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

TL;DR

This paper provides the first observational evidence of a retrograde orbit for exoplanet HAT-P-7b, supporting dynamic migration theories like planet-planet scattering and Kozai migration.

Contribution

It reports the first measurement of a retrograde orbit for HAT-P-7b using Rossiter-McLaughlin effect observations, confirming predictions of dynamic migration models.

Findings

HAT-P-7b has a spin-orbit angle of approximately -133 degrees.

The retrograde orbit supports planet migration theories involving scattering or Kozai mechanisms.

The measurement was made using Subaru HDS during a transit in 2008.

Abstract

We present the first evidence of a retrograde orbit of the 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 UT 2008 May 30. Our best-fit model shows that the spin-orbit alignment angle of this planet is \lambda = -132.6 (+10.5, -16.3) degrees. The existence of such a retrograde planet have been predicted by recent planetary migration models considering planet-planet scattering processes or the Kozai migration. Our finding provides an important milestone that supports such dynamic migration theories.