Instabilities in compact multiplanet systems: dynamical insights from numerical experiments

Observational surveys have revealed that multiplanet systems are common, but generally differ substantially from the solar system. Typical multiplanet systems contain several planets orbiting close-in to the star with low inclinations and eccentricities. To be observable, planets in these systems have survived over long timescales; less-fortunate planets have instead collided, fallen into their host stars, or been ejected from their systems. Understanding the dynamical sculpting of planetary systems is crucial for interpreting observations. However, the mechanisms that cause compact multiplanet systems to destabilize are not yet fully understood. In fact, several different dynamical mechanisms have been proposed as the primary driver of instabilities in compact multiplanet systems, including secular chaos, three-body mean-motion resonances (MMRs), and the secular modulation of two-body MMRs. To this end, we have performed a suite of N-body simulations for systems of five equally-spaced planets spanning a range of eccentricities. With a novel approach of building successive Hamiltonian models and comparing results with that of N-body simulations, we are able to deduce the relevant dynamics. Our results provide insight into the causes of instability in compact multiplanet systems and demonstrate the usefulness of this numerical approach to dynamics, made easy by the open-source Celmech code.