🪐 Are Uranus and Neptune really ice giants? It's not so certain

Billions of miles (kilometers) from the Sun, Uranus and Neptune are the most distant planets in our Solar System. Long cataloged as simple "ice giants," these planets might hold a major surprise.

A recent modeling study, published in Astronomy & Astrophysics, proposes a renewed view of their internal architecture, suggesting a predominance of rocky rather than icy materials. This unexpected perspective questions planetary formation scenarios and offers new keys to understanding the diversity of worlds discovered around other stars.


The exploration of these two planets still relies heavily on data collected by the Voyager 2 probe during its brief flybys in the 1980s. Models developed since were based on the idea of a thick mantle of water, ammonia, and methane ices, enveloping a rocky core. This classification seemed logical given their distance from the Sun and the temperatures reigning in these remote regions. However, the scarcity of direct observations left considerable room for uncertainty regarding their exact composition.

A new look at the planets' interiors

The team of researchers from the University of Zurich adopted an innovative methodology to uncover these secrets. Rather than assuming an ice-rich structure, they developed thousands of "agnostic" interior models, imposing no predefined composition. These models, generated randomly but physically consistent, were then compared to the only available observational data, primarily the measured gravity fields. Their goal was to identify the most plausible internal structures.

The results of these simulations open a much wider range of possibilities than expected. The compositions that best fit the observations are not necessarily dominated by ice. On the contrary, they could indicate a significantly larger proportion of rocky material than envisioned in the classical model. The scientists emphasize that this hypothesis of a more rocky interior had been proposed several years ago, but now finds a robust numerical framework to support it.

This reinterpretation fits into a broader understanding of objects in the outer Solar System. It notably aligns with observations from the New Horizons mission which revealed that Pluto has a predominantly rocky composition. It thus appears plausible that the building blocks that formed Uranus and Neptune were richer in silicates and metals than in volatile ices, challenging the image of a distant environment solely dominated by freezing.

A key to chaotic magnetic fields

One of the most persistent mysteries concerning these planets lies in the atypical nature of their magnetic fields. Unlike Earth's relatively ordered dipole field, those of Uranus and Neptune are complex, multipolar, and strongly offset from their axis of rotation. The new models provide a promising explanatory clue for these singularities.

The proposed hypothesis is based on the existence of ionic water layers located at different depths inside the planets. These conductive layers could host convection movements, thereby generating localized and independent magnetic dynamos. The superposition of these dynamos could explain the observed multipolar and disordered structure. The models also suggest that the source of Uranus's magnetic field lies deeper than that of Neptune's.

This theoretical advance does not remove all uncertainties. Physicists acknowledge that the behavior of matter under the extreme pressures and temperatures of planetary cores remains poorly understood. These exotic conditions could influence the models' results. The study's authors therefore insist on the absolute necessity for new dedicated space missions, capable of precisely measuring gravitational and magnetic fields to definitively decide between the scenarios.