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

We constrain the true spin-orbit alignment of the KOI-89 system by numerically fitting the two Kepler photometric light curves produced by the transiting planets KOI-89.01 and KOI-89.02. The two planets have periods of 84.69 and 207.58 days, respectively. We find that the two bodies are low-density giant planets with radii of 0.45 ± 0.03 R_jup and 0.43 ± 0.05 R_jup and spin-orbit misalignments of 72° ± 3° and {73}_-5^{^\circ +11}, respectively. Through 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.