VISITOR FROM THE KUIPER BELT
Pluto is an odd object. Its 248-year orbit is the most eccentric and inclined of all the planets, and, for part of it, Pluto travels closer to the Sun than Neptune does.
Astronomers think Pluto is an object that wandered in from the Kuiper Belt, a region of the Solar System beyond the zone of the planets. The belt begins at roughly 35 astronomical units and may extend to about 1,000 astronomical units. The bodies in the Kuiper Belt are icy planetesimals— comets without tails. These never accreted into larger objects like Neptune because they orbit too slowly and there are not enough of them to make planet-building collisions likely.