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Pluto demoted -- from 9th planet to just a dwarf
Astronomers vote in Prague, setting rules to determine status of bodies in solar system

David Perlman, Chronicle Science Editor

Friday, August 25, 2006

Pluto is no longer the ninth planet in our solar system. It's only a "dwarf."

Its fate was determined Thursday by the world's astronomers, who for the first time created a set of rules defining just what a planet is -- and what it is not.

Pluto got the shaft.

That leaves the solar system with its original eight planets and countless other objects that now must be called "small solar system bodies."

The historic vote by the International Astronomical Union in Prague ended more than two years of controversy that started after California Institute of Technology astronomer Mike Brown announced that his team had discovered a 10th planet orbiting the sun, and astronomers began puzzling over just how to define it.

It also reversed a week-old recommendation by a high-level committee of the 9,000-member organization that would have allowed Pluto to keep its full planetary status and added three more planets to the solar system: Ceres, an asteroid that orbits the sun between Mars and Jupiter; Charon, one of Pluto's moons; and a far-out object Brown had nicknamed Xena, 10 billion miles away in a distant realm where hundreds of similar objects lie.

The new definition of a true planet was arrived at after a week of vigorous debate.

There are now three planetary criteria: A planet is anything in the heavens that's massive enough for its own gravity to keep it roughly round, that orbits a star on its own and is not a satellite, and that has cleared away any loose cosmic rubble from its neighborhood.

Pluto, discovered in 1930 by astronomer Clyde Tombaugh, didn't make the cut.

That leaves the solar system with the eight planets every schoolkid has always known: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

"Pluto is still Pluto, and this decision is really a celebration of our increasing scientific understanding of the solar system," Richard Binzell, an MIT planetary astronomer and member of the astronomical union's committee that drafted the definition of planets, said in a phone interview from Prague.

"But there are many more Plutos just waiting to be discovered."

For schoolchildren and new textbooks, the new mnemonic for quick learning in elementary astronomy will have to go something like this: "My Very Earnest Mother Just Served Us Nothing" -- instead of the old way that used to end in "Nine Pizzas."

Like Pluto, Pizzas are out.

Gibor Basri, a UC Berkeley astronomer who has written extensively on planetary definitions, was by no means satisfied with the idea of distinguishing between planets and dwarfs. Astronomers use many informal adjectives to describe planets now, he said, like "terrestrial planets" for the rocky ones near Earth and "giant planets" like far-off Jupiter and Saturn.

So the term "dwarf," Basri said in an interview, should merely be an adjective rather than part of a formal term. Keeping Pluto off the list of full-fledged planets may be justified, he said, because it hasn't cleared away its tiny neighborhood of some 30 little nearby objects called "plutinos" that share its orbit.

"We already have a bunch of dwarf planets, including Pluto," he said, "and no one has to remember all their names. But in Prague, they did the right thing the wrong way. I'm afraid they stepped in mud."

The newly designated dwarf planet Pluto is tiny, barely 1,800 miles in diameter and smaller than Mercury. Its orbit is so skewed that sometimes it flies inside Neptune's orbit and sometimes outside it.

Thus, the world's astronomers in Prague decided to call Pluto the first in a growing category of "trans-Neptunian objects" as well as a dwarf. Brown's Xena, scientifically cataloged as 2003UB313, also fits those categories and, in the future, so will all 40 other far-off objects Brown's team has found in the Kuiper Belt, with unofficial names like Sedna and Quaoar and Varuna. Brown expects to find hundreds more.

The Kuiper Belt, countless billions of miles from the sun, also is the home of the short-period comets, like Halley's, that frequently blaze brilliantly every few years as their orbits carry them into the inner solar system. Those comets, in fact, can now be called "small solar system bodies," according to the astronomers' definition.

In a telephone interview from his office at Caltech in Pasadena, Brown said he wasn't the least bit disappointed that the objects his team discovered aren't being recognized as true planets, and he agrees that Pluto doesn't deserve full planetary status, either.

"Scientifically, it's the right decision, it's sensible, it's acceptable and it's streamlined. There may be a lot of gnashing and wailing because Pluto is demoted, but Pluto overwhelmingly is not a planet, and it was a mistake to call it one when it was discovered more than 75 years ago."

One problem the astronomers faced was what to do about Ceres, the largest object in the asteroid belt where thousands of rocky chunks orbit the sun mostly between Mars and Jupiter. It is round and firm and fully packed, and many astronauts thought it should be called a planet. But Ceres is stuck with the status of dwarf planet, too.

Pluto's tiny moon Charon will also be a dwarf. The committee of astronomers proposed linking it to Pluto and calling both together a "double planet." But the voters in Prague turned that term down and also abolished the old-fashioned term "classical" that has often been used to describe the eight planets that were discovered long before Pluto.

The astronomers finessed at least one big question. By using the term "solar system," they avoided defining the nearly 200 monster "exoplanets" now orbiting stars far beyond our own system -- the ones Geoffrey Marcy at UC Berkeley, his colleagues and others have been discovering for the past 10 years. Another meeting of the Astronomical Union three years from now must face that issue.

Pluto may have fallen on hard times, but it will not be forgotten completely.

It is due for exploration in 2019. A thousand-pound NASA spacecraft named New Horizons is now on a 3-billion-mile flight to the icy dwarf and its moon Charon, where the mission will be to explore the inner edges of the Kuiper Belt for the first time in the history of space exploration.

E-mail David Perlman at dperlman@sfchronicle.com.