orbital resonance
Push a child on a swing at just the right moments and small shoves add up to a big motion. Planets and moons do something similar to one another: when their orbital periods fall into a simple whole-number ratio, their gravitational tugs line up over and over in the same place, and the small kicks accumulate. That repeated, reinforcing alignment is an orbital resonance.
A resonance occurs when two bodies complete orbits in a ratio of small integers — 2:1 (one body orbits twice for every one orbit of the other), 3:2, 4:3, and so on. Because the closest approaches then always happen at the same point in the orbit, the tiny gravitational nudges do not average out to nothing; they add coherently. The effect can be protective, gently herding bodies into a stable lockstep, or destructive, slowly pumping up eccentricities until orbits cross and bodies are flung away or crash.
Resonances sculpt the architecture of planetary systems. They carve the Kirkwood gaps in the asteroid belt (where Jupiter's 3:1 and other resonances clear out asteroids) and the Cassini Division in Saturn's rings; they lock Jupiter's moons Io, Europa, and Ganymede into a 1:2:4 chain that keeps Io's orbit eccentric and its volcanoes burning; and chains of resonant exoplanets reveal that planets migrated through their gas disks. Resonance is gravity's way of imposing order — and sometimes chaos — on a crowd.
Pluto is locked in a 2:3 resonance with Neptune — it orbits the Sun twice for every three Neptune orbits — so even though Pluto's path crosses Neptune's, the resonance guarantees they never come close, and Pluto's orbit stays stable for billions of years.
A resonance can protect crossing orbits by ensuring the two bodies are never in the same place at once.
Resonance is not automatically stabilizing or destabilizing — the same ratio can shepherd bodies into safe lockstep or steadily drive them onto crossing orbits, depending on the geometry. Which way it goes is the subtle heart of long-term dynamics.