The Moon is Earth's only natural satellite. Together with Earth it forms the Earth–Moon satellite system. It is about one-quarter of Earth in diameter. In the Solar System it is the fifth largest satellite, larger than any of the known dwarf planets and the largest satellite of a planet relative to the planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under the geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's. Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.