Table 6.

Possible Categories for Probability of Life Detection on Individual Exoplanets

Confidence level for detection of life	Posterior probability P(life | data, context)	Evidence	Suggesitve but purely illustrative examples
Level 1: very likely inhabited	90–100%	Multiple lines of evidence for life. Given current understanding of planetary processes, no known abiotic process can plausibly explain all observed features.	An O2-rich atmosphere with other biosignature gases, including CH4 and N2O, and a liquid ocean identified on an Earth-size exoplanet in the HZ.
Level 2: likely inhabited	66–100%	The body of evidence is consistent with the presence of life.	Atmospheric O2 detected together with CO2 and water vapor on an exoplanet in the HZ.
Level 3: about as likely as not inhabited (inconclusive)	33–66%	Some evidence for life, but insufficient contextual information to draw a definitive conclusion because plausible alternative abiotic explanations cannot be ruled out.	O2 detection in isolation; or an organic haze with abundant CH4; or pigment-like biosignatures; or N2–CO2 atmosphere. Circumstantial evidence for liquid water on a planet in the conventional HZ.
Level 4: likely uninhabited	0–33%	Observational evidence that the planet is habitable, but no biosignatures detected despite an exhaustive search.	Planet is in the HZ and has an atmosphere with abundant water vapor features. But no biosignatures are detected despite extensive data.
Level 5: very likely uninhabited	0–10%	Criteria for habitability are not met or atmospheric antibiosignatures are detected.	CO2-rich, desiccated planets; or CO2–H2 antibiosignature atmosphere; or abundant CO antibiosignature

Examples are illustrative and based on current thinking in the field. As noted in the second column, specific exoplanet cases would have to be quantified by calculating a Bayesian posterior probability to determine the exact level.