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...