Breaking Giant Chains: Early-Stage Instabilities in Long-Period Giant Planet Systems

Orbital evolution is a critical process that sculpts planetary systems, particularly during their early stages where planet-disk interactions are expected to lead to the formation of resonant chains. Despite the theoretically expected prominence of such configurations, they are scarcely observed among long-period giant exoplanets. This disparity suggests an evolutionary sequence wherein giant planet systems originate in compact multi-resonant configurations, but subsequently become unstable, eventually relaxing to wider orbits–a phenomenon mirrored in our own solar system's early history. In this work, we present a suite of N-body simulations that model the instability-driven evolution of giant planet systems, originating from resonant initial conditions, through phases of disk-dispersal and beyond. By comparing the period ratio and normalized angular momentum deficit distributions of our synthetic aggregate of systems with the observational census of long-period Jovian planets, we derive constraints on the expected rate of orbital migration, efficiency of gas-driven eccentricity damping, and typical initial multiplicity. Our findings reveal a distinct inclination towards densely-packed initial conditions, weak damping, and high giant planet multiplicities. Furthermore, our models indicate that resonant chain origins do not facilitate the formation of Hot Jupiters via the coplanar high-eccentricity pathway at rates high enough to explain their observed prevalence.