clipped from: www.universetoday.com If aliens visited our Solar System, it would only take them a moment to figure out which planet is the one with all the life on it. That's because our atmosphere has a high percentage of oxygen in its atmosphere. The presence of oxygen in our atmosphere has given scientists the key to searching for life on other worlds. But what if there are purely natural processes, that could confuse the search for life, fooling powerful new space observatories like the Terrestrial Planet Finder and Darwin.
Don't worry. A new simulation by a team of US researchers shows that no natural process on a habitable world with liquid water could keep high levels of oxygen and ozone present in an alien atmosphere. If there's oxygen, there's life.