Defining "Planet" III

After having ten days to think about, and discuss the issue, the International Astronomical Union General Assembly has declared itself on the definition of planet. (You can read the on-line part of the discussion in this electronic supplement to the GA newspaper.)

Their discussions have led to a revised set of definitions. These are in two resolutions, each with a possible amendment.

Resolution 5 (Resolutions 1-4 relate to coordinate systems in space and time and IAU housekeeping) is the planet definition resolution. It divides the solar system into three groups: Planets, dwarf planets, and Small Solar System Bodies, as follows:

(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.

What's new here is clause (c). The idea here is that a planet dominates the mass distribution in its immediate vicinity. There can be a small amount of stuff left (e.g. the Trojan asteroids associated with Jupiter), but any number of similarly-sized objects in similar orbits violates this clause. Thus both Ceres and Pluto are excluded. Historically, this is how Ceres was demoted the first time, and now Pluto has joined it in the next category. The end result is that there are eight currently known planets. (The amendment would have added "classical" to this definition, leaving the word "planet" on its own still undefined, but this was defeated.)

(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.

The new clauses here are (c) and (d). Clause (c) distinquishes the dwarf planets, while (d) excludes Charon in particular. There are no double planets under this definition.

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

This term replaces "minor planet" now that the dwarf planets have been segregated out. This term is very unwieldy and I suspect something else will come into general use. Half the time I get the "S"s mixed up and say "Solar System Small Bodies".

Resolution 6 defines a new class of object dwarf planet with Pluto as the prototype. Up until now we've been calling them they've been included under the rubric of "Kuiper Belt objects", but the failed amendment would have changed this to called them "plutonian objects".

Left undecided was what we should call the current asteroid belt, now that it is made up of a combination of dwarf planets and small bodies. Shall these be '"ceresian objects" travelling in the "ceresian object belt"?

UPDATE: except satellites in paragraph 3 was a late amendment according to my source in Prague who has also clarified the purpose of Resolution 6. The amendment was very narrowly defeated, and an "IUA process" will be established to come up with a better name.