Dwarf Planets

PLUTO'S DOWNFALL
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 doing that though is that Pluto is too large to be a comet. 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, it 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.

Orbiting the Sun are several planets. These worlds are split into two categories: Traditional Planets and Dwarf Planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune are Planets. They are all round, all orbit the Sun, and some of them have moons. At the moment, there are three worlds in the Solar System which are classed as Dwarf Planets. They are Pluto, Ceres and Eris. Like the traditional planets, they are round, orbit the Sun, and two of them even have moons. So what makes them different? Here is the story of how a meeting of scientists in 2006 changed the Solar System.

It was Summer 2006 and astronomers from all over the world decided to hold a meeting in Prague, the capital of the Czech Republic. They were going there to discuss space, which is pretty much what you'd expect a bunch of astronomers to discuss. This group of astronomers are members of the International Astronomical Union (IAU) whose job is to classify objects in space and give them names. For several years, these astronomers had been getting more and more concerned about state of the Solar System. The main reason? Pluto. The tiniest planet in the Solar System, the most distant one, and the only one discovered in the Twentieth Century, had given astronomers headaches for years. Although it was officially one of the nine planets orbiting the Sun, it didn't quite fit in. The four closest planets to the Sun are small, rocky worlds. The next four are large gas giants. And after them, there is Pluto, a small, icy world, the tiniest of them all. Added to that is the fact that it doesn't even orbit the Sun in a circle! The other eight planets stay in a fairly regular orbit, not even coming close to crossing another planet's orbit. Pluto doesn't. Its orbit crosses Neptune's, meaning that for about 20 years of its 248 year journey around the Sun, it is closer to the Sun than Neptune. However, Pluto's elliptical orbit also takes it twice as far away from the Sun than Neptune. Finally, Pluto orbits on a different plane. If you could see all of the planets from the side, they would appear to spin around the Sun almost level with each other. Pluto doesn't. It goes above and below the other planets in a tilted orbit. However, despite these differences, Pluto couldn't fit in any other category. It can't be a Moon because it has its own orbit around the Sun rather than directly orbiting another planet, and it can't be a comet because it is too large and its orbit isn't elliptical enough. So, because like the other planets in the Solar System, Pluto is round and orbits the Sun, it too had to be a planet, and since its discovery in 1930, it has officially been recognised as the ninth planet in the Solar System.

However, a discovery in 2005 eventually led to the cure for the astronomers' headaches and also the downfall of Pluto. Modern technology has brought us telescopes that can see deeper and deeper into space. It has been known for several years that Pluto isn't alone in its region of the Solar System, and that there are other small, asteroid-sized objects, orbiting the Sun at similar distances to Pluto. This area is known as the Kuiper Belt and these objects are known as Kuiper Belt Objects. No objects the same size as Pluto had yet been found. That was until 2005 when scientists at the California Institute of Technology were looking through images taken in 2003 of a distant area in the Solar System. They noticed an object which, in different images, appeared in different locations in relation to the stars. This suggested that the object could be a comet or a planet-sized object (or a speck of dust on the photograph!). Further observations revealed that this object was actually larger than Pluto and could be the Solar System's tenth planet!

But, the IAU were not happy. The new discovery, provisionally named 2003 UB313, and nicknamed Xena because it was easier to say, has an even stranger orbit than Pluto's. It is elliptical, taking it into an orbit closer than Neptune to the Sun but which also goes twice as far away as Pluto can get. The orbit is also much more tilted than Pluto's. As well as that, other similar round objects had also been found beyond the orbit of Neptune, sharing similar characteristics. But, because Pluto was a planet, these would also have to be planets.

DWARF PLANETS MENU	WHAT IS A DWARF PLANET	PLUTO	PLUTO'S MOONS
PLUTO'S DOWNFALL	NAMING PLUTO	CERES	ERIS