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.)
   • Large-scale eddies in Solar Nebula.
   • Particles settle due to gas drag high density, low velocities
     ........... planetesimals