Defining 'planet': Newfound world forces action

By Robert Roy Britt

updated 4:49 p.m. ET Aug. 2, 2005

The claim Friday that a 10th planet has been discovered in our solar system has set off a fresh round of debate and international talks aimed at defining the most vexing term in astronomy: the word planet.

A formal proposal could come within a week or two. But some astronomers see no easy resolution.

Now, the guy who stirred the latest dust is trying to snuff the whole debate by repositioning planet as a cultural term that no longer has any scientific meaning.

"Scientists have for the most part not yet realized that the term planet no longer belongs to them," says Caltech's Mike Brown, who led the discovery of the new larger-than-Pluto object.

Brown's new view comes after contemplating six years of mostly fruitless scientific arguments that began when the public became outraged over a rumor that scientists planned to demote Pluto, a rumor rooted in the fact that some astronomers had already stopped calling Pluto a planet by the late 1990s.

"I finally realized the mistake we astronomers had been making all along," Brown told SPACE.com yesterday. "The word planet is simply not a scientific word, it is a cultural word. Once you get over that trap the rest becomes easy."

The problem
At the heart of the problem is small world that should never have been called the ninth planet when it was found 75 years ago.

Pluto is small, its orbit very noncircular, and it travels 17 degrees outside the main plane of the solar system where the other planets roam. In recent years, several other round worlds at least half as big as Pluto have been found on similar offbeat paths, including two announced last week in addition to 2003 UB313, whose orbit is inclined a whopping 45 degrees.

Most astronomers view all of them, Pluto included, as members of the Kuiper Belt (other terms are used, too, to describe the increasingly complex outer solar system).

The newfound object, temporarily named 2003 UB313, is perhaps 1.5 times the diameter of Pluto and appears to have a similar surface rich in frozen methane. So Brown called it the 10th planet in a hastily arranged teleconference with reporters Friday evening.

NASA, which funded the research, appeared to endorse the label by using Brown's terminology in its official press release.

But yesterday, NASA's Paul Hertz said, "It's not NASA's job to decide what is and what is not a planet." Hertz, chief scientist in the agency's Science Mission Directorate, acknowledged the task falls to the International Astronomical Union (IAU).

"We anticipated there would be a difference of opinions," Hertz said in a telephone interview.

Wildly different, it turns out.

If 2003 UB313 is a planet, one argument goes, then so are those other round things out there. So the new kid on the block would have to go to the back of the line, numerically. It might be No. 12 or No. 24, depending on whose scheme you like.

Proposal soon?
Efforts to craft an official definition have dragged on for years.

The IAU, responsible for nomenclature of all things beyond Earth, has been mulling a planet definition since at least 1999. An IAU Working Group specifically set up to develop a recommendation has been stalled for the past six months.

But most of the dozen members in the group were "exchanging a lot of email this weekend," said Alan Stern of the Southwest Research Institute, who is on the committee.

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