Uranus

Uranus is an unusual ice giant that orbits the Sun lying on its side. It was the first planet discovered with a telescope, and only one spacecraft has ever visited this mysterious world.

What kind of planet is it?

Uranus is the seventh planet, orbiting at an average distance of about 19.2 AU from the Sun. It's about 51,118 km across, roughly four times Earth's diameter, and takes about 84 years to go around the Sun once. British astronomer William Herschel discovered it with a telescope in 1781, making it the first planet found with a telescope. It's so faint that it's barely visible to the naked eye, and only on very dark, clear nights, so before then it had been taken for a star.

Uranus and Neptune are classified as ice giants. The outside is covered by an atmosphere of hydrogen and helium, but beneath it lies a thick layer where water, ammonia and methane are densely mixed under great heat and pressure, and there's thought to be a rocky core at the center. Astronomers call these substances “ices,” but they're actually in a state quite unlike the hard ice we know.

The most striking thing about Uranus is its axial tilt of about 97.77°. It essentially rolls around the Sun lying on its side. The leading hypothesis is that it was knocked over by a collision with a large body long ago, but the exact cause is still unknown. A day (rotation) lasts about 17 hours 14 minutes, and because its tilt is more than 90°, it's sometimes described as spinning in the opposite direction to the other planets. Surface gravity is about 0.89 times Earth's.

Atmosphere and weather

Uranus's atmosphere is mainly hydrogen and helium with a little methane. Methane absorbs red light and leaves blue, giving Uranus its soft blue-green color. The average temperature is about -195°C (-319°F), and temperatures below -220°C have been recorded in the upper atmosphere, making it the coldest planetary atmosphere in the Solar System. It's this cold even though it's closer to the Sun than Neptune, apparently because very little heat leaks out from its interior.

Its sideways tilt makes for extreme seasons. Since its orbit takes about 84 years, each pole gets about 42 years of continuous sunlight followed by about 42 years of darkness. When Voyager 2 flew by in 1986, Uranus looked bland and almost featureless, but later observations by the Hubble Space Telescope and ground-based telescopes have caught bright clouds, storms and a bright cap over the pole appearing as the seasons change.

Moons and rings

As of 2025, Uranus has more than 25 known moons. While the moons of other planets are mostly named after mythological figures, Uranus's moons are named after characters from the works of William Shakespeare and Alexander Pope. The five major moons are:

• Miranda: The smallest and innermost of the five. It's famous for its jumbled terrain, which looks like different pieces stitched together, and for its dizzyingly tall cliffs.
• Ariel: Has the youngest-looking, brightest surface of the five.
• Umbriel: Has the darkest surface of the five.
• Titania: The largest moon of Uranus, about 1,580 km across.
• Oberon: The second-largest moon, and the outermost of the five.

Uranus has rings too. They were discovered in 1977, when astronomers watched starlight flicker several times as Uranus passed in front of a star. Thirteen rings are known so far. Unlike Saturn's, they're dark and narrow, so they're hard to spot. Because the planet lies on its side, its rings are tilted along with it as it orbits the Sun.

Exploration

To date, only one spacecraft has visited Uranus: Voyager 2. Launched in 1977, it passed Jupiter and Saturn before flying by Uranus in January 1986, learning a great deal in a short time.

• It discovered 10 new moons and 2 new rings.
• It found for the first time that Uranus's magnetic field is tilted about 59° from its rotation axis and is also strongly offset from the planet's center.
• It took the first close-up images of the major moons, including Miranda's bizarre terrain.

Since then, the Hubble Space Telescope, large ground-based telescopes and the James Webb Space Telescope have kept watching Uranus, recording changes in its rings and atmosphere. The US planetary science community has recommended a Uranus orbiter and atmospheric probe as its top-priority large mission for the coming years. If it goes ahead, it will be the first long, close-up look at Uranus since Voyager 2.

Fun facts

• Uranus was the first planet discovered with a telescope. Herschel at first thought it was a comet.
• Herschel wanted to name it the Georgian Star (Georgium Sidus) in honor of King George III, but in the end it took the name of Uranus, the Greek god of the sky.
• At Uranus's poles, a summer of about 42 years when the Sun never sets alternates with a winter of about 42 years when it never rises.
• One hypothesis says that deep inside Uranus and Neptune, high pressure could turn carbon into diamonds that fall like rain. This hasn't been directly confirmed.

See it in Planetica

In Solar System view, Uranus appears on the seventh orbit, beyond Saturn. It's placed at its real position for today's date, calculated from J2000 orbital elements, but with an orbital period of about 84 years it moves so slowly that it completes barely one lap in a human lifetime.

Pick Uranus and fly in close to see it wearing its thin rings. Moons approximating the real ones circle with it, drawn larger than their true proportions so they're easy to see. Size, gravity, day length, axial tilt (97.77°), average temperature and orbital period are based on real values.

The surface patterns are created by Planetica's planet generator to suit the planet type. Keep in mind that the real Uranus looks more like a smooth, nearly featureless blue-green ball.

Other planets

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• Mercury
• Venus
• Earth
• Mars
• Jupiter
• Saturn
• Uranus
• Neptune