A new study modeled whether we could find intelligent life on another planet—by looking for its pollution.
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.
Determining the photochemical reactions happening in a faraway atmosphere takes an advanced and extremely fine-tuned telescope fit with a spectrograph. Astronomers have to focus this telescope on a (relatively) miniscule and fast-moving planet as it transits in front of its host star. During this brief window, the telescope can capture the light beaming through the planet’s atmosphere and break it apart with a prism. The bands of the prism tell us the composition of the atmosphere by way of a unique spectral signature that each element displays, almost like a fingerprint. If an alien civilization had polluted its skies with NO₂, the way we have ours, this would clue us in to their existence.
Kopparapu and Haqq Misra concluded that among Earth-like planets, orbiting sun-like stars within a small band called the “habitable zone” that supports liquid water, we could find this signal, if it exists, using the next generation of advanced telescopes. Two highly anticipated NASA telescope concepts, LUVOIR and HabEx, designed with significant improvements in sensitivity and spatial resolution, as well as spectrographs that can focus on multiple objects simultaneously, would be capable of carrying out these types of observations. With the instrumentation earmarked for these missions, we could confidently affirm a signal 30 light years away, after roughly 400 hours of observation.
This may sound like a long time, but Kopparapu points out that the Hubble Space Telescope used at least twice that amount of time over a period of three years to conduct the Frontier Fields observations, which took the most detailed photos ever attained of the early universe— thousands of galaxies pixelated across the dark expanse of 13-billion-year-old space.
“If we have a really good candidate in the habitable zone of a planet, around a sun-like star, then we can potentially spend more time on this planet,” says Kopparapu. “The number looks big, but within the context of what we have done before, it is not.”
Even with enough observation time, there could still be a number of complications. Clouds and aerosols in a planet’s atmosphere absorb light within the same wavelength region as NO₂, making it plausible that they could mimic the signal outright. At the same time, planets around stars that are slightly smaller than our sun, K and M type stars, could produce a stronger NO₂ signal, since these stars produce less ultraviolet light which can break apart this gas in the atmosphere. That might lead to an overestimate of its prevalence—and an indicator of civilization where there may be none.
Biological processes like soil nitrification, wildfires, and lightning also produce NO₂, but research on Earth suggests that these sources provide far less of the overall total than anthropogenic sources—namely the burning of fossil fuels. Still, there are only a small handful of atmospheres from within our own solar system that we’ve been able to study in any detail that would provide a helpful basis for comparison.
Renyu Hu, a planetary scientist and expert in exoplanet atmospheres at the NASA Jet Propulsion Laboratory, says the biggest challenge he sees in identifying NO₂ as a technosignature has to do with the chemical lifetime of the gas in our atmosphere. On Earth, most NO₂ is broken apart by the sun or “rained out” when it transforms into nitric acid, or HNO₃, within 5 to 10 days of being produced. But on other planets, this may differ. “In exoplanet atmospheres, since their atmospheric conditions could be quite different than Earth, perhaps this NO₂ will have a longer lifetime, and therefore accumulate at a higher abundance,” Hu says. If this exoplanet’s atmosphere doesn’t have the same sinks that exist on Earth, it could theoretically mimic the sustained, strong NO₂ signal that we’d look for as a sign of pollution and an industrial society.
For a follow-up study, Kopparapu’s team is planning on using a more advanced 3D model that would more accurately simulate the atmospheric dynamics we’d expect to find on another planet. Instead of a single column of atmosphere, the model would simulate the atmosphere as a whole, including more physically accurate cloud heights and movements—improving the researchers ability to vet whether such an NO₂ signal could be mimicked by clouds.
Before any of this is possible, NASA needs to prioritize and fund at least one of several next generation telescope concepts, like LUVOIR or HabEX. Both were studied for the forthcoming Planetary Science and Astrobiology Decadal Survey, which will provide research and investment recommendations to NASA and Congress and guide the efforts of the broader scientific community when it is released in spring, 2022. But even if both are prioritized by the survey, they are a long way off—we likely won’t see these missions get underway before the 2030s.
A few decades ago, federal funding for SETI (the search for extraterrestrial intelligence) and for possible radio signals was at an all-time high. In 1961, astronomer Frank Drake published his famous Drake Equation, a formula for estimating the odds of detectable, intelligent life in space, using variables like the number of planets with environments suitable for hosting life, and how many of them might give rise to intelligent creatures. But people following that equation have gotten results ranging anywhere from none to millions. (Last year a set of researchers in the United Kingdom calculated, with unusual specificity, that there are at least 36 communicating intelligent civilizations in the Milky Way alone.)
The SETI Institute was founded in 1984 to help search for that life; in 1999, researchers at the Berkeley SETI Research Center launched the SETI@home project, which allowed people to use their personal computers to help parse patterns in radio telescope data. There was promising astronomical news, too. In 1992, Alexander Wolszczan and Dale Frail, using the Arecibo Observatory’s radio telescope, discovered the first planets outside of our solar system orbiting a pulsar in the Virgo system. Thousands more were discovered once the Kepler Space Telescope became operational in 2009. On the heels of this came a new line of inquiry: Now that the existence of exoplanets was confirmed, what might they be like? Astronomers began theorizing about the composition of alien atmospheres, specifically what they might be composed of if a planet harbored life. Textbooks are now devoted to the atmospheric physics of faraway worlds, and potential biosignatures—chemical signs of life we could observe in exoplanet atmospheres—are reviewed in top astronomy journals.
But some in the scientific community have always been skeptical of technosignatures. Even if there are other civilizations out there, how long could we expect them to send out radio signals? And would we be around to receive them? Over the 13.5 billion year life of our galaxy alone, it’s entirely possible we’d miss another civilization’s lifespan, like ships passing in the night.
Plus, the search for extraterrestrial radio signals has recently taken some serious blows. The Arecibo telescope was badly damaged last August by a falling cable, and will now be demolished. The public-facing aspect of the SETI@home project was halted in March, 2020, so that researchers can now crunch through two decades of data.
Kopparapu calls the work he’s doing now “atmospheric SETI,” an alternative to searching for radio signals from another civilization. “With atmospheric technosignatures, they don’t have to do anything actively to communicate with us. They can just go on about their lives and can be completely unaware that we exist while we are observing their planet,” says Kopparapu. “In the next 20 or so years, we may launch space telescopes that could look at the atmospheres and potentially image far away habitable planets. If we can do this within 150 years of industrial civilization, and less than 100 years of developing radio communication capability, how many civilizations have already done this to us within our Earth’s history of billions of years?”
Last August, Haqq Misra, the paper’s coauthor, gathered more than 50 participants, including many prominent astrobiologists and astrophysicists, together for Technoclimes, an online conference at which presenters talked about the latest research into technosignatures, and discussed the focus and framework that such research might follow. “We’re kind of in the era now where it’s possible to ask these questions and not get laughed at by too many astronomers,” says Haqq Misra.
“The idea that we can have life on other planets has moved from being science fiction to more close to science reality,” says Kopparapu.
Kopparapu and Haqq Misra are now at work on a paper that will look at whether an exoplanet with Earth’s current level of atmospheric chlorofluorocarbons (CFCs), the ozone-depleting chemical present in older refrigerants and aerosols, would be detectable using a future space telescope that takes observations in that wavelength. (Current conceptions of LUVOIR and HabEX would not be capable of this.) Yet there’s a slight complication—CFCs are industrially produced, and could be a clear indicator of some sort of technological capability. But on Earth we’ve been fighting for decades to purge them from our atmosphere. The same is true for NO₂—and really all planet-warming pollutants. To survive, we on Earth will have to drastically curtail our emissions to avoid making the planet inhospitable to most life. If this is the case on other planets, and they’re either fighting to clean up their atmosphere or dying off because they failed to do so, that would further shrink the window of time in which we can actually detect these types of signals—for CFCs, that’s anywhere from 50 to 150 years, practically a fraction of a second on astrological timescales
Haqq Misra points out that there are some situations in which a planet could have high levels of CFCs without dooming its inhabitants. Increasing their quantity might actually be desirable on a planet with very little atmosphere, especially if the inhabitants wanted to create an environment that could retain liquid water by doing some large-scale planetary engineering. “They’re a potent greenhouse gas, so if we wanted to terraform Mars, one possibility is to put CFCs in the atmosphere,” he says. “Or maybe CFCs aren’t toxic to whatever organism they are. Or maybe they’re not biological, they’re AI.”
Sara Seager, an astrobiologist at MIT who’s studied biosignatures and exoplanets for decades, says she’s glad there’s another tool in the arsenal. Still, she says that photochemistry is a difficult field and there’s no magic bullet. Last year, she was part of a team of researchers who announced they’d found a phosphine signal in Venus’s atmosphere. On Earth, phosphine is produced by bacteria involved in decomposition, and a small amount is manufactured artificially for use in fumigants or biological weapons. So when astronomer Jane Greaves found what appeared to be phosphine’s signature on our nearest neighbor, she enlisted an entire team of astrobiologists and chemists to confirm and study the signal. They published their findings in Nature Astronomy in September, 2020, and though there hasn’t been a single follow-up paper that offered an explanation of how the chemistry in Venus’s upper atmosphere could produce phosphine non-biologically, scientists are still far from a consensus on the signal and whether it’s truly phosphine.
“People still don’t believe—and the people who wrote the paper, we’re not believing it’s a sign of life, either—even though we don’t know any way that we can produce phosphine without life in an environment like Earth or Venus,” Seager says. “We’re still not ready to say it’s life.”
And Venus is in our backyard. On an exoplanet tens of light-years away, a phosphine signal would be even more difficult to find, and require a higher concentration to be detectable in the first place. “There’s this kind of reality check now that it’s going to be hard to find, and even if we get a strong signal, one could probably always come up with a different explanation,” Seager says.
For what it’s worth, there has been some momentum behind a new search for life in space—and it’s coming from Congress. In the 2018 House Appropriations Bill, Congress directed NASA to include technosignatures as part of its research portfolio, which had not been the case for several decades. Later that year, NASA hosted a three-day Technosignatures Workshop in Houston, bringing together leaders in a number of scientific disciplines to assess the current state of the field and determine a path forward. The new Biden administration has thrown its full support behind the moon-bound Artemis program and the Space Force, but it's not yet clear whether that support will extend to the search for life in the cosmos. Still, Kopparapu is optimistic about the growing support for this kind of research, including the influx of private money from organizations like the Breakthrough Initiative, a multimillion-dollar suite of space programs geared toward discovering extraterrestrial life.
A solid signal of alien life, or even technological life, is likely far off—and our acceptance of it further still. Regardless, that notion is what drives many researchers in the field, like Kopparapu and Haqq Misra, to continue their research. Asked what it would feel like to identify a potent signal from a faraway planet, Kopparapu is initially at a loss for words. “It’s not a question of if, but when,” he says. And maybe also: Who will believe it?