To find an extraterrestrial civilization, look for traces of pollution

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To probe the presence of an advanced extraterrestrial civilization in our near environment, a study suggests that we focus on the presence of nitrogen dioxide gas. And for good reason, on Earth, this gas is mainly produced by the combustion of fossil fuels.

To date, there are more than 4,000 exoplanets in our Galaxy. Out of this sample, some worlds could possibly support an advanced life form, like ours. Because these planets are so far away, it is unfortunately impossible for us to send probes directly there to see, or not, the presence of this extraterrestrial life. On the other hand, from Earth, we could use powerful telescopes to probe the composition of their atmosphere. Future instruments, such as the James Webb Telescope (October 2021) or the Giant European Telescope (2025), will then be able to support us.

A possible indication of the presence of life in these atmospheres (biosignature) could be a combination of gases such as oxygen and methane. These could indeed testify to the presence of microorganisms or plants on the surface (but not always).

Another area of ​​research consists in probing this time not bio-signatures, but traces testifying to the presence of advanced technologies. We then speak of technosignatures. After all, here on Earth the most obvious signs of life are far from natural.

Nitrogen dioxide as a technosignature

As part of a recent study, researchers from NASA’s Goddard Space Flight Center suggest that we focus on one gas in particular: the nitrogen dioxide (NO2).

And for good reason, if this gas can actually be produced by non-industrial sources (biological processes, lightning, volcanoes), on Earth, “Most of nitrogen dioxide is emitted by human activity (around 76%, editor’s note)”, underlines Ravi Kopparapu, main author of this work, citing combustion processes such as vehicle emissions and fossil fuel power plants.

Based on this consistent principle, the observation of NO2 on a habitable planet could potentially indicate the presence of an industrialized civilization.

“Other studies have looked at chlorofluorocarbons (CFCs) as possible technosignatures, which are industrial products widely used as refrigerants until phased out due to their role in ozone depletion.”, continues Jacob Haqq-Misra, co-author of the article.

“To our knowledge, CFCs are not produced by biology at all, so they are a more obvious technosignature than NO2. However, CFCs are very specific manufactured chemicals that may not be found elsewhere. NO2, by comparison, is a general by-product of any combustion process ”.

Artist’s impression of the planet TOI 700 d, discovered by TESS in January 2020. Credits: NASA

A gas visible up to 30 light years

In their study, the team also relied on computer modeling to predict whether extraterrestrial NO2 pollution can produce a signal strong enough to be detected with our current and future telescopes.

They discovered that for an Earth-like planet orbiting a Sun-like star, a civilization producing the same amount of NO2 as ours could be detected by a powerful telescope specializing in the wavelengths visible in a radius of thirty light years, after about 400 hours of observation.

Finally, the researchers also found that stars cooler than our Sun, such as K- and M-type stars, could produce a stronger, more easily detectable NO2 signal. Indeed, these types of stars produce less ultraviolet light capable of “breaking” the molecules of NO2. Another positive point: these stars are also much more common in the Galaxy than Sun type stars.

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