Over the history of life on Earth there have been many extinction events, but the top 5 mass extinctions are the big ones. Fortunately, such events don’t happen often. Understanding what caused these massive die-offs is inherently interesting, just so that we better understand the world, but might also provide some insight into potential future threats. A recent study suggests an interesting potential cause for one of these mass extinctions – nearby supernova.

These 5 mass extinction events are:

Ordovician-Silurian extinction – 444 million years ago – this is thought to have been caused by global cooling, resulting in increased polar ice, dropping of the ocean which reduced shallow habitats and changed its chemistry, allowing for more toxic minerals and less oxygen. This extinction saw the loss of 85% of species.

Late Devonian extinctions – 383-359 million years ago – this extinction correlates with a dramatic decrease in ocean oxygenation. The cause of this drop is not well understood, but candidates include asteroid impact, volcanic eruptions, and increased soil weathering due to the evolution of land plants. The species loss in this extinction was 70-80%.

Permian-Triassic extinction – 252 million years ago – this is the biggest mass extinction on Earth ever. Over 96% of sea species, and 75% of land species went extinct over about 60,000 years. Ecosystems did not recover for millions of years. These numbers actually underestimate the devastation, as these are the loss of species. But if you look at individual creatures, almost everything on Earth died, which just the slightest residue of life left. This was probably triggered by massive volcanic activity, releasing CO2, warming the planet, and causing acid rain.

Triassic-Jurassic extinction – 201 million years ago – this was caused also by global warming from an increase in atmospheric CO2 by a factor of four. This was in turn also caused by volcanic activity – this time from the Central Atlantic Magmatic Province. In this extinction about 80% of species were lost.

Cretaceous-Paleogene extinction – 66 million years ago – this is the one everyone knows about, because it saw the end of the non-avian dinosaurs. This was almost certainly caused by a large impact, but there persists a minority opinion regarding the contribution of volcanic activity from the Deccan Traps in what is now India. Along with the non-avian dinosaurs, 76% of species on Earth went extinct.

Some themes emerge from these mass extinctions. First, there often appears to be multiple potential causes. This might be because for such a mass extinction to occur, multiple factors have to conspire to stress life beyond its limits. Four of the five events also resulted at least in part from extreme climate change, two with cooling and two with warming. These in turn involved changes in atmospheric CO2 and ocean oxygenation and chemistry. One mass extinction, the K-Pg, resulted primarily from a massive impact. But impacts may occasionally play a role in other extinctions.

The new study involves the Late Devonian extinction, the one whose cause is the least well understood. Actually, this is a series of extinction events over 24 million years.  Again, we know these events correlates with a significant drop in ocean oxygenation. There are many theories as to what caused this drop. For example, increased flow of nutrients from the land to the sea could cause algae blooms which in turn cause dead zones of decreased oxygenation. This may also correlate with a period of climate change, probably global cooling, but also warming at other times.

One other factor that seems to be playing a role is the decrease in the ozone layer. The ozone layer blocks out certain kinds of radiation (including harmful UV light) and a dramatic decrease would certainly be a stressor on life. The evidence for this comes from pollen from the time period which shows an increase in malformations, known to be due to ionizing radiation. This then leads to the question of what caused the depletion of the ozone layer. This is where the new paper comes in – the authors hypothesize that ozone depletion could have been triggered by nearby supernova. This would make the Late Devonian extinction events the only ones known to be cause by such astronomical phenomena.

They write:

“Here we study an alternative possible cause for the postulated ozone drop: a nearby supernova explosion that could inflict damage by accelerating cosmic rays that can deliver ionizing radiation for up to  100 ky. We therefore propose that the end-Devonian extinctions were triggered by supernova explosions at ∼20 pc, somewhat beyond the “kill distance” that would have precipitated a full mass extinction.” (100 ky = 100,000 years; 20 pc = 20 parsecs or about 67 light years).

It makes sense that sometime in the last few billion years our sun would have been relatively close to a supernova. Right now there are no stars within our “kill distance” that will go supernova anytime soon. Perhaps the closest such star is Betelgeuse, which is 600 light years away – a safe distance. The radiation from one or more supernova, they argue, could have depleted the ozone, resulting in a series of extinctions until the ozone could be repleted.

If you want to delve into the details of how the ionizing radiation from a nearby supernova would deplete the ozone, read the original paper. It is surprisingly accessible. Basically they propose that increased surface temperatures increased water vapor in the stratosphere, which lead to an increase in free radical formation, which chemically degraded ozone.

At this point this should be considered a hypothesis, but a plausible one with some evidence. But the Late Devonian extinctions remain perhaps the most complex and least understood of the extinction events.

This idea also raises the broader question of – what are the implications of this for the possibility of life throughout the universe? The more densely packed a stellar system’s local area is packed with other stars, the more likely it is that they will be hit by a nearby supernova. Or looked at another way – the more frequently this will happen. If the supernova is close enough, within the “kill distance”, then any life on planets in that system could be 100% wiped out – entirely sterilized. The same would happen if a relatively nearby gamma ray burst was aimed in your direction. So it is certainly safer, from this perspective, to live in a system that is relatively far away from other stars.

However, the conditions for life formation might not be best in the outer reaches of galaxies. The “matallicity” of stars varies based on their location in the galaxy. High metallicity (containing lots of heavier elements from previous stars) is good for the formation of life. Higher metallicity stars tend to exist in denser parts of the galaxy, because metallicity correlates with supernova seeding gas clouds that form later star systems. So life both needs and is threatened by supernova. But there is a sweet spot where there is enough metallicity to form life, but enough distance not to get sterilized on a regular basis. And of course, our system is in this goldilocks galactic zone. But we don’t really know for sure what the prospects are for life in other zones of the galaxy.

One interesting thing to consider is that about 50% of all stars in the universe are not even in galaxies. They are drifting between galaxies. Yet, it is likely that they formed in galaxies but where then flung out of their galaxies by gravitational interactions. This could be the best-case scenario for life – a star that forms in a high metallicity zone, then gets flung out to the vast emptiness of intergalactic space, safe from any nearby supernova or gamma ray bursts. Most life in the universe, therefore, may exist in intergalactic rather than galactic systems. We may be a rare exception.

In fact, there is a lot to be said for being far away from the dense galactic rings. There are fewer interstellar objects that could impact your planet, or disturb the Oort cloud and send comets into the inner stellar system causing more impacts. A rogue planet may wander by and change the orbits of planets in your system. There is also less of a chance of a close encounter with another star, also causing gravitational havoc. In the right location, you might even be gifted with a beautiful face-on view of a spiral galaxy.

The downside is that your neighbors would be far away, so interstellar travel would be even more difficult. But given the laws of physics, this may not ultimately matter.

It makes sense, living as we are at the edge of the safe zone of our galaxy, that we would have had some close calls in the past. I do wonder how lucky we have been. What are the odds that life on Earth has survived for almost 4 billion years? We came super close to complete extinction 252 million years ago. Do most systems in our situation not survive as long as we have?