planetplanet

The story

From Middle English planete, from Old French planete, from Latin planeta, planetes, from Ancient Greek πλανήτης (planḗtēs, “wanderer”) (itself an ellipsis of ἀστέρες πλανῆται (astéres planêtai, “wandering stars”)), from Ancient Greek πλανάω (planáō, “wander about, stray”), of unknown origin.1uncertain The source flags this step as uncertain. 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."

Full etymology on Wiktionary