An Overview of Exoplanet Biosignatures

Brief

Schwieterman and Leung (2024) survey the full landscape of proposed exoplanet biosignatures across three categories: gaseous molecules (O2, O3, CH4, N2O, DMS, NH3, PH3), surface features (vegetation red edge, pigment reflectance, polarization), and temporal signals (seasonal gas and albedo modulation). The review establishes three required attributes for a compelling biosignature, detectability, survivability, and specificity, and stresses that all small molecules have abiotic sources, making contextual interpretation mandatory for any claimed detection. JWST cannot detect the O2 A-band at 0.76 μm in transit at modern Earth-like abundances, and detecting O3 at 3σ on TRAPPIST-1e would require more than 100 transits, straining or exceeding the telescope's five-year nominal lifetime. Prior models predicting robust abiotic O2 accumulation from CO2 photolysis on M-dwarf planets were shown by Ranjan et al. (2023) to be erroneous, capping realistic abiotic O2 below 1%.

Metadata

Category

Search

Venue

Reviews in Mineralogy and Geochemistry

Type

Peer-reviewed

Year

2024

Authors

Edward W. Schwieterman, Michaela Leung

DOI

10.2138/rmg.2024.90.13

arXiv

2404.15431

Access

Open access

Length

10.8 M

Programs

Habitable Worlds Observatory, LUVOIR, LIFE

Instruments

JWST NIRSpec, JWST MIRI, Extremely Large Telescopes (ELTs), Virtual Planetary Laboratory (VPL)

Data sources

HITRAN 2020 database, PNNL spectral database

Tags

biosignature, astrobiology, exoplanet, spectroscopy, atmospheric-chemistry, SETI-adjacent

Key points

• Over 5,000 exoplanets have been confirmed; JWST has opened a new era of atmospheric characterization, revealing composition and chemical processes in unprecedented detail.p.1
• A compelling biosignature requires three attributes: detectability (spectral interaction with light), survivability (resistance to depletion), and specificity (separability from abiotic sources).p.3
• Earth's transmission spectrum includes the industrial pollutants CFC-11 (CCl3F, 11.8 μm) and CFC-12 (CCl2F2, 10.8 μm), demonstrating that technosignature-class atmospheric contaminants are in principle remotely detectable.p.5
• The O2 A-band at 0.76 μm is too narrow and weak to be detected by JWST via transit transmission spectroscopy at modern Earth-like abundances.p.7
• The Habitable Worlds Observatory (HWO), a proposed space-based UV/Vis/IR direct-imaging telescope, is identified as the most viable platform for detecting Earth-like O2 levels in exoplanetary atmospheres.p.7
• Detecting O3 at 3σ significance on TRAPPIST-1e with JWST NIRSpec or MIRI requires more than 100 transits, at or beyond JWST's entire nominal five-year lifetime.p.9
• O3 is a non-linear amplifier of sub-detectable O2 levels and could signal biotic O2 too low for direct detection, but false positives from abiotic photochemistry significantly complicate this interpretation.p.10
• Ranjan et al. (2023) corrected prior models predicting robust abiotic O2 from CO2 photolysis on M-dwarf planets; the errors stemmed from setting the model atmospheric top at too low an altitude, confining photolysis to one numerical layer; realistic abiotic O2 stays below 1%.p.12

Verbatim

• “Any purported exoplanet biosignature will require further vetting beyond the first observation to confirm a biological origin.”

  p.3

• “A compelling biosignature has the following attributes: detectability, survivability, and specificity (Meadows 2017; Meadows et al. 2018b).”

  p.3

• “Because all small molecules have abiotic sources—though some are much more limited than others—the interpretation of any of these molecules as a sign of life highly depends on the context in which they are found (Krissansen-Totton et al. 2022).”

  p.5

• “Ranjan et al. (2023) have found that some past results predicting robust abiotic O 2 accumulation from CO 2 decomposition on terrestrial planets orbiting M dwarf host stars were erroneous and caused by instituting a model "top" at too low of an altitude (too high of a pressure), which effectively confined CO 2 photolysis to one numerical layer.”

  p.12

Most interesting

• Earth's own cloud-free transmission spectrum contains detectable CFC-11 and CFC-12 absorption features, meaning a civilization monitoring Earth from outside the solar system could already identify its industrial pollution signature.
• The pre-main-sequence phase for M dwarf stars can last hundreds of millions of years, compared to roughly 50 Myr for the Sun; inner habitable-zone planets around these stars may lose their surface water entirely before the star stabilizes.
• O2 is chemically unusual: it is a symmetric, homonuclear diatomic molecule yet still possesses significant electronic transitions that produce detectable spectral features, unlike N2, which has almost no remotely detectable bands.
• Oxygenic photosynthesis may be the most productive metabolism possible on any planet orbiting a star, because it requires only H2O, CO2, and photons, all universally available, whereas other photosynthetic pathways are limited by scarcer reductants such as H2S or Fe2+.
• A prior generation of models predicting robust abiotic O2 buildup on M-dwarf planets were found to be artifacts of numerical grid design: placing the model atmosphere's upper boundary too low confined CO2 photolysis to a single computational layer, artificially inflating oxygen yields.
• The 4.7 μm O3 feature detectable by JWST NIRSpec can be constrained twice as precisely as the canonical 9.65 μm O3 band detectable by MIRI, suggesting NIR wavelengths may be the more practical near-term window for ozone detection.