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

TL;DR

This paper reveals a false positive in biosignature detection involving two chemical species on exoplanets with moons, showing that combined spectra can mimic disequilibrium signals, and discusses the limitations of spectral resolution.

Contribution

It introduces a new false positive scenario for biosignature detection involving exoplanet-moon systems and analyzes the spectral resolution needed to distinguish true biosignatures.

Findings

Combined spectra of planet and moon can mimic disequilibrium biosignatures.

Spectral resolution of upcoming space telescopes is generally insufficient to resolve this degeneracy.

Both chemicals can be confirmed only if their absorption features are unambiguously detected together.

Abstract

The detection of strong thermochemical disequilibrium in the atmosphere of an extrasolar planet is thought to be a potential biosignature. In this article we present a new kind of false positive that can mimic a disequilibrium or any other biosignature that involves two chemical species. We consider a scenario where the exoplanet hosts a moon that has its own atmosphere and neither of the atmospheres is in chemical disequilibrium. Our results show that the integrated spectrum of the planet and the moon closely resembles that of a single object in strong chemical disequilibrium. We derive a firm limit on the maximum spectral resolution that can be obtained for both directly-imaged and transiting planets. The spectral resolution of even idealized space-based spectrographs that might be achievable in the next several decades is in general insufficient to break the degeneracy. Both chemical species can only be definitively confirmed in the same object if absorption features of both chemicals can be unambiguously identified and their combined depth exceeds 100%.