Astronomy

That's really interesting!
I just discovered a theory about the cause of the 'late heavy bombardment', which is thought to have delivered water to earth via comets.

Essentially the gas giants all orbited much closer, but Jupiter and Saturn got into resonance and flung Uranus and Neptune way out (and Saturn too). Uranus and Neptune flew out into the path of a heap of ice, and their gravity pulled the ice into an orbit that collided with the terrestrial planets.