i made a background for a tank once with planets. its really easy.
black background, take white spray paint and lightly spatter it so it comes out in little specs. then take a plastic cup, cut the bottom off, set it on the rim down and spray in whatever color(s) you want into the hole on the top of the cup, reds/black, blues/whites seem to work good, but any color works. then with the cup still in place, lightly spray the middle of the outside of the cup so the overspray creates a "halo" around the planet. these easily create your planets. different size circular items/cups make different size planets.
ive attached a picture of a painting i did so you can get an idea. hope its not as confusing at it sounds.