On March 13, 1781, British astronomer William Herschel pointed his telescope at the night sky and discovered something no one had seen before: a new planet. It was Uranus, the seventh planet from the Sun, and it doubled the known size of our solar system overnight. Yet Uranus remained so dim and distant that for decades, scientists didn't even agree it deserved to be called a planet. About 70 years after its discovery, astronomers finally settled on naming it after Ouranos, the ancient Greek god of the sky and one of the primordial deities. Today, Uranus remains one of the solar system's greatest mysteries.

Uranus is an ice giant, a category of planet fundamentally different from the rocky worlds closer to the Sun or the gas giants Jupiter and Saturn. The planet's thick atmosphere of hydrogen and helium surrounds a deep, churning layer of water, ammonia, and methane so compressed and hot that these substances exist in a "supercritical" state, neither quite liquid nor gas. Inside all that sits a rocky core. The planet's icy cyan color comes from methane in its atmosphere, which absorbs red light and reflects blue and green. With a diameter third-largest in the solar system and mass fourth-largest, Uranus is genuinely enormous: you could fit 63 Earths inside it.

The most extreme feature of Uranus is its radical tilt. The planet rotates on an axis tilted 82.23 degrees from vertical, so extreme that it essentially rolls around the Sun like a ball rather than spinning like a top. This bizarre orientation means that each of Uranus's poles experiences 42 years of continuous sunlight, followed by 42 years of complete darkness, all within one 84-Earth-year orbit. Scientists still don't fully understand why Uranus tilts so drastically; the leading theory suggests a collision with an Earth-sized object billions of years ago knocked it sideways. The planet also displays puzzling climate behavior: winds reach 900 kilometers per hour (560 miles per hour), yet Uranus produces almost no internal heat, and its clouds form and vanish unpredictably.

Uranus possesses rings, 29 known moons, and a magnetosphere, though none shine as brightly as those of Saturn. Its ring system is remarkably dark, reflecting only 2 percent of incoming light. Five major moons dominate the system: Miranda, Ariel, Umbriel, Titania, and Oberon, each named after characters from Shakespeare and Alexander Pope. Fourteen smaller inner moons and ten distant irregular moons complete the family. Despite being visible to the naked eye on dark nights, Uranus has been visited by only one spacecraft: Voyager 2 flew past in 1986 and sent back humanity's only close-up images. Scientists are eager to return. In 2023, the Planetary Science Decadal Survey declared a Uranus Orbiter and Probe mission a top priority for the next decade, and China's space agency has proposed sending a subprobe to investigate this tilted, distant world.