Pluto crosses Neptune's path yet never gets close, thanks to rhythm
Pluto's orbit cuts inside Neptune's, and yet the two can never collide. Pluto goes around the Sun twice for every three Neptune laps, and that fixed rhythm guarantees Neptune is always far away when Pluto swings closest. Countless bodies without such timing were flung out of that zone long ago.
This is orbital resonance: bodies whose orbital periods form a ratio of small whole numbers, so they tug on each other at the same points again and again. The effect resembles pushing a child on a swing. Small shoves, perfectly timed, add up. Most often the result is instability, with the bodies trading momentum until the pattern breaks, but occasionally the arrangement corrects itself and becomes remarkably stable.
Examples of both are everywhere. Jupiter's moons Io, Europa and Ganymede circle in a one, two, four pattern of periods that Pierre-Simon Laplace worked out, and the same ratio links three planets around the star Gliese 876. Clumps of asteroids sit comfortably in some resonances with Jupiter, including the numerous Trojans that share its orbit. Other resonances act as sweepers: in the main asteroid belt, the spots where an asteroid would orbit a simple fraction as often as Jupiter are nearly empty lanes, the Kirkwood gaps. Saturn's rings show the same carving. The Cassini Division was cleared by a resonance with the moon Mimas, and little moonlets embedded in the rings keep narrow gaps open.
Slower resonances work over millions of years by syncing the gradual wobble of orbits rather than the orbits themselves. One probably explains why Saturn is tipped over by 26.7 degrees while Jupiter stands nearly upright: Saturn's spinning axis seems to have locked onto the slow wobble of Neptune's orbit and been tilted further as the two stayed in step. Data from the Cassini spacecraft suggest that lock has since broken, perhaps because a lost moon was destabilised about 100 million years ago. Another such resonance with Saturn helps define the inner edge of the asteroid belt.
Simulations even suggest that billions of years from now a resonance between Mercury and Jupiter could stretch Mercury's orbit and unsettle the inner planets. Long before Newton, thinkers imagined orbital ratios as a music of the spheres; resonance turned out to be the real harmony.
Source: Orbital resonance
