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

Abstract

JWST is revolutionizing the field of exoplanet atmospheres by delivering unprecedented spectroscopic constraints on their chemical compositions. It has provided tight constraints on the abundances of dominant carbon- and oxygen-bearing species on numerous warm-to-hot exoplanets with hydrogen-dominated atmospheres. Under thermochemical equilibrium, many of these exoplanets should have abundant methane (CH4); however, CH4 has, so far, only been spotted in a few cases. Here, we present a simple, geochemistry-inspired framework to explore whether elevated intrinsic temperatures (T$\rm_{int}$) can account for the CH4 depletions. Instead of using computationally expensive, forward grid models, our fast analytical framework focuses on two key chemical equilibria: CO-CH4 and CO-CO2, allowing us to quickly constrain the minimum Tint that is consistent with JWST-observed abundances of H2O, CO, CH4, and CO2. Applying this framework to 12 warm-to-hot exoplanets, we find that several targets require minimum Tint values exceeding those predicted by standard evolution models, while others remain consistent with lower Tint solutions or exhibit degeneracies with other solutions. Our sample enables an initial exploration of population-level trends: while many exoplanets broadly follow an empirical Teq-Tint relation derived from the hot Jupiter mass-radius population, a subset of targets lie well above this trend. The apparent need for hotter interiors suggests that while Ohmic dissipation is probably an important heat source among the general population, additional heating processes, such as tidal heating, may also play important roles for some planets. Our results demonstrate the diagnostic power of atmospheric chemistry as a complementary probe of exoplanet interiors.

Figures (16)

• Figure 1: "Missing" methane on volatile-rich exoplanets motivates this study. JWST-measured volume mixing ratios (VMRs) of CH$_4$ are shown as a function of radiative equilibrium temperature for exoplanets with T$\rm_{eq}\leq1600$ K. The solid, dashed, and dotted lines indicate equilibrium CH$_4$ mixing ratios computed with FASTCHEM2018MNRAS.479..865S2022MNRAS.517.4070S2024MNRAS.527.7263K at 0.01 bar (a representative pressure level probed by transmission spectroscopy, e.g., 2024Natur.630..831S), assuming 3$\times$, 30$\times$, and 100$\times$ protosolar metallicities with the protosolar C/O ratio. Symbol colors correspond to equilibrium temperatures of these planets. Several annotated exoplanets deviate significantly from the expected trend of increasing CH$_4$ abundance with decreasing T$\rm_{eq}$. A complete list of the exoplanets included in this figure is provided in Table \ref{['table:target']}.
• Figure 2: Summary of our method and comparison to the traditional grid modeling approach.
• Figure 3: Inferring the intrinsic temperature T$\rm_{int}$ of "missing methane" exoplanets, with WASP-107 b as an example. Colored curves show atmospheric pressure-temperature (P-T) profiles computed for different T$\rm_{int}$ values, based on 30$\times$ solar metallicity and a C/O ratio of 0.25$\times$ solar. The gray shaded region between the black curves represents the chemically-derived P-T relationships that are consistent with JWST-observed H$_2$O-CO-CH$_4$-CO$_2$ abundances under chemical equilibrium (using the ATMO free retrieval mixing ratios). Solid colored curves represent P-T profiles that intersect the chemically-derived P-T region, while the dashed colored curves represent atmospheric P-T profiles with lower T$\rm_{int}$ values that are incompatible with the measured composition. Quench points (gray plus signs for T$\rm_{int}=450$ K and T$\rm_{int}=600$ K profiles as examples) mark the first intersections between the solid P-T profiles and the chemically-derived P-T region. The minimum T$\rm_{int}$ is defined as the coolest P-T profile that intersects the chemically-derived P-T region (450 K for WASP-107 b). At altitudes above the quench point, species abundances are vertically mixed and remain "frozen," producing the atmospheric compositions observed in transmission spectroscopy.
• Figure 4: Volume mixing ratios of key species for WASP-107 b as predicted by forward chemical disequilibrium modeling with VULCAN, using different values of K$\rm_{deep}$ for the eddy diffusion coefficient (K$\rm_{zz}$) parameterization. JWST-observed abundances and 1$\sigma$ error bars are shown based on the ATMO free retrieval results from 2024Natur.630..831S. Thick gray lines represent chemical equilibrium profiles. Solid, dashed, dashed-dotted, and dotted lines correspond to K$\rm_{deep}$ values of 10$^{7.8}$, 10$^{8}$, 10$^{9}$, and 10$^{10}$ cm$^2$ s$^{-1}$, respectively. This plot illustrates how methane goes "missing" around the 1 mbar level, which can be explained by quenching of deeper CO-CH$_4$ equilibrium.
• Figure 5: Top panel: Same as Figure \ref{['fig:wasp107b_atmo']}, but with elemental abundances of the climate model P-T profiles and the chemically-derived P-T region calculated using the NEMESIS free retrieval results. The gray region at the upper-left is very small here. Because there is no overlap between the colored curves and this region, chemical equilibrium cannot create the observed molecular speciation of C-H-O species. Bottom panel: Same as Figure \ref{['fig:wasp107b_forward']}, but with VMRs of key species predicted using VULCAN with photochemistry and compared to JWST-observed abundances (with 1$\sigma$ error bars) from the NEMESIS free retrieval. Solid, dashed-dotted, and dotted lines correspond to K$\rm_{deep}$ values of 10$^{7}$, 10$^{8}$, and 10$^{9}$ cm$^2$ s$^{-1}$, respectively.