Why is Pluto not a planet anymore

When we ask the question of what is and isn’t a planet we are really just talking about how we define the word “planet”. In ancient times we didn’t really need a definition Astronomers know five stone like objects: Mercury, Venus, Mars, Jupiter and Saturn They didn’t move around the other stars but seemed to wander through the sky The word planet comes from an ancient Greek word that meant wanderer Eventually we figured out that the Earth is a planet too, orbiting around the Sun just like the others and now it’s pretty much it for the Solar System until the age of telescopes. The next planet to be discovered was Uranus since 1781, followed by Neptune in 1846. These planets are huge but just too far away for ancient astronomers to see. But did you know that in 1801 astronomer Joseph Piacci discovered another planet or so he thought? Piacci found a spherical body about six hundred miles across orbiting between Mars and Jupiter. The planet which is smaller than our moon was called Ceres. In this case it wasn’t too distant to see but too small. The problem, however, was that Ceres had buddies, lots of them. The very next year a smaller object named Pallas was discovered and two years after that, another named Juno. We now know about thousands of similar bodies Instead of adding thousands of new planets to the Solar System, astronomers decided it made more sense to group these objects together they named a new class of objects, asteroids, and Ceres got demoted. Something similar has just happened to Pluto. Astronomer Clyde Tombaugh stumbled across Pluto in 1930 but since then it’s status as a planet has been called into question too. First of all Pluto’s orbit is strange, wildly offset from the other planets and even intersecting the orbit of Neptune. Also it turns out that Pluto isn’t bigger than the Earth as Tombaugh initially thought. Instead it´s got a huge moon nearby, in fact some people called them a binary planet or a double planet and, just like with Ceres, astronomers discovered that Pluto isn’t alone. Beginning in the early 1990s they started finding other objects similar to Pluto, some like Quaoar and Sedna were almost as big. Then in 2005 astronomers announced that they´d found an object named Eris even bigger than Pluto. So, are all these planets?. Astronomers wonder the same thing, and in August of 2006 astronomers from all over the world met and finally decided on a formal definition of a planet. Pluto and its buddies didn’t make the cut and now they are known as dwarf planets. But don’t be upset that Pluto isn’t a full fledge planet anymore. Instead, think how exciting it is that there is still so much to learn about the Solar System and that science can continue to set the record straight even when our initial ideas turn out to be wrong. From Ask an Astronomer, I’m Doctor Robert Hurt and that is Spitzer Science Center.