Not planets had last value. Not all planets as we know today, have been open by medieval astronomy. Then people simply did not know about Uranium and Plutona existence. It is important to remember that for medieval astronomers, planets have been apprehended as visible as a star, instead of as objects which reflect a sunlight as we know today. By the end of the medieval period, many astronomers have started to make changes in an official science confirmed by Church. They took for a basis of changes of a note and Aristotle's theory. While changes concerned that Aristotle was right, nobody paid special attention to changes in the theory.
Though the medieval astronomy has been rigidly limited by Church, it has brought many concepts about stars and planets. The majority of people during the medieval period still believed that the Earth is the Universe centre. However astronomers could observe movement of stars and the Sun in relation to the Earth, and their schedule did not correspond to assumptions. Discrepancy of these rotations, finally, has forced people to believe that the Earth rotates round the Sun, rather the reverse.