New Horizons spurs new calls to make Pluto a planet once again

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Pluto and its moon Charon, compared to Earth(Credit: NASA)

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After nearly a decade in the wilderness of celestial classification, Pluto is on the rise again. In 2006, the International Astronomical Union (IAU) voted to adopt a new definition of what makes a body a planet, and to specifically demote Pluto to the status of dwarf planet. Now, with new data and images streaming in from New Horizons showing that Pluto is not only a little larger than previously thought, but also home to some remarkable geological features (including what may be some of the solar system's youngest mountain peaks, reaching to 11,000 ft/3,353 m high), many are saying it's time to restore the ninth planet to its previous station.

Perhaps not surprisingly, some of the most prominent advocates for Pluto are scientists working on the New Horizons mission, which reached the closest point of its long-awaited Pluto fly-by on July 14.

"We are free to call it a planet right now," Philip Metzger, a planetary scientist on the New Horizons mission, told DW.com. "Science is not decided by votes ... the planetary science community has never stopped calling bodies like Pluto 'planets'."

Indeed, science – like journalism – is not a democratic process, which tends to make it a little messy at times. But for the sake of standardization and communication, we all cut corners now and then by agreeing to imperfect definitions of concepts. This lets us avoid constantly having to refer to things like Pluto in terms such as "an object possessing many traditional qualities ascribed to planets but also sporting more anomalous characteristics, like its sharing of an orbital centerpoint with its moon Charon and lying in a region of deep space occupied by trans-Neptunian objects likely originating from the nearby Kuiper Belt."

The real trouble for Pluto arguably started with the 2005 discovery of Eris. Eris is one of those aforementioned trans-Neptunian objects (things that orbit the sun from a point beyond Neptune, on average) that was thought to be significantly larger than Pluto. If Pluto is a planet, then surely larger Eris must be a planet, and if things smaller than Eris like Pluto are planets, then there could literally be hundreds of trans-Neptunian objects deserving of being added to the family of planets.

For whatever reason, be it the chaos of rewriting countless astronomy textbooks or a hidden lobby favoring preservation of an elevated status for gas giant planets within the IAU, the body voted instead to basically demote all potential planets orbiting in the more crowded stretches of the solar system – notably the asteroid belt between Mars and Jupiter, and the Kuiper Belt beyond Neptune – to dwarf planet status or something lesser.

It should be noted, however, that this 2006 vote was relatively close. In the end, 237 astronomers voted to officially demote Pluto, 157 voted against the measure and 17 abstained. The vote to define a planet as a body orbiting the sun that has a nearly round shape and has "cleared the neighborhood around its orbit" was not nearly as close, though. And even with the remarkable new data coming back from New Horizons' cruise by Pluto, which seems to show that it is actually bigger than Eris after all, it still does not meet the third criterion for once again being named a planet under the IAU's current definition.

In fact, it was Eris' discoverer Mike Brown who became one of the villains in the eyes of Pluto proponents following the 2006 vote, but today he still says the demotion was the right thing to do.

Still, some Pluto enthusiasts see it as fortuitous that New Horizons' much-hyped encounter has taken place just weeks before the next general assembly of the IAU, which only happens once every three years and is set to take place this August in Hawaii. At the moment, Pluto is not on the IAU agenda next month, but that hasn't stopped Hollywood director Paul Feig – director of Spy, Bridesmaids and the upcoming Ghostbusters reboot - from starting a petition on Change.org to pressure the IAU to reconsider returning the planet count to nine.

"In the coming weeks, New Horizons will collect unprecedented amounts of data about the Pluto system, calling in to question the definition set forth by the IAU," the petition reads.

Others aren't waiting for the IAU to change its mind. If Pluto has a hometown, it might as well be Alamogordo, New Mexico, home to the Museum of Space History and just down the road from New Mexico State University, where Pluto's discoverer, Clyde Tombaugh, was a longtime professor. The town held a "Pluto-palooza" party to celebrate the New Horizons fly-by earlier this month, where the mayor was on hand to declare Pluto a planet once again.

New Horizons mission leader Alan Stern is also on record saying the planet definition "stinks" and his colleague Metzger certainly supports the view of Alamogordo over the IAU when it comes to Pluto. "Start calling Pluto a planet right now. Add to the consensus, because that's how science makes progress, by one person at a time being convinced of the truth and adopting it," Metzger says. "You are not required to submit to nonsense."

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