Either there is a tenth planet or there are only eight.
I think it's more like eight, nine or many planets. There are untold numbers of Kuiper Belt objects. Pluto is just a very close one, that was found early.
Although Pluto more nearly matches the pattern of KB object, one way to define planets is by roster: everything on a certain canonical list is a planet, everything else is something else, asteroid, comet, KB object, what have you.
BTW, I don't think calling Pluto a KB object means he can't be a planet as well, the sets are not necessarily disjoint.
I like to think of Pluto as the St. Christopher of planets. He was a planet when I was a boy reading "Our Sun and the Worlds Around It" and "Lives of the Saints". I'd like him to remain a planet in my days. I thought it was unkind of the Vatican to remove St. Christopher from the Canon merely because their is no historical evidence that he ever lived. Picky, picky.