August 24th, 2006


The IAU Resolution on Pluto

As many of you know, I've been following the International Astronomical Union's debate over the definition of the word "planet" with great interest. It is no secret that I'd like to see a definition that keeps Pluto as a planet.

Well, as of this morning, the IAU has posted the final resolution on the definition of the word "planet," which they will be voting on today. For convenience, I am quoting the entire resolution (technically four different resolutions) here:

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So if the IAU passes these four resolutions, what does that mean? Well, it means that we would now have two different categories of planet, "classical planet" and "dwarf planet." The classical planets would be Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto would be considered a dwarf planet and the prototype of a new category of "plutonian objects." Nothing else would be called a planet. It would even seem likely that 2003 UB313 (popularly known as Xena) would not be a planet.

While the resolutions do seem well thought out, it does leave us with an interesting question: Is Pluto a planet? Technically, it is, since it now is considered a new type of planet, a "dwarf planet." But does that mean that we'll refer to an eight-planet solar system once more, or will we still consider our solar system to have nine?

Here's my prediction. How we view the solar system depends on what we learn in school, and Pluto's reduced status as a dwarf planet does not demote it enough to eliminate it from the list entirely. Although textbooks will have to be revised, to make it clear that Pluto is in a new category of planet, I suspect that pictures, tables, and charts of the solar system will continue to show all nine of the planets that we have come to know and love since the 1930s.

IAU Resolution 5B Fails

Resolution 5B, which would have created the term "classical planet" to describe the big eight, just failed after a standing vote. What this means is that the solar system now officially has eight "planets" and (should resolution 6A pass) one "dwarf planet," which is Pluto. In essence, Pluto has lost the right to be called a planet along with the others. From now on, if resolution 6A passes, it will have to be known as a dwarf planet.

A bittersweet result for us Pluto supporters.

Pluto's Demotion: My Initial Response

A short time ago, the IAU passed resolutions 5A and 6A, defining what is and isn't a planet. For those who wish to read them, here they are, reposted from the IAU's website:

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So what does this mean for Pluto?

Well, had Resolution 5B passed, we would have had the overall classification of a "planet" that would have had two sub-classifications: "classical planet" and "dwarf planet." By those classifications, we could have used the sentence "Pluto is a planet" with complete scientific accuracy.

But the IAU rejected Resolution 5B. What this means for our solar system is that there are now two separate categories: "planet" and "dwarf planet." Pluto is now a dwarf planet; and it would be technically incorrect to refer to it as a "planet" without the preceding adjective of "dwarf."

I have to say that I am disappointed with this news. I grew up with a nine-planet solar system, and as I was growing up I marveled at the Voyager spacecraft missions that brought us new knowledge and those spectacular pictures of the planets. I was disappointed that we didn't know what Pluto looked like, and I went on record for many years as hoping for a mission to Pluto. I was delighted when the New Horizons spacecraft launched back in January, as we would finally get to see pictures of the planet Pluto.

But by relegating Pluto to "dwarf planet" status, the IAU has changed the emotional impact that such a mission can have.

I believe the IAU's vote could conceivably stifle the imagination of those of us who still wonder at the glories of our solar system, and who reach for the stars. And I hope that in 2009, when the IAU meets again in Rio de Janeiro, Brazil, they will reconsider the vote that they have taken today in Prague.