There appears to be growing disgruntlement with the IAU's definition of a planet. A large group of astronomers, about equal to the paltry amount that were present for the original vote, have launched a protest movement, against the definition as well as the way it was defined. The vote took place with an extreme minority of astronomers, against the proposal of the tasked working group, and without significant discussion, as usually occurs in science. But the definition itself has issues. Astronomers are pointing out that by this definition, even Jupiter isn't a planet, as it hasn't cleared it's orbit of all the asteroids. Perhaps the Earth isn't a planet, as the moon is present- the definition is inherently vague.
And even if a Mars-sized object were found beyond Pluto, it wouldn't be a planet under the new definition. This is because it wouldn't have cleared it's orbit, the key difference between a planet and a dwarf planet under the new guidelines- it hasn't gotten rid of all the other objects in it's orbit due to it's gravity. We know this because there is just so much space out there, the further you go from the central star, that you have to be really massive to clear out everything from your orbit. This results in the rather unscientific, inprecise guideline that, the further out you are, the more massive you must be, in order to clear everything from your orbit, and be a planet.
The result of all this: in attempts to define, and in ignoring the working group, the IAU seems to have thrown the definitions in disarray. Now some are recommending that students not be taught there are 8 planets, or 12- but rather that, ironically, the controversy be taught, as, unlike other controversies in science out there, this is actually a scientific argument, where scientists are arguing about the best approach. It gives students an opportunity to understand how scientific disagreement actually works, and how it gets results. They can learn what the different positions are, and what the different evidence is, and learn how to argue both sides in class, to see the merits of the different scientific viewpoints.
The alternate conference for the Pluto-supporters will be held next year, and we'll see where that goes- but the IAU meets only every three years, and so doesn't have a formal setup to change it's vote in the short-term. Science doesn't work quickly at times. But we are dealing here not so much with what's out there, but how to anthroporphically define it- how do we as a (scientific) society wish to define a large object in space? In the Genesis myth, Adam named the animals, but he didn't create them. Whatever we call it, it will still be there.