An international panel has unanimously recommended that Pluto retain its title as a planet, and it may be joined by other undersized objects that revolve around the sun.
Some astronomers had lobbied for reclassifying Pluto because it is so tiny. And at least one major museum has excluded Pluto from its planetary display. But sources tell NPR that under a proposal to be presented at a big meeting of astronomers in Prague next week for a vote, Pluto would become part of a new class of small planets. Several more objects could be granted membership.
When Pluto was first discovered in 1930, its planethood was not in question. Early estimates put it at perhaps five times the size of the Earth. Over the years, measurements have consistently shrunk.
Does a Planet's Size Matter?
Pluto is now thought to be smaller than Earth's moon. It has a cockeyed orbit. And most damningly, astronomers now know it is just one of hundreds of rocky things at the edge of the solar system called Kuiper Belt objects. Pluto is larger than most, but one recently discovered Kuiper Belt object -- UB313 -- seems to be larger than Pluto.
So earlier this year, the International Astronomical Union, which has decided tricky nomenclature issues since it was formed in 1919, appointed a panel to try to define the word "planet."
Seven experts, including a science writer and a variety of astronomers, met in Paris this past June. Under the guidance of Owen Gingerich, a historian and astronomer emeritus at Harvard, they debated for two days.
Gingerich would not discuss the conclusions, but says "I think we have done something that will make the Plutocrats and the children of the United States happy."
NPR interviewed five of the seven panel members. All but one said they thought of Pluto as a planet, or had made statements in the public record to that effect.
Dava Sobel, the writer on the panel, was sympathetic to Pluto's cause. In her book The Planets, Pluto merits a chapter.
"People love Pluto, children identify with its smallness," she writes. "Adults relate to its inadequacy, its marginal existence as a misfit." Sobel has several solar system models in her house. Asked if she had torn Pluto off any of them, she said "No, Pluto is definitely there."
New Category: Dwarf Planets
Several panel members have favored dividing planets into categories: terrestrial planets (Mercury, Venus, Earth, Mars), giant planets (Jupiter, Saturn, Uranus, Neptune) and a third class that would include Pluto.
"We'll call them dwarf planets or something," says Iwan Williams, an astronomer at the University of London who favors the idea and also served on the panel.
Sources say the panel's new definition for planets would, in fact, create a third category embracing Pluto.
It's unclear what astronomers will make of the new definition or how they will vote on it. Observers say the definition will have to be concise and unambiguous. What is too small to be a dwarf planet? Do moons count? What about round comets?
Some panel members say they favor counting any object which is large enough that its gravity has made it round. If the object is spinning, a small bulge would be tolerated. "We're talking about no more than four or five new planets," says Iwan Williams.
Small potato-shaped asteroids wouldn't make the cut. But Ceres, a big round asteroid between Mars and Jupiter, might qualify.
The panel's recommendation is being reviewed by the International Astronomical Union's executive committee. In an interview last week, executive committee member Bob Williams said the definition proposed by the panel had some potential problems, and he was not at all sure if the astronomers voting in Prague this month would approve it.
"At this point, I don't feel confident enough to bet in favor of it," he said.
It may be that the objects of the solar system are too varied to be put into neat human categories. Williams is hopeful though.
And he hopes the final definition will fit on two pages.
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