by Jean AUDOUZE, Director of Research at the Institut d'astrophysique de Paris

The International Astronomical Union's (IAU) General Assembly got off to a flying start in Prague (Czech Republic) last August when it proposed including three new stars in the category of planet.

This would have carried the number of planets in our Solar System from nine to twelve, a veritable revolution. The reaction was not slow in coming. After a lively debate, the astronomers voted on 24 August to create a new category of planet, the dwarf planet or pluton in reference to the smallest of the planets in our Solar System, an icy star just 2,300 km in diameter.

That is how Pluto found itself downgraded to a dwarf planet and our Solar System dropped from nine planets to eight.

Director of Research at the Institut d'astrophysique de Paris, Scientific Advisor to the President of the French Republic from 1989 to 1993 and laureate of the Kalinga Prize for the Popularization of Science awarded by UNESCO in 2004 for his role at the head of the Palais de la decouverte in Paris, Jean Audouze explains why defining the notion of planet is no easy task, even for the experts.

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- In early August our Solar System counted nine planets, in mid-August twelve and by the end of August just eight? What happened?

- Two factors entered into play. The community of astronomers meets every three years. This year, they were in Prague, in 2009 they will be in Rio de Janeiro in Brazil. Major ongoing projects are presented at these meetings but they are also an occasion to codify the nomenclature and put a bit of order in our classifications.

Astronomers tackle questions concerning the planets in much the same way a Parliament functions. In Parliament, you have a Commission made up of a group of parliamentarians who examine a bill and make recommendations on it. The full Parliament then either endorses or rejects these recommendations.

It works the same way for planets. The IAU asked a small group of astronomers to submit a recommendation. In this particular case, that recommendation was not followed by the community of astronomers.

- Is it the discovery of Xena in 2005, a star on the outer edges of our Solar System like Pluto, which put the cat among the pigeons?

- Of course. After a team of astronomers discovered the star you describe, which has since been named Eris, they said, 'here's a new object which orbits the Sun. We propose that it be the tenth planet.' At the time, people were not yet talking about adding two others but I shall come back to that.

- Is it true that Eris measures 100 km in radius more than Pluto?

- Yes, 110 km more to be exact. It also lies three times farther from the Earth than Pluto, at a distance of 16 billion kilometers. This compares to a gulf between Pluto and Earth of just 6 billion kilometers. Eris possesses characteristics which are fairly comparable to those of Pluto in terms of chemical composition. However, Eris reflects the Sun's light much more effectively than Pluto and this light irradiates Eris like any other planet.

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Let's move on to the first stage, shall we. The group of seven experts convenes. The group is keen to add Eris then thinks, 'but if we want to include Eris, we shall have to add Charon, Pluto's only known satellite, as well as Ceres, a massive asteroide lying between Jupiter and Mars.' The group of experts thus proposes moving from nine to twelve planets.

When this proposal is presented to the full contingent of astronomers, other voices clamour, 'Let's take a less permissive definition. Let's say that, on the one hand, the planets of our Solar System have to orbit the Sun and that, on the other hand, they have to follow the same trajectory. As none of the four candidates-Pluto, Charon, Ceres and Eris - satisfies these restrictive criteria, we prefer moving from nine to eight planets.' When it comes to the vote, the majority of astronomers follow this reasoning. There you have the story.

- What differentiates the eight planets from Mercury to Neptune from the other four?

- Firstly, they vary in size, these four being smaller than the other planets. Secondly, the Sun is not the only cause of their movement. Remember that the eight planets orbit the sun in an ecliptic plane. They follow an elliptic movement in line with Galileo's theory. We have known for ages that Pluto follows an orbit that is completely out of line with the ecliptic plane. It has been common knowledge since 1932 when Pluto was discovered by an American amateur astronomer by the name of Percival Lowell.

- Is it true that, according to the definition adopted by the IAU on 24 August, any celestial body moving in the orbit of a planet, like Charon, cannot lay claim to the title of planet?

- Yes, that's right. According to this new definition, the main agent of a planet's movement must be the Sun. According to one hypothesis, Pluto used to be one of Neptune's satellites, until it steered off course; having left Neptune's orbit, it then began orbiting the Sun. This is another reason why Pluto has been downgraded. The new definition excludes the celestial bodies in the asteroid belt between Mars and Jupiter from the category of planet, as well as the more distant celestial bodies in the Kuiper belt, one of which is Pluto. This is more convenient, since new celestial bodies are being discovered in these two belts every year.

- Is it also because Pluto Is the only icy planet in our Solar System that it has been downgraded, when the other planets are either telluric like Mercury, Venus, Earth and Mars, or gaseous like Jupiter, Saturn, Uranus and Neptune?

- I don't honestly think so, especially since Pluto and Eris are both telluric bodies. That is to say, both are stars which possess solid ground like Earth that is surrounded by an atmosphere.

There is one point I would like to stress. Pluto is no less interesting for having lost its planet status. It is not the name we give a star which necessarily determines its degree of interest. We have just explored Titan, even though Titan is not considered a planet. This hasn't prevented Titan from being the object of a major spatial mission called Cassini-Huygens. Titan is Saturn's principal satellite. Titan is bigger than either Mercury or Pluto and has an atmosphere which is fascinating to study because it could resemble that of Earth at the beginning of our own planet's history.

Coming back to Pluto, we shall continue studying it. NASA has already dispatched a satellite which should fly over Pluto in 2015. It boils down to a question of nomenclature. It is as if we were botanists trying to categorize a flower. As we keep discovering planets around other stars though, the word 'planet' has to have a precise meaning.

- In that case, should we set about revising school textbooks without delay or is it possible that, in a year or two, a new discovery will come along which throws everything into doubt again?

- It is true that things are moving very fast. At the same time, for the teacher, books need to last a while. To avoid having to revise the list of planets with each new discovery, I would proceed with a mini-revision of school books. As there is no need to redraw the diagrams of the Solar System, I would simply add a mention at the bottom of the page to the effect that Pluto has been considered as a planet up until now but that, from now on, it belongs to the new category of dwarf planets like Eris. This new definition simply gives the notion of planet a more precise meaning. Even if school textbooks are not modified, it will make little difference to our conception of the sky and the universe.

- But if a teacher quizzes the class on the number of planets in our Solar System, what answer should the pupils give?

- In my opinion, teachers should accept both eight and nine as being the correct answer. The right answer is for a pupil to say that there are eight planets in a strict sense but that there is also a ninth which astronomers consider for the time being as a 'planet with a difference'. That's nothing new. I could have said that Pluto was a planet with a difference 20 years ago. What is new is that Pluto has been classified in another category.

You are totally right on one point. I would not be surprised to see us return to 12 planets in two or three years' time. Teaching needs long lapses of time, unlike research. School books should change as little as possible. If we can avoid confusing young minds, so much the better. All we need to tell youngsters is that Pluto is a special type of planet which astronomers classified differently in 2006.