Kepler-80 Revisited: Assessing the Participation of a Newly Discovered Planet in the Resonant Chain

The Kepler-80 system was thought to be a system of five transiting short-period planets with the outer four in a resonant chain. Recently, a sixth planet was discovered by Shallue & Vanderburg (2018). We consider the chain of resonances in the Kepler-80 system, and we evaluate the impact that the additional planet has on the dynamics of the system and the physical parameters that can be recovered by a fit to the transit timing variations (TTVs). Ultimately, we calculate the mass of Kepler-80 g to be 0.8 ± 0.3 M⊕ when assuming all planets have zero eccentricity, and 1.0 ± 0.3 M⊕ when relaxing that assumption. We show that the outer five planets are in successive three-body mean-motion resonances (MMRs). We assess the current state of two-body MMRs in the system and find that the planets do not appear to be in two-body MMRs. We find that while the existence of the additional member of the resonant chain does not significantly alter the character of the Kepler-80 three-body MMRs, it can alter the physical parameters derived from the TTVs, suggesting caution should be applied when drawing conclusions from TTVs for potentially incomplete systems. We find that while different fit methodologies and underlying assumptions can result in different measured orbital parameters, the most secure conclusion is that which holds true across all lines of analysis: Kepler-80 contains a chain of planets in three-body MMRs but not in two-body MMRs.