Back in 2006, Pluto was at the centre of a scientific row. Was it a planet or not? It had always been recognised as one since its discovery in 1930, but had also always been the odd planet out. However, within the last decade or so, a number of other objects similar to Pluto had been found scattered about the Solar System in its distant depths. As Pluto was classified as a planet, it would mean that these newly-discovered objects should also become planets. But, it was feared that this would result in the Solar System containing tens, possibly hundreds, of insignificant planets. So rather than classifying them as planets, members of the International Astronomical Union decided at a meeting in 2006 they would be classify the new Pluto-like worlds as dwarf planets. This would also mean that Pluto would have to be reclassified as a dwarf planet, officially losing its status as a regular planet. It is now regarded as incorrect to call Pluto a planet and to state that the Solar System contains nine planets. That is, unless, you live in Illinois.....
In February last year, the State of Illinois passed a bill to re-establish Pluto's "full planetary status" and to declare March 13th 2009 as Pluto day to mark the 79th anniversary of the date that its discovery was announced. The reason for Illinois' defiance of the IAU's ruling? Clyde Tombaugh, the man that discovered Pluto, was born in Streator, Illinois. And, while Pluto passes through the skies over Illinois, it must be a planet. The bill also states that Tombaugh is the only American ever to have discovered a planet. Maybe somebody should point out that many Americans have discovered many planets. Okay, so none of them orbit the Sun. Instead they orbit other stars. Illinois' bill has been met with mixed reactions. There are of course many people who don't want to accept Pluto's new status, including members of the Society for the Preservation of Pluto as a Planet, who encourage supporters to email the IAU in disgust and throw Pluto Parties. Quite what you would do at a Pluto Party remains a mystery but they sound like they could be fun nevertheless! Supporters of Pluto being a planet, including the state of Illionois and the Society for the Preservation of Pluto as a Planet, state that the decision to demote Pluto was made unfairly since only 5% of the total number of members of the IAU actually voted. Other people believe that Illinois' decision, which is more of a way of honouring Clyde Tombaugh rather than being a scientific protest, is silly as it does nothing more than confuse people. Even now, almost four years after the IAU's ruling, many people aren't sure of the true definition of Pluto or know how many objects are officially classed as planets in the Solar System.
Personally I felt it was a shame that Pluto did get demoted but that is probably more to do with the fact that I'd always known it to be a planet rather than not agreeing with the way it became demoted. It was as if somebody had said to me, "You know all that stuff you were told about Pluto being the ninth planet in the Solar System. Well, it's all rubbish." It also made me realise that what we know about the universe is only what we think we know and that any discovery can change our understanding of it almost overnight. The reason we knew Pluto to be a planet was because we didn't know of other objects like it and despite its differences to the other planets, it was more similar to them than to other objects in the Solar System. It couldn't be a star, a moon, a comet, or an asteroid, so it had to be a planet. As soon as other objects were found similar to Pluto, they would need to be put into a group. I agree that grouping these objects into the same group as the rest of the planets could devalue the status of the word "planet" and could eventually lead to too many objects becoming planets, most of which would be quite small and insignificant. Creating a new group for these objects is logical. But it wouldn't be logical to not put Pluto into the group. And I really don't understand why people refuse to accept its new status - the dwarf planet definition was created to make it easier to understand the Solar System. Why oppose it?
For more information about how Pluto was discovered and how it lost its planetary status, visit this page.