Collisions within the nebula remove random momentum, leading to a disc that is flattened perpendicular to the rotation axis. Depending on the temperature and pressure in the nebula, various materials are able to condense as it cools. In the hottest central portion, only refractory substances such as iron and other metals and compounds condense. Small grains of these substances stick together when they collide forming larger particles that ultimately grow faster when their mutual gravitational attraction pulls them together. This process is called accretion .
Planetesimals combine to form planets because of their mutual gravity.
Many of these captures may be cataclysmic. Late captures of large planetesimals are thought to have been important in forming the moon and perhaps in tipping Uranus on its side. The objects formed at distances greater than 2.8 AU will be larger since they have available the much larger amount of material available in the form of ices. These cores become large enough that they can attract and hold on to the abundant hydrogen and helium in the nebula. This explains the difference in properties of Jupiter, Saturn, Uranus, etc.