Intruder Alert: Breaking Resonant Chains with Planetesimal Flybys

Abstract The orbital architectures of compact “Kepler-multi” (KM) exoplanet systems record their complicated dynamical histories. The “breaking-the-chains” hypothesis proposes that compact systems typically form in chains of mean-motion resonances and subsequently break out on a ∼100 Myr timescale. We investigate a scenario for breaking the chains through intermittent flybys of planetesimals originating from a distant reservoir. Using N -body simulations and semianalytical calculations, we find that the probability of escaping resonance through flybys depends mainly on the product of the typical planetesimal mass and the square root of the number of flybys, or “root-cumulative mass” (RCM); an RCM ≳ 50 M ⊕ is required to disrupt >50% of compact KM chains. We perform long-term integrations to find the fraction of chains rendered unstable by flybys; we find that an RCM ∼ 50–100 M ⊕ reproduces the late-time (≳100 Myr) occurrence rate of resonant KM systems. This mechanism operates in systems with both a sufficiently massive reservoir and an efficient mechanism for planetesimal injection. Consequently, we predict an anticorrelation between resonant inner systems and dynamically active outer configurations.