What discovery in 2005 accelerated the need for a formal definition of a planet?

Answer

Eris was found to be initially thought more massive than Pluto

The scientific imperative to establish a rigorous, binding definition for a planet was drastically heightened by discoveries occurring beyond Neptune, particularly within the Kuiper Belt. The discovery of Eris in 2005 served as the tipping point. Initial observations suggested that Eris was not only comparable to Pluto but potentially more massive. This presented a clear dilemma: if Pluto was to retain its planetary status, Eris, being similar or larger, would logically require the same classification. Extending this logic further, if Eris qualified, then dozens or possibly hundreds of other large Kuiper Belt Objects (KBOs) would also need to be elevated to planetary status, leading to an unmanageable census of planets in the solar system, thereby rendering the old, descriptive system obsolete.

What discovery in 2005 accelerated the need for a formal definition of a planet?

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