Thermal Routing Windows and Structural Habitability Classes

The search for life beyond Earth is often guided by chemical inventories, surface conditions, and stellar habitability zones. While these criteria identify environments where life might exist, they substantially overpredict biological viability. This paper proposes a complementary, structure-first classification that emphasizes negative constraints—conditions under which life as a thermodynamically active process cannot persist. Building on prior work that reframes heat as a structural transit problem and identifies structural heat retention as a necessary precondition for sustained non-equilibrium, we classify planetary and small-body environments according to their capacity to retain, route, and release heat within bounded regimes. This framework applies explicitly to life as a heat-mediated, non-equilibrium physical process, without asserting universality or excluding alternative forms of organization that may not depend on heat. Within this scope, we show that habitability is governed by a narrow structural routing window bounded by insufficient and excessive heat retention. The resulting classification explains why chemically complete environments are frequently sterile and provides a conservative, falsifiable method for excluding non-viable targets in astrobiological surveys. Keywords: Structural habitability, heat-mediated life, non-equilibrium systems, structural heat routing, habitability constraints, energy persistence, negative constraints on life, structure-first physics, thermodynamic life, planetary habitability classification, energy gradients, magnetic structure.