The invariable plane is perpendicular to a system's total angular momentum. For an isolated planetary system, it is fixed while individual orbital planes precess.
The term "invariable plane" is commonly used in celestial mechanics and can refer to the concept of a plane that remains fixed in space during the orbital motion of a celestial body, such as a satellite or a planet. Specifically, in the context of celestial mechanics, the invariable plane is defined as the plane that contains the total angular momentum vector of a system of bodies. For a system of celestial bodies, the invariable plane is often considered with respect to the center of mass of the system.
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