Soot-Rich Planets: Carbon-Rich Worlds

• Soot-rich planets are exoplanetary bodies with high refractory carbon inventories from polyaromatic and CHON-rich macromolecules.
• They exhibit nonstandard mass–radius relations with layered interiors, including metallic cores, silicate mantles, and soot shells.
• Their atmospheres feature high CH4, PAH-driven haze, and muted spectral features, offering distinctive diagnostic signatures.

Soot-rich planets are planetary bodies whose interiors and/or atmospheres possess a significant fraction of refractory organic carbon—soot—originating from condensing polyaromatic and CHON-rich macromolecules in the proto-planetary disk. These worlds are distinguished from water-rich or rocky planets by their substantial carbon inventories, distinctive chemistry and aerosol processes, and unique mass–radius and spectral properties. Soot-rich exoplanets can form as both terrestrial and sub-Neptune-class objects, and account for several observed features among transiting exoplanets, such as low bulk densities, extreme C/O atmospheric ratios, and muted or featureless transmission spectra.

1. Formation Environments and the Soot Line

The formation of soot-rich planets is set by temperature-dependent condensation fronts in protoplanetary disks, notably the “soot line”—the radial zone beyond which refractory organic carbon survives thermal decomposition. The soot line, typically at midplane temperatures $T_\mathrm{mid}\sim500$ K (corresponding to radii $r_{\rm soot}\sim0.3$–1 AU in solar-type disks), marks the inner disk boundary for the condensation and incorporation of macromolecular organics into accreting solids. Planets forming in the interval between the soot line and the water ice line ($T_\mathrm{mid}\sim170$ K; $r_{\rm w}\sim2$–5 AU) can accrete both silicates and refractory carbon, yielding compositions with soot mass fractions $f_{\mathrm{soot}}$ up to $\sim0.4$ and variable water content. Interior to the soot line, organics are irreversibly destroyed, while beyond the snow line, bodies can additionally incorporate water ice, forming “soot-water” worlds (Li et al., 22 Aug 2025, Bergin et al., 2023, Dorn et al., 18 Jun 2026).

The prevailing processes in this condensation interval include inheritance of interstellar CHON-rich grains, in-disk polyaromatic synthesis and downward transport, and thermal/chemical processing of solids. This partitioning directly determines the planet’s bulk carbon inventory and establishes the physical context for subsequent outgassing, volatile retention, and atmospheric evolution.

2. Interior Structure, Composition, and Equations of State

Soot-rich planets exhibit highly nonstandard internal structures, with mass–radius relations that are notably distinct from both silicate–metal and water-rich worlds. Their interiors can be stratified as a differentiated series:

• Metallic core
• Silicate mantle
• Soot (macromolecular solid-phase organic) shell
• Water-ice shell (if beyond the snow line)

Mixing models are also considered, wherein silicate, soot, and water are distributed throughout the mantle and crust. The carbon-rich layer is physically and chemically diverse, spanning compressibility regimes from amorphous–water-like to diamond-like, with density estimates of $\rho_\mathrm{soot}\sim1.32$ g cm$^{-3}$ (C:H:O =100:78:17 reference) (Li et al., 22 Aug 2025).

For sub-Neptunes, Lin & Seager (Lin et al., 20 Aug 2025) propose a four-layer spherically symmetric model governed by the mass conservation and hydrostatic equilibrium equations:

$$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$

with shells of iron–silicate core, graphite/diamond soot, H/He envelope, and possible ice. Increasing the soot mass fraction at fixed mass inflates the planetary radius (though less efficiently than a thick H/He envelope), facilitating bulk densities that would otherwise require large water inventories.

Mechanical and thermal consequences of a soot layer include increased viscosity and thermal conductivity (for diamond-like phases), which can suppress convective heat transport, impede plate tectonics, and alter differentiation and dynamo processes. Interior structure models show that up to 40 wt% soot, or combined soot+water with $f_{\rm soot}+f_{\rm H_2O}\leq0.5$, can explain most observed sub-Neptune densities (Li et al., 22 Aug 2025, Lin et al., 20 Aug 2025).

3. Atmospheric Chemistry, Soot Formation, and Aerosol Processes

Atmospheres of soot-rich planets reflect their unique bulk chemistry. Outgassing from soot-rich mantles or envelopes supplies high hydrocarbon abundances—most notably CH$r_{\rm soot}\sim0.3$0, with additional C$r_{\rm soot}\sim0.3$1H$r_{\rm soot}\sim0.3$2, C$r_{\rm soot}\sim0.3$3H$r_{\rm soot}\sim0.3$4, NH$r_{\rm soot}\sim0.3$5, H$r_{\rm soot}\sim0.3$6, and HCN. For high soot content and strongly reducing conditions, C/O ratios can approach or exceed unity, fundamentally shifting equilibrium composition away from H$r_{\rm soot}\sim0.3$7O and CO$r_{\rm soot}\sim0.3$8 dominance (Lin et al., 20 Aug 2025, Dorn et al., 18 Jun 2026, Bergin et al., 2023).

Atmospheric structure calculations employ global chemical equilibrium, radiative–convective equilibrium (via, e.g., HELIOS/HELIOS-K), and photochemical transport models (e.g., VULCAN, Photochem, and FastChem), culminating in self-consistent P–T profiles, species vertical distributions, and opacity grids (Dorn et al., 18 Jun 2026, Lin et al., 20 Aug 2025).

Soot and haze formation in the atmosphere follow two principal pathways:

• Deep-atmosphere thermochemistry: In sub-Neptunes with $r_{\rm soot}\sim0.3$9–800 K, the lower atmosphere acts as a “soot factory,” converting methane through successive hydrocarbon steps (involving C$T_\mathrm{mid}\sim170$0H$T_\mathrm{mid}\sim170$1, PAHs) into macromolecular soot precursors. Quench-level mixing establishes a bell-shaped abundance profile of polycyclic aromatic hydrocarbons (PAHs, peaking at $T_\mathrm{mid}\sim170$2 K), which are then lofted into observable layers (Yang et al., 13 Apr 2026).
• Photochemistry: UV-driven polymerization of small hydrocarbon fragments generated by CH$T_\mathrm{mid}\sim170$3 photodissociation further augments haze mass and produces optically dominant fractal aggregates. Dominant opacity sources are sub-micron soot grains, which yield grey (wavelength-independent) absorption from the near-IR to visible (Bergin et al., 2023, Zhang et al., 4 Sep 2025).

On some extreme objects, such as PSR J2322–2650 b, direct JWST spectroscopy finds emission features from C$T_\mathrm{mid}\sim170$4 and C$T_\mathrm{mid}\sim170$5 clusters—solid carbon precursors—confirming active high-temperature carbon polymerization:

$T_\mathrm{mid}\sim170$6

with C/O $T_\mathrm{mid}\sim170$7 and C/N $T_\mathrm{mid}\sim170$8 (Zhang et al., 4 Sep 2025).

4. Observable Signatures and Diagnostic Criteria

Detection and characterization of soot-rich planets exploit both bulk and spectral diagnostics:

• Bulk properties: Mass–radius measurements alone can indicate a significant soot fraction when densities are lower than rock-only worlds but cannot be fully attributed to H$T_\mathrm{mid}\sim170$9O or H/He.
• Transmission/emission spectra: Soot and haze opacity, especially for PAH-dominated atmospheres, mute spectral features below 3 μm, producing featureless or “flat” transmission spectra (e.g., GJ 1214 b, LTT 9779 b). Strong CH$r_{\rm w}\sim2$0 and C$r_{\rm w}\sim2$1H$r_{\rm w}\sim2$2 absorption at 1.7–3.5, 3.3, and 7.7 μm, and absence/muting of H$r_{\rm w}\sim2$3O and CO$r_{\rm w}\sim2$4 bands, are expected.
• Two-dimensional diagnostics: Combining H$r_{\rm w}\sim2$5O/CH$r_{\rm w}\sim2$6 band ratios with mean molecular weight ($r_{\rm w}\sim2$7) measurements in the ($r_{\rm w}\sim2$8, $r_{\rm w}\sim2$9) plane provides a classification metric, distinguishing soot–water–rock planets (low H$f_{\mathrm{soot}}$0O/CH$f_{\mathrm{soot}}$1, high $f_{\mathrm{soot}}$2) from water–rock and rock-only worlds (Dorn et al., 18 Jun 2026).
• Sooting propensity parameter ($f_{\mathrm{soot}}$3): Defined as

$f_{\mathrm{soot}}$4

this metric captures the chemical–aerosol link underlying the observed parabolic trend in spectral muting for $f_{\mathrm{soot}}$5 K (Yang et al., 13 Apr 2026).

A summary of formation regimes and their signatures:

Zone	f$f_{\mathrm{soot}}$6	f$f_{\mathrm{soot}}$7	f$f_{\mathrm{soot}}$8_2$f_{\mathrm{soot}}$9	Atmospheric trait
Rocky	$\sim0.4$01	0	0	Low C/O, strong H$\sim0.4$1O
Soot planet	$\sim0.4$20.6	$\sim0.4$30.4	0	CH$\sim0.4$4-dom., high $\sim0.4$5
Soot-water-rock	$\sim0.4$60.55	$\sim0.4$70.2	$\sim0.4$80.25	CH$\sim0.4$9 + H$\rho_\mathrm{soot}\sim1.32$0O, high Z
Water planet (ocean)	$\rho_\mathrm{soot}\sim1.32$10.4	0	$\rho_\mathrm{soot}\sim1.32$20.5	H$\rho_\mathrm{soot}\sim1.32$3O-dominated

5. Notable Examples and Observational Case Studies

Pulsar-planets—“Ultra-carbon planets”

PSR J2322–2650 b, observed by JWST/NIRSpec, exhibits a helium-rich, carbon-cluster-dominated atmosphere (C$\rho_\mathrm{soot}\sim1.32$4/C$\rho_\mathrm{soot}\sim1.32$5 emission, C/O $\rho_\mathrm{soot}\sim1.32$6 100), sub-micron soot grains, and dynamical signatures of westward hot-spot offsets and ultra-fast rotation (Zhang et al., 4 Sep 2025). This sets an upper bound on carbon enrichment and illustrates the physics of soot nucleation under extreme C/O.

Sub-Neptunes and Super-Earths

• TOI-270 d: Consistent with a broad range ($\rho_\mathrm{soot}\sim1.32$7–$\rho_\mathrm{soot}\sim1.32$8 wt%) of soot in the interior and a C/O near unity; moderate metallicity envelopes, CH$\rho_\mathrm{soot}\sim1.32$9 absorption, and discernible clouds/hazes (Lin et al., 20 Aug 2025).
• K2-18 b: Spectral fits permit up to $^{-3}$0 wt% soot; high C/O and metallicity requirements are met, but enhancement of CO$^{-3}$1 above model predictions likely requires an active soot outgassing source (Lin et al., 20 Aug 2025, Dorn et al., 18 Jun 2026).
• GJ 1214 b: Flat transmission spectrum requires $^{-3}$2 solar metallicity; carbon-rich interiors are disfavored unless the current envelope is secondary and not representative of interior C/O (Lin et al., 20 Aug 2025, Dorn et al., 18 Jun 2026).
• TOI-836 c: Tentative CH$^{-3}$3-rich, high-$^{-3}$4 candidate, possibly an empirical example of a soot–water–rock planet (Dorn et al., 18 Jun 2026).

Population-level Trends

The parabolic dependence of spectral feature amplitude on $^{-3}$5 for sub-Neptunes ($^{-3}$6–$^{-3}$7 K) is accounted for by the thermochemical formation and vertical quenching of soot precursors (PAHs), modulated by C/O and metallicity. Planets in this regime with elevated C/O and [M/H] show the most muted spectra due to maximal PAH/soot haze production (Yang et al., 13 Apr 2026).

6. Evolutionary, Chemical, and Habitability Implications

Soot-rich planets provide a mechanism for forming low-density, methane-rich atmospheres without requiring large primordial H/He envelopes. High levels of refractory carbon favor reducing conditions and facilitate prebiotic chemistry by persistent outgassing of CH$^{-3}$8, H$^{-3}$9, and NH$$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$0, while photochemical hazes may deliver complex organics to surfaces. However, interiors with high soot or diamond content may hinder plate tectonics and magnetic dynamo generation due to enhanced viscosity and thermal conductivity, potentially restricting surface habitability (Li et al., 22 Aug 2025, Bergin et al., 2023).

In the context of origin models, both classical “black widow” stripping and merger scenarios fail to reproduce the most extreme C/O or C/N ratios observed in objects like PSR J2322–2650 b (Zhang et al., 4 Sep 2025). This suggests the need for exotic evolutionary paths or previously unrecognized condensation and fractionation mechanisms.

7. Future Prospects and Observational Strategies

JWST’s spectral reach (1–5 μm with NIRISS/NIRSpec; 5–12 μm for MIRI) and sensitivity enable the direct identification of soot-rich planets via characteristic muted spectra, CH$$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$1 and C$$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$2H$$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$3 absorption, and haze continua. High-resolution ground-based spectroscopy can access complementary CH$$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$4 lines. Reflected-light imaging by next-generation ELTs or missions such as HabEx/LUVOIR will further constrain soot-rich planet occurrence by measuring albedo suppression unique to thick organic hazes (Lin et al., 20 Aug 2025, Yang et al., 13 Apr 2026).

Future cycles of JWST/ELT transit and eclipse spectroscopy, combined with ALMA mapping of carbonation in disks, will refine the abundances and chemistry of refractory carbon and enable robust statistical mapping of soot-rich planets in exoplanetary population studies (Bergin et al., 2023).

Emerging diagnostics—such as the $$\frac{dm}{dr}=4\pi r^2\rho(r),\qquad \frac{dP}{dr}=-\frac{Gm(r)\rho(r)}{r^2},\qquad P=f(\rho,T)$$5 plane and sooting propensity parameter—anchor a physically motivated observational framework for identifying, classifying, and contextualizing soot-rich planets within the broader architecture of exoplanetary formation and atmospheric evolution (Dorn et al., 18 Jun 2026, Yang et al., 13 Apr 2026).