From a science standpoint, the lesson gives us a chance to expand the students' fresh understanding of solar systems to those that are now being discovered far from our own sun. We discuss the hundreds of new planets orbiting distant stars and currently being discovered by astronomers and how scientists convey what these "exoplanets" might look like with the help of professional artists.
Returning to our own art, we look at the professional planetary renderings and conclude that the artists who drew them made the planets look spherical (gave form to shape) through the use of shadows on one side and highlights on the other (essentially, different color values). Students then use circle tracers to make four or five planets of different sizes on white cardstock. Then they use oil pastels to color in their planets with a "crescent moon" shape of shadow on one side. They then drag this color (with fingers or thumb) across their planets to leave behind increasingly lighter values and then go back and re-emphasize their shadow side.
After cutting out their planets, the artists play with various compositions on a 9x12 piece of black construction paper. They are encouraged to consider visual balance. Of course, the planets must also be arranged so that all their highlight sides and shadow sides face in the same direction. Finally, with white colored pencils, the students add astronomical features such as distant stars, moons, comets, meteors, and galaxies. I'm always pleased with how these turn out, and I think the lesson works well both to reinforce a number of important art concepts, with the bonus of being able to explore a science topic a little more deeply.