Researchers in the United States have proposed using telescopes to look for city lights on other planets.
A report published on the Cornell University Library website suggests telescopes could examine the kind of light coming from other planets and tell whether it is artificially generated.
If alien cities exist, they would be particularly easy to spot when their lit, night side is pointing towards Earth.
The authors from Harvard and Princeton Universities say existing telescopes are only powerful enough to spot cities within the solar system but in the future the method could be used to spot intelligent life orbiting distant stars.
