Coined by the science fiction writer, James Blish, this is another term used to describe the 4 outermost planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. While the ‘giant’ part of the name is undeniable, especially when comparing these planets with the terrestrial planets of the inner Solar System, the ‘gas’ part is somewhat of a misnomer.

Although Jupiter and Saturn are composed primarily of hydrogen and helium, due to the enormous pressures inside these planets, the majority of this exists in liquid rather than gaseous form. In addition, Uranus and Neptune have large icy mantles surrounding their cores and only a relatively thin outer atmosphere. For this reason, they are occasionally labeled ‘ice giants’, but this terminology is not as widespread as ‘gas giant’.

Current detection methods for extrasolar planets are biased towards finding massive planets orbiting close to their parent star. For this reason, the majority of the extrasolar planets discovered so far have also been labeled as gas giants.

See also:jovian planet.