Global nuclear war would be hard to notice from a distance (Image: Everett Collection/Rex)

There are billions of potentially habitable planets out there, so why haven’t we seen signs of life beyond the solar system? Perhaps because the aliens have all destroyed themselves. A new analysis of various apocalyptic scenarios suggests that we may be able to detect distant worlds where life has been wiped out by nuclear war or nanotechnology run amok.

The disparity between the vast potential for alien life and our lack of extraterrestrial contact is known as the Fermi paradox, after physicist Enrico Fermi, who asked why we appeared to be alone. It might be that microbes evolve on many worlds, but intelligent life is rare, or that most civilisations choose not to communicate and even actively hide from the rest of the universe.


A third possibility is that civilisations don’t stick around very long – blink, and you’ll miss them, at least on galactic timescales. “If that’s correct, there should be some signs of dead alien civilisations all over the place,” says Duncan Forgan of the University of St Andrews, UK.

Some aliens will be wiped out by “natural” causes – a massive asteroid impact, or their star going supernova – but these could also happen on worlds with no intelligent life, so these kinds of signatures won’t reveal ET. Instead, Forgan and his colleagues have catalogued various technological ways to end the world. “We thought quite hard about the ways that we could snuff ourselves out,” he says.

Apocalypse then

Global nuclear war would be hard to notice from a distance, they say. Aliens would have to simultaneously deploy nuclear weapons a billion times more powerful than Earth’s entire stockpile for us to see the gamma-ray burst from the explosion, and even then it is so brief that we’re unlikely to be looking at the right time. The subsequent fallout and nuclear winter would turn the planet’s atmosphere hazy, which we could detect, but some planets are very cloudy anyway so this wouldn’t be a definitive sign of alien apocalypse.

A killer plague or other form of biological warfare is more promising, as long as it kills every living thing on the planet in short order. Their decaying corpses could release enough methanethiol and ethane gas to briefly show up in the atmosphere, but again we’d have to be looking at the right time.

Nanotechnology gone horribly wrong could also reveal a formerly inhabited world. Researchers and science fiction authors have worried about self-replicating nanomachines going rogue and consuming the entire planet, covering it in “grey goo“.

If this happened on an alien world, the uniformly sized nanodust would absorb or emit light at a specific wavelength, showing up as an odd signature in the planet’s reflected light. “You’re effectively creating an artificial desert across the entire landmass,” says Forgan. “This would be quite unusual sand because it would all be grains of the same size.”

Signs of life

Other scenarios the team explored include artificial rings of space debris, revealing a civilisation that has fallen victim to a Gravity-like space junk cascade on a grand scale, or the total destruction of the planet. Both of these are hard to see from a distance, and to distinguish from natural causes. In fact, perhaps the easiest signature to detect from a once-thriving civilisation is one we are fighting against here on Earth: death by pollution, which could show up as large amounts of CFCs in the atmosphere.

“They go through some very interesting ideas,” says Ignas Snellen of Leiden University in the Netherlands, but he thinks the short timescale of these apocalyptic signatures means we would have to get very lucky to see them. The team is not proposing that we mount dedicated missions to hunt for dead aliens, but instead piggyback on other astronomy missions and search through data for anything unusual. That’s the best approach, agrees Snellen. “It would be a little strange if we didn’t even look.”

Besides revealing whether we’re alone in the universe, putting a figure on the number of dead aliens could tell us how long we’ve got to live. “It will tell us what we expect the typical lifetime of a civilisation might be,” says Forgan. “It can give us some kind of context whether we are a young or old civilisation, in galactic terms.”

Journal reference: arxiv.org/abs/1507.08530