There are tens of billions of galaxies in the observable universe, each one a sea of stars. When astronomers watch these stars closely, they see them wobbling, the way our sun wobbles when its planets spin around it, tugging on its center of gravity. Astronomers also see these stars dimming ever so slightly, as though an objects were passing in front of them, and this dimming occurs at predictable intervals, as though these objects were moving around the stars in regular orbits. For these reasons and others, astronomers now believe that nearly all stars play host to planets, and they are making plans to image these planets directly, by catching the faint light they give off with huge, ultra-sensitive telescopes.

What will this light tell us? A remarkable amount, it turns out. Light encounters all kinds of molecules as it makes its way through the universe, and it keeps a close record of these encounters, in its spectra. If sunlight were to beam through Earth’s atmosphere, and then out into the stars, it would travel with this detailed chemical record in tow. If, after some millennia, this earth-kissed light fell into a distant astronomer’s telescope, that astronomer would be able to determine what sorts of chemicals were present in our planet’s atmosphere. They would know that water vapor was present, and life too, because Earth’s atmosphere contains methane gas, breathed out by the trillions of organisms that live on its surface. Indeed, it’s precisely these sorts of “biosignatures” that Earth’s astronomers hope to find in the atmospheres of extrasolar planets.

Light from extrasolar planets might also tell us whether our universe is home to other tool-making beings. After all, some of our pollutants leave behind chemical traces that would never occur naturally. If we glimpsed these pollutants in a distant planet’s atmosphere, we could be reasonably certain that technological life lived on its surface at one time or another. And according to Adam Stevens, Duncan Forgan, and Jack O’Malley James from Cornell’s Carl Sagan Institute, we might be able to know whether they used their technology to destroy themselves.

In July, Stevens, Forgan, and James published a paper that asked what a distant, “self-destructive civilization” might look like through the business end of a telescope. To do so, they gamed out several dystopian science fiction scenarios in great detail. They calculated the brightness of the gamma rays that would flash out from a massive exchange of nuclear weapons. They asked themselves what would happen if an engineered pathogen ripped through a large population of human-sized animals. What gases would fill a planet’s atmosphere, if its surface were strewn with rotting corpses? And would those gases be detectable across interstellar distances?