A burst of space radiation might have caused the mass extinction (artist’s impression) at the end of the Ordovician period some 445 million years ago. Credit: Richard Jones/Science Photo Library

A resident’s guide to avoiding lethal blasts from the Galaxy

Astronomers map out the most benign environments for life in the Milky Way over the past 13 billion years.

The Milky Way is a dangerous place, full of exploding stars and bursts of γ-rays that could strip off a planet’s protective atmosphere and irradiate any life on the surface. An analysis shows that the Galactic neighbourhood safest from these cosmic killers has shifted over time, moving from the Galaxy’s outskirts towards its centre.

Riccardo Spinelli at the University of Insubria in Como, Italy, and his colleagues calculated how often supernovae and γ-ray bursts have occurred throughout the Galaxy, and whether those explosions might have happened near rocky planets. From the Milky Way’s formation more than 13 billion years ago until 6 billion years ago, the most sheltered place for life was the Galaxy’s outskirts.

But around 4 billion years ago, the planet-dense region stretching roughly 2,000–8,000 parsecs from the Galaxy’s centre became the lowest-risk zone. Earth is on that zone’s outer edge. One or two lethal radiation bursts could have reached Earth in the past 500 million years, the scientists say — supporting the idea that a γ-ray blast caused the mass extinction at the end of the Ordovician period, around 445 million years ago.