Chemical Anomaly Index (CAI): Extending Adaptive SETI Through Astrochemical Techno-signatures

The Search for Extraterrestrial Intelligence (SETI) has tradionally relied on electromagnetic technosignatures, particularly radio transmissions. While these approaches remain scientifically valuable, they assume that technologically advanced civilzations continue to produce detectable communication signals over extended timescales. This study proposes an alternative framework based on the hypothesis that sufficiently advanced civilizations may become increasingly electromagnetically inconspicuous while leaving persistent chemical signatures through long-term planetary-scale environmental regulation. We introduce the Chemical Over-Optimisation Hypothesis (COH), which suggests that the sustained technological activity may produce statistically improbable chemical states that are difficult to explain through known natural processes alone. To quantify such anomalies, we develop the Chemical Anomaly Index (CAI), a statistical framework integrating standardized chemical deviations, disequillibrium, persistence, and regulation into a unified metric for prioritizing potential technosignatures. Rather than serving as evidence of extraterrestrial intelligence, a high CAI identifies planetary environments whose chemistry warrants further scientific investigation. The proposed framework complements, rather than replaces, traditional SETI methodologies and extends Adaptive SETI by shifting focus from intentional communication to persistent environmental consequences. Future observational validation using high-resolution spectroscopic surveys and atmospheric modeling will determine the applicability and limitations of the proposed index.