“It is thought that the giant planets did not form where they are now – instead their orbits drifted for hundreds of million of years. Eventually, the two giants of the Solar System, Jupiter and Saturn, fell into a resonant order – where Jupiter was orbiting exactly twice in the time it took Saturn to complete one lap of the Sun.
This resonance had such a powerful gravitational surge that it distorted the orbits of all the outer planets.
It’s believed that Neptune was catapulted from its orbit into the heart of the Kuiper Belt, the ring of icy comets surrounding the Solar System, where it caused chaos.
For a hundred million years, a rain of comets was fired from the Kuiper Belt into the inner Solar System. It was called the Late Heavy Bombardment.”
from the article “Saturn’s beauty and power,” published March 2010 on BBC News