Why did God create so many planets? The argument that He had to in order to make the universe work is just opinion based because He could or can change all laws of nature and the universe and create them whatever way He wants. But According to the RCC, why is there so many planets?
To the best of my knowledge and research, the RCC does not express a position on this issue. Like many other mysteries of the physical universe this subject is not specified mentioned in Scripture nor does it have any bearing on faith in so far as knowledge about God would affect how we live in relation to him. While the RCC does have doctrines about lots of scientific details¹ like this for one reason or another, even they haven't felt the need to explain this².
¹ In fact they have frequently erred on the side of being too explicit about matters not pertaining to faith and not specified in Scripture only to be later contradicted by scientific developments. If any mainline branch of Christianity was going to have a doctrine about this it would be the RCC.
² Or perhaps even if it had come up, they recognized the lack of sources on the matter.