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.
For two groups carrying angular momenta with mutual inclination , resolving total angular momentum parallel and perpendicular to the second group's normal gives its orbital inclination relative to the invariable plane:For a positive denominator, . The larger-angular-momentum group lies closer to the invariable plane.
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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.