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One dark night 20 years ago, a small boy we knew was taken outside to view Halley’s Comet as it flashed fuzzily by on its once-in-a-lifetime visit to the inner solar system. He gazed skyward through his grandfather’s binoculars for a long time, then lowered them solemnly and pronounced in tones of awe: “I didn’t see any Halley’s, but I saw Pluto.”

Ah, Pluto. Whether he saw it or not — and it is safe to say not — the then-ninth planet from the sun was the one celestial body that gripped his 4-year-old imagination. And why shouldn’t it? Pluto was far out. Literally. So far out, in fact, that it put a whole new spin on the newly chic word edgy. Small, cold, dark and lonely, graced with a funny name, it radiated an appeal that none of its more robust fellow planets did. It was the shy little kid of the planet family.

But it also had the status of being the solar system’s official unmanned checkpoint. Inside Pluto’s orbit, you were mentally at home in the universe. Here, you knew what was what and pretty much where. Pass beyond it, and, well, who knew what black hole or red giant might grab you? Pluto marked an existential boundary.

Readers of Russell Hoban’s minor classic of 1967, “The Mouse and His Child,” will recall the mouse’s fascination with the idea of “the last visible dog.” Popping up all through the book is a can of Bonzo dog food that features a picture of a dog displaying a can of Bonzo dog food, featuring the same picture of a smaller dog displaying a smaller can and so on and on, receding to an implied and, by definition, terrifying infinity.

Pluto, named for the Roman god of the underworld on its discovery in 1930, was our solar system’s last visible dog, the guardian at the gate of the void. Except that, unfortunately, it wasn’t, as the International Astronomical Union recognized recently when it voted after a week of acrimonious debate to eject little Pluto from the classical planet family and put it at the head of its own, clearly inferior family of so-called dwarf planets. Turns out that far from being the last visible dog, Pluto is one of a swarming pack. Astronomically speaking, when it comes to “trans-Neptunian objects,” it’s a puppy farm out there.

The debate — or at least the acrimony — was never really about the science of the decision. Even the discoverer of the object nicknamed Xena (who stood to benefit had the gathering voted to keep Pluto as a planet and add Xena and a couple of other bodies to the list) didn’t object to the outcome. “Somehow the right answer was stumbled on,” professor Michael E. Brown said. “Science is self-correcting eventually, even when strong emotions are involved.”

The emotions were more like the emotions of 4-year-olds — and they were indeed strong. The public’s expressions of interest and dismay last week were surprising, until one stopped to consider how much we human beings have invested in things just staying the same. Or at least some things, especially those we got to know in childhood. Lopping off Pluto, the satisfying last term in so many mnemonic strings, feels a bit like lopping December off the roll-call of months. Or banishing Saturday from the week. Or losing a first tooth. The tongue trips over that bloody new space in the known world.

Luckily, it’s only those of us who have spent time with Pluto who will miss it. Next year’s kindergartners won’t give a hoot about the recent decision. It’s a tiny bit like what happened when countries adopted the metric system (are you listening, America?) or switched their currencies to the euro (hello, Britain!). People swear they’d rather keep things the old way, and then they see the new way has its merits, too.

Apparently this happens all the time in science, the prime battlefield in that never-ending clash between humanity’s impulse to order what it knows of the world and the messy world’s resistance to being ordered. Just ask botanists, who can hardly keep up with the changing nomenclature — or technically, the taxonomy — of plants, with the “splitters” reportedly gaining ascendancy after several decades in the shadow of the “lumpers.” It is actually comforting to know that a similar impulse is all that’s behind Pluto’s demotion — scientists responding as they learn more about the solar system’s complexities. It’s a good thing they do that, too — otherwise we’d still be stuck with just the five planets the ancients could see with the naked eye.

Meanwhile, Pluto sails on as cold and remote as ever, oblivious to our agitation. It hasn’t changed. We have. Or our mental universe has. But that’s OK. Which would you rather be: the rock, or the excitable creature that couldn’t decide what to call the rock?

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