Extremely intense stellar explosions could be responsible for past mass extinction events on Earth, a new study has found. In addition, these dramatic events may have prevented life from developing until around 5 billion years ago, and likely restricted the locations that life could potentially emerge throughout the universe to the fringes of galaxies.
Gamma-ray bursts (GRBs) are fleeting, intense flashes of radiation which, for a brief moment, light up the sky on a daily basis. In just a few seconds, a GRB will spew out as much energy as our sun does in its entire lifetime. It’s thought that these events could be caused by giant exploding stars, or by collisions between ancient remnants of stars known as neutron stars.
If a GRB occurred close enough to Earth, our planet would be in serious trouble. It would frazzle our protective ozone layer, exposing us to harsh UV rays that would wipe out the majority of life. However, it’s generally believed that our planet’s threat from these events is minimal. That’s because the vast majority of GRBs are the long-duration kind, and these tend to occur in galaxies very different from our own—galaxies with low concentrations of metals and other heavy elements, or low metallicity galaxies. These galaxies are thought to produce more of the kind of stars that explode as GRBs. The Milky Way, however, is a high metallicity galaxy.
But recent observations have indicated that, although rare, long GRBs do actually occur in galaxies like ours. This was enough to prompt a duo of researchers to examine the threat that these events could pose to our planet, and how they could have affected planets in the past.
As described in Physical Review Letters, the researchers used recently gathered data on GRBs to estimate that the rate of these explosions in the Milky Way is approximately one-tenth that of the average rate in the universe. Their calculations also suggest that there was a 60% chance that a GRB triggered a mass extinction event in the past 1 billion years, and a 90% chance within the last 5 billion years. This means it’s possible that a GRB could have triggered one of the known mass extinction events on Earth, such as the Ordovician extinction.
They also found that areas toward the center of the Milky Way face a much greater threat of GRBs due to the high density of stars in this region, suggesting life is probably restricted to the outskirts of our galaxy. Furthermore, early on in the universe, galaxies were much more compact, meaning the fringes were not far enough to be protected from deadly GRBs, which were also much more frequent in the past. This means that life would have probably struggled to emerge prior to 5 billion years ago.
This, the researchers say, could partly explain the Fermi paradox, the apparent contradiction between the high chance of extraterrestrial life existing and the lack of evidence for it. While there may be many planetary systems similar to our own, it’s possible that GRBs could have ravaged them and thus prevented life from developing.