"You can leave it blank in the CelestialObject definition" is false. I didn't 'refuse' anything, I just followed what you told me to do there, and noted that it isn't a good practice to simply draw an unwanted object offscreen- that's just eating extra RAM for no reason.
I also got that I have the sun overlapping the earth, as stated in the code block, where it notes that the sun and earth are overlapping. I just wanted to know how to adjust them so as to prevent this in a best-practices manner.
1- Render culling will prevent any drawing to happen outside the screen, so there's no CPU impact per say. As for RAM consumption, you're not looking at the right place to improve its usage. Consider doing some profiling so you can focus on what really consume resources.
2- There's a template GoogleSheet to help you design the planets, it's linked in the wiki for the legacy Transition Planes system but still applies here.