planet gear vs planetary gear

planet gear and planetary gear both are nouns.

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

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

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

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

planet gear (noun)	planetary 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