<P> However, that meaning should not be confused with the process of accretion forming the planets . In this context, accretion refers to the process of cooled, solidified grains of dust and ice orbiting the protostar in the protoplanetary disk, colliding and sticking together and gradually growing, up to and including the high - energy collisions between sizable planetesimals . </P> <P> In addition, the giant planets probably had accretion disks of their own, in the first meaning of the word . The clouds of captured hydrogen and helium gas contracted, spun up, flattened, and deposited gas onto the surface of each giant protoplanet, while solid bodies within that disk accreted into the giant planet's regular moons . </P>

Which theory offers an explanation of how the solar system formed