Chandra’s Sight Sheds New Light: Gathering Insight on Earth, Life, and the X-ray Bright

Ian Brunton.

We welcome Ian Brunton, a research scientist currently at NASA Johnson Space Center in the Astromaterials Research and Exploration Science Division as our guest blogger. In this post, he describes his team’s work below on the effects that a nearby supernova may have on an Earth-like planet and its biosphere. Ian first became involved with this area of research as an astronomy student of Brian Fields at the University of Illinois. He will soon be continuing his academic studies as a PhD student at Caltech in the Division of Geological and Planetary Sciences.

Much has been said about the extraordinary advancements throughout the field of astronomy, particularly regarding the innovative ways in which we can now observe the universe across the electromagnetic spectrum. Chandra has of course been one of the instruments at the forefront of this exploration for the last couple of decades, illuminating the universe in the X-ray band. These new ways of looking at our universe have served to confirm, alter, or entirely upend our prior notions of certain astrophysical processes.

What I personally find most intriguing is how these new observations can then be integrated into the knowledge and pursuits of other scientific disciplines, be it planetary science, atmospheric chemistry, geology, etc.

One of the most fascinating processes (if I may say so myself…) that orbital X-ray telescopes are especially handy for are supernovae, i.e., exploding stars! I’ll elaborate a bit on exactly why below, but first, some background on nearby supernovae and Earth is needed since our project really builds upon a lot of previous work in the field.

Everyone loves a good astronomical explosion, and supernovae — typically characterized by the wondrous spectacle of their initial outbursts — are some of the best explosions in the known universe. In the blink of an eye, these monstrous events can outshine the entire combined output of stars in a galaxy, launching neutrinos, photons, and stellar material out into the abyss of the interstellar medium.

That said, much like an explosion that may occur here on Earth, one will most happily observe these events when placed at a safe distance away. Or more simply put, there is only so close you can get to an explosion before your child-like wonder turns from, “Oh, cool!” to “Oh, ----!” My co-authors and I are most interested in the latter; essentially, the harmful effects that may occur if life is too close to the explosion.

The damage that a nearby supernova may inflict on Earth has been investigated as far back as the 1950s and 60s, and several of my coauthors (while not that old) have been closely involved in this research for the past few decades. In short, if a supernova occurs in the vicinity of a planet (within its galactic neighborhood), then the ionizing radiation emitted can have a formidable impact on the planet’s surface and/or atmospheric processes. In the case of modern-day Earth, this radiation would alter the planet’s atmospheric chemistry, which then poses a threat to the biosphere in several ways.

Much of the previous literature on the topic has primarily looked at two phases of harmful emission that are prevalent in all supernovae explosions: (1) the distinct outburst of harmful radiation at the beginning of the event (mostly gamma-rays), and (2) the influx of destructive cosmic rays that propagate outwards through the interstellar medium.

However, there are some supernovae that have a rather distinct phase of emission which is especially luminous in the X-ray band. This emission occurs as a byproduct of the progenitor star having shed an exorbitant amount of its mass prior to its explosive supernovae death. When the supernova then does occur, the resultant shock wave collides with the deposited stellar material, heating it up, and causing high energy X-rays to propagate further outwards. Because of the delay between the explosion and the shock wave propagation, there is a lag in the X-ray emission, from a few weeks, to as long as a few years. This is where the innovative work done by the astronomers working with Chandra and other X-ray telescopes comes in. These telescopes have now observed numerous X-ray luminous supernovae, which has thus allowed us to gain keen insight into the magnitude and evolution of these unique types of stellar deaths. And as the astronomical community has developed a clearer picture of these X-ray emission profiles, so too can we develop a clearer picture into how these X-rays may impact a nearby planet.

Brian Fields has done a lot of previous work in relation to past supernovae, so when Connor O’Mahoney and I expressed interest in working with him on a project related to this, he then had the idea to look at these X-ray data to see what we could find. The project initially began with us scouring the literature to consolidate and analyze all the observations that have been made. From there, we could then assess the potential damage this high-energy emission may have on Earth-like biospheres.

This is why it was great to collaborate with Adrian Melott and Brian Thomas, two co-authors on the paper who have conducted extensive research into the terrestrial consequences of astrophysical radiation from all sorts of transient events. And since this was the first time anyone has focused the analysis specifically on the X-ray profiles, we had to look at what the Earth system’s response would be in a different light (no pun intended). For example, the depth at which the X-rays will interact with the atmosphere will be distinct from the previous gamma-rays and cosmic rays that most of the literature had analyzed previously. This, and other factors, serve to alter the combined effects that would be imposed on the biosphere.

Now that we’ve taken a first look at the effects that these X-ray luminous events may have on Earth, there are plenty of paths we could take next. One path forward is to further utilize modern datasets of transient emission profiles to run against modern global climate models of Earth and other exoplanets. Of course, what would be of great benefit for our research is to continue the wonderful work being done by Chandra, and to increase the cadence of astronomical observations in the X-ray band!