If there is life on other planets, in addition to looking for it through habitable zone liquid water or artificial signals, perhaps we can also judge from whether there is a polluting gas such as nitrogen dioxide (NO2) in the atmosphere.
In the lower atmosphere (10-15 km) of the earth, most nitrogen dioxide (NO2) comes from industrial processes, such as vehicle exhaust and fossil fuel power plants, and millions of tons of nitrogen dioxide are emitted every year In the atmosphere, it is the main air pollutant; other non-biological sources are lightning and volcanic activity.
So far, astronomers have discovered more than 4,000 planets orbiting other stars, but they are far away from the Earth. It is basically impossible to send probes to these planets to search for life or alien civilizations. They can only use powerful functions first. The telescope adds and subtracts to analyze the atmosphere of exoplanets.
A recent NASA study pointed out that if we can observe nitrogen dioxide in the atmosphere of a planet in the habitable zone, it may indicate that there is also industrial civilization there.
In the past, other studies proposed to use Chlorofluorocarbons (CFCs) as gas to test extraterrestrial life. CFCs are not produced through biological processes at all. They are a more obvious technical feature than nitrogen dioxide, except for chlorofluorocarbons. It is a very special man-made chemical on the earth and may not be common elsewhere. In contrast, nitrogen dioxide is a general by-product of any combustion process and is easier to identify than chlorofluorocarbons.
Through computer modeling, it is found that the James Webb Space Telescope scheduled to be launched this year can detect planets with the same amount of nitrogen dioxide as the Earth within 30 light-years, but the observation time is very long, requiring 400 hours.
Of course, even if nitrogen dioxide is observed on another planet in the future, it cannot be determined immediately that there is an alien civilization. The maximum amount of nitrogen dioxide that may be emitted from non-industrial processes must be excluded first, and the excess content can be attributed to the industry. Activities; secondly, the wavelength of light absorbed by clouds or aerosols is also similar to that of nitrogen dioxide, which may cause misjudgment. Other models will be needed to analyze cloud cover to eliminate any interference factors.
When searching for life outside of the Earth, there is always the possibility of false alarms. For example, scientists originally claimed that phosphine from microorganisms was found in the clouds of Venus, but subsequent research argued that it was just ordinary sulfur dioxide. The new paper will be published in the Astrophysical Journal, and the preprint can be viewed in arXiv.