Maybe we haven’t seen any aliens because they’re all dead

Last year Jack O’Malley-James, an astrobiologist at Cornell University, and his colleagues Adam Stevens and Duncan Forgan published their analysis of this macabre possibility. Just as astrobiologists have started to look for possible biosignatures—evidence of life, such as water or oxygen in planetary atmospheres—they might also look for necrosignatures, the remains of annihilated civilizations. And if we ever do find them, it would not augur well for our own prospects, because what makes us think we could avoid the fate that befalls all other intelligent beings in the galaxy?

Ironically, the most dramatic scenario—nuclear annihilation—would be the least conspicuous. Nuclear explosions produce gamma radiation, which could be detected with the same instruments used to observe cosmic gamma-ray bursts. “The strongest signature that would come from that would be a very short-lasting radiation burst as the weapons are detonated,” O’Malley-James says. But even that would be almost impossible for us to see. If aliens around nearby stars exploded the equivalent of the superpowers’ entire arsenal, all at once, the gamma pulse would fall short of what our gamma-ray burst detectors could register by a factor of a billion. We might have more luck with detecting radioactive fallout, which would ionize the air and give it a blue or green tint. But even this signature lasts only a few years. “You really would have to be looking at the right place at the right time to see that happen, so you’d have to be very lucky,” O’Malley-James says.

Global biological warfare would create a longer-lasting signal. If a deadly pandemic left a planet covered in rotting bodies, anaerobic microbes would fill the air with gases such as methanethiol, which, conveniently, isn’t produced by abiotic sources and is therefore an unambiguous signal of biological scavenging. The miasma produced by seven billion moldering bodies would be detectable for up to a year. If the pandemic crossed species barriers and destroyed not just the civilization but also all other higher organisms, we could see the effects for 30 years.