Scientists think Pluto is part of the Kuiper belt because it's surface is covered by methane ice. Kuiper belt is a disk-like zone beyond Neptune's orbit populated by thousands of rocky, icy bodies. Its orbit as well is from a different origin than the planets. There are recent discoveries as well that there are objects in the Kuiper belt similar to the size of Pluto's moon, Charon. So larger objects could be a part of it too, one of which is Pluto.
