Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets

ABSTRACT

The search for life on planets outside our own solar system is among the most compelling quests that humanity has ever undertaken. An often suggested method of searching for signs of life on such planets involves looking for spectral signatures of strong chemical disequilibrium. This article introduces an important potential source of confusion associated with this method. Any exoplanet can host a moon that contaminates the planetary spectrum. In general, we will be unable to exclude the existence of a moon. By calculating the most optimistic spectral resolution in principle obtainable for Earth-like planets, we show that inferring a biosphere on an exoplanet might be beyond our reach in the foreseeable future.