Measurement of spin-orbit alignment in an extrasolar planetary system

Joshua N. Winn, Robert W. Noyes, Matthew J. Holman, David Charbonneau, Yasuhiro Ohta, Atsushi Taruya, Yasushi Suto, Norio Narita, Edwin L. Turner, John A. Johnson, Geoffrey W. Marcy, R. Paul Butler, Steven S. Vogt

Research output: Contribution to journalArticlepeer-review

256 Scopus citations


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 (λ = -4°4 ± 1.°4), and the line-of-sight rotation speed of the star (v sin I * -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 v sin I* is a rare case in which rotational line broadening has been isolated from other broadening mechanisms.

Original languageEnglish (US)
Pages (from-to)1215-1226
Number of pages12
JournalAstrophysical Journal
Issue number2 I
StatePublished - Oct 1 2005

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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