How dangerous are nearby supernovae for life on Earth?

Life and supernovae don’t go together.

From afar, supernova explosions are fascinating. A star more massive than our sun runs out of hydrogen and becomes unstable. Eventually, it explodes, releasing so much energy that it can eclipse its 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 are likely to explode without affecting living beings.

But a new study shows that one type of supernova has a longer range than previously thought. And it could have consequences for planets like ours.

Supernovae are not alien to Earth. They weren’t close enough to sterilize Earth, but there is evidence that supernovae have affected life on Earth.

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

Another paper showed up to 20 supernovae in the last 11 million years in the Scorpius-Centaurus OB association. Some of these were as far 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 damage 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 that supernovae emit during the explosion. They also know about the cosmic rays that can arrive hundreds or thousands of years later. When this happens close enough to a planet like Earth, cosmic rays can deplete the ozone layer and increase surface muon radiation.

An X-ray-luminous Type IIn supernova differs from other supernovae. When a supernova explodes, it immediately emits gamma rays and other photons. An X-ray-luminous supernova emits gamma rays and photons, but some of the radiation from the explosion interacts with a dense circumstellar medium surrounding the progenitor star. This creates X-rays that can be fatal up to a distance of 160 light years.

In a scenario where an SN explodes near Earth, it could take months or years after the initial explosion for the X-rays to arrive. Interactions with the circumstellar debris cause a lag. The X-rays can deplete the Earth’s ozone layer, allowing harmful UV radiation from the sun to reach the planet’s surface.

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

Researchers are unsure of the deadly 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 to 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 for the first time confirmed the visible light energy source of the SN. The long-lasting glow after an SN explosion has been proven to be radioactive.

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

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 to nearly 100 light-years.

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 sphere of influence. This shows that these SN could significantly affect the Earth’s biosphere from a greater distance.

This research has some implications for Earth.

Our solar system is located inside the so-called local bubble. It is a cavity carved out of the ISM in the Milky Way’s Orion arm. Several supernovae explosions have created the bubble over the past 10 to 20 million years. Did these SN affect the Earth?

Advances in X-ray astronomy will shed more light on implications for terrestrial planets, and the authors believe there is much more to discover. But their observations show that “…the interacting X-ray phase of SN evolution can have significant consequences for terrestrial planets. We limit further speculation until further developments in X-ray astronomy are made; However, the evidence presented here points with certainty that this process can have deadly consequences for life at tremendous distances.”

Scientists know that supernovae have had some impact on Earth. The presence of the radioactive isotope ⁶⁰Fe has a half-life of 2.6 million years, but researchers found it doesn’t decay ⁶⁰Fe in marine samples dated 2 to 3 million years ago. It should have decayed to nickel long ago. Supernovae can form ⁶⁰Fe by nucleosynthesis when they explode.

But other things can be created ⁶⁰feet Asymptomatic giant branch stars can also make it, so by itself it’s not compelling evidence of a nearby supernova.

Researchers also found ⁵³Mn in the same samples of ferromanganese crust containing the ⁶⁰feet It’s also a radioactive isotope that should have decayed by now. not how ⁶⁰For example, only supernovae can form ⁵³Mn. Its presence is clear evidence of nearby supernovae in the recent geological past.

It is not the presence of these radioactive isotopes that pose a threat to life. It’s the radiation that must have hit the earth, and if the supernova that produced the isotopes was close enough to propagate them to the earth, then the radiation must have hit the earth as well.

Ionizing radiation from supernovae can alter Earth’s atmospheric chemistry from considerable distances. The initial burst of energy from an SN poses a threat, as does the cosmic rays that arrive and linger hundreds or thousands of years later. But this research adds another threat: X-rays arriving months or years after the initial eruption. “Therefore, a consequence of the formidable threat found here is that this alters the timeline that we know a SN can affect a nearby planet, adding an additional phase of adverse effects.”

What effect did it have exactly?

“If we combine these results with our threat assessment here, it is possible that one or more of these SNe interacted and thus injected a high dose of X-rays into the Earth’s atmosphere. This would mean that SN X-rays had a notable impact on Earth and may have played a role in the evolution of life itself,” they write.

SN bursts have almost certainly hit our planet. The exact consequences are difficult for scientists to unravel. But if the radiation weakened the ozone layer and allowed more UV radiation to reach the earth’s surface, it would have caused mutations. It’s called UV mutagenesis, which may have driven molecular evolution and was crucial to the emergence of sex. In fact, mutation is the main driver of evolution.

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

“We therefore conclude by noting that further exploration of SN X-rays is of value not only for stellar astrophysics, but also for astrobiology, paleontology, and earth and planetary sciences in general.”

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. Because supernovae can be life-threatening if they’re close enough, regions with many stars that have the potential to explode as supernovae are less habitable. If this research is correct, then supernovae can be deadly at a greater distance than thought and become fatal within months or years of the initial eruption due to the X-rays. This changes the shape and location of a galaxy’s GHZ.

The researchers are urging a longer-term study of supernovae, months and years after an eruption, and advocate further advances in X-ray observation to support the study. “These observations and innovations will shed light on the physical nature of SN X-ray emission and the danger these events pose to life in our galaxy and other star-forming regions,” they write.

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