The writings of Aristotle explained that the heavens were composed of the ether, also known as the quintessence. This ether had been postulated to explain the workings of the universe that surrounded the earth and was supposed to be some sort of medium inside of which the universe existed.
The original debates in ancient times were over the nature of the ether, but the general agreement was that it existed. The ether was believed to be divided in spheres, naturally perfect, rotating without friction. No one, or at least very few, posed questions as to whether the spheres were hard or soft. The planets were regions of high density in the transparent ether. There were debates over earthly matter and heavenly matter, if one believed that the ether was a form of matter. The medieval period kept alive the belief in the ether since Aristotle's works were the basis for education. The only changes were in what was believed to be the character of the ether and the addition of a quantitative approach to its analysis.