We all grew up learning in school that we have 9 planets in our solar system. And the most popular and mysterious of them is Pluto. No one loves the planet Pluto more than kids! In fact, after Pluto was discovered in 1930, the name was suggested by an 11 year old schoolgirl living in Oxford England who was very interested in classical mythology and astronomy. No, Pluto is not named after the lovable friend of Mickey Mouse, Pluto is the Roman god of the underworld. In fact, Pluto the Disney dog was also created in 1930 and named after the planet Pluto!

Here are some more cool facts. Did you know that Pluto is smaller than our moon? And because it’s so small and far away, even our strongest telescopes can’t get a good picture of it. So you wont see it when you’re star gazing, in fact no one really knows what it looks like! But if all goes well, when the New Horizons spacecraft flies by Pluto in 2015, we will finally have up-close pictures of the most popular former planet. Have you memorized the planets and their order from the sun? If not, here’s an easy way to remember: My (Mercury) Very (Venus) Educated (Earth) Mother (Mars) Just (Jupiter) Served (Saturn) Us (Uranus) Nine (Neptune) Pizzas (Pluto). This is called a mnemonic device (pronounced ni-mon-ik) which means it helps your memory. We have solar system model kitsthat teach this and science activity books as well!

In 2005, astronomers discovered what they thought was the 10th planet, and temporarily named it after the TV inspired character – Xena (pronounced – Zee-nuh) the Princess Warrior. In September of 2006 they officially gave Xena the name Eris from the Greek goddess of discord and strife. Because Eris is about 5% bigger than Pluto,astronomers decided it was obvious that either they are both planets, or they are both something else. After Eris was discovered the IAU (International Astronomical Union) decided to wait to make a decision on the status of Eris until it was decided what a “planet” actually was.

So what happened to Pluto? Why is it no longer considered a planet? Believe it or not, this debate has been going on for a long time. In August of 2006, 424 astronomers finally got together to vote on the definition of “planet.” The definition they came up with would no longer consider Pluto a planet, but rather a dwarf planet. The astronomers decided on 3 criteria that make up a “planet”:

1) The object must be in orbit around the Sun. Well, Pluto does that!

2) Size was the second factor. Remember that Pluto is actually smaller than our moon. But who is to say exactly how big a planet needs to be? Well, the astronomers decided that to be a “planet”, the object has to be big enough that gravity squeezes it into a round shape (a sphere) like an orange or a basketball. An asteroid will have an odd shape, often like a potato, because it’s just not that big. When a body is large enough to be squeezed into a round shape, it’s called hydrostatic equilibrium.

3) The third thing they decided on also has to do with size. They agreed that an object must be big enough that it’s gravity removes other objects that occupy the same orbit. That means that as Pluto follows a certain path around the Sun, no asteroids or other objects can occupy the same path around the Sun. But Pluto does not meet this criterion. Pluto is in a section of our solar system called the Kuiper belt which is made up of ice asteroids.

So according to the IAU, Pluto meets only 2 of the 3 criteria and therefore is considered a dwarf planet, along with Xena (now Eris).

Also, Pluto has an orbit that is eccentric (oval-shaped), yet the other 8 planets have circular orbits. This is something that has bothered astronomers for a long time. It means that Pluto’s orbit brings it closer to the sun twice during its 248 year revolution around the sun. If you want to visualize this get a pencil or Crayon and a piece of paper and first draw the sun, make it about the size of a penny. Then draw a large circle around it. Try to make it as close to a perfect circle as you can! This will be Neptune’s orbit around the sun. Remember, Neptune is the 8th planet from the Sun. Now draw an oval that overlaps Neptune’s orbit so that two sides of the oval are inside the circle and two sides are outside the circle. This is Pluto’s orbit. As you can see Pluto’s orbit comes closer to the sun than Neptune’s. It’s not one of the 3 planet criteria, but it’s an oddity.

Many people were very happy that Pluto was no longer considered a planet, but other people were so upset they started petitions and websites to try and convince the IAU to reinstate Pluto as a planet. There were even bumper stickers made up that said “Honk if Pluto is still a planet!”

So what do you think? Are you convinced that Pluto is a dwarf planet based on what the astronomers decided, or do you think Pluto should be a planet just like the other 8? This debate will likely go on for a long time!