Anyway, when I was having my brainstorm, I remembered seeing this cute project all over the Internet:
This is the best idea for the planets I can think of. I'll use different sized balloons to make the planets, along with yarn in colors that represent each planet. This is basically just taking the yarn, dipping it in watered down glue, and wrapping it around and around the balloon. Then a second coating of glue can be brushed on. Once it's all dry, you pop the balloon, pull it out the area you left open, and viola! If you really want to see how to do this project, visit this great tutorial by Raechel Myers.
My planets won't be light fixtures, they'll just hang from the cross beams of the skylight. I plan to stagger them out from the Sun, not necessarily in a straight line, but in different orbits.