It isn't completely certain that Mickey Mouse's dog Pluto was named after the planet. As it happens, Pluto was a fairly common name for dogs and horses in the early part of the 20th century, so the choice of the name for the dog wouldn't have seemed odd in any event. But Disney's bloodhound first appeared under the name Pluto in the cartoon The Moose Hunt in 1931, just a year after the announcement that Clyde Tombaugh of the Lowell Observatory in Arizona had discovered a new planet beyond the orbit of Neptune. And it's hard to believe that Disney wouldn't have tried to take advantage of the Plutomania that was sweeping the nation.
After the announcement, thousands of people wrote in to suggest names for the planet. The popular story has it that the name Pluto was submitted by an 11-year-old British schoolgirl, though the credit for the name most likely belongs to the Italian astronomers who corroborated the discovery photographically.[1]
Meanwhile, Tombaugh himself was lionized as an American hero on the model of Charles Lindbergh -- a 22-year-old aw-shucks Kansas farm boy and self-taught astronomer who had beat the best scientists of the rest of the world in the search for the elusive "Planet X" that was thought to be perturbing the orbit of Neptune. And the public's imagination quickened ever more when it was announced, wrongly as it happens, that the new planet was the earth's near twin in mass -- as the press put it, it was the only other planet a human being could go without changing his weight.
This had all happened before. William Herschel's discovery of Uranus in 1781 so excited the world that from that moment on the detection of a new planet epitomized the ecstasy of scientific discovery -- the feeling that Keats famously captured in the lines from "On First Looking Into Chapman's Homer": "Then felt I like some watcher of the skies/When a new planet swims into his ken." And the discovery of Neptune in 1846 sparked another popular sensation, along with a spirited controversy when the French and English both claimed credit for the first sighting.
But it's not likely any of that will ever happen again. The restrictive definition of a planet that was adopted last Thursday at the meeting of the International Astronomical Union in Prague didn't simply demote Pluto from a planet to what is now called a "dwarf planet." As several astronomers pointed out, it also made it unlikely that any new objects will be found in the Solar System that can qualify for the planet label.[2]
The new definition was chosen over one that would have kept Pluto a planet but would also admit several other bodies to planethood, almost certainly with more in the offing. From here on in, the number of the planets is necessarily eight -- what the astronomers now refer to as the "classical planets," which is just another way of saying a planet that was known in the day of Jules Verne.[3]
Actually, both the winning and losing definitions of a planet had a legalistic feel to them. The astronomers were clearly less interested in carving nature at the joints than in finding plausible-sounding criteria that would give due honor to the cultural significance that people have always accorded to the notion of a planet. The astronomers implicitly conceded that the winning definition was ad hoc when they stipulated that it would only apply to objects in our own solar system, which is like hearing biologists propose a definition of mammals that only holds for North America.
It's a good example of what legal theorists disparage as result-oriented jurisprudence. You couldn't help being reminded of the Supreme Court's decision in Bush v. Gore, another result-oriented decision that was "limited to the present circumstances," as the Court put it, and which couldn't be cited as precedent in other cases.
The interesting question was why it was necessary for the astronomers to go through the whole business in the first place, and why anybody else should care.
After all, language routinely recognizes natural categories that have no good scientific basis. There's no geological reason why we should consider Europe a separate continent from Asia, and no botanical reason why we should refer to tomatoes as vegetables rather than as fruits. There's no way to define the lily that doesn't include a lot of tulips as well.[4] And other words like shrub and weed don't have any kind of scientific definition at all. So why can't we just keep using planet however we damn well please?
But then the meaning of planet has always depended on what science tells us. You can be interested in lilies or tomatoes without caring much about botany. But being interested in planets pretty much guarantees you're a astronomy buff. Since Herschel's day, the idea of other planets has given schoolchildren their first taste of the romance of science. And it was no doubt in the hope of preserving that romance that the astronomers decided to adopt a definition that closed off the club and expelled Pluto, rather than using one that included Pluto but would also require them to acknowledge a new member every time some astronomer discovered a biggish ice ball in the far reaches of the Kuiper belt.
The irony is that nobody will ever again thrill to the news of a planetary discovery in our solar system. Astronomers will continue to locate curious objects orbiting the sun beyond Neptune, and the discoveries will be given respectful mention on the science pages. But none of those objects is going to have a cartoon character named after it.
1. For more on this, see my post at LanguageLog. Return
2. Some scientists disagreed, however, saying that the region between10 and 100 billion km from the sun could contain planet-sized objects that have not yet been detected. Return
3. Well, okay -- I don't mean "necessarily" in that particular way. But there was an air of inevitability if not logical necessity about the definition the astronomers adopted, in the sense that it had less to do with specifying necessary and sufficient conditions for planethood than in finding some ad hoc set of properties that would admit those very objects and no others to planetary standing. Return
4. This example is drawn from John Dupre's "Natural kinds and biological taxa," Philosophical Review, 90 (1981), which gives a number of other examples of this type. Return
