There are several different theories about the origin of the Moon. One theory, called the fission' theory states that early in the life of Earth, a piece broke off, and that plece became the Moon. A second, closely related theory is that the Moon is composed of several pieces of Earth that broke away from our planet. Yet another theory is that the formed elsewhere in the solar system and was captured by Earth's final theory states that a huge piece of planetary rock stauck Earth and broke up into pieces. One of the pieces became the Moon.