New findings shake up the ‘ice giant’ label
For decades, scientists have labeled Uranus and Neptune as the solar system’s “ice giants,” a description rooted in early models of planetary formation. Those models imagined vast interiors rich in water, ammonia, and other volatile ices that would freeze under the frigid conditions of the outer solar system. But a new line of research is prompting a rethink: our understanding of what lies inside these distant worlds is far more uncertain than we believed.
Unlike Jupiter and Saturn, which benefited from dedicated missions such as Cassini and Juno, Uranus and Neptune have not hosted a dedicated spacecraft in over 30 years. This means we rely on indirect clues—magnetic fields, atmospheric features, and the subtle gravitational tugs of their moons—to infer their inner structure. To date, those clues have yielded a picture that is robust in some aspects but tantalizingly ambiguous in others.
A radical approach to interior modeling
A recent pre-print in Astronomy & Astrophysics took an unusual approach. Rather than building a single model anchored to specific assumptions about composition, researchers generated a large ensemble of random interior models for Uranus and Neptune. They then cross-checked these models against a wide array of observational constraints, creating a database of all interior configurations that could plausibly fit what we see from Earth and orbiting observatories.
The results offered two expected points: both planets appear to have less than a quarter of their mass in hydrogen and helium, and both harbor layers of electrically conductive material, consistent with magnetic field generation. These elements align with some solar-system formation ideas and the measured densities of Uranus and Neptune. Yet the study also delivered a startling implication: the data do not strongly constrain what lies between the surface and core. In other words, our confidence in the exact internal arrangement is weaker than commonly assumed.
From ice to rock? What the models imply
One of the most striking findings concerns the rock-to-water ratio. Uranus could, depending on the model, be almost as watery as it is rocky, with a rock-to-water ratio ranging from 0.04 (mostly water) up to 3.92 (dominated by rock). For Neptune, the possibilities are similarly broad—anywhere from five times as much water as rock to twice as much rock as water. Such a wide spread means there is no unique, well-defined interior profile to pin down with current data.
Given these uncertainties, the term “ice giants” may be a misnomer. If a substantial portion of the bulk is rocky, these planets could be considered more accurately as “rocky giants” in the sense that their interiors may be more rock-dominated than previously thought. That doesn’t mean they are dry or sunless, but it does upend a neat, ice-dominated narrative that has guided models of their formation and evolution for years.
Implications for solar system formation theories
The notion that Uranus and Neptune could be more rock-rich has important consequences for how we understand the outer solar system’s early history. If rocky material played a larger role than expected, scientists may need to revisit how and where these planets accreted material in the protoplanetary disk. Were there regions in the outer solar system that concentrated rocky debris more effectively? How did this affect the eventual mass and composition of these giants?
Resolving these questions will require data we simply cannot obtain from Earth-based observations alone. A dedicated mission to Uranus or Neptune—ideally an orbiter with instruments capable of probing magnetic fields, gravity, and atmospheric composition—could provide the high-fidelity data needed to disentangle the interior puzzle. Until then, researchers must continue to refine models and look for indirect clues that can validate or challenge the new “rocky giants” perspective.
What comes next?
In the near term, the scientific community will likely pursue more sophisticated simulations and exploit every available observation, from ground-based telescopes to any future flybys. The prospect of a mission to the ice giants has long been debated in space agencies around the world; the current work adds urgency to those discussions. If a future orbiter arrives, it could illuminate the exact distribution of rock, ice, and possibly metallic layers inside Uranus and Neptune, finally turning the current conjecture into a coherent, testable theory.
