Pluto: round and round

So... the ad hoc sub-subcommittee of the standing subcommittee for the Naming of Names has reported out: the Federation of People Who Believe No Really Important Discoveries are Made West of the Mississippi are reeling in defeat, while the Alliance of the Crusade for Consistency is handed a token superficial victory.

Millions of elementary school teachers, and their pupils, are in tears.

A Good Day for Science, indeed.

Phil has the lowdonw and here is the official word

Various Science^Bloggers chime in, here and here and here

Well, it is all over but the voting, which comes later this week.
Not sure if this will be accepted. We'll find out the hard way.

So, what do we make of it: the obvious inconsistency of "how round is round" - is not actually there, the resolution speficies hydrostatic equilibrium, so figures of rotation are included, not just spheres.
The Earth is about 0.1% bumpy, the bottom size range of the planets will be potentially more bumpy; some nitpicker is going to want to quanity how "nearly" round something has to be to be "round".

Let me be the first to propose 1%. Anything with bumps larger than 1% vertical displacement from hydrostatic equlibrium is Right Out.
What surface you ask - well, fortunately, most of the marginal categories will be airless, so we have solid surfaces to deal with, and the fluid planets will mostly have hydrostatic equilibrium at all the higher isopotentials.

Phil notes that this makes the definition of a planet composition dependent, not strictly size dependent; but it is worse than that. The definition is also temperature dependent - compression moduli and shear strengths are quite sensitive to temperature in the regime of concern, and equlibrium shapes depend not just on current temperature but the thermal history!
Annoyed that your favourite object is not a planet? No problem, just melt it slowly, let it achieve equlibrium and then cool it rapidly. Voila, a new planet!

I am chuffed, if slightly puzzled, that the new "consistent" definition puts the critical radius at ~ 800 km, which makes Ceres finally a planet.
This restores the Titius-Bode Law to its true full glory of numerological happenstance, and, bonus!, really screws up any astrologer with intellectual integrity.

The "binary planet" issue is whacky. Again, as Phil notes, this implies the Moon will become a planet Real Soon Now, as it recedes from the Earth and the barycenter exits the surface of the Earth.
We can of course fix that by making the "round" definition such that the Moon is too lumpy, but we must do that without losing any of the useful planets, like Mars. Tricky. Still, the Moon is just way too lopsided...
Or we can just dump Charon. Please.

People with Unusually Large Telescopes can now go discover lots of planets in the Kuiper belt; or "plutons" as we call them, to the everlasting irritation of geologists who had a perfectly good use for the word
Of course plutino was already taken... I recommend we go with Plution instead.
Good news is Astronomers can now do lots of press releases announcing their discovery of new planets (yawn), the Bad news is that they can no longer give them funky names when they feel like it like they were some common or garden asteroid. No, the subcommittee on Naming Names will be busy. Very busy. And Very Important. For a Long Time.
Which is the purpose of all ad hoc sub-subcommittees, ultimately, to validate the continued existence and importance of their parent committee, of course.

Sadly, the orthodoxy that Free-Floaters Must Not Be Planets is maintained. Poo.

However, the real sneaky business is this "...is in orbit around a star": the words "main-sequence" are, thankfully, missing from this sentence.
(see here and here - a powerful faction in the "I Did Too Discover Planets" wars wants to have planets be planets only if they orbit main-sequence stars, not ex-stars, or proto-stars.
Under such a definiton, the Earth would abruptly cease being a planet as and when the Sun stopped thermonuclear fusion and became a white dwarf (apparently shell-fusion and post-main sequence RGB and AGB stars are ok for planets. But No Further. Pah.).

Text of the Resolution

Contemporary observations are changing our understanding of the Solar System, 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 force us to create a new definition, which we can make using currently available scientific information. (Here we are not concerned with the upper boundary between "planet" and "star".)

The IAU therefore resolves that planets and other Solar System bodies be defined in the following way:

(1) A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape1, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.2

(2) We distinguish between the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane, and other planetary objects in orbit around the Sun. All of these other objects are smaller than Mercury. We recognize that Ceres is a planet by the above scientific definition. For historical reasons, one may choose to distinguish Ceres from the classical planets by referring to it as a "dwarf planet."3

(3) We recognize Pluto to be a planet by the above scientific definition, as are one or more recently discovered large Trans-Neptunian Objects. In contrast to the classical planets, these objects typically have highly inclined orbits with large eccentricities and orbital periods in excess of 200 years. We designate this category of planetary objects, of which Pluto is the prototype, as a new class that we call "plutons".

(4) All non-planet objects orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".4

1 This generally applies to objects with mass above 5 x 1020 kg and diameter greater than 800 km. An IAU process will be established to evaluate planet candidates near this boundary.

2 For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.

3 If Pallas, Vesta, and/or Hygeia are found to be in hydrostatic equilibrium, they are also planets, and may be referred to as "dwarf planets".

4 This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets. In the new nomenclature the concept "minor planet" is not used.

the ad hoc sub-subcommittee... Andre, Iwan, Junichi, Richard, Catherine, Dava and Owen

Possible future planets - Ixion should be easy, but remember 2002 AW₁₉₇ kids! It will be on the test.