Jupiter's northern high latitudes photographed by JunoCam during the 69th flyby. Turbulent cloud belts show wind-driven disturbances at their edges. Credit: NASA/JPL-Caltech/SwRI/MSSS, Image processing: Jackie Branc (CC BY).
Jupiter's northern high latitudes, captured by JunoCam during the spacecraft's 69th flyby of the giant planet on January 28, 2025. The turbulence along the edges of Jupiter's cloud belts is driven by winds moving in opposite directions. Beyond these visible storm systems, the planet's magnetic field creates a bow shock millions of kilometers wide, a natural particle accelerator that Juno flew through. Credit: NASA/JPL-Caltech/SwRI/MSSS, Image processing: Jackie Branc (CC BY).

On October 1, 2023, NASA's Juno spacecraft crossed an invisible boundary around Jupiter and found something that was never supposed to be there.

The boundary is called a bow shock. It forms where a planet's magnetic field slams into the stream of charged particles flowing outward from the Sun, creating a standing shock wave like the bow wave in front of a ship. Juno had crossed Jupiter's bow shock dozens of times before. Each crossing produced the usual data: some turbulence, a spike in magnetic field strength, nothing unusual.

This time was different. As Juno passed through the foreshock, the turbulent region just upstream of the shock, its instruments detected a burst of electrons moving at nearly the speed of light. Relativistic electrons, carrying energies above one million electron volts. Particles moving fast enough that their mass had measurably increased, exactly as Einstein predicted.

They had no business being there. The standard model of how particles gain energy near planets could not explain electrons that energetic. Something else was doing the accelerating.

Three years later, on June 3, 2026, the results landed in the journal Nature. What Juno recorded was the first direct confirmation of a mechanism astronomers have suspected for decades but could never prove: foreshock transients, the same structures that accelerate particles at Earth's bow shock, also work at Jupiter. And they scale. The bigger the shock, the faster the particles get. Keep scaling up, and the same physics might explain how cosmic rays form across the entire universe.

The 100-year-old question

Cosmic rays were discovered in 1912 when Austrian physicist Victor Hess carried an electroscope up in a balloon and found that radiation increased with altitude, not decreased. Whatever it was, it was coming from space. He won the Nobel Prize for it in 1936.

More than a century later, scientists know cosmic rays come from many sources: the Sun, exploding stars, active galactic nuclei. They know the most energetic cosmic ray ever detected, nicknamed the Oh-My-God particle in 1991, carried roughly the kinetic energy of a baseball thrown at 90 kilometers per hour, packed into a single subatomic particle.

What they have never fully understood is the exact mechanism that accelerates particles to those staggering energies. The leading theory, diffusive shock acceleration, or DSA, works well on paper. Particles bounce back and forth across a shock front, gaining a little energy with each crossing, eventually reaching relativistic speeds. But DSA has a problem called the injection problem: it only works on particles that are already moving fast enough to cross the shock. Where do those first energetic particles come from?

That is where foreshock transients enter the story.

A natural particle accelerator hiding in plain sight

When the solar wind hits a planet's magnetic field, it does not strike it cleanly. The collision creates a turbulent foreshock upstream of the bow shock, filled with reflected particles, tangled magnetic fields, and large-scale structures scientists call foreshock transients: hot flow anomalies, foreshock bubbles, and spontaneous hot flow anomalies.

At Earth, NASA's MMS and THEMIS missions have watched these transients trap and accelerate electrons to about one MeV. The mechanism is a kind of cosmic pinball: particles get reflected between magnetic structures, scattered by turbulence, and geometrically trapped inside the transient while it grows. Each bounce adds energy. The transients effectively serve as the first stage of a multi-stage particle accelerator, solving the injection problem by getting particles moving fast enough for DSA to take over.

The question was whether this worked the same way everywhere, or whether it was a quirk of Earth's relatively small magnetic environment. To find out, scientists needed a bigger shock. A much bigger one.

Advertisement

Testing the idea at Jupiter

Jupiter's bow shock is enormous. The planet's magnetic field is roughly 20,000 times stronger than Earth's, and its magnetosphere stretches millions of kilometers into space. If it were visible from Earth, it would appear larger than the full Moon in the night sky.

On October 1, 2023, Juno was inbound on an orbit that took it across the duskward flank of Jupiter's bow shock. The crossing happened around 18:05 UTC. The spacecraft's particle detectors recorded a population of relativistic electrons with energies exceeding one MeV, far higher than any electrons previously seen near a planetary bow shock.

The electron energies matched a clear scaling relationship. At Earth, foreshock transients accelerate electrons to about one MeV. At Jupiter, with a bow shock roughly 100 times larger in linear scale, the transients accelerated electrons to significantly higher energies, consistent with the increase in system size. The bigger the accelerator, the more energetic the particles.

"This finding demonstrates that the physics of particle acceleration near planets is universal," the research team wrote. The process that works at Earth also works at Jupiter. By extension, it should work everywhere.

From Jupiter to the galaxy

The implications reach far beyond our solar system. Foreshock transients are fundamental properties of collisionless shocks, and collisionless shocks exist throughout the universe. They form around young stellar objects, in protostellar jets, and, most dramatically, in the expanding blast waves of supernova remnants.

A supernova remnant shock can be light-years across, billions of times larger than Jupiter's bow shock. If the same scaling relationship holds, those vast shocks should accelerate particles to the energies seen in galactic cosmic rays, the particles that stream through the Milky Way and occasionally strike Earth's atmosphere at nearly the speed of light.

The Nature paper takes this idea and runs with it. The authors propose a framework based on what they call a foreshock transient scaling law. Given the size of a shock system and its upstream medium, you can estimate the maximum particle energy it can produce. The law is empirically grounded: it matches data from Earth's bow shock, Jupiter's bow shock, and extrapolates to match observed cosmic ray energies from known astrophysical sources.

It is not a complete theory of cosmic ray acceleration. But it bridges the gap between two fields that rarely talk to each other: heliophysics, which studies particle acceleration inside the solar system with direct spacecraft measurements, and astrophysics, which studies the same processes at galactic scales with telescopes and theory. Juno provided the missing middle step.

Why Juno was the right tool for the job

Juno was not designed to study cosmic rays. It was designed to probe Jupiter's interior, map its gravity and magnetic fields, and study its atmosphere and auroras. But its suite of particle detectors, magnetometers, and plasma instruments turned out to be exactly what scientists needed to catch relativistic electrons in the act of being accelerated.

The spacecraft launched in 2011, arrived at Jupiter in 2016, and has been orbiting the planet ever since. Its highly elliptical orbit carries it from just above Jupiter's cloud tops out to the fringes of its magnetosphere, crossing the bow shock repeatedly. Each crossing is a chance to sample the particle environment in a different region under different solar wind conditions.

The October 2023 crossing happened to catch the right transient at the right time with the right instrument configuration. In space physics, as in most science, luck is the residue of persistence.

What it means for the future

The result opens several lines of investigation. It suggests that planetary bow shocks across the solar system can serve as natural laboratories for studying processes that also operate at galactic scales, but are too distant to measure directly. Future missions to the outer planets could carry instruments specifically designed to probe foreshock transients and test the scaling law in even larger magnetic environments.

It also reinforces the value of long-duration missions like Juno, which has now been operating at Jupiter for a decade. The discovery relied on a single bow shock crossing among hundreds, and required years of data analysis to confirm. Without the extended mission, the October 2023 crossing would not have happened, and the scaling law would still be a hypothesis waiting for evidence.

The Nature paper is the latest in a growing line of work that treats the solar system as a cosmic laboratory. We cannot visit a supernova remnant. We cannot fly through the shock of an active galactic nucleus. But we can fly through Jupiter's bow shock and watch the same physics play out at scales we can actually measure. Juno just showed us how.

Sources

The hero image of Jupiter's northern high latitudes was captured by JunoCam on January 28, 2025, during Juno's 69th close flyby, and processed by citizen scientist Jackie Branc. Credit: NASA/JPL-Caltech/SwRI/MSSS, Image processing: Jackie Branc (CC BY). NASA images are generally in the public domain. This article describes peer-reviewed research published in Nature on June 3, 2026.