Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment (1705.07560v1)

Abstract: Here we review how environmental context can be used to interpret whether O2 is a biosignature in extrasolar planetary observations. This paper builds on the overview of current biosignature research discussed in Schwieterman et al. (2017), and provides an in-depth, interdisciplinary example of biosignature identification and observation that serves as a basis for the development of the general framework for biosignature assessment described in Catling et al., (2017). O2 is a potentially strong biosignature that was originally thought to be an unambiguous indicator for life at high-abundance. We describe the coevolution of life with the early Earth's environment, and how the interplay of sources and sinks in the planetary environment may have resulted in suppression of O2 release into the atmosphere for several billion years, a false negative for biologically generated O2. False positives may also be possible, with recent research showing potential mechanisms in exoplanet environments that may generate relatively high abundances of atmospheric O2 without a biosphere being present. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. Similarly our ability to interpret O2 observed in an exoplanetary atmosphere is also crucially dependent on environmental context to rule out false positive mechanisms. We describe future photometric, spectroscopic and time-dependent observations of O2 and the planetary environment that could increase our confidence that any observed O2 is a biosignature, and help discriminate it from potential false positives. By observing and understanding O2 in its planetary context we can increase our confidence in the remote detection of life, and provide a model for biosignature development for other proposed biosignatures.

• The paper demonstrates that oxygen’s role as a biosignature depends on detailed planetary and stellar context to distinguish biological sources from abiotic mimics.
• It highlights the use of transit spectroscopy and direct imaging to detect oxygen’s unique spectral features in exoplanet atmospheres.
• The study advocates comprehensive observational frameworks combining multi-layered data to enhance biosignature assessment for future missions.

Understanding Oxygen as an Exoplanetary Biosignature

The paper of exoplanetary biosignatures is an essential aspect of astrobiological research, focusing on identifying conditions conducive to life beyond Earth. The paper "Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment" explores the potential of molecular oxygen (O₂) as a biosignature. It explores the complexity of interpreting remote observations of O₂ in exoplanetary atmospheres. Molecular oxygen, a byproduct of oxygenic photosynthesis, has long been considered a robust biosignature due to its linkage to biological processes on Earth. However, the paper underscores the need to contextualize O₂ detections within specific planetary environmental conditions to avoid both false negatives and false positives.

Reliability, Survivability, and Detectability of Oxygen

In the context of biosignatures, molecular oxygen fulfills the three primary criteria: reliability, survivability, and detectability. Oxygen is a volatile byproduct of photosynthesis, indicative of biological processes. It is considered reliable due to the historical absence of significant abiotic sources on modern Earth. Oxygen's survivability as a biosignature is supported by its accumulation to high atmospheric levels over geological timescales despite potential losses through photochemistry and surface interactions. The detectability criterion is addressed through oxygen’s spectrally distinct features, which are accessible via current transit spectroscopy and forthcoming direct imaging techniques.

False Positives and Negatives

The evolution of our understanding highlights potential false positives and negatives associated with O₂. False positives involve abiotic processes that could mimic the presence of biological oxygen. These include mechanisms like photolysis of water and the subsequent atmospheric oxygen accumulation driven by stellar interactions, especially concerning M dwarf stars. Earth-like planets in such systems might experience high atmospheric oxygen levels from abiotic origins, challenging the reliability of O₂ as a pure biosignature. Conversely, false negatives consider scenarios where biological activity is present but undetectable due to environmental factors like volcanic activity or planet surface composition, which can suppress oxygen levels, despite active biological production.

Implications for Future Research and Observations

The paper proposes strategies for the contextual evaluation of oxygen detections to distinguish between biological and abiotic sources. It suggests comprehensive observational frameworks that incorporate multi-layered data: stellar characteristics, planetary composition, atmospheric conditions, and potential secondary biosignatures. The synthesis of these elements could increase confidence in detecting genuine biosignatures. It also emphasizes the importance of understanding the planetary context to formulate observation priorities and target selection for future missions, including the James Webb Space Telescope (JWST) and large ground-based observatories.

Conclusion

The meticulous paper of oxygen as a biosignature facilitates a deeper understanding of its potential and limitations in exoplanetary environments. This work advances the broader framework for biosignature research by integrating planetary science, atmospheric chemistry, and stellar interactions. As the field progresses, it calls for ongoing comparative planetology, leveraging insights from Earth's history and diverse planetary systems to refine methodologies for identifying life beyond our planet.