Nitrogen dioxide is part of a group of gaseous air pollutants produced due to road traffic and other fossil fuel combustion processes. 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 industrialized civilization.
Until now, astronomers have detected over 4,000 planets orbiting other stars. Some of these planets are habitable; some have conditions suitable for life. Since exoplanets are so distant, scientists cannot look for signs of life or civilization by sending spacecraft to these distant worlds.
The presence of a combination of gases like oxygen and methane in the atmosphere could be a sign of life or biosignature. Likewise, a sign of technology on an exoplanet, called a techno signature, could be what’s considered pollution here on Earth — the presence of a gas that’s released as a byproduct of an overall industrial process, such as NO2.
A new NASA research suggests that we might detect advanced extraterrestrial civilization using its atmospheric pollution. This study is the first time NO2 has been examined as a possible technosignature.
Jacob Haqq-Misra, a co-author of the paper at the Blue Marble Institute of Science, Seattle, Washington, said, “Other studies have examined chlorofluorocarbons (CFCs) as possible technosignatures, which are industrial products that were widely used as refrigerants until they were phased out because of their role in ozone depletion. CFCs are also a powerful greenhouse gas that could terraform a planet like Mars by providing additional warming from the atmosphere. As far as we know, CFCs are not produced by biology, so they are a more obvious technosignature than NO2. However, CFCs are particular manufactured chemicals that might not be prevalent elsewhere; NO2, by comparison, is a general byproduct of any combustion process.”
In this study, scientists used computer modeling to predict whether NO2 pollution would produce a practical signal to detect with current and planned telescopes. Atmospheric NO2 strongly absorbs some colors (wavelengths) of visible light, which can be seen by observing the light reflected from an exoplanet as it orbits its star. 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. One light-year, the distance light travels in a year, is almost 6 trillion miles (about 9.5 trillion kilometers). For comparison, the closest stars to our Sun are found in the Alpha Centauri system, a little over four light-years away, and our galaxy is about 100,000 light-years across.
They likewise found that cooler and far more common stars than our Sun, for example, K and M-type stars, will deliver a stronger, more easily detected NO2 signal. This is because these kinds of stars produce less ultraviolet light that can split up NO2. More abundant stars increase the chance that an extraterrestrial human advancement may be found.
Giada Arney of NASA Goddard, a co-author of the paper, said, “Since NO2 is also produced naturally, scientists will have to carefully analyze an exoplanet to see if there is an excess that could be attributed to a technological society. On Earth, about 76 percent of NO2 emissions are due to industrial activity.”
“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, and future work will be needed to ensure confidence in distinguishing true positives from false positives.”
This work was funded by NASA Goddard’s Sellers Exoplanet Environments Collaboration (SEEC) and the NASA Exobiology program. SEEC is supported by NASA’s Planetary Science Division’s Research Program. This work was performed as part of NASA’s Virtual Planetary Laboratory through the NASA Astrobiology Institute and by the NASA Astrobiology Program as part of the Nexus for Exoplanet System Science (NExSS) research coordination network.
- Ravi Kopparapu et al. Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology. arXiv:2102.05027