There are several mechanisms that can lead to the formation of planetesimals:

1. Accretion: accretion occurs when small particles in the solar nebula collide and merge, growing larger and more massive over time. This process continues until the particles become large enough to become planetesimals.
2. Gravitational collapse: Small particles in the solar nebula can collapse under their own gravity, forming small, solid bodies called planetesimals.
3. Fragmentation: Large particles in the solar nebula can break apart into smaller pieces, which can then grow into planetesimals.
4. Condensation: Material from the solar nebula can condense and become part of a planetesimal.