The composition of Uranus and Neptune may be less icy than previously thought, according to a new study by scientists at the University of Zurich.
Uranus could be an ice giant (left) or a rock giant (right), depending on the model's assumptions. Image credit: Keck Institute for Space Studies/Chuck Carter.
“The classification of ice giants is too simplistic because Uranus and Neptune are still poorly understood,” said Luca Morf, Ph.D. student at the University of Zurich.
“Physics-based models were too loaded with assumptions, and empirical models were too simplistic.”
“We combined both approaches to produce interior models that are both 'agnostic' or unbiased, yet physically consistent.”
In their study, the scientists started with a random density profile of the planet's interior.
They then calculated the planetary gravitational field consistent with the observations and inferred the possible composition.
Finally, they repeated the process to get the best fit between the models and the observed data.
Using their new agnostic yet fully physical model, scientists have discovered that the potential internal composition of our solar system's ice giants is not just limited to ice.
“This is something we first proposed almost 15 years ago, and we now have the numerical basis to demonstrate it,” said University of Zurich professor Ravit Helled.
“The new range of internal composition shows that both planets could be either water-rich or rock-rich.”
The research also opens new perspectives on the mysterious magnetic fields of Uranus and Neptune.
While Earth has clear North and South magnetic poles, the magnetic fields of Uranus and Neptune are more complex and have more than two poles.
“Our models have so-called 'ionic water' layers that generate magnetic dynamos at locations that explain the observed non-dipolar magnetic fields,” Professor Helled said.
“We also discovered that Uranus' magnetic field has a deeper origin than Neptune's.”
Although the results are encouraging, some uncertainty remains.
“One of the main problems is that physicists still barely understand how materials behave under the exotic conditions of pressure and temperature that exist at the center of the planet, and this could affect our results,” Morf said.
Despite the uncertainties, the new results also open the way to a new potential internal composition scenario, challenge decades-old assumptions and guide future research into materials in planetary environments.
“Both Uranus and Neptune could be rock giants or ice giants, depending on the model's assumptions,” Professor Helled said.
“Current data is currently insufficient to distinguish between these two phenomena, and therefore we need dedicated missions to Uranus and Neptune that can reveal their true nature.”
A paper a description of this study was published this week in the journal Astronomy and astrophysics.
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Luka Morf and Ravit Helled. 2025. Icy or rocky? Convective or stable? New interior models of Uranus and Neptune. A&A 704, A183; two: 10.1051/0004-6361/202556911
