Uranus and Neptune: Rockier Than Thought?

A groundbreaking new study from the University of Zurich suggests that Uranus and Neptune, long categorized as "Ice Giants," may actually possess significantly more rock in their interiors than previously assumed. This research, published recently in Astronomy & Astrophysics, challenges decades-old assumptions about the composition of these distant worlds, as reported by scitechdaily on December 13, 2025.

The findings could revolutionize how astronomers classify planets both within and beyond our solar system, offering fresh insights into planetary formation and evolution. According to Space Daily on December 11, 2025, the study indicates that these planets can be consistent with either ice-rich or rock-rich internal structures, broadening our understanding.

Lead author Luca Morf, a PhD student at the University of Zurich, explained that the traditional "ice giant" classification is oversimplified, given how poorly understood Uranus and Neptune remain. Morf noted that previous models were either too assumption-heavy or too simplistic, as quoted by Sci.News on December 11, 2025.

Professor Ravit Helled, who initiated the project at the University of Zurich, highlighted that their new modeling framework demonstrates that the internal composition is not limited to ice. Helled stated that the planets could be either water-rich or rock-rich, a possibility she first suggested nearly 15 years ago, according to uzh News on December 10, 2025.

This re-evaluation stems from a novel "agnostic" modeling approach developed by the Zurich team, which combines physically motivated equations with flexible numerical procedures. scitechdaily reported on December 13, 2025, that this method allowed researchers to generate a wide range of internal density profiles.

The study's implications extend to understanding the puzzling magnetic fields of both planets, which differ significantly from Earth's. ScienceDaily noted on December 10, 2025, that the models include layers of ionic water that could explain these complex, non-dipolar magnetic field geometries.

Ultimately, the research underscores that current observational data on mass, radius, and gravity are insufficient to definitively determine the precise internal makeup. As SWI swissinfo.ch reported on December 10, 2025, Professor Helled emphasized the need for dedicated missions to Uranus and Neptune to uncover their true nature.

• Challenging Decades-Old Classification: For generations, Uranus and Neptune have been classified as "ice giants," implying their interiors are predominantly composed of water, ammonia, and methane ices. This classification distinguishes them from the rocky inner planets and the gas giants like Jupiter and Saturn, as Universe Space Tech explained on December 11, 2025. The new study from the University of Zurich directly challenges this long-held assumption, suggesting a much greater proportion of rock is plausible.

• Innovative Modeling Methodology: The research team, led by Luca Morf and Professor Ravit Helled, developed a unique "agnostic" modeling framework to investigate the planets' internal structures. This approach avoids predefined layering or simplified empirical density profiles, instead generating numerous internal density profiles and comparing them against observed gravitational moments. sci.news reported on December 11, 2025, that this method allows for a more unbiased and physically consistent exploration of possible compositions.

• Implications for Planetary Formation: A rock-dominated composition for Uranus and Neptune would necessitate a revision of current models for planet formation in the outer Solar System. If these planets can host a large rock fraction, the understanding of how icy and rocky material accumulated in the protoplanetary disk may need to be re-evaluated. Space Daily highlighted on December 11, 2025, that this could significantly alter theories on how such massive planets acquire their building blocks.

• Impact on Exoplanet Categorization: The findings have significant implications for the study of exoplanets, particularly those with similar masses and radii to Uranus and Neptune. If our own "ice giants" can vary so widely in their rock-to-water ratios, then comparable-sized exoplanets could also possess a broad spectrum of internal makeups. This means planets with similar bulk properties might have very different internal compositions, complicating their classification and understanding, as noted by SciTechDaily on December 13, 2025.

• Explaining Magnetic Field Anomalies: The study also offers new perspectives on the peculiar magnetic fields of Uranus and Neptune, which are known for their complex, non-dipolar, and strongly tilted configurations, unlike Earth's relatively simple magnetic field. The models developed by the Zurich team include layers of ionic water that can generate magnetic dynamos in locations consistent with these observed non-dipolar fields. Professor Helled stated that Uranus's magnetic field appears to originate deeper than Neptune's, according to sci.news on December 11, 2025.

• Need for Future Missions: Despite the advanced modeling, the researchers emphasize that current observational data are insufficient to definitively distinguish between a rock-rich or ice-rich interior. Professor Ravit Helled concluded that dedicated space missions to Uranus and Neptune are essential to gather the necessary data to resolve these compositional ambiguities and unlock their true nature, as reported by ZME Science on December 12, 2025.