Reason why Pluto was dismissed is because it's orbit. The way it orbits, astronomers were able to deduce that it did not form normally like the other planets (I.E. a mass left over orbiting after the sun ignited, that cooled and thus became a planet.)
Pluto's orbit suggests it is either an escaped moon, or an object from the Kiper belt (Large belt of rocky and icy bodies orbiting beyond pluto) that got hit by another object knocking it closer and into an orbit.
Plus there are bodies in the Kiper Belt that are larger than pluto, but they aren't planet's.
Pluto has been categorized now as a dwarf planet: too small to be a planet, too big to be an asteroid.