I would like to explain my insatisfaction with the present-day definition of planet. This recent definition was approved by the International Astronomical Union (IAU) in 2006. This date was not picked ramdomly, it was an emergency meeting (my interpretation) to not classify Eris as planet (Eris was announced in the 29th July of 2005).
The previous definition of planet (non-official) was simply:
a) Orbits a star
b) It is not a brown dwarf nor a star nor any other massive celestial body
c) It is bigger than Pluto
During 75 years no other object in the Solar System other than the traditional planets (with Pluto) fulfilled these requirements. The discoveries of Quaoar (2002) and Sedna (2003) were announced as the discovery of the "tenth planet" but several entities quickly rejected these ideas, saying that these bodies were smaller than Pluto. However the discovery of more and more celestial bodies in the Edgeworth-Kuiper belt predicted what would follow...
In these 75 years another discoveries put to the test the definition of planet. In the sixties van de Kamp proposed the existance of planets in the Barnard's Star (one of the Sun's closest stars).
That means that for the first time the name planet was applied for bodies outside the Solar System. Later analyses disproved the existance of these planets. However the extrasolar planets would appear later. In 1988 Campbell, Walker and Yang proposed the existance of a planet around Gamma Cephei, but this was only confirmed in 2003. In 1991 and 1992 planets were propoded around two pulsars. Only in 1995 the most skeptical scientific community was shattered by the spectacular discovery of 51 Pegasi b (representation here with its star 51 Pegasi). Since then the number of these plantes grown. When the techniques of detection allow the discovery of small planets I think it will be shown that most of the stars have planets.Let's get back to the definition of planet. In 2005 the announce of Eris immediately created shock waves. Unlike Quaoar and Seda, Eris was bigger than Pluto. By the definition used at that time it was a planet. I don't understand the reasons why it was so hard for them to accept this classification. I understand that a lot of people would not want a high number of planets as the existance of other bigger-than-Pluto bodies is likely. But to do so they need to do something some people accused of blasphemy: if Eris was not a planet, Pluto was not either. As a symbol of this classification (for some an humiliation) they gave Pluto its asteroid number (134340). Interestingly this was not the first time a planet was demoted: it happened to the biggest asteroids of the Main Belt.
The essential question here is to consider Eris and its possible bigger-than-Pluto friends or to not consider none of them, including Pluto. In my opinion the existance of 10, 20 or 30 planets would not cause any problems and it would be accepted without much trouble. Obviously a school child would not be forced to memorize the name of this planet legion, just like he's not forced to know the name of all the moons of traditional planets (Jupiter alone has more than 60). The same applies to other situations. For me this desire for a low number of planets is nothing but a dogmatic imposition of the IAU.
The declassification of Pluto (this is the best picture we have of it) has severe effects. People
question the power the IAU has to judge something that a lot of people considered a part of their planetary convictions (like a lot of IAU astronomers). If the IAU defines a planet, who controls IAU's definitions? Even NASA is suffering the consequences as New Horizons, that is now flying (in almost vaccum, you understand what I mean...) to Pluto, in an emotional perspective, lost some of its importance. It will not visit a planet now, it will visit one of the numerous asteroids of the Solar System.Some people (like me) think that the IAU, that for so much time considered Pluto a planet of its own right, now decided to exclude him just because of Eris and friends' existance and not because of its features. It seems that this scientific definition (that was needed) was forced by a dogma and not because of need nor scientific responsability... The mood now is that Pluto is not a planet de jure but is it a planet de facto...
In a future post (not too much in the future, I promise!) I will speak about the definition of planet that came out from the 2006 meeting and its other consequences.
