Earth Science is my favorite subject in school, so I'd know about that arguement
They say Pluto may and may not be a planet for one particular reason, asteroids. Beyond Pluto, but not by that far, is a field of asteroids and the debate or theory is that one of the asteroids was knocked away from it's belt and pulled into the field of gravitation of the other planets. That's why some think it may not be one, though no one's for certain.