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More and more, it seems that the existence and persistence of life on Earth is the result of sheer luck. According to a new analysis of the history of the Milky Way, the best time and place for the emergence of life is not here, not now, but more than 6 billion years ago on the outskirts of the galaxy.
This specific location in space and time would have offered a habitable world the best protection against gamma-ray bursts and supernovae that exploded space with deadly radiation.
About 4 billion years ago, the central regions of the galaxy (which include the solar system) became safer than the peripheries – safe enough for life to emerge, if not as safe as the peripheries.
“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 numerous explosive events capable of triggering 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 radiation flying through space; so intense is the result that it can be fatal for life.
Earth was also not immune. Mass extinctions throughout our history have been linked to supernovae, including the Late Pliocene extinction 2.6 million years ago and the Late Devonian extinction 359 million years ago. ‘years. Gamma-ray 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 main sequence lifespan, or a white dwarf accretion material becomes unstable, reignites, and melts. Both scenarios result in a massive explosion of stellar material in space.
Gamma-ray bursts are thought to be spat out of stars that collapse into neutron stars or black holes, and we know they can occur when neutron stars merge. We’ve never seen one in the Milky Way; those we detect are 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 the Ordovician, before the age of the dinosaurs.
“Supernovae are more common in star-forming regions, where massive stars form,” said astronomer Giancarlo Ghirlanda of INAF.
“Gamma-ray bursts, on the other hand, prefer regions of star formation that are still poorly engulfed by heavy elements. In these regions, massive stars formed by a gas poor in metals lose less mass during their lifetime due to stellar winds. Therefore, these stars are able to keep in rapid rotation, a condition necessary to be able to launch, once a black hole has formed, a powerful jet. “
To determine the safest places for life, the research team carefully modeled the evolutionary history of the Milky Way, paying attention to the emergence of regions most likely to harbor supernova or burst activity. gamma.
Their model predicted that the interior 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 slowed down, but increased in the outer regions of the galaxy.
When the Universe was young, it was mostly filled with hydrogen and helium – the gases that the first stars were made of. Heavier elements were built from the stellar fusion of nuclei; and even 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 the very central regions, within 6500 light years of the galactic center, where supernova explosions are more frequent, our study suggests that the evolutionary pressure in each epoch is primarily determined by the GRBs,” Spinelli said. .
“Although these are much rarer events than supernovae, GRBs are capable of causing mass extinction at greater distances: being the most energetic events, these are the bazookas with the longest range. . “
Although the Milky Way suburbs were once safer than the intermediate regions are now, the news is improving – for us, anyway. According to the team’s analysis, over the past 500 million years, the outskirts of the Milky Way would likely have been sterilized by two to five long gamma-ray bursts. The location of our solar system, on the other hand, has become more secure than it ever was.
But even the relative danger and repeated exposure to cosmic explosions could have been fortuitous for us.
“We note that the very existence of life on planet Earth today demonstrates that mass extinctions do not necessarily exclude the possibility of complex development of life,” the researchers wrote in their article.
“On the contrary, the mass extinctions occurring at the right pace could have played a central role in the evolution of complex life forms on our home planet.”
So perhaps “security” should be taken with a grain of salt.
The research was published in Astronomy and astrophysics.
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