Ever since its discovery, Pluto has never really fitted in.
After the pale and glowing giant Neptune, it is little more than a cosmic dust mite, swept through the farthest reaches of the solar system on a plane wildly tilted relative to the rest of the planets. It is smaller than Neptune's largest moon, and the arc of its orbit is so oval that it occasionally crosses its massive blue neighbor's path.
For years, it has been seen as our solar system's oddest planet. Monday, however, scientists released perhaps the most convincing evidence yet that Pluto, in fact, is not a planet at all.
For the first time, astronomers have peered into a belt of rocks beyond Pluto unknown until 10 years ago and found a world that rivals Pluto in size.
The scientists posit that larger rocks must be out there, perhaps even larger than Pluto, meaning Pluto is more likely the king of this distant realm of space detritus than the tiniest of the nine planets.
When discovered in 1930, "Pluto at that point was the only thing [that far] out there, so there was nothing else to call it but a planet," says Mike Brown, an astronomer at the California Institute of Technology in Pasadena. "Now it just doesn't fit."
In one sense, the question of Pluto's planetary status is arcane, the province of pocket-protected scientists and sun-deprived pen pushers determined to decide some official designation for a ball of dust and ice 3 billion miles away.
Yet it is also unquestionably something more. From science fair dioramas to government funding, planets hold a special place in the public imagination, and how Pluto is eventually seen by kids and Congress alike could shape what future generations learn about this mysterious outpost on the edge of the solar system.
The debate has split the astronomical community for decades. Even before the distant band of rocks known as the Kuiper Belt was found, Pluto's unusual behavior made it suspicious.
Elsewhere, the solar system fit into neat families: the rocky inner planets, the asteroid belt, the huge and gaseous outer planets. Pluto, though, was peculiar.
With the discovery of the Kuiper Belt countless bits of rock and ice left unused when the wheel of the solar system first formed Pluto suddenly seemed to have cousins.
Yet, until yesterday, it held to its planetary distinction because it was far larger than anything located there.
The rub now is Quaoar (pronounced KWAH-oar), 1 billion miles beyond Pluto and roughly half as large. Named after the creation force of the tribe that originally inhabited the Los Angeles basin, Quaoar forecasts problems for the erstwhile ninth planet, says discoverer Dr. Brown: "The case is going to get a lot harder to defend the day somebody finds something larger than Pluto."
To some, the problem is not with Pluto, but the definition of "planet." In short, there is none.
To the Greeks, who coined the term, it meant "wanderer," describing the way that the planets moved across the night sky differently from the stars behind them.
Today, with our more nuanced understanding of the universe, the word no longer has much scientific meaning.
New York's Hayden Planetarium caused a commotion two years ago by supposedly "demoting" Pluto, lumping it with the Kuiper Belt objects in its huge mobile of the solar system.
In reality, however, the planetarium was making a much broader statement, says Neil Degrasse Tyson, an astrophysicist there. The textbooks of the future should focus more on families of like objects than "planets."
The discovery of Quaoar strengthens this idea: "Everyone needs to rethink the structure of our solar system," he says. "We've just stopped counting planets."
Still, many are loath to part with the planet Pluto. They note that Pluto, in fact, is distinct from many Kuiper Belt objects. It has a thin atmosphere, for one. It reflects a great deal of light, while most Kuiper Belt objects are very dark. And unlike all but a handful of known Kuiper Belt objects, it has a moon.
"Maybe Pluto, then, should be representative of a new class of planets," says Mark Sykes, an astronomer at the University of Arizona in Tucson.
"It's the first example, and we are just beginning to find this category."