log(CO2) and log(CH4) typically falling between −3 and −1.

C/O ratios are generally approximately solar to super-solar, with individual retrieval cases reaching values as high as 3.21+6.1 −1.9 (Felix et al. 2025b), and atmospheric metal mass fractions are consistently high, with representative values around Zatm ∼0.6.

The CO2 abundances further confirm the conclusion drawn from H2O/CH4 in Section 3.2: both planets strongly disfavour water-poor formation pathways (cases 1 and 2).

In water-poor scenarios, carbon is sequestered deep in the planetary interior rather than outgassed into the atmosphere, keeping log(CO2) ≲−6 and placing it well outside the observational constraints for both planets.

Only waterrich initial compositions reproduce the observed CO2 abundances, C/O ratios, and atmospheric metal mass fractions within the correct order of magnitude.

We note that this conclusion differs from Nixon et al. (2025), who employed a similar interior–atmosphere chemical equilibrium framework but without carbon partitioning into the metallic phase.

In that end-member treatment, carbon was mainly present by construction in the atmosphere, which naturally promotes higher atmospheric carbon abundances in volatile-poor scenarios.

Our treatment explicitly allows carbon to partition into the metal, revealing that sequestration into the deep interior becomes the dominant fate of carbon when volatile-poor building blocks are assumed.

Both approaches confirm that magma ocean–atmosphere equilibration can reproduce the observed atmospheric composition of planets such as TOI-270 d; the two frameworks differ primarily in the predicted carbon budget of volatile-poor formation scenarios, where the inclusion of metal–carbon partitioning shifts carbon away from the atmosphere and into the interior.

CO2 also offers a potential discriminant between cases 3 and 4.

Case 4 predicts log(CO2) between roughly −5 and −2 at low atmospheric pressures (Figure 4), whereas case 3 tends to yield lower values spanning approximately −8 to −3.

For TOI-270 d, the relatively high and tightly constrained CO2 abundance, together with sub-unity C/O ratios, appears more consistent with case 4, suggesting that a soot component is not required to explain its atmosphere.

For K2-18 b, the picture is less clear: while some retrievals are compatible with the higher C/O and lower CO2 characteristic of case 3, the low mean molecular weight favours case 4.

This preference is reinforced by interior modeling suggesting a possible magma ocean interface temperature of TAMOI ≈3000 K or higher for K2-18 b (Rigby et al. 2024); at this temperature, case 4 produces a tight MMW distribution consistent with observations, whereas reducing TAMOI toward ∼2000 K introduces a large spread in predicted MMW (Figure 5) that is harder to reconcile with observational constraints.

Taken together, the available evidence tentatively suggests that both K2-18 b and TOI-270 d can be explained without invoking a significant soot component.

However, given the substantial observational uncertainties and model dependencies in current retrievals, this interpretation remains speculative.

A tighter constraint on TAMOI from interior modeling, and improved retrieval precision on CO2 and C/O for K2-18 b in particular, would help to more robustly distinguish cases 3 and 4.

|  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- |
|  |  | TOI-421 b |  |  |  |  |
|  |  |  |  | GJ3470 b | G | J9827 d |
|  |  |  |  |  |  |  |
|  | Cases TAMOI |  |  | TOI-270 d LP 791-18 c | (if not hazy) |  |
|  | Case 1 3 20 00 00 0 K K Case 2 3 20 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) |