An extraterrestrial spacecraft lurking in a satellite's orbit near Earth would be able to see city lights and pollution in our atmosphere. But what if it searched for signs of life on Earth from afar?
This question has great pertinence to those searching for other Earths outside of our solar system. NASA's Kepler space telescope is among a fleet of telescopes and spacecraft searching for rocky planets similar to our own. Once the size and location of these worlds are plotted, the next step is examining the chemical composition of their atmospheres.