What physical state was implicitly assumed for objects classified as planets before the 2006 formalization?

Hydrostatic equilibrium (pulled into a round shape)

Even though hydrostatic equilibrium—the state where an object’s own gravity is strong enough to pull it into a nearly round shape—was not explicitly stated as the defining criterion before 2006, it was a characteristic strongly associated with the eight classical planets. The text indicates that this physical state was an implicit assumption within the older system. Most acknowledged planets were large enough to overcome material strength and achieve hydrostatic equilibrium. However, this characteristic alone was insufficient for classification, as evidenced by the fact that Pluto was accepted despite this similarity, while certain smaller, rounder Kuiper Belt Objects were not. The implicit assumption meant that while roundness was expected, the decisive factor differentiating a planet from a large asteroid or a KBO was often related to an arbitrary threshold of size or distance rather than this physical shape alone.