This light will be different from natural illumination. On Earth, artificial lighting falls into two types: thermal lighting in the form of incandescent light bulbs and quantum lighting in the form of LEDs and fluorescent lights. “The spectra of artiﬁcial lights on distant objects would likely distinguish them from natural illumination sources,” say Loeb and Turner. “Artiﬁcial illumination may serve as a lamppost which signals the existence of extraterrestrial technologies and civilizations.”
But how easy would it be to spot a city on another planet? Clearly, this light will have to be distinguished from the glare from the parent star and Loeb and Turner suggest a way to do this. Their idea is to look at the change in light from an exoplanet as it moves around its star.
Given that its orbit will be elliptical, the amount of reflected light will change with the distance from its star. But the amount of artificial light will remain constant. So the total flux from a planet with city street lighting will vary in a way that is measurably different from a planet that has no streetlights.
There’s a caveat, however. “For this signature to be detectable, the night side needs to have an artiﬁcial brightness comparable to the natural illumination of the day side,” say Loeb and Turner. That seems rather unlikely given that Earth’s night time illumination is some 100,000 times less its day time lighting.
But it’s early days for this entirely new form of SETI. Other techniques for spotting cities as they blink on and off in the extraterrestrial night will surely emerge.
There is another search that could be done closer to home. With the help of some back-of-the-envelope calculations, Loeb and Turner say that today’s best telescopes ought to be able to see the light generated by a Tokyo-sized metropolis at a distance of about 50 AU, that’s roughly the distance to the Kuiper belt.
So if there are any cities out there, we ought to be able to see them now. “Artiﬁcially-lit Kuiper Belt objects might have originated from civilizations near other stars,” say Loeb and Turner who suggest they could have been ejected from their own systems and ended up here. They may even have passed near Earth on their way through the Solar System before the age of telescopes.
For that reason, they argue it’s worth studying the spectra from Kuiper belt objects, just in case.