Bayesian Biosignature Update

Detecting extraterrestrial life requires strict application of Bayes' Theorem to mitigate false-positive cascades. The posterior probability of a biosignature depends heavily on the pre-assigned prior probability of life, alongside instrument sensitivity and false-positive rates. This mathematical framework forces a rigorous, objective evaluation of anomalous spectral data. It systematically prevents observational bias from inflating the statistical significance of borderline chemical disequilibrium detections.

This tool reveals something that surprises most people: how sharp your instrument is matters far less than how rare life is to begin with. If only 1 in 1000 planets host life, even a 90% sensitive detector with a 2% false-positive rate flags mostly lifeless worlds — the posterior probability lands near 4%, because false alarms drown out the rare real find. The takeaway: before trusting a positive signal, ask how likely life was before you looked.

Reading a positive as "life confirmed" commits the prosecutor's fallacy — swapping P(signal | no life) for P(no life | signal). Courts have made exactly this slip: in the 1999 Sally Clark case, an expert's tiny probability of the evidence arising by chance was wrongly presented as the probability of her innocence, helping convict a mother later exonerated in 2003. Your detector's low false-positive rate is not the chance the world is lifeless.