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Astronomers Sharply Divided on New Planet Definition

by Robert Roy Britt, Senior Science Writer   |   August 17, 2006 10:41am ET

UPDATED 8:45 a.m. ET Friday Aug. 18

A 12-person committee representing the world's largest group of planetary scientists today threw its support behind a new planet-defintion proposal that would increase the tally of planets in our solar system to 12.

More dissent emerged, too, from several prominent planet experts.

Straw Poll

SPACE.com conducted an informal straw poll of respected astronomers who study planets and other small objects in our solar system and around other stars. Not all of them are at the IAU meeting where they can vote, but the question was this:

How would you vote on the planet definition proposal?

Yes 8
No 7
Undecided 0

(Updated 8:45 a.m. ET Aug. 18)

The definition, proposed yesterday at a meeting of the International Astronomical Union (IAU) in Prague, preserves Pluto's planet status and essentially classifies as planets all round objects that orbit the Sun and do not orbit another planet. The tally of planets is expected to eventually soar into the hundreds if the resolution is passed by a vote next week.

The Division for Planetary Sciences (DPS), a group within the American Astronomical Society, has the opposite view. The 12-member DPS Committee, elected by the membership, "strongly supports the IAU resolution," according to a statement released today.

"The new definition is clear and compact, it is firmly based on the physical properties of celestial objects themselves, and it is applicable to planets found around other stars. It opens the possibility for many new Pluto-like planets to be discovered in our solar system," the DPS statement reads.

An informal SPACE.com survey of astronomers who study planets in and out of our solar system found eight in favor of the resolution and seven against. A separate private straw poll being conducted by the National Academies of Sciences has so far yielded an overwhelming "No" response, a source told SPACE.com.

Original Story / Gallery of the "Planets" / Fixing Textbooks & Toys

'Terrible definition'

Clearly no consensus has emerged, however.

"I think it's a terrible definition," said David Charbonneau, a researcher at the Harvard-Smithsonian Center for Astrophysics who searches for and studies planets around other stars. Charbonneau joins two other astronomers close to the issue who sharply criticized the plan [see yesterday's story].

More Views

In email interviews, several experts in planetary science share their views:

"The definition itself is not that important.  There are lots of interesting bodies out there for us to study.  We need to have a definition, though, because it makes it easier for people to understand what we mean."
-Amy Simon-Miller, NASA scientists and member of the DPS Committee that endorsed the IAU resolution

"I think most astronomers agree that there are eight planets, and (like myself) are not particularly passionate about either Pluto's status or the outcome of the 'debate.' It's clear, however, that removing Pluto from the list rouses strong emotions within the public (who ultimately pay the bills). So I would just retain the eight planets plus Pluto."
-Gregory Laughlin, University of California, Santa Cruz extrasolar planet researcher

"It [the definition] makes a lot of sense. There has to be a physically meaningful definition for a planet since we are finding lots of KBOs and planets around other stars.  If you had an arbitrary cutoff at say Pluto or even Mercury, how would you justify it when looking for other bodies in the solar system or in other stellar systems?"
- Larry Lebofsky, senior research scientist at the Lunar and Planetary Laboratory

"My prejudice is to restrict the definition of planet and put Pluto and its large Kuiper Belt cousins in a different class...with a name to be determined (planetoid seems to serve well)."
-Jonathan Lunine, professor of planetary science and of physics at the Lunar and Planetary Lab

"The whole debate, with Pluto as the pivot point, seems a bit silly to me, to make such a big deal of it. If planets are round, then there are a whole lot more than 12 of them."
-Laurance Doyle, SETI Institute extrasolar planet researcher

"Although it will mean major revisions to my classroom presentations, its a definition that I can live with without any qualms."
-Nadine Barlow, Northern Arizona University, member of DPS Committee

"The astronomers who oppose the resolution on pure or ostensibly pure science grounds find the criterion that makes Charon a planet -the center of mass is outside the body of the more massive partner-most objectionable.  I also think that this criterion is new to them and they might find it less objectionable after it gets to be a familiar rule."
-Stephen Maran, retired NASA astronomer and author of " Astronomy for Dummies"

Charbonneau said the definition was motivated by a desire to determine whether Pluto and another object, 2003 UB313, are planets. But the IAU now says there are a dozen other objects that might be planets but need further study.

"It is ironic that we are left with more, not fewer objects for which we are uncertain of their 'planetary' status," Charbonneau told SPACE.com. "Perhaps astronomy will undergo a schism, with sects of astronomers proclaiming different numbers of planets."

"As representatives of an international community of planetary scientists, we urge that the resolution be approved," said the DPS statement, signed by chairman Richard French of Wellesley College.

In an email interview, French said he supports the defintion but realizes its shortcomings.

"My own personal definition would have been different from the final IAU resolution, but scientists have been stalemated for years by defending their own pet definitions," French said. "I understand the appeal of a simple declaration that Pluto is no longer a planet and that the solar system has only eight, but I also think there is value in the present definition that has applicability to planets around other stars as well."

The DPS has about 1,300 members, at least one-quarter of which are outside the United States. The statement does not represent the views of all members, said DPS Press Officer Sanjay Limaye. "There has been some feedback saying, 'I don't like it,'" he said.

'Worst' decision

The definition would make a planet of the asteroid Ceres and also reclassify Pluto's moon Charon as a planet, on the logic that the center of gravity around which Charon and Pluto orbit is not inside Pluto but rather in the space between them. (Earth's Moon orbits our planet around a center of gravity that is inside Earth.)

Pluto and Charon would be called a double planet, and they'd also be termed "plutons" to distinguish them from the eight "classical" planets. Ceres would be termed a dwarf planet.

The definition entirely misses the key element of a solar system object, namely its role in the formation of the solar system," Charbonneau said. "There are eight fully formed planets. The other objects-Ceres, Pluto, Charon, [2003 UB313], and hundreds of thousands of others, are the fascinating byproducts of the formation of these eight planets."

David Jewitt, an astronomer at the University of Hawaii who searches for objects in the outer solar system, told SPACE.com that the proposal is "the worst kind of compromised committee report."

Jewitt has long avoided the whole debate over whether Pluto is a planet "because I think it is essentially bogus and scientifically it is a non-issue." He waded in reluctantly this week.

"Scientifically, whether Pluto is also a planet is a non-issue," Jewitt writes on his web site. "No scientific definition of planet-hood exists or is needed. Is that a boat or a ship? It doesn't matter if you are using it to float across the ocean. Scientists are interested in learning about the origin of the solar system, and setting up arbitrary definitions of planet-hood is of no help here."

Geoff Marcy, who has led the discovery of more planets around other stars than anyone, called the definition arbitrary.

"Pluto, its moon, and large asteroids cannot suddenly be deemed planets," Marcy said in an email interview. "How would we explain to students that one large asteroid is a planet but the next biggest one isn't?"

Astronomers made a mistake when they deemed Pluto a planet in the 1930's, Marcy and many other astronmoers say. "Scientists should show that they can admit mistakes and rectify them," he said.

'Just might work'

However, one mild endorsement came today from Brian Marsden, who heads the Minor Planet Center where asteroids, comets and other newfound solar system objects are catalogued.

Marsden was on an IAU committee of planetary scientists that tried for a year but failed to come up with a definition for the word "planet," which was never needed until recent discoveries of Pluto-sized worlds out beyond Neptune. The newly proposed defintion was crafted by a second IAU committee of seven astronomers and historians.

Marsden is a firm believer that there are eight planets, but the new proposal has him sounding more flexible than in the past.

In an email message from Prague, Marsden said the new definition is "intended to satisfy the eight-planet traditionalists (such as myself) and the 'plutocrats.'" He added that he's "not against" the idea of using roundness as a determining factor.

The IAU proposal will be voted on by IAU members Aug. 24.

"It all just might work," Marsden said.

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