Comets are bodies of ice, rock, and organic compounds that can be several miles in diameter.
Comets are thought to originate from a region beyond the orbits of the outermost planets. Some, called long-period comets, are in elliptical orbits of the Sun that take them far out beyond the planets and back. Others, called short-period comets, travel in shorter orbits nearer the Sun. When comets venture into the more intense sunlight of the inner solar system, the ices in the comet nucleus begin to vaporize and fall away. The evolved gas forms a tenuous atmosphere around the nucleus called a coma, while the dust previously in the nucleus forms a tail that can be thousands of miles long and sometimes can be seen from Earth.