[Sa'ar Chasm]

I took an astronomy class in undergrad several years ago, and the prof mentioned then that Pluto had been demoted to a Kuiper Belt Object, part of a cloud of chunky bits leftover from the formation of the solar system. It makes sense, really. Given the highly elliptical and inclined nature of its orbit, it probably didn't form from the planetary accretion disk that gave rise to the other eight planets.

Disclaimer: I am not an astronomer, and I don't even play one on TV. I haven't kept up with all the semantic redefinitions over the years, so absorb this information at your own risk. That is all.