Well, as of this morning, the IAU has posted the final resolution on the definition of the word "planet," which they will be voting on today. For convenience, I am quoting the entire resolution (technically four different resolutions) here:
Resolution 5A is the principal definition for the IAU usage of "planet" and related terms. Resolution 5B adds the word "classical" to the collective name of the eight planets Mercury through Neptune.
Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. Resolution 6B introduces the name "plutonian objects" for this class. The Merriam-Webster dictionary defines "plutonian" as:
Main Entry: plu • to • ni • an
Usage: often capitalized
: of, relating to, or characteristic of Pluto or the lower world
After having received inputs from many sides -- especially the geological community -- the term "Pluton" is no longer being considered.
IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation 'planets'. The word 'planet' originally described 'wanderers' that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:
(1) A planet1 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) shape2, (c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
(3) All other objects3 orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
Insert the word "classical" before the word "planet" in Resolution 5A, Section (1), and footnote 1. Thus reading:
(1) A classical planet1 is a celestial body . . .
1The eight classical planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
IAU Resolution: Pluto
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.
The following sentence is added to Resolution 6A:
This category is to be called "plutonian objects."
So if the IAU passes these four resolutions, what does that mean? Well, it means that we would now have two different categories of planet, "classical planet" and "dwarf planet." The classical planets would be Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto would be considered a dwarf planet and the prototype of a new category of "plutonian objects." Nothing else would be called a planet. It would even seem likely that 2003 UB313 (popularly known as Xena) would not be a planet.
While the resolutions do seem well thought out, it does leave us with an interesting question: Is Pluto a planet? Technically, it is, since it now is considered a new type of planet, a "dwarf planet." But does that mean that we'll refer to an eight-planet solar system once more, or will we still consider our solar system to have nine?
Here's my prediction. How we view the solar system depends on what we learn in school, and Pluto's reduced status as a dwarf planet does not demote it enough to eliminate it from the list entirely. Although textbooks will have to be revised, to make it clear that Pluto is in a new category of planet, I suspect that pictures, tables, and charts of the solar system will continue to show all nine of the planets that we have come to know and love since the 1930s.