We are in the year of the dwarf planet. NASA’s Dawn space probe has just taken up a yearlong orbital residence at Ceres, the largest object in the asteroid belt, and this summer the agency’s New Horizons mission will fly past Pluto, one of the largest objects in the Kuiper Belt, a sparse disc of icy chunks out beyond Neptune. We have never seen a dwarf planet up close before; their nature and origin are still poorly understood. Nobody knows, for instance, what the two closely spaced bright spots on Ceres, which resemble a pair of headlights, could be. And nobody knows much at all about Pluto. “It could have ice geysers or ice volcanoes,” Alan Stern, the lead scientist for New Horizons, told me recently. “It could have a subterranean ocean. It’s anybody’s guess at this point.”

Whatever the probes find, it probably won’t help untangle the tortuous reasoning that led to Pluto and Ceres being labelled as dwarf planets in the first place. That happened in 2006, a few months after New Horizons launched and about a year before Dawn did, at a meeting of the International Astronomical Union, the organization that is in charge of classifying and naming celestial objects. The I.A.U. defines a dwarf planet according to four criteria: it must orbit the sun, it must be spherical, it must not be a satellite of another planet, and it must not have “cleared the neighborhood” of other objects of comparable size.

Ceres has a diameter of fewer than six hundred miles, Pluto of about fourteen hundred miles. By comparison, Mercury, now the smallest official planet in our solar system, is more than three thousand miles across. So it’s not unreasonable, Stern says, to call Pluto both a planet and a dwarf, provided that one doesn’t cancel out the other. “I’m the one who originally coined the term ‘dwarf planet,’ back in the nineteen-nineties,” he told me. “I’m fine with it. But saying a dwarf planet isn’t a planet is like saying a pygmy hippopotamus isn’t a hippopotamus. It’s scientifically indefensible.”

Why, then, did the I.A.U. demote Pluto? As David Spergel, the head of the astrophysics department at Princeton University, explained to me, once scientists discovered the Kuiper Belt, which includes several Pluto look-alikes, and once they discovered Eris, a dead ringer for Pluto, the organization became worried about a slippery slope. If Pluto was a planet, Eris would have to be, too, along with any number of Kuiper Belt objects. Things risked getting out of hand. Fifteen or twenty or fifty planets was too many—who would be able to remember them all? That last question may sound absurd, but in a debate held last year at the Harvard-Smithsonian Center for Astrophysics, Gareth Williams, the astronomer representing the I.A.U.’s position, couldn’t come up with a better argument. “You’d need a mnemonic to remember the mnemonic,” he said. “We really want to keep the number of planets low.” He lost the debate on the merits, but the demotion had already been won.

The degradation of Pluto distressed scientists and non-scientists alike. Neil deGrasse Tyson’s book on the episode, “The Pluto Files: The Rise and Fall of America’s Favorite Planet,” sold briskly (although so did “How I Killed Pluto and Why It Had It Coming,” by Mike Brown, the Caltech astronomer who first spotted Eris). There wasn’t any controversy, however, over the I.A.U.’s classifying Ceres as a dwarf planet. Though considered a full-fledged planet when it was discovered, in 1801, it was soon grouped with the asteroids. The 2006 decision, then, was effectively a promotion.

It makes no practical difference, of course, what we call Pluto and Ceres, just as it makes no practical difference whether Australia is the biggest island or the smallest continent. Both are believed to be leftovers of a population of thousands of smallish, roundish objects that mostly collected together to form larger ones—Mars, Jupiter, Earth, and the rest. Examining them up close and in detail will tell planetary scientists about what the solar system looked like more than four billion years ago. It’s as though a couple of Neanderthals had survived to the present, long after their fellows had been absorbed into our own population. We might get into an argument about precisely who and what they were, but we wouldn’t forget what they could teach us about ourselves.