Planetesimals are the fundamental building blocks of the planets as well as the ancestors of asteroids and comets. To understand them and their importance, one must first understand how planets form.
The solar system formed 4.6 billion years ago from an interstellar cloud of gas and dust. When this gaseous cloud became unstable, it collapsed under the force of its own gravity and became a flattened, spinning disk of hot material. The region with the greatest concentration of mass became the Sun. The rest of the mass, perhaps only a little more than the mass of the Sun, eventually cooled enough to allow solid grains to condense, with rocky ones close to the Sun and icy ones farther away. The grains settled near the midplane of the disk, where mutual collisions allowed them to slowly grow into pebble-sized objects. At this point, the story is less clear. Some astronomers claim particle velocities in the disk remained low enough to allow the pebbles to stick to one another. Others argue that pebbles are generally not very sticky, and gravitational forces alone could cause concentrated swarms of pebbles to coalesce. In either case, the process eventually produced planetesimals, which measured a few kilometers across.
Models of planetesimal disks suggest that low relative velocities between the bodies produce accretion (objects hit and stay together) rather than fragmentation (objects break up and disperse). As planetesimals grow still larger, their gravitational attraction increases, allowing them to become even more effective at accreting nearby planetesimals. This process, called runaway growth, allows some planetesimals to reach the size of the Moon or even Mars. These so-called protoplanets were the precursors of the current planets of the solar system. For reference, 2 billion planetesimals, each one being 10 kilometers (6 miles) in diameter, are needed to make an Earth-sized planet.
At this point in solar system evolution, the disk mass is dominated by protoplanets, planetesimals, and gas. Mutual gravitational interactions force most protoplanets to collide and merge, eventually producing small planets such as Earth. If a protoplanet grows large enough, however, it can also gravitationally capture enormous amounts of the remaining gas. This explains why Jupiter and Saturn are so much larger than Earth.
The same interactions that cause protoplanets to collide also stir up the remaining planetesimals. Most of these objects end up impacting existing protoplanets or are thrown out of the solar system. The leftovers that managed to stay in the stable regions of the solar system until planet formation ended are now called asteroids and comets. The asteroid belt is a population of rocky planetesimals located between the orbits of Mars and Jupiter. The Kuiper belt and Oort cloud are populations of icy planetesimals located beyond the orbit of Neptune. Even now, mutual collisions between and among asteroids and comets as well as planetary interactions cause pieces of the survivors to escape their small body reservoirs. A few of these multi-kilometer or smaller objects strike Earth. Small impactors deliver meteorites, while large ones infrequently wreak global devastation.
see also Asteroids (volume 2); Comets (volume 2); Gravity (volume 2); Impacts (volume 4); Kuiper Belt (volume 2); Meteorites (volume 2); Oort Cloud (volume 2); Orbits (volume 2); Small Bodies (volume 2).
Safronov, Viktor S. Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets. Moscow: Nauka Press, 1969. Trans. NASA TTF 677, 1972.
Ward, Wilham R. "Planetary Accretion." In Completing the Inventory of the Solar System, eds. Terrence W. Rettig and Joseph M. Hahn. San Francisco: Astronomical Society of the Pacific, 1996.