How Big is Uranus?
Uranus and its five major moons are depicted in this montage of images acquired by the Voyager 2 spacecraft. The moons, from largest to smallest as they appear here, are Ariel, Miranda, Titania, Oberon and Umbriel.
Credit: NASA/JPL

The seventh planet from the sun, Uranus is the larger of the ice giants. The blue body contains an icy atmosphere that, like Neptune, differs dramatically from the other large planets.

"Uranus and Neptune are really unique in our solar system. They're very different planets than the other ones we think of," planetary scientist Amy Simon said on NASA's Gravity Assist podcast. "Part of the reason we call them ice giants is because they actually have a lot of water ice. So, while some of the other gas giant planets are mostly hydrogen and helium, they're predominately water and other ices."

The mean radius of Uranus is 15,792 miles (25,362 kilometers), giving a diameter four times that of Earth. 

"If Earth were a large apple, Uranus would be the size of a basketball," NASA's Science website says.

But like many other bodies in the solar system, the rapid spin of Uranus causes a slight bulge around the center. At the poles, Uranus has a radius of 15,517 miles (24,973 km), but at the equator, it expands to 15,882 miles (25,559 km). This bulge gives Uranus a shape known as an oblate spheroid.

If you were to take a walk around the equator of Uranus — a trip that might be challenging since the planet has no solid surface — you would travel 99,018 miles (159,354 km).

Although Uranus, discovered in 1781, is only four times the physical size of Earth, it is significantly more massive, weighing in at 86 septillion kilograms (just under one trillion trillion trillion). That makes it more than 14.5 times as massive as our rocky home.

The planet has a volume of 6.83x1013 cubic kilometers.

The density of Uranus is 1.27 grams per cubic centimeter, making it the second least dense planet in the solar system. Its low density indicates that it is predominantly composed of ice rather than gas. The icy composition of Uranus and Neptune both differ from the heavier gas giants, Jupiter and Saturn, and have caused them to be labeled "ice giants." The distance to Uranus from the sun is significant, resulting in the coldest atmosphere in the solar system and accounting for the icy temperatures.

"These planets formed much farther out in the solar system where there was a lot of ices available," Simon said. "And they didn't quite form as big as, say, Jupiter or Saturn. So, they couldn't pull in quite as much gas. And so, that's kind of part of why we believe they're so different."

Although not as famous as Saturn, Uranus does display a set of rings around its middle. The rings around Uranus are made up of tiny dark particles smaller than a meter. Only two of the 13 rings are larger than six miles across.

Although the second ring system to be discovered, the rings around Uranus weren't found until 1977, when astronomers attempted to study the planet's atmosphere as it crossed in front of a bright star. Instead of gradually fading out, as a body with an atmosphere would do, the star disappeared and reappeared several times, indicating rings. It wasn't until NASA's Voyager 2 visited Uranus in 1986 that the rings were imaged.

Uranus' outermost ring shines a bright blue. Saturn is the only other world in the solar system with a blue ring. The blue rings of both worlds are associated with moons, Saturn with Enceladus and Uranus with Mab.

"The outer ring of Saturn is blue and has Enceladus right smack at its brightest spot, and Uranus is strikingly similar, with its blue ring right on top of Mab's orbit," Imke de Pater, professor of astronomy at the University of California, Berkeley, said in a 2006 statement.

The rings of Uranus encircle the equator of the planet, but to observers on Earth, they appear to stand almost straight up and down. This is because the planet is tipped almost completely on its side in relation to the plane of the solar system. Scientists think a collision soon after Uranus' formation caused the intriguing misalignment.

The impact may have done more than tipped the planets on its side. It may also have created some of the 27 moons.

"Material from the two bodies is ejected in a debris disk, and finally satellites are formed from the debris disk," researcher Yuya Ishizawa, of Japan's Kyoto University, told Space.com. "It is possible to explain the axial tilt and the formation of the regular satellites of Uranus simultaneously."

Waves in the ring hint that the planet may have even more satellites.

"At the edges of the rings … it's almost like the amount of stuff is going up and down in a periodic fashion that looks kind of like a wave, with crests and troughs," then-graduate student Robert Chancia, of the University of Idaho, told Space.com. "It seems consistent with something disturbing the rings there."

"Based on the amplitude of this wave pattern and that distance from the ring … and our attempts to find the moon in images, it basically points toward if they exist, they're pretty tiny," Chancia said. He estimated that the moons, if they exist, are likely smaller than 3 miles (5 kilometers) in radius.

In addition to pointing toward potentially unseen moons, the thin rings of narrow can also help researchers to understand more about the planet.

"Rings are great because they're one way that we actually can do kind of the equivalent of seismology on the planets," Simon said. "We can look at how the rings oscillate and how their shapes change and learn a little bit about the inside of the planets."

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