Jupiter, the largest planet in our solar system, is a gas giant dominated by hydrogen and helium, mirroring the Sun’s composition. Its immense size and powerful gravitational field suggest a complex interior structure, long a focus of planetary science. The central question has been the physical state of its innermost region: does Jupiter harbor a compact, Earth-like solid core, or something far stranger? Recent data from the Juno mission has provided a definitive, yet unexpected, answer, moving the discussion beyond a simple solid-or-liquid debate.
Jupiter’s Internal Structure: The Layered Giant
The traditional view of Jupiter’s structure imagined a distinct, layered planet, consistent with the long-standing core accretion theory of giant planet formation. Moving inward from the visible cloud tops, the outer atmosphere is a vast envelope of molecular hydrogen and helium gas. Beneath this gas, extreme pressures cause the hydrogen to transition into a liquid state, forming an enormous ocean of liquid molecular hydrogen.
Deeper still, at depths where the pressure is millions of times greater than Earth’s sea-level pressure, hydrogen atoms are squeezed so tightly that their electrons detach, creating an electrically conductive fluid. This forms the thick inner layer known as liquid metallic hydrogen, which generates Jupiter’s powerful magnetic field. The traditional model posited that beneath this metallic layer resided a dense, compact core of heavy elements, primarily rock and ice, accumulated during the planet’s formation.
This compact core was thought to represent the original “seed” that rapidly accreted the massive hydrogen and helium envelope. Its composition would consist of silicates, iron, and water ices compressed to extraordinary densities. While the immense temperature and pressure conditions might prevent the material from being conventionally “solid,” the boundary between this dense center and the surrounding metallic hydrogen was expected to be sharp and well-defined.
The Modern View: Defining the “Fuzzy” Core
The concept of a sharply defined, rocky center has been replaced by the idea of a “dilute” or “fuzzy” core, based on modern measurements. This fuzzy core is not a compact, discrete sphere but an extended region where heavy elements are blended with the surrounding liquid metallic hydrogen. It represents a gradual transition zone rather than a distinct boundary separating the core from the envelope.
In this region, the dense material—composed of silicates, iron, and various ices—is not centrally concentrated but appears dissolved or mixed into the lighter hydrogen and helium. This diffuse zone spans a vast area, potentially extending to nearly half of Jupiter’s total radius. While the heavy elements are most concentrated at the center, the proportion slowly decreases moving outward into the surrounding metallic hydrogen layer.
The total mass of heavy elements within this dilute core is estimated to be between 10 and 25 times the mass of Earth. This structure directly challenges classic planet formation models, which predicted a much smaller, compact core that should have remained intact. The existence of this mixed, diffuse core suggests a more complex formation history, possibly involving a massive impact early in the planet’s life or a gradual accretion process that continuously mixed materials.
Probing the Depths: The Role of the Juno Mission
The shift in understanding from a compact core to a fuzzy one is directly attributable to the precise data collected by NASA’s Juno spacecraft. Since entering Jupiter’s orbit in 2016, Juno has performed close, polar flybys, allowing for unprecedentedly accurate measurements of the planet’s gravitational field. The Gravity Science experiment on Juno tracks tiny variations in the spacecraft’s velocity—down to a fraction of a millimeter per second—as it orbits.
These minute changes in velocity are caused by subtle differences in Jupiter’s gravitational pull, which are determined by the internal distribution of mass. By mapping these variations, scientists could effectively “see” the density profile deep inside the planet. The gravity data revealed that the planet’s density changed far more gradually than a compact core model would predict.
The measurements indicated that the heavy elements were not confined to a small, central region, but were instead distributed outward, diluted and mixed into the surrounding layers. This confirmed the model of a dilute core that lacks a sharp interface with the overlying metallic hydrogen. The gravity data also unexpectedly showed that the atmospheric winds penetrate thousands of kilometers deep into the planet.