Revisiting the Architecture of the KOI-89 System

Abstract

While high stellar obliquities observed in exoplanetary systems may be attributed to processes that tilt the planetary orbits, it is also possible that they reflect misalignments between protoplanetary disks and stellar spins. This latter hypothesis predicts the presence of co-planar multi-planetary systems misaligned with their central stars. Here we re-evaluate the evidence of such an architecture that has been claimed for the KOI-89 system. KOI-89 is an early-type star with one validated transiting planet KOI-89.01/Kepler-462b (period 84.7 days, radius $3.0\,R_\oplus$) and one transiting planet candidate KOI-89.02 (period 207.6 days, radius $4.0\,R_\oplus$), where the latter exhibits transit timing variations (TTVs). A previous modeling of the stellar gravity-darkening effect in the transit light curves inferred a high stellar obliquity of $\approx70^\circ$. We perform a photodynamical modeling of the Kepler transit light curves, and use the resulting constraints on the orbital configuration and transit times to update the gravity-darkened transit model. As a result, we find no firm evidence for gravity darkening effect in the transit shapes and conclude that stellar obliquity is not constrained by the data. Given evidence for low orbital eccentricities from the dynamical analysis, the system architecture can thus be consistent with many other multi-transiting systems with flat, near-circular orbits aligned with the stellar spin. We find that the TTVs imparted on its neighbor imply that KOI-89.01 has a mass $\gtrsim20\,M_\oplus$. This would render it one of the densest known sub-Neptunes, mostly composed of a solid core. Lower masses are possible if the TTVs are instead due to an unseen third planet.