Last March, when Ravi Kopparapu was still working from his office at Goddard Space Center in Maryland, he came across a press release from NASA’s Earth Observatory. Nitrogen dioxide (NO₂) levels had fallen above China since the country of 1.4 billion people instituted strict home stay 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 sightings had piqued Kopparapu’s interest, and two months later, still pondering the ways modern societies pollute the air on their planet, he read an article on the effect of public health measures linked to the pandemic on air pollution. Researchers have found the same effect in other highly industrialized countries, such as South Korea and the United States. The level of NO₂ in urban centers decreased by 20-40% from January to April 2020, when many governments followed China’s lead and demanded that citizens stay at home. Nitrogen dioxide is one of the most prevalent pollutants, resulting from the combustion and use of fossil fuels as well as natural biological processes such as emissions from the ground and lightning. But Kopparapu was not interested in NO₂ because of its effect on Earth. Its target was light years away, in the atmospheres of more than 4,000 known exoplanets in our region of the Milky Way galaxy.
The shutdown had shown what atmospheric scientists had struggled to measure accurately up to this point: that the majority – around 65% – of Earth’s NO₂ comes from non-biological sources, a combined result of our transportation, manufacturing and refining of gases and metals. If so, Kopparapu wanted to know, would it be possible to detect this gas in the distant atmospheres of exoplanets? And if so, could we envision a civilization similar to ours, which used its own fossil fuels to drive a technological revolution?
“We produce three times more nitrogen dioxide than what biology and lightning produce together,” says Kopparapu of our own planet. “So if we see an Earth-like planet and the nitrogen dioxide signal, and we do a model for all possible biological and atmospheric sources, and we still can’t explain how much we’re seeing on the planet, then one possibility is that there could be a technological civilization.
Kopparapu is at the forefront of an emerging field of astronomy which aims to identify technosignatures, or technological markers that we can look for in the cosmos. No longer conceptually limited to radio signals, astronomers are looking for ways to identify planets or other space objects by looking for things like atmospheric gases, lasers, and even hypothetical structures surrounding the sun called Dyson spheres. The technosignatures could be observed from Earth or by some of our more ambitious probe concepts, like Starshot—A luminous laser veil 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 co-author. Their paper, published at the end of 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 chances 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 on 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 are the reactions that give us ozone.) When radiation, or light, from the sun heats molecules in the atmosphere, they enter a temporarily excited state in which a number of things can happen: they can separate, or they can bond together – and on the ground, they can become plant food. Different types of radiation, coming from other types of stars, could cut or stimulate a NO₂ signal.