A moon is a natural satellite consisting of solid material that naturally orbits a planet or dwarf planet. Although the Earth has just one moon, there are other planets that have more, and some that even have none. Moons and their planets often have very symbiotic, or close relationships.
There are seven moons in the solar system as big or bigger than the Earth's moon. Titan on Saturn and Triton on Neptune are among the largest, and the most studied outside of the Earth's moon. Some moons have active volcanoes. The discovery of the massive volcano Io on Jupiter was a major astronomical discovery in the later 20th century.