Uranus

A planet knocked on its side

Most planets spin like tops, roughly upright as they circle the Sun. Uranus rolls around its orbit like a ball, with its axis tilted about 98 degrees. Many scientists think a huge collision early in its history, with a body possibly larger than Earth, knocked it over.

The result is the strangest seasons in the solar system. Uranus takes 84 Earth years to orbit the Sun, so each pole gets about 42 years of continuous sunlight, followed by about 42 years of darkness. Near the solstices, the Sun hangs almost overhead at one pole while the other half of the planet sits in a decades-long night.

An ice giant

Uranus and Neptune are called ice giants. Beneath a hydrogen and helium atmosphere, most of their mass is a hot, dense fluid of water, ammonia and methane, the materials astronomers call ices, wrapped around a small rocky core.

Methane in the upper atmosphere absorbs red light, giving Uranus its soft blue-green color. Despite being closer to the Sun than Neptune, Uranus has recorded the coldest temperatures of any planet, dipping to about -224 °C, likely because it releases very little heat from its interior.

Uranus is the seventh planet from the Sun and the third largest by diameter. Its atmosphere looked almost featureless to Voyager 2, but modern telescopes reveal bright storms, bands of cloud and a hazy cap over the sunlit pole that changes as the planet's long seasons shift.

Rings, moons and Shakespeare

Uranus has 13 known rings, mostly dark and narrow, discovered in 1977 when they blocked the light of a background star. The James Webb Space Telescope now shows them glowing clearly in infrared light.

It also has 29 known moons, most named after characters from Shakespeare and Alexander Pope. Miranda looks like a patchwork world, with cliffs about 20 km tall, among the highest in the solar system. Larger moons such as Titania and Oberon may hide thin oceans beneath their icy crusts.

Uranus also has a magnetic field that is tilted far from its spin axis and offset from the planet's center, so its magnetic environment wobbles wildly as it rotates. Researchers are still puzzling over what deep inside the planet produces such an unusual field.

Visited only once

William Herschel discovered Uranus with a telescope in 1781, the first planet found in modern times. Only one spacecraft has ever visited: Voyager 2 flew past in January 1986, discovering new moons and rings and measuring a lopsided magnetic field.

Much of what we know since comes from Hubble, Webb and large ground-based telescopes. A dedicated orbiter and atmospheric probe have been ranked by planetary scientists as a top priority for a future NASA flagship mission, to finally study this tilted world up close.

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