planet (n.) Look up planet at Dictionary.com
late Old English planete, from Old French planete (Modern French planète), from Late Latin planeta, from Greek planetes, from (asteres) planetai "wandering (stars)," from planasthai "to wander," of uncertain etymology, possibly from PIE *pele- (2) "flat, to spread" on notion of "spread out," which would make it a relative of plane (n.1), "but the semantics are highly problematic," according to Beekes, who adds, "The meaning strongly recalls" plazein "to make devious, repel, dissuade from the right path, bewilder" (see plague (n.)) "but 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; modern scientific sense of "world that orbits a star" is from 1630s. An enlarged form of Greek planes, planetos "who wanders around, wanderer," also "wandering star, planet," in medicine "unstable temperature."