Gas planet

A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars.

Gas giants can be subdivided into different types. The "traditional" gas giants, Jupiter and Saturn, are composed primarily of hydrogen and helium. Uranus and Neptune are sometimes considered a separate subclass called ice giants, as they are mostly composed of water, ammonia, and methane; the hydrogen and helium in Uranus and Neptune is mostly in the outermost region. Among extrasolar planets, Hot Jupiters are gas giants that orbit very close to their stars and thus have a very high surface temperature; perhaps due to the relative ease of detecting them, Hot Jupiters are currently the most common form of extrasolar planet known.

Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass. Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with a probable rocky core. The outermost portion of the hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The metallic hydrogen layer makes up the bulk of each planet, and is described as "metallic" because the great pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.

Uranus and Neptune have distinctly different interior compositions from Jupiter and Saturn. Models of their interior begin with a hydrogen-rich atmosphere that extends from the cloud-tops down to about 85% of Neptune's radius and 80% of Uranus'. Below this point is predominantly "icy", composed of water, methane and ammonia. There is also some rock and gas but various proportions of ice/rock/gas could mimic pure ice so the exact proportions are unknown.

Very hazy atmosphere layers with a small amount of methane gives them aquamarine colors such as baby blue and ultramarine colors respectively. Both have magnetic fields that are sharply inclined to their axes of rotation.

Unlike the other gas giants, Uranus has an extreme tilt that causes its seasons to be severely pronounced.