Planetesimal accretion formation

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

Collisions can either result in net accretion, as demonstrated by the assembly of planets from planetesimals, or disruption, as in the case of the formation of the Moon. Even accretional collisions can redistribute materials far and wide, altering and perhaps blurring the distributions of compositionally distinct surface materials and exposing subsurface compositions. 

Most young stars with optically thick disks appear to be accreting gas, which suggests that gas is continually flowing inwards through the disk and onto the star (Calvet et al. 2004). This inward viscous accretion is probably caused by turbulence in the differentially rotating disk. The existence of turbulence and the strength of the turbulent eddies can have a substantial impact on planetesimal formation, as we will see in Section 10.2.2. 

---