Formation of Uranus and Neptune (Standard Model)

• If we put planetary embryos beyond Saturn and intergrates their orbits nothing happens!
  
  
• However, the standard model works for the terrestrail planets!
  
  
• So, what is different?
  • Strong gravitational scattering can only occur if mutual attraction is stronger than attraction of the Sun.
  • If objects are close to the Sun then this distance is small
    
  • If objects are far from the Sun then this distance is large
    
  • So far from the Sun, two objects are more likely to scatter than hit.
  • So the system gets excited and nothing hits.