Celestial sprawl: 12 planets?

Proposed definition secures Pluto's status, expands cosmic roster

Jim Erickson, Rocky Mountain News

Wednesday, August 16, 2006

The number of planets in our solar system would jump from nine to 12, and puny, belittled Pluto would remain among them under a proposal by an international body of astronomers meeting in Prague.

The definition of planet being proposed today by the International Astronomical Union, if approved, would rewrite the textbooks and discard the simple nine-planet solar system that schoolchildren have memorized for decades.

Under the proposed definition, our solar system would gain three planets immediately. A dozen or more additional planets could be added later, if they qualify.

Supporters of the plan say a burst of discovery over the past decade has forced astronomers to face the fact that our solar system is a far more complicated, messy place than we once thought.

Critics say the new system would be confusing and that the proposed 12-planet cutoff is arbitrary. One prominent planetary scientist labeled the proposal "insane."

Nearly 2,500 IAU members in Prague are expected to vote on the proposal Aug. 24. The IAU is the official arbiter of issues related to astronomical nomenclature.

"The solar system is different than we thought it was in the 20th century. There are just planets all over the place," said Boulder planetary scientist Alan Stern.

"Life's complicated. Get over it," said Stern, a supporter of the proposed definition and the lead scientist on NASA's New Horizons mission to Pluto and the Kuiper Belt, a band of icy, sun-orbiting bodies beyond Neptune.

Scientists have argued about Pluto's status for years. Some say it's nothing more than a large Kuiper Belt Object and should be tossed from the family of planets.

The need for a bulletproof definition of a planet gained urgency last year after the discovery of a Kuiper Belt Object designated 2003 UB313 and nicknamed Xena.

Hubble Space Telescope photos show that Xena has a diameter of 1,490 miles, plus or minus 60 miles. Pluto is 1,422 miles across.

If Xena rivals Pluto in size, isn't it a planet?

Two IAU-appointed panels spent about two years trying to write a definition. The second committee came up with the unanimous recommendation that will be presented today.

The proposed definition has two parts: A heavenly body would be considered a planet if it is big enough that its own gravity pulls it into a roughly spherical shape. In addition, it must orbit a star but cannot be a star or a planetary satellite.

If the IAU resolution passes, 2003 UB313 would become a planet, as would the large asteroid Ceres. Asteroids are rocky bodies that orbit the sun between Mars and Jupiter.

Charon, Pluto's largest moon, also would be elevated to planetary status. Charon would qualify because Pluto and Charon are relatively close in size and form a double-planet system, according to the IAU.

So the 12 planets in the new solar system would be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313.

More planets will likely join them later. The IAU said it is evaluating 12 "candidate planets" that could be added to the list. Others could follow.

"I think it's insane," said Caltech astronomer Michael Brown, who discovered 2003 UB313. Brown said he has found 15 roughly spherical Kuiper Belt Objects that qualify as planets under the proposed definition.

If a full census of the Kuiper Belt were conducted, the number of IAU-defined "planets" in the solar system would exceed 100, he said.

"I think the IAU is actually deceiving themselves about what their proposal really implies," Brown said. "They seem to actually believe that the number of planets they're adding is small, but it would be more than 100."

MIT planetary scientist Richard Binzel, a member of the seven-person panel that drafted the definition in June, defended its wording.

"Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor," Binzel said. "Nature decides whether or not an object is a planet."

Under the proposed definition, Pluto would be the prototype for a new class of planet called "plutons" - icy, Pluto-like objects that inhabit the outer fringes of the solar system.

The other eight planets discovered before 1900 - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - would be referred to as the "classical" planets under the IAU scheme. The asteroid Ceres, initially deemed a planet when it was discovered in 1801, would be called a "dwarf planet."

University of Colorado planetary scientist Larry Esposito views Pluto and its Kuiper Belt kin as cosmic debris unworthy of planethood.

He'd like to see Pluto demoted. But the IAU plan is a "plausible compromise" because "it doesn't put Pluto on exactly an equal footing" with the classical planets, he said.

Boulder planetary scientist Heidi Hammel said she has a problem with the term "plutons" because there's already a category of small, Pluto-like objects called "plutinos."

But Hammel said she likes using self-gravity as a measure of planethood. And increasing the number of planets from nine to 12 sends the right message, she said.

"It sets the stage for people to understand that science changes all the time, and our understanding of our solar system changes all the time," Hammel said.

But what about schoolchildren. How can they be expected to memorize the names of dozens of planets?

"I think they'll learn the major planets - which they'll be able to name - and they'll learn the fact that there's a whole bunch of smaller ones and that we're still discovering more," said David Grinspoon, curator of astrobiology at the Denver Museum of Nature & Science.

"It's like rivers," Grinspoon said. "Kids know about the Amazon and the Mississippi and the Nile, and then they know that there are a lot more rivers."

ericksonj@RockyMountainNews.com or 303-954-5129