Comets are clouds of icy debris that come from beyond planet Earth. Comets have irregular orbits around the Sun and could easily collide with other objects in the Solar System. Astronomers believe that comets may have formed when the planets Uranus and Neptune were forced outside of their normal orbits into the Kuiper Belt. Once these planets entered the Kuiper Belt they would have forced rocks out of the belt and the rocks would begin their own orbit around the Sun.
Some rocks would become comets, but others would crash into planets in a period called the Late Heavy Bombardment. It is believed that these crashing comets may have even brought the essential amino acids needed to start life on Earth. To learn more about comet formation and the Late Heavy Bombardment, view Comets Bombard the Early Earth from eMediaVA℠.