Uranus

Uranus is the seventh planet from the Sun and the third-largest planet in the solar system by diameter. Classified as an ice giant, Uranus is fundamentally different from the gas giants Jupiter and Saturn. While it has a hydrogen-helium outer layer, much of its mass is composed of heavier elements -- primarily 'ices' of water, methane, and ammonia in a hot, dense fluid surrounding a small rocky core. With a diameter of 50,724 km, Uranus is about four times the width of Earth. The most remarkable feature of Uranus is its extreme axial tilt of 97.77 degrees. The planet essentially rolls around the Sun on its side, likely the result of a collision with an Earth-sized object early in its history. This extreme tilt means that during its 84-Earth-year orbit, each pole alternately points almost directly at the Sun, resulting in extreme seasons where one hemisphere receives 42 years of continuous sunlight followed by 42 years of darkness. Despite this, Uranus's equator is actually warmer than its poles, a phenomenon not yet fully explained. Uranus was the first planet discovered using a telescope, found by William Herschel on March 13, 1781. Its distinctive blue-green color comes from methane in the atmosphere, which absorbs red light and reflects blue-green wavelengths. Uranus has 13 known rings, which were discovered in 1977, and 28 known moons, all named after characters from the works of William Shakespeare and Alexander Pope. The only spacecraft to visit Uranus was Voyager 2, which flew by in January 1986.