The Ice Giants (Uranus and Neptune)

FAS Astronomers Blog, Volume 33, Number 10.

As far as we can tell, our solar system formed some 4 ½ billion years ago. It began as a cloud of gas, ice, and dust. Over time, gravity slowly consolidated most of this material into the center and a star was born. However, some material remained, and it slowly formed a disk rotating about the Sun. Gravity continued to exert its influence and consolidated most of this remnant material into the planets and outer moons. Some left over material also formed the asteroid and Kuiper belts. 

The four rocky terrestrial planets (Mercury, Venus, Earth, and Mars) developed inside the “frost line” where there was little gas or ice available – most of this material was burned off by the heat of the Sun. Heavier metallic material sunk to the cores of these planets, while the remaining rocky material became the crusts and mantles.

The four outer planets (Jupiter, Saturn, Uranus, and Neptune) formed outside the “frost line” where there was plenty of ice and gas. It is thought that these planets have rocky/metallic cores. However, out here gas and ice were plentiful allowing much larger planets to form. The gas giants (Jupiter and Saturn) are composed primarily of hydrogen with interiors of metallic hydrogen. The ice giants (Uranus and Neptune) are more “mid-sized” and are also composed of hydrogen, but with icy material (water, methane, and ammonia) in their interiors.

There are, of course, five official dwarf planets including the ex-planet Pluto. You can find more on this in our articles, Pluto, The Kuiper Belt, and the Outer Solar System and Reclassifying Pluto.

According to the Nice model, both Uranus and Neptune formed in closer to the Sun. However, around 4 billion years ago, Gravitational interactions pushed both of these planets out farther to where we find them today.

• Uranus is 1.8 billion miles (19.2 AU) from the Sun with an 84-year orbit and a 17.2-hour rotational period.
• Neptune is 2.8 billion miles (30.1 AU) from the Sun with a 164.8-year orbit and a 16-hour rotation. For twenty out of 248 years, Neptune is farther from the Sun than Pluto. The last time was from 1979 to 1999. Neptune also completes three orbits for Pluto’s two orbits. 

In addition to the planets, most moons in the Solar System are found around these giant outer planets. The theory is that as the planets formed, there was more leftover material that flattened into a disk around each planet and, over time, gravity consolidated this material into moons. Uranus has five relatively large moons: Miranda, Ariel, Umbriel, Titania, and Oberon. Although not all moons were created this way. Several, particularly small irregular moons, might be captured asteroids. In addition, Neptune’s one large moon, Triton, orbits Neptune in the opposite direction of Neptune’s rotation and is thought to be a dwarf planet that was captured by Neptune’s gravity in the distant past. 

The number of moons keeps changing as new ones are discovered. We’ll try to keep up here – New Moons for the Solar System. As of October 2025, Uranus has 29 moons (named after characters from British literature) and Neptune 16 (named for lesser gods of the sea). 

Uranus and Neptune, unlike Jupiter and Saturn, which were known by the ancients, are the two planets that were discovered in modern times. It took the invention of the telescope to bring these planets into view.

• Uranus was discovered by William Hershel in 1781. He initially named it Georgium Sidus in honor of King George III. This didn’t sit well with the rest of Europe, and the planet eventually became Uranus – the Greek god of the sky. It is the only planet with a Greek rather than a Roman name. The real challenge is the learn how to pronounce its name. You can find this story in What’s in the name Uranus?
• Neptune came later after a mathematical analysis of Uranus’ orbit and was first observed by Johann Galle in 1846. Although two other astronomers, Englishman John Couch Adams and Frenchman Urbain Le Verrier, did the math pointing out its location. Neptune was named for the Roman god of the sea. For more on this and the controversy associated with the discovery, see The Discovery of Neptune. 

Uranus was likely hit by an Earth sized object, which knocked it on its side. It rotates like a ball while most planets spin like tops. Because of this, each pole directly faces the Sun, while the other pole faces away from the Sun for 21 years. Neptune, on the other hand, has a 28^o tilt giving it seasons like the Earth, although each season is 41 years long.

In 1952, Science Fiction author James Blish, in his story “Solar Plexus”, first coined the term gas giants to describe all four of the outer planets. Because all the large outer planets are made of gas, the term caught on and has been used ever since.

In the 1990s, astronomers began to discover that Uranus and Neptune differed somewhat in composition from Jupiter and Saturn. Jupiter and Saturn have a much larger proportion of hydrogen gas than Uranus and Neptune. Well below the outer layer, both Jupiter and Saturn have a band of metallic hydrogen possibly surrounding a rocky core. Uranus and Neptune, on the other hand, have a thinner layer of hydrogen surrounding a layer of slushy icy material – probably mostly water, but with ammonia and methane. This slushy inner layer makes up around 80% of the planets’ mass. 

These two planets also have some methane in their outer layer, which absorbs red light from the Sun and gives these planets a blue color. Most images show Uranus with a much lighter color than Neptune. However, a recent study concluded that Neptune might be a much lighter blue – more like Uranus.

Jupiter’s surface clouds show many storms such as the Great Red Spot and alternating bands of dark belts and light zones going in opposite directions. Saturn has a much calmer surface, although storms such as the hexagon at its northern pole are visible. 

Uranus is the blandest of all. It appeared to be mostly a pale blue when Voyager 2 passed by in the 1980s. After the Voyager 2 mission, it was thought that Uranus is the only one of the four outer planets to radiate less heat than it absorbs from the Sun. However, recent studies contract this, and Uranus might actual radiate heat more like the other giants. The folks at NASA provide a couple additional facts. The temperature on its surface clouds can drop as low as -224.2 degrees Celsius. It has a lopsided magnetic field, which is tilted and off center from the plane of rotation. 

Neptune is a bit more active with storms such as the Great Dark Spot discovered by Voyager 2 and winds up to 1,200 miles per hour (per NASA). These storms, unlike Jupiter’s red spot, appear and disappear over time. Neptune’s magnetic field is also tilted off from the planet of rotation.

There is some evidence that both Uranus and Neptune rain diamonds. The methane (CH₄) in the atmosphere might be crushed down so much that carbon is squeezed into diamonds. 

Saturn, as we all know, is known for its big, beautiful rings. Saturn’s rings might look to be solid, but they’re not, they are composed mostly of water ice. Jupiter also has rings, but these are mostly dust and are only visible from the spacecraft that have visited the giant planet. The rings of Uranus and Neptune are more prominent that those surrounding Jupiter, although they are not nearly as bright as what Saturn has to offer. 

According to the folks from NASA, Uranus has two sets of rings. The inner set is reddish and dusty looking, while the outer set has a bluish color more like Saturn’s E-ring. These sets of rings are further divided into individual rings – named Zeta, 6, 5, 4, Alpha, Beta, Eta, Gamma, Delta, Lambda, Epsilon, Nu, and Mu.

Neptune, again per NASA, has five rings – named Galle, Leverrier, Lassell, Arago, and Adams. Its outermost ring, Adams, displays four brighter arcs of dust – named Liberté (Liberty), Egalité (Equality), Fraternité (Fraternity), and Courage).