Astronomers suggest that artificial illumination creates a signature that could point to the existence of civilizations on other worlds — and they say we should get started on a survey of the edges of our own solar system, just in case.

The suggestion comes from Harvard's Abraham Loeb and Princeton's Edwin Turner, in a research paper submitted to the journal Astrobiology. A version of the paper appears on the preprint server and sparked a write-up today on Technology Review's Physics arXiv Blog.

Loeb, who chairs Harvard's astronomy department and is affiliated with the Harvard-Smithsonian Center for Astrophysics, acknowledged that detecting aliens by looking for the glow of their cities would be a long shot. But he pointed out that the cost of the exercise would be low.

"We say that we can piggyback on existing surveys that people are doing anyway. There's no need to use extra resources. ... My philosophy is simple: If we can do it, why not do it and check? Why put blinders on ourselves?" Loeb told me today.

Here's how the idea could work: An object's brightness varies with distance, but the relationship between those two factors will depend on whether the brightness is due to reflected sunlight or due to illumination from the object itself. For a self-illuminated object, the brightness varies by a factor of 1 over the distance squared, but "if you have an object that reflects light from another source ... the flux dies out like 1 over the distance to the fourth power," Loeb said.

Monitoring the changes in the brightness of an object on the edge of our solar system, in a broad disk of icy material known as the Kuiper Belt, could provide a "very simple test" to determine whether extraterrestrials have turned on the lights, Loeb said.

"We conclude that existing telescopes and surveys could detect the artificial light from a reasonably brightly illuminated region, roughly the size of a terrestrial city," on a Kuiper Belt object, Loeb and Turner write.

How likely is it that E.T. would be found on the edges of our own solar system? Not that likely, but Loeb and Turner speculate that it could happen. "Artificially lit KBOs [Kuiper Belt objects] might have originated from civilizations near other stars," they write. "In particular, some small bodies may have traveled to the Kuiper Belt through interstellar space after being ejected dynamically from other planetary systems."

In addition to the E.T. search, Loeb said the Kuiper Belt survey would also be useful for studying how Kuiper Belt objects reflect light at different points in their orbits. "Even if the answer is, 'No, there is nothing peculiar,' we can still learn something from doing that," he told me. "And if there's something out ther worth finding, that could change our perception of reality."

The technique could conceivably be extended to other stars once next-generation telescopes such as the James Webb Space Telescope and the Giant Magellan Telescope come online, over the next decade or so. There's been a lot of debate over whether the traditional search for radio signals from alien civilizations might be fruitless if E.T. moved beyond analog radio transmissions — and the search for artificial illumination could be worth checking out as a new frontier.

Someone could even try looking for the spectral signature of artificial light. (Do aliens use incandescent bulbs, compact fluorescent or LEDs?) But that particular kind of search would not be easy.

"For this signature to be detectable, the night side needs to have an artificial brightness comparable to the natural illumination of the day side," Loeb and Turner write. And when you consider that Earth's day side is about 600,000 times brighter than the night side, that means E.T. would have to cope with one heck of an electric bill.

What do you think about the search for E.T.'s city lights? Feel free to add your comment below.

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