Theory and experiment, for the first time, agreed.
Beneath the crushing depths of gas giants like Jupiter and Saturn, matter exists in states that have long defied direct observation, leaving scientists to rely on theory alone. Now, a team of researchers from Lawrence Livermore, UC Berkeley, and partner institutions has brought those conditions into the laboratory, using diamond anvil cells and the OMEGA laser to compress helium to pressures between 270 and 360 gigapascals — nearly double any previous experiment. What they found was not a sudden transformation, but a gradual, continuous ionization, helium quietly becoming an electrically conducting fluid, just as first-principles simulations had long suggested. In confirming that theory and experiment can finally agree at these extremes, the work opens a window not only into planetary interiors, but into the physics of the universe's most compressed objects.
- For decades, models of Jupiter, Saturn, and white dwarf stars have rested on theoretical foundations that no experiment could verify — a quiet but consequential uncertainty at the heart of planetary science.
- Helium's atomic simplicity made it the ideal subject, but getting it into a usable experimental state required cooling it to near absolute zero, tripling its density in a diamond anvil cell, and then firing a high-powered laser to drive shock waves through it.
- A redesigned diamond window just 200 micrometers thin allowed the laser energy to couple efficiently into the sample, pushing shock pressures and temperatures to record levels that previous helium experiments had never approached.
- The data revealed helium was more compressible and far more reflective than several existing models predicted, pointing to continuous ionization rather than any sharp phase transition.
- For the first time at these extreme conditions, experimental results and first-principles computer simulations agreed — a landmark alignment that validates the theoretical tools scientists use to model planetary and stellar interiors.
- The team now aims to push further still, targeting densities found in white dwarf atmospheres to test whether their simulation methods hold at even more violent extremes.
At the cores of Jupiter and Saturn, hydrogen and helium are crushed under pressures millions of times greater than anything on Earth's surface. In those depths, the two elements may separate, and that separation shapes how heat moves through the planet, how its interior is structured, and how its magnetic field behaves. For decades, scientists have modeled these conditions theoretically. Now, for the first time, they have experimental confirmation.
A team from Lawrence Livermore National Laboratory, UC Berkeley, France's CEA, and the University of Rochester published results this month in Physical Review Research showing helium behaving in ways many existing models did not anticipate. The choice of helium was deliberate: unlike hydrogen, helium exists as individual atoms rather than paired molecules, making it a cleaner subject for studying how pressure strips electrons from matter — a process called ionization — without the complication of molecular bonds breaking first.
The experimental challenge was considerable. Gaseous helium is too diffuse to shock efficiently, and liquid helium compresses too easily on its own. The team's solution was to precompress liquid helium in a diamond anvil cell to roughly three times its normal cryogenic density, then fire the OMEGA laser at the University of Rochester at the sample to drive a shock wave through it. A key innovation was thinning the diamond window to just 200 micrometers, allowing the laser energy to couple more efficiently into the material. The resulting shock pressures and temperatures — 270 to 360 gigapascals and 50,000 to 80,000 Kelvin — nearly doubled what previous helium experiments had achieved.
Analyzing the data, the team found the helium was more compressible and far more reflective than several theoretical models had predicted. The pattern pointed to continuous ionization — a gradual change of state rather than a sharp transition — and confirmed that the shocked helium was behaving as an electrically conducting fluid. Crucially, these results aligned with first-principles computer simulations built from fundamental physics. Theory and experiment, at these extreme conditions, finally agreed.
The team now plans to push further, investigating helium's electrical conductivity at densities approaching those found in white dwarf star atmospheres — extending the laboratory's reach deeper into the physics of the universe's most compressed and violent objects.
At the center of Jupiter and Saturn, hydrogen and helium exist in a state almost impossible to imagine on Earth—crushed under pressures millions of times greater than what we experience at sea level. In these depths, the two elements may separate from each other, and that separation ripples outward, shaping how heat flows through the planet, how its interior is structured, and even how its magnetic field behaves. To understand how planets form and evolve, scientists need to know what happens to matter under these conditions. For decades, they've relied on theory. Now, for the first time, they have experimental proof.
A team of researchers from Lawrence Livermore National Laboratory, UC Berkeley, France's Commissariat à l'Énergie Atomique et aux Energies Alternatives, and the University of Rochester's Laboratory for Laser Energetics published results this month in Physical Review Research that show helium behaving in ways most existing models did not predict. The findings matter because helium, unlike hydrogen, exists as individual atoms rather than paired molecules. That simplicity is the key to understanding what extreme pressure actually does to matter at the atomic level.
"Helium is interesting because its atoms don't form molecules," explained Marius Millot, a physicist at Lawrence Livermore. "That lets us isolate how pressure and temperature strip electrons from atoms—what we call ionization—without the complication of molecular bonds breaking first." Hydrogen has to go through that intermediate step. Helium does not. It responds to pressure more directly, making it a cleaner subject for study.
But getting helium to the right state for experimentation posed a practical puzzle. Gaseous helium is too thin to shock efficiently. Liquid helium, cooled to near absolute zero, compresses too easily and doesn't provide a suitable starting point on its own. The team's solution was to precompress liquid helium in a diamond anvil cell—a device that squeezes a sample between two diamond tips. They pushed it to between 7,600 and 12,000 atmospheres, making it three times denser than ordinary cryogenic liquid helium. Then they aimed the OMEGA laser at the University of Rochester at the compressed sample, using the laser's energy to drive a shock wave through it.
The critical innovation was redesigning the diamond anvil cell with a thinner diamond window, just 200 micrometers thick, so the laser energy could couple more efficiently into the sample. The shock pressures and temperatures reached between 270 and 360 gigapascals and between 50,000 and 80,000 Kelvin—nearly double what previous helium experiments had achieved. During the shocks, two diagnostic systems tracked the shock wave's speed and measured how hot and reflective the shock front became.
Michael Wadas, a summer fellow working under Millot and Jon Eggert, analyzed the data. What he found surprised the team: the helium was more compressible and far more reflective than several existing theoretical models had predicted. The pattern suggested that helium was undergoing continuous ionization—changing state gradually rather than through a sharp, sudden transition. The conductivity values inferred from the temperature and reflectivity data confirmed that the shocked helium was behaving as an electrically conducting fluid. More importantly, the results matched first-principles computer simulations built from fundamental physics rather than empirical guesses. Theory and experiment, for the first time at these extreme conditions, agreed.
The implications extend beyond Jupiter and Saturn. The team plans to push even further, investigating helium's electrical conductivity at densities approaching those found in the outer atmospheres of white dwarf stars. "We want to figure out if our numerical simulation methods are reliable at those extreme conditions," Millot said. By bringing the extreme physics of planetary and stellar interiors into the laboratory, researchers are building a foundation for understanding not just how gas giants work, but how the most violent and compressed objects in the universe behave.
Notable Quotes
Helium is interesting because its atoms don't form molecules, letting us isolate how pressure and temperature strip electrons from atoms without the complication of molecular bonds breaking first.— Marius Millot, Lawrence Livermore National Laboratory physicist
The team plans to investigate helium's electrical conductivity at even higher densities to reach conditions found in white dwarf star atmospheres.— Marius Millot
The Hearth Conversation Another angle on the story
Why does it matter that helium doesn't form molecules like hydrogen does?
Because it means you can study ionization in isolation. Hydrogen has to break its molecular bond first before electrons start coming off. Helium skips that step entirely. You're watching a cleaner process.
So you're saying the experiment is simpler because of helium's nature?
Exactly. It's like comparing a single instrument to an orchestra. Helium lets you hear what pressure alone does to atoms without all the other complications getting in the way.
The pressures they reached—270 to 360 gigapascals—how does that compare to what's actually inside Jupiter?
That's the whole point. These are the conditions that exist deep in Jupiter's interior. For the first time, they've recreated those conditions in a lab and measured what actually happens. No guessing.
And the results contradicted what the models predicted?
Not contradicted exactly. The helium was more compressible and more reflective than the broad-range models expected. But the first-principles simulations—the ones built from fundamental physics—those matched perfectly. So the old models were incomplete, but the new physics-based approach was right.
What does it mean that helium became electrically conducting?
It means the electrons were stripped from the atoms. That's ionization. And once you have free electrons moving around, you have a conductor. That changes everything about how heat and magnetic fields move through the planet.
Where does this lead next?
They want to push even further—to the densities you'd find in white dwarf stars. If they can validate their simulations at those extremes too, they'll have a tool that works across the entire universe.