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The detection of strong thermochemical disequilibrium in the atmosphere of anextrasolar planet is thought to be a potential biosignature. In this article wepresent a new kind of false positive that can mimic a disequilibrium or anyother biosignature that involves two chemical species. We consider a scenariowhere the exoplanet hosts a moon that has its own atmosphere and neither of theatmospheres is in chemical disequilibrium. Our results show that the integratedspectrum of the planet and the moon closely resembles that of a single objectin strong chemical disequilibrium. We derive a firm limit on the maximumspectral resolution that can be obtained for both directly-imaged andtransiting planets. The spectral resolution of even idealized space-basedspectrographs that might be achievable in the next several decades is ingeneral insufficient to break the degeneracy. Both chemical species can only bedefinitively confirmed in the same object if absorption features of bothchemicals can be unambiguously identified and their combined depth exceeds100%.

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