What is the asteroid belt? The asteroid belt is a region of space between the orbits of Mars and Jupiter where most of the asteroids in our solar system are found orbiting the sun. The asteroid belt probably contains millions of asteroids. Astronomers think that the asteroid belt is made up of material that was never able to form into a planet, or of the remains of a planet which broke apart a very long time ago. The asteroids in the asteroid belt come in a varity of sizes. Some are very small (less than a mile across), while others are quite large. The largest asteroid is called Ceres. It is about one-quarter the size of our moon.