A new study from the University of Zurich challenges the traditional view of Uranus and Neptune as ice giants, suggesting they could be dominated by rock. Researchers developed a hybrid modeling approach to explore possible interior structures. The findings also shed light on the planets' irregular magnetic fields.
The Solar System's classification groups planets by composition, with Uranus and Neptune long considered ice giants due to their distant, blue atmospheres. However, a team at the University of Zurich has proposed that these worlds might contain significantly more rock than previously thought. Led by PhD student Luca Morf and Professor Ravit Helled, the research questions the assumption of ice-heavy interiors as the sole explanation supported by data.
To investigate, the scientists created an innovative simulation combining physics-based and empirical models. This 'agnostic' approach starts with a random density profile for each planet's interior, then adjusts it to match observed gravitational fields and infer compositions. The process iterates until the model aligns with all available measurements. 'The ice giant classification is oversimplified as Uranus and Neptune are still poorly understood,' Morf explained. 'Models based on physics were too assumption-heavy, while empirical models are too simplistic. We combined both approaches to get interior models that are both unbiased and yet physically consistent.'
The results indicate that either planet could feature water-rich layers or a rockier structure, broadening the range of possible interiors. This aligns with observations of Pluto, a dwarf planet dominated by rock. The study also addresses the planets' bizarre magnetic fields, which lack Earth's clear bipolar pattern and instead show multiple poles. Helled noted, 'Our models have so-called ionic water layers which generate magnetic dynamos in locations that explain the observed non-dipolar magnetic fields. We also found that Uranus' magnetic field originates deeper than Neptune's.'
Despite these insights, uncertainties persist, particularly in how materials behave under extreme planetary pressures and temperatures. 'One of the main issues is that physicists still barely understand how materials behave under the exotic conditions of pressure and temperature found at the heart of a planet, this could impact our results,' Morf said. Helled emphasized the need for future missions: 'Both Uranus and Neptune could be rock giants or ice giants depending on the model assumptions. Current data are currently insufficient to distinguish the two, and we therefore need dedicated missions to Uranus and Neptune that can reveal their true nature.'
The work was published in Astronomy in 2025.
