To scientists from a distant galaxy, Earth is just a small rocky planet orbiting an average star. These scientists would probably note Earth's bluish color, or its atmosphere, or even its moon. But chances are, Earth would seem unremarkable to them. Now imagine these scientists on a journey to our planet. As they draw closer, they detect radio signals. Artificial satellites come into view—along with tons of orbiting debris. Pollution is seen everywhere. All these observations add up to one thing: Earth supports life