Simulated transmission spectrum for warm exo-Titan TRAPPIST-1e (pCH4 = 3 × 10−2 bar) forced by D. J. Wilson et al. (2021) SED. — astro-ph.EP
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 N2-CH4 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 CH4 lifetime τCH4 relative to true Titan by orders of magnitude, reducing the probability of observing them. We constrain the τCH4 on a warm exo-Titan to be ≤0.1× (and most likely ≤0.02×) 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 CH4-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.
Sukrit Ranjan, Nicholas F. Wogan, Ana Glidden, Jingyu Wang, Kevin B. Stevenson, Nikole Lewis, Tommi Koskinen, Sara Seager, Hannah R. Wakeford, Roeland P. van der Marel
Comments: Submitted to AAS Journals. Comments welcome at [email protected]
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2509.10611 [astro-ph.EP](or arXiv:2509.10611v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.10611
Focus to learn more
Submission history
From: Sukrit Ranjan
[v1] Fri, 12 Sep 2025 18:00:05 UTC (538 KB)
https://arxiv.org/abs/2509.10611
Astrobiology,