Planetesimals

Planetesimals are solid bodies that form in protoplanetary disks and serve as the initial building blocks of planets. They typically range from about 1 kilometer to hundreds of kilometers in diameter, though in some environments larger bodies can form. Planetesimals arise after microscopic dust grains collide and stick together into larger aggregates, a process that may proceed gradually through sticking of pebbles or rapidly by the gravitational collapse of a dense concentration of solids via streaming instability or related instabilities.

Composition reflects formation location: rocky and metallic bodies dominate the inner disk, while icy and volatile-rich bodies form beyond the snow line. As the disk evolves, planetesimals grow by accreting surrounding material and by colliding with one another, yielding protoplanets and, ultimately, full-fledged planets in a process called planetesimal accretion. In the outer solar system, collisions among planetesimals produced asteroids, icy bodies, and comets; in the inner regions, similar processes led to terrestrial planets and cores of giant planets.

Evidence for planetesimals comes from meteorites and asteroids. Primitive meteorites contain chondrules and tiny inclusions dating to the first few million years of the solar system, setting a timescale for planetesimal formation. Observations of protoplanetary disks around other stars show ringed structures and gaps consistent with the trapping and growth of solids into planetesimals. Planetesimals remain a key concept in theories of planet formation and in the study of planetary system architectures.