planet gear vs cogwheel

planet gear and cogwheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
planet gear	Yes	No	No	No
cogwheel	Yes	No	No	No

As nouns, cogwheel is a hypernym of planet gear; that is, cogwheel is a word with a broader meaning than planet gear:

planet gear: an outer gear that revolves about a central sun gear of an epicyclic train
cogwheel: a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion

Other hypernyms of planet gear include gear, gear wheel, geared wheel.

planet gear (noun)	cogwheel (noun)
an outer gear that revolves about a central sun gear of an epicyclic train	a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion