Planetary Common State (PCS): A Testable Common Macroscopic State Representation for Coupled Earth-Civilization Systems

Planetary-scale questions frequently involve interactions among geophysical, biological, infrastructural, and informational processes that are modeled in different variables, units, resolutions, and disciplinary frameworks. This paper formulates Planetary Common State (PCS) as a testable macroscopic state-representation framework for such coupled systems. PCS is not a replacement for disciplinary governing equations and is not presented as a new fundamental physical theory. Instead, it defines an application-specific state-construction problem in which heterogeneous observations are mapped to a common state S(t) = H(Ω₀:ₜ) whose components remain traceable to their original physical or domain-specific meanings. State construction is guided by four SONE criteria: Sufficiency, Operational Observability, Non-redundancy, and Extensibility. These are empirical design criteria rather than axioms of physics. The framework separates the scientific state representation from software or data-infrastructure implementation, preserves uncertainty and provenance, and represents cross-domain interaction through application-specific coupling maps rather than assuming that heterogeneous variables share a universal flux law. A scalar planetary index is explicitly optional and should be rejected when its interpretation is unstable under reasonable normalization or weighting choices. PCS is evaluated through reconstruction, hindcasting, baseline comparison, ablation, sensitivity analysis, uncertainty calibration, and, where appropriate, conservation checks. A companion Earth-System Physics Core study provides an example of this validation principle: a four-coordinate residual state was retained while scalar compression was withheld after a predeclared validation gate failed. The contribution of PCS is therefore methodological: it specifies how a common macroscopic state can be posed, tested, revised, or rejected without asserting that one unique or universal planetary state must exist.