A mushroom cloud rises with ships below during Operation Crossroads nuclear weapons test on Bikini Atoll, Marshall Islands in this 1946 handout provided by the U.S. Library of Congress.
Nuclear war would be nigh on impossible for us to detect if it were happening billions of light years away (US Library of Congress)

New analysis has looked at why we are yet to come across signs of life on other planets across the universe, despite the fact that there are potentially billions of habitable planets.

The study, from Cornell University's Carl Sagan Institute, says that rather than looking for signs of life, maybe we should look for signs of apocalypses on other planets, suggesting that some alien civilisations may have wiped themselves out with nuclear war or the overpowering of nanotechnology which could be detectable.

Nuclear war

Global nuclear war would be nigh on impossible for us to detect if it were happening billions of light years away, says the report published. Extraterrestrial beings would have to deploy all of their nuclear weapons – which would have to be at least a billion times stronger than Earth's nuclear weapons – at the same time to make the gamma-ray burst visible to telescopes. Even if that were to happen, though, the impact would be so brief that astronomers would need to be looking in the right place at the right time.

What would be easier to see however is spectral signatures that nuclear weapons give off and persist for much longer. "Global nuclear war therefore potentially offers several spectral signatures that could be observed," the study reads, adding that: "To confirm that a planet had been subject to a global nuclear catastrophe would require the observation of several independent signatures in short succession."

The rise of nanotechnology

The rise of nanotechnology would be easier for us to see as the self-replicating machines would consume the entire planet, creating a machine-made desert across the whole planet that would be a grey goo. The authors write in their study: "In the case of grey goo, we may expect there to be a substantially larger amount of 'dust', as well as a fixed grain size. This will be deposited as sand dunes or suspended in the atmosphere, with similar spectral signatures [to a nuclear winter]."

Biological or chemical war

Many fear that this could ultimately be our downfall as governments could hypothetically release disease into populations to wipe out the public of any country during war. So why would another civilisation not be capable of the same stupidity that humans are?

The report states that a bioweapon could destroy chunks of the planet's biosphere and the resulting decay could be detectable from here.

The study's lead author, Jack O'Malley-James, told io9: "For something like this, the signs would be indirect. It would be the mass dying-off of alien people and animals that would be detectable. If this happened suddenly, over a short period of time, it could be argued that we were witnessing some kind of advanced biowarfare, but we could never be completely certain."

Are we the worst?

However, if you believe the sixth man to walk on the moon, Edgar Mitchell, then it is the aliens who are looking out for our signs of nuclear activity. He believes that extraterrestrial beings visited the US at the start of the Cold War to warn them about the devastation that nuclear war would bring.

He told the Mirror Online: "White Sands was a testing ground for atomic weapons – and that's what the extraterrestrials were interested in. They wanted to know about our military capabilities. My own experience talking to people has made it clear the ETs had been attempting to keep us from going to war and help create peace on Earth. Other officers from bases on the Pacific coast told me their missiles were frequently shot down by alien spacecraft."