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  • Jul 31, 2014
  • Updated: 3:13pm

Worlds collide

PUBLISHED : Monday, 04 September, 2006, 12:00am
UPDATED : Monday, 04 September, 2006, 12:00am

It has been dubbed the pipsqueak of the planetary system, had a Walt Disney cartoon dog named after it, and now produced a new generation of Plutocrats peddling T-shirts with slogans calling for its salvation.


It used to be the ninth planet, the smallest member of our solar system and known to schoolkids the world over since its discovery in 1930.


Now it's just a lump of rock among thousands, 4.6 billion miles from the Sun in a region of space beyond the planet Neptune known as the Kuiper Belt. It's tough being Pluto.


Not that its demotion from a planet to a member of a new group of objects called dwarf-planets was a simple affair, as recent events at the top-dog star gazers' convention - the International Astronomical Union (IAU) - in Prague last month testified.


There has long been controversy over what constitutes a planet and the meeting, representing about 10,000 astronomers worldwide, set about fixing the definition.


At first it seemed easy. A planet had to go round the sun, the governing committee agreed. It also had to have enough self-gravity to be roughly spherical and could not host thermo-nuclear reactions or it would be by definition a star. Those criteria allowed Pluto, at less than half the Earth's diameter, to remain a planet and let in three other Kuiper Belt objects as well, including the largest, called UB313, nicknamed Xena, discovered by California Institute of Technology's Michael Brown in 2003.


Suddenly there were 12 planets. That was August 16. By August 24 everything had changed.


The definition and criteria were opposed by many astronomers, who felt it was too general and did not take into account the serious physical differences between the traditional planets and thousands of other objects, including Pluto, beyond Neptune.


One of those astronomers was Julio Fernandez, from the University of Montivideo, Uruguay, who proposed what is likely to be remembered as an historic amendment. Another was Sun Kwok, the new dean of science at the University of Hong Kong who, although he was unable to make it to Prague because of academic duties, felt the 12-planet definition to be deeply flawed.


'Many people were unhappy - including myself - because we didn't think the 12 were homogeneous. It was part of the committee's motivation to preserve Pluto as a planet that led to it,' he told a CNN news report at the time.


Professor Kwok, 56, who specialises in the study of planetary nebulae, said he supported Professor Fernandez's amendment, but understood why it was controversial.


'This is a normal process for us, science is about progress, getting to know things better but it's different for the public because it is in kids' early thinking,' he said. 'They were told when they were just a few years old that there were nine planets as a fact and that's as far as most people go in terms of understanding the solar system. It's quite a shock.


'In our case, we saw it coming. The issue started in 1992 when David Jewitt at the University of Hawaii and his collaborator Jane Luu discovered the first Kuiper Belt (trans-Neptunian) object. But that was just the beginning, now there are thousands of them.'


Professor Kwok said that Professor Fernandez's resolution, voted into history by just 300 of the 2,500 astronomers from 75 countries attending the conference, added a new criterion for membership of the elite planet club.


'The amendment said that planetary objects have to clear all other objects in their orbit around the sun. That eliminates four of the 12, leaving eight. Bodies like the Earth suffered collisions in the past but cleared their orbits of other objects in time,' he said.


Although Kuiper Belt objects maintained elliptical orbits around the Sun, they did not necessarily clear each other and were often on a collision course.


'Some people think it's a little vague,' he added. 'For example, what do we mean by 'clear'?'


To call the IAU's resolution controversial is an understatment.


Shortly after news of Pluto's change of status was announced there was an outcry. Amateur space buffs, cosmic hippies and an army of bloggers led the furore, with T-shirts sporting slogans such as 'I miss Pluto', 'Stop planetary discrimination' and 'Pluto, you'll always be a planet to me' on sale within days. An internet bumper sticker campaign beseeched the faithful to 'Honk if Pluto is still a planet' and there were reports this week that Pluto was about to become a star in its own right, courtesy of a pop song about to hit the charts.


The popular satirical website stopthebleating.com even proposed a planetary protest. 'Pluto's demotion from the status of planethood is further evidence of the western imperialistic oppression of the little people of the world. On behalf of all Plutonians, we call for protests against the league of planets' assault on Pluto's sovereignty. The protest will be held on Uranus at 5pm, March 15, 2100 (that is how long it will take to get there),' it said.


But the protests have not been confined to the weird and whacky. Harvard scientist Owen Gingerich, chair of the seven-member IAU panel that recommended expanding the number of planets to 12 in the first place, was reported to have said after the u-turn: 'I think astronomers blew it this morning', and 93-year-old Patricia Tombaugh, widow of the man who discovered Pluto, said: 'I don't know how you just handle it. It kind of sounds like I just lost my job.'


Most scathing of all, was Nasa's Alan Stern, head of the US$700 million nine-and-a-half-year New Horizons space mission to Pluto, which lifted off earlier this year. Criticising Professor Fernandez's 'cleared orbit' criterion, he said: 'It's a sloppy definition. It's bad science. It ain't over.' He also said he was 'embarrassed'. He has joined forces with more than 300 other scientists to issue a petition, which says, 'We, as planetary scientists and astronomers, do not agree with the IAU's definition of a planet, nor will we use it. A better definition is needed.' The scientists argue that even Earth has not 'cleared' all of the asteroids that cross its path and say Jupiter, the largest planet, has asteroids known as Trojans sharing its orbit.


But Professor Kwok said he stood by the new criterion, while conceding 'different perspectives' on the issue. 'Alan Stern also wrote to me asking me to sign, but I won't sign,' he said.


Professor Kwok said that with four of the remaining eight planets rocky and four gaseous, scientists could have come up with totally different definitions and names for the bodies had they been starting from scratch.


'But we are not in that position,' he said. 'We are bound by history. We want to do the minimum to preserve the status quo but still have a system that is meaningful. We can tolerate the eight. When it comes to Pluto, though, it is a different matter because there are thousands of other objects like it. It has more in common with a comet.


'You hear people say they love Pluto, it is their favourite planet and that we want to get rid of it. No, continue to love Pluto, it is still there, nothing has changed. This is scientific progress.'


Professor Kwok said the demotion of Pluto was nothing compared with two of the greatest cultural shocks to have resulted from astronomy in the past, the first being the discovery of the sixth planet after there being five for about 2,000 years.


'You know which one it was?' he asked. 'It was Earth. Copernicus realised that Earth was no different from the five planets. Previously we thought Earth was a special place created by God, but he turned the whole thing round and showed that Earth was no different from Mars. The definition was objects orbiting the Sun and you could no longer make a special case for Earth.


'That was a real cultural shock. We're going through the same thing here. It's not a negative thing. We didn't make a mistake 70 years ago, it's not that we goofed and are now correcting our errors. No, we are making progress.'


Professor Kwok said the second great shock came 150 years ago, when we discovered the Sun was a star no different from all the other ones we see in the sky. 'Of course it will always be special to us,' he added. 'But this reclassification of Pluto is the same kind of thing.'


And HKU's dean of science said there was a very important educational lesson to be learned from Pluto's fate. So-called facts were not sacrosanct.


'I want to use this opportunity to say something as an educator because this is a very good chance for us to convey to our young people that science is not about studying facts, such as remembering that the solar system has nine planets.


'It's about the why, about the process. We are changing all the time, we shouldn't be too attached to so-called facts.


'Why bother learning facts? With the internet now, you can look things up. The most important thing is to be able to connect 'a' to 'b' to 'c' by rational thought or by mathematics.'


Professor Kwok said that once people realised that facts were not set in stone but ever changing, as the Pluto saga had shown, the sky was the limit.


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