Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

Kazumi Ozaki,Peng K Hong,Christopher T Reinhard,Edward W Schwieterman,Chester E Harman,Stephanie L Olson,Timothy W Lyons

doi:10.3847/1538-4357/ab05e1

Abstract

Abstract Some atmospheric gases have been proposed as counter indicators to the presence of life on an exoplanet if remotely detectable at sufficient abundance (i.e., antibiosignatures), informing the search for biosignatures and potentially fingerprinting uninhabited habitats. However, the quantitative extent to which putative antibiosignatures could exist in the atmospheres of inhabited planets is not well understood. The most commonly referenced potential antibiosignature is CO, because it represents a source of free energy and reduced carbon that is readily exploited by life on Earth and is thus often assumed to accumulate only in the absence of life. Yet, biospheres actively produce CO through biomass burning, photooxidation processes, and release of gases that are photochemically converted into CO in the atmosphere. We demonstrate with a 1D ecosphere-atmosphere model that reducing biospheres can maintain CO levels of ∼100 ppmv even at low H2 fluxes due to the impact of hybrid photosynthetic ecosystems. Additionally, we show that photochemistry around M dwarf stars is particularly favorable for the buildup of CO, with plausible concentrations for inhabited, oxygen-rich planets extending from hundreds of ppm to several percent. Since CH4 buildup is also favored on these worlds, and because O2 and O3 are likely not detectable with the James Webb Space Telescope, the presence of high CO (>100 ppmv) may discriminate between oxygen-rich and reducing biospheres with near-future transmission observations. These results suggest that spectroscopic detection of CO can be compatible with the presence of life and that a comprehensive contextual assessment is required to validate the significance of potential antibiosignatures.

Highlights

There are currently over 3900 known exoplanets,11 many of which are rocky and orbiting within the circumstellar habitable zone (HZ) of their host star (e.g., Kane et al 2016)
Our work shows that the combination of biological sources of carbon monoxide (CO), molecular flux limitations imposed by the ocean-atmosphere interface, and photochemistry around late-type stars may produce simultaneously high levels of CO2, CH4, and CO even for planets with a productive biosphere
This possibility is most relevant for oxygen-rich planets like the modern Earth, which could produce high CO fluxes through biomass burning and photooxidation of dissolved organic matter in the surface ocean

Summary

Introduction

There are currently over 3900 known exoplanets, many of which are rocky and orbiting within the circumstellar habitable zone (HZ) of their host star (e.g., Kane et al 2016). We examine the common presumption that CO is a spectroscopic antibiosignature as a test case and find that spectroscopically detectable CO is compatible with an inhabited biosphere in some cases We expand from this example to argue that there are no “smoking-gun” antibiosignatures, where a detection of a single gas above a prescribed threshold is definitive evidence for a sterile planet. In these cases, CO mixing ratios may approach those of the equivalent abiotic case, showing that antibiosignature arguments drawn from the Earth–Sun case are not generally applicable.

Antibiosignatures

Example 1

Example 2

Findings

Discussion and Conclusions