For decades, Jupiter was supposed to be simple: a huge ball of gas, maybe with a solid core, maybe without one.

Two competing theories fought quietly in textbooks… until NASA’s Juno spacecraft flew in and smashed both of them at once.

Juno didn’t find a neat, rocky heart or a gas-only interior.

Instead, it revealed something no one had ever seriously expected—a vast, fuzzy core that blurs the line between rock and gas, order and chaos.

This strange discovery doesn’t just rewrite Jupiter’s story.

It challenges our understanding of gas giants across the entire universe.

The Core Debate That Juno Killed
Before Juno, scientists were split between two big ideas about how Jupiter formed and what lay at its centre: Gravitational Collapse
In this scenario, Jupiter formed when part of the early solar nebula—just a cold pocket of gas—collapsed directly under its own gravity.

That would make Jupiter more or less gas all the way down, with no solid core at all.

Just hydrogen, helium, and swirling layers from surface to centre.

Core Accretion
This model argued that Jupiter began like a giant rock or icy planet.

A solid core of heavier elements—iron, silicon, oxygen, carbon—formed first.

Once this seed got big enough, its gravity pulled in enormous amounts of hydrogen and helium, eventually building the gas giant we see today.

That would leave behind a well-defined, dense core buried deep inside.

Both theories had support.

We knew from earlier missions like Galileo that Jupiter is enriched with heavy elements, so it’s not “just gas.

” We also knew its magnetic field is generated by liquid metallic hydrogen, not a solid iron core like Earth’s.

But the structure of its interior—sharp core or fully mixed gas—remained frustratingly out of reach.

Then Juno arrived…

How Juno Weighed a Planet from Space
Launched in 2011 and in orbit around Jupiter since 2016, Juno carries one of the most precise gravitational instruments ever sent to another world.

Instead of drilling or landing, Juno used something far subtler: tiny changes in its own speed.

As Juno swooped just 4,200 km above Jupiter’s cloud tops in a highly elliptical polar orbit, variations in Jupiter’s gravity tugged on the spacecraft—speeding it up or slowing it down by as little as 0.01 millimetres per second.

Juno continuously sent radio signals back to Earth, and by analysing tiny Doppler shifts in those signals, scientists could map Jupiter’s gravitational field in exquisite detail.

Why gravity? Because gravity is a direct reflection of how mass is distributed inside a planet.

If Jupiter had a small, dense core, its gravity field would look different from that of a world with a diffuse interior.

Juno’s job was to reveal that hidden architecture.

What it found stunned everyone.

The Fuzzy Core: Neither Solid Nor Absent
Juno’s data ruled out both the fully solid and fully coreless models.

Jupiter does have a central concentration of heavy elements—but it is not a compact ball of rock.

Instead, its “core” is:
Diffuse rather than sharply defined

Spread over about half of Jupiter’s radius

Gradually blending into the surrounding hydrogen and helium

This is what scientists mean by a “fuzzy core.

” It’s as if the heavy elements that should form a solid centre are smeared out, mixed deeply into the planet’s interior rather than locked in a tight lump at the bottom.

This discovery created an immediate problem.

If gas giants form around solid cores, how did Jupiter’s core get so diluted?
If gas giants don’t need solid cores, why does Jupiter have any central concentration at all?
For a moment, theorists thought they had an answer.

The Giant Collision That Didn’t Quite Work
One of the first big ideas to explain the fuzziness was dramatic:
Maybe Jupiter used to have a solid core… until something smashed it.

In this scenario, a massive protoplanet—perhaps ten Earth masses or more—slammed into early Jupiter.

The impact would have shattered the solid core and stirred heavy elements upward, mixing them into the surrounding gas and creating the broad, diffuse core we see today.

Up to half of Jupiter’s core, some suggested, might even come from this intruding planet.

It was a neat theory.

Too neat.

When scientists ran detailed supercomputer simulations of such a collision, they ran into a problem.

The impact did mix the interior—for a while.

But after the shockwaves faded and Jupiter settled, the heavy materials slowly sank again, re-forming a dense core over time.

The models kept giving the same answer:
You might fuzz the core temporarily, but you can’t keep it that way for billions of years.

The collision explanation couldn’t fully match Juno’s data.

So researchers turned their attention somewhere else…
Right next door.

Saturn’s Rings Whisper the Same Secret
In 2014, astronomers discovered something extraordinary:
waves in Saturn’s rings are caused by oscillations inside Saturn itself, like quakes detected through floating ice.

By combining that “ring seismology” with gravitational data from the Cassini mission, scientists concluded that Saturn, too, has a fuzzy, diffuse core—not a sharp boundary between core and envelope.

Even more importantly, they found that Saturn’s deep interior doesn’t fully convect.

That means heat isn’t rising and mixing all the way through like boiling soup.

Instead, there’s a stable gradient, with heavy elements more concentrated toward the centre but gradually fading outward.

Sound familiar?
It’s exactly the kind of structure Juno is seeing in Jupiter.

Suddenly, the “giant collision” theory looked less like a one-off freak event and more like we were glimpsing a common feature of gas giant interiors.

A New Picture of Gas Giants Is Emerging
The emerging view is that gas giants like Jupiter and Saturn:
Do start with solid or icy cores

Do accrete thick envelopes of gas

But then undergo long-term mixing, diffusion, and partial stratification

Over billions of years, heavy elements may slowly dissolve into the hydrogen-helium envelope, creating a blurred compositional gradient.

This leads to:
No sharp core boundary

A “fuzzy” central region with higher heavy-element concentration

Layers that are partly stable and non-convective

This complexity challenges our simple models, which treated gas giants as neat two-layer systems: solid core + pure gas.

Now we know the truth is messier, richer, and far more dynamic.

And it has consequences—not just for Jupiter and Saturn, but for every gas giant exoplanet we’ve detected.

Their cooling rates, radii, and even observable atmospheres may all be influenced by deep fuzzy cores.

The Magnetic Field Puzzle: Fuzziness Makes It Harder, Not Easier
Jupiter’s magnetic field is a monster.

It’s: The strongest of any planet

Enormous—its magnetosphere can stretch as far as Saturn’s orbit

Weirdly lopsided, with strange spots and multiple magnetic “south poles”

We know it’s powered by a dynamo in the layer of liquid metallic hydrogen deep inside Jupiter.

But how the fluid motions actually generate such a complex field is still unclear.

Some scientists hoped the fuzzy core would explain Jupiter’s bizarre magnetism.

Instead, it made things more complicated.

Simulations suggest:
A stable, non-convective layer in the upper interior does help explain features of the observed field.

But a fully stable fuzzy core doesn’t match the field we see.

In other words:
The fuzzy core alone cannot explain Jupiter’s magnetism.

Now, researchers think Jupiter’s dynamo might depend on:
Subtle variations in density and conductivity

Complex flows where helium rains out of hydrogen

Multiple interacting layers, not one smooth convective region

One thing is clear: Jupiter’s interior is not simple, and its magnetic field reflects that wild complexity.

What we can enjoy with certainty, though, are the results—the brightest, most powerful auroras in the Solar System, blazing endlessly at Jupiter’s poles like cosmic neon storms.

What We Still Don’t Know… Yet
Despite Juno’s breakthroughs, big questions remain:
How exactly did Jupiter’s fuzzy core form—and how stable is it?

Will its heavy elements eventually re-settle into a sharper core, or is the fuzziness permanent?

How do diffuse cores affect the evolution and appearance of giant exoplanets?

Can we integrate fuzziness into a unified model of planet formation that works for all gas giants?

Every answer Juno gives us seems to open two new mysteries.

And that’s exactly what makes this so exciting.

The more precisely we probe Jupiter, the more alien it becomes—yet the more it reveals about worlds across the galaxy.

Our models weren’t wrong so much as incomplete.

Now, with fuzzy cores and wild magnetospheres in the mix, we’re finally starting to see just how complex “a ball of gas” can really be.