Bonzer Words!: Good Heavens - Pluto's Gone!
Wendy Ogbourne considers the "demotion'' of Pluto.
On 24th August 2006, a small group of scientists at the IAU (International Astronomical Union) voted to expel Pluto from our Solar System. For those of us who have grown up knowing that our sun had nine planets in orbit around it, this was a huge shock. Without having done anything to deserve it, Pluto was to be relegated to the status of Dwarf Planet, and all the textbooks would have to be rewritten.
How could this happen? Pluto was first discovered in 1930 and named by an 11-year-old girl, after the God of the Underworld in Roman mythology. It was always known to be different from the other 8 planets, and there has been continuing controversy among scientists as to its origins and status as a full member of the sun's family.
Pluto is far smaller than any of the other planets, even smaller than our earth's moon, only 2360km across. It's a frozen world of rock and ice, with a surface temperature of -233 C, though it does have a thin atmosphere. It also has 3 tiny moons. The orbit of Pluto is highly elliptical, unlike the other planets, whose orbits are almost circular. This means that at times it is actually closer to the sun than Neptune, its next nearest neighbour, but at other times far out in an area called the Kuiper Belt, which is the home of many of the asteroids which occasionally stray closer to the sun and can pose the threat of collision with the other planets. Pluto's orbit is also tilted well above and below the flat plane that the other planets inhabit, which again suggests that it was probably not formed at the same time or in the same way as the other planets, from the same swirling cloud of gas.
So what actually makes a planet a planet? The original definition was basically that it was an object orbiting around a sun, large enough to have formed a round shape, rather than just a chunk of rock, but not generating its own heat internally by nuclear fusion. In the time of Galileo this definition was quite sufficient and covered all the satellites that could be observed with early telescopes. Even until the 20th century this definition stood. However, with the advent of more sophisticated telescopes such as Hubble and better tracking equipment, computers and technology of all sorts, scientists realised that there were thousands of objects out there which could be called planets under the existing rules, many larger than Pluto. Clearly the definition needed to be refined, and the condition was added that to be called a planet, the body must have 'cleared its orbit of other objects'. This just means that it stands alone and has sucked in, by means of its gravity, all the debris surrounding it. And by this definition, it was decided that Pluto had failed.
However, that is far from the end of the matter. Astronomers are divided as to the sense of the new definition and many do not accept the IAU's ruling. Some museums and observatories have changed their displays and models to adopt the new ruling and others have refused to do so. Many people feel surprisingly sentimental about little Pluto and do not want it to be demoted. When the change was announced, car bumper stickers appeared in the US saying 'Honk if Pluto is still a planet'. It seems that people care about Pluto. So the controversy continues to rage. Maybe we will wake up one day soon to read that Pluto has been reinstated and there are again 9 planets in the solar system, or maybe others will need to be included - there may be 10 or 20 or more.
In 2015 a NASA spacecraft will fly past Pluto and will send back information which will help us to solve the riddle of its past and perhaps its future. Is it a planet like the others, or an asteroid; was it a moon of Neptune that got kicked out of orbit, or just a piece of space debris from a passing comet? Whatever the verdict, Pluto will continue on its lonely way, orbiting the sun once every 248 years, a sun that would appear from its surface as just another point of light in the blackness of space. The arguments between the astronomers of earth will not affect it in the slightest.
© Wendy Ogbourne
**
Wendy writes for Bonzer! magazine. Please visit www.bonzer.org.au
