Besides the short lifespan of nitrogen dioxide, there is also the problem that there are too many natural sources that produce it – from lightning to… Forest firesSo finding that might not be strong evidence that an alien civilization developed internal combustion engines, for example, or any other technology for spewing out nitrogen dioxide. However, Arne and her co-authors argue that natural sources on Earth alone would not produce enough nitrogen dioxide to make it detectable from afar. Therefore, observing it in another planet’s atmosphere may actually indicate that there is industrial activity of some kind afoot.
Scientists are already studying light reflected from distant planets to try to determine what chemicals might be present there. In September, researchers reported that they had used the James Webb Space Telescope To detect methane and carbon dioxide In the atmosphere of a planet called K2-18b, which orbits a dwarf star about 120 light-years from Earth. This in itself was a major discovery because it suggested that the planet could be surrounded by an ocean of water.
“This is the first time ever that we’ve been able to come this close to saying that we can say there’s an ocean on an exoplanet,” says Nikku Madhusudhan of the University of Cambridge.
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But he and his colleagues also found evidence of a more exciting compound known as dimethyl sulfide, or DMS, in K2-18b’s atmosphere. On Earth, only life produces this compound, although it is also emitted from some industrial sources, for example, pulp mills that process wood.
Madhusudan stresses that the result is provisional and requires further confirmation to confirm that DMS actually exists. But in principle, it could be a biosignature – a sign of life – and who knows, perhaps even a technological fingerprint, too, if aliens process materials in a similar way to the way we process wood on Earth. However, Madhusudan adds that he finds it difficult to imagine an alien industrial civilization in a world that might be completely covered by water.
However, the search for life elsewhere in the universe, he says, must take into account possibilities that might surprise us: “We should expect the unexpected.”
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