Dependence of Lifetimes of Closely-Spaced Planetary Systems on Number of Planets

We compare the lifetimes of geometrically-spaced systems of multiple 1-Earth-mass planets orbiting a 1-solar-mass star for systems of three planets with those of higher-multiplicity planetary systems. Lissauer and Gavino (2021, Icarus 364, 114470) found that a nontrivial fraction of very closely-spaced three-planet systems survive for orders of magnitude longer than most systems with similar orbital separation; no analogously long-lived closely-spaced five-planet systems were found in the extensive study conducted by Obertas, Van Laerhoven, and Tamayo (2017, Icarus, 293, 52-58). Consistent with previous studies, we find that systems with more planets generally have shorter lifetimes, with the biggest drop going from three to four planets. We do not find any four-planet systems that are anomalously long-lived. We quantify differences in lifetimes of otherwise identical systems (same initial separations between neighboring planets) with different multiplicities and identify resonances between planets orbiting far from one another that are responsible for the largest reductions in lifetime.