IS PLUTO A PLANET? WHAT IS A PLANET?

August 24 Update

The IAU put up these resolutions regarding planet-hood on August 24 and they were voted upon later that day. Resolutions in white, comments in yellow, votes in lime.

RESOLUTION 5A
(1) A 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, and (c) has cleared the neighbourhood around its orbit.

Comment
This is basically good. It gives us 8 planets. Thanks, Julio.

(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.

Comment
This seems reasonable to me. We've always had "Minor Planets" and everybody understood them to be something else (namely, "Asteroids"). Dwarf planets will go the same way, most of them devolving to what they really are - big Kuiper Belt Objects.

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

Comment
Can't argue with that.

The IAU Vote: Yes.

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 . . .

Comment
This is superfluous: a planet is a planet by 5A and doesn't need the qualifier.

The IAU Vote: No.

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.

Comment
Ah, if only this had been done in 1930.

The IAU Vote: Yes.

RESOLUTION 6B
The following sentence is added to Resolution 6A: This category is to be called "plutonian objects."

Comment
Don't you mean...Kuiper Belt Objects? That's what they've been called all along without ambiguity. Is a round KBO now a Plutonian object but not a Kuiper Belt object?

The IAU Vote: No.

Bottom Line
IAU's initial proposal for planet-hood went over like a lead balloon mostly because their definition was overly complicated. They then pulled themselves together with a more rational (and more comprehensible) alternative that passed the vote. The deed is done: let's move on.

Comet Jewitt Kuiper Irregular Satellites