A Dynamical Framework for the Origin and Long-Term Evolution of Orbital Motion Through Differential Infall and the First Close Passage

Abstract Orbital motion is a universal phenomenon observed in planetary systems, satellite systems, binary stars, and many other gravitational systems. Although the conservation of angular momentum successfully explains the long-term maintenance and stability of already established orbits, it provides only a limited direct explanation for the dynamical process through which initially independent mass bodies first acquire orbital motion around a central mass.This study proposes a new conceptual dynamical framework in which the origin of orbital motion is interpreted as the outcome of continuous gravitational evolution within a nonuniform three-dimensional convergent environment. During the early stage of gravitational evolution, mass bodies possessing different masses, initial positions, infall distances, infall timing, and approach velocities undergo Differential Infall toward a growing central mass. Under these conditions, close gravitational interactions among neighboring bodies naturally become more frequent. Some bodies may consequently transition from nearly radial infall to non-radial motion and experience a First Close Passage around the central mass without direct collision. This study interprets the First Close Passage as the dynamical beginning of orbital motion.The first orbit is not interpreted as the final stable orbital configuration. Instead, orbital motion continues under the persistent gravitational influence of the central mass while successive orbital revolutions occur under progressively changing dynamical conditions. Accordingly, this study proposes that the locations of periapsis and apoapsis may gradually change during repeated orbital evolution, allowing the orbital geometry itself to evolve over time. Through this long-term process, initially highly eccentric orbits may progressively evolve toward lower-eccentricity and more stable orbital configurations.This study further proposes that long-term orbital evolution need not produce identical outcomes for all bodies. Variations in body mass, the location of the initial interaction, the remaining infall distance, approach velocity, and continuously changing dynamical conditions may lead to different evolutionary pathways, including long-term orbital stability, renewed infall toward the central mass, or complete escape from the gravitational system. Accordingly, the present planetary system may represent only those bodies that survived prolonged dynamical evolution.The proposed framework also does not assume that orbital motion initially formed within a preferred orbital plane or common orbital direction. Instead, first close passages may initially occur from many different directions within a non-uniform three-dimensional gravitational environment. The common orbital direction and the ecliptic plane observed in the present Solar System are therefore interpreted as possible outcomes of long-term dynamical evolution and selective survival rather than as primordial initial conditions.Overall, this study presents a unified conceptual framework linking the origin of orbital motion, long-term orbital evolution, orbital diversity, selective survival, orbital branching, and the formation of the ecliptic plane within a single dynamical interpretation. The proposed mechanism remains a conceptual model and will require future numerical simulations and comparison with observational data to evaluate its quantitative validity and broader applicability.