A Five-Planet Resonant Chain: Reevaluation of the Kepler-80 System

Abstract: Since the launch of the Kepler space telescope in 2009 and the subsequent K2 mission, hundreds of multi-planet systems have been discovered. The study of such systems, both as individual systems and as a population, leads to a better understanding of planetary formation and evolution. Kepler-80, a K-dwarf hosting six super-Earths, was the first system known to have four planets in a chain of resonances, a repeated geometric configuration. Transiting planets in resonant chains can enable us to estimate not only the planets' orbits and sizes but also their masses. Since the original resonance analysis and TTV fitting of Kepler-80, a new planet has been discovered whose signal likely altered the measured masses of the other planets. Here, we determine masses and orbits for all six planets hosted by Kepler-80 by direct forward photodynamical modeling of the lightcurve of this system. We then explore the resonant behaviour of the system. We find that the four middle planets are in a resonant chain, but that the outermost planet only dynamically interacts in $\sim14$\% of our solutions. We also find that the system and its dynamic behaviour are consistent with \emph{in situ} formation and compare our results to two other resonant chain systems, Kepler-60 and TRAPPIST-1.