Since the search for extraterrestrial intelligence started, scientists have been “listening” to stars, looking for artificial radio emissions from other planets. While we keep searching for such signals, there may as well be other ways to detect alien civilizations. Light itself may in the future become an efficient way to find another inhabited and civilized world.
Researchers at SETI are using radio signals in order to detect other possible civilizations elsewhere in the Galaxy for various reasons, the main one being our own civilization: we are emitting considerable amounts of electromagnetic radiation as a byproduct of communications, and many radio frequencies penetrate the atmosphere quite easily.
However, you might argue that this doesn’t mean any other civilization would do the same, especially considering that technology changes quickly, and that our own emissions are constantly decreasing. Also, any signal would have to be pretty strong to be detectable.
Abraham Loeb of Harvard-Smithsonian Center for Astrophysics, and Edwin Turner of Princeton University have now suggested a new way for detecting alien civilization, and it is pretty simple: we should look for their city lights. So if it is that simple, why aren’t we already doing it? The only limitation is technology: to glimpse the light of alien cities, researchers would have to be able to distinguish it from the glare of their parent star. According to the two scientists, the slight change in light from an exoplanet as it moves around its star should be detectable. Indeed, an inhabited exoplanet with city lighting would emit more light than one without artificial lighting during a dark phase (when orbiting its star the planet would go through phases similar to those of our Moon).
With our current technology, the team estimates that we should be able to detect a Tokyo-sized metropolis on Pluto. Obviously, it’s very unlikely that there is any alien civilization out there or anywhere in the Kuiper Belt (the region in which Pluto is orbiting the Sun). However, by the time the first Earth-like exoplanets are found, our technology will have improved and should be able to detect the artificial lights of potential nearby Earth twins.
Although this technique also relies on the assumption that any alien civilization would use Earth-like technologies, it seems rather difficult to avoid artificial lighting, unlike radio signals. And that’s why I personally hope to see such research being done in the near future.
Abraham Loeb, Edwin L. Turner. Detection Technique for Artificially-Illuminated Objects in the Outer Solar System and Beyond: arXiv:1110.6181v1