Something very odd seems to be going on out beyond Pluto. Astronomers have known for more than two decades that the tiny former planet is not alone at the edge of the solar system: it is part of a vast cloud of icy objects known collectively as the Kuiper belt. But unlike most of their fellow travelers, and unlike the planets and most asteroids, which orbit between Mars and Jupiter, a small handful of Kuiper belt objects, or KBOs, have orbits that are decidedly weird. For one thing, they take unusually elongated paths around the sun, unlike the roughly circular orbits of most planetary bodies.