Memo to scientists. Looking for intelligent life? Have you tried checking for worlds with a lot of industrial pollution?

Astronomers have been offered a cheat sheet on the kinds of things to look out for – from air pollution to energy production – when hunting for signs of intelligent life on faraway exoplanets.

“We have no idea whether intelligence is something very common in the Universe or, on the contrary, whether it is extremely rare,” said physicist Hector Socas-Navarro on Wednesday.

"For that reason we cannot know whether these searches have any chance of success. There is no choice but to search and see what we find, because the implications would be tremendous."

But what exactly should we be searching for? Radio signals? Intergalactic re-runs of Big Bang Theory? Glad you asked: Socas-Navarro, who works at the Institute of Astrophysics of the Canary Islands, is the first author of a paper published in Acta Astronautica detailing the so-called technosignatures scientists ought to be trying to pick up when looking for intelligent alien life. You can get a preprint list of these signature concepts here.

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These signatures are more than a simple wish list for some distant alien race. Technosignature research is gathering momentum, with NASA looking into it since 2018 and sponsoring a virtual conference on the topic last August, which formed the basis for this latest paper. We're told that by piggybacking a few instruments onto future exoplanet telescopes, it should be possible to perform scientifically valid observations for signatures of advanced life.

It’s highly unlikely that colonies of microbes floating in a sea or lodged in rocks will be able to build things like rockets or nuclear power plants that we can detect from our corner of the galaxy, although they may, for instance, be emitting abnormal levels of gases that we could identify. Older and wiser species are more likely to build technology we can monitor the void for, however. We're talking about things like industrial gases and signs of surface artifacts as well as radio transmissions; all clues pointing to the presence of intelligent life. If said aliens leave any trace at all.

“It is often argued that advanced civilizations are likely to build large geoengineering or stellar system engineering megastructures,” the paper stated.

“There might be advanced civilizations that do not leave a large imprint on their planetary or space environment and will thus be very difficult to detect. And conversely, there might be younger civilizations that announce themselves very conspicuously, [for example] by broadcasting radio signals or releasing large amounts of industrial pollutants into their atmospheres.”

Science fiction to science fact?

The researchers are particularly interested in the technosignatures of a Dyson sphere, a hypothetical structure built around a star to harvest its energy. Scientists could detect these spheres – popularized by physicist Freeman Dyson – by inspecting the infrared spectra around stars to see if its heat was being funneled elsewhere, the paper suggests.

“The idea of searching for technosignatures draws upon the technology we have on Earth today and possible extensions of our technology into the future,” said Jacob Haqq-Misra, coauthor of the article and a researcher at the Blue Marble Space Institute of Science.

“This does not necessarily mean that any extraterrestrial technology must be like our own, but imagining plausible extensions of our own future is one place to begin thinking of astronomical searches we could actually do to look for possible technosignatures.”

NASA said it will, over the course of three years, bankroll a team of boffins led by Harvard University to look for ways to find these signs of life.

The lead researcher on the team, Avi Loeb, chairman of Harvard University’s Department of Astronomy, has argued that Earth has already witnessed the product of an alien civilization: ‘Oumuamua, the first-known foreign interstellar object to visit the Solar System. ®