Supernova shower: A deadly rain of X-rays threatens planets

New research shows that supernovae may pose an additional, previously unidentified threat to nearby planets. These are intense X-rays produced when a supernova’s shock wave strikes dense surrounding gas, potentially damaging the atmospheres of planets up to 160 light-years away and possibly leading to mass extinctions. Earth is not currently at risk but may have been at risk in the past.

An exploded star may pose more risks to nearby planets than previously thought, according to a new study[{” attribute=””>NASA’s Chandra X-ray Observatory and other X-ray telescopes. This newly identified threat involves a phase of intense X-rays that can damage the atmospheres of planets up to 160 light-years away.

Earth is not in danger of such a threat today because there are no potential supernova progenitors within this distance, but it may have experienced this kind of X-ray exposure in the past.

Before this study, most research on the effects of supernova explosions had focused on the danger from two periods: the intense radiation produced by a supernova in the days and months after the explosion, and the energetic particles that arrive hundreds to thousands of years afterward.

However, even these alarming threats do not fully catalog the dangers in the wake of an exploded star. Researchers have discovered that, in between these two previously identified dangers, lurks another. The aftermaths of supernovae always produce X-rays, but if the supernova’s blast wave strikes dense surrounding gas, it can produce a particularly large dose of X-rays that arrives months to years after the explosion and may last for decades.

The calculations in this latest study are based on X-ray observations of 31 supernovae and their aftermath mostly obtained from Chandra, NASA’s Swift and NuSTAR missions, and ESA’s (European Space Agency’s) XMM-Newton. The analysis of these observations shows that there can be lethal consequences from supernovae interacting with their surroundings, for planets located as much as about 160 light-years away.

“If a stream of X-rays swept across a nearby planet, the radiation would seriously alter the planet’s atmospheric chemistry,” said Ian Brunton of the University of Illinois at Urbana-Champaign, who led the study. “For an Earth-like planet, this process could wipe out a significant portion of the ozone, ultimately protecting life from its host star’s dangerous ultraviolet radiation.”

If a planet with Earth’s biology were struck by sustained high-energy radiation from a nearby supernova, particularly one that interacts strongly with its surroundings, it could result in the demise of a wide range of organisms, particularly marine organisms at the base of the food chain. These effects can be significant enough to trigger mass extinctions.

“Earth is not currently at risk from such an event because there are no potential supernovae within the X-ray hazard zone,” said co-author Connor O’Mahoney, also of the University of Illinois. “However, such events may have played a role in Earth’s past.”

There is strong evidence – including the discovery of a radioactive form of iron in various places around the world – that supernovae occurred near Earth about two to eight million years ago. Researchers estimate that these supernovae were about 65 to 500 light-years from Earth.

Earth is in the “local bubble,” a still-expanding bubble of hot, low-density gas surrounded by an envelope of cold gas that stretches about 1,000 light-years. The outward extension of stars near the surface of the Local Bubble suggests that it formed about 14 million years ago from a burst of star formation and supernovae near the center of the bubble. The massive young stars responsible for the supernova explosions were then much closer to our planet than such stars are today, putting Earth at much greater risk from these supernovae in the past.

While this evidence does not link supernovae to any specific mass extinction event on Earth, it does suggest that cosmic explosions have impacted our planet throughout its history.

Although the Earth and the solar system are currently in a safe space in relation to possible supernova explosions, many other planets are in the[{” attribute=””>Milky Way are not. These high-energy events would effectively shrink the areas within the Milky Way galaxy, known as the Galactic Habitable Zone, where conditions would be conducive for life as we know it.

Because the X-ray observations of supernovae are sparse, particularly of the variety that strongly interact with their surroundings, the authors argue that follow-up observations of interacting supernovae for months and years after the explosion would be valuable.

“Further research on X-rays from supernovae is valuable not just for understanding the life cycle of stars,” said co-author Brian Fields of the University of Illinois, “but also has implications for fields like astrobiology, paleontology, and the earth and planetary sciences.”

The paper describing this result appears in the April 20, 2023 issue of The Astrophysical Journal. The other co-authors of the paper are Adrian Melott from the University of Kansas and Brian Thomas from Washburn University in Kansas.

For more on this study, see NASA’s Chandra Unmasks New Cosmic Threat to Planetary Life.

Reference: “X-Ray-luminous Supernovae: Threats to Terrestrial Biospheres” by Ian R. Brunton, Connor O’Mahoney, Brian D. Fields, Adrian L. Melott and Brian C. Thomas, 19 April 2023, The Astrophysical Journal.
DOI: 10.3847/1538-4357/acc728

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.