© ALAMY Pluto: 'More like Earth than Earth is like Jupiter'
The number nine has a special significance for Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona. Nine is the number of planets in the Solar System, and Sykes is one of several leading astronomers who want to keep it that way.
Unfortunately, the International Astronomical Union (IAU), which adjudicates on these matters, has ruled there are no longer nine planets in the Solar System, after a decision two years ago to downgrade Pluto to the lowly status of a "dwarf planet".
But in 2009, Dr Sykes and his like-minded colleagues hope to get the ruling overturned at the next general assembly of the IAU, to be held in Rio de Janeiro in August.
"The IAU is not the Holy Mother Church, so its pronouncements are not followed by everybody," said Dr Sykes. "To me and many like me, Pluto remains a planet and there are still nine planets in the Solar System.
"The one thing that was particularly bad about the IAU's decision is that normally it makes pronouncements that are a mark of a general consensus, but here it has tried to impose its view on the rest of us."
The row over Pluto's downgrading has been simmering since the astronomy organisation voted to relegate it in August 2006 in Prague. It was agreed at the last vote of that conference - after many scientists had left.
It was particularly galling for Alan Stern, principal investigator on a Nasa mission, New Horizons, which had launched a nuclear-powered probe to Pluto six months earlier.
Dr Stern and Nasa found that their £460m New Horizons spacecraft, due to arrive at Pluto in 2015, was no longer going to visit the Solar System's most distant planet, but just one of many chunks of rock in the Kuiper belt of asteroids beyond Neptune.
"The IAU definition is so flawed on so many levels," Dr Stern fumed. "It's an awful definition; it's sloppy science and it would never pass peer review."
Nasa states: "Most people call Pluto a planet because it orbits the Sun and it is large enough for its own gravity to pull it into a spherical shape."
Pluto is a strange world: its surface is frozen at about minus 233C - just 40 degrees above the "absolute zero" of minus 273C - and it is so far from the Sun that its daytime could be compared to Earth during a full moon night. In fact, there is a strong scientific case for calling Pluto something other than the name given to the eight other planets, which fall either into the terrestrial "rocky" planets, notably Earth, Venus, Mercury and Mars, or the Jovian "gaseous" planets of Jupiter, Saturn, Uranus and Neptune.
"Pluto is just one of the largest members of the Kuiper belt of objects - the dregs of planet formation," said Hal Levison of the South Western Research Institute in Boulder, Colorado who has studied the dynamics of planet definition.
In other words, Pluto is just one of many large icy bodies around the Sun. The IAU has a new term, "plutoid", for such objects, which, while massive enough to form a near-spherical shape, do not have gravitational influence to clear the neighbourhood around their orbit of other objects.
"If you took Earth out of the Solar System, the other planets would care. If you took Pluto out, it would make no difference to the orbits of the other planets," said Dr Levison. Dr Sykes disagrees: "Pluto is far more like Earth than Earth is like Jupiter. Jupiter is a gas planet. It doesn't even have a surface or topography, unlike Pluto."
Both men, though, believe the argument over Pluto is more than an arcane discussion for experts. Astronomy and science are about organising observations of nature and a major aspect of this is how scientists agree on a system of classification.
"The argument over Pluto is a demonstration that scientists can disagree and that science is not some dictatorial project - it's dynamic," said Dr Sykes. "I think if the IAU changes its mind, that would be fine. If it doesn't, its credibility will be harmed."
Earth's most distant neighbour
1930 The year Pluto was discovered by the American astronomer Clyde Tombaugh
3,600 Pluto's average distance (in million miles) from the Sun
247.9 Years it takes Pluto to orbit the Sun
-233C Astronomers' estimate of the temperature on its surface
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