Original: The Nice model popular among planetary scientists postulates that the gas giant planets migrated in orbit at this time , causing objects in the asteroid belt and\/or Kuiper belt to be put onto eccentric orbits that reached the terrestrial planets .
Simplified: One popular theory is that the gas giant planets migrated in orbit at this time , causing objects in the asteroid belt and\/or Kuiper belt to be forced into orbits which reached the terrestrial planets .