Orbital Resonance

Orbital resonance occurs when two orbiting bodies exert a regular, repeating gravitational influence on each other because their orbital periods form a simple, whole-number ratio, such as 2:1 or 3:2, causing their gravitational tugs to consistently reinforce one another at the same points in their orbits rather than averaging out randomly over time. One of the most famous examples in our own solar system involves three of Jupiter's large moons, Io, Europa, and Ganymede, which are locked in a 4:2:1 resonance: for every one orbit Ganymede completes, Europa completes exactly two and Io completes exactly four, a precise, repeating gravitational dance that has persisted for billions of years and generates significant internal heating in Io and Europa through the resulting tidal forces. Resonances can also actively clear gaps rather than lock objects together: the Kirkwood gaps in the asteroid belt are regions conspicuously empty of asteroids because any object that wandered there would fall into an unstable resonance with Jupiter, and repeated gravitational nudges over long timescales would eventually fling it out of that orbit entirely. Orbital resonances appear throughout the solar system, from Neptune and Pluto's stable 3:2 resonance, which actually prevents them from ever colliding despite their orbits crossing, to the intricate, tightly packed resonant chains discovered in some exoplanet systems around other stars.