Rolling sideways and spinning backwards: Uranus is much smaller than Jupiter and Saturn, but with a diameter of 51,118 km, it is still over four times that of Earths. It would take 15 of our planet to equal the mass of Uranus. Though Uranus is the next planet out from Saturn, it is over twice Saturn's distance from the Sun. Being 28.69 x 108 km from the Sun, Uranus is a cold world with an average temperature of -221 C above cloud tops. And clouds are all we have seen of this distant cold world. Even the Voyager spacecraft could not see beyond those clouds. Uranus was not known to the ancient astronomers who knew only those planets out to Saturn. Uranus was too distant to be seen with the unaided eye. This planet had to wait until 1781 when the astronomer William Herschel discovered it while hunting for comets.