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, the 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>CH</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> relative to true Titan by orders of magnitude, reducing the probability of observing them. We constrain the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>CH</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> 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 &lt;0.1 and likely &lt;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 toward 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.