Bold claim first: Uranus and Neptune may be far more rocky or watery than we’ve long supposed, challenging the traditional ice-giant label.
New research from the University of Zürich suggests that these two outer planets could host interior compositions that aren’t limited to ice alone. Depending on the assumptions used, Uranus might be categorized as an ice giant or a rock giant, and the same ambiguity applies to Neptune. This work argues that the common classification is an oversimplification and that a more nuanced picture is possible.
The researchers describe an approach that blends two ends of the modeling spectrum. On one side are physics-based models that can be heavy with assumptions; on the other are empirical models that can be overly simplistic. By merging these, they built interior models that are both agnostic (unbiased) and physically consistent.
Here’s how they approached the problem. They began with a randomly generated density profile for the planets’ interiors. Next, they computed the gravitational field that would arise from that profile and checked how well it matched observational data. They then iterated the process to find models that best align with what we observe.
The result is a flexible framework that allows a broader range of interior possibilities. The study concludes that the internal makeups of Uranus and Neptune are not confined to ice alone; instead, both planets could be water-rich or rock-rich depending on the chosen model. This echoes a view the team first proposed about 15 years ago, now supported by a numerical method that demonstrates the idea more robustly.
The new perspective also touches the planets’ unusual magnetic fields. Unlike Earth’s relatively simple dipole field, Uranus and Neptune exhibit complex magnetic configurations with multiple poles. The researchers’ models include layers of “ionic water” that can drive magnetic dynamos in locations that account for the observed non-dipolar fields. They also find that Uranus’ magnetic field may originate deeper inside the planet than Neptune’s does.
Despite these promising insights, some uncertainties remain. A key challenge is our limited understanding of how materials behave under the extreme pressures and temperatures found in planetary interiors, which could influence the results. Nevertheless, the study opens up new interior scenarios and invites fresh material-science research under planetary conditions, while prompting a reevaluation of long-held assumptions.
Ultimately, both Uranus and Neptune could be rock giants or ice giants, depending on the model. With current data, it isn’t yet possible to definitively distinguish between the two. The authors advocate for dedicated missions to Uranus and Neptune to reveal their true nature.
A peer-reviewed paper describing this work is published in Astronomy & Astrophysics:
Luca Morf & Ravit Helled. 2025. Icy or rocky? Convective or stable? New interior models of Uranus and Neptune. A&A 704, A183; doi: 10.1051/0004-6361/202556911
Would you like this rewritten piece to emphasize more beginner-friendly explanations, or to lean into the provocative questions at the end for engagement?
