We may have had a 10th and 11th planet that collided. It's also possible that the outward migration of Neptune scattered comets and small planet bodies, inducing collisions in the asteroid belt. The close passing of a neighboring star could have had a similar effect.  -   Robert Duncan     Quotes
We may have had a 10th and 11th planet that collided. It's also possible that the outward migration of Neptune scattered comets and small planet bodies, inducing collisions in the asteroid belt. The close passing of a neighboring star could have had a similar effect. Robert Duncan