Life on the Edge: Using Planetary Context to Enhance Biosignatures and Avoid False Positives

We use a probability theory framework to discuss the search for biosignatures. This perspective allows us to analyse the potential for different biosignatures to provide convincing evidence of extraterrestrial life and to formalise frameworks for accumulating evidence. Analysing biosignatures as a function of planetary context motivates the introduction of 'peribiosignatures', biosignatures observed where life is unlikely. We argue, based on prior work in Gaia theory, that habitability itself is an example of a peribiosignature. Finally, we discuss the implications of context dependence on observational strategy, suggesting that searching the edges of the habitable zone rather than the middle might be more likely to provide convincing evidence of life.