All this talk about habitable zones brings us back to the inescapable fact that what we are really looking for, at the heart of all these searches, is life on other planets. And one way I have been saying to look for that life is to search for signs of a civilization’s atmospheric pollution. Now, NASA has released a study to be published in The Astrophysical Journal that agrees with me.

As lead author Ravi Kopparapu explains: On Earth, most of the nitrogen dioxide (NO2) is emitted from human activity — combustion processes such as vehicle emissions and fossil-fueled power plants. 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.

This is the first time a study has looked at nitrogen dioxide as a possible technosignature for extraterrestrial life. The team used computer modeling to determine if this particular form of pollution could produce a detectable signal. Well, at least a signal detectable using those advanced telescopes we talked about earlier. It’s what we have to work with at the moment or will have in the near future. 

Per the press release: They found that for an Earth-like planet orbiting a Sun-like star, a civilization producing the same amount of NO2 as ours could be detected up to about 30 light-years away with about 400 hours of observing time using a future large NASA telescope observing at visible wavelengths. This is a substantial but not unprecedented amount of time, as NASA’s Hubble Space Telescope took a similar amount of time for the famous Deep Field observations.

Additionally, stars that are cooler and more common than our Sun, what we call K and M-type stars, or red dwarfs, will actually produce a more easily detected nitrogen dioxide signal. Since those stars are more abundant, we have more chances for finding technosignatures from planets in orbit around them.

I have to note, though, that as exciting as this study is for me, nitrogen dioxide is also produced naturally. So this isn’t the substantial result we hoped for, but it’s a step in the right direction. Now we’ll just have to figure out how to distinguish naturally occurring amounts from technologically produced amounts and double-check any positive signals we find. If we find them. I really hope we find them.

More Information

NASA press release

“Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology,” Ravi Kopparapu et al., to be published in The Astrophysical Journal (preprint on arxiv.org)