Redefining interiors and envelopes: hydrogen-silicate miscibility and its consequences for the structure and evolution of sub-Neptunes

Abstract

We present the first evolving interior structure model for sub-Neptunes that accounts for the miscibility between silicate magma and hydrogen. Silicate and hydrogen are miscible above $\sim 4000$K at pressures relevant to sub-Neptune interiors. Using the H${}_2$-MgSiO${}_3$ phase diagram, we self-consistently couple physics and chemistry to determine the radial extent of the fully miscible interior. Above this region lies the envelope, where hydrogen and silicates are immiscible and exist in both gaseous and melt phases. The binodal surface, representing a phase transition, provides a physically/chemically informed boundary between a planet's "interior" and "envelope". We find that young sub-Neptunes can store several tens of per cent of their hydrogen mass within their interiors. As the planet cools, its radius and the binodal surface contract, and the temperature at the binodal drops from $\sim 4000$K to $\sim 3000$K. Since the planet's interior stores hydrogen, its density is lower than that of pure-silicate. Gravitational contraction and thermal evolution lead to hydrogen exsolving from the interior into the envelope. This process slows planetary contraction compared to models without miscibility, potentially producing observable signatures in young sub-Neptune populations. At early times ($\sim 10$-$100$Myr), the high temperature at the binodal surface results in more silicate vapour in the envelope, increasing its mean molecular weight and enabling convection inhibition. After $\sim$Gyr of evolution, most hydrogen has exsolved, and the radii of miscible and immiscible models converge. However, the internal distribution of hydrogen and silicates remains distinct, with some hydrogen retained in the interior.

17 pages, 11 figures, resubmitted to MNRAS after revision

References in corpus (47)

• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler
• The false positive rate of Kepler and the occurrence of planets
• Prevalence of Earth-size planets orbiting Sun-like stars
• Kepler planets: a tale of evaporation
• The evaporation valley in the Kepler planets
• Probabilistic Forecasting of the Masses and Radii of Other Worlds
• An asteroseismic view of the radius valley: stripped cores, not born rocky
• The Role of Core Mass in Controlling Evaporation: the Kepler Radius Distribution and the Kepler-36 Density Dichotomy
• Core-powered mass loss and the radius distribution of small exoplanets
• Sculpting the Valley in the Radius Distribution of Small Exoplanets as a by-product of Planet Formation: The Core-Powered Mass-Loss Mechanism
• A Framework for Quantifying the Degeneracies of Exoplanet Interior Compositions
• Super-Earth Atmospheres: Self-Consistent Gas Accretion and Retention
• Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond
• Probabilistic Mass-Radius Relationship for Sub-Neptune-Sized Planets
• To Cool is to Accrete: Analytic Scalings for Nebular Accretion of Planetary Atmospheres
• Atmospheres of low-mass planets: the "boil-off"
• Density and Eccentricity of Kepler Planets
• On the Minimum Core Mass for Giant Planet Formation at Wide Separations
• Signatures of the Core-Powered Mass-Loss Mechanism in the Exoplanet Population: Dependence on Stellar Properties and Observational Predictions
• A new equation of state for dense hydrogen-helium mixtures
• Constraining the initial entropy of directly-detected exoplanets
• The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age
• Condensation-inhibited convection in hydrogen-rich atmospheres: Stability against double-diffusive processes and thermal profiles for Jupiter, Saturn, Uranus, and Neptune
• Atmosphere Origins for Exoplanet Sub-Neptunes
• Earth shaped by primordial H${}_2$ atmospheres
• Effects of Helium Phase Separation on the Evolution of Extrasolar Giant Planets
• Conclusive evidence for a population of water-worlds around M-dwarfs remains elusive
• Water on Hot Rocky Exoplanets
• Vertically resolved magma ocean-protoatmosphere evolution: H${}_2$, H${}_2$O, CO${}_2$, CH${}_4$, CO, O${}_2$, and N${}_2$ as primary absorbers
• Distinguishing oceans of water from magma on mini-Neptune K2-18b
• On the Role of Dissolved Gases in the Atmosphere Retention of Low-Mass Low-Density Planets
• How planets grow by pebble accretion. III. Emergence of an interior composition gradient
• Explaining the low luminosity of Uranus: A self-consistent thermal and structural evolution
• How planets grow by pebble accretion II: Analytical calculations on the evolution of polluted envelopes
• Atmospheres as windows into sub-Neptune interiors: coupled chemistry and structure of hydrogen-silane-water envelopes
• The importance of silicate vapor in determining the structure, radii, and envelope mass fractions of sub-Neptunes
• A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18 b
• Convective inhibition with an atmosphere, I: super-critical cores on sub-Neptune/super-Earths
• Magma ocean evolution at arbitrary redox state
• The effect of a small amount of hydrogen in the atmosphere of ultrahot magma-ocean planets: atmospheric composition and escape
• How planets form by pebble accretion V. Silicate rainout delays contraction of sub-Neptunes
• Rocky sub-Neptunes formed by pebble accretion: Rain of rocks from polluted envelopes
• The Evolution and Internal Structure of Neptunes and Sub-Neptunes: The importance of thermal conductivity in non-convective regions
• The miscibility of hydrogen and water in planetary atmospheres and interiors
• Phase equilibria of sub-Neptunes and super-Earths
• Assessing Core-Powered Mass Loss in the Context of Early Boil-Off: Minimal Long-Lived Mass Loss for the Sub-Neptune Population

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• The Effects of Non-ideal Mixing in Planetary Magma Oceans and Atmospheres
• Using observations of escaping H/He to constrain the atmospheric composition of sub-Neptunes