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New definition of planet



Editors: Guillem Anglada-Escudé [envoy to the meeting at Prague] & Salvador J. Ribas [Barcelona] (University of Barcelona)

These last weeks of August, a new has arrived to the media which has had a great impact all over the world: the International Astronomical Union would approve of, on 24th August in a meeting at Prague, a resolution to change the definition of planet, so that Pluto still qualifies. Besides, other objects, such as Charon, Ceres (nowadays classified as an asteroid) and 2003UB313, would become new planets. So, the Solar System would no longer consist on 9 members, but 12 which would be continuously increasing due to very likely detections of new objects of similar features.

After some meetings, held during the General Assembly of the IAU, this first resolution, which seemed to be firm, was ruled out. Then, another alternative proposal came up from different sectors of the IAU with a clear intention: create a new category, called "dwarf planets", set aside for including Pluto, the other three objects mentioned above and those to be discovered with similar features. Finally, this new proposal was left to discussion and put to vote on thursday 24th August. A synthesis of the results of this controversy can be read in the chronicle by Guillem Anglada-Escudé from Prague:

Chronicle from Prague:

The final decision about the definition of the planets of the Solar System has already been made. Finally,the resolutions 5 and 6 have been partially approved, in spite of the main part of the definitive content has been accepted. The definitive text reads:

Resolution 5A The IAU therefore resolves that planets and other bodies in our Solar System, except for satellites,be defined into three distinct categories in the following way: $ 1.-A "planet"1 is a celestial body that:: (a) is in orbit around the Sun, (b) has enough mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. $ 2.-A "dwarf planet" is a celestial body that:: (a) is in orbit around the Sun, (b) has enough mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2, (c) has not cleared the neighbourhood around its orbit, and (d)is not a satellite. $ 3.-All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".: $ (3) These currently include most of the Solar System asteroids, most TNOs , comets, and other small bodies.:

This resolution was approved by a wide majority, in spite of that, before the vote, a turn of "any other business" was opened, which was used by assistants to express their opinions for more than ¾ an hour. These ones spread from sarcasm and angry critic to the demand of unity in the Assembly.

The next resolution is referred to add the adjective "classical" to the planets included in 5A, section 1. This would imply that both the "classical planets" and the "dwarf planets" would be under the same generic definition of planet, in such a way that resolution 5A, already approved, would become just a simple semantic specification.

To illustrate this, the president of the IAU executive comittee (who was playing the role of chairperson through the debate) used a balloon (planet), a cuddly toy of the dog Pluto, a cereal box (dwarf planets) and a lemon ("Small Solar System Bodies"). She graphicly explained the content and the consequences of this resolution placing the cuddly toy, the ballon and the cereal box under an umbrella with the word "Planets" on it.

Resolution 5B Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading: $ 1.- A classical "planet" is a celestial body...:

It has NOT been approved after only 97 among approximately 400 assistants with the right to vote supported it. In order to avoid discrediting Pluto as a peculiar object (both physicaly and historically), the following couple of resolutions, from which only 6A was approved, this time by a very tight result (231 Yes - 157 No - 30 abstentions), were proposed.

Resolution 6A The IAU further resolves: Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of TNOs .

Previous resolution intended to stablish the name of "plutonian objects" for the new cathegory. The shortly attractive sound of the word drove many not English-speaking to reject this name. The nominal outcome of the vote was 187 supporting, 186 rejecting and 30 abstentions. The absolute lack of consensus drove to ask to the assistants for a new vote on the same issue, in order to achieve a significant majority. The very most of assistants, tired because of the hours of intense discussion, decided not to vote again, leaving the deffinite decision to a comittee of experts or to the time. Perhaps future discoveries will provide with a more appropriate noun to be pronounced over the world.

CONCLUSIONS Strictly speaking, planets are just 8: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto, Ceres, Charon (until now a Pluto's satellite), and 2003UB313 (called "Xena" by its discoverer, in a clair reference to the popular heroine of the TV series with the same name, but not internationally accepted) remain classified (NOT DEMOTED) as "dwarf planets". This list will probably increse A LOT (several hundreds?) during next years.

If the new classification described on resolution 5A has been accepted quite unanimously, the efforts of some sectors (mainly anglosaxon) in order to achieve the inclusion of Pluto and the new dwarf planets in the cathegory of planets have been received with scepticism and mostly rejected.

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