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When we think of alien life, images of little green men, Martians, or perhaps even spaceships carrying them to Earth are the first things that come to mind. Yet when you think about life on Earth, how it arose, and how it came to be the way it is today, you’ve got to recognize that the evidence of an inhabited world was present for billions of years before humans ever arose. The first signs of alien life that we’re going to discover are unlikely to be sentient beings arriving at Earth, but rather the subtle signs we’d see if someone had been searching for Earth as it was hundreds of millions (or more) years ago.

While it’s going to require some huge advances in technology before we can directly image the surface of an alien world for signs that it’s inhabited, there’s a surefire signature that organic processes have been happening on a planet’s surface or in its oceans for a long period of time: the existence of certain chemicals in its atmospheric composition. And thanks to the unique properties of molecules, combined with the astronomical techniques we’ve pioneered over the last two decades, we have every reason to believe the first signs of life beyond our Solar System will show themselves in the coming two or three decades, tops.

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