  Isotopologue ratios are anticipated to be one of the most promising signs of
life that can be observed remotely. On Earth, carbon isotopes have been used
for decades as evidence of modern and early metabolic processes. In fact,
carbon isotopes may be the oldest evidence for life on Earth, though there are
alternative geological processes that can lead to the same magnitude of
fractionation. However, using isotopologues as biosignature gases in exoplanet
atmospheres presents several challenges. Most significantly, we will only have
limited knowledge of the underlying abiotic carbon reservoir of an exoplanet.
Atmospheric carbon isotope ratios will thus have to be compared against the
local interstellar medium or, better yet, their host star. A further
substantial complication is the limited precision of remote atmospheric
measurements using spectroscopy. The various metabolic processes which cause
isotope fractionation cause less fractionation than anticipated measurement
precision (biological fractionation is typically 2 to 7%). While this level of
precision is easily reachable in the laboratory or with special in situ
instruments, it is out of reach of current telescope technology to measure
isotope ratios for terrestrial exoplanet atmospheres. Thus, gas isotopologues
are poor biosignatures for exoplanets given our current and foreseeable
technological limitations.
