How do accretion disks form into planets?
Chunks of the accretion disk clump up by what are essentially electrostatic forces: the clumps, when they get big enough, are called “planetesimals”. Over a period of time, they collide with each other: some collisions break them apart, while other collisions cause sticking together of pieces. Eventually, some lumps become so large that they are resistant to breaking apart from collisions. These ones tend to accrete more and more of the chunks they run into (which they attract ever more strongly by gravitational forces). These are the lumps that become planets.
