The mod simply renders the solar system planets in Minecraft's skybox during the night. In a way, the MC skybox is a planetarium if you think about it. The planet textures are made by @Crackers0106 (PR #1) and @lxly9 (PR #2).
The project source is licensed under the MIT license whereas the project assets are licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License.
I personally welcome third party ports to previous minecraft versions, but I respectfully ask that all third party ports remain Quilt exclusive. That means no ports to (neo)Forge, Fabric or any other non-Quilt modloader.