A team of researchers from the University of Zurich and the NCCR PlanetS is challenging our understanding of the Solar System planets interior. The composition of Uranus and Neptune, the two outer most planets, might be more rocky and less icy than previously thought.

The planets in the Solar System are typically divided into three categories based on their composition: the four terrestrial rocky planets (Mercury, Venus, Earth and Mars), followed by the two gas giants (Jupiter and Saturn), and finally two ice giants (Uranus and Neptune). According to the work carried by the UZH scientific team, Uranus and Neptune might actually be more rocky than icy. The new study does not claim the two blue planets to be one or the other type, water- or rock- rich, it rather challenges that ice-rich is the only possibility. This interpretation is also consistent with the discovery that the dwarf planet Pluto is rock-dominated in composition.

The team developed a unique simulation process for the interior of Uranus and Neptune. “The ice giant classification is oversimplified as Uranus and Neptune are still poorly understood,” explains Luca Morf, PhD student at the University of Zurich and lead author of the study. “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 “agnostic” or unbiased and yet, are physically consistent.” To do so, they first start with a random density profile for the planetary interior. Then they calculate the planetary gravitational field that is consistent with observational data and infer a possible composition. Finally, the process is repeated to obtain the best possible match between the models and the observational data.

A whole new range of possibilities

With their new agnostic, and yet fully physical model, the University of Zurich team found the potential internal composition of the “ice giants” of our Solar system, is not limited at all to only ice (typically represented by water). “It is something that we first suggested nearly 15 years ago, and now we have the numerical framework to demonstrate it” reveals Ravit Helled, Professor at the University of Zurich, and initiator of the project. The new range of internal composition shows that both planets can either be water-rich or rock-rich.

The study also brings new perspectives on Uranus’ and Neptune’s puzzling magnetic fields. While the Earth has clear North and South magnetic poles, the magnetic fields of Uranus and Neptune are more complex having more than two poles. “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,” explains Ravit Helled.

The need for new space missions

While the results are promising, some incertitude remains. “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,” says Luca Morf who plans to expand the models in the future.

Despites the uncertainties, the new results also pave the way for new potential interior composition scenario and challenge decade-old assumptions and guide future material science research at planetary conditions. “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”, concludes Ravit Helled.

Literature
Luca Morf and Ravit Helled. Icy or rocky? Convective or stable? New interior models of Uranus and Neptune. Astronomy & Astrophysics, 10. December 2025. Doi: 10.1051/0004-6361/202556911