Gas giants

Introduction[]

The term "gas giant" refers to a planet which is composed mainly of hydrogen and helium and whose mass exceeds that of an average terrestrial planet by several times. The term is a bit of a misnomer insofar as throughout most of the volume of giant planets the pressure is so high that matter is not in gaseous form.

Unlike rocky planets, which have a clearly defined difference between atmosphere and surface, gas giants do not have a well-defined surface; their atmospheres simply become gradually denser toward the core.

Usually, the interior structure of gas giants comprises two main layers that surround a rocky or nickel/iron core. The first layer is a mixture of metallic hydrogen fluid and neutral helium atoms, while the second layer consists mainly of molecular hydrogen and its outer portion is filled with many strata of visible water and ammonia clouds.

The upper mass limit of a gas giant planet is approximately 1.32 × 10²⁹ kg (70 M♃). Above this point, the intense heat and pressure at the planet's core begins to induce nuclear fusion and the planet ignites to become a red dwarf star.

A more in-depth article on this subject: Gas giants