Item of sf Terminology invented by James Blish; it proved so useful that it is now often used by astronomers. It refers to the fact that four of the planets of our solar system are not comparatively small and dense, like Earth and Mars, but extremely large, and consist mainly of substances like hydrogen, helium, methane and ammonia. The helium content suggests the possibility of "mining" gas giants for helium-3 fuel to be used in nuclear-fusion Power Sources. Even in the cold at the outer edge of our solar system, these planets are of low density, being essentially globes of gas and liquid. The four gas giants – often called the Jovian Planets – in our solar system are Jupiter, Saturn, Uranus and Neptune (see Outer Planets). The fact that there are two kinds of planet in the solar system is of great interest to scientists constructing theories of its evolution; of the numerous extrasolar planets detected or inferred by modern astronomical techniques, many are of gas-giant size (unsurprisingly since greater mass and volume both increase the chances of detection).
An alien gas giant, complete with moons, has drifted into Earth's solar system in Iris (1990) by William R Barton and Michael Capobianco. The moons of gas giants are occasionally chosen for Terraforming and/or Colonization of Other Worlds scenarios: examples outside our solar system (for which see Outer Planets) include Anee in Dave Wolverton's Serpent Catch (1991), the titular world of Allen Steele's Coyote sequence, opening with Coyote: A Novel of Interstellar Exploration (fixup 2002), and Pandora in Avatar (2009). Galactic networks of Alien-inhabited, Wormhole-linked gas-giant planets feature in Kevin J Anderson's The Saga of Seven Suns: Book 1: Hidden Empire (2002) and Iain M Banks's The Algebraist (2004). [PN/DRL]
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