Not All Sub-Neptune Exoplanets Have Magma Oceans

TL;DR

The paper addresses whether sub-Neptunes universally host magma oceans by linking envelope–mantle boundary conditions to bulk and atmospheric properties through a large grid of interior structure models. Using the SMILE framework, it demonstrates that high envelope mass fractions and high mean molecular weight atmospheres can push the boundary into solid silicate phases, reducing magma-ocean prevalence. Case studies and a broad parameter exploration show that roughly a third of configurations yield solid surfaces, with age proxies ($P_{ m rc}$) and temperature playing key roles. The findings provide critical context for interpreting JWST observations, suggesting atmospheric composition measurements (mean molecular weight) can constrain the presence of magma oceans and influence our understanding of sub-Neptune evolution and structure.

Abstract

The evolution and structure of sub-Neptunes may be strongly influenced by interactions between the outer gaseous envelope of the planet and a surface magma ocean. However, given the wide variety of permissible interior structures of these planets, it is unclear whether conditions at the envelope-mantle boundary will always permit a molten silicate layer, or whether some sub-Neptunes might instead host a solid silicate surface. In this work, we use internal structure modeling to perform an extensive exploration of surface conditions within the sub-Neptune population across a range of bulk and atmospheric parameters. We find that a significant portion of the population may lack present-day magma oceans. In particular, planets with a high atmospheric mean molecular weight and large envelope mass fraction are likely to instead have a solid silicate surface, since the pressure at the envelope-mantle boundary is high enough that the silicates will be in solid post-perovskite phase. This result is particularly relevant given recent inferences of high-mean molecular weight atmospheres from JWST observations of several sub-Neptunes. We apply this approach to a number of sub-Neptunes with existing or upcoming JWST observations, and find that in almost all cases, a range of solutions exist which do not possess a present-day magma ocean. Our analysis provides critical context for interpreting sub-Neptunes and their atmospheres.

Figures (4)

• Figure 1: Pressure-temperature profiles of GJ 1214 b with different compositions ($300\times$ and $500\times$ solar metallicity) and haze properties (maximally reflective hazes and tholin hazes), from Nixon_2024. The profiles are overlaid on the phase diagram of MgSiO$_3$, showing the solid-liquid and bridgmanite-postperovskite (Brg-PPv) boundaries. Circles indicate the pressure and temperature at the envelope-mantle boundary for each model. In all cases, the boundary occurs in the solid phase. The dotted orange line shows the best-fit isothermal-adiabatic profile (i.e., the parameterization used in this work) to the $300\times$ solar, maximally reflective haze model.
• Figure 2: Mass-radius curves for a selection of model planets within the model grid. Each panel shows the effect of varying a given parameter on the mass-radius relationship and the presence (or absence) of a magma ocean. The purple line represents the minimum value of each parameter, and the yellow line represents the maximum. Intermediate colors represent evenly-spaced values between the minimum and maximum, using either linear or log-spacing according to Table \ref{['tab:grid']}. Other parameters are held constant at fixed fiducial values, denoted in the legend. Red triangles indicate model planets with magma oceans, while blue circles indicate a solid silicate layer at the boundary.
• Figure 3: Location of the envelope-mantle boundary in pressure-temperature space as a function of parameters within the model grid. Each curve represents the locus of boundary locations across a different parameter range, with an upwards facing triangle representing the maximum value and a downward triangle the minimum. In some cases, the model radius exceeds 4 $R_{\oplus}$ before the maximum (or minimum) is reached when $\mu=2.35\,$g/mol, meaning the following models are not shown at that MMW: $T_0>500\,$K, $P_{\rm rc}<31.6\,$bar and $x_{\rm env}>3.5\%$. Additionally, we do not include the model with $\mu=18\,$g/mol and $T_0=300\,$K as this is impacted by water condensation. The phase diagram of MgSiO$_3$ is shown to indicate the phase at the envelope-mantle boundary for each model in the grid, including the solid-liquid and bridgmanite-postperovskite (Brg-PPv) boundaries. Different line colors represent the MMW of the model. Fiducial values for parameters which are not varied in a given plot are shown below the panels.
• Figure 4: Boundaries for the presence or absence of a magma ocean in $x_{\rm env}$--MMW space. Each line represents the boundary with a particular $M_p$ (represented by color) and $T_0$ (represented by line style). Different panels show different values of $P_{\rm rc}$. Models that lie above and to the right of the colored lines lack magma oceans, whereas those that lie to the left and below the lines do possess magma oceans. Blue shading represents regions of the parameter space for which all models feature a solid silicate surface, and red shading represents regions where all models feature a magma ocean (liquid silicate surface). Grey shaded regions represent models whose radius exceeds 4 $R_{\oplus}$. We do not include models with $T_0 = 500\,$K, $P_{\rm rc}=100\,$bar since they are impacted by water condensation.