JuanCastillo29/space-mission-intelligence-data huggingface dataset:
Recent JWST detections of CH4 and CO2 in sub-Neptune atmospheres point to a link between atmospheric composition and the nature of planetary building blocks – rock, water, or refractory carbon (“soot”) – yet this connection remains poorly understood. Here we investigate how different formation environments shape the coupled interior and atmospheric compositions of sub-Neptunes. We model planets assembled from varying proportions of rock, water, and soot and compute the global chemical equilibrium and the overlying atmospheric structure. We find that planets formed from water-poor material produce atmospheres strongly depleted in carbon-bearing species, with log(CH4) and log(CO2) below −4. In contrast, planets assembled from water-rich building blocks naturally develop methane- and carbon-dioxide-rich atmospheres with elevated metal mass fractions and C/O ratios. The presence of refractory carbon (soot) further enhances methane production and can lead to methane-dominated atmospheres. Comparison with JWST observations suggests that water-rich formation is sufficient to explain K2-18 b and TOI-270 d with no soot component required, while TOI-421 b and GJ3470 b are consistent with water-poor formation inside the water ice line. The ratio H2O/CH4 combined with the mean molecular weight (MMW) provides a powerful two-dimensional diagnostic linking atmospheric composition to formation environment, with departures from the predicted trends explained by water condensation in temperate atmospheres or fractionated atmospheric mass loss.
Key words. Exoplanet structure, Exoplanet atmospheric structure , Exoplanet atmospheric composition
capable of reproducing the observed M–R relationships of sub-Neptunes (Lin & Seager 2025). In early planet-forming disks, the main carriers of carbon are organic ices and macromolecular refractory carbonaceous dust. The final carbon budget of a planet may undergo substantial processing and volatile loss (Bergin et al. 2015). The availability of carbon in the upper atmosphere is strongly influenced by the formation location in the disk, the degree of chemical processing, and the fraction of carbon sequestered into the deep interior. Soot-dominated planets, and variants that also incorporate water ice, termed soot-water worlds, may therefore possess chemically distinct interiors and atmospheres enriched in hydrocarbons such as methane. Such compositions can also promote haze and cloud formation (Mahapatra et al. 2017; Lee et al. 2025), with important implications for atmospheric observability and habitability. In parallel, Lin & Seager (2025) investigate sub-Neptunes with carbon-rich interiors consisting of an ironsilicate core, an intermediate carbon layer, and envelopes spanning metallicities from 1 to 1000 times solar. Their models reproduce the M–R relationships and transmission spectra of key exoplanets, including TOI-270 d and K2-18 b. In their framework, the atmospheres exhibit high metallicities and elevated C/O ratios, reaching up to 100 times solar. A central assumption of this model is that the atmospheric C/O ratio is identical to that of the deeper interior and thus to the planetary bulk. However, when chemically active interiors are considered, the C/O ratios
Recent JWST detections of CH4 and CO2 in sub-Neptune atmospheres point to a link between atmospheric composition and the nature of planetary building blocks — rock, water, or refractory carbon — yet this connection remains poorly understood. Traditional mass–radius (M–R) interpretations invoke mixtures of rock, water, and gaseous envelopes to explain observed densities of sub-Neptunes. However, recent studies point to alternative scenarios: planets enriched in refractory organic carbon, termed soot, and formed between the soot line and the water ice line (Bergin et al. 2014), may represent a common and previously underappreciated class of low-density worlds (Li et al. 2026; Lin & Seager 2025). High-precision atmospheric measurements from JWST now provide an opportunity to test these formation scenarios directly. Li et al. (2026) argue that bodies forming in carbon-rich regions of protoplanetary disks naturally accrete substantial amounts of soot in addition to rock, analogous to the composition of Solar System comets. Soot, together with water ice, may therefore constitute viable components of the building blocks of sub-Neptunes. Because both soot and water have low intrinsic densities, and because bulk compositions inferred from M–R measurements are highly degenerate (Rogers & Seager 2010; Dorn et al. 2017), these materials are plausible and potentially significant constituents
⋆Corresponding author: dornc@ethz.ch
Article number, page 1 of the bulk, interior, and atmosphere can differ from each other by orders of magnitude (Seo et al.
of Solar System comets. Soot, together with water ice, may therefore constitute viable components of the building blocks of sub-Neptunes. Because both soot and water have low intrinsic densities, and because bulk compositions inferred from M–R measurements are highly degenerate (Rogers & Seager 2010; Dorn et al. 2017), these materials are plausible and potentially significant constituents
⋆Corresponding author: dornc@ethz.ch
Article number, page 1 of the bulk, interior, and atmosphere can differ from each other by orders of magnitude (Seo et al.

2024; Werlen et al. 2025b) because of volatile sequestration (incl. hydrogen, oxygen, and carbon) in the deep interior (Kite et al. 2016; Dorn & Lichtenberg 2021; Schlichting & Young 2022; Luo et al. 2024; Bower et al. 2025). Taken together, Li et al. (2026) and Lin & Seager (2025) underscore the importance of carbon as a central component in interpreting sub-Neptune interiors and spectra. A key gap remains: neither study explicitly models interior–atmosphere chemical equilibration to connect the composition of planetary building blocks to observable atmospheric signatures. This paper aims to fill that gap. We investigate the chemical equilibration mechanisms in soot-enriched planets thereby encompassing chemical reactivity in melts, gases, and between phases, volatile solubility in silicates, partitioning in metals , and outgassing of hydrocarbon species. By coupling thermochemical models with atmospheric structure models, we assess how soot-rich interiors may evolve chemically after formation and how these equilibrium states influence atmospheric signatures. The main underlying assumption is that the interior and the gas are in chemical equilibrium. We adopt complete interior–atmosphere chemical equilibration as a deliberate end-member. In reality, convection inhibition and mean molecular weight gradients may restrict chemical exchange on a global scale (e.g., Leconte et al. 2017; Spaargaren et al. 2020; Misener & Schlichting 2022; Markham et al. 2022; Leconte et al. 2024), such that sub-Neptunes likely achieve equilibration only in certain regions. Current sootplanet models typically adopt the opposite end-member of chemically inactive, decoupled interiors (Li et al. 2026; Lin & Seager 2025); our study explicitly examines the consequences of interior–atmosphere equilibration between these two limits. With JWST now delivering transmission spectra precise enough to robustly detect CH4, CO2, and H2O in sub-Neptune atmospheres, these predictions may be directly tested against a rapidly growing observational sample. This paper is structured as follows. In Section 2, we describe the models of both global chemical equilibrium and atmospheric structure. In Section 3, we present the predicted trends in atmospheric properties that stem from different bulk core compositions. In Section 3.2, we compare model predictions with JWST observations of characterized sub-Neptunes (incl., K2-18 b, TOI-776 c, TOI-270 d, TOI-836 c, GJ3470 b, GJ9827 d, TOI 421 b, TOI-732 c, and LP791-18 c). Finally, we discuss limitations in Section 4 and conclude in Section 5.
10 1
12.5
0.6
10 2
10.0
7.5
0.4
10 3
10 2
5.0
10 4
0.2
2.5
0.02 0.04 0.06 0.08
0.02 0.04 0.06 0.08
0.02 0.04 0.06 0.08
0.02 0.04 0.06 0.08
Case 2, Mp = 10M
Case 3, Mp = 10M
Case 4, Mp = 10M
Mp =10M  w/ H2O condensation

[Figure: Fig. 2. Atmospheric properties of the four modeled planet cases after chemical equilibration: (a) atmospheric C/O ratio, (b) atmospheric mass fraction, (c) atmospheric metal mass fraction Z, and (d) mean molecular weight (MMW). The volatile-rich cases (3 and 4) display markedly higher atmospheric C/O ratios and metal mass fractions than the volatile-poor cases (1 and 2). Case 3 also exhibits an atmospheric mass fraction roughly an order of magnitude larger and a significantly higher MMW than in the other cases, including the water-rock case (4). In panel (d), semi-transparent lines with crosses show the MMW after accounting for water condensation; only Case 4 is visibly affected, as water is negligible in the other cases.]
nents, along with the total number of moles in each phase,
by simultaneously enforcing chemical equilibrium, mass bal-
ance, and elemental conservation in each phase. Our nu-
merical scheme is (Grimm et al. 2026) has significantly im-
proved in computational efficiency upon the original imple-
mentation in (Schlichting & Young 2022). For a full descrip-
tion of the equilibrium formulation, including the reaction
network and governing equations, we refer the reader to the
Appendix A, as well as Grimm et al. (2026).
can reach wt% levels (Miozzi et al. 2025; Horn et al. 2025;
Gilmore & Stixrude 2026; Young & Werlen 2026), we have
this choice represents a key source of model uncertainty.
2.2. Atmospheric profiles
We formally couple the deep atmosphere to the observ-
able upper layers of the atmosphere. The deep atmosphere
composition and pressure are set by its chemical equi-
librium with the magma ocean. Those atmospheric con-
molar gas fractions
0.02 0.05 0.08 10
0.02 0.05 0.08
0.02 0.05 0.08
0.02 0.05 0.08
accreted mass fraction of primordial gas
Species H2 gas CH4 gas
CO2 gas CO gas
O2 gas H2O gas
Fe gas SiO gas
Mg gas

[Figure: Fig. 3. Molar mixing ratios of atmospheric species in equilibrium with the underlying magma ocean. Although the gas composition]
mospheric models: FastChem (Stock et al. 2018), HELIOS
(Malik et al. 2017, 2019), VULCAN (Tsai et al. 2017), and
HELIOS-K (Grimm et al. 2021). These models together en-
able the simulation of chemical equilibrium, photochem-
istry, and radiative transfer with species-dependent opaci-
ties over the pressure range of the atmosphere and up to
der local thermodynamic conditions. HELIOS is a one-
dimensional radiative-convective model used to compute
pressure-temperature (P–T) profiles. HELIOS-K calculates
wavelength-dependent opacities for given gas mixtures.
VULCAN is a solver for kinetic reaction networks that incor-
porates thermochemistry, photochemistry, vertical mixing,
and condensation. Together, these models allow us to simu-
late atmospheric structures from the magma ocean interface
to the upper observable atmosphere.
core composition cases and investigate how the resulting
atmospheric properties depend on formation environment.
Figure 2 summarizes the bulk atmospheric properties (i.e.,
C/O ratio, atmospheric mass fraction, metal mass fraction,
and mean molecular weight) as a function of the accreted
primordial gas fraction.
Planetary cores assembled from volatile-rich material
(cases 3 and 4) are characterized by markedly higher at-
mospheric C/O ratios and metal mass fractions than their
volatile-poor counterparts (cases 1 and 2). The C/O ra-
tios of the pure rock (case 1) and soot–rock (case 2) cases
fall at least three orders of magnitude below those of the
water-rich cases, which reach solar to super-solar values.
This result may appear counterintuitive, since the addition
of water introduces oxygen and might therefore be expected
to lower the atmospheric C/O ratio. However, in the water-
rich cases the added oxygen is preferentially incorporated
into the deep interior. In addition to the dominance of oxy-
gen in silicates, large amounts of oxygen partition into the
iron-dominated metal phase, reaching tens of weight per-
cent, while simultaneously yielding increased abundances
deep atmosphere between 10 and 1000 bar.

These dynamics are reflected directly in the mean molecular weight profiles (bottom panel).

Under strong mixing, the MMW remains nearly constant with pressure across all cases.

Under weak mixing, the MMW can vary by up to one order of magnitude in case 3, while changes are moderate for other cases.

The sensitivity of the MMW to mixing strength is therefore most pronounced in case 3, where photochemistry dramatically lightens the upper atmosphere.

Despite this sensitivity to vertical mixing, the fundamental chemical differences between cases identified at depth remain discernible within the pressure range Article number, page 5

Case 2
Case 3
Case 4
low Kzz high Kzz
Pressure [bar]
1000 2000 3000 Temperature [K]
1000 2000 3000 Temperature [K]
1000 2000 3000 Temperature [K]
Pressure [bar]
H2 CH4 CO2
CO H2O
low Kzz high Kzz
Mixing Ratio
Mixing Ratio
Mixing Ratio
low Kzz high Kzz
Pressure [bar]
2 3 5 10 22 Mean Molecular Weight [g/mol]
2 3 5 10 22 Mean Molecular Weight [g/mol]
2 3 5 10 22 Mean Molecular Weight [g/mol] Fig. 4. Atmospheric profiles connecting the AMOI interface with the upper observable part of the atmosphere. Profiles are shown for the soot-rock case 2 (left), the soot-water-rock case 3 (middle), and the water-rock case 4 (right). Top: Pressure–temperature profiles before and after convergence. Middle: Vertical mixing ratio profiles of the considered gas species. Bottom: Vertical profiles of MMW. Here, we consider planets of Mp = 6 M⊕. All panels show results for two different eddy diffusion coefficients, kzz = 104
and 107 cm2s−1, which control vertical mixing in the atmosphere. The stellar spectrum is modeled as a blackbody.
probed by transmission spectroscopy (∼10−2 to 10−5 bar) (e.g., Benneke et al. 2024). Soot–rock planets (case 2) are predicted to exhibit H2–H2O–CH4 dominated upper atmospheres in that order, whereas the soot–water–rock case (case 3) may maintain methane dominance with CH4–CO–H2 (strong mixing) or H2–CH4–H2O under weak mixing, in both instances retaining methane as a prominent species. Water-rock planets (case 4) similarly transition from H2–CO–CH4 under strong mixing to H2–CH4–H2O
00 00 0 K K |  |  |  |  |  |
|  | Case 3 3000 K 2000 K Case 4 3000 K 2000 K |  |  |  |  |  |
|  |  |  |  |  |  |  |
|  | K2-18 b |  |  |  |  |  |
|  | TOI-836 c (if aer |  |  | osols) TOI-776 c (if | TOI clear) (if | -836 c clear) |

|  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
|  |  | TOI-421 b fractionate |  | d mass loss |  |  |  |
|  |  |  |  | GJ3470 b |  |  | GJ9827 d |
|  |  |  |  |  |  |  |  |
| v | Core compositions pure rock olatile |  |  |  | TOI-270 d |  |  |
| v | poor olatile rich | soot-rock water-rock water-soot-rock |  | LP 791- |  | 18 c |  |
|  | 2O sation |  |  | 2O sation |  |  |  |
|  | H conden |  |  | H conden |  |  |  |
|  | K2-18 b |  |  |  |  |  |  |

H2O/CH4
H2O/CH4
Teq [K]
10 1
10 2

[Figure: Fig. 5. Molar mixing ratios of CH4/H2O versus mean molecular weight (MMW) as a diagnostic of sub-Neptune formation location
- together with inferred atmospheric characteristics of sub-Neptunes from JWST data. Right: Model predictions for all four core composition cases across variations in TAMOI, showing how volatile-poor (cases 1 and 2) and volatile-rich (cases 3 and 4) formation scenarios occupy distinct regions of this parameter space. Left: Simplified representation of the right panel showing approximate regions occupied by each case, together with locations of characterized sub-Neptune atmospheres. Gray arrows illustrate how two evolutionary processes, i.e., water condensation in temperate atmospheres and fractionated atmospheric mass loss, can displace planets from the main equilibrium trend. TOI-732 c is omitted as constraints on MMW and water abundances are too weak.]
Context. The long-term evolution of planetary systems around solar-type stars is governed by the interplay between stellar expansion, tidal interactions, and mass loss during the red giant branch (RGB) and asymptotic giant branch (AGB) phases. However, tidal dissipation efficiencies and AGB mass-loss rates both remain poorly constrained, leading to significant uncertainty in predicting the fate of planetary systems, in particular, that of the Earth orbiting the ageing Sun. Aims. We reassess the survival of the Earth and the inner Solar System planets during the post-main-sequence evolution of the Sun, focusing on the impact of updated tidal dissipation prescriptions and varying AGB mass-loss rates. Methods. We modelled the orbital evolution of the Earth using stellar evolution tracks for a solar-mass star that incorporate the most recent tidal dissipation prescriptions derived from ab initio calculations. We compared these results with outcomes obtained using previously published and commonly adopted tidal prescriptions, and we explored a range of AGB mass-loss rates. Results. We find that the predicted fate of the Earth is highly sensitive to the tidal model and the assumed mass-loss rate. Based on updated tidal dissipation prescriptions, Earth survives the RGB and AGB phases of the Sun. In contrast, the use of earlier prescriptions of tidal dissipation prescriptions leads to engulfment during the AGB phase. Furthermore, low AGB mass-loss rates result in engulfment, and vice versa. Using the observed mass-loss rates of the AGB star L2 Pup as a proxy for the Sun’s future AGB mass-loss rate results in the survival of the Earth during the AGB phase when combined with our tidal dissipation evaluation. Conclusions. Currently, the survival of the Earth and the inner Solar System is not robustly determined and critically depends on the treatment of tidal dissipation and stellar mass loss. Given the current observational uncertainties in AGB mass-loss rates, the ultimate fate of the Earth remains uncertain, highlighting the need for improved constraints on the late-stages of stellar evolution. However, considering observational proxies for the Sun during the AGB phase, it is likely that the Earth will survive the Sun’s red giant phase.
Key words. methods: numerical — planet-star interactions — binaries: close — stars: evolution — planetary systems — Earth
sumptions.
The most fundamental is complete interior–atmosphere
chemical equilibration. Convective inhibition and mean
molecular weight gradients may restrict chemical exchange
on a global scale (e.g., Leconte et al. 2017; Spaargaren et al.
2020; Misener & Schlichting 2022; Markham et al. 2022;
Leconte et al. 2024). In particular, mean molecular weight
gradients
in
the
atmosphere
may
operate
over
long
timescales and limit how far one can make interpretations
about the bulk composition from the observable part of
the atmosphere. The true atmospheric composition of any
individual planet likely lies between the fully equilibrated
state modeled here and the chemically decoupled interior
assumed in previous soot-planet models. The vertical mix-
ing sensitivity explored in Figure 4 only partially accounts
for this limitation; ideally, evolutionary models that track
compositional gradients, their erosion, and layered convec-
tion (Markham et al. 2022) would be employed, but this is
beyond the scope of the present work.
A second key uncertainty is the magma ocean interface
and demonstrate that the main compositional trends are
preserved at 2000 K, though the predicted MMW distri-
bution for case 4 significantly broadens at lower tempera-
tures, reducing the discriminating power of that diagnostic.
The choice of upper bound is physically motivated: recent
work suggests that at temperatures approaching 4000 K,
gases and refractories become miscible (Young et al. 2025),
dissolving the distinct magma ocean–atmosphere interface
that our model requires. The thermodynamic data underly-
ing our GCE model would also require substantial extrap-
Neptunes carry a direct imprint of their formation environ-
ment, and that global chemical equilibration between plan-
etary interiors and atmospheres is essential for correctly
interpreting this imprint. By modeling four end-member
core compositions — rock, soot–rock, soot–water–rock, and
water–rock — spanning the range of building blocks avail-
able at different locations in the protoplanetary disk, we
identify a set of atmospheric diagnostics that link JWST
observables to planetary origins.
Planets formed from water-poor material produce atmo-
spheres strongly depleted in carbon-bearing species, with
and GJ3470 b, while GJ9827 d may have evolved from such
a state through preferential hydrogen loss. Planets assem-
tions and C/O ratios, consistent with TOI-776 c, TOI-270 d,
bon is sequestered deep in the planetary interior rather
than outgassed, a result that depends critically on allow-
ing carbon to partition into the metallic phase – a key dif-
ference from otherwise similar magma ocean frameworks
(Nixon et al. 2025)– and explains why only water-rich for-
C.D acknowledges support from the Swiss National Science Foundation under grant TMSGI2_211313. This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grant 51NF40_205606. We acknowledge the use of large language models (LLMs), including ChatGPT, to improve the grammar, clarity, and readability of the manuscript. S.J. acknowledges funding support from ETH Zurich and the NOMIS Foundation in the form of a research fellowship. The NOMIS Foundation ETH Fellowship Programme and respective research are made possible thanks to the support of the NOMIS Foundation.
the chemical reaction network are adopted from the compilation presented in Schlichting & Young (2022).

In addition to their framework, we explicitly account for carbon dissolution into the metallic phase by implementing metal–silicate partition coefficients from Blanchard et al. (2022).

Hydrogen solubility in silicate melt is described using the revised formulation introduced by Werlen et al. (2026b), which combines the experimental constraints of Hirschmann (2012) with DFT–MD results from Gilmore & Stixrude (2026).

Deviations from ideal solution behavior are considered only within the metallic phase.

For Si, O, and H, we follow the interaction model outlined in Young et al. (2023) and applied in Werlen et al. (2025a), ultimately based on Article number, page 11 the formalism of Badro et al.

(2015). Carbon activity coefficients in metal are taken from Fischer et al. (2020), consistent with the implementation described by Werlen et al. (2025a). We assume ideal mixing in both the gas and silicate melt phases. As discussed by Werlen et al. (2026b), introducing non-ideality in only a subset of phases can produce artificial compositional trends, while robust high-pressure, high-temperature activity data for multicomponent silicate melts remain limited.
Article number, page 12
M. Esseldeurs1⋆, S. Mathis2, and L. Decin1 1 Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium 2 Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France