Uranus (planet)

Uranus was recognized as a planet after the English astronomer William Herschel observed it in 1781. It was the first planet discovered with the aid of a telescope rather than known since antiquity. It is the seventh planet from the Sun, orbiting at an average distance of about 19 astronomical units and taking roughly 84 Earth years to complete one orbit. A Uranian day, measured by the planet’s rotation, lasts about 17 hours.

Uranus’s rotation axis is tilted by about 98 degrees relative to its orbital plane, so the planet appears to roll around the Sun on its side. This orientation produces extreme seasonal patterns: for part of Uranus’s long year, sunlight can shine for years over one pole while the other experiences a long winter. Uranus also rotates in the opposite direction to most planets. The cause of its extreme tilt is not known with certainty; a major collision early in its history is one proposed explanation.

Uranus is an ice giant, not a terrestrial planet with a solid surface. Its atmosphere consists mostly of hydrogen and helium, with methane and smaller quantities of other compounds. Methane absorbs red wavelengths of sunlight, contributing to the planet’s blue-green colour. Beneath the atmosphere, models describe a hot, dense interior rich in water, ammonia and methane, surrounding a smaller rocky core. Uranus is extremely cold in its upper atmosphere, and winds can reach hundreds of kilometres per hour.

Uranus has a system of narrow, faint rings and 28 known moons in NASA’s current overview. The planet’s moons are named mainly after characters in works by William Shakespeare and Alexander Pope. Its magnetic field is also unusual: its axis is strongly tilted relative to Uranus’s rotation axis and offset from the planet’s centre. These properties make the Uranian system unlike the more familiar configurations around Earth and the gas giants.

NASA’s Voyager 2 flew past Uranus in January 1986, providing the only close-up spacecraft observations of the planet to date. The encounter revealed new moons and rings and measured an unexpectedly complex magnetic environment. Later analysis of the Voyager measurements has shown that the spacecraft may have encountered Uranus during unusual solar-wind conditions, a reminder that a single flyby gives only a limited snapshot of a changing planetary system. Uranus remains a target of scientific interest for future exploration.