The way I heard it was that in the early 'wild west days' in the early epoch of the great bombardment the solar system had a giant stuggle between Jupiter and Saturn and they started a voyage closer to the Sun. They sling shot one another inwards for a time and then back out again. It was against the odds like sinking half the balls in a billiards break so there is room for other theories.

The Earth only exists as it does today because of a Mars sized, perhaps Oberon, collision at a very unusual angle to produce our Moon and thus its stability.