Photo credit: KidsKnowIt.com
Make a Solar System from kidsastronomy.com is a simulator that allows kids to create solar systems of their own. They can select the types of planets, the numbers of planets, the position of the planets, and they can even add comets and an asteroid belt. When the solar system gets too crowded, students do see that planets crash & the smaller ones disappear. It is implied that gravity is responsible for the impact.