Planetary Migration & Orbital Resonance: Structures & Vocabulary

Planetary orbits can change substantially after formation. Migration occurs when a planet exchanges angular momentum with a gas disk or surrounding smaller bodies, while orbital resonance occurs when two orbital periods maintain a near-integer ratio that allows repeated gravitational interactions to build coherently.

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.

Mean-motion resonance occurs when orbital periods lie near ratios of small integers, such as 2:1 or 3:2. Resonant angles can librate instead of circulating. Disk-driven migration, planet-planet scattering, and planetesimal interactions can all move planets, while resonant chains can lock several bodies into coupled architecture.