For decades, scientists have believed that Uranus and Neptune are “ice giants,” planets made mostly of icy materials like water, ammonia and methane.
But a new study from the University of Zurich and the NCCR PlanetS challenges this long-standing assumption.
According to the research team, the two distant planets might actually contain far more rock than previously thought, making them much less icy on the inside.
Traditionally, the solar system is grouped into three types of planets: the rocky planets like Earth and Mars, the gas giants like Jupiter and Saturn, and the ice giants—Uranus and Neptune.
The icy label comes from the belief that these planets formed far from the sun, where frozen materials were more common.
But the new study, published in Astronomy & Astrophysics, suggests that this classification may be too simple. Uranus and Neptune might be rock-rich instead of ice-rich, or possibly somewhere in between.
The research team developed a new way to simulate the planets’ interiors.
Instead of relying heavily on assumptions, as many traditional physics-based models do, or oversimplifying the problem with purely observational models, the team created a hybrid approach.
They began by generating random internal density profiles for each planet. Then they calculated how each profile would affect the planet’s gravitational field and compared those predictions with real spacecraft data.
By repeating this process many times, they created models that were both unbiased and physically realistic.
Their results revealed a wide range of possible interior compositions for Uranus and Neptune. Rather than being limited to water-rich models, the planets could contain large amounts of rock.
This idea lines up with other discoveries, such as the finding that Pluto—another distant, icy-looking world—is actually dominated by rock.
The study also provides new insights into the strange magnetic fields of Uranus and Neptune. Unlike Earth, which has two clearly defined magnetic poles, both planets have complex magnetic fields with multiple poles.
According to the new models, layers of “ionic water”—a form of water that exists under extreme pressure—may create magnetic dynamos deep inside the planets. The researchers also found that Uranus’s magnetic field might originate from a deeper layer than Neptune’s, helping explain why the planets have such unusual magnetic behavior.
Even with these advances, some uncertainty remains. Scientists still do not fully understand how materials behave under the extreme pressures and temperatures found deep inside giant planets. These unknowns could influence the accuracy of the models. The team hopes to expand their work as new laboratory data become available.
Despite the challenges, the study opens up an entirely new set of possibilities for understanding Uranus and Neptune.
The researchers say that to truly solve the mystery, new space missions are needed. Current data are not detailed enough to determine whether these planets are mostly rock or mostly ice. Future spacecraft could finally reveal what lies beneath their beautiful blue clouds—and reshape our understanding of the outer solar system.
