Stellar Spin-Orbit Alignment for Kepler-9, a Multi-transiting Planetary system with Two Outer Planets Near 2:1 Resonance

TL;DR

This study measures the spin-orbit alignment of Kepler-9's planet b, finding it to be aligned, and adds to evidence that multi-planet systems tend to have aligned orbits.

Contribution

First Rossiter-McLaughlin measurement for Kepler-9, supporting the trend of aligned orbits in multi-planet systems.

Findings

Sky-projected spin-orbit angle is approximately -13 degrees.

System strongly disfavors misaligned, polar, and retrograde orbits.

Supports the hypothesis that multi-planet systems are generally aligned.

Abstract

We present spectroscopic measurements of the Rossiter-McLaughlin effect for the planet b of Kepler-9 multi-transiting planet system. The resulting sky-projected spin-orbit angle is $\lambda=-13^{\circ} \pm 16^{\circ}$, which favors an aligned system and strongly disfavors highly misaligned, polar, and retrograde orbits. Including Kepler-9, there are now a total of 4 Rossiter-McLaughlin effect measurements for multiplanet systems, all of which are consistent with spin-orbit alignment.