Uranus: The Ice-Gas Giant in the Outer Solar System

Uranus as seen in infrared light. Its atmosphere has storms churning around and the planet is encircled by a thin set of rings. NASA

Everyone's heard about Uranus as a planet in the solar system. Chances are, most folks have giggled about its name and how to pronounce it. But, this world is way more than a funny joke. Uranus is a distant planet with strange moons, gigantic storms, and a thin ring system that are still being studied by planetary scientists in an effort to understand how it came to be.

Uranus was first explored close-up only a few decades ago. Before that, it wasn't actually thought to be a planet until the late 1700s, even by its discoverer. By the time the Voyager 2 spacecraft gave astronomers a good close-up view of this chilly world in 1986, they knew it was a planet, but they didn't know how weird it really was.. Since then, astronomers have used ground-based and space-based observatories (such as Hubble Space Telescope to observe it. Here are the top 10 facts they've found out about Uranus. 

Edited and updated by Carolyn Collins Petersen.


Okay, so let's get the funny name out of the way first. Most commonly the word Uranus is pronounced ū·rā′·nəs (said like "your anus", emphasis placed on the second syllable) which always seems to get a giggle. For this reason, most in academia have taken to pronouncing it ūr′·ə·nəs (said like "urine iss", emphasis on the first syllable). Either way is acceptable, but when around children, or those that act like children, the latter pronunciation is a safer bet.  More »

Uranus was named for the Greek god of the sky.

Though many names were suggested for the planet, including Herschel after its discoverer, the logical choice of Uranus was chosen. The name actually comes from the ancient Greek god Uranus, who was the grandfather of Zeus, the greatest of all gods.

While known about for centuries, Uranus was thought to be a star until 1781.

Because Uranus is so far from Earth, it doesn't move very quickly in its orbit. It appears to travel very slowly across the night sky. Therefore, many early astronomers thought that it was a star due to its seeming lack of motion. Later observers, including its "discoverer" William Herschel, thought it might be a comet instead. However, in 1781 Herschel and others determined it was a planet after carefully charting its very, very slow motion across the sky, using the most powerful telescopes of the time. 

Uranus is the farthest planet from Earth visible to the naked eye.

Rim Of Uranus
Space Frontiers - Stringer/Archive Photos/Getty Images

Uranus lies 2.5 billion kilometers (1.7 billion miles) from the Sun. Only Neptune orbits at a greater distance, but it was not discovered by direct observation as Uranus was. Instead, Neptune's existence was calculated first and then observed later.

Uranus is the fourth most massive planet in our solar system.


With a mass of approximately 8.6810 × 1025 kg (about fourteen and a half Earths), the planet Uranus lags behind Jupiter, Saturn and Neptune in terms of heft.

Uranus is the third-largest planet in our solar system.

At 25,559 ± 4 km (about four times radius of Earth), Uranus is larger than every planet except Jupiter and Saturn. However, since it is less massive than Neptune, it is not very dense; trailing all other planets except Saturn in terms of density.

Uranus is the seventh planet from the Sun.

Despite efforts from some to get Ceres, effectively a large spheroidal asteroid between Mars and Jupiter, classified as a planet, the number of planets in our solar system is static. So after Mercury, Venus, Earth, Mars, Jupiter and Saturn, Uranus is the 7th planet from the Sun.

The rings of Saturn are well known. But as it happens, all of the Jovian planets (Jupiter, Saturn, Uranus and Neptune) have rings. However, they are generally very difficult to see and require specialized instruments to study. They are generally made from small pieces of ice, rock and dust that are held in orbit by the planet's immense gravity. More »

Uranus has at least 27 moons.

All of the Jovian planets (Jupiter, Saturn, Uranus and Neptune) have large numbers of moons. Some of them are very small, asteroid sized objects that are difficult to detect. So the exact number of moons is unknown, but at least 27 have been observed.

Uranus nearly rolls along in its orbit.

With an axis tilted at about 97.7 degrees, Uranus rolls on its side as it orbits around the Sun. This is highly unusual and probably means that the planet was ploughed over by a titanic collision with another world early in its history. 

The 7th Planet Remains Mysterious

Uranus did not give up its secrets easily. While Voyager gave us our first up-close look at this distant world, in the years since then astronomers have studied its hazy atmosphere from Earth and Earth orbit. Over the years they have learned more about weather changes on Uranus as it rolls around the Sun.
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Millis, John P., Ph.D. "Uranus: The Ice-Gas Giant in the Outer Solar System." ThoughtCo, Nov. 29, 2017, thoughtco.com/top-facts-about-uranus-3074102. Millis, John P., Ph.D. (2017, November 29). Uranus: The Ice-Gas Giant in the Outer Solar System. Retrieved from https://www.thoughtco.com/top-facts-about-uranus-3074102 Millis, John P., Ph.D. "Uranus: The Ice-Gas Giant in the Outer Solar System." ThoughtCo. https://www.thoughtco.com/top-facts-about-uranus-3074102 (accessed January 16, 2018).