A robust detection of life on another world would be a landmark event. A common approach towards life detection focuses on observing key chemical species in atmospheres that are uniquely associated with life. However, an alternative method involves identifying chemical disequilibrium: atmospheric chemistry that has been perturbed from thermodynamic equilibrium by some biological process. Amber Young and colleagues have simulated a present-day Earth orbiting another star to determine whether our ‘out of whack’ atmosphere would be detectable by JWST — and the answer is yes, under certain circumstances.
Under this method, chemical disequilibrium is established by calculating the available Gibbs free energy (GFE) of a planet, which is the difference between the observed GFE and a theoretical equilibrium GFE for the same system. The observed GFE is a combination of observationally derived quantities, such as gas mixing ratios, global surface pressure, effective temperature, and physical properties of clouds, aerosols, and hazes in the atmosphere. The theoretical estimate is derived from essentially putting planetary ingredients into a ‘box’ and letting them settle. Earth’s main atmospheric ‘perturbation’ from equilibrium relates to the CH4–O2 system, where methane is a waste gas from metabolism. JWST/MIRI could not detect the ∼1 J mol−1 GFE offset of an Earth–Sun system. However, if the Earth were to transit an M dwarf (a TRAPPIST-1 analogue), the ~320 J mol−1 GFE offset would be detectable if a challenging part-per-million noise level could be achieved.
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