planet gear(n = noun.artifact) epicyclic gear, planet wheel, planetary gear - an outer gear that revolves about a central sun gear of an epicyclic train;
is a kind of cogwheel, gear, gear wheel, geared wheel
is a part of epicyclic gear train, epicyclic train

bevel gear, chafing gear, change gear, clash gear, differential gear, epicyclic gear, epicyclic gear train, first gear, fishing gear, foul-weather gear, gear, gear case, gear lever, gear mechanism, gear to, gear up, gear wheel, head gear, high gear, in gear, inferior planet, irreversible steering gear, jovian planet, landing gear, low gear, major planet, minor planet, out of gear, outer planet, pinion and ring gear, planet, planet wheel, planetary gear, red planet, reduction gear, second gear, spur gear, stable gear, steering gear, sun gear, superior planet, terrestrial planet, third gear, wedge gear, worm gear