"This would be like if botanists had found something between trees and bushes and invented the word 'animal' to describe it."
is what Allen Glazner, a geologist at UNC Chapel Hill, has to say about the new IAU proposal on the definition of planets.
As is well known, the IAU is set to vote on a proposal in the next few days regarding the definition of the word "planet". The discovery of several trans-Plutonic objects that are of comparable size, or even larger, than the 9th planet of the solar system brought to surface the fact that there exists no consensus on what a planet exactly is. This lack of a definition cannot really be blamed on the astronomers: they just never have had a need for a precise definition until now. For nearly a century, the common nomenclature of applying the word "planet" to the set of nine large (comparatively speaking) celestial objects whose primary orbit is the sun is quite enough. No large objects were expected to exist in the Kuiper belt (except for Pluto), and no one really needed a lower bound in size for the term planet when discussing objects that orbit around distant stars, because due to the technology involved, we can only extrasolar planets when they are about the size of Jupiter or larger. In short, whether inside or outside of the solar system, there has never been a need to consider how to draw the line between a planet and an asteroid.
The discovery of Xena changed all that. 2003 UB313 was announded by Michael Brown et al. of Caltech in January of 2005, and an initial estimate was that it could be larger than Pluto. Subsequent observations reduced the size of the object, putting it at just 4% larger than Pluto. Furthermore, an object was shown to be in orbit about Xena (the moon is nicknamed Gabrielle).
What's more, the object was announced at the same time as two other large trans-Neptunian objects, 2005 FY9 and 2003 EL61, both of which barely smaller than Pluto. Together with the previously known Kuiper belt objects Sedna and Quaoar, both of which larger than the large moon, Charon, of Pluto, we have ourselves a neat set of large KBOs.
This caused a problem: what is a planet? If we go purely by equatorial diameter, then Xena has to be included, since it is larger than Pluto, and there should be no conceivable reason why the current size of Pluto would be a lower cut-off line for the planet definition. If we leave out Pluto by size, many people would be upset at the demotion.
The International Astronomical Union thus set up a committee to come up with a recommendation as to what constitutes a planet. Simplistically speaking, their proposal says that a planet is something that is roughly spherical because of gravitational self-interactions and that it must primarily orbit a star. Under this definition, there won't be a strict size bottom limit, since denser objects can obtain the spherical shape with a smaller diameter, but it would successfully rule out most asteroids (clumps of dirt held together by electromagnetic [intramolecular] forces) and comets (clumps of ice and dirt held together by ditto). It would, however, give planetary designation to Ceres, the largest object in the asteroid belt, which is around 950 kilometers across and contains 1/3 of the total mass of the asteroid belt. (The story of Ceres is a fun one: it was first observed on January 1, 1801 by Giuseppe Piazzi. Piazzi fell ill in February and paused his observations. Soon the object moved behind the sun [from the vantage point of the earth] and it was doubtful whether men could find it again [it was the early 19th century, mathematics and astrophysics weren't as developed back then]. Carl Friedrich Gauss was enlisted to help, and he, at the tender age of 24, developed a method of orbit determination that requires only three data points. He finished his calculations in a few weeks and sent his predictions to some prominent astronomers of the time. By December 31 of that year, the asteroid was "recovered". The saga doesn't end here, however. Ceres was originally considered as a planet [the missing one predicted by Johann Daniel Titius]. It wasn't until the discovery of additional asteroids fifty years later [well, Pallas, Juno, and Vesta were discovered soon after, but they were thought to be it] that Ceres was demoted, and the name asteroid was coined.)
On the other hand, another sure candidate for planethood would be, strangely enough, Pluto's moon. While Charon does indeed "orbit" Pluto, it fails the criterion because the center of mass of the system sits outside of Pluto (Charon is rather big). Under the new rules, Pluto and Charon would be considered a binary planet system. It is interesting to note that the Earth and the Moon might suffer the same fate in a couple billion years. Our Moon is an exceptionally large satellite (only Charon has a higher ratio against its host's mass), and is known to be slowly drifting away from the Earth (a phenomenon that is possibly explained by the theory that the Moon is formed from an impact that sent a gigantic chunk of the the Earth into orbit). Even though the barycenter of the system is right now far below the surface of the Earth, a day would arrive (if both the Earth and the Moon survive that long; several Ga in fact [Ga means Giga-annum, the fancy way for scientists to say a billion years, partly because the Brits and the Americans disagree on how big a billion actually is]) when the barycenter moves above the surface of the Earth. In that case, Earth and Moon would be considered as a binary planet system. (There are some other arguments as to why they should be considered a double-planet.)
Anyway, back to the subject at hand: why are the geologists pissed?
Well, part of the IAU proposal actually considers giving a designation to the large Kuiper Belt Objects that have planetary status to distinguish them from other planets. And unfortunately, they chose the word "Pluton".
For geologists (or even anyone who has an amateur interest in such), pluton is a fairly common word. From WordNet:
pluton n : large mass of intrusive igneous rock believed to have solidified deep within the earth [syn: {batholith}, {batholite}, {plutonic rock}]It is in most common dictionaries, and is included in the OED. (Incidentally, the name pluton is indeed derived from Pluto ... the Roman god of the underworld, as that's where those rocks are formed.) So on the 18th of August, Allen Glazner (remember Allen? This story is about Allen) sent a letter to the IAU "accusing them of 'hijacking our perfectly good [definition] and causing endless confusion."
And confusion it will cause. Astronomy (especially that of the planetary variety) and geology are closely related (it is quite different if the two disciplines involved are as disjoint as, say, computer science and barrista science *cough*java*cough*): MIT, a school known for its science departments, even grouped both under course 12! Should this proposal go through, one might very well imagine reading a paper on "pluton formation in plutons" and be very confused.
To add insult to injury, the committee members responsible for the draft not only admitted that they knew of such a term in geology, they used it anyway because they didn't think it was important enough. And their judgement of importance? Not by asking any geologist friend of theirs nor looking it up in a dictionary. No, they chose to go by the MS Word Litmus test:
"Since the term is not in the MS Word or the WordPerfect spell checkers, we thought it was not that common," [Owen] Gingerich wrote in an e-mail to news@nature.com.(Gingerich is the chair of that IAU committee and also astronomer at Harvard.)
Now you ask, why did the committee chose such a name? One possible explanation is that some of the other good names are already taken. Planetoid is out as it is used to denote the large class of small things that is between a meteroid and a planet (though the definition of the latter is imprecise at the moment, the former has a upper limit of about 10 meters across). Asteroids are a subclass of planetoids. Another good term might have been plutino, but that has also been taken. Plutinos are trans-Neptunian objects that has a 2:3 orbital resonance with Neptune. Basically, they are objects that, like Pluto, are dragged around by Neptune. (In particular, Pluto is a Plutino, which, in my opinion, is pretty funny nomenclature.) (There's also a name for TNOs that are not in resonance with Neptue; they are called Cubewanos. The funny name comes from the first such object, 1992 QB1.)
As a mathematician, I think I'll go grab another cup of tea, sit on the sideline, and jeer.