Well if you're a committed Aristotlelian you think they're perfect spheres made up of the ethereal 5th element which only exists within the celestial realm.

I'm genuinely confused as to why you pick out Galileo as a major point of change as regards thoughts on the physical nature of the planets. Certainly he calls into question their perfection (ie observation of the moon, ears of Saturn) and discovers a second center in our solar system (moons of Jupiter) but he doesn't provide much in terms of a replacement regarding the "what the planets are."