Ice giant

An ice giant is a special kind of giant planet made mostly of elements heavier than hydrogen and helium, like oxygen, carbon, nitrogen, and sulfur. The Solar System has two ice giants: Uranus and Neptune. These planets are different from other giant planets like Jupiter and Saturn, which are called gas giants because they are mostly made of hydrogen and helium.

Scientists learned in the 1990s, mostly from the Voyager 2 spacecraft, that Uranus and Neptune are a unique group of planets. Unlike gas giants, ice giants do not have solid surfaces. They are mostly made of gases and liquids. The materials that make up these planets were once solids, either as ice or trapped in water ice, when the planets formed. Today, most of the water inside them is not ice but exists as a special kind of fluid called supercritical fluid. Uranus and Neptune have much less hydrogen and helium—only about 20%—compared to Jupiter and Saturn, which are more than 90% hydrogen and helium.

Terminology

In 1952, a writer named James Blish created the term gas giant to describe big planets in our Solar System that are not like Earth. But we now know that Uranus and Neptune are different from Jupiter and Saturn. These two planets are made mostly of materials heavier than hydrogen and helium, so they are called ice giants. This name came about because these planets formed from icy materials and gases. The term became popular in the 1970s.

Formation

The smaller planets in our Solar System, like Earth, formed by crashing together small pieces called planetesimals. Bigger planets, such as Jupiter and Saturn, started with solid cores and then pulled in lots of gas from around them.

But it is harder to explain how Uranus and Neptune formed because they are far from the Sun. One idea is that these planets formed closer to the Sun and then moved farther out. Another idea is that the cloud of gas and dust around the young Sun became unstable and helped create these planets quickly.

Since 2004, scientists have found many possible ice giants around other stars, suggesting that they might be common in our galaxy, the Milky Way.

Characteristics

Ice giants are a special kind of giant planet in our Solar System. Unlike gas giants, which are mostly made of hydrogen and helium, ice giants are made of heavier elements like oxygen, carbon, nitrogen, and sulfur. They have some hydrogen, but it makes up less than 20% of their mass and never gets deep enough to turn into a special form called metallic hydrogen.

Both Uranus and Neptune, the two ice giants in our Solar System, are thought to have a huge ocean of water and ammonia deep inside them. This ocean is so hot and squished that it acts like a super hot, super pressured liquid, and it makes up about two-thirds of their total weight.

Atmosphere and weather

The outer layers of ice giants have some of the same weather features as gas giants, like fast winds and big swirling spots. Neptune has a huge dark spot that appears and disappears every few years, unlike Jupiter’s Great Red Spot, which has been around for centuries. Neptune gives off a lot of heat from inside, more than any other planet in our Solar System. Uranus, on the other hand, gives off very little heat, which might be because it spins on its side.

Interior

Because ice giants are so big and don’t let heat escape easily, the pressure and temperature inside them get extremely high.

Magnetic fields

Uranus and Neptune have magnetic fields that are tilted and not centered like Earth’s. These fields are stronger than Earth’s but weaker than those of the gas giants. Scientists think these magnetic fields come from a special layer of liquid inside the planets that is moving and creating electric currents.

Exploration

The Voyager 2 spacecraft visited both Uranus and Neptune, giving us important information about these distant worlds.

Scientists have suggested many ideas for future missions to explore these planets. Some of these ideas include MUSE, a NASA Uranus orbiter and probe, OCEANUS, ODINUS, an Outer Solar System mission, a Triton Hopper, a Uranus Pathfinder, and a Neptune Odyssey. These ideas have been considered by space agencies in different years, showing ongoing interest in learning more about Uranus and Neptune.