Between the orbits of Mars and Jupiter are swarms of tiny planets called asteroids—some mere shapeless hunks of rock a few miles across, the largest about a fifth the size of the moon. It was once believed that the asteroids are the remains of a planet that was destroyed by a collision or some other tragedy. Astronomers now believe that these are the remnants of a planet that never was. At the distance of the asteroid belts, it would be difficult for a planet to form because of the gravitational interference of Jupiter. The asteroids are planetesimals that shattered and have broken each other by collisions. This theory would explain why the silicate rich asteroids are concentrated in the inner belts, and the carbonaceous type asteroids are in the outer belts. In the cooler outer regions, carbonaceous material would form more easily than in the inner belts.

Asteroid Ida and moon Dactyl, Galileo flyby, courtesy NASA/JPL