Spotting alien cities
Bright city lights on other worlds could help astronomers find ET, according to scientists.
Powerful next-generation telescopes may be able to spot extraterrestrial cities from the light they generate at night, it is claimed.
Cities such as London, New York and Tokyo illuminate the Earth when it is facing away from the sun, and cities built by intelligent aliens could be detected by measuring the effect they have on their planets' light emissions, say the researchers.
Spotting such a tiny signal from a nearby star would require telescopes more powerful than any in use today.
But present-day technology could allow a trial city search to be conducted among icy bodies in the Kuiper Belt.
This is the region at the edge of the solar system occupied by Pluto and thousands of smaller "planetoids".
It should be possible to detect any extraterrestrial Kuiper Belt outpost the size of Tokyo or larger, say the US scientists who outline their plan in the journal Astrobiology.
"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," said astronomer Dr Edwin Turner, from Princeton University.
Colleague Dr Avi Loeb, from the Harvard-Smithsonian Center for Astrophysics, said: "Looking for alien cities would be a long shot, but wouldn't require extra resources. And if we succeeded, it would change our perception of our place in the universe."
Until now most attempts to search for extraterrestrial intelligence have relied on listening out for signals beamed to the Earth, or eavesdropping on broadcast transmissions.
However, transmissions from Earth have fallen dramatically in recent decades as cable, fibre optics and other advances replace TV and radio broadcasts.
Intelligent aliens on other planets may have gone through the same changes, making them harder to detect from "leaked" radio signals, the scientists believe. If this is true, looking for brightly lit cities might be the best way to spot them.
"Artificial illumination may serve as a lamp post which signals the existence of extraterrestrial technologies and thus civilisations," the researchers wrote.
Weird Science Factoid: The horizon is around 11 miles from where you are standing on a beach.