Have the children color the big Styrofoam ball using different markers. They can color it whatever colors they like or you may have them copy a picture of an actual planet depending on your lesson plan. For older kids you could use paint, but markers are easy for preschoolers to use and they are easy to clean up.
Have the children also color the smaller Styrofoam balls with the markers. These can be used as the planet's moons or as smaller surrounding planets.
Attach the small balls to the big one using the toothpicks.
Use the pipe cleaners to create rings around the planet. You can glue the pipe cleaners directly to the planet or you may glue them to the tips of toothpicks and then stick the toothpicks into the Styrofoam ball. This will give the illusion that the rings are not attached to the planet, but floating around it as they actually are in space.
Add any decorations that the children may want to add. These might include pieces of decorative or colored paper, stickers and glitter. Glitter will add another dimension to the planet and make it look more like an object from outer space.