I have an idea, but I haven't tried it so I'm not sure if it will work. Go into the files, choose the planet you want to retexture, find the texture for that planet, remove it and put your own texture in, making sure to rename it so that it is the same as the original texture. Take the original texture out, and maybe it will work. (don't try if you don't know if you're doing or else you might end up with rainbow pixel planets)