  Methane has been proposed as an exoplanet biosignature. Imminent observations
with the James Webb Space Telescope may enable methane detections on
potentially habitable exoplanets, so it is essential to assess in what
planetary contexts methane is a compelling biosignature. Methane's short
photochemical lifetime in terrestrial planet atmospheres implies that abundant
methane requires large replenishment fluxes. While methane can be produced by a
variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and
impacts, we argue that, in contrast to an Earth-like biosphere, known abiotic
processes cannot easily generate atmospheres rich in CH$_4$ and CO$_2$ with
limited CO due to the strong redox disequilibrium between CH$_4$ and CO$_2$.
Methane is thus more likely to be biogenic for planets with 1) a terrestrial
bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host
star; 2) an abundance of CH$_4$ that implies surface fluxes exceeding what
could be supplied by abiotic processes; and 3) atmospheric CO$_2$ with
comparatively little CO.
