Kavli Affiliate: Sara Seager
| First 5 Authors: Sukrit Ranjan, Sukrit Ranjan, , ,
| Summary:
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 $tau_textCH_4$
relative to true Titan by orders of magnitude, reducing the probability of
observing them. We constrain the $tau_textCH_4$ on a warm exo-Titan to
be $leq0.1times$ (and most likely $leq0.02times$) 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.
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