Our fundamental understanding of Neptune and Uranus may be completely wrong, say researchers at the University of Zurich, whose new work suggests that an entire category of planets in our solar system could be eliminated.
Our solar system is presently divided between three categories: terrestrial rocky planets, gas giants, and ice giants. According to the new research, Neptune and Uranus, currently recognized as the two ice giants in our planetary neighborhood, may not be so icy, after all.
Rethinking the Solar System
Mercury, Venus, Earth, and Mars make up the rocky planets, while Jupiter and Saturn are classified as gas giants. The possibility that Neptune and Uranus are instead rocky giants is consistent with recent findings that the dwarf planet Pluto also appears to be of a rocky constitution. Still, the Zurich team emphasizes that their results only suggest this possibility; they do not offer definitive proof.
To investigate, the researchers used computer simulations to probe the interiors of Neptune and Uranus—but approached the problem in an unconventional way. Rather than relying solely on traditional physical models or purely empirical ones, the team developed a hybrid method designed to strike a balance between the two extremes.
“The ice giant classification is oversimplified as Uranus and Neptune are still poorly understood,” said lead author Luca Morf, a PhD student at the University of Zurich. “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.”
Running the Experiment
The team’s models were run many times, exploring various random density profiles to describe the planets’ interiors. Additionally, the team calculated planetary gravitational fields for these profiles that matched observational data from the planets, thereby enabling them to infer a likely material composition. At the conclusion of their runs, the team identified the best fit between the simulated model and real-world data.
The results revealed something at odds with the traditional understanding of Uranus and Neptune. Instead of being composed solely of water ice, the data suggested that the planets could be primarily composed of water or rock.
“It is something that we first suggested nearly 15 years ago, and now we have the numerical framework to demonstrate it,” said co-author Ravit Helled, Professor at the University of Zurich.
Magnetic Uranus and Neptune
Beyond describing the compositions of Uranus and Neptune, the work also offers a fresh look at their unusual magnetic fields. Unlike Earth, which has only two opposite poles, both planets exhibit complex magnetism that generates 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,” Helled explained. “We also found that Uranus’ magnetic field originates deeper than Neptune’s.”
“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 added.
Future Solar System Exploration
“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,” Helled said.
The Zurich team plans to refine its model as new data becomes available. While the results highlight how much remains to be learned about the solar system’s outer planets, only future spacecraft capable of directly probing their atmospheres and gravitational fields will ultimately confirm their true composition.
The team’s recent paper, “Icy or Rocky? Convective or Stable? New Interior Models of Uranus and Neptune,” appeared in Astronomy and Astrophysics on December 5, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.