A team of astrophysicists has renewed theories that suggest certain extinctions that have shaped the history of Earth’s biosphere would be the product of supernovae explosions in the Sun’s suburbs. This also has implications for Exo-Earths in the Milky Way and the notion of a galactic habitable zone around them.
If we count the supernovae in other galaxies over a year, all types together (especially SN II and SN Ia), we estimate that our Milky Way should have an average of three to four supernovae per century. Since our galaxy is about 100,000 light-years across, one might think that this would have no particular impact on Earth, but on the scale of tens of millions of years, it is safe to assume that certain stars exploded dangerously close to our solar system. In fact, before the discovery of the famous black clay layer separating the Late Cretaceous from the Tertiary, and also the famous Chicxulub astrobleme in Yucatán, some had claimed that it was the radiations and cosmic rays of a supernova that exploded about 66 million years ago in the Close to the sun and killed the dinosaurs.
The question of the danger of supernovae to biospheres in the Milky Way comes to the fore today with an article published in the Astrophysical Journal by a group of American astrophysicists who have looked at the data collected in the field of X-rays on supernovae in our Milky Way and beyond ( 31 in total), with a battery of telescopes in space, Chandra of course, but Swift and NuSTAR from NASA and XMM-Newton from ESA (European Space Agency). Calculations now suggest that the interaction of supernovae with their environment can have deadly consequences for planets up to about 160 light-years away. This is enough to prompt exobiology to reconsider the habitability of exoplanets in the Milky Way, particularly in the context of the so-called galactic habitable zone.
It’s a concept that can be traced back to the early 1980s, but was most notably developed in 2001 by Donald Brownlee and Peter Ward in the context of their famous rare earth hypothesis. Basically, certain regions of the galactic system would not have the heavy elements needed to produce habitable terrestrial planets, and a planet too close to the galactic center would be subject to many supernovae whose radiation is harmful to life. Many biospheres would thus have been sterilized in galactic history.
Supernovae near the sun less than 10 million years ago
The article published today clarifies this last point. But rest assured, there aren’t any stars in the solar system’s suburbs that could turn into supernovae in the near future. However, this does not mean that the Earth has not been affected by supernovae explosions in the recent past, possibly with some extinctions.
In fact, for about 20 years, isotopic anomalies with iron have been found in certain sediment layers that formed 2 to 3 million years ago, exactly at the time when about a third of the large species of marine animals disappeared. . The 60Fe radioisotopes can be well explained in terms of supernovae that exploded a few hundred light-years from the Sun in relation to the so-called “local bubble”, a kind of envelope of about 1,000 light-years that surrounds our solar system. This is exactly where it seems to have been created a few million years ago by several explosions of massive stars in supernovae.
As I said, the idea that supernovae are deadly is not new. But it used to be thought that the danger came from only two sources. The first is the X-rays and gamma rays produced by an ongoing supernova for a few months at most, which radiation travels at the speed of light. The second potentially deadly source of radiation would arrive hundreds or even thousands of years later, with cosmic rays in the form of material particles associated with the original explosion, traveling considerably slower than light, albeit at considerable speeds.
UV-protective ozone layers destroyed by X-rays
But according to the new calculations, the effect of shock waves from explosions in the interstellar medium had not been taken into account. The collisions that heat the material must actually cause it to radiate so intensely in X-rays, and have done so for decades. Lethal doses of this radiation to living organisms would thus be produced up to 160 light-years after the explosion.
In the case of an exoplanet like Earth, these X-rays could wipe out a significant portion of the ozone, which ultimately protects life from its host star’s ultraviolet radiation. This could thus lead to the disappearance of a wide variety of organisms, particularly marine organisms at the base of the food chain, leading to extinction.
As the artists’ images below show, even after years of lethal X-ray exposure, large amounts of nitrogen dioxide can be produced, leading to a brown haze in the atmosphere and “grading” of land masses due to plant damage.