Saturn and Mimas from space. Computer artwork showing the planet Saturn (upper right) with Mimas (lower left), one of its moons. Saturn is the sixth planet from the Sun and the second largest planet in the Solar System. Mimas was discovered in 1789 by William Herschel. With a diameter of 396 kilometres it is the twentieth-largest moon in the Solar System and is the smallest astronomical body that is known to be rounded in shape because of self-gravitation.