The Photochemical Plausibility of Warm Exo-Titans Orbiting M-Dwarf Stars

Abstract

The James Webb Space Telescope (JWST) has begun to spectrally characterize small exoplanets orbiting M-dwarf stars, but interpretation of these spectra is ambiguous, with stellar, instrumental, or atmospheric origins possible for apparent spectral features. Consequently, interpretation of JWST small exoplanet spectra follows a Bayesian approach, with less theoretically plausible interpretations facing a higher burden of proof. Here, we use photochemical modeling to evaluate the plausibility of warm exo-Titans, exoplanets with N$_2$-CH$_4$ atmospheres analogous to Titan but orbiting closer to their host stars. Consideration of warm exo-Titans is motivated by arguments from planet formation, as well as tentative evidence from observations. Using TRAPPIST-1e as a case study, we show that the higher instellation experienced by warm exo-Titans reduces their CH$_4$ lifetime $\tau_{\text{CH}_{4}}$ relative to true Titan by orders of magnitude, reducing the probability of observing them. We constrain the $\tau_{\text{CH}_{4}}$ on a warm exo-Titan to be $\leq0.1\times$ (and most likely $\leq0.02\times$) true Titan, implying the absolute probability of detecting a warm exo-Titan is $<0.1$ and likely $<0.01$. This finding is consistent with recent JWST nondetections of CH$_4$-dominated atmospheres on warm terrestrial exoplanets. The low prior probability means that the standard of proof required to claim a warm exo-Titan detection is high, and we offer specific suggestions towards such a standard of proof. Observation of oxidized carbon species would corroborate a putative warm exo-Titan detection. Confirmed detection of warm exo-Titans would signal the need to fundamentally rethink our understanding of the structure, dynamics, and photochemistry of Titan-like worlds.