Uranus
Uranus is the seventh planet from the Sun and the third-largest in our solar system. It's classed as an "ice giant" because it's made mostly of icy fluids around a small rocky core. Uranus is famous for being tipped on its side. While most planets spin like spinning tops, Uranus rolls around the Sun like a ball. It's also one of only two planets that astronomers discovered with a telescope, rather than having known about since ancient times.
- Position 7th planet first ice giant
- Distance from Sun 2.9 billion km 19 AU
- Diameter 50,724 km 4 times wider than Earth
- Day length 17h 14m spins on its side
- Year length 84 Earth years very long orbit
- Moons 27 named after Shakespeare characters
Where Uranus sits
Uranus orbits the Sun at a vast distance of 2.9 billion km, around 19 times further than Earth. It takes 84 Earth years for Uranus to make one complete orbit. That means most people born today will live their entire lives without seeing Uranus return to the same place in the sky.
Uranus vs Earth
An ice giant compared to home
Uranus is 4 times wider than Earth. You could fit approx. 63 Earths inside Uranus.
Uranus has 14.5 times the mass of Earth, but is less dense than rocky planets because so much of it is ice and gas.
Surprisingly, Uranus's gravity is slightly weaker than Earth's, despite being a much bigger planet. A 50 kg person would weigh 45 kg "on" Uranus (though there's no surface to stand on).
A year on Uranus is longer than most human lives. Each pole gets 42 years of sunlight followed by 42 years of darkness.
Earth's axis is tilted at a gentle 23.5°, which gives us seasons. Uranus is tilted at a whopping 98°. It's lying on its side compared to its orbit.
All 27 of Uranus's moons are named after characters from the plays of Shakespeare and the poems of Alexander Pope.
The planet tipped on its side
Uranus is unique in our solar system because of its 98 degree axial tilt. Most planets' axes are tilted only a little. Earth is tilted at 23.5 degrees, which is what gives us seasons. Uranus is tilted so far that its axis points almost directly at the Sun. This means Uranus essentially rolls around the Sun on its side, like a ball rolling along a flat surface.
As a result, each pole of Uranus spends 42 Earth years in continuous sunlight, followed by 42 years of complete darkness. Halfway between, there's a more "normal" period when the Sun rises and sets every 17 hours like on most planets.
What knocked Uranus over?
Scientists believe Uranus was probably knocked on its side billions of years ago when it was struck by an Earth-sized object during the chaotic early days of the solar system. The collision was so violent that it tipped the whole planet, and Uranus has been rolling around the Sun ever since. The same collision may also have given Uranus its strangely off-centre magnetic field.
The coldest planet
Uranus is the coldest planet in the solar system, despite Neptune being further from the Sun. Temperatures in Uranus's atmosphere can drop to −224°C. The reason for this is mysterious. Other planets give off internal heat, but Uranus barely does. Some scientists think the ancient collision that tipped it over also knocked out most of its internal heat.
Why is Uranus blue-green?
Uranus has a beautiful pale blue-green colour. The colour comes from methane in its atmosphere. Methane absorbs red light from the Sun but reflects blue and green light back into space. The same gas gives Neptune its slightly darker blue colour.
Discovery by telescope
Uranus was the first planet ever discovered with a telescope. The British astronomer William Herschel spotted it in 1781 while studying the night sky from his back garden in Bath. At first he thought he'd found a new comet. It took some time for astronomers to realise it was actually a brand new planet, the first one discovered in recorded history.
Herschel wanted to name his new planet "Georgium Sidus" (George's Star) after King George III. Other astronomers refused. The planet was eventually named Uranus, after the Greek god of the sky.
The moons and rings of Uranus
Uranus has 27 known moons, all named after characters from the plays of William Shakespeare and the poems of Alexander Pope. The five biggest are:
- Miranda, which has the strangest surface of any moon in the solar system, looking like it was shattered and stuck back together by an inexperienced repair team.
- Ariel, with deep valleys and bright icy plains.
- Umbriel, the darkest of the major moons.
- Titania, the largest, named after the queen of the fairies in A Midsummer Night's Dream.
- Oberon, named after the king of the fairies in the same play.
Uranus also has 13 thin, dark rings, much fainter than Saturn's. They are made of larger chunks of debris (between 1 metre and several metres across) rather than the fine particles in Saturn's rings.
Missions to Uranus
Only one spacecraft has ever visited Uranus: NASA's Voyager 2, which flew past in January 1986. Voyager 2 photographed Uranus, its rings, and many of its moons in just a few hours. Everything we currently know about Uranus up close comes from that single flyby.
A new mission to Uranus is now being planned by NASA, possibly launching in the 2030s and arriving in the 2040s. It would orbit Uranus for several years and study its rings, moons, and atmosphere in much more detail.