Pluto is not a planet

In 1992, Jewitt & Luu identify the first Trans-Neptunian Object: 1992QB1. The Kuiper Belt, whose existence was speculated since the 30's, had been discovered. Presently, more than 1200 objects were discovered in the Kuiper Belt. Being a member of this belt, Pluto saw its classification as planet compromised. After the discovery announcement of Eris (2003UB313) by Brown, Trujillo & Rabinowitz, in 2005, Pluto was definitely doomed. The question might seem strange but a strict definition of planet did not exist. The distinction between planet and star is still a subject of debate for the extra-solar planets. However, regarding to the Solar System we all believed to know who was and who was not a planet. A simple meeting was enough to conclude we do not know it as we would like to.

The International Astronomical Union (IAU), on its XXVI General Assembly, in Prague, from 16 to 24 of August, 2006, proposed to establish a planet definition. The initial proposal, essentially, established only the mass and sphericity as criteria to an object which was not a satellite to be a planet. In a way it was established that only size matters: Pluto would continue to be a planet. However, after a large debate it was determined to take also into account, as an additional criteria, the gravitational control of its orbital region. That is, it was also necessary to have cleared its orbital region of other objects: Pluto would not be a planet now. Also, it was decided that the planet definition would regard only to Solar System objects. The extra-solar planets continue, still, without a strict definition.

On the 24th, the Resolution 5A was approved [see IAU website]:

[...] The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet" (see footnote 1) is a celestial body that (a) is in orbit around the Sun, (b) has sufficient 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 sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape (see footnote 2), (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects (see footnote 3) except satellites orbiting the Sun shall be referred to collectively as "Small Solar-System Bodies".

Footnote 1: The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Footnote 2: An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
Footnote 3: These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

It was also approved the 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 trans-Neptunian objects (see footnote 1).

Footnote 1: An IAU process will be established to select a name for this category."

I confess that the demotion of Pluto pleases me . If Pluto had been discovered after 1992 it would never had been classified as planet. Objectively or not, words are important. However, the new definition possesses its problems. Subjectively, the term "dwarf planet" does not seem to be a very good choice for something that it is not a planet. Objectively, items 1b-2b and 1c-2c are more vague than they seem and problems will most likely arise. Not that I have heard a better idea. The problem here may be resumed to the Sorites Paradox, attributed to Eubulides of Miletus, that Doctor Nuno Crato likes to quote. That is: at which point a heap of wheat stops being a heap if I continuously remove wheat grains?

If borderline objects between dwarf planets and small bodies exist, as probably they will, we will have situations where one day an object is a small body by 1 km of diameter and on the next day it will be a dwarf planet, due to a more accurate measurement of its diameter. The question of clearing the neighbourhood will also be delicate. What is the maximum number and the minimum size of dust / small bodies allowed in a neighborhood so that it is considered clean? Problems like these will be delightful for the miscontent with Pluto's demotion.

But... from the scientific point of view, is this question truly important? In my opinion: in theory no, in reality yes. No: because in theory Science will not be neither more nor less interested on Pluto, or any other object, due to a subjective classification. Yes: because Science also lives on social and political impacts. However, from this point of view, I think that the balance will be positive. Pluto may have lost some importance to the eyes of society but, suddenly, all the other dwarf planets had gained it.


Notes: There are a huge amount of articles and opinions on this issue. There are, however, two sites one has to visit: David Jewitt's web page, with particular emphasis on the article "On Pluto, perception & planetary politics", and Mike Brown's web page, one of the miscontent with the demotion.