London: Future astronomers in the hunt for alien life forms on planets would be better off looking for pollution trails, as they would indicate the presence of technology.
During most of the 20th century, our television transmission antennas leaked a lot of their energy into space.
More recently, they have begun to be supplanted by satellites that beam their transmissions at the ground, as well as by cable.
Inquisitive aliens searching for signs of intelligent life on Earth may soon have to look elsewhere.
But, light pollution from cities might still give us away.
"Observed over interstellar distances, they would reveal to the observer the presence of a technology," according to a team of astronomers led by Jean Schneider of the Paris Observatory at Meudon, France.
Now, according to a report in New Scientist, scientists suggest that we should look for a similar glow on alien planets.
But, it wouldn`t be easy. Even if all the electricity we generate was used to produce light, it would still be thousands of times fainter than the glint of sunlight reflected from Earth`s surface.
To reliably detect even this massive amount of artificial light on a planet orbiting a relatively nearby star - say 15 light years away - would require an array of telescopes with a combined light-collecting area of 1.5 square kilometres, Schneider`s team calculated.
Our presence on Earth also leaves other traces that could be observed from afar.
The chemicals known as CFCs strongly absorb infrared light at characteristic wavelengths, making them detectable in the atmosphere even when present at concentrations of only parts per trillion.
CFCs do not form naturally, so detecting them on a world orbiting another star would be good evidence of alien technology.
"CFCs are a very interesting idea to look for advanced civilizations," said Lisa Kaltenegger of Harvard University.
But an exceptionally sensitive telescope would be needed to pick them up - more sensitive even than NASA`s Terrestrial Planet Finder and the European Space Agency`s Darwin mission, the most ambitious space telescopes now being planned.
According to Kaltenegger, it may be feasible "in the far future with a flotilla of infrared telescopes in space".
So, looking for CFCs might be a way to look for other civilisations - if aliens make the same mistakes we did.
Other artificial compounds, including less damaging substitutes for CFCs, also have characteristic infrared fingerprints, according to Jim Kasting of Pennsylvania State University, University Park.
"There`s a whole host of things we make industrially as solvents, cleaners and refrigerants - they certainly have absorption lines. If you had a big enough telescope, you could detect them," he added.