planet

Etymology

From Middle English planete, from Old French planete, from Latin planeta, planetes, from Ancient Greek πλανήτης (planḗtēs, “wanderer”) (ellipsis of πλάνητες ἀστέρες (plánētes astéres, “wandering stars”).), from Ancient Greek πλανάω (planáō, “wander about, stray”), of unknown origin. Cognate with Latin pālor (“wander about, stray”), Old Norse flana (“to rush about”), and Norwegian flanta (“to wander about”). More at flaunt. Perhaps it is from a nasalized form of Proto-Indo-European *pleh₂- (“flat, broad”) on the notion of "spread out", "but the semantics are highly problematic", according to Beekes, who notes the similarity of meaning to πλάζω (plázō, “to make devious, repel, dissuade from the right path, bewilder”), but adds, "it is hard to think of a formal connection". So called because they have apparent motion, unlike the "fixed" stars. Originally including also the moon and sun but not the Earth; modern scientific sense of "world that orbits a star" is from 1630s in English. The Greek word is an enlarged form of πλάνης (plánēs, “who wanders around, wanderer”), also "wandering star, planet", in medicine "unstable temperature."

noun

1. (now historical or astrology) Each of the seven major bodies which move relative to the fixed stars in the night sky—the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn.
   The moon[…]began to rise from her bed, where she had slumbered away the day, in order to sit up all night. Jones had not travelled far before he paid his compliments to that beautiful planet, and, turning to his companion, asked him if he had ever beheld so delicious an evening? 1749, Henry Fielding, Tom Jones, Folio Society, published 1973, page 288
   Another of Boehme's followers, the Welshman Morgan Llwyd, also believed that the seven planets could be found within man. 1971, Keith Thomas, Religion and the Decline of Magic, Folio Society, published 2012, page 361
2. (astronomy) A body which is massive enough to be in hydrostatic equilibrium (generally resulting in being an ellipsoid) but not enough to attain nuclear fusion and, in IAU usage, which directly orbits a star (or star cluster) and dominates the region of its orbit; specifically, in the case of the Solar system, the eight major bodies of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
   A Discovrse concerning a New Planet. Tending to prove, That 'tis probable our Earth is one of the Planets 1640, John Wilkins, A Discovrse concerning a New Planet. Tending to prove, That 'tis probable our Earth is one of the Planets, title
   Their decision will force a rewrite of science textbooks because the solar system is now a place with eight planets and three newly defined "dwarf planets"—a new category of object that includes Pluto. 22 December 2006, Alok Jha, The Guardian
   2009 December 1st, Keiichi Wada, Yusuke Tsukamoto, Eiichiro Kokubo, “Planet Formation around Supermassive Black Holes in the Active Galactic Nuclei”, in The Astrophysical Journal, volume 886, number 2, article 107:
3. construed with the or this: synonym of Earth.
   It is tempting to speculate about the incentives or compulsions that might explain why anyone would take to the skies in [the] basket [of a balloon]:[…]; perhaps to moralise on the oneness or fragility of the planet, or to see humanity for the small and circumscribed thing that it is;[…]. 2013-06-07, David Simpson, “Fantasy of navigation”, in The Guardian Weekly, volume 188, number 26, page 36