A broad term used for the four massive planets of the kind found in outer Solar System, as well as in orbit around countless other stars. The name Jovian refers to the planet Jupiter, but in the Solar System there are also three other planets of this kind: Saturn, Uranus and Neptune. Because of variations in their internal structures, these planets are typically subdivided into two more specific categories: gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune).
Beneath their turbulet cloud layers, gas giants like Jupiter and Saturn are composed primarily of hydrogen. The immense mass of these planets causes the hydrogen to exist in a liquid state throughout much of the planet's body, and in a metallic state near its core. The core itself is thought to be a comparatively small body - though still probably several larger than Earth - composed of rock and ice.
The ice giants Uranus and Neptune have a different composition, with mantles consisting primarily of water, ammonia and methane, surrounded by outer layers primarily consisting of hydrogen, helium and methane. As with the gas giants, planets like this are thought to contain a rocky or icy core at the centre of the planet.
Beyond the Solar System, there are numerous examples of giant planets in orbit around other stars, varying widely in structure and properties. One notable class of extrasolar Jovian is the so-called 'hot Jupiter', a planet with a Jupiter-like mass and composition, but following a very close orbit around its star. An equivalent class of extrasolar planet to the ice giants also exists, and planets of this kind are commonly known as 'hot Neptunes'.