Is there a way to create new planets with or without the use of mods? Like, is there a file I can make/add to add another planet to the the solar system? And if so, how can I make new terrain and add atmosphere and orbit and things like that?
Use Kopernicus mod to change or add new planets to game.
Kopernicus is framework for loading new and modifying existing planets. Its data files are based on Module Manager patches. It will take care of orbit, gravity, atmosphere and so on. Textures are simply .dds files.
You can find bunch of examples on Kopernicus Github page or just look on existing planet packs.
While small modifications are quite easy, all you need is notepad, creating fully functioning planet takes a while. New planet mod requires:
- Orbit, parent body, atmosphere height and density curve, temperature gradient, and so on. All can be done in mm patch files.
- Height map, textures, biomes, science definitions.
You can find a lot of tutorials on KSP official forum, but be sure to check for which version of KSP/Kopernicus it was made.