New Research Challenges the 'Ice Giant' Classification of Uranus and Neptune

The long-held classification of Uranus and Neptune as 'ice giants,' primarily composed of water, methane, and ammonia, is now being called into question by a novel study. Published in December 2025 in the journal Astronomy & Astrophysics, the research presents findings that suggest these outer planets might possess a significantly more rocky composition than previously assumed. Scientists from the University of Zurich (UZH), doctoral candidate Luca Morf and Professor Ravit Heled, spearheaded this investigation. This potential shift in understanding carries considerable weight for planetary formation models, especially considering that a vast number of known exoplanets share similar sizes with Uranus and Neptune.

For decades, these planets, situated beyond the solar system's gas giants, were categorized as ice giants based more on theoretical assumptions than on extensive empirical evidence. Direct observation of these distant worlds has been sparse; only the Voyager 2 probe provided close-up data during its flybys in 1986 and 1989. The UZH researchers employed a new modeling approach described as compositionally agnostic. This methodology allowed them to generate thousands of random density profiles, selecting only those that aligned with the actual data gathered by Voyager 2. This contrasts sharply with earlier models that often imposed strict layered structures or relied on simplified empirical profiles.

The resulting best-fit compositional models strongly imply that these planets could be predominantly rocky. The analysis indicates that the rock-to-water mass ratio for Uranus could be nearly ten times greater than that of Neptune, highlighting an internal divergence between the two planets. This more rock-heavy interpretation aligns reasonably well with the known composition of Pluto, a Kuiper Belt Object, which is understood to be approximately 70 percent rock and metal. Furthermore, the range of permissible models for Uranus spans a hundredfold difference in its rock-to-water mass ratio, stretching from as low as 0.04 up to nearly 4.

The new models also offer a plausible explanation for the chaotic, multipolar magnetic fields observed on both planets. The research team determined that layers of 'ionic water' situated at various depths could independently generate magnetic dynamos. This mechanism accounts for the non-dipolar field geometries, unlike Earth's relatively straightforward dipole field. Professor Heled pointed out that the study suggests Uranus’s magnetic field originates deeper within its interior compared to Neptune’s. Nevertheless, the researchers caution that substantial uncertainties persist, largely due to the incomplete understanding of how materials behave under the extreme internal pressures and temperatures found there.

Professor Heled stressed that the current dataset is insufficient to definitively settle the debate between rocky or icy giant classifications, underscoring the need for dedicated missions to uncover their true internal architectures. Future exploration remains a high priority for space agencies worldwide. NASA’s Uranus Orbiter and Probe (UOP) concept is listed as a highest-priority Flagship-class mission according to the 2023–2032 Decadal Survey, although its projected launch date has been pushed back to the mid-to-late 2030s due to plutonium production shortfalls. Meanwhile, China is preparing its Tianwen-4 mission, which includes a Uranus flyby tentatively scheduled for around March 2045, following a launch planned for approximately 2030.