Uranus is unique among the planets in our solar system because it rotates on its side, with its axis tilted at an angle of 98 degrees relative to its orbit. This unusual tilt is thought to have resulted from a collision with a large object early in the planet's history. Uranus is also notable for its icy composition, with a thick atmosphere of hydrogen and helium surrounding a mantle of water, ammonia, and methane ice. The methane in Uranus' atmosphere absorbs red light, giving the planet its distinctive blue-green color. Uranus has 27 known moons, and its largest moon, Miranda, has a diverse and geologically active surface with features such as cliffs, valleys, and enormous craters. Uranus also has a faint ring system, discovered in 1977 by astronomers observing the planet from Earth.