What happened to Pluto?

For 76 years, Pluto was the smallest, cutie-pie planet at the end of the solar system until all of a sudden, it wasn't considered a planet anymore. I never knew the hows and whys of this, so I looked it up for today's learning...

On 24 August 2006, the International Astronomical Union, passed a resolution that revoked Pluto's planetary status.

It provided this definition of "planet" (which hadn't been clearly defined up to this point - isn't that incredible?):

A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood [sic] around its orbit.


Pluto is relatively round and orbits the Sun but it does not meet the criteria because its orbit crosses Neptune's orbit.

 

It also established two new categories of objects in orbit around the Sun: dwarf planets and small solar-system bodies. According to the resolution, a dwarf planet is:

A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood [sic] around its orbit, and (d) is not a satellite.

Hello Pluto! You may not be a full-fledged planet, but you're still a cutie-pie dwarf-planet.