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Note to the Moon: You May Already be Pre-Approved!!!

In the wake of the recent series of discoveries of previously unknown trans-Neptunian objects of similar size as (and in some cases perhaps even larger than) the granddaddy of the Kuiper Belt Objects, Pluto, beginning in late August of 2006, the International Astronomical Union's Working Group on Planetary System Nomenclature and Committee on Small Bodies Nomenclature held a series of meetings in order to attempt to clarify the scientific definition of what constitutes a "planet."

From a nice article about the Pluto-as-a-planet wrangle by LanguageLog's Geoff Pullum, "Gay marriage and counting the planets:":

Next week astronomers will vote on a newly proposed definition (not redefinition — we've never really had a definition before). There are various positions out there, including a restrictive one that says Pluto is an eccentrically-orbited snowball from the Kuiper belt that should never have been included, so there are 8 planets, and a very inclusive one that allows at least 53 spherical sun-orbiting bodies to count as planets (an idea some astronomers refer to contemptuously as "No Snowball Left Behind"). The official proposal is to say that a planet is a body that meets the following four conditions.

1. it has sufficient mass to assume near-spherical shape because of its own gravity, and
2. it is in orbit around a star, and
3. it is not itself a star, and
4. it is not a satellite of a planet in the sense of having an orbit that goes around a center of gravity that is located inside a body that is independently a planet.

Under this definition, the number of planets will go up from 9 to at least 12, and unless a committee keeps a lid on things, the number could go way up from there to 53 or more.

The last of the four clauses quoted looks a little bit gerrymandered, doesn't it? It turns out to allow Charon to be a planet: Charon looks a lot like the largest moon of Pluto, but in fact it does not orbit around a center of gravity inside Pluto; it revolves around a center of gravity determined by both bodies and located in the space between them, so you can see the two of them as a pair of small planets orbiting the sun but at the same time slowly twirling around each other like a couple waltzing around the outer edge of a ballroom. (Our own moon's orbit is around a center of gravity located deep within the earth, so the moon definitely can't count as a planet.)

I have my own problems with the IAU proposal, some of which overlap Pullum's objections, but some quite different.

The part of the proposed definition dealing with the location of the center of mass of a planet and its moon(s), though somewhat arbitrary (like most other aspects of any such definition - how do we determine at which point a hydrogen-dominated gas giant becomes a star, for instance) would make the Pluto/Charon system a "double planet," which nomenclature is actually quite reasonable by analogy to multiple-star systems. However, the COM rule is not without its problems, which include the following:

• (1) Relative masses (in the absence of large-body collisions) are more or less fixed, whereas relative orbital distances change over time - for instance, the Moon is drifting away from the Earth at an average of around 3.8 cm (1.5 inches) per year;

• (2) Since the COM of a 2-body system depends not only on the relative masses but also on the *distance* between the bodies, a very small moon which happens to be orbiting very far from its parent would also qualify for planethood. By way of an interesting illustration of this, the COM of the Earth/Moon system is actually not as far from the Earth's surface (currently at roughly 0.8 Earth radii from Earth's center, according to Wikipedia) as the above LL post might lead one to believe, so based on the aforementioned drift rate, in around 2 billion years the moon will have increased its orbital distance by the roughly 20% needed to assume full planetary status. Someone should put together a "You may already be pre-approved as a planet!!!" mailing for our dear neighbor.

The bit about having "sufficient mass to assume near-spherical shape because of its own gravity" (part 1 of the proposal) is similarly not only arbitrary (e.g. just how much deviation from sphericity do we allow?), but a bit silly based on planetary physics. Any reasonably large assemblage of loose rubble or pebbles will get pulled into a quasi-sphere under its own gravitational self-attraction, whereas a much more massive object which happens to made of more solid stuff would not. Even a quite small liquid sphere could achieve "planethood" according to this criterion -- it need only be large enough that gravity dominates surface tension as far as the relative forces maintaining its shape are concerned - for liquid water, that occurs when the sphere's radius exceeds a few meters or so. Does anyone in their right mind really think a swimming-pool-sized blob of water deserves to be called a planet? A much better criterion here would have been one based on whether the object is large and massive enough to have a differentiated core, i.e. has sufficient gravity to pull the denser elements toward the center during its formation. That reflects not only size, but basic processes of planetary differentiation and geology.

By way of a replacement for the COM criterion I suggest a simple relative-mass criterion: any body in mutual orbit around another (or around others) which is less than (say) 5% the mass of the largest body in the lot is considered to be a moon of the larger. Thus Earth's moon remains a moon irrespective of its outward orbital drift. The Charon/Pluto system (in which Charon is slightly > 10% the mass of Pluto) would be co-planets, but flunks the orbital-eccentricity-and-inclination criterion. Ceres is geologically undifferentiated, reflecting its evolutionary history as an asteroid.

I also dislike the fact that the IAU committee proposal completely ignored fundamentals of planetary orbital mechanics (frozen slushballs tend to have highly eccentric and/or inclined orbits w.r.to the inner 8 planets) and formational geology - why use some arbitrary "quasispherical" criterion when one could look at fundamental geological properties like "is it massive enough to have a differentiated core?" (This would be a planetary analog of the stellar criterion, "Is it massive enough to initiate and sustain hydrogen fusion?") There seems to be some hope that at least the first of these will be remedied in the final proposal, but at this point I'm not holding my breath.

About the differentiatedness criterion, my friend Paul Leyland writes:

The Gallilean satellites would count as planets under this criterion.

which is an excellent point, but I had in mind that the differentiatedness criterion would only apply in an "and" sense, i.e. if the body in question doesn't flunk any of the others.

So to be clear, let me summarize "Ernst-a.k.a.-random-dilettante-off-the-street's criteria for planethood:"

• 1) It is in orbit around a star or within a gravitationally bound group of stars, and is not itself a star; and

• 2) Has an orbital eccentricity and inclination which lie within reasonable bounds (to be defined) relative to the other planets of the system and/or the rotational plane of the central star, in single-star systems; and

• 3) Is sufficiently massive to have a geologically differentiated core, except in the case of giant gas planets or ones sufficiently close to a star to maintain them in molten state; and

• 4) If it is part of a gravitationally bound group of non-stellar bodies, has a mass at least 5% that of the most massive member of the group.

My love for our picked-on ninth planet is deeply, perhaps embarrassingly, personal ... [but] ... Even I was a little abashed last week when the International Astronomical Union tried to protect Pluto’s status by proposing an absurdly broad definition of planethood that encompasses moons, asteroids and trans-Neptunian objects — in other words, pretty much any half-formed hunk of frozen crud that can pull itself together into a ball long enough to get photographed by the Hubble.

For longtime Pluto partisans, there was something almost punitive about this proposal: happy now?

I guess I always knew, in my heart, that Pluto didn’t “belong.” Pluto is idiosyncratic — neither a dull, domestic terrestrial planet nor a surly, vainglorious gas giant. It’s mostly ice. It’s smaller than our own Moon, and has an orbit so eccentric that it spends 20 years of its 248-year revolutionary period inside Neptune’s orbit. It’s tilted at a crazy 17-degree angle to the ecliptic, and its satellite, Charon, is so disproportionately large that it’s been called a double planet.

Pluto is what my old astronomy textbook rather judgmentally called a “deviant,” and I’ve always felt a little defensive on its behalf.

I’ve long regarded Saturn’s misty tantalizing moon Titan as the Homecoming Queen of the solar system, courted and fawned over, stringing us along with teasing glimpses under her atmosphere, while Pluto was more like the chubby Goth chick who wrote weird poems about dead birds and never talked to anybody.

• The August 2006 LanguageLog Archives include multiple postings about the "I say Plan-ee-to, You say Plan-ah-to" kerfuffle.