Uranus

Uranus spins on its side, giving it the most extreme seasons in the solar system. What an ice giant is made of, how Uranus may have been knocked over, and why we know so little about it.

Diameter: 51,118 km (4 × Earth)
Mass: 14.5 × Earth
Gravity at cloud tops: about 0.9 × Earth
Distance from Sun: 19.2 AU (about 2.9 billion km)
Year: 84 Earth years
Day: about 17 hours 14 minutes
Axial tilt: about 98°
Moons: 29 known (as of 2025)

The build: a different kind of giant

Uranus and Neptune are called ice giants. Unlike Jupiter and Saturn, which are mostly hydrogen and helium, they are thought to be mostly heavier materials — water, ammonia and methane — in a hot, dense fluid state around a rocky core, wrapped in a relatively thin hydrogen–helium atmosphere. “Ice” here refers to what the ingredients were made of when the planet formed, not to frozen solid ice inside today.

Far from the Sun, building material was spread thin and growth was slow. The ice giants probably finished growing their cores when much of the disk gas was already gone, so they could only collect a modest atmosphere.

Knocked on its side

Uranus's axis is tilted about 98°, so it essentially rolls around the Sun on its side. The most popular explanation is one or more giant impacts with Earth-sized bodies during its formation, although other ideas involve the slow gravitational influence of a former moon or of the other planets.

The result is the most extreme seasons in the solar system. Over its 84-year orbit, each pole spends about 42 years in continuous sunlight followed by about 42 years of darkness.

Cold and quiet

Uranus releases surprisingly little heat from its interior, unlike the other giant planets. Its atmosphere has recorded temperatures near −224 °C, among the coldest in the solar system. Methane in the upper atmosphere absorbs red light, giving the planet its pale blue-green colour.

Visited only once

The only spacecraft to visit Uranus was Voyager 2, which flew past in 1986. It discovered new moons and rings and measured an oddly tilted, off-centre magnetic field. A dedicated Uranus orbiter and probe was ranked the top priority for a new NASA flagship mission in the 2022 planetary science decadal survey, but no mission has launched yet.

Uranus also has a set of thin, dark rings, discovered from Earth in 1977. Its larger moons, such as Miranda with its jumbled cliffs, may hold clues to a violent past.

Lessons for planet builders

Far from the star, planets grow slowly and may miss most of the gas.
A late giant impact can change a planet's tilt — and with it, every season.
Ice giants may be among the most common planet types in the galaxy, so understanding Uranus matters well beyond our solar system.