When most people think about the search for alien life, the first thing that usually pops into mind is SETI (Search for Extraterrestrial Intelligence). Primarily a search for extraterrestrial radio signals, another more recent facet of SETI is now looking for laser pulses as a conceivable means of communication across interstellar distances. But now, a third option has been presented: looking for sources of artificial light on the surfaces of exoplanets, like the lights of cities on Earth.
According to Avi Loeb at the Harvard-Smithsonian Center for Astrophysics, “Looking for alien cities would be a long shot, but wouldn’t require extra resources. And if we succeed, it would change our perception of our place in the universe.”
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Like the other SETI initiatives, it relies on an assumption that an alien civilization would use technologies that are similar to ours or at least recognizable. That assumption itself has been the subject of contentious debate over the years. If an alien society was thousands or millions of years more advanced than us, would any of its technology even be recognizable to us?
That aside, how easy (or not) would it be to spot the signs of artificial lighting on an alien planet light-years away from us? The suggestion is to look at the changes in light from an exoplanet as it orbits its star. Artificial light would increase in brightness on the dark side of a planet as it orbits the star (as the planet goes through its phases, like our Moon or other planets in our own solar system), becoming more visible than any light that is reflected from the day side.
That type of discovery will require the next generation of telescopes, but today’s telescopes could test the idea, being able to find something similar as far out as the Kuiper Belt in our solar system, where Pluto and thousands of other small icy bodies reside. As noted by Edwin Turner at Princeton University, “It’s very unlikely that there are alien cities on the edge of our solar system, but the principle of science is to find a method to check. Before Galileo, it was conventional wisdom that heavier objects fall faster than light objects, but he tested the belief and found they actually fall at the same rate.”
The paper has been submitted to the journal Astrobiology and is available here.