A planetesimal is a solid body participating in the growth and collisional evolution of a planetary system. Its collisions can build larger bodies or fragment existing ones; the result depends on relative velocity, material strength and self-gravity. Gravitational encounters can increase random motion, while gas drag and dynamical friction can reduce it.
The category marks a physical role rather than one sharply defined diameter. A population can feed planetary embryos during planet formation or act as the parent bodies of a later debris disk. The same measured total mass can have very different collision behaviour when distributed among many small bodies or a few large ones.
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