i remember talking about this controversy in astronomy class. They took out pluto because they found a better planet and they rather drop the last planet (pluto) and replace it with the new planet they found. I dont know, if that's correct but thats wat i heard clearly from myteacher.
So far they have not announced a replacement for Pluto, nor are there any other Planets in our system. There are some big *** moons, but no other planets so I am not sure what your prof is talking about