A natural satellite is a celestial body that orbits another celestial body of greater mass (e.g., a planet, star or dwarf planet), which is called its primary. For example, the Moon is a natural satellite of Earth, Earth is a natural satellite of the Sun and the Sun is a natural satellite of the Milky Way Galaxy. Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites.