Saturn has 53 named moons, and another nine which are still being studied. Many of the moons are very small: 33 are less than in diameter and 13 moons are less than . Seven moons are large enough to be a near perfect sphere caused by their own gravitation. These moons are Titan, Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas. Titan is the largest moon, larger than the planet Mercury, and it is the only moon in the Solar System to have a thick, dense atmosphere. Hyperion and Phoebe are the next largest moons, larger than in diameter.