The planets Neptune and Uranus may be better described as "magma-ocean giants" rather than "ice giants," according to a team of researchers from the University of California. Gizmodo reports: While the Voyager flyby confirmed the planets' classification as ice giants... [a]s the least explored planets in the solar system, the two planets have never been thoroughly investigated. Therefore, scientists aren't sure where the planets originally formed in the early solar system or the reason for their wildly chaotic magnetic fields. A long-standing hypothesis suggests that both worlds have a hydrogen/helium atmosphere that covers a vast mantle of ices, made primarily of water, ammonia, and methane, with a rocky core. The new study, however, notes that the three-layer model of an ice giant's interior structure is not the only way to explain the properties of the two planets.

The researchers also point out that objects found in the Kuiper Belt, which are thought to preserve evidence of the material in the outer Solar System where Uranus and Neptune formed, are primarily composed of rock rather than ice. For the recent study, the researchers simulated different models for the interior processes and composition of Uranus and Neptune. The model that best fits Uranus's and Neptune's different properties suggests the two planets have a well-mixed magma ocean with dissolved hydrogen at the bottom and a hydrogen-dominated envelope at the top. The model suggests that at high pressures, hydrogen gas can dissolve into magma, forming a well-mixed fluid. This mixing might help explain Uranus's and Neptune's density, which has traditionally been interpreted as evidence for an ice-rich interior.
The article notes that the theory "could also help scientists understand the interior structure of sub-Neptune planets in the Milky Way, which have thus far remained a mystery."