Hey Deep-Fold, awesome, awesome tool!
It gives me inpiration to work on a game, and I'm reusing your planet scenes directly from a fork of your github repo, as I love the flexibility compared to just using sprite sheets. Thank you for sharing!
Your code is very clean, and it's easy to just instanciate a planet and benefit from its methods; but I noticed a thing that annoyed me: I don't know if it is intentional or not but Asteroid scene and NotAtmosphere scene have a 'custom_minimum_size' of (100,100) instead of (0,0) for the other scenes. No impact on your tool, but for people trying to generate several planets on a screen and resize them, it makes it harder to align correctly compared to the other scenes.
Another tiny thing that can help is also to set all shader materials 'local_to_scene' to allow several different planets of the same type on the screen. I did it programmatically (I hate the UI) within the _ready() func of planet.gd with:
'for c in get_children():
c.material.resource_local_to_scene = true'
Anyway, once more: Thank you for this tool, thank you for the github repo, I hope I will be able to share an exciting demo someday with you :)