Giant planet

Giant planets are massive worlds that are much larger than Earth.

They are often called jovian planets, which refers to the Roman god Jove, another name for Jupiter. These planets are typically composed of volatiles, which are materials with low boiling points. Unlike Earth, they are not primarily made of rock or solid matter. In our Solar System, there are four such giants: Jupiter, Saturn, Uranus, and Neptune. Astronomers have also identified many extrasolar giant planets orbiting other stars.

Scientists often divide these worlds into two main categories. Jupiter and Saturn are frequently called gas giants because they are mostly made of hydrogen and helium. Uranus and Neptune are often called ice giants because they consist mainly of water, ammonia, and methane.

However, some experts argue that these names are misleading. Throughout most of a giant planet's volume, the pressure is extremely high. This pressure pushes matter past its critical point. At this point, there is no distinction between a liquid and a gas. Therefore, these planets are actually composed of fluids.

Inside a gas giant like Jupiter, the structure is quite complex. The outermost part is an atmosphere of molecular hydrogen. Below this, the immense pressure creates a layer of liquid metallic hydrogen. This material is called metallic because the pressure turns the hydrogen into an electrical conductor.

At the very center, there is likely a molten core made of heavier elements. This core may be partially or completely dissolved into the surrounding envelope. In Jupiter and Saturn, these heavier elements make up between 3 and 13 percent of the total mass. The temperatures in these deep cores can reach 20,000 K.

Ice giants like Uranus and Neptune have different internal structures. Their hydrogen-rich atmospheres extend down to about 80 percent of Uranus's radius and 85 percent of Neptune's radius. Below these layers, the planets are predominantly made of ices like water and methane. These planets often have hazy atmospheres containing small amounts of methane. This gives them a light aquamarine color. Both planets also have magnetic fields that are sharply inclined to their axes of rotation. Uranus is unique because it has an extreme tilt, which causes very severe seasons.

While we call them gas or ice giants, these planets lack a solid surface. The gases and fluids simply become thinner as you move away from the center. Eventually, the atmosphere becomes indistinguishable from the interplanetary medium. This means landing on a giant planet depends on the size and composition of its core. Some extrasolar planets, called hot Jupiters, orbit very close to their stars. These worlds have very high surface temperatures compared to the giants in our own Solar System.

There is also a debate regarding the boundary between giant planets and brown dwarfs. A brown dwarf is an object that is larger than a planet but smaller than a star. One way to tell them apart is through the process of nuclear fusion. Objects with a mass above 13 Jupiter masses can start deuterium fusion. This 13-Jupiter-mass limit is a rule of thumb rather than a precise physical law. The amount of deuterium a body burns depends on its mass and its chemical composition, especially its helium levels.

Beyond the standard giants, astronomers have found even stranger worlds. Mega-Earths are massive solid planets with densities higher than Earth's. They are defined as having radii between 2.1 and 5.0 Earth radii. These worlds are likely made of rock, metal, or ice. There are also super-puffs, which are planets with very low densities. For example, some planets around Kepler-51 are Jupiter-sized but have densities below 0.1 g/cm3.

These diverse discoveries help scientists understand the many ways planets can form in the universe.