The 5 visible planets were known to be something separate from the celestial sphere on which the stars were thought to be positioned. in Ptolemy's Almagest, he describes the motions of the heavens being a complex set of concentric crystalline spheres on which the planets rest. I don't think there were many besides Bruno who imagined them as worlds, but they knew that they were not regular ol' stars. The name planets is said to derive from the Greek planetes or wanderers.