The International Astronomical Union (IAU) is expected to accept a new definition of the word "planet" today. It's almost certain that Pluto will be given a different status from the eight other planets (Mercury through Neptune), but exactly what status is still uncertain.
Quoting from the last page of
the official IAU newspaper, the latest proposed definition reads:
The IAU [...] resolves that planets and other bodies in our Solar System 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 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 [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects [3] orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".
[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.
There is an additional proposal that modifies point (1) and footnote [1] above, by inserting the word "classical" before the word "planet". That seems like pure semantics, but there's more to it. The IAU newspaper gives arguments for and against this additional proposal.
For:
Compromise. Achieving a planet definition has been all about compromise. There are two equally valid descriptions of what should be the principal criterion for defining a planet. One is dynamical, an object that has "cleared out its zone." The other is based on the physical nature of the body itself. The pendulum of argument has swung both ways during the General Assembly discussions. But now it has swung too far.
Resolution 5B is all about finding the middle ground. Using qualifiers gives equal status to both points of view and leaves open the possibility to define other types of planets in our Universe. Resolution 5B restores the "global and cultural points of view" that the Planet Definition Committee had responsibility to achieve. The public recognizes Mars, for example, as a "planet" not because it has cleared out its zone, but because it is a fascinating world.
To illustrate why Resolution 5B is cultural, and not silly semantics, consider how you must answer two questions: How many planets are there? Is Pluto a planet? A vote in favor of 5B yields: "There are 8 classical planets and many dwarf planets yet to be discovered" and "Pluto is a planet, but in the dwarf planet category." These answers highlight and communicate the tremendous revolution of new discoveries in our outer Solar System. Further, it saves enormous public backlash by still being able to say the words "Pluto is a planet, but". Do not underestimate the global cultural importance of these first four words. The word "planet" deserves to be shared equally.
Against:
Resolution 5B represents a small but significant change to Resolution 5A.
The key issue is the definition of "planet". Resolution 5A is close to the version agreed by consensus on Tuesday evening where it was made clear that three distinct categories of objects orbiting the Sun were being defined: planets, dwarf planets, and small bodies. The logical implications from the rules of grammar cannot be ignored. By using the name "planet" with two different adjectives "classical" and "dwarf" a larger category of planets is implied. This contradicts the first paragraph of both Resolutions 5A and 5B and transforms three distinct categories into two (planets and small bodies) and two sub-groups of planets.
To the question "is Pluto a planet?" the two resolutions give different solutions--"Yes" for 5B and "No" for 5A. To the question "How many planets are there?" Resolution 5A gives 8, Resolution 5B currently gives 12 and soon at least 50.
The total number of planets may not matter to scientists, it is critical for education and the dissemination of science. For scientists, it is relevant that dynamical and cosmogonical criteria, which are now the source for the definition of planets, would in Resolution 5B be relegated to a secondary role. In Resolution 5A the arguments from geophysics and from dynamical astronomy are given equal weight. Such a balanced solution had received very strong support in the meeting of Division III (Planetary Systems Science) and the Planet Definition Information Meeting. Resolution 5B is misleading and should be rejected.
If I were able to vote, I would accept the primary proposal and reject the additional one.
As for the astronomers at the IAU, I'm pretty sure the primary proposal will be accepted, but I have no idea on the additional one.