The nighttime light of cities could be a new target in the search for extra-terrestrial life. A recent paper by Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Edwin Turner of Princeton University suggests that we should be scouring faraway planets for urban metropolises.
It’s an idea that relies on a lot of assumptions being true, but the basic premise is that if there are planets with human-like intelligent life, they'll likely have invented their way out of the dark nights caused by revolving around a sun. In other words, cities full of lights shining out, waiting to be seen.
But if there are alien mega-cities – and we can only hope that there are – this approach might offer a way to track them down and identify places in the universe where other intelligent life has evolved. Loeb and Turner suggest that the lights of big cities would be a relatively easy tell. The Harvard-Smithsonian Center for Astrophysics explains:
As the planet orbits, it goes through phases similar to those of the Moon. When it's in a dark phase, more artificial light from the night side would be visible from Earth than reflected light from the day side. So the total flux from a planet with city lighting will vary in a way that is measurably different from a planet that has no artificial lights.
This sort of observation of far-off planets isn’t possible yet, but telescopes are improving. The researchers note that through their method, telescopes today would be able to observe a Tokyo-sized city as far away as Pluto. Once the hardware is improved, astronomers and scientists might consider pointing their telescopes to the 1,235 Earth-sized planets NASA’s Kepler mission has so far identified.
Loeb concedes that finding an alien city is a long-shot, but that no extra resources would be needed to look. And as our own planet becomes increasingly urbanized, maybe it will be us that gets discovered.