Physicists Propose the Amaterasu Particle Is Heavier Than Iron, Reframing the Universe's Most Energetic Cosmic Rays as Ultraheavy Nuclei

Overview

The most energetic cosmic ray detected in three decades may not be a lone proton or an iron nucleus, but something heavier than any element found in iron’s group on the periodic table. That is the central proposal of a Penn State-led study published in Physical Review Letters, which argues that the highest-energy cosmic rays — including the celebrated Amaterasu particle — could be ultraheavy atomic nuclei that lose energy more slowly than lighter particles as they cross intergalactic space, according to Penn State. If correct, the idea would reshape both what these particles are made of and where they come from.

The paper, titled “Ultraheavy Ultrahigh-Energy Cosmic Rays,” was published on May 7, 2026, and drew renewed attention through science press in early June, as reported by ScienceDaily.

What We Know

Cosmic rays are charged particles that strike Earth’s atmosphere from space, and the rarest of them carry almost unimaginable energy. Ultrahigh-energy cosmic rays are those above 100 exa-electron volts, per Penn State — roughly seven orders of magnitude, or 10 million times, more energetic than the particles accelerated in CERN’s Large Hadron Collider, the university said.

The Amaterasu particle, named after the sun goddess in Japanese mythology, was detected in 2021 by the Telescope Array in Utah, according to Penn State. The paper itself records its energy as 244 EeV, per the study on arXiv. That places it just below the Oh-My-God particle, detected in 1991 at around 320 exa-electron volts, as reported by Tech Times.

What has long made Amaterasu a puzzle is its apparent origin. Its inferred arrival direction pointed back toward a cosmic void — an almost empty region of space — with no obvious source capable of producing it, according to ScienceDaily.

The new work, led by Kohta Murase, professor of physics and of astronomy and astrophysics at Penn State’s Eberly College of Science, approaches the problem through the particle’s composition rather than its direction. The team ran simulations of how nuclei of different masses lose energy while traveling across the universe and found that nuclei heavier than the iron group — which the authors define as “ultraheavy” — shed energy more slowly than protons or intermediate-mass nuclei at these extreme energies, as reported by ScienceDaily.

“Our research showed that at energies comparable to that of the Amaterasu particle, ultraheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei,” Murase said, according to Penn State. The study examined specific heavy elements as candidates — among them selenium, tellurium, and platinum, extending up to uranium — per the paper on arXiv.

If the Amaterasu particle is such an ultraheavy nucleus, its slower energy loss means it could have traveled from farther away than a proton or iron nucleus of the same observed energy, per the study on arXiv. That, in turn, could relax the requirement that its source lie within the nearby void.

Where Such Particles Could Come From

Producing a nucleus heavier than iron and then accelerating it to these energies demands extraordinary environments. “Ultrahigh-energy cosmic rays can only be accelerated by some of the most powerful sources in the universe,” Murase said, according to Penn State.

The study points to massive star deaths that involve explosive collapse into black holes or strongly magnetized neutron stars, as well as binary neutron-star mergers, as the most promising sites, according to Penn State. Those same neutron-star mergers are known gravitational-wave emitters, tying the cosmic-ray question to the broader field of multi-messenger astronomy.

Murase framed the stakes in historical terms. “The origins and acceleration mechanisms of ultrahigh-energy cosmic rays have been among the biggest mysteries in the field for more than 60 years,” he said, according to Penn State. He added: “These highest-energy cosmic rays are thought to come from extreme astrophysical sources, like two neutron stars colliding or a massive star collapsing,” the university said.

The paper was authored by B. Theodore Zhang of Kyoto University’s Yukawa Institute for Theoretical Physics, Murase, and collaborators Mukul Bhattacharya at Penn State along with Nick Ekanger and Shunsaku Horiuchi at Virginia Tech, according to Penn State.

What We Don’t Know

The proposal remains a theoretical interpretation, not a measurement of the Amaterasu particle’s composition. Current detectors infer the energy and rough direction of an ultrahigh-energy cosmic ray from the cascade of secondary particles it produces in the atmosphere, but pinning down the mass of any single event is far harder. The decisive test will come from instruments designed to measure composition directly.

The upgraded AugerPrime configuration of the Pierre Auger Observatory is built to measure individual cosmic-ray mass composition and could indicate whether ultraheavy nuclei appear at the highest energies, as reported by Tech Times. Looking further ahead, the planned Global Cosmic Ray Observatory is among the next-generation efforts that could sharpen the picture, according to Universe Today.

Until such measurements arrive, the ultraheavy interpretation stands as a compelling but unconfirmed answer to a question physicists have chased for decades: what, exactly, is hurling the universe’s most violent particles toward Earth, and from where.

Analysis

The appeal of the ultraheavy hypothesis is that it attacks two problems at once. The composition question — why a particle this energetic survives its journey at all — and the source question — why Amaterasu seems to come from an empty patch of sky — are normally treated separately. By showing that heavier nuclei lose energy more slowly, the Penn State team links them: a slower-decaying particle can originate beyond the local void, loosening the constraint that made Amaterasu so confounding in the first place.

That elegance is also the reason to be cautious. A model that resolves an anomaly by inference is only as good as the data that can confirm it, and composition measurement at these energies is precisely where the field has been weakest. The value of this study may lie less in declaring the mystery solved than in handing observatories like AugerPrime a concrete, falsifiable target to aim at.