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 (> 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). Galactic networks of 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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