If extraterrestrial beings dwell in cities as well lit as the ones on Earth, future generations of telescopes should be able to detect them. That's a new theory posited by Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Princeton University's Edwin Turner, who think looking for light from alien cities, while a long shot, is worth doing because it wouldn't require extra resources. "And if we succeed, it would change our perception of our place in the universe," Loeb said. If an alien civilization builds brightly-lit cities like those shown in this artist's conception, future generations of telescopes might allow us to detect them. This would offer a new method of searching for extraterrestrial intelligence elsewhere in our galaxy. (Image: David A. Aguilar/CfA) In the past, astronomers searching for alien life looked for radio signals and ultrashort laser pulses, but the new technique is based on the assumption that aliens would use Earthlike technologies for illumination during hours of darkness. 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. Loeb and Turner calculate that today's best telescopes should be able to see the light generated by a Tokyo-sized metropolis at the distance of the Kuiper Belt — the region occupied by Pluto, Eris and thousands of smaller icy bodies. By looking, astronomers can hone the technique and be ready to apply it 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 said. “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, said the researchers. If the same is true of extraterrestrial civilizations, then artificial lights might be the best way to spot them from afar. Loeb and Turner's work has been submitted to Astrobiology and is available online. For more information, visit: www.cfa.harvard.edu