A New Study Questions the Nature of Uranus and Neptune: What if They Aren't Icy Giants?

A group of researchers from the University of California has challenged one of the most entrenched classifications in planetology: that of Uranus and Neptune as the icy giants of the Solar System.

The debate over their internal composition has never really been settled, and in the new study presented in The Astrophysical Journal, the team led by Edward Young proposes to rename them as "magma ocean giants," describing their interior as an ocean of magma rich in hydrogen, rather than a shell of water, ammonia, and methane ice.

The currently dominant model proposes three layers: an atmosphere of hydrogen and helium, a broad mantle of ice, and, at the center, a rocky core. Almost everything we know about the two planets comes from a single flyby of the Voyager 2 spacecraft, the only one to have ever visited them. On January 24, 1986, it skimmed past Uranus at an altitude of 81,500 kilometers, discovering 11 new moons and two rings. On August 25, 1989, it flew over Neptune's north pole, identifying another six moons and four ring arcs.

Uranus and Neptune remain the least explored planets of the Solar System, and no spacecraft has reached them since. Scientists still lack a shared explanation for where they formed in the early life stage of the Solar System, as well as for the causes of their magnetic fields, described as extremely chaotic and irregular.

Magma Instead of Ice, According to New Studies on Uranus and Neptune
Along with Sarah Marcum, Aaron Werlen, and Paula Wulff, Young compared the three-layer model with an alternative: a supercritical magma ocean, with dissolved hydrogen at the bottom and a shell dominated by the same element on the surface. Only three adjustment parameters for each planet are needed for this scheme to reproduce the observed radius, density, moments of inertia, gravitational field, internal heat, and atmospheric composition of Uranus and Neptune.

The same authors note that the origin of the two planets would thus follow a path parallel to that of the small gas planets known outside the Solar System. The initial clue comes from the Kuiper Belt. The objects that populate it, considered witnesses to the material from which the two planets formed, are found to be composed largely of rock rather than ice. Under high pressures, gaseous hydrogen dissolves into the magma, forming a homogeneous fluid, a reaction that would explain the density of Uranus and Neptune without resorting to an ice-rich interior, as previously hypothesized.

The work, dated June 2026, substantially expands and revises a previous, shorter version that circulated as a preprint on Research Square: a process that indicates how the results have already undergone initial scrutiny by the scientific community before reaching the journal.

The label of icy giants, the authors admit, is unlikely to exit common language despite the new calculations. However, the magma ocean model offers a tool for studying sub-Neptunes, the exoplanets whose internal structure remains a mystery so far. Uranus and Neptune thus become two accessible test cases for verifying hypotheses previously conceived only for worlds outside the Solar System.