In the search for extraterrestrial intelligence, astronomers have hunted for radio signals and ultra-short laser pulses. In a new paper, Avi Loeb (Harvard-Smithsonian Center for Astrophysics) and Edwin Turner (Princeton University) suggest a new technique for finding aliens: look for their city lights.
“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,” said Loeb.
As with other SETI (Search for ExtraTerrestrial Intelligence) methods, this technique relies on the assumption that aliens would use Earth-like technologies. This seems reasonable because any intelligent life that evolved in the light from its nearest star is likely to have artificial illumination that switches on during the hours of darkness.
How easy would it be to spot a city on a distant planet? Clearly, this light will have to be distinguished from the glare from the parent star. Loeb and Turner suggest looking at the change in light from an exoplanet as it moves around its star.
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.
Spotting this tiny difference in brightness would require future generations of telescopes. However, the technique could be tested now on objects closer to home—at the edge of our own solar system.
Loeb and Turner calculate that today’s best telescopes ought to be able to see the light generated by a Tokyo-sized metropolis at the distance of the Kuiper Belt—the region of our own Solar System occupied by Pluto, Eris, and thousands of smaller icy bodies. So, if there are any cities out there, we ought to be able to develop techniques that could see them now. By looking at these closer objects, astronomers can hone the techniques and be ready to apply them when the first Earth-sized worlds are found around distant stars in our galaxy.
“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,” Turner says.
As an example of how important it is, in science, to find methods that can test accepted beliefs, Turner adds that “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.”
As our technology has moved from radio and TV broadcasts to cable and fiber optics, we have become less detectable to aliens. If the same is true of extraterrestrial civilizations, then artificial lights, rather than radio signals, might be the best way to spot them from afar.
Loeb and Turner’s work has been submitted to the journal Astrobiology and is available at arxiv.org:
You can contact Bob Eklund at [email protected]