Creating Pileups of Eccentric Planet Pairs Wide of Mean Motion Resonances through Divergent Migration

Abstract Observed pileups of planets with period ratios ≈1% wide of strong mean motion resonances (MMRs) pose an important puzzle. Early models showed that they can be created through sustained eccentricity damping, driving a slow separation of the orbits, but this picture is inconsistent with elevated eccentricities measured through transit-timing variations (TTVs). We argue that any source of divergent migration (tides, planet–disk interactions, etc.) will cause planets that encounter an MMR to both jump over it (piling up wide of resonance) and get a kick to their eccentricities. We find that normalizing observed deviations from resonance by the value at which the pair would encounter and jump over the resonance leads to a significantly clearer trough narrow of the resonance. We additionally find that the jumps in eccentricity expected from slow MMR crossings are sufficiently large to explain the scale of eccentricities measured through TTVs, though some residual eccentricity damping would be required to quantitatively match observations.