Just a few years ago, the mnemonic My Very Excellent Mother Just Sent Us Nine Pizzas worked just fine to remember the names and order of the planets in our solar system. All that changed when the International Astronomical Union (IAU) decided to kick Pluto out. Not of the solar system, just out of the elite club of astronomical bodies we call planets.
When Pluto was demoted, it caused quite a stir. It was hotly debated, even between astronomers.
Those who supported Pluto’s demotion argued that Pluto’s moon Charon does not technically orbit the body. It’s so large in relation to Pluto itself, that they orbit each other. Pluto’s relative puniness was also noted. Pluto only has 0.24% the mass of the Earth. They also pointed out that Pluto’s orbit is very different from the nearly circular orbits of the other planets. It’s very eccentric and it’s orbital plane is tilted. The odd orbit of Pluto even sometimes moves it inside the orbit of Neptune. They also likened it to many other icy objects swarming around outside Neptune’s orbit in an area called the Kuiper Belt and felt if we let Pluto stay, we'd have an untold number of planets as time went on and we found more large objects in the Kuiper Belt.
Pluto supporters claimed that like all the other planets, it was massive enough to be squeezed into a nearly rounded shape by gravity. Like most other planets, it had a moon and of course it has a regular- albeit long- orbit around the sun. To Pluto supporters, size didn’t matter. It fit all the characteristics of what we think of as a planet.
In 2006, the IAU decided to vote on an official definition of a “planet,” and with the version they voted on, Pluto didn’t make the cut. They ultimately decided that a planet’s gravitational pull must be strong enough to clear the debris from its orbit, meaning it must be able to throw out or suck up any space junk nearby.
To this day, many still do not accept this definition. Many claim that only 4% of the IAU voted on this definition, and it does not reflect the majority opinion.
As an act of protest, the state of Illinois, where Pluto’s discoverer Clyde Tombaugh was born, passed a resolution that stated that within the state borders, Pluto will remain a planet. They’ve declared that March 13 (the anniversary of the announcement Pluto had been discovered) will be Pluto Day in Illinois. New Mexico, where Tombaugh lived in later years, adopted a similar idea. In the state, March 13 is designated “Pluto Planet Day.” Both states have granted full planetary status to Pluto when it is overhead in Illinois or New Mexico skies.
Where it Stands
Pluto is now classified as a dwarf planet, along with several other Kuiper Belt objects. Scientists agree that even though it is no longer a planet, it is still a fascinating world. The New Horizons spacecraft will reach Pluto in 2015 bringing us the first up-close pictures of the now dwarf planet.