Title: Breaking mean-motion resonances during Type I planet migration,
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.