Increasingly, it seems that the existence and persistence of life on Earth is the result of pure luck. According to a new analysis of the history of the Milky Way, the best time and place for the appearance of life is not here or now, but over 6 billion years ago, on the periphery of the galaxy.
This specific location in space and time would have given the habitable world the best protection against gamma-ray bursts and supernovae that threw space with deadly radiation.
About 4 billion years ago, the central regions of the galaxy (including the solar system) became safer than the periphery – safe enough for life to occur, if not as secure as the periphery.
“Our work shows that until 6 billion years ago, excluding the peripheral regions of the Milky Way, which had relatively few planets, due to the formation of high stars and low metallicity, the planets were subjected to many explosive events capable of triggering mass extinction. , ”Explained astronomer Riccardo Spinelli of the University of Insubria and the National Institute of Astrophysics (INAF) in Italy.
Cosmic explosions are no joke. Incredibly energetic events, such as gamma-ray bursts and supernovae, transmit cosmic radiation flying through space; so intense is the result that it can be deadly to life.
The Earth was not immune either. Mass extinctions throughout our history have been linked to supernovae, including the end of the late Pliocene 2.6 million years ago and the late Devonian extinction 359 million years ago. Gamma explosions, which are much rarer but much more powerful than supernovae, would be just as devastating.
Both events are related to the life cycles of the stars. Supernovae occur when a massive star reaches the end of its life in the main sequence, or a material that gathers a white dwarf becomes unstable, renews, and merges. Both scenarios result in a massive explosion of stellar material in space.
It is believed that gamma-ray bursts are thrown from stars that collapse into neutron stars or black holes, and we know that they can occur when neutron stars fuse. I’ve never seen one in the Milky Way; the ones we detect come from other galaxies millions of light-years away – the most energetic electromagnetic events in the universe.
Scientists believe that a gamma-ray burst 450 million years ago could have triggered the mass extinction of Ordovician before the age of the dinosaurs.
“Supernovae are more common in star-forming regions, where massive stars are forming,” said INAF astronomer Giancarlo Ghirlanda.
“Gamma explosions, on the other hand, prefer regions of star formation that are still poorly swallowed by heavy elements. In these regions, massive stars that are made up of metal-poor gas lose less mass during their lifetime due to stellar winds. Therefore, these stars are able to stay in rapid rotation, a necessary condition to be able to launch, once a black hole has formed, a strong jet. “
In order to find the safest places to live, the research team carefully modeled the evolutionary history of the Milky Way, paying attention to the emergence of regions with the highest risk of hosting supernova or gamma-ray burst activities.
Their model predicted that the inner regions of the galaxy would have formed faster than the periphery; therefore, the inner Milky Way would have been much more active in both star formation and cosmic explosions. Over time, the rate of star formation in the inner region has slowed, but increased in the outer regions of the galaxy.
When the universe was young, it was filled mainly with hydrogen and helium – the gases that formed the first stars. The heavier elements were built from the stellar fusion of the nuclei; and heavier elements from supernova explosions.
As the stars lived and died, the central region of the Milky Way became richer in elements and heavier metals.
In turn, this would have reduced the frequency of gamma-ray bursts, making the central region – between about 6,500 and 26,000 light-years from the galactic center – safer than it had been.
“Excluding very central regions, less than 6,500 light-years from the galactic center, where supernova explosions are more common, our study suggests that the evolutionary pressure in each epoch is determined primarily by GRB,” Spinelli said.
“Although they are much rarer events than supernovae, GRBs are capable of causing mass extinction from greater distances: being the most energetic events, they are the bazookas with the longest range.”
Although the periphery of the Milky Way was once safer than the middle regions are now, the news is getting better – for us, anyway. According to the team’s analysis, in the last 500 million years, the edge of the Milky Way has probably been sterilized by two to five long gamma-ray bursts. The location of our solar system, on the other hand, has become safer than it has ever been.
But even the relative danger and repeated exposure to cosmic explosions could have been fortuitous to us.
“We note that the very existence of life on planet Earth today demonstrates that mass extinctions do not necessarily rule out the possibility of complex development of life,” the researchers wrote in their paper.
“On the contrary, mass extinctions that occur at the right pace could have played a key role in the evolution of complex life forms on our home planet.”
So maybe the “safety” needs to be taken with a grain of salt.
The research was published in Astronomy and astrophysics.