When a particle the size of a grain of sand crashed into Earth's atmosphere above Utah in May 2021, it carried the kinetic energy of a speeding tennis ball—a distinction no cosmic visitor had matched in three decades. Scientists named it the Amaterasu particle after the Japanese sun goddess, honoring both its luminous power and its mystery. For nearly a century, researchers have puzzled over where such ultrahigh-energy cosmic rays originate and what they actually are. Now, scientists at Penn State may have found the answer hiding in an unexpected place: the periodic table.
The Amaterasu particle's energy—roughly 240 exa-electron volts—places it among the most powerful cosmic-ray events ever observed, a rare club it shares with the famous "Oh-My-God particle" detected in 1991. Yet what makes it truly baffling is that when astronomers trace its path backward through space, it appears to come from nowhere, arriving from a cosmic void with no obvious source powerful enough to have launched it. That paradox has haunted astrophysicists for more than 60 years, ever since ultrahigh-energy cosmic rays were first discovered.
Kohta Murase, a professor of physics and astronomy at Penn State, led a team that ran detailed computer simulations to investigate how different types of particles survive the journey through intergalactic space. Their research, published in Physical Review Letters and conducted with collaborators at institutions including the Yukawa Institute for Theoretical Physics in Japan and Virginia Tech, revealed a surprising possibility: the cosmic rays may not be protons at all, but ultraheavy atomic nuclei—elements heavier than iron.
The physics is elegant. As particles travel across the universe, they lose energy through interactions with radiation and magnetic fields. Murase's team discovered that ultraheavy nuclei lose energy more slowly than protons or lighter nuclei under these extreme conditions. This means an ultraheavy nucleus could survive the multibillion-year journey to Earth while still retaining the staggering energies needed to be detected at such extraordinary levels. To put the Amaterasu particle's power in perspective: it carried roughly ten million times more energy than particles accelerated in the Large Hadron Collider, the world's most powerful human-made accelerator.
"When we detect individual cosmic-ray particles such as the Amaterasu particle here on Earth, we can often use their energies, arrival directions and expected magnetic deflections to infer their possible cosmic sources," Murase explained. If some of the highest-energy cosmic rays are indeed ultraheavy nuclei, it fundamentally changes how scientists search for their origins—pointing them toward the universe's most violent phenomena.
The most likely cosmic forges for such particles would be the deaths of massive stars that collapse into black holes, encounters between neutron stars powerful enough to emit gravitational waves, or the mergers of stellar remnants so violent they trigger gamma-ray bursts. These cataclysmic events are among the most energetic explosions known to exist.
The team was careful not to claim that all ultrahigh-energy cosmic rays are ultraheavy nuclei. Instead, their work sets new boundaries on how much these heavy particles could contribute to the overall population of ultrahigh-energy cosmic rays observed on Earth. But by solving one 60-year mystery, they may have cracked open doors to understanding the universe's most extreme and violent corners.