Most asteroids are part of the Main Belt, a collection of thousands of rocky and metallic bodies revolving more or less together between the orbits of Mars and Jupiter (between 2.0 and 3.5 AU from the sun). Astronomers once thought that the Main Belt was the debris ring left over from a destroyed planet. But it's much more likely that such a planet was, in fact, prevented from ever forming by Jupiter's strong gravitational pull.