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 that get squeezed under pressure.

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 so hot and squished that 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 a big 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 was first used to describe big planets made mostly of hydrogen and helium, like Jupiter and Saturn. Even though these planets are called "gas" giants, most of their material isn’t really gas—it’s actually in a special state where it’s not exactly liquid or gas. Scientists still 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 different types based on what their atmospheres look like. These types are: ammonia clouds, water clouds, cloudless skies, alkali-metal clouds, and silicate clouds. In our Solar System, Jupiter and Saturn both belong to the ammonia clouds type. There is also a special group called Hot Jupiters, which fall into the alkali-metal clouds or silicate clouds types.

Extrasolar

Gas giants can also exist far beyond our Solar System. Some of these planets, called cold gas giants, are similar in size to Jupiter but can sometimes shrink due to strong gravity if they are very massive.

Smaller gas planets, known as gas dwarfs, can have thick layers of hydrogen and helium but are not as large as Jupiter or Saturn. These planets often have a rocky core and can be closer to their stars, causing 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 larger than Earth but not as massive.

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, even though gas giants don't have oceans or wet ground.

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

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.