I think they created a new class, called "Plutons." Seriously.

The reason they changed Pluto is that astronomers have discovered 2-3 bodies farther out than Pluto that are almost as big as Pluto, so if they wanted to count Pluto as a planet they'd have to count these others too. And then we'd have an 11-12 planet solar system, and that would just be crazy.

So now we have a solar system with 8 planets, 3 plutons, and a bunch of asteroids and comets.