If you were an alien looking at Earth from a great distance, it would have looked mostly the same right up until 10,000 years ago. That’s when humans invented agriculture, and the planet has never been the same since. Farming allowed humans to proliferate, and the scale at which we now produce food has changed the way Earth would appear to an outside observer. The same could be true of an inhabited exoplanet, and it just so happens we finally have a tool that could detect these conditions: the James Webb Space Telescope. A team of astronomers recently explained how the new observatory might be able to spot “exofarms” when it begins scanning the heavens later this year.
A study exploring this possibility is available on the preprint arXiv server. It comes courtesy of the Blue Marble Space Institute of Science in collaboration with NASA’s Goddard Space Flight Center and the University of California. The analysis assumes a few things about agriculture on exoplanets, including that the biosphere would take advantage of the free energy raining down from their suns. If there’s something like photosynthesis, then the large-scale farming of plants would produce recognizable signatures as it does on Earth.
The team points to nitrogen as one of the key identifiers of agriculture. On Earth, nitrogen makes up the majority of the atmosphere. However, atmospheric N2 is a very stable molecule. Natural processes can “fix” nitrogen to make it available to plants, but agriculture calls for vast amounts of nitrogen fertilizer. Producing fertilizer leads to the release of ammonia, but those molecules would fall back to the ground in a matter of days on an Earth-like planet. Webb, with its keen mid-infrared eyes could detect high levels of ammonia in a planet’s atmosphere, which would signify ongoing agricultural activity.
The use of nitrogen fertilizer causes the build-up of nitrous oxide, as well. Unlike ammonia, nitrous oxide could remain detectable for 100 Earth years, and it’s also a byproduct of combustion. If we assume that aliens, like humanity, determine that combustion is not a sustainable method of energy production, perhaps they would phase it out. Thus, nitrous oxide in an exoplanet’s atmosphere could indicate large-scale farming operations. The same may be true of methane, which is produced in large volumes by agriculture on Earth.
The study concludes that Webb could potentially scan exoplanet atmospheres with enough sensitivity to confirm water, oxygen, and carbon dioxide, along with traces of ammonia and nitrous oxide. That, the team says, is the “technosignature for extraterrestrial agriculture.”
The James Webb Space Telescope launched at the tail end of 2021, and NASA has spent the intervening months deploying and calibrating the instruments. Everything has gone perfectly since the launch, so we expect the first science operations to kick off this summer.
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