Creating Pileups of Eccentric Planet Pairs Wide of MMRs Through Divergent Migration

TL;DR

This paper investigates how divergent migration causes planet pairs to pile up just outside strong mean motion resonances, explaining observed period ratio distributions and eccentricities.

Contribution

It introduces a new perspective that divergent migration leads to resonance crossing jumps and eccentricity excitation, aligning models with observed eccentricities.

Findings

Divergent migration causes planets to jump over MMRs and gain eccentricity.

Eccentricity jumps from slow MMR crossings can explain observed free eccentricities.

Non-adiabatic crossings and residual damping can reconcile models with observations.

Abstract

Observed pileups of planets with period ratios $\approx 1\%$ 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. 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 free eccentricity. We find that the jumps in eccentricity expected from slow MMR crossings are sufficient (but mostly too large) to explain the free eccentricities measured through TTVs. We argue that this mechanism can be brought in line with observations if MMR crossings are not adiabatic and/or through residual eccentricity damping.