The HAT-P-13 exoplanetary system: Evidence for spin-orbit alignment and a third companion

Joshua N. Winn, John Asher Johnson, Andrew W. Howard, Geoffrey W. Marcy, Gáspár Bakos, Joel Hartman, Guillermo Torres, Simon Albrecht, Norio Narita

Research output: Contribution to journalArticlepeer-review

65 Scopus citations


We present new radial velocity (RV) measurements of HAT-P-13, a star with two previously known companions: a transiting giant planet "b" with an orbital period of 3 days and a more massive object "c" on a 1.2 yr, highly eccentric orbit. For this system, dynamical considerations would lead to constraints on planet b's interior structure, if it could be shown that the orbits are coplanar and apsidally locked. By modeling the Rossiter-McLaughlin effect, we show that planet b's orbital angular momentum vector and the stellar spin vector are well aligned on the sky (λ = 1.9 ± 8.6 deg). The refined orbital solution favors a slightly eccentric orbit for planet b (e = 0.0133 ± 0.0041), although it is not clear whether it is apsidally locked with c's orbit (Δω = 36+27-36 deg). We find a long-term trend in the star's RV and interpret it as evidence for an additional body "d," which may be another planet or a low-mass star. Predictions are given for the next few inferior conjunctions of c, when transits may happen.

Original languageEnglish (US)
Pages (from-to)575-582
Number of pages8
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 2010
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • C
  • D)
  • Planets and satellites: dynamical evolution and stability
  • Planets and satellites: formation
  • Planets and satellites: individual (HAT-P-13b
  • Planets and satellites: interiors
  • Stars: rotation


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