Caption. The midplane (lavender) is a plane within a body (a purple sphere in this case) that lies at z = 0. It is perpendicular to the rotational axis of a body. Credit: G. O. Hollyday.
The midplane is an xy plane that is positioned at z = 0 (see image above). In other words, the midplane intersects the boundary between the northern and southern hemispheres of a planetary body. It is a two-dimensional plane that lies along the middle of the body and is perpendicular (90\(^\circ\)) to the planetary body’s rotational (z) axis.
The midplane in a synestia is a region in which moons form. Moonlets are pulled towards the midplane (either down towards the midplane if they are above it, z \(>\) 0, or up towards the midplane if they are below it, z \(<\) 0) as they move around in a synestia. It is a place of gravitational equilibrium in z.