How dangerous are nearby supernovae to life on Earth?

Life and supernovae don’t mix.

From a distance, supernova explosions are fascinating. A star more massive than our Sun runs out of hydrogen and becomes unstable. Eventually, it explodes and releases so much energy that it can outlast the host galaxy for months.

But space is vast and largely empty, and supernovae are relatively rare. And most planets don’t support life, so most supernovae probably explode without affecting living things.

But a new study shows how one type of supernova is more far-reaching than we thought. And it could have consequences for planets like ours.

Earth is no stranger to supernovae. One wasn’t close enough to sterilize Earth, but there is evidence that supernovae have affected life on Earth.

A 2018 paper presented evidence of a near-Earth supernova explosion about 2.6 million years ago. It was about 160 light years away. The authors of this paper linked the supernova to the extinction of the Pliocene marine megafauna. In this case, up to a third of Earth’s large marine species disappeared, but only in shallow coastal waters.

Another paper showed up to 20 supernovae in the last 11 million years in the Scorpius-Centaurus OB link. Some of them were as close as 130 light years from Earth. The paper’s authors say that about 2 million years ago, one of the supernovae exploded close enough to our planet to destroy the ozone layer.

But there are different types of supernovae. Some of them have a much longer range and a much longer duration. Scientists have long known about the powerful gamma rays released by supernovae during explosions. They also know about cosmic rays that can arrive hundreds or thousands of years later. If this happens close enough to a planet like Earth, cosmic rays can destroy the ozone layer and increase muon radiation at the surface.

A bright X-ray type IIn supernova is different from other supernovae. When a supernova explodes, it immediately emits gamma rays and other photons. In a bright X-ray supernova, gamma rays and photons are emitted, but some of the radiation from the explosion interacts with a dense peristellar medium surrounding the progenitor star. This creates X-rays that can be deadly up to 160 light years away.

In a scenario where a SN exploded near Earth, it may take months or years after the initial explosion for the X-rays to arrive. Interactions with circumstellar debris cause a delay. X-rays can destroy the Earth’s ozone layer, allowing harmful UV radiation from the Sun to reach the planet’s surface.

After X-rays arrive, cosmic rays arrive, similar to other SNs. This is a double whammy for the Earth’s ozone layer.

Researchers are not sure about the lethal distances of supernovae. There are many variables, both in the progenitor star and in its environment. The mass loss of the progenitor star is particularly important. But by characterizing the lethal X-ray dose for Earth’s stratosphere and the energy output of some of the brightest SNs, the authors calculated the lethal distance for some known supernovae.

SN 1987A exploded in the Large Magellanic Cloud and the light reached Earth in 1987. Scientists observed the explosion and confirmed the energy source for the SN’s visible light for the first time. He proved that the long-lasting glow after an SN explosion is radioactive.

SN1987A was not very lethal, according to the authors. They say the SN was only lethal at a distance of less than a light year. It was the least dangerous SN of the 31 the team characterized.

The deadliest of the 31 was SN2006jd. It exploded in the galaxy NGC 4179, about 57 million light-years away, and the light reached Earth in 2006. According to the researchers, SN2006jd was deadly nearly 100 light-years away.

The five deadliest SNs in this study are all type IIn supernovae, as are seven of the top ten.

Type IIn supernovae also have the largest range of influence. This indicates that these SNs could significantly affect the Earth’s biosphere from greater distances.

This research has some implications for Earth.

Our Solar System is inside what is known as the Local Bubble. It is a cavity carved by the ISM in the Orion arm of the Milky Way. Multiple supernova explosions created the bubble over the past 10 to 20 million years. Did these SNs affect Earth?

Advances in X-ray astronomy will shed more light on the implications for the terrestrial planets, and the authors believe there is much more to uncover. But their observations show that “… the interacting X-ray phase of a SN’s evolution can have important consequences for the terrestrial planets. We reserve any further speculation until further developments in X-ray astronomy are made. However, the evidence presented here certainly shows that this process is capable of imposing lethal consequences on life at tremendous distances.”

Scientists know that supernovae have had some effect on Earth. The presence of the radioactive isotope ⁶⁰Fe has a half-life of 2.6 million years, yet the researchers found that it has not decayed ⁶⁰Fe in ocean samples dating from 2 to 3 Myr ago. It should have decayed to nickel a long time ago. Supernovae can create ⁶⁰Fe through nuclear fusion when they explode.

But other things they can create ⁶⁰Fe. Asymptotic giant branch stars can also do this, so by itself, it is not a smoking gun for a nearby supernova.

The researchers also found ⁵³Mn in the same ferromanganese crust samples that retain the ⁶⁰Fe. It is also a radioactive isotope that should have decayed by now. Different ⁶⁰Fe, only supernovae can create ⁵³Mr. Its presence is definite evidence of nearby supernovae in the recent geological past.

It is not the presence of these radioactive isotopes that is life threatening. It’s the radiation that must have hit Earth, and if the supernova that created the isotopes was close enough to spread them to Earth, then the radiation must have hit Earth too.

Ionizing radiation from supernovae can change Earth’s atmospheric chemistry from significant distances. The initial burst of energy from a SN is a threat, as are cosmic rays that arrive hundreds or thousands of years later and linger. But this research adds another threat: X-rays that arrive months or years after the initial outbreak. “Therefore, one implication of the formidable threat found here is that this changes the timescale by which we know a SN can affect a nearby planet, adding an extra phase of adverse effects.”

What exactly did it do?

“Combining these findings with our threat assessment here, it is possible that one or more of these SNe interacted and thus produced a high dose of X-ray radiation in the Earth’s atmosphere. This would mean that SN X-ray emission had a notable impact on Earth and potentially played a role in the evolution of life itself,” they write.

SN explosions have almost certainly hit our planet. The exact implications are difficult for scientists to disentangle. But if the radiation weakened the ozone layer, allowing more UV to reach the Earth’s surface, it would have caused mutations. It’s called UV mutagenesis, which may have driven molecular evolution and was critical to the origin of sex. In fact, mutation is the main driver of evolution.

The fact that supernovae can lead to mutations is the background for the authors’ concluding remarks.

“We thus conclude with the comment that further investigation of SN X-ray emission has value not only for stellar astrophysics but also for astrobiology, paleontology, and the Earth and planetary sciences as a whole.”

This research also has implications for habitability across the galaxy. The Galactic Habitable Zone (GHZ) is a region in a galaxy where habitability is most likely. Since supernovae can be fatal to life if they are close enough, regions with many stars that can potentially explode as supernovae are less habitable. If this research is correct, then supernovae may be deadly at greater distances than previously thought, and may be deadly within months or years after the initial burst due to X-rays. This changes the shape and location of the GHZ of a galaxy.

The researchers urge more long-term study of supernovae for months and years after an outburst and call for more advances in X-ray observation to aid the study. “These observations and innovations will shed light on the physical nature of SN X-ray emission and clarify the risk these events pose to life in our galaxy and other star-forming regions,” they write.

More: