In between Mars and Jupiter is the main asteroid belt. There are thousands of small, rocky bodies in this region. Some people think that these may be part of a planet that broke up because of Jupiter's enormous gravity. Others feel that they are part of a planet that never formed, for the same reason. There is another region of asteroids beyond Pluto, called the Kuiper Belt. It contains many more bodies than the asteroid belt. It may also serve as the origin for comets.