Comets in our solar system usually come from the Oort cloud. The Oort cloud is a sphere of ice that goes outside of the Kuiper belt. A comet is a very small solar system body made mostly of ices mixed with smaller amounts of dust and rock. Most comets are no longer than a few kilometers across. The main body of the comet is called the nucleus, and it can contain water, methane, nitrogen and other ices.