Gas giant

A gas giant is a giant planet made mostly of hydrogen and helium. In our Solar System, the two gas giants are Jupiter and Saturn.

These huge planets look different from Earth because they don’t have a solid surface like we do. Instead, they have thick atmospheres and deep layers of gas.

Jupiter and Saturn have a special kind of hydrogen deep inside them that acts like electricity. This layer is called “metallic hydrogen” because it can conduct electricity. Scientists think these planets also have a rocky core in the middle, but it’s very hot and squished, so we don’t fully understand it yet.

Sometimes, scientists talk about other objects in space that are almost like stars but not quite. These are called brown dwarfs, and they can be close in size to gas giants. But there is debate about what exactly makes something a gas giant versus a tiny star.

Terminology

The term gas giant was created in 1952 by a science fiction writer named James Blish. It describes big planets made mostly of hydrogen and helium, like Jupiter and Saturn. Even though they are called "gas" giants, most of their material is in a special state—not quite liquid or gas. Scientists use the term because it’s a simple way to talk about planets made from certain building blocks. In our Solar System, planets like Uranus and Neptune are called ice giants because they are made more of water, methane, and ammonia instead of hydrogen and helium.

Classification

Main article: Sudarsky's gas giant classification

Gas giants can be grouped into five types based on their atmospheres. The types are: ammonia clouds, water clouds, cloudless skies, alkali-metal clouds, and silicate clouds. In our Solar System, Jupiter and Saturn are both in the ammonia clouds type. There is also a special group called Hot Jupiters, which belong to the alkali-metal clouds or silicate clouds types.

Extrasolar

Gas giants can be found far outside our Solar System. Some of these planets, called cold gas giants, are about the same size as Jupiter but might get smaller if they have very strong gravity.

Smaller gas planets, called gas dwarfs, have thick layers of hydrogen and helium but are not as big as Jupiter or Saturn. These planets often have a rocky center and can be closer to their stars. This can cause them to lose some of their atmosphere over time. One example is Kepler-138d, a small planet with a thick layer of gas that is bigger than Earth but not as heavy.

Main article: Mini-Neptune

Precipitation and meteorological phenomena

Heat rising from storms drives the weather on gas giants like Jupiter and Saturn. Big storms, such as Jupiter's famous Great Red Spot, form when warm air moves upward. These storms can create lightning, similar to thunderstorms on Earth.

The Great Red Spot is a huge, swirling storm in Jupiter's southern hemisphere. It moves very fast and can absorb smaller storms. Its reddish color may come from special chemicals.

On Saturn and Jupiter, helium can change into liquid and fall as rain deep inside the planet. This process releases energy and may help explain some of the heat we see coming from these planets.