Scientists propose searching for nitrogen dioxide (NO2) in exoplanet atmospheres as a potential indicator of advanced extraterrestrial civilizations—much like the pollution from fossil fuel combustion on Earth.
Over 4,000 exoplanets have been discovered in our galaxy, some potentially habitable. Direct probes are impossible due to vast distances, but powerful telescopes like the James Webb Space Telescope (launched October 2021) and the Extremely Large Telescope (2025) can analyze their atmospheres from Earth.
Biosignatures, such as oxygen and methane combinations, might signal microbial or plant life—but not always reliably.
A promising alternative: technosignatures, traces of advanced technology. On Earth, human activity leaves unmistakable industrial markers.
Researchers from NASA's Goddard Space Flight Center, led by Ravi Kopparapu, highlight nitrogen dioxide (NO2) as a prime candidate.
While NO2 can arise from natural sources like biology, lightning, or volcanoes, on Earth about 76% comes from human activities—primarily vehicle emissions and fossil fuel power plants.
Detecting NO2 on a habitable exoplanet could thus signal an industrialized civilization.
"Previous studies explored chlorofluorocarbons (CFCs) as technosignatures—industrial refrigerants phased out for ozone depletion," notes co-author Jacob Haqq-Misra. "CFCs are purely artificial, making them clear indicators. But NO2 is a universal byproduct of combustion, more likely across diverse civilizations."
Using computer models, the team assessed NO2 detectability. For an Earth-like planet around a Sun-like star producing Earth-level pollution, a specialized visible-wavelength telescope could spot it within 30 light-years after ~400 hours of observation.
Cooler K- and M-type stars—far more common than Sun-like ones—yield stronger signals, as their lower UV output preserves NO2 molecules.
