Asteroids, meteoroids, and comets are objects in the solar system that never gathered to form planets or the Sun. They are composed of material that is the same as that from which the solar system formed. They are the oldest objects in the solar system, giving us opportunities to study the solar system formation from the beginning. There are many small asteroids and only a few large ones because the asteroid belt is a place where collisions are common. When asteroids collide, they often break apart and form smaller asteroids or large meteoroids.
Some outside force cause the nebula to start to rotate and to collapse.
The center of the collapsed area began to heat up as it became more and more dense. The faster rotation caused the nebula to flatten out to a rotating disk. At the center was the protosun. The protosun continued to gather material and grow, and the outer edges of the spinning disk started to cool. The planets grew by a process called accretion - the gathering of matter around a small rocky core much as an oyster creates a pearl around a grain of sand.