We used to think Pluto was all on it’s lonesome out in it’s far away orbit.  It turns out, we just got lucky in finding it, and it’s part of a class of objects, now called Trans Neptunian objects.  As more and more objects got discovered in the Kuiper belt it became pretty clear that there were good odds we’d find something larger than Pluto.  In 2005 we did, it’s now called Eris.