Why is perfect sphericity considered the 'baseline expectation' for planetary nebula geometry based on simple physics?
It is the mathematically and physically simplest outcome of an isotropic outflow from a single central point expanding into space.
Sphericity represents the unperturbed, fundamental state for an expanding cloud originating from a single source. If the material loss (mass ejection) is uniform in all directions—isotropic in both velocity and density—the resulting shape will naturally conform to a sphere, which minimizes complexity. This outcome is the direct, unadulterated expression of a solitary star undergoing uniform mass loss. Any visual deviation from this simple spherical template signifies that secondary physical processes, such as interaction with a companion or internal magnetic anomalies, have introduced turbulence or anisotropy into what should have been a smooth expansion.
