Can Carbon Fractionation Provide Evidence for Aerial Biospheres in the Atmospheres of Temperate Sub-Neptunes?

The search for signs of life on other worlds has largely focused on terrestrial planets. Recent work, however, argues that life could exist in the atmospheres of temperate sub-Neptunes. Here we evaluate the usefulness of carbon dioxide isotopologues as evidence of aerial life. Carbon isotopes are of particular interest, as metabolic processes preferentially use the lighter 12 C over 13 C. In principle, the upcoming James Webb Space Telescope (JWST) will be able to spectrally resolve the 12 C and 13 C isotopologues of CO 2 , but not CO and CH 4 . We simulated observations of CO 2 isotopologues in the H 2 -dominated atmospheres of our nearest (