The contentious new planet definition passed by the International Astronomical Union leaves Pluto out.

A now-outdated rendering of the solar system.  Credit: Murat Baysan

On Thursday, Aug. 24, as the 26th General Assembly for the International Astronomical Union reached its conclusion, its members passed a resolution that redefines the word “planet” and reclassifies Pluto as a “dwarf planet.”

The decision followed a contentious process that some astronomers have deemed a “total disaster.”

“The fact that they came up with an acceptable result is shocking and maybe a function of luck,” said Mike Brown, the Caltech astronomer, who discovered the Kuiper Belt object 2003 UB313, which catalyzed the need for a revision of the solar system. “It was a circus for the last week or so.”

According to the new definition, in order to be a planet, a celestial body must orbit the sun, be nearly round in shape and have cleared the neighborhood around its orbit. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune retain their status as planets.

Pluto does not—it meets the first two criteria, but since its orbit crosses Neptune’s as well as those of several other Kuiper Belt objects, it has been relegated to a “dwarf planet.” The asteroid Ceres and 2003 UB313 (nicknamed Xena) join Pluto as the first official “dwarf planets.” A dozen other candidates are currently listed on the IAU’s “dwarf planet” watchlist. All other objects, except for satellites, will be known as “small solar-system bodies.”

“In general, the IAU bungled this whole affair very badly: They never really opened the workings of the two definition committees to the community,” said one US astronomer.

“There are already polls of people who say most people think it’s a really bad idea,” said Brown. “I understand everybody’s feeling that everybody loves Pluto and nobody wants to get rid of Pluto, and we haven’t gotten rid of Pluto. We’ve just categorized it correctly. And it really is the right thing to do.”

This resolution comes after over a week of heated debate among astronomers.

At the start of the General Assembly on August 16th, the IAU’s Planet Definition Committee proposed a resolution that would have included not only Pluto but also Ceres, UB313 and Pluto’s moon Charon as planets. The proposed definition emphasized that a qualifying body must have “sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape” and included the new planet category of “pluton” for objects that take longer than 200 years to complete an orbit around the sun. The proposal was the result of two years of official deliberations.

Brown said he had two major problems with the first definition. First, he said it was inconsistent: Only 12 objects would be named planets instead of the 53 that he said qualified under the first set of criteria. He called this inconsistency “a little bit either intentionally or unintentionally dishonest.” Second, he said allowing 53 objects to qualify as a planet would rob the word of its special meaning.

“That one, I thought, was a total mess,” he said. “It actually did damage to what we think of as the solar system.”

MIT professor Richard Binzel, a member of the Planet Definition Committee, said he was glad that the IAU members had come to a consensus, even if it was not the one his committee had proposed.

“We chose to start with self-gravity, a body’s own gravity,” he said. “Over the course of discussions, it was a matter of how much does its self-gravity matter, and how much does the way it formed or the way it orbits the sun matter? And we came to a little more emphasis on the orbiting the sun part than the self-gravity part.”

Paul Weissman, an astronomer at Jet Propulsion Laboratory, said that the IAU began with a terrible definition but, through the efforts of its members, came to a satisfactory one.

“The formal definition I had suggested was that a planet was massive enough to have cleared its dynamical zone over the age of the solar system,” Weissman wrote in an email. “This is now essentially a part of the definition. All of the first eight planets can do this. Ceres and Pluto have not.”

Despite the result, many astronomers were unhappy with the whole process of having a select committee draft a resolution behind closed doors.

“In general, the IAU bungled this whole affair very badly: They never really opened the workings of the two definition committees to the community; there was no community-wide call for input to the committees,” said one US astronomer who wished to remain anonymous. “My experience was that the IAU executives were more concerned with decorum than with getting it right.”

Brown agreed that the process left a lot to be desired.

“They tried to have it secret until when the meeting started, and so there was no chance for debate,” he said. “Even as of today, when I woke up this morning, I still didn’t know what they were going to be voting on. So, [for] this historic vote on what a planet is, basically no one had more than about five hours to even read the text of what was going to be voted on. In the end, there was still really no room for debate.”

Planet Definition Committee-member Binzel argued that the positive conclusion shows the success of the IAU’s method.

“There’s always different ways, different approaches that one might take,” he said. “I think we came to the same end result, regardless of the path.”

Even for the parties who approve of the final definition of a planet, some issues remain.

Brown said he is slightly disappointed that the object he discovered, 2003 UB313, will not be a planet. Both he and Weissman lament having to use the term “dwarf planets” for objects previously referred to as “planetoids” or “trans-Neptunian objects.” Weissman also said he regrets that the public will be unhappy with the demotion of Pluto.

Binzel, for his part, encourages fans of Pluto not to mourn.

“Pluto is still Pluto from a scientific point of view,” he said. “Everything we want to learn about Pluto—and we have a spacecraft headed there now called New Horizons, it gets there in 2015—the science questions that we want to ask, are all the same.”

“It’s still the same world today that it was yesterday.”

Originally published August 25, 2006

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