The search for life on alien planets focuses largely on radio signals – on keeping an ear out for transmissions from far-off civilizations. But what if we kept an eye out instead, in the hope that the aliens literally have left a light on? If intelligent beings have indeed evolved around distant stars, they likely have developed artificial lighting systems for the dark hours, and those systems could help us spot them, say Abraham Loeb of Harvard University and Edwin Turner of Princeton University. Looking for artificial lighting instead of radio signals could help scientists spot extraterrestrial civilizations such as those depicted in this artistic representation. Courtesy of David A. Aguilar, Harvard-Smithsonian Center for Astrophysics. Humans have moved from exclusive use of radio and television signals to broadcasting through cable and fiber optics as well. If aliens have done the same, their signals might not be as easy to pick up as before. So, as long as they haven’t evolved to see in the dark, artificial lighting might be a better way to spot them. Loeb and Turner have proposed a mathematical method that could use ground- and space-based telescopes to detect light patterns from other planets. All we would have to do is measure the changes in light from a planet as it moves around its star. Artificial light sources such as incandescent lightbulbs, LEDs and fluorescent lamps have different spectral properties than sunlight, they said, and such emissions could show up right away if we only look for them, lighting the way to alien life. When the planet is in its dark phase, more artificial light from the night side would be visible from Earth than reflected light from the day side. The total flux from a planet with city lighting would vary in a way that is measurably different from a planet with no artificial lighting. Today’s state-of-the-art telescopes should be able to pick up the light from a Tokyo-size city as far away as the Kuiper Belt, so if the aliens are that close, we already have the technology to find them. “It’s very unlikely that there are alien cities on the edge of our solar system,” Turner said, “but the principle of science is to find a method to check.” Tokyo glows a cool blue-green color at night, thanks to widespread use of mercury vapor lighting. Today’s telescopes could spot a Tokyo-size city in the Kuiper Belt. Courtesy of NASA; photo taken by International Space Station astronaut Daniel Tani in 2008. The key to extending the search beyond our solar system will be next-generation telescopes that can detect phase modulation. But the artificial brightness of a planet’s night side would have to equal the natural brightness of the day side, which means that aliens outside our solar system would have to use brighter, more extensive artificial lighting than we do on Earth if we ever hope to find them. If unusual light patterns should appear, the findings can be followed up with a complementary sweep for artificial radio signals, Loeb and Turner point out in their article titled Detection Technique for Artificially-Illuminated Objects in the Outer Solar System and Beyond, which has been submitted to the journal Astrobiology.