The 41/2 Steps of Terrestrial Planet Formation in the Solar System.

Step 0.5: Once the the disk forms the dust settles to a the mid-plane.

Step I: A miracle occurs, Planetesimal Formation (i.e. 1-10km objects form)
  1. Standard two-body accretion forms planetesimals.
    • Particles spiral into the Sun?
    • How do you get two rocks to stick?

  2. Gravitational instability (Goldreich & Ward).
    • As dust layer settles, dust disk becomes unstable planetesimals.
    • But, dust will orbit faster than gas turbulence stabilization? (Weidenschilling)

  3. Particle-Trapping in Eddies (Cuzzi et al.; Barranco & Marcus)
    • Large-scale eddies in Solar Nebula.
    • Particles settle due to gas drag high density, low velocities
      ........... planetesimals