Well, if you notice, there are no other planets that reside in an asteroid belt, so that should give you a clue. Another clue should be the fact that Pluto was closer than Neptune for a brief while in 1989. You see, Pluto fits the qualifications for the planet in that its large enough and has an elliptical orbit, though it's elliptical should be [url=http://i.kinja-img.com/gawker-media/image/upload/s--MbrCIEdK--/c_scale,fl_progressive,q_80,w_800/1353965312331823975.jpg]another hint[/url]. The thing that Pluto [i]doesn't[/i] do is clear its orbit, which means that there is more mass in its pathway, than there is on Pluto. It's very possible that Pluto won't even exist in the future due to the asteroids in its path. Neptune is a planet because even though Pluto is technically in its path, even though they wouldn't hit each other, Neptune is big enough to simply swallow Pluto and move on if it were to happen. We find a lot of objects in the Kuiper Belt. Calling Pluto a planet would force us to call a minimum of 5 more objects planets.