Cosmic rays from a nearby supernova may help explain Earth-like planets

How common are Earth-like planets in the universe? When I started working on supernova explosions, I never imagined that my research would eventually lead me to ask a question about the origin of Earth-like planets. Yet that is exactly where it brought me.

For decades, planetary scientists have believed that the early solar system was enriched with short-lived radioactive elements—such as aluminum-26—by a nearby supernova. These radioactive elements played a crucial role in forming water-depleted rocky planets such as Earth. Their decay heated young planetesimals, causing them to lose much of their originally accreted water and other volatile materials.

There was just one problem that kept bothering me.

The classic “injection” scenario requires an extraordinary coincidence. The supernova must explode at just the right distance—close enough to deliver radioactive material, but not so close that it destroys the fragile protoplanetary disk. The geometry must also be finely tuned so that the material is injected efficiently. In other words, Earth’s birth appeared to depend on an event that was possible, but extremely rare.

As a researcher working on supernova physics and cosmic rays, this explanation felt incomplete.

Supernovae are not just explosions that eject material. They are also some of the most powerful particle accelerators in the universe. Their shock waves generate enormous numbers of high-energy particles—cosmic rays—that propagate far beyond the expanding debris. Yet in most models of solar system formation, these particles were largely ignored.

I began to wonder: what if the young solar system was not simply hit by supernova ejecta, but was also immersed in a bath of cosmic rays?

In our recent study published in Science Advances, my colleagues and I explored this idea using numerical simulations of cosmic-ray acceleration and nuclear reactions. When cosmic rays interact with the protosolar disk, they can trigger nuclear reactions that naturally produce short-lived radioactive elements, including aluminum-26.

What surprised us was how well this mechanism worked.

The required amounts of radioactive elements are produced at distances of about one parsec from a supernova—a distance that is entirely typical in star clusters. At these distances, the protosolar disk remains intact. There is no need for an extraordinarily lucky injection event. The young solar system simply had to exist in the same stellar nursery as a massive star that later exploded.

We call this mechanism a “cosmic-ray bath.”

From a physical perspective, it is a much more universal process. Many sun-like stars form in clusters. Many clusters contain massive stars. Many massive stars explode as supernovae. If cosmic-ray baths are common in such environments, then the thermal histories that shaped Earth’s interior may be common as well.

This realization has broader implications.

If Earth-like planets require an extremely rare supernova encounter, then water-depleted rocky planets might be exceptional. But if cosmic-ray immersion is sufficient—and common—then the conditions that helped shape Earth may arise around a large fraction of sun-like stars.

Of course, our work does not claim that supernova guarantees every habitable planet. Many factors still matter, including disk lifetime, cluster structure, and stellar dynamics. But what our results show is that Earth’s formation may not have depended on an almost miraculous coincidence.

For me, this study was a reminder of how interconnected astrophysical processes are. A phenomenon usually studied in high-energy astrophysics—cosmic-ray acceleration—turns out to be central to questions in planetary science and habitability. Sometimes, the key to understanding where we come from lies not in adding more complexity, but in noticing what we have been overlooking.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

I am an astrophysicist (ICRR fellow, Ph.D) at the Institute for Cosmic Ray Research, University of Tokyo. My research focuses on supernova explosions, cosmic-ray acceleration, and nuclear processes in astrophysical environments. By combining models of high-energy astrophysics with planetary science, I study how violent astrophysical events influence the formation and evolution of planetary systems. I am particularly interested in connecting supernovae with the origin of Earth-like planets and habitability.