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NATURE|Vol 442|31 August 2006 NEWS

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The ayes have it: astronomers have criticized the vote to define a planet. Pluto: the backlash begins

The future of the Solar System — or at least investigator for NASA’s New Horizons mis- resolution, thinks the IAU had no choice but to that of some of its nomenclature — may be sion to Pluto, he has a particular interest in its bend to the dynamicists’ demands. “They may thrown into turmoil by scientists who are status. But he says the issue is not really Pluto’s not have had a majority for anything positive, calling for a boycott of a new definition of status so much as the idea of putting objects in but they could rouse a strong negative major- a planet. orbital contexts. “We do not classify objects in ity simply because there are so many little On 24 August, delegates at the general astronomy by what they are near,” he says. “We fiefdoms,” he explains. assembly of the International Astronomical classify them by their properties.” “The dynamics part of the definition is a Union (IAU) in Prague voted to define the The day after the new definition was rather complex one,” says Ron Ekers, past IAU planets of the Solar System by three criteria. unveiled, Stern was among a dozen scientists president. Couching the idea in terms of a To deserve planet status, the who launched a petition to con- planet ‘clearing its orbit’ was intended to make assembly agreed, a chunk of “We do not classify test it. By e-mail, they sought the issue easier for the public to understand. rock or ball of gas must be big objects in astronomy the support of their colleagues But it may well end up confusing matters. enough for its gravity to have for the following statement: Some organizations have already said they made it round, must orbit the by what they are near, “We, as planetary scientists and will accept the IAU’s new definition. Encyclo- Sun but not be a satellite of we classify them by astronomers, do not agree with paedia Britannica, for example, issued a state- another planet, and must have their properties.” the IAU’s definition of a planet, ment saying that some of its articles on Pluto cleared other bits of debris nor will we use it. A better defi- and the Solar System were updated online from its orbit. Round objects, —Alan Stern nition is needed.” More than the same day the IAU’s pronouncement was including Pluto, that failed on the final count 200 people had added their names to the peti- made. According to a spokesperson, later became not planets but ‘dwarf planets’. tion as Nature went to press on Tuesday. revisions may reflect any uncertainty, but “the The definition originally proposed on 16 Stern thinks that requiring a planet to have vote by the IAU is considered binding — until August by the IAU would have had just two ‘cleared its orbit’ rules out some of the Solar the next vote, whether it’s next year or next criteria — roundness and not being a moon. System’s other eight planets. These include century”. NASA, too, promised to abide by the This was rejected by members at the meeting Neptune, the orbit of which is crossed by Pluto, definition, adding that it will “continue pursu- where the three-part definition was voted on and Jupiter, which shares its orbit round the ing exploration of the most scientifically inter- as the final word on the subject. But many IAU Sun with the Trojan asteroids. esting objects in the solar system, regardless of members were not in Prague for the vote, and The ‘clearing’ criterion was introduced how they are categorized”. some are furious at the outcome. when astronomers who study the dynamics Others are waiting to see how strong the coun- “I am just disgusted by the way the IAU, of the Solar System insisted that the definition ter-movement becomes. A black ribbon was which is meant to represent the best in sci- should recognize their idea of what constitutes tied around the Pluto panel at the Smithsonian ence, handled this matter,” says Alan Stern, a a planet — an object with a mass that domi- Institution’s National Air and Space Museum planetary scientist at the Southwest Research nates its orbital zone. Owen Gingerich, chair in Washington DC when the IAU’s verdict was Institute in Boulder, Colorado. As principal of the committee that proposed the 16 August announced, says curator Andrew Johnston. But

it has since been removed. “We’re going to let things calm down for at least a few weeks before Diary of a planet’s demise we decide to make any changes,” he says. One thing that particularly irks critics is the way the decision was made. The IAU has nearly While attending the International Astronomical Union’s meeting in 9,000 members, but only 2,500 people attended Prague, Jenny Hogan kept the world up to date on the Pluto debate the Prague meeting and only a few hundred were present for the vote. The IAU should have used through our newsblog. Edited excerpts: the Internet to gauge wider opinion, and then allowed electronic voting, according to those who Monday 21 August about the proposal in the original definition oppose the definition. The proposal to define a planet as anything to use ‘pluton’ to mean an object in the same “The IAU seems to be rooted in the pre- round that isn’t a moon, and thus increase the class as Pluto. Pluton is a term of long-stand- Internet age,” says Mark Sykes, director of the tally in our Solar System to 12, is scheduled ing and wide use in geology, where it refers Planetary Science Institute in Tucson, Arizona, for discussion at lunchtime tomorrow. But to an intrusion of igneous rock. who instigated the petition. “The rules of the many astronomers have already conveyed Another problem has emerged in transla- IAU say that resolutions are passed by those their objections to the executive committee tion. The French name for Pluto is — you’ve present and voting,” says Catherine Cesarsky, of the International Astronomical Union guessed it — Pluton. The definition com- director of the European Southern Observa- (IAU) by e-mail — and some are supporting mittee thought this linguistic borrowing tory and newly elected president of the IAU. a second, rival definition. would give the pluton label special appeal for Sykes admits that a “better definition” might This alternative definition argues that a French-speaking astronomers, but apparently be hard to come by, but is still pressing for the planet, as well as being round, must also be some of them object. current one to be scrapped. He thinks the IAU “by far the largest object in its local popula- All this leads to speculation that tomor- would be better off without any definition at all tion”. This definition knocks Pluto off its row’s revised definition, whatever other rather than the one they have chosen. “If they planetary pedestal (although it offers it con- changes it contains, will include a replace- can determine that this process was flawed and cessionary ‘dwarf planet’ status), and destroys ment word for ‘pluton’. nullify it, then I think that would be in their the chances of promotion for Ceres, queen of best interests,” he says. the asteroid belt. Tuesday 22 August “If enough people are completely unhappy, Of the 100 people in the closed meeting 15:00 For people who often tell journalists we could go through the process again,” says last Friday where the alternative definition that defining a planet is a meaningless label- Ekers. But a new resolution would have to wait was floated, a show of hands showed about 50 ling exercise, astronomers actually seem to for the next general assembly in 2009 in Rio de for it and only 20 for the IAU’s suggestion. care a great deal. The open discussion on what Janeiro. The IAU may issue a clarifying state- makes a planet stopped just short of fisticuffs. ment in the next week or two, but is hesitat- 23:00 My dinner companions tonight The official resolution has been divided ing to do so now. “Perhaps we need to make include some (very tired) members of the into three parts, each of which will be voted our next statement when things are a little less Planet Definition Committee. They say they on separately on Thursday at the closing cere- emotional,” says Ekers. ■ have received hundreds of e-mails over the mony. These cover the requirement of round- Jenny Hogan past few days from geologists complaining ness; the distinction between a binary planet JHUAPL/SWRI Dwarf planet in quotes “I’m here. I’m a sphere. found something between “The comments were Get used to it.” trees and bushes and intelligent, but they came Pluto itself, talking to Gady invented the word ‘animal’ with a passion that makes Epstein of the Baltimore Sun to describe it.” me think this debate has a about its recent demotion. Allen Glazner of the non-intelligent dimension.” University of North Carolina, Paul Murdin, Cambridge “I don’t know about the Chapel Hill, on the proposal astronomer, at the annual public, but… the astrologers to call dwarf planets ‘plutons’ meeting of the International will be upset.” — a term that geologists Astronomical Union. Patrick Moore, astronomer have long used to describe and veteran presenter of the certain bodies of rock. “It’s over, it’s done.” BBC’s The Sky at Night. Richard Binzel, an astronomer “Since the term is not in at the Massachusetts “Please don’t turn Pluto the Microsoft Word or Institute of Technology into a dwarf planet because WordPerfect spellcheckers, and member of the Planet that makes me sad. I’ll miss we thought it was not that Definition Committee, on the Pluto a lot.” common.” Prague vote. Daniel Dauber, aged six, on Owen Gingerich, chairman Nature’s Newsblog. of the Planet Definition Sources: Baltimore Committee, which proposed Sun, Guardian, Nature, NASA’s New Horizons probe should reach Pluto “This is as if botanists had the use of the term pluton. news@nature. in 2015, regardless of whether it is still a planet.