Measurement of spin-orbit alignment in an extrasolar planetary system

We determine the stellar, planetary, and orbital properties of the transiting planetary system HD 209458 through a joint analysis of high-precision radial velocities, photometry, and timing of the secondary eclipse. Of primary interest is the strong detection of the Rossiter-McLaughlin effect, the alteration of photospheric line profiles that occurs because the planet occults part of the rotating surface of the star. We develop a new technique for modeling this effect and use it to determine the inclination of the planetary orbit relative to the apparent stellar equator (gamma = -4 degrees.4 +/- 1 degrees.4), and the line-of-sight rotation speed of the star (vsinI(*) = 4.70 +/- 0.16 km s(-1)). The uncertainty in these quantities has been reduced by an order of magnitude relative to the pioneering measurements by Queloz and collaborators. The small but nonzero misalignment is probably a relic of the planet formation epoch, because the expected timescale for tidal coplanarization is larger than the age of the star. Our determination of vsinI(*) is a rare case in which rotational line broadening has been isolated from other broadening mechanisms.