The safest place and time for life to flourish in our Milky Way galaxy is not on Earth or now, but at the outskirts of our galaxy, billions of years ago.
Researchers from Italy's INAF national astrophysics institute and University of Insubria reached the conclusion after analysing gamma-ray bursts (GRBs) and supernova explosions – violent events that can wipe out emerging life-forms.
The researchers believe that around 6 billion years ago, the outskirts of our galaxy remained relatively safe from these devastating blasts.
Gamma-ray bursts release as much energy in a second or so as the sun will release over its entire lifetime – and could sterilise entire planets, the researchers say.
The researchers say their study supports the idea that a gamma-ray burst may have caused the first of the five great mass extinctions on Earth, which occurred 445 million years ago.
The research was published in Astronomy and Astrophysics.
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Riccardo Spinelli, a PhD student at the University of Insubria, said: “Our work shows that, until 6 billion years ago, planets were subject to many explosive events able to trigger a mass extinction.”
The possibility of life surviving in the Milky Way is largely governed by gamma-ray bursts, Spinelli says.
This meant that the safest place in the Milky Way’s history was the outskirts of our galaxy, about 6 billion years ago.
Spinelli said: “Excluding the very central regions, less than 6,500 light-years from the galactic centre, where supernova explosions are more frequent, our study suggests that evolutionary pressure in each epoch is determined by GRBs mainly.
“Although they are much rarer events than supernovae, GRBs are able to cause a mass extinction from larger distances: being the most energetic events, they are the bazookas with the longest range.”
On Earth, the effect of a gamma-ray burst would be catastrophic, Spinelli says.
A gamma-ray burst within 3,300 light years from Earth would destroy the ozone layer in the atmosphere, leaving our planet exposed to the sun’s ultraviolet radiation, which could trigger the extinction of almost all life forms on the surface.
Later on, from 4 billion years ago, the increase of heavy elements produced by later stellar generations reduced the frequency of the bursts, ensuring a safer environment in the most central regions of the galaxy,
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“Supernovae are more frequent in star-forming regions, where massive stars are formed,” Giancarlo Ghirlanda, co-author and INAF researcher in Milan, said.
“GRBs, on the other hand, prefer star-forming regions that are still poorly engulfed by heavy elements. In these regions, massive stars that are formed by metal-poor gas lose less mass during their life due to stellar winds.
"Therefore, these stars are able to keep themselves in rapid rotation, a necessary condition to be able to launch, once a black hole has formed, a powerful jet.”
In the last 500 million years, the Milky Way became safer than in earlier epochs, with the peripheral regions being more sterilised by lethal GRBs and the central ones, within 6,500 light years from the galactic centre, mostly exposed to supernovae.
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