Oxygen can be a false-positive biosignature without methane context
Earth’s air is rich in oxygen largely because life continually replenishes it — so O₂ in an exoplanet spectrum is an exciting clue, not automatic proof of biology.
Abiotic chemistry can also build oxygen: ultraviolet light can split water vapor, and hydrogen can escape to space, leaving O₂ behind without plants or microbes. Models stress pairing oxygen with reducing gases like methane, plus climate and stellar context.
A lonely oxygen line can mislead. Careful biosignature work treats gases as a chemical story, not a single green checkmark.
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