Today’s the day on which astronomers will decide whether or not the world gets a new definition of planet. The final text of the resolution — to be voted on this afternoon at the meeting’s closing ceremony — is posted in today’s edition of the conference newspaper, Nuncio Sidereo III. It is preceded, after a turbulent week of revisions, by the warning: “only minor corrections can be accommodated at this stage”.
According to this resolution (version four by my count), the solar system has eight top-flight planets, with Pluto in a second class of dwarf planets. Separate votes will be held on whether to label these top-flight planets “classical planets” and what, if anything, to do about putting Pluto and other round trans-neptunian rocks into a “plutonian object” category.
A short opinion article arguing for the “classical” category says it’s good because it allows people to say “Pluto is a planet, but in the dwarf planet category”. A counterpart article putting the opposing viewpoint says it confuses the answer to the simple question “How many planets are there?” and encourages astronomers to reject the idea.
You can read these pieces in full in edition 9 of the newspaper here (on page 8). For readers’ convenience, I have also copied over the fold the full text of the resolution, which is in two parts, each with two sub-parts, and has added footnotes.
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 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.
(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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects 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.
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 . . .and
1 The 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.”