Is planet a proper noun or common noun or collective noun?

The noun 'planet' is a common noun, a general word for any body moving in an orbit around a star.

A proper noun is the name of a specific person, place or thing. A proper noun for the common noun planet is the name of a specific planet, such as Mars, Jupiter, Earth, etc.

A collective noun is a word used to group people or things taken together as one whole in a descriptive way. The collective noun for planets is a quincunx of planets.