Mercury and Venus have no moons. Mars has two small moons, Phobos and Deimos, which are thought to be captured asteroids rather than bodies that formed with Mars. Jupiter, Saturn, Uranus and Neptune all have large systems of moons, but none of these systems is as similar to the Earth-Moon system as the Earth-Moon system is to itself.
The rarity of Earth-Moon systems in the solar system suggests that their formation may have been a relatively rare event. One possible explanation for the relative rarity of Earth-Moon systems is the fact that the Moon is thought to have formed from a giant impact between the Earth and a Mars-sized body called Theia. Such a collision would have been a rare occurrence, and it may be that this type of collision is necessary for the formation of a large natural satellite.
