Impact of Cold Jupiter Scattering on the Mean-motion Resonance of Inner Small Planets

Abstract A key feature of close-in, multiple super-Earth (SE) systems is the tendency for adjacent planet pairs to lie just wide of low-order mean-motion resonances (MMRs). This period ratio distribution has motivated numerous theoretical studies, particularly those invoking postdisk processes that perturb initially resonant architectures. We investigate whether orbital instability among cold Jupiters (CJs) can perturb inner SE systems initially in MMR. We show that a single pericenter passage of a highly eccentric CJ can disrupt inner resonances once a critical perturbation strength is exceeded, increasing the libration amplitude of the resonant angles. However, N -body simulations show that the deep penetration of CJs into the inner system is uncommon, with ≲10%–20% of cases reaching ≲10% of the initial semimajor axis of the innermost CJ. Motivated by these results, we use secular perturbation theory to quantify the impact of time-dependent forcing from scattering CJs on the eccentricity and resonant-angle evolution of inner SEs. We find that for typical systems (e.g., with SEs at ∼0.1 au and CJs at a few astronomical units), such forcing can efficiently disrupt resonances, driving resonance-angle circulation in most systems (≳60% for 2:1 and ∼85% for 3:2 configurations). Thus, even when the “final” CJ has little effect on the “current” SEs, its earlier scattering history can leave significant imprints on the system architecture. This mechanism and similar ones involving more abundant cold Neptunes provide a natural source of dynamical “kicks” and offer a pathway for producing the observed trough–peak structure in the period ratio distribution of Kepler multiplanet systems.