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arxiv: 2512.01805 · v3 · submitted 2025-12-01 · 🌌 astro-ph.EP · astro-ph.SR

A window for water-hydrogen demixing on warm metal-rich sub-Neptunes

Caroline Piaulet-Ghorayeb , Daniel P. Thorngren , Eliza M.-R. Kempton , Justin Lipper , Leslie Rogers , Fernanda Correa Horta , Shi Lin Sun This is my paper

Pith reviewed 2026-05-17 02:30 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.SR
keywords sub-Neptuneswater-hydrogen demixingenvelope metallicityTOI-270 dinterior modelsatmosphere composition
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0 comments X

The pith

Warm sub-Neptunes with metallicities 150 to 700 times solar can have water-hydrogen demixing in their envelopes.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates whether warm sub-Neptunes, which receive much more stellar irradiation than Uranus or Neptune, can still experience separation of hydrogen and water in their atmospheres. Previous work suggested only cold planets could have this stratification, but the authors show that in water-rich conditions the demixing occurs at higher temperatures, opening a window for metal-rich sub-Neptunes. Using a new framework called ATHENAIA that links atmosphere and interior models, they find this applies to planets like TOI-270 d. If true, this means current models assuming everything mixes fully may be underestimating the total metal content in these planets. It also suggests more planets could have molten interiors due to the greenhouse effects from metals.

Core claim

We find that the higher temperatures at which hydrogen and water demix in water-rich environments open a window for demixing on sub-Neptunes with bulk envelope metallicities of ∼150 to 700× solar, compatible with TOI-270 d. Demixing is easier to achieve on more massive and colder planets, but still broadly affects warm (≃330 to 450 K) metal-rich sub-Neptunes.

What carries the argument

The ATHENAIA interior-atmosphere composition inference framework that couples radiative-convective atmosphere models to interior structure models to assess demixing potential.

If this is right

  • Demixing becomes possible on warm sub-Neptunes that have high bulk envelope metallicities.
  • Assuming fully-miscible envelopes when interpreting atmosphere metallicities can lead to underestimated bulk metallicities and mass fractions.
  • Metal-rich atmospheres increase the greenhouse effect and alter the adiabatic gradient, widening the range of compositions where molten mantles are expected.
  • Models of sub-Neptune evolution should account for the onset of metallicity gradients.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Observations of atmospheric composition on planets like TOI-270 d could test for signs of stratification if demixing occurs.
  • This finding connects to broader questions about how volatiles separate in exoplanet interiors beyond just Uranus and Neptune analogs.
  • Future evolutionary models incorporating demixing could predict different thermal histories for these planets.

Load-bearing premise

The phase boundaries for hydrogen-water demixing and the accuracy of the coupled atmosphere-interior models hold at the high pressures and temperatures inside these planets.

What would settle it

A direct measurement of the interior structure or atmospheric profile of TOI-270 d showing no evidence of demixing despite the predicted metallicity range would challenge the claim.

Figures

Figures reproduced from arXiv: 2512.01805 by Caroline Piaulet-Ghorayeb, Daniel P. Thorngren, Eliza M.-R. Kempton, Fernanda Correa Horta, Justin Lipper, Leslie Rogers, Shi Lin Sun.

Figure 1
Figure 1. Figure 1: Illustration of the demixing window concept. Top panel: Water phase diagram (black lines) with labeled re￾gions; the critical curve (blue) indicates the highest tempera￾ture for single-phase H-H2O mixing, compiled from low-pres￾sure data (dashed, Seward & Franck 1981) and high-pressure data (solid, Gupta et al. 2024), with interpolation (dotted). Envelope T-P profiles below the blue line can experience dem… view at source ↗
Figure 2
Figure 2. Figure 2: Illustration of the workflow for the construction of coupled interior-atmosphere models with ATHENAIA. For each composition, atmosphere models are calculated with SCARLET (top left) and interior models following the model from Thorngren & Fortney (2019). Then we couple them with ATHENAIA by finding the Rref (atmosphere model) and Tmod (interior model) that minimize δTPR (see Eq. 5). The radius at 20 mbar i… view at source ↗
Figure 3
Figure 3. Figure 3: The adiabatic pressure-temperature profiles used for the water/H/He mixtures in our interior models at vari￾ous compositions, which were computed through integration of the computed adiabatic gradient. These are compared against the approximation that the P-T profile is the same as for Z=0 (pure H/He, dotted line). The densities only differ significantly at low pressures, but the temperature gradient is si… view at source ↗
Figure 4
Figure 4. Figure 4: Coexistence curves for hydrogen and water in pressure-metallicity space, for 4 different temperatures. Ver￾tical lines indicate 1 to 1000× solar metallicity. The corre￾sponding mean molecular weight values are indicated at the top. Demixing is easier to achieve in colder envelopes with moderate metal enrichments. core/mantle and the envelope happens at pressures that are shallower than the RCB, or shallowe… view at source ↗
Figure 5
Figure 5. Figure 5: Impact of physical parameters varied across the grid on the pressure-temperature and pressure-radius profiles. The left, middle, and right panel illustrate the impact of varying the planet mass, envelope metallicity, and envelope mass fraction respectively. Aside from the parameter varied in each panel, all other parameters are kept fixed to AB = 0, Mp = 5.14M⊕, and fenv = 8.79%. Increasing the planetary a… view at source ↗
Figure 6
Figure 6. Figure 6: Joint constraints on the envelope mass fraction and envelope metal content of TOI-270 d. The red (blue) contours spanning the full range of potential envelope metal￾licities represent the 1 and 2σ composition constraints for a Bond albedo of 0 (0.4), based on the mass, radius, and insola￾tion of TOI-270 d obtained with ATHENAIA. The contours in the top right corner outline the region of the parameter space… view at source ↗
Figure 7
Figure 7. Figure 7: Contours delineating the region of parameter space where demixing occurs for different planet masses and Bond albedos, for a TOI-270 d-like irradiation level. The color encodes the planet mass, and the line style encodes Bond albedo(solid lines for AB = 0, dashed for AB = 0.2). Vertical gray lines indicate different atmosphere metallicities (labeled). None of the models shown cross the water conden￾sation … view at source ↗
Figure 8
Figure 8. Figure 8: Mass-radius diagram of planets across the sub-Neptune size range (circles, colored by zero-Bond albedo equilibrium temperature) and their susceptibility to demixing for Zenv = 0.01 (left, 1× solar metallicity), 0.55 (middle, 120× solar metal￾licity), and 0.75 (right, 250× solar metallicity). Circle sizes encode the favorability of planets to transmission spectroscopy via the transmission spectroscopy metri… view at source ↗
read the original abstract

Sub-Neptunes represent the largest exoplanet demographic, yet their bulk compositions remain poorly understood. Recent studies suggested that only very cold planets, such as Uranus and Neptune, could experience stratification of volatiles in their envelopes. Transiting warm sub-Neptunes, with $10^3$ to $10^4$ times more stellar irradiation, were therefore believed to have fully-miscible compositions. Here, we present ATHENAIA, an interior-atmosphere composition inference framework we leverage to assess the potential for water-hydrogen demixing on warm sub-Neptunes and for the 350 K planet TOI-270 d as a case study, using radiative-convective atmosphere models coupled to interior models. We find that the higher temperatures at which hydrogen and water demix in water-rich environments open a window for demixing on sub-Neptunes with bulk envelope metallicities of $\sim 150$ to $700\times$ solar, compatible with TOI-270 d. Demixing is easier to achieve on more massive and colder planets, but still broadly affects warm ($\simeq $330 to 450 K) metal-rich sub-Neptunes. Therefore, combining atmosphere metallicities with models of fully-miscible envelopes may lead to underestimated bulk envelope metallicities and mass fractions. Further, we find that considering the increased greenhouse effect in metal-rich atmospheres in concert with the composition-dependent adiabatic gradient in the convective envelope increases the range of compositions under which molten mantle conditions should be expected on sub-Neptunes. This work encourages a reconsideration of the current paradigm for linking sub-Neptune atmospheres to their interiors and motivates evolutionary modeling describing the onset of metallicity gradients in sub-Neptune envelopes.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper introduces the ATHENAIA framework coupling radiative-convective atmosphere models to interior structure models to evaluate water-hydrogen demixing on warm sub-Neptunes. Using TOI-270 d as a case study, it reports that higher demixing temperatures in water-rich mixtures open a bulk envelope metallicity window of ∼150–700× solar where demixing can occur, compatible with the planet, and argues this affects a broad range of warm (330–450 K) metal-rich sub-Neptunes while also increasing the likelihood of molten mantle conditions.

Significance. If the central result holds, the work challenges the prevailing view that warm sub-Neptunes maintain fully miscible envelopes and shows that demixing windows exist for metal-rich compositions on planets receiving 10^3–10^4 times the irradiation of Uranus/Neptune. The ATHENAIA coupling of composition-dependent adiabatic gradients with enhanced greenhouse effects in metal-rich atmospheres is a clear methodological advance that could lead to revised bulk metallicity estimates and motivates evolutionary models of gradient onset.

major comments (2)
  1. [ATHENAIA framework description and results for TOI-270 d] The demixing phase boundaries and two-phase region intersections that define the 150–700× solar window are taken directly from prior literature without new ab initio calculations, experimental validation, or sensitivity tests at the metallicities, pressures (∼1–100 bar), and temperatures (∼330–450 K equilibrium, higher in the convective zone) relevant to warm sub-Neptunes. This assumption is load-bearing for the central claim; a 50–100 K downward shift in the boundaries would close the reported window for TOI-270 d and similar planets.
  2. [Results section on demixing window and TOI-270 d case study] The compatibility statement for TOI-270 d and the broader 330–450 K range rest on the specific coupling between the radiative-convective P-T profiles and the composition-dependent adiabatic gradient; the manuscript does not report explicit tests of how variations in these profiles or in the imported phase boundaries alter the intersection points.
minor comments (2)
  1. [Abstract] The abstract states the metallicity window and temperature range but does not reference the specific prior studies supplying the demixing curves; adding these citations would improve traceability.
  2. [Methods and figures] Figure captions or the methods section could clarify the exact pressure levels at which the atmosphere-interior coupling is performed to allow readers to assess the relevance of the imported boundaries.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for their constructive and detailed review. The comments identify key areas where additional clarification and analysis will strengthen the manuscript. We address each major comment below and describe the revisions we will make.

read point-by-point responses

  1. Referee: The demixing phase boundaries and two-phase region intersections that define the 150–700× solar window are taken directly from prior literature without new ab initio calculations, experimental validation, or sensitivity tests at the metallicities, pressures (∼1–100 bar), and temperatures (∼330–450 K equilibrium, higher in the convective zone) relevant to warm sub-Neptunes. This assumption is load-bearing for the central claim; a 50–100 K downward shift in the boundaries would close the reported window for TOI-270 d and similar planets.

    Authors: We agree that the phase boundaries are adopted from existing literature rather than newly computed in this study. The primary advance of the manuscript is the ATHENAIA framework that couples these boundaries to radiative-convective atmospheres and composition-dependent interiors for warm sub-Neptunes. To address the sensitivity concern directly, the revised manuscript will include a dedicated sensitivity analysis subsection. We will shift the demixing temperature boundaries by ±50 K and ±100 K, recompute the resulting metallicity windows for TOI-270 d and the 330–450 K population, and discuss how these shifts affect the reported conclusions. revision: yes

  2. Referee: The compatibility statement for TOI-270 d and the broader 330–450 K range rest on the specific coupling between the radiative-convective P-T profiles and the composition-dependent adiabatic gradient; the manuscript does not report explicit tests of how variations in these profiles or in the imported phase boundaries alter the intersection points.

    Authors: The referee is correct that the manuscript presents the nominal coupling but does not include explicit variation tests. In revision we will add quantitative tests of how changes to the radiative-convective P-T profiles (arising from different opacity or metallicity assumptions) and to the adiabatic gradient affect the intersection with the phase boundaries. These tests will be shown for TOI-270 d and summarized for the broader temperature range, either in the main text or as a new appendix figure. revision: yes

standing simulated objections not resolved
  • New ab initio calculations or experimental validation of the water-hydrogen demixing boundaries at the metallicities, pressures, and temperatures relevant to warm sub-Neptunes

Circularity Check

0 steps flagged

No significant circularity in model-derived demixing window

full rationale

The paper introduces ATHENAIA as a new coupling of radiative-convective atmosphere models to interior structure models and applies it to locate intersections between P-T profiles and imported H2-H2O demixing boundaries for metallicities 150-700x solar. This produces the reported window as a computational output rather than a re-expression of inputs by definition. Phase boundaries are external literature inputs (not self-citations or fits performed inside this work), and the derivation chain remains self-contained against those benchmarks without reducing the central claim to a tautology or load-bearing self-reference. No steps meet the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim depends on standard assumptions from planetary interior modeling plus the specific demixing phase diagram adopted for water-hydrogen mixtures. No new particles or forces are introduced.

free parameters (1)
  • bulk envelope metallicity range
    The 150-700× solar window is the output range reported for demixing; it is not an input fit but emerges from the model grid.
axioms (2)
  • domain assumption Water-hydrogen demixing phase boundaries from prior literature remain valid at the pressures and metallicities relevant to sub-Neptune envelopes.
    Invoked when mapping the higher demixing temperatures in water-rich environments to the 330-450 K window.
  • domain assumption Radiative-convective equilibrium and composition-dependent adiabatic gradients can be coupled without additional free parameters beyond those already calibrated in the literature.
    Used to link atmosphere greenhouse effects to interior temperature profiles.

pith-pipeline@v0.9.0 · 5644 in / 1553 out tokens · 70668 ms · 2026-05-17T02:30:14.103892+00:00 · methodology

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Forward citations

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