Migration of Planets Into and Out of Mean Motion Resonances in Protoplanetary Discs: Overstability of Capture and Nonlinear Eccentricity Damping

TL;DR

This paper investigates how nonlinear eccentricity damping affects the stability and outcomes of planet migration into mean motion resonances, revealing new phenomena like eccentricity overshoot that can lead to planet ejection.

Contribution

It introduces a more realistic parametrization of planet-disk interactions based on hydrodynamical simulations, enhancing understanding of resonance capture and stability.

Findings

Nonlinear eccentricity damping increases equilibrium eccentricities.

Resonance captures tend to be more stable with nonlinear damping.

Eccentricity overshoot can cause planet ejection, explaining observed planetary system distributions.

Abstract

A number of multiplanet systems are observed to contain planets very close to mean motion resonances, although there is no significant pileup of precise resonance pairs. We present theoretical and numerical studies on the outcome of capture into first-order mean motion resonances (MMRs) using a parametrized planet migration model that takes into account nonlinear eccentricity damping due to planet-disk interaction. This parametrization is based on numerical hydrodynamical simulations and is more realistic than the simple linear parametrization widely used in previous analytic studies. We find that nonlinear eccentricity damping can significantly influence the stability and outcome of resonance capture. In particular, the equilibrium eccentricity of the planet captured into MMRs become larger, and the captured MMR state tends to be more stable compared to the prediction based on the simple migration model. In addition, when the migration is sufficiently fast or/and the planet mass ratio is sufficiently small, we observe a novel phenomenon of eccentricity overshoot, where the planet's eccentricity becomes very large before settling down to the lower equilibrium value. This can lead to the ejection of the smaller planet if its eccentricity approaches unity during the overshoot. This may help explain the lack of low-mass planet companion of hot Jupiters when compared to warm Jupiters.