Dynamical Friction
Dynamical friction plays an important role in planetary formation. The basic notion is equipartition of energy - not by collisions, but by gravitational interactions. When the protoplanetary disk has reached the stage of being populated by a variety of bodies of various masses, random gravitational encounters will tend to slow down the big ones and speed up the small ones - relative to the mean orbital speed. Being slower is an advantage from the standpoint of growth, as larger bodies with smaller relative velocities merge more readily, so the big get bigger faster than the small. The fast moving smaller planetesimals either get ejected or ultimately crash into bodies large enough to hang on to them. Encounters between the large bodies, whether planets or protoplanets, and the smaller, statistically tend to change the semi-major axis of the little guys, but unless the little guys are ejected completely, they are doomed to return to the point of encounter with the big guy. Comets...