19 October 2009 by David Shiga
DO ALIENS pollute their planets? Let's hope they do, as this would give us a promising way of spotting where they live.
Radio noise may be too short-lived to help us find aliens, if our own activity is any guide. 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.
Light pollution from cities might still give us away. "Observed over interstellar distances, they would reveal to the observer the presence of a technology," say a team of astronomers led by Jean Schneider of the Paris Observatory at Meudon, France. In a paper to appear in Astrobiology, they suggest we should look for a similar glow on alien planets.
This 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 calculates.
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 civilisations," agrees 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. Kaltenegger says it may be feasible "in the far future with a flotilla of infrared telescopes in space".
There is, of course, no guarantee that any alien civilisations will have been spewing CFCs into their planet's atmosphere. The damage CFCs have done to Earth's ozone layer in the few decades they have been used led to a worldwide ban on their manufacture, and they are slowly disappearing from our atmosphere. "Do all intelligent civilisations make the same mistakes?" Kaltenegger wonders.
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, says 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," he says. "If you had a big enough telescope, you could detect them."