Breaking mean-motion resonances during Type I planet migration

TL;DR

This study uses hydrodynamical simulations to show that planets migrating in protoplanetary discs can break out of mean-motion resonances due to overstable librations, impacting planetary system formation.

Contribution

It demonstrates the resonance-breaking mechanism during Type I migration through hydrodynamical simulations, highlighting differences from previous analytic models.

Findings

Resonance breaking occurs within a few hundred orbits.

The mechanism is effective in both viscous and inviscid discs.

It may explain the scarcity of resonances in observed planetary systems.

Abstract

We present two-dimensional hydrodynamical simulations of pairs of planets migrating simultaneously in the Type I regime in a protoplanetary disc. Convergent migration naturally leads to the trapping of these planets in mean-motion resonances. Once in resonance the planets' eccentricity grows rapidly, and disc-planet torques cause the planets to escape resonance on a time-scale of a few hundred orbits. The effect is more pronounced in highly viscous discs, but operates efficiently even in inviscid discs. We attribute this resonance-breaking to overstable librations driven by moderate eccentricity damping, but find that this mechanism operates differently in hydrodynamic simulations than in previous analytic calculations. Planets escaping resonance in this manner can potentially explain the observed paucity of resonances in Kepler multi-transiting systems, and we suggest that simultaneous disc-driven migration remains the most plausible means of assembling tightly-packed planetary systems.