 Okay, so what happened to our friend Ceres, though? It's got a lot of asteroids near it. How come they haven't combined together and turned into a bigger planet? So the asteroid belt did not become a big planet because of the presence of Jupiter. Really? Okay. Jupiter is a very massive planet, so the tides from Jupiter end up exciting the orbits. So that is much more likely that when two planetesimals in the asteroid belt collide, they're going to fragment.