Increasingly, it seems that the existence and persistence of life on Earth is the result of great 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, nor now, but more than 6 billion years ago on the outskirts of the galaxy.
This specific location in space and time would have allowed a habitable world the best protection against gamma-ray bursts and supernovae that exploded space with deadly radiation.
For about 4 billion years, the central regions of the galaxy (including the solar system) became safer than the outskirts, safe enough for life to emerge, if not as safe as the outskirts had been.
“Our work shows that until 6 billion years ago, excluding the peripheral regions of the Milky Way, which had relatively few planets, due to high star formation and low metallicity, the planets went be the subject of many explosive events capable of causing 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 send cosmic radiation flying through space; so intense is the output that it can be deadly to life.
The Earth has not been immune either. Mass extinctions throughout our history have been associated with supernovae, including the late Pliocene extinction 2.6 million years ago and the late Devonian extinction 359 million years ago. Gamma-ray explosions, which are much rarer but much more powerful than supernovae, would also be devastating.
Both events are related to the life cycles of stars. Supernovae occur when a massive star reaches the end of its life in the main sequence or when a white matter that accumulates a white dwarf becomes unstable, resuscitates, and becomes a fugitive fusion. Both scenarios result in a massive explosion of stellar material in space.
Gamma rays are believed to escape from stars collapsing into neutron stars or black holes, and we know that they can occur when neutron stars fuse. We have never seen them towards the Milky Way; those we detect come from other galaxies millions of light-years away: the most energetic electromagnetic events in the Universe.
Scientists believe a gamma-ray burst 450 million years ago could have caused the massive extinction of Ordovician before the age of the dinosaurs.
“Supernovae are more common in star-forming regions, where massive stars form,” said INAF astronomer Giancarlo Ghirlanda.
“Gamma rays, on the other hand, prefer star-forming regions that are still poorly absorbed by heavy elements. In these regions, massive stars formed by 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, a powerful jet has formed. “
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 most likely to host supernova or gamma-ray burst activities.
His model predicted that the inner regions of the galaxy would have formed faster than the outskirts; 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, but increased in the outer regions of the galaxy.
When the Universe was young, it was mainly filled with hydrogen and helium, the gases of which the first stars were formed. The heaviest elements were constructed 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 heavier elements and metals.
In turn, this would have reduced the frequency of gamma-ray bursts, making the central region (about 6,500 to 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 most frequent, our study suggests that evolutionary pressure at each epoch is primarily determined by GRBs,” Spinelli said.
“While they are much rarer events than supernovae, GRBs are capable of causing mass extinction at greater distances: being the most energetic events, they are the most far-reaching bazookas.”
While the outskirts of Milky Ways were safer once the middle regions now, the news is getting better, anyway. According to the team’s analysis, in the last 500 million years the outskirts of the Milky Way would probably have been sterilized by two to five long gamma-ray bursts. The location of our solar system, on the other hand, became safer than ever.
But even the relative danger and repeated exposure to cosmic explosions could have been fortuitous to us.
“We observe that the very existence of life on planet Earth shows today that mass extinctions do not necessarily rule out the possibility of a complex development of life,” the researchers wrote in their article.
“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.”
Therefore, it may be necessary to take “safety” with a grain of salt.
The research has been published in Astronomy and astrophysics.