Unusually Hot Interiors Could Reconcile the Missing Methane Problem for Warm-to-Hot Exoplanets with Hydrogen Atmospheres

The James Webb Space Telescope (JWST) has been revolutionizing the field of exoplanets by delivering high-precision spectra that constrain atmospheric compositions with unprecedented detail. JWST is best at constraining the atmospheric compositions for warm-to-hot exoplanets with hydrogen-dominated atmospheres and large-scale heights. To date, JWST has already provided constraints on the abundances of dominant carbon and oxygen carrier gases in over a dozen such targets. Under standard assumptions, many of these exoplanet targets are expected to have abundant methane (CH₄) in their observed atmosphere. However, methane was only spotted on a few of these existing targets, giving rise to the so-called "missing methane" problem.

Here, we use published JWST results in combination with a simple, geochemistry-inspired model to explore whether elevated internal temperatures (T_int) can account for the CH₄ depletion in these exoplanet atmospheres. Instead of using computationally intensive forward grid models to search for the optimum parameters (in this case, T_int) to fit the observed spectra, we construct a fast analytical framework that focuses on two key chemical equilibria: CO-CH₄ and CO-CO₂. The equilibrium constants are calculated using Gibbs energies from the NIST-JANAF Thermochemical Tables (Chase, 1998), and the quench temperatures of these two chemical equilibria are linked by a relationship provided in Glein et al. (2025): T_CO-CO2 ≈ 0.8T_CO-CH4. Following a similar and revised methodology as Glein et al. (2025), we can translate JWST-constrained CHO species abundances into a pressure–temperature (P–T) space consistent with equilibrium chemistry.

We generate a suite of P-T relationships consistent with the observed bulk atmospheric composition and a second set of P-T profiles with varying internal temperatures (characterized by T_int). For each target, we identify the minimum T_int required for the P–T profile to intersect the constrained chemical equilibrium region, which corresponds to the condition under which the observed H₂O, CO₂, CH₄, and CO abundances can be reproduced. We first tested this approach against WASP-107 b, for which detailed forward modeling exists, and found our derived minimum T_int (minimum T_int = 500 K) to be consistent with results from the forward grid modeling approach (T_int = 460±40 K, Sing et al. 2024).

We apply this method to eleven warm-to-hot exoplanets with reliable JWST-retrieved constraints on the volume mixing ratios of H₂O, CO₂, CH₄, and CO. For six of these targets, including WASP-107 b, HD 189733 b, HIP 67522 b, WASP-69 b, HAT-P-18 b, and GJ 3470 b (data from Sing et al. 2024; Welbanks et al. 2024; Fu et al. 2022, 2024; Thao et al. 2024; Schlawin et al. 2024), we can derive minimum internal temperatures consistent with the observed CH₄ depletion. The inferred T_int values range from 70 K to 900 K for these targets (see Figure 1). We then compare these values to predictions from standard planetary evolution models, finding that several targets require additional internal heat sources (e.g., tidal dissipation and/or ohmic dissipation) to sustain such elevated T_int values at their current ages and masses.

In particular, for the five planets exhibiting strong CH₄ depletion (WASP-107 b, HD 189733 b, HIP 67522 b, WASP-69 b, and HAT-P-18 b), high T_int leads to deeper quench levels at higher temperatures where CO dominates over CH₄ (which also naturally requires rapid mixing) to explain the observed missing methane. In contrast, for targets like WASP-80 b, where CH₄ is detected, or for very hot planets where quenching occurs high up in the atmosphere in the radiative zone, internal temperature plays a less critical role and cannot be constrained with this method.

Our framework offers a computationally efficient and consistent method to quickly infer internal temperatures of exoplanets based on chemical equilibrium constraints. While complementary to the forward grid modeling approach, our approach provides intuitive visualizations that allow fundamental trends to be identified. Its simplicity and speed make it well-suited for analyzing a much broader sample as JWST and future space telescopes such as ARIEL continue to deliver high-quality atmospheric data across a growing and diverse target population.

Figure 1: Pressure–temperature (P–T) profiles for exoplanet targets where internal temperature (T_int) can be constrained. Solid colored curves indicate predicted profiles consistent with the retrieved CH₄-CO-CO₂-H₂O volume mixing ratios from JWST observations (the allowed parameter space is in gray). Dashed curves represent P–T profiles with lower T_int values that are incompatible with the observed atmospheric compositions.

References:

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Chase, M. W., Jr. 1998, NIST-JANAF Thermochemical Tables, Parts I and II (4th ed.; Woodbury, NY: American Institute of Physics)

Fu, G., et al. (2022). Water and an escaping helium tail detected in the hazy and methane-depleted atmosphere of HAT-P-18b from JWST NIRISS/SOSS. The Astrophysical Journal Letters, 940(2), L35.

Fu, G., et al. (2024). Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet. Nature, 632(8026), 752-756.

Glein, C. R., Yu, X., & Luu, C. N. (2025). Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d, in press at ApJ.

Schlawin, E., et al. (2024). Multiple Clues for Dayside Aerosols and Temperature Gradients in WASP-69 b from a Panchromatic JWST Emission Spectrum. The Astronomical Journal, 168(3), 104.

Sing, D. K., et al. (2024). A warm Neptune’s methane reveals core mass and vigorous atmospheric mixing. Nature, 630(8018), 831-835.

Thao, P. C. et al. (2024). The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST. The Astronomical Journal, 168(6), 297.

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