Interesting Facts About Uranus | Universe Today

Written by Fraser Cain

Want to learn more about Uranus? Here are 10 interesting facts about Uranus. Some of these Uranus facts you might already know, and others will be completely new.

1. Uranus is the coldest planet in the Solar System

Uranus is the 7th planet from the Sun, orbiting at a distance of 2.88 billion km. But it's still much closer than Neptune, which averages a distance of 4.5 billion km from the Sun. But there's something strange about Uranus – it's really cold. Unlike the other large planets in the Solar System, Uranus actually gives off less heat than it absorbs from the Sun. The other large planets have tremendously hot cores, and radiate infrared radiation. But something made the core of Uranus cool down to the point that it doesn't radiate much heat. The temperature of the cloud tops on Uranus can dip down to 49 K (?224 °C).

2. Uranus is turned over on its side

All of the planets in the Solar System rotate on their axis, with a tilt that's similar to the Sun. Sure, the planets are a little tilted. For example, the axis of the Earth's rotation is tilted 23.5-degrees away from the Sun's plane; and Mars is similar. But the tilt of Uranus is 99-degrees. In other words, the planet is rotating on its side. All the planets look a bit like spinning top as they go around the Sun, but Uranus looks more like a ball rolling on its side.

And this leads to another strange fact about Uranus…

3. Summer on Uranus lasts one long day – 42 years

A day on Uranus is only about 17 hours. But the tilt of Uranus works out so that one pole or the other is usually pointed towards the Sun. This means that a day at the north pole of Uranus lasts half of a Uranian year – 84 Earth years. So, if you could stand on the north pole of Uranus, you would see the Sun rise in the sky, circle around for 42 years, and finally dip down below the horizon. Then you would have 42 years of darkness.

4. Uranus is the second least dense planet

The least dense planet in the Solar System is Saturn. In fact, Saturn has such a low density that it would float on water if you could find a pool large enough. Uranus comes in second as the least dense planets in the Solar System. The density of Uranus is only 1.27 g/cm³. In other words, Uranus would sink in water (water is 1 g/cm³. This low density has an interesting side effect. Even though Uranus is 14.5 times as massive as the Earth, you would only experience about 89% the force of gravity if you could stand on its surface.

5. Uranus has rings

Obviously Saturn has rings; you can see them in any backyard telescope. But Uranus has the second most dramatic set of rings in the Solar System. Unlike Saturn's particles which are made of bright ice, the rings of Uranus are very dark. They're also narrow, only measuring a few km wide. Astronomers think that the rings of Uranus are very young, and probably formed relatively recently, and not with the planet.

6. The atmosphere of Uranus is boring… or is it?

Seen in visible light, Uranus is a boring blue ball. You can't see the amazing bands of clouds and storms that we see on Jupiter and Saturn. But look closer in other wavelengths, like infrared, and you'll see that Uranus does have bands and cloud patterns. Early observations of Uranus didn't show anything, but improved telescopes in the 1990s showed that Uranus has bright regions in its atmosphere. Some of these clouds only last for hours, while others have been around since the Voyager flyby in 1986. Astronomers have also been able to chart the wind speeds on Uranus, and have found they can 250 m/s.

7. Uranus has 27 moons

Like all of the giant planets, Uranus has its share of moons. Astronomers now count 27 natural satellites. But they're actually a pretty lightweight group of moons. If you could add up all their mass, they would account for less than half the mass of Triton, Neptune's largest moon. The largest moon, Titania, has a diameter of about half the diameter of the Earth's moon.

8. Uranus was the first planet discovered in the modern age

Most of the planets are visible to the unaided eye, and were known in ancient times. Uranus was the first planet discovered after the invention of the telescope. It was first recorded in 1690 by John Flamsteed, who thought it was a star in the constellation Tauri. But it wasn't until Sir William Herschel made his observations in 1781 that astronomers finally realized it was a planet. Herschel originally wanted to call Uranus "George's Star" after King George III, the astronomical community settled on Uranus, and the name stuck.

9. You can see Uranus with the unaided eye

You might be surprised to know that you can see Uranus without a telescope; just with your eyes. At magnitude 5.3, Uranus is just within the brightness scale that a human eye can perceive. Of course, you've got to have extremely dark skies and know exactly where to look to see Uranus. Get a pair of binoculars, and Uranus is possible to see even in brighter skies. It's amazing to think that Uranus was right there, ready for anyone to discover, but nobody knew what to look for.

10. Uranus has only been visited once

Only one spacecraft in the history of spaceflight has ever made a close approach to Uranus. NASA's Voyager 2 zipped pass Uranus in January, 1986, coming within 81,000 km of the surface of Uranus. It took thousands of photographs of Uranus and its moons, and then sped off onto towards its next target: Neptune. No other spacecraft have ever been sent towards Uranus, and there are no plans to send any more. So, get used to the few pictures we've got.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

Filed under: Astronomy