Not a planet. It was only discovered about 80 years ago. It's not like it's been a planet since prehistory. Uranus and Neptune are also fairly new, discovered in the 16th and 17 centuries, but they qualify as planets.
Science develops. Definitions get changed, and the you get on with it. Anyone with any experience with science knows that it is not static; taxonomy changes, biological concepts get refined, and physics may completely change with new evidence of particles that can be accelerated past the speed of light.
What was science fiction in my not to distant childhood is now the stuff of routine medicine. I'm old enough to remember the first IVF, and how out there it all seemed at the time; now it is a completely normal, although last resort, treatment for infertility.