Classifying Pluto

(Bill over at The Evil Eyebrow had invited me to participate in his celebration of the second anniversary of Pluto’s demotion from “planet” to “dwarf planet” status. Unfortunately, despite my fascination with time, I’m unable to actually hit a deadline. It would be a shame to waste the two pages of actual notes I wrote out longhand, so I’m submitting an entry anyway.)

Two years ago, the International Astronomical Union (IAU) made headlines when its constituent astronomers redefined the word “planet” to no longer include Pluto. The decision was described by the mainstream media as if a murder had occurred. Elementary school teachers cursed that the tiny bit of science they’d been required to commit to their brains actually changed. I even have a shirt commemorating the occasion. For $14.99 and a trip to thinkgeek.com, you can too.

At the center of the issue is one of the core ideas of science — classification. You could even make the argument that science itself is classification. In order to figure out how some facet of nature works, we must compare the objects and actions that compose it, and compare those properties with the properties of other phenomena we’ve observed in the past.

This idea seems horribly anal-retentive. But these simple classification schemes have led to great advances in all the sciences. In biology, Linnaeus’ taxonomy of species led to our current system of binomial nomenclature. (That is, the system by which living things are assigned two Latin names, like Homo sapiens or Felis domesticus. This does not include the ones describing Wile E. Coyote and the Roadrunner in cartoons.)

Dmitri Mendeleev, a Russian chemist, posited the idea that elements could be grouped in order of their mass, with elements with similar properties arranged vertically. His “periodic table” is the single most important tool that led to modern chemistry.

Classification is even ingrained in the human brain. A toddler at the stage of emergent language will frequently name unknown objects the same as a similar-looking known object. For instance, three fruits that are roughly the same shape but different colors might all be called “lemons” by a young child, even though adults would call them “orange”, “lemon”, and “lime”. Child development theorist Jean Piaget even described the hallmarks of his stages of learning in terms of classification. According to Piaget, students entering the concrete operational stage of development develop significantly-increased classification skills. It’s easy to see how this has been marketed. Toys aimed for 7-9 year olds include Pokémon and Beanie Babies (a few years ago), two collections of “creatures” that deserve their own periodic tables.

At the root of the Pluto debate is classification, and that classifications are not static. Where an object or phenomenon sits in its hierarchy must change as new information is gained.

When Clyde Tombaugh discovered the planet Pluto in 1930, he knew roughly where it needed to be in the sky to cause the “wobble” that had been observed in Neptune’s orbit. Tombaugh found Pluto by comparing photographic plates of that small corner of the sky, taken several days apart.

Tombaugh saw virtually nothing of Pluto. He saw a dot. The dot happened to be moving faster than the background stars — proof positive that the object was in our solar system.

Tombaugh’s best guess is that Pluto was a planet. It was too large to be a comet, and the only asteroids that had been observed were located between Mars and Jupiter. Pluto was a planet, the ninth from the sun.

As time progressed, telescopes and imaging improved. It wasn’t even until the mid-1970s that we learned that Pluto even had a moon, Charon. And, even more surprisingly, we learned in 2003 (yes, only five years ago!) that Pluto actually has three moons, the latter two named Nix and Hydra. Improvements in visual resolution enabled us to barely resolve the tiny moons from Pluto’s own shape.

But the ability of astronomers to resolve smaller and smaller objects led to some unexpected discoveries. More objects, that incidentally looked a heck of a lot like Pluto, were discovered. There was a tenth planet. And an eleventh. And a twelfth. Pluto shared more in common with these newly-discovered objects (called Kuiper-belt objects, or KBOs) than it did with nearby Uranus or Neptune. We now know of more than 70,000 Kuiper belt objects.

My very enterprising mother just shut up because she’s in a quandary. How many planets do you want? Eight? Or 70,008 (and increasing daily)?

Back to our old friend classification. We now can see more closely than we ever have. Pluto does not at all look like the other planets, and it has a whole bunch of friends with whom it shares many properties. The only sensible thing to do is to change our classification based on the new information.

Despite the public outrage, you’d think this was even an unprecedented step. Actually, the asteroid Ceres had been downgraded from planet status to a mere “asteroid” in the mid-nineteenth century, for a similar argument.

Classification allows us to group objects based on their properties. Classifying it in the wrong place just doesn’t make sense, even if for the most sentimental reasons. Ultimately, whether you choose to call Pluto a planet or a dwarf planet is a matter of semantics. It’s still the same rock, floating billions of miles away in an orbit that takes almost a quarter of a millennium to complete.

To me it’s still the same old Pluto, a frozen world waiting to be explored.

3 Comments

• 

  By Laurel Kornfeld, August 25, 2008 @ 11:35 pm

  Pluto, Eris, MakeMake, and Ceres actually look and are a lot more like the “classical planets” than like the asteroids or most Kuiper Belt Objects (KBOs). Asteroids and the majority of KBOs are shapeless, inert, tiny objects while Pluto, Eris, Ceres, and Makemake are in a state of hydrostatic equilibrium, meaning they have enough gravity to have pulled themselves into a round shape. When objects reach the mass that enables hydrostatic equilibrium, geophysical functions such as plate tectonics and volcanism, pheonomena seen on planets, begin. This is why we cannot simply lump these objects with the other KBOs. Placing objects in hydrostatic equilibrium in the same category as those that are not amounts to classifying them in the wrong place. Most of the 70,000+ KBOs are not round, so including the ones that are will not lead to our solar system having 70,008+ planets.

  The discovery of other objects similar to Pluto does not mean Pluto is not a planet; it simply means there is yet a third class of planets–the ice dwarfs–in addition to the two classes we now recognize, the terrestrial planets and the gas giants.

  It’s interesting that you mention the demotion of Ceres because it turns out that that demotion was wrong, specifically because Ceres is round. Of course, 19th century astronomers had no way of knowing this, no way of telling that Ceres was different from the other objects between Mars and Jupiter. It was not until 1995, when Ceres was imaged by Hubble, that it was recognized as being in a state of hydrostatic equilibrium. This means its initial demotion was wrong, and that based on the new information, it should be promoted back to the status of planet.

  A major problem with the IAU definition is that it states dwarf planets are not planets at all. This makes absolutely no linguistic sense. It’s like saying a grizzly bear is not a bear. The problem could easily be remedied by rescinding this portion of the definition and simply reclassifying dwarf planets as a subcategory of planets. That would take into account their dynamical differences from the classical planets while at the same time recognizing they are more akin in their geological makeup to the classical planets than to the asteroids and shapeless KBOs.

  Semantics do matter because definitions inform how we understand and perceive the world around us. Many people’s objections to the IAU decision stem not from sentiment but from the fact that the IAU (actually only four percent, in a very controversial process) created a sloppy, vague, inaccurate classification that fails to take into account the diversity of objects that still have enough features in common to fall under the broad category of “planet.” As Dr. Mark Sykes said at the Great Planet Debate in Laurel, MD, earlier this month, “that’s why God invented subcategories.”

Other Links to this Post

1. The Evil Eyebrow » Pluto is Two Years Dead — August 26, 2008 @ 2:29 pm

2. Sorry, Laurel — September 8, 2008 @ 7:25 am

RSS feed for comments on this post. TrackBack URI

Leave a comment