Rings of trees may contain clues to impacts of distant supernovas on Earth

Massive explosions of energy occurring thousands of light years from Earth may have left traces in the biology and geology of our planet, new research from geoscientist Robert Brakenridge of the University of Colorado at Boulder finds .

The study, published this month in the International Journal of Astrobiology, probes the impacts of supernovas, some of the most violent events in the known universe. In the space of just a few months, just one of these flares can release as much energy as the sun will during its lifetime. They are also brilliant – really brilliant.

“We are constantly seeing supernovas in other galaxies,” said Brakenridge, senior associate researcher at CU Boulder’s Institute for Arctic and Alpine Research (INSTAAR). “Through a telescope, a galaxy is a small hazy dot. Then all of a sudden a star appears and can be as bright as the rest of the galaxy.”

A very nearby supernova might be able to wipe human civilization off the face of the Earth. But even from further afield, these explosions could still wreak havoc, Brakenridge said, bathing our planet in dangerous radiation and damaging its protective ozone layer.

To study these possible impacts, Brakenridge searched the rings of trees on the planet for fingerprints of these distant cosmic explosions. His findings suggest that relatively nearby supernovas could theoretically have triggered at least four disruptions to Earth’s climate in the past 40,000 years.

The results are far from conclusive, but they offer tantalizing clues that when it comes to the stability of life on Earth, what happens in space doesn’t always stay in space.

“These are extreme events, and their potential effects seem to match the records for dark circles,” Brakenridge said.

Radiocarbon tips

His research revolves around the case of a curious atom. Brakenridge explained that carbon-14, also known as radiocarbon, is an isotope of carbon that only occurs in small amounts on Earth. It is not from here either. Radiocarbon is formed when cosmic rays from space bombard our planet’s atmosphere almost constantly.

“There is usually a constant amount year over year,” Brakenridge said. “Trees take up carbon dioxide, and some of that carbon will be radiocarbon.”

Sometimes, however, the amount of radiocarbon picked up by trees is not constant. Scientists have discovered a handful of cases in which the concentration of this isotope inside tree rings increases suddenly and for no apparent earthly reason. Many scientists have speculated that these multi-year peaks could be due to solar flares or huge energy ejections from the sun’s surface.

Brakenridge and a handful of other researchers have looked at events much further from home.

“We are seeing earthly events that call for an explanation,” Brakenridge said. “There are really only two possibilities: a solar flare or a supernova. I think the supernova hypothesis was dismissed too quickly.”

Attention Betelgeuse

He noted that scientists have recorded supernovas in other galaxies which produced an incredible amount of gamma radiation – the same type of radiation that can trigger the formation of radiocarbon atoms on Earth. While these isotopes are not inherently dangerous, a spike in their levels could indicate that energy from a distant supernova has traveled hundreds to thousands of light years across our planet.

To test the hypothesis, Brakenridge looked to the past. He put together a list of supernovas that have occurred relatively close to Earth over the past 40,000 years. Scientists can study these events by observing the nebulae they left behind. He then compared the estimated ages of these galactic fireworks to the record of the tree ring on the ground.

He found that of the eight closest supernovas studied, all appeared to be associated with unexplained peaks in radiocarbon recordings on Earth. He considers four of them to be particularly promising candidates. Take the case of a former star of the constellation Vela. This celestial body, which once rested about 815 light years from Earth, became a supernova about 13,000 years ago. Shortly after, radiocarbon levels jumped nearly 3% on Earth – a staggering increase.

The results don’t come close to a smoking gun, or a star, in this case. Scientists are still struggling to date supernovas from the past, making the timing of Vela’s explosion uncertain with a possible error of up to 1,500 years. It is also unclear what the impacts of such a disturbance on plants and animals on Earth were at the time. But Brakenridge believes the issue deserves much more research.

“What keeps me going is when I look at the earthly records and say, ‘My God, the predicted and modeled effects seem to be there.'”

He hopes humanity won’t have to see these effects for themselves anytime soon. Some astronomers believe they have detected signs that Betelgeuse, a giant red star in the constellation Orion, may be on the verge of collapsing and becoming a supernova. And it’s only 642.5 light years from Earth, much closer than Vela.

“We can hope that is not what will happen because Betelgeuse is really close,” he said.