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.

Astrobiologists have released a report outlining a number of speculative scenarios where Extraterrestrials might contact Earth, including the possibility that they may detect our global warming problems and view as us a potential resource threat.

The portrayal of extraterrestrials in movies and popular fiction strikes some scientists as unrealistic. Should any real visitors — or conquerors — from space come to our planet, the scientific odds strongly suggest the aliens will be completely artificial forms of life.

Proof of extraterrestrial intelligence could come within 25 years, an astronomer who works on the search.

Experts who, like Hawking, have devoted their careers to thoughtful exploration of the possibilities of alien contact say that we have little reason to be concerned that extraterrestrial civilizations are planning to invade Earth.

For more than 50 years, many have taken the so-called Fermi Paradox to indicate that the existence of intelligent alien civilizations is an impossibility. However, a recent re-examination of the paradox points out that, rather than discounting the spread of an intelligent civilization, the Fermi Paradox merely points out that advanced civilizations with exponential growth are unlikely to exist.

A search for colossal feats of alien engineering called "Dyson spheres" has so far found no convincing candidates within 1000 light years of Earth, but Richard Carrigan, formerly with the Fermi National Accelerator Laboratory, has combed through data from the Infrared Astronomical Satellite and has identified 17 possible "spheres," four of which seem most promising.

Advanced ground and space-based telescopes are discovering new planets around other stars almost daily, but an environmental scientist from England believes that even if some of those planets turn out to be Earth-like, the odds are very low they'll have intelligent inhabitants.

A team of American scientists argue that recent astrophysical discoveries suggest that we should find ourselves in the midst of one or more extraterrestrial civilizations.