The Ice Giants Were Never Ice

What Uranus and Neptune Reveal About the Liquid Star Model

A new study calls for renaming two planets. The Liquid Star Model says the pattern goes much further.

What the Study Actually Found

The team modeled Uranus and Neptune against six independent observational constraints: their radii, their bulk densities, their gravitational harmonics, their normalized moments of inertia, their intrinsic luminosities, and key features of their atmospheric compositions. These are the standard tests any interior model must pass to be considered physically realistic.

The ice giant model — ice mantle over rocky core — has long been the accepted fit for these constraints. The new study shows it is not the only fit. A model built around a magma ocean interior, with hydrogen dissolved into supercritical molten rock at depth and a hydrogen-rich gas envelope above, fits all six constraints equally well. In the researchers’ view, it fits more simply — requiring fewer free parameters to explain the observed data.

The study also points to an important piece of external evidence. Objects in the Kuiper Belt — the region of the outer solar system where Uranus and Neptune are thought to have formed — are composed primarily of rock, not ice. If the raw material available where these planets formed was predominantly rocky, a magma ocean interior is not just a mathematical possibility. It may be the physically expected outcome.

A model built around supercritical magma oceans with dissolved hydrogen fits all six observational constraints of Uranus and Neptune at least as well as the traditional ice giant model — and does so with fewer free parameters.

The Temperature Pattern: A Crucial Clue

There is a second pattern embedded in this sequence that deserves specific attention, because it connects the planetary evidence directly to one of the Liquid Star Model’s most important proposals about the Sun.

As you move from the surface of any large body toward its interior, temperature increases. That is expected — internal heat from radioactive decay, gravitational compression, and residual primordial energy all produce hotter conditions at depth. Earth’s core is hotter than its surface. Jupiter’s interior is hotter than its cloud tops.

But in several large bodies, another pattern emerges at the outermost layer: the atmosphere or outer envelope is hotter than the surface immediately below it.

On Earth, this is a localized effect in the upper atmosphere — the thermosphere, far above the troposphere where weather occurs, reaches temperatures of hundreds to thousands of degrees Celsius, far hotter than the surface below it. This upper atmospheric heating is driven by the absorption of solar ultraviolet radiation and by interaction with the solar wind.

On the Sun, this phenomenon is dramatic and unexplained by the standard model. The photosphere — the visible surface — sits at approximately 5,500 degrees Celsius. The corona above it reaches one to three million degrees. This is the coronal heating problem, discussed in detail in Chapter 38.

The new magma ocean study adds relevant context to this pattern. Uranus and Neptune, in the new model, have supercritical magma interiors transitioning through a hydrogen-rich envelope to an outer atmosphere. The pressure and temperature conditions at the boundary between the magma ocean and the hydrogen envelope represent exactly the kind of phase transition — a change in the state of matter from liquid to supercritical fluid to gas — that produces distinct temperature profiles and structural boundaries.

Phase transitions matter for temperature profiles. When matter changes state — from solid to liquid, from liquid to gas, from supercritical fluid to gas envelope — the temperature behavior at that boundary is not simple or smooth. It involves latent heat, changes in thermal conductivity, and changes in how energy is transported. These boundaries are where anomalous temperature behavior is most likely to appear.

The Liquid Star Model proposes that the boundary between the Sun’s liquid metallic surface and its overlying plasma atmosphere is exactly this kind of phase transition boundary — and that the anomalous temperature behavior observed at the corona is a direct consequence of the physics operating at that phase transition, including arc discharge processes described in earlier chapters. The new study on Uranus and Neptune, by establishing that large planetary bodies contain phase transition boundaries between magma ocean interiors and gas envelopes, makes this proposed solar analog considerably more physically credible.

What This Means for the Sun

The solar implications of the new study are worth stating directly.

If Uranus and Neptune — bodies that formed in the cold outer solar system from material now known to be predominantly rocky — have magma ocean interiors rather than icy ones, then the assumption that interior composition is well-constrained by formation location becomes much weaker. The outer solar system produced magma interiors. The inner solar system, where temperatures were higher and rocky material dominated even more completely, certainly produced differentiated metallic and silicate interiors in every body studied.

The Sun formed at the center of the entire system, from the densest concentration of the most massive material in the solar nebula. If every other large body in the system differentiated into layered interiors dominated by rock and metal at depth, the proposal that the Sun alone is a uniform ball of hydrogen and helium gas throughout its interior requires a specific reason why differentiation would not have operated at solar scale.

No such reason has been established. The standard solar model assumes a uniform gaseous interior not because differentiation is known to have failed in the Sun, but because the model was built before the full extent of planetary differentiation was understood, and before studies like this one began challenging the compositional assumptions built into planetary categories.

The Liquid Star Model does not require Uranus and Neptune to be magma-ocean worlds for its own proposals about the Sun to be valid. But the convergence is meaningful. Each time a planetary body previously assumed to have a simple, cold, undifferentiated interior turns out to have a hot, liquid, structured one, the solar continuum argument becomes stronger.

The Escanor Project publishes long-form science articles and videos exploring the Liquid Star Model and related topics in solar and planetary physics. Read more at escanorproject.wordpress.com.