There's no surefire way to find evidence of alien civilizations — indeed, they're basically all long shots — but a new method could reveal the existence of aliens just like us. And we've almost got the technology needed to do it.
It's always dangerous to assume that another intelligent species would evolve along the same general lines as humans, but there are some reasonable assumptions that we can make. For instance, it's a reasonably safe bet that other intelligent species would evolve on the surface of their planets, meaning that they're at the mercy of their sun's night and day cycle. As these hypothetical aliens built more complex structures, they would probably want to be able to see in the dark.
Yes, this method relies on spotting alien city lights. It's the equivalent of spotting the nighttime glow of Manhattan from hundreds of light-years away — not impossible, maybe, but pretty damn close to it. But astrophysicists Avi Loeb and Edwin Turner think it's possible for future telescopes that can spot extremely tiny fluctuations in the total light emitted from alien solar systems.
Basically, as an alien planet revolves around its star, it goes through different phases relative to Earth, much like how we see the Moon. During certain phases, we would see more light from the dark side of the planet than what's reflected from the day side. If an alien planet has no intelligent species on it, then the reflected daytime light should pretty much overwhelm what's visible from the night side. But if there are alien cities on the planet, they would emit enough light from the dark side of the planet for sufficiently advanced telescopes here on Earth to notice.
Right now, Loeb and Turner calculate that our very best telescopes can spot the light of a Tokyo-sized city off in the Kuiper Belt. That means that, if there happens to be a secret city on Pluto or Eris, we should be able to spot it. The pair acknowledge that there almost certainly aren't any such cities, but they still want to refine their method by testing it against the worlds of the Kuiper Belt.
What seems like a bit of a waste of time now could well illuminate the existence of faraway alien cities a few decades from now. Plus, now that they've put the idea in my head, the thought of a secret city on Pluto is too awesome not to double-check.
Via Astrobiology. Image by David A. Aguilar (CfA).