Planets form in protoplanetary discs from dust grains that collide and stick together. However, as the solids grow to sizes of a few ten cms, the head wind from the slightly sub-Keplerian gas causes these rocks and boulders to drift rapidly through the disc.

Mass accretion through protoplanetary discs is likely due to turbulence caused by the magnetorotational instability. Zonal flows appearing in magnetorotational turbulence locally alters the rotation profile of the disc, and the radial drift of the solids is stopped or slowed down in regions of super-Keplerian rotation. As solids pile up at these points of converging radial flow, the local density may become high enough to trigger a gravitational instability in the solid component. This way 1000 km dwarf planets form directly from cm-m sized rocks and boulders.

Magnetorotational turbulence is also responsible for inducing collisions between the solid particles. Collision speeds higher than 10 m/s likely lead to the destruction of the colliding bodies, so the fact that planets exist may indicate an upper limit to the amount of turbulence in the solar nebula and in protoplanetary discs in general.