A model for the accretion of the planetary bodies from the primitive solar nebula (PSN), in which the rate of accretion of solid particles into the planets is slow relative to the rate at which the PSN cools. The consequence is that the surface layer of each body at any one time is in equilibrium with the pressure and temperature conditions prevailing in the nebula, and thus each planet accretes successive ‘onion-skin’ layers of material with different compositions. According to this model, the layered structure of planets may be partly of primary origin. Compare homogeneous accretion.