HAT-P-7: A Retrograde or Polar Orbit, and a Third Body

TL;DR

This paper reveals that exoplanet HAT-P-7b has an extremely tilted, possibly retrograde orbit, and provides evidence for an additional planetary or stellar companion based on precise radial velocity measurements.

Contribution

It demonstrates the first detailed measurement of a highly tilted or retrograde orbit for HAT-P-7b and suggests the presence of a third body influencing its orbit.

Findings

HAT-P-7b has an orbit inclined at least 86 degrees, likely retrograde.

Evidence of a third body from radial velocity variations.

Inversion of Rossiter-McLaughlin effect indicating extreme orbital tilt.

Abstract

We show that the exoplanet HAT-P-7b has an extremely tilted orbit, with a true angle of at least 86 degrees with respect to its parent star's equatorial plane, and a strong possibility of retrograde motion. We also report evidence for an additional planet or companion star. The evidence for the unparalleled orbit and the third body is based on precise observations of the star's apparent radial velocity. The anomalous radial velocity due to rotation (the Rossiter-McLaughlin effect) was found to be a blueshift during the first half of the transit and a redshift during the second half, an inversion of the usual pattern, implying that the angle between the sky-projected orbital and stellar angular momentum vectors is 182.5 +/- 9.4 degreees. The third body is implicated by excess radial-velocity variation of the host star over 2 yr. Some possible explanations for the tilted orbit are a close encounter with another planet, the Kozai effect, and resonant capture by an inward-migrating outer planet.