August 24 is a special holiday called Pluto Demoted Day.
It isn’t exactly a holiday. No one gives presents, and nobody gets the day off. Many people don’t even know about Pluto Demoted Day. But some people do, because it’s a day that changed school books and charts of the solar system. It also changed a clever saying that people used to remember the planets in order.
The saying went like this: My Very Eager Mother Just Served Us Nine Potatoes. The first letter of each word was also the first letter of a planet.
Starting with Mercury, the closest planet to the Sun, the saying helped people remember Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
On August 24, 2006, scientists changed the rules for what makes a planet a planet. And suddenly, Pluto was demoted from being one of our nine planets to being a dwarf planet.
Demoted (rhymes with he voted) means lowered in rank. When Pluto was demoted, this left us with eight instead of nine planets. A new saying said: My Very Educated Mother Just Served Us Noodles.
Many people, including many scientists, were upset when Pluto was demoted. They felt that the rule should not have been changed and that Pluto should have stayed a planet. After all, Pluto had been our ninth planet for 76 years.
In the early 1900s, telescopes were not nearly as powerful as ones are today. So seeing a tiny planet so very far away (40 million miles!) wasn’t easy. But our story starts even earlier than that.
Clear back in the 1840s, a scientist named Urbain Le Verrier noticed occasional wobbles in the orbit of Uranus. He predicted that the wobbles were being caused by gravity from another, yet undiscovered, planet beyond Uranus. That planet turned out to be Neptune.
In the late 1800s, astronomers began to think maybe there was another planet beyond Neptune that was also adding to the wobble of Uranus. They couldn’t see another planet, but were sure that there was one. They began searching for what they called “Planet X.”
In 1930, a man named Clyde Tombaugh took photos of a moving object that turned out to be the mysterious Planet X. The discovery made headlines around the world.
The newly discovered planet needed a name, and more than 1,000 suggestions were made. An eleven-year-old schoolgirl named Venetia Burney suggested the name Pluto, the ancient Greek god of the underworld. Her suggestion was chosen.
So from 1930 until 2006, Pluto was our ninth planet. It was large enough that its gravity caused it to become ball-shaped and it orbited the Sun. Those were the two requirements for a space object to be a planet.
But in 2006, the International Astronomical Union (called the IAU) added a third rule: To be a planet, an object must have cleared its neighboring region of other objects.
Here’s the problem. Pluto has five moons: Charon, Styx, Nix, Kerberos, and Hydra. The last four are small. But Charon is almost as big as Pluto. And Charon doesn’t orbit Pluto the way our Moon orbits the Earth. Instead, Pluto and Charon do a sort of dance, both orbiting around a point between the two.
So astronomers at the IAU said that Pluto hadn’t really cleared Charon from its region, therefore Pluto is not a planet.
The IAU called Pluto a dwarf planet.
Many scientists, including many astronomers and astrophysicists, were not happy with the IAU’s decision and chose to ignore it. But schools and the general public believed that what the IAU said must be right or they wouldn’t have said it, so they dropped Pluto from the list of planets. It was removed from school books and from wall charts.
To this day, many people believe the IAU decision was wrong and are sad that Pluto is no longer a planet. And so every year on August 24, they observe Pluto Demoted Day, a day to honor our former ninth planet.
This, however, is not the end of the story. Other space objects are joining Pluto in a growing collection of dwarf planets.
Also, some astronomers believe there may be another real planet further out, and if it’s found, it will again give us nine planets.
• Some people used to use this sentence to remember the planets: My Very Elegant Mother Just Sat Upon Nine Porcupines.
• Workers at Disney Studios believe that in the 1930s, Walt Disney named Mickey Mouse’s dog Pluto after the newly discovered planet.
• Pluto’s rotation is backwards from Earth’s. For us, the Sun rises in the east and sets in the west. On Pluto, it rises in the west and sets in the east.
• It takes Pluto 247 of our years to orbit the Sun, so since its discovery back in 1930, it has yet to complete a full orbit.