... it all has to with much, i.e.: The bubble and gas at the time our sun and planets were born, metalticity of our star and gases/noble and otherwise in the nebula that gave rise to it all and momentum.
Every star system is unique. Some have much in common, but seem to have no hope for life and some as ours with our mediocre sun have are rich in metals and now it seems with water ice (Mars and Enceladus and our many comets...).
The good point that you made, Raoul is that there seem to be quite a few systems out there that DO harbor comets and even asteroid belts, so the possibility for their deliverance of water-ice to some inner rocky planet is there.
As Phil said, Kepler should answer some of these questions (Kepler is loooooong overdue for liftoff!).
Space/Science » in reply to Our solar system's planets being pushed around
re: Our solar system's planets being pushed around