But five years after the ruling that demoted the icy object to dwarf planet, people continue struggling with the definition, and the debate over what exactly should be called a planet remains as contentious as any political divide.
"Maybe it's just an argument over semantics, but we ought to be worried about semantics. We learned that lesson when the definition was changed," said Marc Kuchner, a planetary scientist at NASA's Goddard Space Flight Center in Maryland.
"After the ruling, astronomers everywhere were besieged by complaints from everyone big and small. A planet is a very personal thing—we think of the Earth, the moon, and the other planets as part of our home, and maybe that's why we got so upset about Pluto."
Since the 2006 ruling, astronomers have also made a number of scientific advances that further cloud the issue, from discoveries of planets that don't orbit stars to new models of how our own solar system may have rearranged itself since birth.
This time next year the International Astronomical Union (IAU)—the organization that originally called for a vote on a planet definition—will hold its general assembly in Beijing, China, and many in the field wonder whether the time is right to revisit the decision.
Birth of the Dwarf Planet
The issue of whether Pluto should be a planet first arose in the 1970s, when scientists were able to refine their estimates for the mass and size of the distant body. With each new measurement, Pluto got lighter and tinier, until astronomers realized that the object is in fact smaller than Earth's moon and has a very low density.
Adding to the oddities, in 1978 scientists announced they'd found a moon of Pluto—but one that's almost half its size, making it the largest moon in relation to its parent body. (Also see "New Moon Discovered Orbiting Pluto.")
Over the decades scientists continued to find similarly large objects in Pluto's neighborhood, a region of the solar system beyond the orbit of Neptune called the Kuiper belt.
The biggest challenge for Pluto came in 2005, when Caltech astronomer Mike Brown announced that he'd found a Kuiper belt object more massive than Pluto—a potential tenth planet provisionally called 2003 UB313.
The discovery prompted the IAU to convene a committee to decide on an official definition of a planet.
"It was a bureaucratic problem, as it had to do with naming rights for these kinds of things," said Owen Gingerich, the Harvard-Smithsonian astronomer who chaired the committee. After all, if 2003 UB313 really was a new planet, it would need a proper name on which everyone could agree.
In drafting a definition, "there were two possible routes to take—one that would deal with the physical nature of these objects, and another that would address dynamically where they fit into the structure of the solar system, with respect to their orbits and so on."
The committee initially proposed that there be two categories of planets: the classical planets and the group of planet-like bodies beyond Neptune, to be called plutons, "as a way of tipping our hat to Pluto," Gingerich said.
The planetlike object Ceres would have to be in a separate class, because it resides in the main asteroid belt, between the orbits of Mars and Jupiter. So the committee suggested it be called a dwarf planet.
The draft definition was put to a vote in 2006 at the IAU general assembly in Prague, the Czech Republic. What emerged from the session is that, to be a planet, an object must:
a) be in orbit around the sun,
b) have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
c) have cleared the neighborhood around its orbit.
Instead of plutons, the IAU members present voted that Pluto, Ceres, and 2003 UB313—now known as Eris—would all be called dwarf planets, and that this term is not for a subclass of planets but is for a unique category of solar system objects.
Exoplanets Complicate Matters
At the time of the ruling, the IAU noted that the new definition does not apply to anything outside the solar system, leaving it unclear how the organization defines the planetary objects found orbiting other stars.
Since 2006 there's been an explosion in the number of these extrasolar planets, or exoplanets, known to exist, with the current count at more than 400 and rising. Many are bigger than the gas giant Jupiter, but astronomers have found an increasing number of worlds close to Earth's mass, some of which may be habitable.
And in the past few years astronomers have even found rocky planets akin to Earth's mass that don't orbit stars at all. (See "Earth-Size 'Lone Wolf' Planets May Host Life.")
By the current IAU definition, none of these objects are official planets, because they violate the first rule about orbiting the sun.
"I was disappointed when I learned that exoplanets were not included in the definition," said NASA's Kuchner.
But for now, he said, the issue is a moot point, because the definition of a planet was necessary mostly to sort out objects that are much smaller than anything we've seen outside our solar system.
The second part of the definition, that planets must be massive enough to be nearly round, helped draw a line between bodies such as Pluto and large asteroids such as 433 Eros, a 21-mile-long (34-kilometer-long) space rock shaped somewhat like a peanut.
"For now we're good at finding exoplanets that are several times bigger than Earth. The smallest planets we've seen around other stars are much bigger than the bodies for which element two of the definition matters," Kuchner said. "And besides, we usually can't see their shapes to tell if they are round or not.
"We're also good at seeing big clouds of small particles of dust. ... But in between dust grains up to a millimeter in size and the smallest planets we can see, we're blind."
According to Kuchner, "the revolution that happened in 2006 was about how dwarf planets are not planets. Instead this is another kind of object in the solar system that we have to be aware of."
He likens the situation to a child adjusting to a new sibling: "You don't know how you're supposed to feel about it at first. I'd like us all to think about the dwarf planets out there as new siblings that we have to get to know and learn to love."
And "it's okay with me if we have another definition for exoplanets. I imagine that's something that will be revised with time."
Orbital Neighborhood Boundary Unclear?
Perhaps the most controversial part of the IAU ruling, though, is whether an object has cleared its orbital neighborhood.
Kuchner, who was a graduate student under Caltech's Brown, thinks this part of the definition is the most subjective.
In baseball, he said, "if you have a foul ball, it's because the ball landed on one side of the line—that's pretty clear. But it's harder to say if something's a strike. ... That relies on someone calling it."
In the case of defining a planet, IAU made the call, and "now we have to use this definition and try to play the game."
Overall, he added, the ruling was crucial for limiting the number of things in the solar system that deserve to be called planets.
"We really didn't have a choice," he said. "It was either going to be eight planets or a whole lotta planets. Nature sort of forced our hand."
But other astronomers aren't in favor of placing those kinds of limits.
"There are more than 200 bones in the human body. Does that mean we should redefine bones to make life easier for medical students?" argued Timothy Spahr, head of the IAU's Minor Planet Center based in Cambridge, Massachusetts.
Instead the IAU definition makes life more complicated for astronomers, he said, because the notion of whether an orbital neighborhood has been cleared remains hazy.
As an example, Spahr points to the increased number of known Earth-crossing asteroids, including roughly 8,000 that are considered near-Earth objects. While these space rocks don't exactly share our planet's orbit, they do cross it, in the sense that when they are closest to the sun, they are inside Earth's orbital path.
"There's certainly no big donut where Earth is, just a big mass of objects" that could be said to share our neighborhood, he said.
This mass of objects will probably always exist, as asteroids in the main belt collide, break apart, and send new material on orbits closer to Earth's.
"In 50 million years our orbital neighborhood will look pretty much the same." By some counts, that means Earth will not ever clear its orbit of debris.
(Related: "Pluto to Make a Star 'Wink Out' Twice.")
Spahr also notes that recent models of the solar system suggest the planets didn't form where they currently reside.
"There's been a lot of work done that says comets and other objects [including many dwarf planets] were formed between Neptune and Jupiter, and as the giant planets migrated, they threw stuff out," he said.
"You can see signatures of Jupiter's migration in the main asteroid belt.
"Probably as Neptune migrated outward, it swept up objects like Pluto and locked them in orbital resonance" out in the Kuiper belt, as they exist today.
And "certainly in the early solar system, not very much had cleared its zone out," he added. "So are you a planet yesterday, today, and tomorrow? I can't tell you that" based on the IAU definition.
Planetary Science Constantly Evolving
For his part, Spahr favors a simpler definition than the current version.
"Orbiting a star and round is a good way to start," he said. Planetary scientist Alan Stern at the Southwest Research Institute calls it the Star Trek criteria, he added: "If you can look out the viewfinder of the Enterprise and see it's round, it's a planet."
From there, Spahr said, planets could be grouped into subclasses: terrestrial planets like Earth, gas giants like Jupiter, and icy outer planets like Pluto. "We could even have a category for rogue planets, to account for the worlds that don't orbit stars."
While NASA's Kuchner thinks the current definition should stand for now, he says he's "happy that we are constantly updating our definitions and revising our vision of universe—that's what science is all about."
And of course, no matter what you call it, many astronomers will continue to see Pluto as one of the most fascinating objects in solar system, the Minor Planet Center's Spahr said.
The IAU definition, "doesn't change the fact that we're going to visit Pluto with [the New Horizons] spacecraft, and scientists are still going to go hog wild over all the data we collect."