Planetary Migration & Orbital Resonance: Physical Mechanisms

Migration can move a planet inward or outward depending on how torques are distributed through a disk or planetesimal population. Resonances such as 2:1 or 3:2 repeat orbital geometry at regular intervals, creating stable configurations in some cases and destabilizing ones in others. Giant planets can reshape belts of smaller bodies through both migration and resonance sweeping.

A planet gravitationally perturbs nearby gas or planetesimals and receives an equal-and-opposite change in orbital angular momentum. Persistent torque changes the planet's semimajor axis. In a resonance, conjunctions recur at preferred orbital phases, so many small gravitational kicks reinforce one another instead of averaging away.

Hot Jupiters demonstrate that some giant planets now orbit far inside the cold regions where they are thought to have formed. Resonant exoplanet pairs, resonant Kuiper-belt objects, gaps in asteroid distributions, and dynamical simulations show that migration and resonance are common consequences of multi-body gravity.

A planet exchanges angular momentum with gas by launching density waves. Depending on disk and planet properties, the net torque can drive inward or outward migration. Convergent migration can capture planets into resonance, while later instability can break resonances and excite eccentricities or inclinations.