planet gear vs planet wheel

planet gear and planet wheel both are nouns.

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

As nouns, planet gear and planet wheel are synonyms defined as:

planet gear and planet wheel: an outer gear that revolves about a central sun gear of an epicyclic train

Other synonyms of planet gear include epicyclic gear, planetary gear.

planet gear (noun)	planet wheel (noun)
an outer gear that revolves about a central sun gear of an epicyclic train	an outer gear that revolves about a central sun gear of an epicyclic train