planetesimal

Planetesimal is a solid object formed by the accumulation of dust and ice within a protoplanetary disk. Planetesimals are typically kilometer-scale or larger and are regarded as the building blocks from which planets grow.

Formation and growth occur as microscopic grains stick together, then gravitational interactions and concentration mechanisms lead to the rapid formation of larger bodies. Processes such as streaming instability can help concentrate material, enabling collapse into planetesimals. Over time, planetesimals collide and accrete to form protoplanets and, ultimately, planets.

In the solar system, many planetesimals survive as asteroids in the main belt and comets in the outer regions. Others become fragments after collisions. The largest remnants may be protoplanets or dwarf planets, depending on their size and environment.

Size and composition vary by location in the disk. Planetesimals may be rocky in the inner disk, icy in the outer disk, or mixed. Typical diameters range from about a kilometer to several hundred kilometers, with the largest known examples exceeding a thousand kilometers in diameter.

Significance arises from their role as the first major step in planetary formation. Understanding their formation and evolution helps explain the distribution and characteristics of small bodies in planetary systems and informs models of solar system history. Evidence for planetesimal processes comes from meteorites, asteroid belt surveys, and observations of circumstellar disks around young stars.