We know that exoplanets are common in our galaxy, but questions of alien life and extraterrestrial civilizations are a matter of speculation. Given that we are yet to find evidence for either, astronomers are currently interested in how we might find them. New research suggests a peculiar method: Look for pollution.
The work is accepted for publication in the Astrophysical Journal and available online as a pre-print. In it, the team discusses how the gas nitrogen dioxide (NO2) could be used to find alien civilization. On Earth, the gas is a byproduct of combustion, and it could be visible in the observations of the atmospheres of distant planets with the next-generation telescopes.
“On Earth, most of the nitrogen dioxide is emitted from human activity – combustion processes such as vehicle emissions and fossil-fueled power plants,” lead author Ravi Kopparapu of NASA’s Goddard Space Flight Center, said in a statement. “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.”
Astronomers are looking for some markers of life in the atmosphere of these exoplanets. These are biosignatures. But now they are also exploring the “technosignatures” – molecules that would be present in the air of those distant worlds in the presence of industrialized civilizations.
“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,” added co-author Jacob Haqq-Misra, from the Blue Marble Institute of Science, Seattle, Washington.
“CFCs are also a powerful greenhouse gas that could be used to terraform a planet like Mars by providing additional warming from the atmosphere. As far as we know, 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 might not be prevalent elsewhere; NO2, by comparison, is a general byproduct of any combustion process.”
Discovering nitrogen dioxide wouldn’t be a “gotcha, we have found alien life.” The gas is also produced naturally, so astronomers will have to estimate if the signal they see is enough to be produced by a technological society.
“On Earth, about 76 percent of NO2 emissions are due to industrial activity,” explained Giada Arney of NASA Goddard, a co-author of the paper. “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.”
Clouds and other aerosols can also create false positives. For the next step into this project, the team wants to create better models for the atmosphere of planets so that can have a more realistic approach to what a nitrogen dioxide signal would look like.