Separately, a group of theorists led by Alessandro Morbidelli at the Observatoire de la Cote d’Azur was beginning to understand the reasons behind the belt’s tremendous complexity. They realized that the early solar system was not an orderly place where planets formed neatly in their present locations, as scientists had long assumed. Conditions around the infant sun were more akin to cosmic pinball, with planets bouncing around and migrating wildly before settling down.
According to the new models, Jupiter drifted inward toward the sun, then tacked back out. As Jupiter moved away, its gravity acted like a giant snowplow, flinging comets and baby planets out into the deep freeze of the Kuiper Belt. If this “Grand Tack” model is correct, Pluto preserves key clues about the solar system’s early days.
In the updated picture, Pluto has a strange dual status, as both the leading representative of the Kuiper Belt and a unique denizen within it. Stern ticks off Pluto’s standout qualities. It has an intricate family of five known satellites, including giant Charon, which is nearly half Pluto’s diameter. “On its own, Charon is one of the largest objects in the Kuiper Belt,” he notes.