If you look at the Earth from space, it’s pretty clear that there’s intelligent life here because you can pick out cities on the night side. Abraham Loeb, of Harvard University and Edwin Turner, from Princeton University, are saying that we might find other civilizations the same way.
The reason is that the way we usually listen for aliens — via radio signals — may not work that well. One issue is that contrary to popular belief, TV transmissions don’t travel all that far. (It would actually be near-impossible to pick up old shows from more than a few light years away).
Our own civilization is actually generating less residual energy as time goes on as communications move to optical fiber. Other civilizations may have done the same, so picking them up actually gets harder as their technology gets better.
The trick is to look for a certain kind of light that would be different from natural starlight. The two note the spectrum of LEDs and streetlights is different from that of the sun. An alien looking at Earth with a sufficiently sensitive telescope would notice that.
The same applies in reverse. Artificial lights will also vary in a different way from reflected light on a planetary surface.
But can we see this on the surface of any known exoplanets from Earth? Current telescopes aren’t sensitive enough. To see a city on another planet the city lights would have to be nearly as bright as the day side, and even here that isn’t the case (yet). But the next generation of telescopes — such as the James Webb Space Telescope, or the ground-based European Extremely Large Telescope — may be able to do it.
Loeb and Turner also calculate that existing telescopes can see a city the size of Tokyo, if it were located on an asteroid or dead comet at about 50 Astronomical units from here, in the Kuiper Belt. So another place to look might be on those objects, to see if anyone has been building in our neighborhood.