The Possibility of Hydrogen-Water Demixing in Uranus, Neptune, K2-18b and TOI-270d

TL;DR

The paper investigates whether hydrogen-water demixing can occur in Uranus, Neptune, K2-18 b, and TOI-270 d by coupling planetary evolution models with ab initio hydrogen-water phase diagrams and introducing a temperature offset $T_{ m offset}$ to account for miscibility uncertainties. It shows that demixing can fully deplete the outer water-rich regions of Uranus and Neptune at sufficiently high offsets (up to $T_{ m offset} ightarrow 1100$ K), causing notable radius inflation and altered thermal structure, and that exoplanets like K2-18 b and TOI-270 d may also experience demixing with partial or full atmospheric water depletion depending on $T_{ m offset}$ (e.g., $500$ K for K2-18 b). These results imply that internal phase separation can link deep interiors to observable atmospheres, affecting interpretations of JWST/ARIEL spectra and requiring its inclusion in interior models. The work highlights the importance of accurate phase diagrams and EOSs, and suggests future refinements including non-adiabatic gradients, condensation, and broader compositional mixtures to better interpret planetary structure and atmospheric data.

Abstract

The internal structures of Uranus and Neptune remain unknown. In addition, sub-Neptunes are now thought to be the most common type of exoplanets. Understanding the physical processes that govern the interiors of such planets is therefore essential. Phase separation between hydrogen and water may occur in cold, water-rich intermediate-mass planets. We assess whether it could occur in Uranus, Neptune, K2-18\,b and TOI-270\,d, and investigate its effect on the planetary evolution and inferred internal structure. We couple planetary evolution models with recent \textit{ab initio} calculations of the hydrogen-water phase diagram, allowing for temperature shifts to account for uncertainties in miscibility gaps. We find that demixing may occur and could lead to a complete depletion of water in the outermost regions of Uranus and Neptune. Temperature offsets of up to 1100~K lead to a depleted region comprising as much as 16\% of the planet's mass, and an increase in planetary radius by nearly 20\%. For K2-18\,b, our models suggest that hydrogen-water demixing is ongoing and may explain the absence of water features in its JWST spectrum. A temperature offset of 500~K is required to get a complete depletion of water in the atmosphere of K2-18\,b. TOI-270\,d may also have experienced hydrogen-water demixing. When applying a similar temperature offset on the phase diagram as for K2-18\,b, we find a partial depletion of water in the atmosphere of TOI-270\,d, consistent with JWST's detection of water. Hydrogen-water immiscibility may play a key role in shaping the structure and evolution of both Solar System giant planets like Uranus and Neptune, and cold/temperate exoplanets such as K2-18\,b and TOI-270\,d. Accounting for such internal processes is crucial to accurately interpret atmospheric observations from current (e.g., JWST) and upcoming (e.g., ARIEL) missions.

Figures (11)

• Figure 1: Miscibility gap of hydrogen and water for different temperatures (color-coded). DFT-MD simulations by bergermann2024 are shown as colored circles while earlier experimental campaigns of seward1981, bali2013, and vlasov2023 are presented by colored diamonds and traingles, respectively.
• Figure 2: Evolution models of Uranus. Each row shows results for a different temperature offset applied to the hydrogen-water phase diagram ($T_{\rm offset}=0$, 600 or 1000 K.) Left panels: temperature-pressure profiles at different ages. Phase curves are shown with grey to black lines for water mass fractions of 0.92 to 0. Grey-shaded areas mark demixing regions. Center panels: water mass fraction as a function of normalized mass, colored by age. Right panels: Radius and effective temperature as a function of age. The horizontal dashed lines show the measured values.
• Figure 3: Evolution models of Neptune. Description is same as for Fig. \ref{['figure:Uranus']}. For comparison, Uranus models are shown with dashed lines.
• Figure 4: Evolution models of K2-18 b. Top panel: planetary radius as a function of age. The black errorbar corresponds to the measured radius and age. Bottom panel: heavy-element mass fraction at 2.4 Gyr (the planet's age) as a function of pressure. We note that condensation was not considered in our models, which explains why some models have a high heavy-element mass fraction at low pressures.
• Figure 5: Evolution models of TOI-270 d. Top panel: planetary radius as a function of age. The black errorbar corresponds to the measured radius and age. Since the age is uncertain, we adopted an age range of 1 to 10 Gyr. Bottom panel: heavy-element mass fraction at the end of the evolution as a function of pressure.