Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life (1705.05791v3)

Abstract: In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a state-of-the-art overview of our current understanding of potential exoplanet biosignatures including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well-known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required for a given atmospheric signature. We focus particularly on advances made since the seminal review by Des Marais et al. (2002). The purpose of this work is not to propose new biosignatures strategies, a goal left to companion papers in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward.

• The paper presents a comprehensive review of biosignatures, analyzing key atmospheric gases, surface features, and temporal variations as signs of life.
• It emphasizes methods to distinguish biological signals from abiotic sources through contextual analysis and high-resolution spectroscopy.
• The review highlights future observational prospects, including JWST and multi-wavelength strategies, to robustly detect exoplanetary life.

An Academic Overview of "Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life"

The paper entitled "Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life," presents a comprehensive examination of biosignatures, which are indirect indicators of life that can be observed in the atmospheres and surfaces of exoplanets. The authors discuss the potential for characterizing these exoplanetary biosignatures through existing and future observational means, considering the significant advances in astrophysical sciences and astrobiology.

Gaseous Biosignatures

Central to the discourse is the analysis of gaseous biosignatures, specifically focusing on Earth's analogs as well as hypothetical extraterrestrial signatures. Oxygen and ozone are highlighted due to their established role as primary biosignatures resulting from oxygenic photosynthesis (OP). However, the paper extends beyond these robust indicators to include a range of gases such as methane (CH4), nitrous oxide (N2O), and others that might serve as complementary or circumstantial biosignatures under various planetary environments. The critical challenge discussed is distinguishing these biosignatures from abiotic sources, which renders a high priority for context and auxiliary analyses involving environmental parameters of candidate exoplanets.

Surface and Temporal Biosignatures

Another aspect scrutinized by the authors is surface biosignatures, with an emphasis on the vegetation red edge (VRE) associated with chlorophyll in terrestrial plants. This spectral feature might differ on exoplanets with photosynthetic organisms adapted to distinct stellar environments. The review highlights the necessity of extensive datasets and high-resolution spectroscopic observations to discern these spectral signatures in potential exoplanetary surfaces. Temporal signatures, such as oscillations in atmospheric gases (e.g., CO2 and CH4) linked to biological activity, are also considered as potentially revealing biosignatures if monitored over time with sufficient depth and regularity.

False Positives and False Negatives

The authors also explore the risks of false positives, where non-biological processes might mimic the anticipated signals of life, emphasizing the importance of understanding planetary system context. Conversely, potential false negatives or scenarios where life is undetected despite its existence, are also a concern, particularly for environments akin to Earth's early history, where biosignatures were weak or absent despite extant life.

Future Observational Prospects

In anticipation of advancements in exoplanet characterization, the paper discusses the capabilities and limitations of upcoming missions such as the James Webb Space Telescope (JWST) and concepts for future observatories that might significantly enhance the range of detectable biosignatures. The paper underscores a multi-faceted observational approach, integrating various wavelengths (UV, visible, NIR, MIR) to discern habitable conditions and potential biological activity robustly.

Conclusion

The paper serves not only as an overview of current scientific understanding but also sets a direction for future research and mission design. It calls for integrating diverse scientific disciplines and leveraging current and next-generation observational platforms to identify and confirm biosignatures beyond Earth. The intricate task of detecting life signatures on exoplanets demands not merely advances in technology but also comprehensive theoretical frameworks to ensure precise interpretation of the data garnered from these distant worlds.