there was an interesting article on space.com about a simulation that was run involving planet formation, in a nut shell the simulation indicated that the formation of terresterial planets and the existance of water on them was directly proportional to the circularity of the orbit of jupiter sized planets in jupiter sized orbits. The more circular the orbit of the Jupiter analog, the more water on the terrestrial planets. some other interesting indications are that sometimes planets can form 4 times the size of the earth, and indications are that earth is middle of the road as far as the ammount of water and size and ammounts of terrestrial planets.