Mean-motion resonance

A mean-motion resonance occurs when orbiting bodies complete orbital periods in a ratio of small integers. Repeated gravitational alignments can then reinforce their orbital perturbations.

Orbital period: The time an orbiting body takes to complete one revolution around another body. Resonance is defined by ratios between the bodies’ orbital periods.
Pluto: A dwarf planet in the Kuiper belt that completes two orbits for every three completed by Neptune. Its 3:2 resonance with Neptune protects it from close encounters despite orbit crossing.
Planetary migration: The change in a planet’s orbit over time through gravitational interactions with other bodies or a disk. Changing orbital periods sweep resonances across small-body populations and can capture them.
Transit timing variation: A departure in a transiting planet’s observed transit times from a strictly periodic schedule. Resonant planets exert repeated perturbations that produce measurable timing variations.
Secular resonance: A resonance involving matching rates of slow orbital precession rather than matching orbital periods. It can alter orbits without the integer period ratio that defines mean-motion resonance.
Orbital resonance: A relationship between orbital motions in which periodic gravitational interactions shape bodies’ orbits. Mean-motion resonance is the orbital form driven by commensurate average orbital rates.
Neptune: The eighth planet from the Sun and the dominant gravitational perturber of many Kuiper belt objects. Its outward migration captured Pluto and other bodies into mean-motion resonances.
Resonance capture: The process by which a body becomes trapped in a resonance as orbital conditions change. It explains how migrating planets build resonant populations rather than merely perturbing them.
Exoplanet: A planet that orbits a star other than the Sun. Resonance patterns help characterize the architecture and dynamical history of exoplanet systems.
Orbital conjunction: An apparent alignment of two orbiting bodies in longitude as viewed from a central body. Conjunctions recur in resonant patterns, but a single alignment is not itself a resonance.

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