Signs of life (Image: Sugimoto Yasuaki/Getty)

Extraterrestrial civilisations could give themselves away by the night-time glow of their cities. And if there is one lurking in the outer reaches of our solar system, we are ready to find it.

Existing telescopes could spot cities the size of Tokyo out to the edge of the solar system, and future telescopes could detect well-lit planets around other stars, a new study suggests. “This opens a new window for a search for extraterrestrial civilisations,” says Avi Loeb of Harvard University.

The mainstream search for extraterrestrial intelligence (SETI), now running for more than 50 years, relies on the hope that aliens will either leak or broadcast radio signals out into space.


In recent years, however, humanity’s own radio voice has softened with the shift to cable and Earthward-pointed satellite broadcasts, while our cities have grown brighter. If we are any guide, alien hunters should look to city lights instead, argue Loeb and Edwin Turner of Princeton University.

In the unlikely event that ET has built a Tokyo-sized city beyond Neptune on an icy object in the Kuiper belt, the Hubble Space Telescope could detect its glittering lights, Loeb calculates.

Round trip

The distance from Earth to objects in the Kuiper belt changes as they all orbit the sun. But the objects’ brightness will vary by different amounts, depending on the source of the light. That’s because light decreases exponentially according to the distance it travels.

So light originating on the object would vary differently to light originating on the sun, travelling to the object and then being reflected back to telescopes. Observing how the object’s brightness changes as its distance varies should let astronomers distinguish an inhabited world from one that’s merely sunny.

Even ground-based telescopes should be able to detect artificial lighting: they could distinguish the spectral signature of sunlight glinting off a Kuiper belt object from the direct glow of alien street lights, he says.

“It should be possible to tell what source of light is being used,” he says. “Existing astronomical facilities are capable of detecting the artificial light of a single city.”

Exocity lights

Loeb admits that finding technologically advanced aliens in the outer regions of our own solar system is a long shot, because they would be far from the life-giving glow of the sun. But he says astronomers should search for them anyway: upcoming surveys, like those planned for the future Large Synoptic Survey Telescope, are already due to collect light from thousands of Kuiper belt objects.

“Many people might say it’s extremely unlikely, so why do it?” says Loeb. “But if we can do it without extra expending of resources, why not just do it?”

Greg Laughlin, who studies extrasolar planets at the University of California, Santa Cruz, thinks this “is a good SETI search strategy”, though he’s sceptical about aliens in the Kuiper belt. “I would be willing to bet money that there are no artificially lit objects in our solar system. But of course, I could lose that bet.”

Future space telescopes could search for artificial light on extrasolar planets, however. “This idea has its best applicability outside the solar system,” Laughlin says.

Of course, the plan could hit some practical snares. When Loeb’s home lost power for three days after a freak snowstorm last weekend, he thought, “I hope the aliens do not use [the same] electric company, or else their lighting will not be consistent.”

Journal reference: arxiv.org/abs/1110.6181