More Like this

1. The interiors of Uranus and Neptune: current understanding and open questions

   https://doi.org/10.1098/rsta.2019.0474  

   Helled, Ravit; Fortney, Jonathan J. (December 2020, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   null (Ed.)

   Uranus and Neptune form a distinct class of planets in our Solar System. Given this fact, and ubiquity of similar-mass planets in other planetary systems, it is essential to understand their interior structure and composition. However, there are more open questions regarding these planets than answers. In this review, we concentrate on the things we do not know about the interiors of Uranus and Neptune with a focus on why the planets may be different, rather than the same. We next summarize the knowledge about the planets’ internal structure and evolution. Finally, we identify the topics that should be investigated further on the theoretical front as well as required observations from space missions. This article is part of a discussion meeting issue ‘Future exploration of ice giant systems’. 

   more » « less
2. The Miscibility of Hydrogen and Water in Planetary Atmospheres and Interiors

   https://doi.org/10.3847/2041-8213/adb631  

   Gupta, Akash; Stixrude, Lars; Schlichting, Hilke E (March 2025, The Astrophysical Journal Letters)

   Abstract Many planets in the solar system and across the Galaxy have hydrogen-rich atmospheres overlying more heavy element-rich interiors with which they interact for billions of years. Atmosphere–interior interactions are thus crucial to understanding the formation and evolution of these bodies. However, this understanding is still lacking in part because the relevant pressure–temperature conditions are extreme. We conduct molecular dynamics simulations based on density functional theory to investigate how hydrogen and water interact over a wide range of pressure and temperature, encompassing the interiors of Neptune-sized and smaller planets. We determine the critical curve at which a single homogeneous phase exsolves into two separate hydrogen-rich and water-rich phases, finding good agreement with existing experimental data. We find that the temperature along the critical curve increases with increasing pressure and shows the influence of a change in fluid structure from molecular to atomic near 30 GPa and 3000 K, which may impact magnetic field generation. The internal temperatures of many exoplanets, including TOI-270 d and K2-18 b, may lie entirely above the critical curve: the envelope is expected to consist of a single homogeneous hydrogen–water fluid, which is much less susceptible to atmospheric loss as compared with a pure hydrogen envelope. As planets cool, they cross the critical curve, leading to rainout of water-rich fluid and an increase in internal luminosity. Compositions of the resulting outer, hydrogen-rich and inner, water-rich envelopes depend on age and instellation and are governed by thermodynamics. Rainout of water may be occurring in Uranus and Neptune at present. 

   more » « less
3. The Promise and Limitations of Precision Gravity: Application to the Interior Structure of Uranus and Neptune

   https://doi.org/10.3847/PSJ/ac60ff  

   Movshovitz, Naor; Fortney, Jonathan J. (April 2022, The Planetary Science Journal)

   Abstract We study the constraining power of a high-precision measurement of the gravity field for Uranus and Neptune, as could be delivered by a low-periapse orbiter. Our study is practical, assessing the possible deliverables and limitations of such a mission with respect to the structure of the planets. Our study is also academic, assessing in a general way the relative importance of the low-order gravity, high-order gravity, rotation rate, and moment of inertia (MOI) in constraining planetary structure. We attempt to explore all possible interior density structures of a planet that are consistent with hypothetical gravity data via MCMC sampling of parameterized density profiles. When the gravity field is poorly known, as it is today, uncertainties in the rotation rate on the order of 10 minutes are unimportant, as they are interchangeable with uncertainties in the gravity coefficients. By the same token, when the gravity field is precisely determined, the rotation rate must be known to comparable precision. When gravity and rotation are well known, the MOI becomes well constrained, limiting the usefulness of independent MOI determinations unless they are extraordinarily precise. For Uranus and Neptune, density profiles can be well constrained. However, the nonuniqueness of the relative roles of H/He, watery volatiles, and rock in the deep interior will still persist with high-precision gravity data. Nevertheless, the locations and magnitudes (in pressure space) of any large-scale composition gradient regions can likely be identified, offering a crucially better picture of the interiors of Uranus or Neptune. 

   more » « less
4. Heat and charge transport in H2O at ice-giant conditions from ab initio molecular dynamics simulations

   https://doi.org/10.1038/s41467-020-17275-5  

   Grasselli, Federico; Stixrude, Lars; Baroni, Stefano (July 2020, Nature Communications)

   Abstract The impact of the inner structure and thermal history of planets on their observable features, such as luminosity or magnetic field, crucially depends on the poorly known heat and charge transport properties of their internal layers. The thermal and electric conductivities of different phases of water (liquid, solid, and super-ionic) occurring in the interior of ice giant planets, such as Uranus or Neptune, are evaluated from equilibrium ab initio molecular dynamics, leveraging recent progresses in the theory and data analysis of transport in extended systems. The implications of our findings on the evolution models of the ice giants are briefly discussed. 

   more » « less
5. Toward a Self-consistent Evaluation of Gas Dwarf Scenarios for Temperate Sub-Neptunes

   https://doi.org/10.3847/1538-4357/ad6c38  

   Rigby, Frances_E; Pica-Ciamarra, Lorenzo; Holmberg, Måns; Madhusudhan, Nikku; Constantinou, Savvas; Schaefer, Laura; Deng, Jie; Lee, Kanani_K_M; Moses, Julianne_I (October 2024, The Astrophysical Journal)

   Abstract The recent JWST detections of carbon-bearing molecules in a habitable-zone sub-Neptune have opened a new era in the study of low-mass exoplanets. The sub-Neptune regime spans a wide diversity of planetary interiors and atmospheres not witnessed in the solar system, including mini-Neptunes, super-Earths, and water worlds. Recent works have investigated the possibility of gas dwarfs, with rocky interiors and thick H₂-rich atmospheres, to explain aspects of the sub-Neptune population, including the radius valley. Interactions between the H₂-rich envelope and a potential magma ocean may lead to observable atmospheric signatures. We report a coupled interior-atmosphere modeling framework for gas dwarfs to investigate the plausibility of magma oceans on such planets and their observable diagnostics. We find that the surface–atmosphere interactions and atmospheric composition are sensitive to a wide range of parameters, including the atmospheric and internal structure, mineral composition, volatile solubility and atmospheric chemistry. While magma oceans are typically associated with high-temperature rocky planets, we assess if such conditions may be admissible and observable for temperate sub-Neptunes. We find that a holistic modeling approach is required for this purpose and to avoid unphysical model solutions. Using our model framework, we consider the habitable-zone sub-Neptune K2-18 b as a case study and find that its observed atmospheric composition is incompatible with a magma ocean scenario. We identify key atmospheric molecular and elemental diagnostics, including the abundances of CO₂, CO, NH₃, and, potentially, S-bearing species. Our study also underscores the need for fundamental material properties for accurate modeling of such planets. 

   more » « less