This false color photograph of Neptune was made from Voyager 2 images taken through three filters: blue, green, and a filter that passes light at a wavelength that is absorbed by methane gas. (Credit: NASA/JPL)

BERKELEY, Calif. — For decades, something strange has lurked inside the icy giants of our solar system. Uranus and Neptune, distant blue worlds 1.7 billion miles from Earth, harbor magnetic fields that behave nothing like those of their planetary siblings. While Earth’s magnetic field resembles a bar magnet neatly aligned with its axis, these planets generate wonky, off-center fields that seemingly defy explanation, until now. A Berkeley physicist has cracked the case by revealing that their interiors naturally separate like oil and water, forever changing our understanding of these mysterious worlds.

Like Oil and Water, But in Space

The research, published in the Proceedings of the National Academy of Sciences, was conducted by planetary scientist Burkhard Militzer from the University of California, Berkeley. He discovered that under extreme conditions inside ice giants, familiar compounds behave in surprising ways.

“We now have a good theory why Uranus and Neptune have really different fields, and it’s very different from Earth, Jupiter, and Saturn,” says Militzer, in a statement. “It’s like oil and water, except the oil goes below because hydrogen is lost.”

When scientists talk about “ices” in these planets, they don’t mean frozen water. In planetary science, “ices” refer to compounds that were frozen in the outer solar system when the planets formed, including water, methane, and ammonia. Inside Uranus and Neptune, these materials exist as hot, dense fluids under crushing pressure.

Previous theories suggested these materials would stay mixed throughout the planets’ interiors, creating uniform layers that should generate Earth-like magnetic fields. But that’s not what Voyager 2 observed.

Better Computer Models

Militzer struggled with this problem for years. A decade ago, his computer simulations with about 100 atoms couldn’t show how layers might form under ice-giant conditions.

Last year, with more powerful computing and machine learning assistance, he ran simulations with 540 atoms that revealed something unexpected: the materials naturally separated into layers.

“One day, I looked at the model, and the water had separated from the carbon and nitrogen,” says Militzer. “I thought, ‘Wow! Now I know why the layers form: One is water-rich and the other is carbon-rich, and in Uranus and Neptune, it’s the carbon-rich system that is below.’”

Two Distinct Layers with Different Properties

The simulations showed planetary ices spontaneously separate into two layers: an upper layer rich in water and a lower layer dominated by carbon and nitrogen compounds. As pressure increases deeper in the planets, the lower layer releases hydrogen, creating a stable pattern where materials don’t mix vertically.

This layered structure explains the magnetic field mystery. The upper, water-rich layer flows and churns, creating the conditions needed to generate a magnetic field. Meanwhile, the lower layer remains still and stratified, preventing it from contributing to the magnetic field generation.

How Planetary Magnetic Fields Work

A planet’s magnetic field is created by flowing, electrically conducting fluid. As a planet cools, cold material sinks while hot material rises, a process called convection. If this flowing material conducts electricity, it generates a magnetic field.

Earth’s field comes from its liquid iron outer core, creating a pattern that makes compasses point north. But Voyager 2 found that Uranus and Neptune have messy, disorganized fields, suggesting their internal structure works differently.

Mapping the Interiors of Ice Giants

Using his findings, Militzer built detailed models of Uranus and Neptune that match both gravity and magnetic field measurements from Voyager 2. His models include four layers:

1. An outer hydrogen-helium atmosphere
2. A water-rich layer where the magnetic field generates
3. A stable carbon-nitrogen-hydrogen layer with decreasing hydrogen content
4. A rocky core

Militzer predicts that below Uranus’ 3,000-mile-thick atmosphere lies a water-rich layer about 5,000 miles thick, followed by a hydrocarbon-rich layer also about 5,000 miles thick. Its rocky core is roughly the size of Mercury. Neptune has a similar structure but with a slightly larger core, approximately the size of Mars.

“If you ask my colleagues what explains the fields of Uranus and Neptune, they may say, ‘Well, maybe it’s diamond rain, or this water property called superionic,’” says Militzer. “From my perspective, this is not plausible. But if we have this separation into two layers, that should explain it.”

Testing the Theory and Future Missions

The phase separation should create specific patterns that could be detected in laboratory experiments using X-ray diffraction to reveal how atoms arrange themselves. The layered structure would also affect how the planets vibrate, something a future space mission could detect with an instrument called a Doppler imager.

Militzer hopes to work with colleagues to test whether these layers form in laboratory conditions. He’s also calculating how a layered planet would vibrate differently than a fully mixed one, which could be confirmed by a future NASA mission to Uranus.

Why This Matters Beyond Our Solar System

If other star systems have similar compositions to ours, ice giants around those stars likely have similar internal structures. Planets about the size of Uranus and Neptune are among the most common exoplanets discovered so far.

This discovery comes at a good time. Planetary scientists have recommended a mission to Uranus as a top priority for the next decade. Militzer’s findings could help shape the scientific goals and instruments for such a mission.

From unusual magnetic fields to distinct internal layers, Uranus and Neptune continue to challenge our understanding of planetary formation. Militzer’s research suggests that inside these distant blue worlds, familiar compounds behave in unfamiliar ways: separating rather than mixing, releasing hydrogen with depth, and generating magnetic chaos from layered order. These ice giants remind us that even after centuries of astronomy, planets still have plenty of secrets to reveal.