SCIENTISTS MEET IN PRAGUE TO DECIDE HOW TO SOLVE A PROBLEM LIKE PLUTO

Pluto, a small distant world orbiting the Sun, is to become an ex-planet. Pluto was discovered in 1930 after it was realised that something beyond Neptune was affecting its orbit. Since then, it has been known as the ninth planet in the Solar System. However, some people don't believe Pluto should be a planet, and in August 2006, astronomers from all over the world stopped stargazing for a little while to meet in Prague, the capital of the Czech Republic, to decide what Pluto actually is, and probably have a few cheap beers while they were there. It has generally been accepted for hundreds of years that a planet is any object large enough to be round and spins around a star. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are all round and they all spin around the Sun so they are all definitely planets. But, Pluto is also round and also spins around the Sun. So, by definition, it should also be a planet. The problem with Pluto is how it makes its journey around the Sun. The other eight planets spin around the Sun in a fairly circular orbit. However, Pluto's orbit is elliptical, so it goes from going as close as 4,443,000,000 kilometres away from the Sun and as far as 7,682,900,000 kilometres away from it (as shown in the diagram to the left) At its closest, Pluto is actually closer to the Sun than Neptune. The other eight planets also orbit on a similar plane to each other, so that if you could see them all from the side, they would look level. Pluto doesn't. Its orbit is tilted by 17 degrees, meaning that it goes up and under the other planets. In fact, Pluto's orbit probably has more in common with a comet than a planet and some scientists have suggested that Pluto should be reclassified as a comet. The problem with Pluto being a comet is that it is too large. Comets are usually about 20km in diameter, and are irregularly shaped balls of rock and ice. They orbit the Sun in elliptical, inclined orbits. Pluto is over 2000 kilometres in diameter and is a regular shape so could never be a comet. 

Scientists have known for years about Pluto's differences, but have tended to ignore these differences and let Pluto continue to be a planet. After all, people have been taught for the 76 years after its discovery that Pluto is the ninth planet in the Solar System and to many people it represents the edge of the Solar System. However, Pluto isn't the furthest point in the Solar System, and in recent years, more and more objects have been discovered orbiting the Sun even further away than Pluto. The area Pluto and these other objects are situated in is known as the Edgeworth-Kuiper Belt. Some of these objects are larger than Pluto, round in shape and, because they orbit the Sun, should also be known as planets. But, instead of orbiting the Sun like the eight traditional planets, they orbit in elliptical, tilted orbits like Pluto, but are too large to be comets. If all of these objects get defined as planets, in a few years, the Solar System could contain up to a hundred planets. Having this many planets in the Solar System takes something away from the importance of the eight traditional planets. It also makes the Solar System appear quite crowded, and if I had to do a page about each new planet on this website, it would never get completed! So scientists have decided that now is the time to put these objects into categories and define different kinds of planet. And, if they are going to put these new objects into a new category, Pluto would also have to be put into it. 

On 24th August 2006, the scientists at the meeting in the Czech Republic (well, those that hadn't gone home back to their telescopes), voted on what Pluto should be. They decided that Pluto should be demoted. it would still remain a planet, but would be reclassified as a "dwarf planet". A traditional planet would still have to be round and orbit the Sun. But, its must have cleared its orbit of all other objects. Because Pluto crosses Neptune's path, it hasn't cleared its orbit. So, Pluto is automatically disqualified from being a proper planet. Pluto is now an example, or a prototype, of what a dwarf planet should be. Other small, round objects in the Solar System, which also orbit the Sun, but are slightly odd in the way they do it, would be a dwarf planet. This includes Ceres, a round object which orbits the Sun in between Mars and Jupiter; Pluto's "moon", Charon (Pluto and Charon appear to spin around an invisible central point between each other Charon doesn't actually orbit Pluto); and 2003 UB313, a world discovered in 2003 which sparked these debates and which should get a proper name one day. Other potential dwarf planets are Sedna and Quaror, which also orbit the Sun as Kuiper Belt objects.