The planet Uranus was discovered on March 13, 1781, by British astronomer William Herschel. While Uranus is visible from Earth with the naked eye under optimal conditions, it was long assumed to be a star due to its dimness and slow orbit.
Herschel's discovery occurred when he was surveying faint stars with a telescope and noticed that one star seemed different than the others. At first, he mistook it for a comet. Within a year, the object was shown to have a round, planetary orbit. Uranus, like Neptune, is classified as an ice giant. It is comprised of a mantle of frozen water, ammonia and methane wrapped around a small iron-nickel core. It has an atmosphere of molecular hydrogen and helium.