Transit Duration Variations in Multiplanet Systems

Abstract

Abstract A planet’s orbital orientation relative to an observer’s line of sight determines the chord length for a transiting planet, i.e., the projected distance a transiting planet travels across the stellar disk. For a given circular orbit, the chord length determines the transit duration. Changes in the orbital inclination, the longitude of ascending node, or both, can alter this chord length and thus result in transit duration variations (TDVs). Variation of the full orbital inclination vector can even lead to de-transiting or newly transiting planets for a system. We use Laplace-Lagrange secular theory to estimate the fastest nodal eigenfrequencies for over 100 short-period planetary systems. The highest eigenfrequency is an indicator of which systems should show the strongest TDVs. We further explore five cases (TRAPPIST-1, Kepler-11, K2-138, Kepler-445, and Kepler-334) using direct N-body simulations to characterize possible TDVs and to explore whether de-transiting planets might be possible for these systems. A range of initial conditions are explored, in which each realization is consistent with the observed transits. We find that tens of percent of multiplanet systems have fast enough eigenfrequencies to expect large TDVs on decade timescales. For the directly integrated cases, we find that de-transiting planets might occur on decade timescales, and TDVs of 10 minutes per decade are expected to be common.