It is thought that most of the water and other volatiles on the Earth's surface were delivered from the outer Solar System by comets, early in the history of the Earth. Now the question is how to build up huge solid bodies out of tiny grains of condensed material. This process can produce fluffy particles up to 1 cm in size. Inelastic collisions of the 1 cm sized particles allow bodies to grow up to approximately 1 km. At this point, two further processes come into play, one relating to the terrestrial planets and one to the gas giants.
Further growth occurs by collisional accretion.
In each zone of the disk, a single large planetesimal begins to dominate, sweeping up the remaining smaller bodies as they collide with it. As the planetesimal gets larger, the efficiency of the sweep-up increases. This is due to gravitational focussing, where the effective collisional cross-section of the body is greater than the actual radius, because of the gravitational attraction. This allows the single largest planetesimal to grow up to the thousand km scale. The impacts of the last few large planetesimals could account for the slight variations in the orbital characteristics of the final planets.