Can drag keep the system cold enough to allow accretion? Drag is the only way.

• We have done experiments that included the effects of two types of drag:
  • Interactions with small particles.
  • Interactions with waves in Solar Nebula.
    • From Ward (1993).
  
  
  
• We required about 400 gm/cm² at 5AU to get ice giants to form.
  • Too massive to be solid particles cannot be comets.
  • About the mass of the Solar Nebula so perhaps the gas can do it.
  
  
  
• Planets only form close to Saturn.
  • Due to radial migration from drag.
  • Keeps the number density up and the relative velocities down.

If True, Uranus and Neptune formed early, before the gas went way.

But, how did they get to their current semi-major axes?