According to NASA, one of the best ways to find alien life on other planets is to look for pollution in their atmosphere – specifically nitrogen dioxide gas (NO2).
Ravi Kopparapu, who works at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and is the lead author of the study, explained this further, “On Earth, most of the nitrogen dioxide is emitted from human activity -- combustion processes such as vehicle emissions and fossil-fueled power plants,” adding, “In the lower atmosphere (about 10 to 15 kilometers or around 6.2 to 9.3 miles), NO2 from human activities dominate compared to non-human sources. Therefore, observing NO2 on a habitable planet could potentially indicate the presence of an industrialized civilization.”
With more than 4,000 exoplanets found so far, experts are eager to find out if any of them host other civilizations but since they are so far away from us, the only way we can search for signs of life is with powerful telescopes that can find out what’s in their atmospheres.
One sign of life would be the presence of combined gases in their atmosphere, such as oxygen and methane. Additionally, a gas such as NO2 may indicate that there is some type of technology on the planet that is creating it (this is called a technosignature).
Jacob Haqq-Misra, who works at the Blue Marble Institute of Science in Seattle, Washington, and is a co-author of the study, noted that previous research focused on chlorofluorocarbons (CFCs) as potential technosignatures since biology can’t produce it. “...However, CFCs are very specific manufactured chemicals that might not be prevalent elsewhere; NO2, by comparison, is a general byproduct of any combustion process,” he explained.
Their research focused in part on how much NO2 pollution could be detected with our telescopes and after extensive studies, they were able to determine that if an intelligent civilization living on an Earth-like exoplanet that orbits a sun-like star were to produce as much NO2 as Earth, it could be detected as far as 30 light-year away with approximately 400 hours of data focused solely on that planet.
That’s a good start, but considering our closest star relatives are located in the Alpha Centauri system which is a bit more than 4 light-years away from us and our entire galaxy is approximately 100,000 light-years across, we will definitely need more powerful telescopes.
There are a few complications with this new information, such as figuring out if the NO2 on other planets are produced naturally or created by another civilization. “If we observe NO2 on another planet, we will have to run models to estimate the maximum possible NO2 emissions one could have just from non-industrial sources. If we observe more NO2 than our models suggest is plausible from non-industrial sources, then the rest of the NO2 might be attributed to industrial activity. Yet there is always a possibility of a false positive in the search for life beyond Earth...,” explained Giada Arney from NASA Goddard and who is another co-author of the study.
Furthermore, if there are clouds or aerosols in an exoplanet’s atmosphere, the wavelengths may falsely appear as NO2. While much more work needs to be done, this is a very interesting step in possibly finding alien civilizations on other planets.
The study was published in arXiv where it can be read in full.