"To me it made no sense to pull one of even a few objects out of the swarm and call them something other than part of the swarm." -Mike Brown, A.K.A. PlutoKiller In our Solar System, the four rocky planets dominate the inner portion, while the four gas giants dominate the outer Solar System. But out beyond Neptune, thousands of icy, rocky worlds -- including Pluto, the former ninth planet -- make up a vast, wide ring known as the Kuiper Belt.