Uranus: The Ice Giant on a Tilted Axis

By: Mark Mancini  | 
Uranus tilt
Uranus, the seventh planet from the sun, has at jaw-dropping axis tilted at a 97.7-degree angle. Compare that to Earth's axial tilt of just 23.5 degrees. Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory

It's funny. There are eight planets in this solar system and Neptune is the farthest planet from the sun. Yet it still releases more heat than Uranus, the seventh planet from the sun.

Four times wider than Earth, Uranus is also about 14.5 times as massive as the world we live on. After the gas giants Jupiter and Saturn, Uranus is the biggest planet in the sun's orbit.


Scientists have coined an appropriate term for large, frozen planets like Uranus: ice giants. Neptune and Uranus are both ice giants and both Jovian planets, but Uranus is quite an odd planet compared to its neighbor. For starters, Uranus' axis spins on an extreme tilt, resulting in some crazy seasons around the poles.

Even the ice giant's name is a bit peculiar, and not just because it makes schoolboys chuckle.


Uranus Is Named for a Greek God

Instead of being named after a figure from Roman mythology, Uranus is named for the Greek god of the sky. NASA/JPL

OK, let's not kid ourselves. You're never too old to enjoy a good Uranus joke. Headline writers certainly don't think so, if article titles like "NASA Wants to Probe Uranus in Search of Gas" and "Scientists Confirm Uranus Stinks" are any indication.

English-speakers tend to pronounce "Uranus" in one of two ways. Some folks say, "urine us," but most prefer the alternative that sounds like "your anus." Comedy gold, right?


(In a bid to minimize rectum comments, Planetary Society blogger Emily Lakdawalla has trained students to point and shout "You're a nuss!" when the name is dropped.)

Puns aside, the planet Uranus represents a break with nomenclatural tradition. Mercury, Venus, Mars, Jupiter, Saturn and Neptune all took their names from Roman gods or deities. However, Uranus — uniquely — was named after a Greek god.

In ancient Greece, Uranus was revered as the primordial Greek god of the sky. He had a son named Cronos and a (more famous) grandson known as Zeus. These figures were later conflated in Roman mythology into two deities: Saturn and Jupiter.

Though astronomer William Herschel discovered the planet Uranus March 13, 1781 — the first planet found with the help of a telescope — Herschel didn't give it the name we use today. A loyal Briton, Herschel wanted to call new planet Georgium Sidus — or "The Georgian Planet" — in honor of King George III.

By nature that name was politically charged. To avoid alienating non-British stargazers, German astronomer Johann Elert Bode suggested calling the planet "Uranus" in 1783. Eventually, the name Uranus won out.


The Ice Giant Spins Sideways

This image shows a representation of Uranus' magnetic field. The yellow arrow points toward the sun, the light blue arrow marks the planet's magnetic axis and the dark blue arrow marks Uranus' rotation axis. NASA/Scientific Visualization Studio/Tom Bridgman

Bode may have been German, but the planet he named looks like it's trapped in a Dutch tilt.

Planets rotate around a magnetic axis, the line connecting their North Pole and South Pole. And speaking of spin cycles, you probably know that all the planets in this solar system are simultaneously orbiting the sun.


Now Earth has an axial tilt of 23.5 degrees. This means there's a 23.5-degree angle between Earth's axis and its plane of orbit around the sun. Without the tilt, our home world wouldn't have seasons (or possibly life).

Uranus is skewed, too — but to a much greater extent. In relation to its orbital plane, Uranus' axis has been tilted at a jaw-dropping 97.7-degree angle. Uranus is the only planet in the solar system with its equator nearly at a right angle to its orbit.

Next to other giant planets Saturn and Neptune, Uranus appears to be on its side. What's up with the seventh planet from the sun's weird orientation?

A computer simulation published in 2018 suggests Uranus was hit by a huge protoplanet around 4 billion years ago. Supposedly, this collision gave the ice giant its exaggerated tilt and caused it to expel most of its primordial heat. And the enormous crash might have done more than force it into its strange tilt.

According to the research, energy from the impact may have formed a thin shell from the debris left behind. That shell traps internal heat radiating from the giant planet's core, which could help explain why Uranus is the coldest planet in the solar system (even colder than Neptune in some places).

Then again, perhaps there were multiple impacts. A long-gone circumplanetary disk might've also played a role here.


Uranus Orbits the Sun Every 84 Earth Years

The tilt in Uranus' axis subjects its magnetic poles to long, dark winters and long, bright summers. It takes 17 hours for Uranus to orbit or rotate completely. But it takes about 84 Earth years (30,687 Earth days) for Uranus to make a complete orbit around the sun.

Because of the planet's slow orbit, the South pole of the ice giant will see continuous sunlight for half of that 84 year orbit, while the North pole is in complete darkness (and vice versa).


In 2033, Uranus orbits around the sun for just the third complete time since it was discovered in 1781.

Uranus' Atmosphere Is Brutally Cold (and Stinks)

Uranus' atmosphere
Processing highlights Uranus' atmosphere in this image taken by Voyager 2. NASA/JPL

Despite the extreme tilt in Uranus' axis, the planet is warmer at the equator than it is at both magnetic poles. But warm is relative here, as Uranus is the coldest planet in the solar system.

The densest part of the planet's atmosphere sees brutal temperatures of minus 243 to minus 370 degrees Fahrenheit (minus 153 to minus 218 degrees Celsius). That's hardly a welcoming environment for future astronauts.


The warm temps at Uranus' equator aren't the planet's only mystery. Jupiter, Saturn and Neptune all radiate more than twice as much heat as they receive from the sun. But Uranus radiates barely any heat. The disparity has long baffled planetary scientists.

As we already mentioned, Uranus and Neptune are both ice giants. Ice giants have rocky cores covered by mantles rich in ammonia and icy water. These giant planets have atmospheres with an outer levels full of hydrogen, helium and even more cloud layers of methane.

The scientific community has found that these cloud layers in Uranus' atmosphere also are full of hydrogen sulfide, a compound responsible for the rotten egg stench we all know and hate. So yes, Uranus literally stinks.

At least the color scheme would be familiar. Earth isn't the only "blue planet" in this solar system: Methane absorbs red light, giving Uranus and Neptune deep-blue complexions. Of the two worlds, Uranus looks slightly greener.


Uranus' Ring System

Uranus' rings
This zoomed-in image of Uranus, captured by Webb’s Near-Infrared Camera (NIRCam) Feb. 6, 2023, reveals stunning views of the planet’s rings. Scientists expect that future Webb images of Uranus will reveal the two faint outer rings discovered with Hubble during the 2007 ring-plane crossing. NASA, ESA, CSA, STScI

We've known since 1977 when Voyager 2 first saw them that Uranus has rings around its equator. This ring system was the first one in the solar system discovered after Saturn's.

To date, astronomers have counted 13 rings encircling Uranus. These are part of an inner and outer rings system.


Its inner system of rings includes nine rings that are narrow and dark. The two outer rings are more distant and were discovered in 1997 by the Hubble Space Telescope. The innermost ring is reddish in color, similar to other rings in the solar system, and the outer ring is blue like one of Saturn's rings.

A 2016 study suggests that the rings of Uranus, Saturn and Neptune could be what's left of Pluto-like dwarf planets that were rippled apart by their gravities after the dwarf planets swung too close long ago.

But Uranus' rings lack the fine particles observed in these other planet ring systems. Instead, the rings around Uranus are composed of debris chunks that are golf ball-sized at minimum. For some reason, smaller material gets exiled into the spaces between these rings.


The Uranian Moons

the Uranian system
This wide view of the Uranian system features the planet Uranus, six of its 27 known moons and several other galaxies. Space Telescope Science Institute

Uranus also has 27 known moons, 25 of which were named after Shakespeare characters like Ophelia, Juliet, Desdemona and Puck. Then there's Miranda, whose namesake appears in "The Tempest" (one of the Bard's later plays).

Geologically complex, Miranda contains the single tallest cliff known to mankind. Dubbed "Verona Rupes," it has an estimated height of 12.4 miles (20 kilometers). If a clumsy human were to fall off the peak, scientists think he or she would plummet for 12 minutes straight before hitting the ground.


Other moons orbiting Uranus include Sycorax and Caliban. While most of Uranus' moons spin in the same direction as the planet does, these two revolve the other way. Scientists think they were once independent objects that the ice giant's gravity ensnared. By the way, Caliban is another "Tempest" character — and Sycorax was said to be his mother.

Oberon and Titania are the largest moons orbiting Uranus. Both were discovered by William Herschel in 1787. Some small moons orbit just outside the rings, while the largest moons orbit beyond the small moons.

From its weird rings to its puzzling climate, Uranus has found plenty of ways to surprise us. Only time will tell what further mysteries the planet holds. To steal a line from "Hamlet," "There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy."


Uranus FAQ

What is so special about Uranus?
Uranus is the only planet whose equator is nearly at a right angle to its orbit, with a tilt of 97.77 degrees — possibly the result of a collision with an Earth-sized object long ago. This unique tilt causes the most extreme seasons in the solar system.
How many moons does Uranus have?
Uranus has 27 known moons, 25 of which were named after Shakespeare characters like Ophelia, Juliet, Desdemona and Puck.
What are Uranus rings made of?
The rings are relatively dim and lack the fine particles observed in other ring systems (like Saturn's). Each ring is composed of debris chunks that are golf ball-sized at minimum. For some reason, smaller material gets trapped in the spaces between these rings.
What is the mass of Uranus compared to Earth?
Uranus is four times wider and about 14.5 times as massive as Earth.
Is Uranus full of water?
According to NASA, Uranus is composed of water, methane and ammonia.