in the transiting eccentric planet HD 17156b

We present simultaneous photometric and spectroscopic observations of HD 17156b spanning a transit on 2007 November 12 (UT). This system is of special interest because of its 21 d period (unusually long for a transiting planet) and its high orbital eccentricity of 0.67. By modeling the Rossiter-McLaughlin effect, we find the angle between the sky projections of the orbital axis and the stellar rotation axis to be 62 degrees +/- 25 degrees. Such a large spin-orbit misalignment, as well as the high eccentricity, could be explained as a relic of a previous gravitational interaction with other planets.