[I]n terms of size and location, Pluto fits neatly in a group with dwarf planets like Eris, Haumea, Makemake, and likely thousands of undiscovered rocks that orbit in the Kuiper belt, outside Neptune. If we really want to call Pluto a planet, our list of planets is going to be expanding dramatically in future years.
The whole idea of a “planet” is, admittedly, an arbitrary category. Our solar system has thousands of objects that orbit the sun, and fundamentally they’re not all that different from one another.
But if you want to single out a handful of these objects as particularly significant ones — i.e., “planets” — then it’s clear that Pluto is very different. The simplest reason is Pluto’s tiny size, in terms of both diameter…and mass. …
If Pluto were its current size but a unique object — the only object of its kind out past Neptune — that would be one thing. But in 1992, astronomers discovered another tiny object out past Neptune, the first one discovered in what’s now the Kuiper belt.
“Most astronomers, by that point, realized that Pluto had probably been mistakenly classified,” Brown says. “It was pretty clear that Pluto was not going to be a singular object.” In the years since, more than a thousand Kuiper belt objects have been found.