planetary gear vs planet gear

planetary gear and planet gear both are nouns.

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

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

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

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

planetary gear (noun)	planet gear (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