First the cloud began to spin, and then flatten into a disk. Clumps orbiting inside the disk evolved into the planets we know today. When the clump in the center of the disk became massive enough, nuclear fusion began and the Sun was born. Solar wind from the new-born Sun cleared away most of the debris, leaving only the nine major planets, and the countless asteroids and comets. To see a scientifically accurate animation of this formation process, click the image above.
Most asteroids are in relatively stable orbits between Mars and Jupiter.
Beyond the belt, there are two clouds of asteroids clustered within the orbital path of Jupiter. Recent studies suggest that these objects are in highly unstable orbits. Finally, there is a third group of asteroids not associated with either the Jovian Asteroids nor the Asteroid Belt. These asteroids are probably the most dangerous, because we usually detect their presence only within a few days of their closest approach. As a result, they tend to appear suddenly, leaving us with very little time to do anything about them.