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The Planetary Society Blog
By Emily Lakdawalla
What's in a Name?
Aug. 21, 2006 | 10:24 PDT | 17:24 UTC
by Jim Bell
All last week I was agonizing about what I could "blog" about while covering for Emily Lakdawalla this week. It's an awesome responsibility, but I was nervous because I've never blogged before and I didn't want to let Emily down (hell hath no fury like a mother scorned). Then, like manna from heaven, the International Astronomical Union rained bloggable bits all over me.
Unless you've been living on some other celestial orb recently, you've no doubt heard all the ballyhoo about the IAU's efforts to redefine the term "planet." The proposal, worked on for years by a committee of astronomers, scholars, and artists, came out in newspapers and websites (and even blogs!) all over the world this past week and made a big splash because, if it is approved, we will suddenly live in a solar system with 12 "planets" (and perhaps more), instead of our familiar 9.
The Greek's came up with the term planetes or "wanderer" for the five objects besides the Sun and Moon that they could see move through the constellations with their naked eyes. When Uranus and Neptune were discovered later by telescopes and found to be much, much larger than Earth, calling those two "planets" made a lot of sense. Then Ceres and Vesta and a host of other objects were found between Mars and Jupiter. Small, starlike... "asteroid" seemed to fit, though formally the IAU and others adopted the term "minor planets."
OK, planets big, asteroids small. No problem. Then, of course, Clyde Tombaugh found Pluto orbiting way way out there past Neptune and the debate became sticky. People began to ask, is Pluto, smallish and in an oddly-tilted orbit different from anything else then know, a planet? Now we know that Pluto is just the easiest-detectable representative of a large population of similar distant, icy objects, variously known as Kuiper belt objects or trans-Neptunian objects. I think that the long hiatus between the discovery of Pluto in 1930 and the discoveries in the 1990s of Pluto's numerous similar cousins provided the opportunity for the public to get comfortable referring to Pluto as a "planet". It was just one more, after all -- an odd duck, but part of the family.
Reactions to the IAU's proposal to formally, quantitatively define the term "planet" are all over the map. Some scientists think it makes sense, others think it's crazy. The media seem mostly amused by the idea. Many people in the general public that I've talked with about this are just plain confused, however. When I've brought up the topic on airplanes, in cafeteria lines, or around the breakfast table, I hear the same kinds of questions: "Why do we need more planets?" "Don't astronomers have better things to do?" "How can Pluto's moon be a planet?"
My own reaction is one of mild frustration, because parts of the definition just don't seem to make sense for our solar system. For example, if the IAU's definition is adopted in voting this week, the (former) asteroid Ceres and (former) moon Charon will both become planets. Both are smaller than, for example, Jupiter's moon Ganymede and Saturn's moon Titan. How can little Ceres be called a majestic planet but already-majestic Titan not be?
Parts of the proposed definition -- like requiring the object to be large enough to have become spherical and to not be a star -- make some scientific sense and are, for the most part, quantifiable. But the part of the definition requiring that the object not be a satellite of a planet seems arbitrary to me. Why not? Would a rogue planet that gets captured by another planet suddenly cease to be a planet? And why Pluto's moon Charon doesn't qualify as a satellite of a planet but instead would be part of a "double planet" system may make sense to orbital dynamicists comfortable with terms like "barycenter", but it's really, really hard to explain to regular people, especially kids.
To be fair to the IAU, coming up with a definition of an ambiguous term like "planet" is really a no-win situation. Is it a crocodile, or an alligator? A donkey or a mule? Planet-hunter Dave Jewitt has compared it to telling the difference between a ship and a boat. None of those words matter if you're bitten, kicked, or sinking fast, however. It's just a word, and words can be fuzzy. Why try to rein it in? I'm surprised the IAU decided to take this debate on, and I'm even more surprised that a committee of scientists and others could come to an agreement about this. Perhaps the old adage about any press being good press was on peoples' minds. If so, it was a good call -- there certainly has been a lot of press!
Ultimately, some people are lumpers, and some are dividers. When it comes to planets, I'm a lumper, perhaps to the extreme. I like our nine common planets, and this view of our solar system family has become ingrained in our culture. But I also think of our Moon as a planet in its own right, with a fascinating geologic and internal history. I lump Jupiter's giant moons into the planet category, and the big ones around Saturn, Uranus, and Neptune, too, as they have been shaped by their own internal forces and either evolved or continue to evolve since the time they were born. By my own personal (perhaps naive and nonscientific) reckoning, we live in a solar system with 25 or so planets, some big, some small, some gassy, some rocky, some orbiting others, and some on their own. Even with them and millions of asteroids and perhaps billions of comets there's still plenty of room for more planets. Bring 'em on! "Planet" is in the eye of the beholder.
For more information about this new "planet" definition, Amir Alexander has a story up on The Planetary Society's site and Wikipedia has great summary of this topic.
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