Reflections on O₂ as a Biosignature in Exoplanetary Atmospheres

Area of Science:

• Astrobiology
• Planetary Science
• Exoplanet Research

Background:

• Oxygenic photosynthesis is Earth's dominant metabolism, utilizing abundant CO2 and H2O.
• Oxygen (O2) is a primary biosignature candidate for habitable exoplanets due to its high abundance and distinct spectral features.
• Previous understanding suggested a lack of abiotic O2 false positives, making it a robust biosignature.

Purpose of the Study:

• To evaluate the robustness of O2 as an exoplanet biosignature.
• To identify and characterize abiotic mechanisms that could produce false positives for O2 and ozone (O3).
• To refine strategies for detecting life on exoplanets by mitigating false positives.

Main Methods:

• Review and modeling of photochemical and atmospheric escape processes.
• Analysis of environmental factors influencing abiotic O2 production, particularly related to host star properties.
• Investigation of complementary biosignatures (O4, CO, H2O, CH4) for distinguishing biological from abiotic O2.

Main Results:

• Abiotic mechanisms, including photochemistry and atmospheric escape, can generate significant O2 and O3 on exoplanets.
• False positive O2 production is often linked to host star properties, especially for planets around M dwarf stars.
• Specific planetary and stellar environmental factors increase the likelihood of abiotic O2.

Conclusions:

• O2 and O3 alone may not be definitive biosignatures due to potential abiotic false positives.
• Target selection (planets in conservative habitable zones, around stars > 0.4 M⊙) can help minimize false positives.
• Combining O2/O3 detection with other spectral indicators (e.g., O4, CO, H2O, CH4) is crucial for confident life detection.