Spin-Orbit Misalignment of Two-Planet-System KOI-89 Via Gravity Darkening

TL;DR

This study measures the true spin-orbit misalignment of the KOI-89 two-planet system using gravity darkening effects on Kepler lightcurves, revealing significant misalignment with the star but potential alignment between the planets.

Contribution

It provides the first detailed measurement of the KOI-89 system's spin-orbit angles and explores possible mechanisms for its misalignment, combining photometric fitting and dynamical stability analysis.

Findings

Planets are low-density giants with radii ~0.45 Rjup.

Spin-orbit misalignments are approximately 72-73 degrees.

The planets may be aligned with each other within 20 degrees.

Abstract

We constrain the true spin-orbit alignment of the KOI-89 system by numerically fitting the two \emph{Kepler} photometric lightcurves produced by transiting planets KOI-89.01 and KOI-89.02. The two planets have periods of 84.69 days and 207.58 days, respectively. We find that the two bodies are low-density giant planets with radii $0.45 \pm 0.03~\mathrm{R_{jup}}$ and $0.43 \pm 0.05~\mathrm{R_{jup}}$ and spin-orbit misalignments $72^{\circ} \pm 3^\circ$ and $73^{\circ+11}_{-5}$, respectively. Via dynamic stability tests we demonstrate the general trend of higher system stability with the two planets close to mutual alignment and estimate their coalignment angle to $20^\circ \pm 20^\circ$ -- i.e. the planets are misaligned with the star but may be aligned with each other. From these results, we limit KOI-89's misalignment mechanisms to star-disk-binary interactions, disk warping via planet-disk interactions, planet-planet scattering, Kozai resonance, or internal gravity waves.