Earth's cities are visible at night from space because of their artificial lights, so populated exoplanets might give off light pollution of their own. But finding it might not be easy. Even if all the world's electricity were used to produce light, it would still be thousands of times fainter than a glint of sunlight reflected off the Earth's surface.



(Image: NASA/GSFC)

We could also hunt for evidence of chemical pollutants in the atmospheres of exoplanets. Artificial compounds such as chlorofluorocarbons, or CFCs, could leave traces that might be observed from afar. Because they strongly absorb infrared light at characteristic wavelengths, CFCs may be detectable even when present at concentrations as low as parts per trillion. But it could take an exceptionally sensitive telescope, far beyond the capabilities of present-day instruments, to pick up such traces.



(Image: NASA/ ESA/G. Bacon/STScI)

Dumping nuclear waste in a star could leave it with suspiciously large abundances of rare elements produced by nuclear fission, such as technetium or neodymium, which we might detect in its starlight. But creating a recognisable signature may require a colossal amount of material – for example, 100,000 tonnes of technetium, according to an estimate by Guillermo Lemarchand of the National University of Quilmes in Argentina. By contrast, Earth's nuclear reactors have produced only 100 tonnes or so of technetium in the past century, says Richard Carrigan.



(Image: JAXA) Advertisement

An extraterrestrial civilisation might also reveal itself through colossal feats of engineering called Dyson spheres, hypothetical structures that would cocoon stars to collect solar energy. A Dyson sphere would partially or fully block a star’s visible light. But because the sphere would still be warmed by its star, it would radiate infrared light that might be detectable from Earth.



Astronomers have found no Dyson spheres for certain – the few candidates they have flagged up could be explained just as easily as clouds of hydrogen gas, dust engulfing ancient stars or even asteroids.



(Image: Vedexent/Wikimedia Commons)

Rather than blocking the light of just one star, a sprawling extraterrestrial civilisation might build Dyson spheres around many stars, creating a noticeable dark patch in its home galaxy called a Fermi bubble. Like individual Dyson spheres, Fermi bubbles would still radiate heat, making them visible in infrared light.



But finding them could still be tricky. Spiral galaxies, like the Pinwheel galaxy (shown here), are full of dust and dark gaps. Galaxies that are naturally more uniform in brightness, like elliptical galaxies, might be better candidates in the hunt for ET.



(Image: NASA/ESA/STScI/K. Kuntz et al.)