Posted Oct. 11, 2008 – Two years ago the International Astronomical Union (IAU) officially defined the word “planet” in a way that gave Pluto the boot. It was quick and seemingly final. NASA’s illustrated card containing all the planets, published for educators, was immediately reissued with Pluto airbrushed out, as if in an old Stalinist photo.
The demotion of Pluto to a “dwarf planet,” as if to join all the other Disney dwarfs, sparked considerable public controversy. Most folks even now lament Pluto’s disappearance; many astronomers have protested the IAU’s decision, as well. But many other planetary astronomers continue to support it.
I’m in the latter group. When we learned in 1978 just how tiny Pluto really is – only a bit more than half the diameter of our moon – and recall that its orbit has a wildly different appearance from all the other planets, we wondered if it truly belonged with the rest. But we stayed silent. Why disturb the long-established order? What was the harm?
But then new bodies started to be found beyond Pluto. Some virtually matched Pluto’s size, and finally astronomers found an even larger one. Suddenly we had Eris, Sedna, Quaoar, and now Makemake, and there was no end in sight. Now we had a problem. If Pluto remained a planet, and these others match or exceed Pluto’s size, and are just as round, they would have to be planets too. We’d have 13 planets, then soon 40, and eventually hundreds. Kids would no longer be able to memorize them.
It made far more sense to create new, separate categories. There’d be eight big planets, all orbiting within 7 ½ degrees of the Earth-Sun plane, with orbits that are more circular than oval. Then we’d have a separate “dwarf” class for much smaller bodies with very elliptical orbits. There’d be a third category for objects with too little weight to even be round in size, like the asteroids or the Kuiper Belt Objects (KBO’s) that probably number in the hundreds or even thousands, beyond Pluto.
Works for me. But some folks won’t give up. That’s why, in recognition of the need for further scientific debate on planet definition, more than 100 scientists and educators representing a wide range of viewpoints converged for three days at the Applied Physics Laboratory of Johns Hopkins University for “The Great Planet Debate: Science as Process” conference, sponsored by NASA, the Planetary Science Institute, The Planetary Society, and other heavy hitters.
Steve Maran, the official spokesperson, whom I know personally, wrote that “different positions were advocated, ranging from reworking the IAU definition (but yielding the same outcome of eight planets), replacing it with a geophysical-based definition (that would increase the number of planets well beyond eight), and rescinding the definition for planet altogether and focusing on defining subcategories for serving different purposes.”
It was almost a food fight. Neil Tyson of New York’s Natural History museum thought that the very word “planet” has outlived its usefulness and should be discarded, and replaced with a new term.
Renu Malhotra, a University of Arizona astronomer said, “I think the IAU made a mistake getting into the business of defining a widely used word, ‘planet,’ and sowing confusion thereby. Scientifically, the useful discussion would be about categories of planets (e.g., gaseous planets, rocky planets, dwarf planets, icy planets. . .and an individual celestial body may fall into more than one category). This approach would address the main practical problem of nomenclature without confusing the public about ‘planet’ itself.”
Still others thought the present situation is the best compromise.
The windup? The conference ended and no consensus was reached. Nothing will change. Too bad Mickey Mouse wasn’t around to sum it all up: Goodbye Pluto.