Condensation Model:

Small eddies formed in the disk material, but since the gas and dust particles moved in almost parallel, near-circular orbits, they collided at low velocities. Instead of bouncing off each other or smashing each other, they were able to stick together through electrostatic forces to form planetesimals. The larger planetesimals were able to attract other planetesimals through gravity and increase in size. This process is called accretion. (http://www.astronomynotes.com/solfluf/s11.htm).