you can google things like "how to create planets in INSERTYOURPROGRAMSNAMEHERE", or if your using something like GIMP, i have found different scripts that you can use that will make them for you, then u just have to give them color usually. The picture in my avatar was made like that, and then i used an online tutorial i came across to make it look like they were colliding (i actually kept switching between the original photoshop tutorial for it and the GIMP tutorial for it, and it ended up looking great)