Can we find extraterrestrial life through its pollution?

Could we someday detect extraterrestrial life not through radio signals broadcast into space, but through a familiar side effect of civilization: environmental pollution? Typically, it concerns Earth's atmosphere. But about a decade ago, a colleague knocked on Gonzalo Gonzalez Abad's door and asked him an unexpected question: "If you were looking for traces of a technologically advanced extraterrestrial civilization light-years away, how would you attempt to do it?"

This article is part of a week-long special report on aliens, dedicated to the upcoming 60th anniversary of the most famous extraterrestrial life form on the BBC, Doctor Who.

Gonzalez Abad, an atmospheric scientist from the Harvard-Smithsonian Center for Astrophysics, paused for a moment before answering, "CFCs" - chlorofluorocarbons. On Earth, various devices, including aerosol cans and refrigerators, emitted these gases for years before we realized that CFCs were depleting the ozone layer.

"They persist for a long time, and, of course, nature doesn't produce them," Gonzalez Abad says. If some extraterrestrial population polluted their world in the same way we polluted ours in the 20th century, telescopes could simply detect the presence of CFCs in their planet's atmosphere. This could potentially be a hint at a technologically advanced culture elsewhere in the Universe. What scientists call a "technosignature."

Perhaps we are not alone... in ruining our planet. Just as human garbage can reveal their secrets, extraterrestrials may expose themselves through their obvious carelessness. For many years, researchers have pondered this and come up with various possible technosignatures based on high levels of extraterrestrial pollution. From excessive light to space debris or harmful gases in the atmosphere of an alien planet, different types of waste could theoretically indicate the presence of a distant civilization.

Now, telescopes are emerging that are powerful enough to detect technosignatures, and many scientists hope to use them in the coming decades to try to find life on other worlds. There might be trash out there. Maybe.

In 2014, Gonzalez Abad became a co-author of a paper discussing the possibility of detecting extraterrestrials through CFC emissions. Researchers estimated that if the concentration of these gases in the atmosphere of a distant planet were about 10 times higher than their concentration on Earth, their presence could be detected using the James Webb Space Telescope, which is set to launch in 2022.

The brown haze that pollutes some regions of Asia can be seen from satellites; could extraterrestrial societies have left similar signs? (Photo: Getty Images)

It's important to note that CFCs can remain in a planet's atmosphere for tens of thousands of years, which means that an extraterrestrial civilization wouldn't necessarily have to produce them for a very long time to leave a trace of CFC activity. Chlorine in Earth's atmosphere today exists due to CFC emissions in recent decades. Today, these gases are banned worldwide, though there is still much work to be done to completely eliminate them.

Detecting CFCs with the James Webb Space Telescope could be possible if the polluted planet orbited a small white dwarf, Gonzalez Abad and his co-authors suggest, as this would increase the chances of useful light levels reaching Earth. Scientists can search for CFCs and various other chemical substances in the atmospheres of distant planets by studying spectra – or specific wavelengths of light – reflected from alien worlds. Since some light is absorbed by chemical substances, while some passes through them, the precise nature of the forward-directed light can show which chemical substances are present on the remote body.

Many of the proposals for extraterrestrial technosignatures put forward to date have been inspired by pollutants created by humans here on Earth. Scientists have wondered if we can find extraterrestrial civilizations by detecting, for example, large amounts of waste heat emitted by industrial sources. Others suggest that if extraterrestrials ever become victims of a large-scale nuclear war on their planet, we might see bright flashes of their exploding warheads from Earth. And then there's space debris – can we see its masses in orbit around another world? Perhaps something from all of this will one day pass by Earth.

Increases and decreases in NO2 concentration can give us an idea of the levels of industrial activity happening on an alien world. Science fiction is full of stories of people exploring space only to stumble upon extraterrestrial garbage. In the 1979 movie "Alien," the crew of the "Nostromo" encounters a crashed extraterrestrial megaship. However, the less said about what happens next, the better.

Returning to gases that could be detected much further away, scientists have contemplated the search for other gaseous pollutants besides CFCs. Take nitrogen dioxide, or NO2. "A large portion of NO2 on our planet is produced as a result of industrial activity," says Jada Arney of NASA's Goddard Space Flight Center, who co-authored a 2021 study on the possibility of discovering extraterrestrial civilizations by searching for NO2 pollution in the galaxy. On Earth, about 65% of NO2 is generated by emissions from cars, ships, airplanes, power plants, and other anthropogenic sources.