When it comes to a collision between two neutron stars, it’s not something you see every day. In fact, scientists estimate that out of the 100 billion stars in the Milky Way galaxy, there are only about 10 neutron star binaries that are destined for a collision.
So far, we have only detected a few kilonova explosions that follow such collisions, and none of them have occurred in our own galaxy. But what if there was a kilonova in the Milky Way? What would that mean for life on Earth?
A team led by physicist Haille Perkins from the University of Illinois Urbana-Champaign has found that if a kilonova explosion were to happen within a certain proximity, it wouldn’t be good news for us. This information may not be helpful if two neutron stars were to collide nearby, but it’s interesting to understand the dangers of space.
The team’s findings, published on the preprint server arXiv, suggest that the biggest danger comes from cosmic rays. If we’re not in the path of the kilonova’s gamma-ray burst, we would be safe within a distance of 35 light-years. However, if we are in the path, we would face a fatal proximity of 300 light-years, but only if we’re in exactly the right spot.
This research not only helps us understand the threats to our own existence but also allows scientists to determine the likelihood of life surviving on other planets near these star-shattering events that release atmosphere-stripping radiation.
Binary neutron star collisions usually involve a short burst of gamma radiation, which is different from the longer supernovae bursts that collapsing single stars produce. The gamma radiation is emitted in a pair of narrow jets on either side of the colliding stars. These jets are surrounded by gamma-ray cocoons that form as the jets try to penetrate the material shed during the kilonova.
When these jets collide with the surrounding interstellar medium, they create powerful X-rays known as the X-ray afterglow. Over time, a bubble of cosmic rays expands from the center of the collision, which is the situation observed in the first detected neutron star collision, GW170817.
Perkins and her team investigated how these events could impact a planet based on the information gathered from GW170817. Their study revealed that any living thing within the narrow range of the jet, up to a distance of 297 light-years, would likely be affected by the intense gamma radiation. However, outside of that range, the threat is somewhat diminished. To be hit by gamma radiation from the cocoons, you would need to be within a distance of around 13 light-years.
Both the intense gamma radiation and X-radiation threats would only persist for a short period. The effects of gamma radiation on Earth’s stratospheric ozone, for example, would take about four years to recover. On the other hand, the team found that the X-radiation is much more lethal since the afterglow lasts longer compared to the gamma-ray emission. However, to be affected by the X-radiation, you would still need to be relatively close, around 16 light-years away.
However, it’s the long-term bubble of cosmic rays that poses the most significant threat. These cosmic rays, accelerated by the kilonova remnant, would strip the ozone layer, leaving Earth vulnerable to radiation for thousands of years. This could result in a devastating mass extinction event. To be affected by this, we would need to be within about 35 light-years from the source.
So for now, we are probably safe from this kind of catastrophe. In fact, we face greater risks from events right here in the Solar System, such as giant flares from our own Sun. After that, we should be concerned about asteroid impacts and supernovae. Kilonovae, despite their dangers, are relatively low on the list of threats to our existence.
“The rarity of binary neutron star mergers combined with a small range of lethality means that they are probably not important threats to life on Earth. We find that the mean recurrence time of lethal mergers at the location of the Sun is much larger than the age of the Universe,” write the researchers in their study.
“However, even if it never induced a mass extinction, a nearby kilonova event would be visible on Earth. It would likely disrupt technology soon after the merger and remain bright in the sky for over a month.”
Well, at least we’ll have something fascinating to witness.
The team’s analysis can be found on the preprint server arXiv.
