Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets.
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 previously unidentified 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%.
Department of Environmental and Physical Sciences, University of Toronto, Toronto, ON, Canada M1C 1A4; hanno.rein@utoronto.ca.
Hanno
H
Rein
Earth-Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro, Tokyo 152-8550, Japan; and.
Yuka
Y
Fujii
Astrophysics Department, Institute for Advanced Study, Princeton, NJ 08540.
David S
DS
Spiegel
D001256:Astronomy; D001272:Atmosphere; D018534:Earth, Planet; D019419:Evolution, Planetary; D018559:Exobiology / Q000379:methods*; D005118:Extraterrestrial Environment / Q000737:chemistry*; D008962:Models, Theoretical; D016081:Moon; D016083:Planets; D013057:Spectrum Analysis; D014867:Water
D014867:Water
astrobiology; biomarker; detection of life; exomoon; habitability