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

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 $0.45\pm 0.03{\mathrm{R}}_{\mathrm{jup}}$ and $0.43\pm 0.05{\mathrm{R}}_{\mathrm{jup}}$ and spin-orbit misalignments $72^{\circ }\pm 3^{\circ }$ and $73_{-5}^{\circ +11}$, 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.

9 pages, 7 figures, 3 tables, accepted in The Astrophysical Journal