So our solar system was originally like many other recently discovered
solar systems with a gas giant close to the star. What caused the disruption
that ejected the gas giant and re-arranged the solar system in its present
life-friendly form? Was it a likely event or something rare and improbable?
I suppose the most likely cause was another star passing close to the solar
system, I've no idea how likely this would be but I imagine most such close
encounters would not result in re-arranging the solar system into a life
friendly form. It is remarkable that after the disruption the solar system
settled into nice stable nearly circular orbits. It also suggests that the inner
planets were not necessarily formed at their current distance from the sun.
Coupled with the need for the "late heavy bombardment" to fill Earth's oceans
the "rare Earth" point of view looks even more convincing.