For decades, Uranus and Neptune have been grouped together in astronomy textbooks as “ice giants,” a category used to distinguish them from the gas giants Jupiter and Saturn. The label was based on the idea that, beneath their thick atmospheres of hydrogen and helium, these distant worlds are dominated by “ices” such as water, ammonia, and methane in frozen or high-pressure fluid states.
However, new research is now challenging that simple classification, suggesting that the interiors of both planets may contain far more rock-rich material than previously assumed. If confirmed, the findings could reshape how scientists understand not only Uranus and Neptune, but also how giant planets form across the universe.
The study, led by researchers including Yamila Miguel of the Netherlands Institute for Space Research, used advanced computer modelling to simulate the extreme pressures and temperatures deep inside the two planets. Instead of relying solely on earlier indirect interpretations, the team explored how different materials—rock, ice, hydrogen, and helium—behave under conditions found thousands of kilometres below the cloud tops.
Their results suggest that the outer shells of Uranus and Neptune may be composed largely of rocky material mixed with hydrogen and helium gas, rather than being dominated by icy compounds as traditionally believed. Miguel explained that this interpretation runs counter to the long-standing assumption that the planets are primarily “icy worlds,” noting that both planets may have significantly more rock-forming elements in their internal structure than previously thought.
The research was partly motivated by comparisons with objects in the Kuiper Belt, such as Pluto and comets, which are known to contain a high proportion of rocky material. Scientists involved in the study questioned whether Uranus and Neptune might have formed from similar building blocks in the early Solar System, rather than from ice-heavy material alone. This led them to test whether rock-rich compositions could realistically explain the planets’ observed properties today.
Using simulations, the team examined how silicate and other rock-forming materials behave under extreme compression. Under such conditions, materials can change phase dramatically, potentially forming dense layers that would not be obvious from surface-level observations. The models indicate that such processes could allow rocky material to play a much larger role in the planets’ interiors than previously accounted for.
Despite these findings, researchers stress that the study does not completely overturn the idea that Uranus and Neptune contain large amounts of “ice” components. Instead, it suggests a more mixed and complex internal structure, where rock, ice, and gases are all present in varying proportions. Miguel noted that while both planets may still contain significant quantities of icy material, they are unlikely to be “pure ice worlds” in the way they were once described.
The study has also revived discussion about whether the term “ice giant” itself is misleading. Some researchers argue that a revised classification perhaps focusing on planetary mass, composition, or formation history rather than simplified categories may better reflect what is now being discovered. Miguel even suggested that labels such as “minor giants” could be more accurate, though such terminology has not been adopted in formal planetary science.
Importantly, Uranus and Neptune have not been officially reclassified. Planetary scientists will need further observational data, likely from future space missions, to confirm the internal structures suggested by these models. Direct exploration of these planets remains limited, meaning much of their composition is still inferred rather than measured.
Still, the study highlights how planetary science continues to evolve as modelling techniques improve. Uranus and Neptune, despite being part of our own Solar System, remain among its least understood worlds. Each new piece of research adds to a growing picture of planets that are far more complex—and perhaps very different from early assumptions—than previously believed.
