plaet - n.
1 a celestial body moving in an elliptical orbit round a star; the earth.
2 esp. Astrol. hist. a celestial body distinguished from the fixed stars by having an apparent motion of its own (including the moon and sun), esp. with reference to its supposed influence on people and events.

planetology n. [ME f. OF planete f. LL planeta, planetes f. Gk planetes wanderer, planet f. planaomai wander]