Last March, when Ravi Kopparapu was still working from his desk at the Goddard Space Center in Maryland, he came across a press release from NASA’s Earth Observatory. Levels of Nitrogen dioxide (NO₂) had plummeted over China since the country of 1.4 billion instituted strict stay-at-home orders more than a month earlier. He texted his colleague Jacob Haqq Misra with the link: “Technosignature?” he wrote. “Oh interesting!” Haqq Misra replied.
The observations had piqued Kopparapu’s interest, and two months later, still thinking about the ways that modern societies pollute their planet’s air, he read a paper on the effect of pandemic-related public health measures on atmospheric pollution. Researchers found the same effect playing out in other highly industrialized nations, like South Korea and the United States. The level of NO₂ over urban centers decreased by between 20 and 40 percent from January to April 2020, when many governments were following China’s lead and mandating that citizens stay at home. Nitrogen dioxide is one of the more prevalent pollutants, a result of combustion and fossil fuel use as well as natural biological processes like soil emissions and lightning. But Kopparapu wasn’t interested in NO₂ because of its effect on Earth. His focus was light-years away, in the atmospheres of the more than 4,000 known exoplanets in our region of the Milky Way galaxy.
The shutdown had shown what atmospheric scientists had struggled to accurately measure up until that point: that the majority—roughly 65 percent—of Earth’s NO₂ is from nonbiological sources, the combined result of our commuting, manufacturing, and gas and metal refining. If this was the case, Kopparapu wanted to know, would it be possible to detect this gas in the faraway atmospheres of exoplanets? And if it was, could we be looking at a civilization not unlike our own, that had made use of its own fossil fuels to drive a technological revolution?
“We are producing three times more nitrogen dioxide than what biology and lightning together are producing,” says Kopparapu of our own planet. “So if we see an Earth-like planet and the nitrogen dioxide signal, and we make a model for all of the biological and atmospheric sources possible, and still cannot explain the amount we are seeing on the planet, then one possibility is that there could be a technological civilization.”
Kopparapu is at the forefront of an emerging field in astronomy that is aiming to identify technosignatures, or technological markers we can search for in the cosmos. No longer conceptually limited to radio signals, astronomers are looking for ways we could identify planets or other spacefaring objects by looking for things like atmospheric gases, lasers, and even hypothetical sun-encircling structures called Dyson spheres. Technosignatures could be observed from Earth or by some of our more ambitious probe concepts, like Starshot—a laser-powered lightsail that could theoretically reach Alpha Centauri in two decades.
Eager to explore further, Kopparapu discussed the idea with his colleagues, including Haqq Misra, a senior researcher at the Blue Marble Space Institute of Science, who soon became his coauthor. Their paper, published in late February by The Astrophysical Journal, explored this question using a computer model that mimicked a single column of atmosphere on an Earth-like planet and calculated the odds that we could find traces of NO₂ on one of our galactic neighbors.
Their model simulates the exposure of atmospheric molecules to sunlight, specifically four different types of sunlight, modeled off of our own sun, an orange dwarf star, and two M-type stars like Proxima Centauri. Each star emits a unique spectrum of light that interacts with the atmospheres of orbiting planets and causes photochemical reactions. (On Earth, these reactions are what give us an ozone.) When radiation, or light, from the sun heats up molecules in the atmosphere, they enter a temporarily excited state in which a number of things can happen: They can break apart, or they can bond together—and on the ground they can become plant food. Different types of radiation, from other types of stars, could mute or stimulate an NO₂ signal.
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