A fixed plane passing through the centre of mass of the Solar System, and oriented perpendicular to the axis about which the angular momentum of the Solar System is measured. The invariable plane is inclined at 1°.58 to the ecliptic, between the orbital planes of Jupiter and Saturn, but its precise location is not known because the masses of all the objects in the Solar System and their positions and velocities at a given time are not yet exactly known. Being fixed, it provides a permanent reference plane, whereas the ecliptic alters with time because of planetary perturbations.