Light-Curve Evolution due to Secular Dynamics and the Vanishing Transits of KOI 120.01

Abstract

Non-Keplerian dynamics of planetary orbits manifest in the transit light-curve as variations of different types. In addition to Transit Timing Variations (TTV's), the shape of the transits contains additional information on variations in the geometry of the orbit. This study presents an analytic approach to light-curve fitting: dynamical variations in the orbital elements are transformed to a light-curve using an analytic function with a restricted set of fitting parameters. Our method requires no N-body integration, resulting in a smaller number of degrees of freedom and a faster calculation. The approach described here is for the case of secular perturbations. By assuming that the orbital motion is dominated by nodal and apsidal precessions, analytic expressions for the light-curve transit parameters are derived as a function of the orbital variations. Detecting and characterizing such dynamical scenarios provides information regarding the possible existence of non-transiting companions, or the non-spherical mass distribution of the host star. The variations may imply forces out of the orbital plane, and thus probe mutual inclinations among components of the system. The derived models successfully reproduce the vanishing transit signals of KOI 120.01, and suggest a possible interesting scenario of a planet orbiting one member of a close-in binary system undergoing unusually rapid nodal regression. The model parameters are degenerate, so we provide relevant information for followup observations, which are suggested in order to place further constraints on this unique Kepler object.