Kuiper belt cluster could be evidence of a departed giant planet

Science : A giant planet that bumped Neptune into its current orbit and was flung from the solar system could explain a cluster of large bodies in the Kuiper belt. The cluster is unusual because the thousand objects are close together and orbit in the planetary plane, unlike many objects in that region of the solar system. A new theory to explain the phenomenon comes from David Nesvorny of the Southwest Research Center in Boulder, Colorado. Nesvorny used computer simulations to model the movements of the cluster through the past 4 billion years. He found that the pieces were pulled together by Neptune’s gravity and orbited the Sun twice for every orbit by Neptune. Then Neptune experienced a major change in its orbit, moving outward 7.5 million km. The cluster couldn’t keep up and settled into its current orbit. For Neptune to have experienced such a movement, it would have to have been pulled by a massive gravitational field. None of the other giant planets could have done it, so Nesvorny suggests a lost fifth giant planet as the culprit.