Gas giants almost certainly have cores enriched in heavy elements, but those cores are probably not the compact, solid rocky balls that textbooks once depicted. The strongest evidence comes from NASA’s Juno spacecraft, which measured Jupiter’s gravitational field with enough precision to show that the planet’s core is likely “dilute,” meaning heavy elements are spread out over a surprisingly large fraction of the interior rather than packed into a tight, well-defined lump. The reality inside a gas giant turns out to be messier and more interesting than the neat layered diagrams suggest.
What Scientists Originally Expected
The leading explanation for how gas giants form, known as core accretion, starts with a rocky or icy seed. Small solid particles in a young solar system clump together into a body roughly ten times Earth’s mass. Once that embryonic core reaches a critical threshold, its gravity becomes strong enough to pull in enormous quantities of hydrogen and helium gas from the surrounding disk, and the planet balloons into a gas giant. This framework naturally predicts a dense, compact core sitting at the center of the finished planet, surrounded by layer after layer of progressively lighter material.
For decades that prediction was untestable. You cannot drill into Jupiter. Ground-based telescopes could measure the planet’s overall size and rough density, but not the arrangement of mass inside it. Scientists built models that were consistent with external observations, and many of those models included a neat solid core of rock and ice weighing somewhere between five and twenty Earth masses. The picture was tidy, widely reproduced, and only loosely constrained by data.
What Juno Actually Found at Jupiter
When Juno arrived at Jupiter in 2016 and began flying extremely close to the cloud tops, it measured the planet’s gravitational field to unprecedented accuracy. Tiny variations in Juno’s speed, tracked by radio signals back to Earth, revealed how mass is distributed beneath the clouds. The results were surprising. Models built on those gravity measurements show that Jupiter’s interior fits better if the core is not a discrete solid sphere but rather a region where heavy elements gradually thin out, extending across a large fraction of the planet’s radius.1Geophysical Research Letters. Comparing Jupiter interior structure models to Juno gravity measurements and the role of a dilute core
This kind of structure is often called a “fuzzy” or “dilute” core. Instead of a sharp boundary between a solid center and a hydrogen-helium envelope, there is a gradual transition zone where the concentration of rock, metal, and ice fades outward. The total mass of heavy elements might still be comparable to what earlier models predicted, but the material is smeared over a much larger volume. Think of it less like a peach pit inside a peach and more like sugar dissolved unevenly in a glass of water, thicker at the bottom but with no clean line between sweetened and unsweetened.
Why a Solid Core Might Not Survive
One reason a compact core could vanish over billions of years is that materials we think of as solids on Earth behave very differently at the pressures found deep inside a gas giant. Jupiter’s central pressure is estimated at roughly 40 to 70 million times the atmospheric pressure at Earth’s surface. Under those conditions, hydrogen itself becomes metallic, a dense, electrically conducting fluid. And that fluid turns out to be a surprisingly good solvent.
Simulations using first-principles physics calculations have shown that water ice, one of the major expected core ingredients, is highly soluble in liquid metallic hydrogen under conditions matching the core-envelope boundary inside Jupiter and Saturn.2arXiv. Solubility of water ice in metallic hydrogen: consequences for core erosion in gas giant planets If the core is literally dissolving into the surrounding fluid over geological time, even a planet that started with a compact solid core could end up with a dilute one after four and a half billion years. The heavy elements would gradually mix upward, eroding the original core and producing exactly the kind of blurred interior that Juno’s measurements suggest.
The dissolution is not limited to ice. Separate calculations at extreme pressures show that heavier noble gases like argon, krypton, and xenon are also soluble in liquid metallic hydrogen, while helium and neon tend to separate out.3arXiv. Noble-Gas Solubility in Solid and Fluid Metallic Hydrogen The deep interior of a gas giant is a chemically active environment, not a passive container holding inert layers in place. Over billions of years, the mixing and separation of different elements reshapes the planet’s internal structure in ways that the original formation process did not determine once and for all.
The Trouble With Jupiter’s Magnetic Field
A dilute core does not just change the picture of what sits at the center of the planet. It also affects how the planet generates its magnetic field. Gas giants produce magnetic fields through dynamo action: electrically conducting fluid moving in convective currents creates and sustains a powerful field. The structure of that field, its shape, symmetry, and strength, depends on where convection happens and where it is suppressed.
Dynamo simulations have shown that a Jupiter with an extended, stably stratified dilute core has difficulty matching the planet’s observed magnetic field and wind patterns. If the dilute core suppresses convection across too large a region, the simulated magnetic field looks wrong. Researchers have found that reconciling a dilute core with the real magnetic data may require the dilute region to be convective itself, or for the interior to have a more complicated multilayered structure than a simple smooth gradient.4Journal of Geophysical Research: Planets. Dynamo Simulations of Jupiter’s Magnetic Field: The Role of Stable Stratification and a Dilute Core In other words, the interior is probably not just “fuzzy” in a simple, uniform way. There may be complicated layers of mixing and stratification whose details are still being worked out.
This tension between the gravity data and the magnetic field data is one of the more productive puzzles in planetary science right now. Each dataset constrains the interior differently, and making a single model that satisfies both is hard. That difficulty is itself evidence that gas giant interiors are more structured and dynamic than any simple cartoon of concentric shells can capture.
Uranus and Neptune Are a Different Story
Jupiter and Saturn are hydrogen-helium dominated, but Uranus and Neptune belong to a different category: ice giants. Their bulk composition is thought to include a much higher fraction of water, ammonia, and methane relative to hydrogen and helium. The question of whether they have solid cores takes on a different character, because so much of their interior may be made of substances that are in exotic physical states rather than clearly “solid” or “liquid.”
At the pressures inside Uranus and Neptune, water enters a phase called superionic ice. In this state, oxygen atoms lock into a crystal lattice while hydrogen nuclei flow freely through it like a liquid. It is simultaneously solid and fluid depending on which atom you follow. Calculations predict that superionic water transitions between different crystal structures at pressures relevant to ice giant interiors, with consequences for how heat moves through the planet.5PubMed. Superionic to superionic phase change in water: consequences for the interiors of uranus and neptune
Modeling Uranus’s interior adds further complexity. Recent work has found that the planet’s interior is not fully convective, meaning heat does not circulate freely from center to surface the way it does in many simple models. Non-adiabatic regions, where the temperature profile does not follow the pattern you would get from free convection, appear to be an important part of how Uranus is structured. Including those regions changes the inferred temperature and composition throughout the planet.6Astronomy & Astrophysics. Uranus’s complex internal structure Whether Uranus has a compact rocky core, a diffuse enrichment zone, or something more exotic remains an open question. It is one of the strongest scientific motivations for a dedicated mission to an ice giant, a priority that planetary scientists have been advocating for years.
Does “Solid” Even Mean What You Think at These Pressures?
Part of the confusion around gas giant cores comes from the word “solid” itself. At room temperature and normal pressure, you can draw a clean line between a rock and a gas. Deep inside a gas giant, those boundaries blur. Hydrogen becomes a metallic fluid. Water becomes a crystal with flowing protons. Rock and ice dissolve into metallic hydrogen the way salt dissolves into water. Asking whether the core is “solid” almost stops being a meaningful question, because the materials involved are in phases that do not map onto the everyday categories of solid, liquid, and gas.
What scientists really want to know is how the heavy elements, everything heavier than helium, are distributed. Are they concentrated in a small central region? Spread out? Layered in ways that affect convection and magnetic field generation? The answer to “is there a solid core” depends entirely on what you mean by solid. If you mean a distinct, rigid, well-defined object at the center, the evidence for Jupiter leans toward no. If you mean a region enriched in heavy elements that is denser than the surrounding material, the answer is almost certainly yes, though it may be fuzzy, extended, and partially dissolved rather than sitting there like a billiard ball.
Exposed Cores Found Around Other Stars
Some of the most striking evidence about gas giant cores comes from exoplanets. In 2020, astronomers announced the discovery of TOI-849b, a planet orbiting a distant star that appears to be the exposed core of a gas giant. It weighs roughly 40 times as much as Earth but is only about 3.4 times Earth’s radius, far too dense to have a thick hydrogen-helium envelope. Any remaining gas makes up at most a few percent of the planet’s mass. The leading interpretations are that TOI-849b either lost its atmosphere through some stripping process or never managed to accumulate one in the first place. Either way, it provides a rare chance to study the kind of massive, heavy-element-rich body that might sit at the heart of a gas giant, except this one is not buried under thousands of kilometers of hydrogen.
At the opposite extreme are the so-called “super-puffs,” planets that have masses comparable to Neptune but radii closer to Jupiter, giving them absurdly low densities. WASP-193b and TOI-1420b are two recent examples. WASP-193b belongs to a class of planets whose extremely low bulk densities are difficult to explain with standard core accretion models.7The Astronomical Journal. The Super-puff WASP-193 b is on a Well-aligned Orbit TOI-1420b has a density of only about 0.08 grams per cubic centimeter, less than a tenth the density of water, and its envelope appears to make up more than 80 percent of the planet’s mass. Its core, if you can call it that, was probably no more than four or five Earth masses when it triggered runaway gas accretion.8The Astronomical Journal. TESS Spots a Super-puff: The Remarkably Low Density of TOI-1420b
These exoplanet discoveries expand the range of what a gas giant core can look like. TOI-849b suggests that cores can be massive, dense, and resilient enough to survive atmospheric loss. Super-puffs suggest that some gas giants may have formed around remarkably small cores. The diversity is enormous, and it makes the assumption that all gas giants share a similar internal layout much harder to defend.
Why Saturn Might Be Different From Jupiter
It is tempting to assume that Saturn, as the solar system’s other gas giant, has the same basic interior as Jupiter scaled down. But there are reasons to think otherwise. Saturn’s lower mass means lower internal pressures, which could slow or alter the rate of core erosion. Saturn also has its own set of gravitational measurements from the Cassini mission, and those data suggest a somewhat different internal structure. Some models favor a more compact core for Saturn than for Jupiter, while others still allow a dilute region.
Saturn has another oddity: its helium abundance in the upper atmosphere is noticeably depleted compared to Jupiter and compared to the solar ratio. The leading explanation is that helium becomes immiscible in hydrogen at certain pressures and temperatures, forming droplets that rain downward into the deeper interior. This helium rain process adds energy and material to the deep interior in a way that Jupiter may not experience to the same degree. It could affect the thermal state of the core region and how heavy elements are distributed, making Saturn’s internal evolution genuinely different from Jupiter’s even though both planets started from broadly similar raw materials.
What a Mission to an Ice Giant Could Settle
Jupiter and Saturn have both been visited by orbiting spacecraft that provided detailed gravity and magnetic field measurements. Uranus and Neptune have only been seen up close once each, during Voyager 2’s brief flybys in the 1980s. The data from those encounters were revolutionary at the time but are too limited to constrain modern interior models in the way Juno has done for Jupiter.
A dedicated orbiter around Uranus or Neptune could measure gravitational harmonics, map magnetic field geometry, and detect seismic-like oscillations that reveal internal density boundaries. Ice giants are particularly interesting for the core question because their heavy-element fractions are so much larger than Jupiter’s or Saturn’s. Whatever core they have is not just a small seed buried under an ocean of hydrogen; it may constitute a major fraction of the planet. Understanding whether that material is concentrated, layered, or mixed into exotic high-pressure phases would fill in a part of the story that Jupiter and Saturn alone cannot tell.
The physics of superionic ice, the possibility of stable stratification preventing convection, and the unexplained tilt of Uranus’s magnetic field all converge on the same gap in knowledge. The interior of an ice giant is where some of the most extreme and least understood states of matter in the solar system exist, and the only way to probe them is to go there with instruments sensitive enough to map the gravity and magnetic fields from orbit.