Physicist: Short answer: Life is the only thing that makes lots of oxygen. So far (as of January 11, 2010) 424 “exoplanets” have been discovered and confirmed in orbit around other stars (update, Nov 10, 2010: 495 planets). Even better, by staring really (really) hard you can actually see light that has passed through the atmosphere of those planets, and now you’re talking chemical analysis! Oxygen, on the other hand, is about as stable as a drunk unicyclist. As soon as oxygen is released it tends to immediately combine with things around it.