Dammit, Planet

So much for the notion that complexity doesn't increase over time. As per Sky & Telescope:

This week our solar system has nine planets. Next week, if astronomers approve a new definition of the word "planet," there will be 12 — with more to come. Newcomers to the list include Ceres, the largest asteroid; Charon, Pluto's largest moon; and 2003 UB313, an icy body more than twice as far from the Sun as Pluto and a little bigger (and not yet graced with an official name).
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Here's the actual wording: "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) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

Under the new system, the eight planets from Mercury through Neptune would be reclassified as "classical planets," while objects smaller than Mercury (including Ceres--now an asteroid--Pluto, Pluto's moon Charon, and 2003 UB313) would be classified as "dwarf planets" (or, perhaps more colloquially, "the Billy Barties"). At the same time, Pluto becomes the standard for a new type of an object called a "pluton" (I'm not wild about the name--it sounds like a subatomic particle) which is defined as "a small object with an orbital period longer than 200 years and a highly elongated path tipped steeply with respect to the ecliptic."

To make it even more confusing, there is this footnote:

"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 barycenter [center of mass] resides outside the primary. Secondary objects not satisfying these criteria are 'satellites.'"

Thus, Charon's moon is a planet and the Pluto-Charon system becomes a "double-planet." In case you were wondering.

Grammar school science classes (assuming there are any anymore; who knows these days?) are going to be a tad less festive, at least for a while.

This whole thing concerns certain people, who I guess are nostalgic for the old, original astronomical ennead and think that the name UB313 lacks poetry (unlike UB40, who at least had "Red Red Wine"). This is all well and good, and no one clings to the past like a barnacle as much as I do, but science is science, and the point of science is discover new information and refine definitions and theories. This disturbs those who like the idea of things being etched and stone and thus immutable and unchanging, but, sorry, that's not what science is or should ever be.

Besides, the definition of "planet" has itself changed over the centuries, and the "pantheon" has seen some objects come and go. In fact, I was actually surprised to find out that there has thus far never been any official definition of what a "planet" is, it being one of those things you know when you see. Before the advent of the telescope, planets were nothing more than "wandering stars" that moved with respect to the fixed stars of the constellations (no one knew they were big round chunks of rock and/or gas). When the telescope was invented in the 1600s, astronomers discovered that planets were actually large, round objects orbiting the Sun and, in fact, Ceres and other asteroids were called "planets" when they were discovered in the early 1800s. (They were downgraded to "asteroids" when a distinct population of similar small objects was found.)

I remember in the 1990s there was some contention over whether Pluto should be demoted from planet status--I think the term "trans-Neptunian object" was bandied about at one point. (In 2002, I visited Lowell Observatory in Arizona, which was the institution that had discovered Pluto in 1930, and they were having none of it. Pluto was a planet. Period. I detect a faint bias on their part....) The problem was that there were all these other objects in the Solar System (like 2003 UB313 and other Kuiper Belt objects) that were bigger than Pluto. So if Pluto was a planet, why not these objects? And if not these objects, then why Pluto?

Hence, the meeting this week to try to sort out all this mess.

Funnily enough:

Michael Brown (Caltech), codiscoverer of 2003 UB313, argues similarly on his Web site, where he also worries that having potentially dozens of planets in our solar system might make sense to astronomers but will be confusing to everyone else.

Like that's ever stopped anyone before! Heck, I've got five phone books; I think I can handle a few extra planets.

I really don't think the definition is a bad one--it delinates the eight "big boys" and sub-categorizes the newer objects (and, yes, even beloved Pluto) as, essentially, "planet-like objects." The only thing I dislike is the term "pluton." But that's strictly an aesthetic point.