So how did the solar system form? What sequence of events lead it to look like it does today? Well, according to this New Scientist article a recently published report suggest Jupiter and Saturn are the key:
In the model, the four planets [Jupiter, Saturn, Neptune, and Uranus] form in 10 million years within the current orbit of Uranus. Surrounding them in a ring are several thousand rocky objects called planetesimals, left over from the formation of the planets.
Gravity pulls the two types of object towards each other, so planetesimals begin to "leak" into the giant planet zone and the orbits of the giant planets gradually change. After 700 million years, Saturn has migrated outward and Jupiter inward to the extent that they reach a "resonance" point. This means they begin to march in lockstep with each other, with Jupiter completing two orbits around the Sun for every one of Saturn's. The resonance allows the pair to greatly disturb the orbits of the other planets.
The rest, it would appear, was cosmic calamatous history. The model neatly accounts for what we see here today, but nobody mentions any testable predictions. Until such predictions surface and are successfully tested, this model is extremely interesting but no more likely than any other competing model that can account for the same observations.