Uranus and Neptune are hiding something big beneath the blue

To better understand what lies inside Uranus and Neptune, the researchers created a specialized simulation technique. "The ice giant classification is oversimplified as Uranus and Neptune are still poorly understood," says Luca Morf, PhD student at the University of Zurich and lead author of the work. "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."

The process begins with a randomly generated density profile representing the interior of each planet. The team then determines the gravitational field that would match observational measurements and uses that information to infer the possible composition. The cycle is repeated until the model best fits all available data.

Expanding the Range of Possible Interiors

Using this unbiased and physics-grounded approach, the researchers found that the interior makeup of the Solar System's so-called ice giants is not restricted to ice (commonly interpreted as water). "It is something that we first suggested nearly 15 years ago, and now we have the numerical framework to demonstrate it," says Ravit Helled, Professor at the University of Zurich and initiator of the project. Their results show that either planet could be dominated by water-rich layers or by a far rockier structure.

The findings also offer new insight into the unusual magnetic fields of Uranus and Neptune. Earth's magnetic field features two well-defined poles, but the fields of these distant planets are more irregular and include multiple poles. According to Helled, "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."

Why Future Missions Are Essential

Although the study provides promising new interpretations, uncertainties remain. "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," explains Morf, who intends to extend the modeling work.

Even with the remaining unknowns, the results open the door to new interior scenarios, challenge long-standing assumptions, and highlight important gaps in material science 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.