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An artist's depiction of an exoplanet.
Supernovas are bad news. They can wreck biospheres and flood planets with deadly radiation. And now, a recent study has added a new potential threat: a special type of supernova that can destroy a planet's ozone layer years after the initial explosion.

When giant stars die in massive explosions called supernovas, they temporarily become some of the most luminous objects in the universe. A single supernova can outshine the combined light of hundreds of billions of stars.

To give you some perspective, the nearby star Betelgeuse is going to explode any day now. (That's an astronomical "any day," meaning sometime within the next few million years.) Even though the star is over 600 light-years from us, when it goes supernova, it will be the brightest object in our sky, second only to the sun. Betelgeuse will be visible during the day, shining brighter than a full moon. For a few weeks, during the peak of the blast, it will be so bright that it will cast shadows in the middle of the night.

Despite the fearsome brightness, the visible light portion of a supernova represents only a tiny fraction of all the energy output. And besides, while intense amounts of visible light may cause blindness, it doesn't have a lot of other serious effects. What's more worrisome is the high-energy radiation associated with the supernova, usually in the form of X-rays and gamma-rays.

X-rays specs

High-energy radiation can catalyze oxygen, stripping away Earth's protective ozone layer. Without the ozone layer, life on the surface of our planet would suffer the full blast of ultraviolet radiation from the sun, which could lead to an extinction event.

The radiation blast happens within the first few seconds of a supernova, but an even bigger threat comes later. Cosmic rays, which are subatomic particles accelerated to nearly the speed of light, eventually burst out of the maelstrom hundreds or thousands of years later. They carry a decent fraction of the total supernova energy with them, and they can also strip ozone layers and soak a planet's surface in deadly radiation.

Such events may have happened in the past. Analysis of lunar regolith and deep-sea cores reveal substantial amounts of iron-60, a radioactive isotope of iron produced only in supernovas. The presence of iron-60 suggests that Earth was hit by supernova ejecta as recently as a few million years ago.

Based on the threats posed by gamma-rays and cosmic rays, astronomers have already concluded that we are relatively safe; there are no nearby supernova candidates that can pose a threat to life on Earth.

But astronomers have found a new potential danger, which they described in a paper posted to the preprint database arXiv in October: A certain class of supernova can release an extra, long-distance form of deadly radiation that poses a serious danger to Earth-like planets.

This special class of supernova occurs when a star approaching the end of its life is surrounded by a thick disk of material. After the initial supernova explosion, a shock wave forms and slams into that disk. The shock wave heats the disk to incredibly high temperatures, which causes the disk to emit high amounts of X-ray radiation.

This radiation can carry away large amounts of energy and travel extremely long distances. In the recent study, the astronomers found that the brightest X-ray supernovas can overwhelm a planet's ozone layer, depleting it by as much as 50%, which is more than enough to trigger an extinction event, out to an incredible distance of 150 light-years.

Shrinking galactic habitable zone

These kinds of supernovas would create a deadly one-two punch. Months or years after the initial outburst, a vulnerable planet would be pummeled by X-rays. Then, hundreds or thousands of years later, the cosmic rays would come, finishing the job before the biosphere had a chance to recover and replenish its protective layer.

Thankfully, Earth remains safe, as we know of no candidate X-ray supernova nearby. But this new study places further limits on the galactic habitable zone, the region in each galaxy that can support life. In the outermost reaches of a galaxy, star formation is too low to build up the necessary ingredients for rocky planets. But the dense cores, where stars live and die at a frenetic pace, are also deadly, because frequent supernovas flood their surroundings with radiation.

The new study shows that the inner edge of the galactic habitable zone is probably farther away from the core of the galaxy than we previously assumed. Despite getting hit here and there, however, Earth is in one of the safest neighborhoods of the entire galaxy.