The Craziness of the KOI-89 Exoplanetary System

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

Authors:

Ahlers et al

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±0.03 Rjup and 0.43±0.05 Rjup and spin-orbit misalignments 72∘±3∘ and 73∘+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∘±20∘ -- 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.