It increasingly seems that the existence and survival of life on Earth is the result of pure happiness. According to a new analysis of the history of the Milky Way, the best time and place for life to arise is not here or now, but more than 6 billion years ago on the edge of the Milky Way.
That specific location in space and time would have provided a habitable world with the best protection against the bursts of gamma rays and supernovae that blasted space with deadly radiation.
Since about 4 billion years ago, the central regions of the galaxy (including the solar system) have become safer than the suburbs – safe enough for life to emerge, if not as safe as the suburbs.
“Our work shows that up to 6 billion years ago, with the exception of the peripheral regions of the Milky Way, which had relatively few planets, due to high star formation and low metallicity, planets were subject to many explosive events that could cause a mass extinction. , said 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 send cosmic rays through space; so intense is the output that it can be life-threatening.
The Earth has not been immune either. Mass extinctions throughout our history have been linked to supernovae, including the end-Pliocene extinction 2.6 million years ago and the late Devonian extinction 359 million years ago. Gamma bursts, which are much rarer but much more powerful than supernovae, are also said to be devastating.
Both events are linked to the life cycles of stars. Supernovae occur when a massive star reaches the end of its life, or a white dwarf that accumulates becomes unstable, re-ignites and goes into runaway fusion. Both scenarios result in a massive explosion of stellar material into space.
Gamma-ray bursts are believed to be emitted by stars collapsing into neutron stars or black holes, and we know they can occur when neutron stars merge. We have 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 eruption could have caused the Ordovician mass extinction 450 million years ago, before the age of the dinosaurs.
“Supernovae are more common in star-forming regions, where massive stars are formed,” said INAF astronomer Giancarlo Ghirlanda.
Gamma bursts, on the other hand, prefer star-forming regions that are still poorly engulfed by heavy elements. In these regions, massive stars formed by low-metal gas lose less mass during their lifetime due to stellar winds. fast rotation, a necessary condition for launching a powerful jet once a black hole has formed. “
To find the safest places for life, the research team carefully modeled the evolutionary history of the Milky Way, focusing on the emergence of regions most likely to harbor supernova or gamma-ray bursts.
Their model predicted that the inner regions of the galaxy would have formed more quickly than the suburbs; therefore, the inner Milky Way would have been much more active in both star formation and cosmic explosions. Over time, the star formation rate decreased in the inner region, but increased in the outer regions of the galaxy.
When the universe was young, it was mainly filled with hydrogen and helium – the gases from which the first stars were made. Heavier elements were built from the stellar fusion of nuclei; and even heavier elements from supernova explosions.
As stars lived and died, the central part of the Milky Way became richer in heavier elements and metals.
This, in turn, 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.
“With the exception of the 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 pressures in each epoch are mainly determined by GRBs,” said Spinelli.
“While much rarer events than supernovae, GRBs are capable of causing mass extinction from greater distances: being the most energetic of events, they are the bazookas with the greatest reach.”
While the outskirts of the Milky Way were once safer than the middle regions are now, the news is getting better – for us anyway. According to the team’s analysis, the outskirts of the Milky Way would likely have been sterilized by two to five long gamma-ray bursts over the past 500 million years. In contrast, the location of our solar system became safer than ever before.
But even the relative danger and repeated exposure to cosmic explosions could have been our coincidence.
“We note that the mere existence of life on planet Earth today shows that mass extinctions do not necessarily rule out the possibility of complex life development,” the researchers wrote in their paper.
“On the contrary, mass extinctions occurring at the right pace may have played a critical role in the evolution of complex life forms on our home planet.”
So maybe “safety” should be taken with a grain of salt.
The research is published in Astronomy and Astrophysics