Astronomers detect best place and time to live in Milky Way

Astronomers have detected the best place and time to live in the Milky Way, in a recent study published in the journal Astronomy and Astrophysics.

More than six billion years ago, the outskirts of the Milky Way were the safest places for the development of possible life forms, sheltered from the most violent explosions in the universe, that is, the gamma-ray bursts and supernovae.

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The study led by researchers from INAF and the University of Insubria in Italy investigates the incidence of these events throughout the evolution of our galaxy.

The universe is ripe with powerful explosions releasing large amounts of energy into its surroundings.

Starting from 4 billion years ago until now, the central regions, embracing also the Solar system, became the safest places.

Not too safe though: this study supports the hypothesis 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 group of researchers led by Riccardo Spinelli, PhD student at the University of Insubria and INAF associate in Milan said “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, planets were subject to many explosive events able to trigger a mass extinction.”

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Both supernovae and GRBs are linked to the life cycle of stars, and in particular to their death. A supernova occurs when a star much more massive than the Sun reaches the end of its life and explodes, or when a white dwarf – the remnant of less massive stars, such as the Sun, explodes after accreting mass from a companion in a binary system.

A GRB, on the other hand, is an intense flash of high-energy radiation emitted when a very massive and rapidly rotating star dies, or when two neutron stars, or a neutron star and a black hole both remnants of massive stars merge.

“Supernovae are more frequent in star-forming regions, where massive stars are formed,” Giancarlo Ghirlanda, co-author and INAF researcher in Milan, explains. “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”.

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“To understand how these events are distributed within our galaxy, we started from a model which describes the evolution of our galaxy”, adds Francesco Haardt, professor at the University of Insubria and INAF associate. “This model predicts that the inner regions, as opposed to the peripheral regions, formed quickly in the early stages of the history of our galaxy. As time passed, the star formation rate decreased in the centre and gradually increased in the periphery.

Consequently, the primordial gas of hydrogen and helium was enriched with heavier elements (oxygen, carbon, nitrogen) quickly in the centre of the Milky Way, while in the periphery it was enriched more gradually, without however reaching the high metallicities of the central regions”.

The energy released by GRBs and supernovae is enormous. A supernova releases, in the high energy band, as much energy as the Milky Way, which contains hundreds of billions of stars, emits in a few hours. A GRB, in 10 seconds, emits what our galaxy does in a century. “Excluding the very central regions, less than 6500 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”, Spinelli says.

“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”.

The effect on a planet like Earth would be catastrophic. Several studies suggest that the gamma radiation released by a GRB within 3300 light-years from Earth would destroy the ozone layer in the atmosphere: without this protection, the planet would be exposed to the Sun’s ultraviolet radiation which could trigger the extinction of almost all life forms on the surface.

“As a secondary effect”, Spinelli continues, “the destruction of the ozone layer would produce nitrogen compounds. These would reduce the visible sunlight thus causing global cooling”. For these reasons, several studies proposed that the first of the five mass extinctions that affected Earth, the Late Ordovician mass extinction, about 445 million years ago, was caused by a GRB. The work by Spinelli and collaborators supports this hypothesis.

Regarding the “recent” past, the study finds that, in the last 500 million years, the Milky Way became globally safer than in earlier epochs, with the peripheral regions being more sterilised by lethal GRBs and the central ones, within 6500 light-years from the galactic centre, mostly exposed to supernovae.

At the distance of the Solar system from the galactic centre, this work estimates there has been at least one lethal GRB in the last 500 million years, possibly associated with the first great extinction. The worst seems to be over.