So how do you go about spotting an alien civilization? Well, you build a really big radio telescope and point it at the stars. But say if ET isn’t transmitting? Say if ET is transmitting, but they are too far away? What if our alien neighbors decided not to transmit radio waves? What if they’re just shy and don’t want the cosmos to know where they live?

The biggest limitation when trying to listen into any kind of signal is the speed of light. After all, mankind has only been transmitting radio waves for about 100 years. This means that any signal we’ve accidentally leaked into space has only travelled 100 light years. In galactic scales, this is very pedestrian; the Milky Way is 100,000 light years in diameter and 1,000 light years thick. Only a small handful of stars have had the pleasure of listening into our noise.

There are a few other ways we can go about searching for intelligent extraterrestrials (such as looking for observational evidence of alien mega-structures), but so far we have had little indication that intelligent life lives anywhere else apart from Earth. The Universe is a huge place, it would be naive to say that we are the only intelligent form of life there is. But the fact remains: it’s getting lonely out here.

Light on Earth

So, we search, and we try to think up other novel ways we can spot ET from 10′s to 100′s to 1,000′s of light years distant. And a team of astronomers led by Jean Schneider of the Paris Observatory at Meudon, France, is currently pondering this challenge.

In paper to appear in the journal Astrobiology, the team decided to investigate whether an alien civilization could give themselves away by the amount of light pollution they produce. Take a look at the orbital images of Earth and you’ll see that this isn’t a difficult thing to understand. On the night-side of Earth the lights of our cities are easily visible; perhaps aliens have cities which, like ours, generate some kind of light pollution?

The problem with this is that when compared with the brightness of the star the alien world is orbiting, city lights will be very hard to distinguish. Schneider et al. estimate that we’d need to build an array of telescopes with the collective area of 1.5 square kilometers. Even then we’d only be able to search for light pollution on exoplanets within 15 light years from Earth.

No, when searching for alien civilizations further afield we’d need to look out for some other indication of a civilization. What kind of pollution doesn’t occur naturally, indicating a civilization is thriving?


The detection of chlorofluorocarbons (or CFCs) in an exoplanet’s atmosphere would be a very good indicator of the presence of alien technology. CFCs do not occur naturally and they are strong absorbers of infrared light. Although a very powerful telescope and spectrometer would be required, the existence of CFCs could be detected by analysing the parent star’s starlight through the exoplanet’s atmosphere.

Although this is a very novel way of looking for ET, we’d be making a huge assumption that the alien civilization has made the same mistakes as mankind in producing CFCs in the first place. After all, CFCs make holes in our ozone layer, this makes CFCs bad, right?

Although CFCs are now banned on Earth (and they will eventually be removed from our atmosphere all together), say if alien civilizations don’t need an ozone layer? (This is a tricky argument to present as ozone absorbs harmful ultraviolet radiation from the sun, thereby allowing life to evolve. But this is terrestrial life, perhaps “life as we don’t know it” evolved differently to us.) Assuming CFCs are bad universally — simply because they eat our protective ozone layer — is another assumption that may not apply to an alien species.

Whether or not we ever detect the presence of an alien civilization, we are certainly giving the search our best shot. Personally, I wonder if we’ll be lonely for much longer…