Adding a already made planet is simple. Making one takes more work. I just got done with a planet mod for SOGE.
Add planets:
Copy .entity files and edit entity.manifest
Copy modified PlanetIcons from textures
Copy planetIcons.brushes from Window (Should be 4 of them) and brush.manifest
Copy textures
Copy meshes
That's it.
Any other questions about modding planets just ask.