Corruptible ("decay-able") bodies don't remain.
Summa Theologica suppl. q. 91 a. 5 = Super Sent. lib. 4 d. 48 q. 2 a. 5.
By "they" at the end of the corpus of the article, St. Thomas means any mixed/corruptible (i.e., composite) body, such as plants, animals, or minerals, all of which are comprised of elements.
An element (στοιχεῖον) is &...
What does St. Thomas Aquinas mean by 'alteration' of heavenly bodies in Summa Theologiae?
What St. Thomas is saying here is that the heavenly bodies are undergoing changes as to the fact that the entire universe is alive, growing and subject to change, just as the earth and it’s inhabitants are.
For example a sun is just a sun. But after a certain amount ...