Hunting for alien life has never before been of such importance to humankind. This is probably because, like never before, we have technologies that allow us to peer deeper into the universe like never before.
To date, we have discovered–confirmed–4341 exoplanets with 5742 pending candidates awaiting confirmation in a total of 3216 star systems. The numbers seem to be increasing by the day.
Life, if it exists, could inhabit one of these worlds. Some of the closest exoplanets discovered to date are those at the Alpha and Proxima Centauri. Proxima Centauri b is a potentially habitable world where life as we know it could have developed. This world orbits its star–a red dwarf–in the so-called habitable zone, a region in space around a star where water can exist in a liquid state.
I believe that life definitely exists somewhere in the cosmos, and there are some good chances it could exist–in terms of cosmic scales–not far from Earth.
If this life developed to a stage similar to that of our civilization today, it would mean that, just as us, these “aliens” are probably searching the universe for other planets like theirs, hoping to find whether there are inhabited planets somewhere out there.
But how can we know, and how can they know that a planet is inhabited?
According to the paper, the answer is pollution. Specifically atmospheric pollution in the form of nitrogen dioxide.
How pollution could reveal alien civilizations
If an advanced extraterrestrial civilization developed on a nearby star system, using our current technology, we might be able to detect by looking at potential air pollution.
As revealed in the NASA-funded study, by looking at the presence of nitrogen dioxide (NO2), which on Earth is produced by burning fossil fuels, we might spot aliens on nearby systems.
However, we must take into consideration that this gas can also be produced in nature. On Earth, NO2 is produced by non-industrial sources such as biology, lightning, and even volcanoes.
“On Earth, most of the nitrogen dioxide is emitted from human activity — combustion processes such as vehicle emissions and fossil-fueled power plants,” explained Ravi Kopparapu of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“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.”
Kopparapu is the lead author of a paper on this research accepted by the Astrophysical Journal and published online in the preprint server arXiv. The paper has yet to be peer-reviewed.
In other words, what the new research is proposing is that astronomers should consider the presence of NO2 as a potential technosignature on nearby exoplanets.
As explained in the new study, a possible indication of life, or biosignature, could be a combination of gases like oxygen and methane in an exoplanet’s atmosphere. Similarly, a sign of technology on an exoplanet–a technosignature–could be what is considered pollution here on Earth: the presence of a gas that is released as a by-product of a widespread industrial process, NO2.
Similarly, advanced alien civilizations inhabiting nearby star systems could use their telescopes and technology to search their exoplanets. If they spot Earth, they might spot air pollution in our planet’s atmosphere and conclude that advanced life likely exists there.
To know whether NO2 exists on a planet artificially or naturally, astronomers would need to analyze the amount of the gas present in the atmosphere. For example, 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.
Earth’s pollution detectable to civilization 30 light-years away
The researchers explain that atmospheric NO2 strongly absorbs certain colors (wavelengths) of visible light, which can be detected by observing the light reflected by an exoplanet as it orbits its star. We are already doing this when looking at exoplanets across the cosmos.
The new research found that for an Earth-like planet orbiting a Sun-like star, an alien civilization producing the same amount of NO2 as ours could be detected up to distances of about 30 light-years away with around 400 hours of observation time using telescopes observing the target at visible wavelengths.
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