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鈥檛 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.
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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鈥檛 stick around very long 鈥� blink, and you鈥檒l miss them, at least on galactic timescales. 鈥淚f that鈥檚 correct, there should be some signs of dead alien civilisations all over the place,鈥� says of the University of St Andrews, UK.
Some aliens will be wiped out by 鈥渘atural鈥� causes 鈥� a massive asteroid impact, or their star going supernova 鈥� but these could also happen on worlds with no intelligent life, so these kinds of signatures won鈥檛 reveal ET. Instead, Forgan and his colleagues have catalogued various technological ways to end the world. 鈥淲e thought quite hard about the ways that we could snuff ourselves out,鈥� 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鈥檚 entire stockpile for us to see the gamma-ray burst from the explosion, and even then it is so brief that we鈥檙e unlikely to be looking at the right time. The subsequent fallout and nuclear winter would turn the planet鈥檚 atmosphere hazy, which we could detect, but some planets are very cloudy anyway so this wouldn鈥檛 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鈥檇 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 鈥�grey goo鈥�.
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鈥檚 reflected light. 鈥淵ou鈥檙e effectively creating an artificial desert across the entire landmass,鈥� says Forgan. 鈥淭his would be quite unusual sand because it would all be grains of the same size.鈥�
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.
鈥淭hey go through some very interesting ideas,鈥� says 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鈥檚 the best approach, agrees Snellen. 鈥淚t would be a little strange if we didn鈥檛 even look.鈥�
Besides revealing whether we鈥檙e alone in the universe, putting a figure on the number of dead aliens could tell us how long we鈥檝e got to live. 鈥淚t will tell us what we expect the typical lifetime of a civilisation might be,鈥� says Forgan. 鈥淚t can give us some kind of context whether we are a young or old civilisation, in galactic terms.鈥�
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