Now that Pluto may have regained its status as the largest object in the outer solar system, should astronomers consider giving it back another former title — that of full-fledged planet?
Pluto was demoted to a newly created category, "dwarf planet," in 2006, partly because of the discovery a year earlier of Eris, another icy body from Pluto's neighborhood. Eris was thought to be bigger than Pluto until Nov. 6, when astronomers got a chance to recalculate Eris' size.
Now it appears that Pluto reigns — though only by the slimmest of margins (the numbers are so close as to be nearly indistinguishable, when uncertainties are taken into account).
The new finding brings renewed attention to Pluto, and to the controversial decision to strip the frigid world of its planet status. Should Pluto be a planet? Should Eris, and many other objects circling the sun beyond Neptune's orbit? Or is the current system, which recognizes just eight relatively large planets, the way to go? [POLL: Should Pluto's Planet Status Be Revisited?]
SPACE.com asked some experts to weigh in on this debate, which affects how astronomers view the solar system, as well as how complicated schoolchildren's planet-memorizing mnemonics must be:
The background: Pluto's demotion
Eris is about 9 billion miles (15 billion kilometers) from the sun at its farthest orbital point, making it about twice as distant as Pluto. Its discovery in 2005 ultimately led astronomers — uncomfortable with the prospect of finding many more planets in the frigid outer reaches of the solar system — to reconsider Pluto's status.
In 2006, the International Astronomical Union came up with the following official definition of "planet:" A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies.
Since Pluto shares orbital space with lots of other objects out in theKuiper Belt — the ring of icy bodies beyond Neptune — it didn't make the cut. Instead, the IAU rebranded Pluto, and Eris, as "dwarf planets."
Dwarf planets are not officially full-fledged planets, so Pluto was stripped of the status it had held since its discovery in 1930. Eight planets officially remain: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The case against Pluto
The scientist who discovered Eris, Caltech astronomer Mike Brown, thinks Pluto's demotion was the right move. Pluto, Eris and the many other Kuiper Belt objects are far too different to be lumped in with the eight official planets, he said. [Mike Brown: Q & A With the Man Who Killed Pluto]
For one thing, they're much smaller. Pluto is about 1,455 miles (2,342 km) wide. The smallest official planet, Mercury, is more than twice as big at 3,032 miles (4,880 km) across.
The dwarfs' orbits tend to be very different, too — much more elliptical and more inclined, relative to the plane of the solar system. And they're made of different stuff, with ices comprising more of their mass.
"It just makes no sense from a classification standpoint to take these objects that clearly belong together and pick one — or two, or a dozen — and say, 'Oh, these belong with the very different, large, planet-like things," Brown said.
The only reason Pluto was ever deemed a planet, Brown added, is because it was first detected so long ago, before people realized that it was just one of a vast flotilla of objects beyond Neptune's orbit.
The Kuiper Belt — which is now known to host more than 1,000 icy bodies, with many more likely to be discovered — wasn't even discovered until 1992.
"It's just a funny historical accident that we found Pluto so early, and that it was the only thing known out there for so long," Brown told SPACE.com. "No one in their right mind would not have called it a planet back then, because we didn't know any better."
Astronomers have a much better sense of what Pluto is now, according to Brown.
"We have progressed so much further in our understanding of what the solar system is that it's pretty obvious," he said. "We can go back and reassess the mistakes of our ancestors."
So Pluto should take its rightful place alongside other Kuiper Belt objects rather than consort with the "real" planets, some astronomers say.
"I group Pluto with the other icy bodies in the Kuiper Belt," said Neil deGrasse Tyson, director of New York City's Hayden Planetarium. "I think it's happier there, actually. Pluto has family in the outer solar system."
Tyson was one of the first to push for Pluto's demotion. A decade ago, when he and the Hayden staff redesigned the planetarium's exhibits, they lumped Pluto with the Kuiper Belt objects rather than with the eight official planets.
The case for Pluto
Yet some astronomers still bristle at the reorganization of the solar system, and not because Pluto's demotion spoiled the popular "My Very Energetic Mother Just Served Us Nine Pizzas" planet-memorizing aid.
Rather, the IAU's planet definition is fundamentally flawed, some astronomers say.
They take particular issue with the "clearing your neighborhood" requirement, for several reasons.
"If you take the IAU's definition strictly, no object in the solar system is a planet," said Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "No object in the solar system has entirely cleared its zone."
The definition also sets different standards for planethood at different distances from the sun, according to Stern, who is principal investigator of NASA's New Horizons mission, which is sending a spacecraft to Pluto.
The farther away a planet is from the sun, the bigger it needs to be in order to clear its zone. If Earth circled the sun in Uranus' orbit, it wouldn't be able to clean out its neighborhood and would thus not qualify as a planet, Stern said.
"It's literally laughable," he told SPACE.com.
In Stern's view, a planet is anything that meets the IAU definition's first two criteria — the bits about orbiting the sun and having enough mass to be roughly spherical, without the "clearing your neighborhood" requirement.
So Pluto should be a planet, as should Eris and the dwarf planet Ceres (the largest body in the asteroid belt between Mars and Jupiter), as well as many other objects.
Such a definition would greatly expand the list of planets in the solar system.
Many astronomers were uncomfortable with this prospect, according to Stern, and that discomfort was a big factor in the decision to demote Pluto. It stemmed from an unscientific desire to keep the numbers low.
"Many people think it's special to be a planet," Stern said.
But adding a bunch of names to the list wouldn't cheapen the ones that had been there forever, he added. It would simply reflect astronomers' increasing understanding of the solar system. In that understanding, small, icy planets far outnumber big gassy or rocky ones.
"There are a large number of planets, and most of them are small," Stern said. "It's the Earth-like planets and the giant planets that are freakish."
For what it's worth, Stern doesn't object to branding Pluto a "dwarf planet" — he said he coined the term — as long as dwarfs are still considered planets.
What's in a name?
The battle over Pluto's planethood may be more semantic than anything else. But words do matter, because they shape how people classify and understand reality.
"You have to be able to sort," Stern said.
Tyson said he tries not to use the word "planet" in its traditional, generic sense too much, because it doesn't convey very much meaningful information. It's more revealing to group objects that are similar in size, composition and other properties.
"The word 'planet' has far outlived its usefulness," Tyson told SPACE.com. "It doesn't celebrate the scientific richness of the solar system."
So Tyson thinks in categories such as gas giants (Jupiter, Saturn, Uranus and Neptune) and terrestrial planets (Mercury, Venus, Earth and Mars) as well as asteroids and Kuiper Belt objects (Pluto, Eris and many others).
For his part, Brown thinks stripping Pluto of its planethood doesn't make the icy body any less interesting or important.
"I think that Pluto as an example of a large Kuiper Belt object is so much more interesting than Pluto as this very weird planet at the outer edge of the solar system unlike anything else," Brown said. "We are going to learn so much more about the solar system with our new understanding of what Pluto is."
Maybe Stern and other scientists fighting for Pluto's planethood would agree. Or maybe not.