First Evidence of a Retrograde Orbit of a Transiting Exoplanet HAT-P-7b

We present the first evidence of a retrograde orbit of a transiting exoplanet HAT-P-7b. The discovery is based on a measurement of the Rossiter-McLaughlin effect with the Subaru HDS during a transit of HAT-P-7b, which occurred on 2008 May 30 UT. Our best-fit model shows that the spin-orbit alignment angle of this planet is lambda = -132 degrees.6(-16 degrees.3)(+10 degrees.5). The existence of such a retrograde planet has been predicted by recent planetary migration models that consider planet-planet scattering processes, or Kozai migration. Our finding provides an important milestone that supports such dynamic migration theories.