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 ( \lambda = -4 \fdg 4 \pm 1 \fdg 4 ) , and the line-of-sight rotation speed of the star ( v \sin I _ { \star } = 4.70 \pm 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 v \sin I _ { \star } is a rare case in which rotational line broadening has been isolated from other broadening mechanisms .