In the earliest days of the solar system, its outer edges consisted of not much more than a thin gruel of icy debris. Given that meager mass, it's bizarre that the sparse disk of small, icy bodies called the Kuiper belt ever managed to take shape. Now there's an explanation: When Neptune migrated outward, it brought the entire contents of the Kuiper belt with it.
It's not surprising that a young Neptune took a stroll. Gravitational interactions between Neptune and the surrounding dense disk of gas, dust, and small protoplanets would have driven the planet outward to its present position. A migrating Neptune would in turn have pushed other bodies ahead of it: A large body can push a small one around when the smaller one is in a resonant orbit, nudging it with repeated pushes at the same point in its orbit. That leaves a big puzzle: Objects on the receiving end of this "neptunian resonance" should in theory have wildly elongated and tilted orbits, yet many Kuiper belt objects have orderly orbits, suggesting they formed in place.
Tossing the standard theory aside, Harold Levison of the Southwest Research Institute in Boulder, Colorado, and Alessandro Morbidelli of the Observatory of the Côte d'Azur in Nice, France, claim that Neptune could have pushed some bodies into the Kuiper belt without greatly distorting the shapes of their orbits. In their simulations, when the total mass that Neptune was pushing outward was high enough, more resonances appeared, causing the trapped bodies' orbital shapes to vary between nearly circular--the ones that survive today--and highly elongated, they report in the 27 November issue of Nature.
The new variation on a Neptune push seems to work. "I think they've put their finger on an important part of the dynamics that was missing," says Renu Malhotra of the University of Arizona in Tucson. "It's getting us a more complete picture."