The darkness of the night sky is one of astronomy’s great puzzles. An infinite universe uniformly filled with stars and galaxies should produce an infinitely bright night sky, Johannes Kepler concluded 400 years ago. So why is the sky dark? It has to do with the finite size of the Milky Way Galaxy and the finite age of an infinite, expanding universe. Now, two astronomers have shown that the dark night sky also tells us about the structure and formation of our solar system. In a paper published in the Astrophysical Journal Letters, Scott J. Kenyon (Smithsonian Astrophysical Observatory) and Rogier A. Windhorst (Arizona State University) have used the dark night sky to set new limits on the amount of material in the outer reaches of the solar system. This observation tells us about how planets like Pluto were formed in our own solar system.