An animation that simulates the first 1.2 billion years of solar system history is available from SwRI at: http://www.swri.org/press/2009/asteroid.htm. The orbits of the 4 giant planets are shown as color ellipses. The green dots show small comet-like objects. A small fraction of these objects become trapped in the asteroid belt when the orbits of the planets become unstable. In the simulation shown, this instability occurs at 880 million years (Myr).
Credit: Southwest Research Institute