How Asteroids Differ From Planets

In a galactic episode of MinutePhysics, narrator Henry Reich explains via whiteboard animation how asteroids differ from planets, thus requiring a different scientific classification.

Asteroids, it turns out, are not just miniature version of the larger planets, but are in fact the  fragments from the collisions and disintegration of fully formed planets (or protoplanets), rather than miniature versions of the larger planets.

It turns out that the composition of asteroids are completely different from that of planets. Scientists discovered in 1953 that asteroids are just broken pieces from other planets and don’t have enough energy to sort the materials properly.

When planets form, the release of gravitational potential energy partially melts their interiors, causing heavy elements to sink to the core and lighter materials to float upwards, sorting into  layers. If asteroids were formed like planets, but smaller, they would each be made of a mix  of materials, and would be too small to have enough energy to sort the materials into layers.  If instead asteroids were relics from collisions  

This discovery caused asteroids to be re-classified and demoted into a secondary class.

So. The asteroids were demoted to  non-planet status, not by a vote, nor from a dislike of a long list of planets, but  due to a scientific discovery that asteroids are, in fact, physically different from everything else  that we call planets. The discovery meant they were split off (in the minds of scientists)  into their own group, and those scientists  organically stopped calling them planets.