In Chapp. LXXXII-LXXXVII St Thomas argues that the heavenly bodies, which he says are "perfect without blending of contraries, being neither light nor heavy, nor hot nor cold," are the instruments whereby God prompts and controls all movement and change in material bodies on earth: that nevertheless they exercise no direct action upon the human understanding, which is something nobler than they, as the incorporeal is nobler than the incorporeal: nor are they arbiters of human will and conduct, except remotely and by occasion, as they affect the human body, under which affection the will makes its free choice: nor do they even determine the course of other terrestrial events absolutely, since much depends upon the condition and capacities of terrestrial physical causes.
Repeatedly in this work St Thomas shows his grievous misgivings as to the later Platonic position, that stars are animals and heavenly spheres have souls. He considered that the stars and their containing spheres, if they were not themselves animate, were moved by angels, which is another thing. Cf. Plato Rep. X, 616 C, sq.
Of God and His Creatures: 3.88