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What Is a Planet?

Nine bodies orbiting the sun? That definition no longer suffices. Does Pluto still count? What about recently discovered spheres that dwarf Jupiter and circle distant stars? Scientists haven't reached a consensus.


It seems the easiest of scientific definitions, something every third-grader knows: What is a planet?

For years, textbooks have maintained that planets are round objects orbiting stars. In our solar system they range in size from 1,413-mile-wide Pluto to massive Jupiter with its 88,846-mile-wide waistline.

Well, it's not so simple any more. One recently discovered extrasolar "planet" circling a distant star may be 40 times the size of Jupiter and more than 12,000 times the size of Earth--a mammoth object that lies far outside the traditional definition of a planet.

This month, British scientists confirmed the existence of "free-floating planets" in the constellation Orion--objects that drift in space without orbiting stars. And research from the edges of our own solar system is challenging Pluto's status as a planet.

"Ten years ago, we thought we knew what the answer was," said Alan Boss, an astrophysicist at the Carnegie Institution in Washington, D.C. "Now we know we don't know."

The muddle has top planetary theorists scratching their heads--and embroiled in emotional arguments--as they attempt to come up with new definitions. "Planet, the word, has no scientific definition," said Neil deGrasse Tyson, an astrophysicist who directs New York's Hayden Planetarium.

Tyson made astronomical waves this year by demoting Pluto in his planetarium's exhibits, claiming it is more accurate to classify the small object as one of hundreds of similar bodies beyond Neptune in a region known as the Kuiper Belt.

When Pluto was first detected in 1930, it was thought to be larger--the size of Neptune or Earth. But better observations have revealed that Pluto is smaller than our moon. It is also the only planet with a freakish orbit, one that is tilted relative to the rest of the planets and crosses Neptune's orbit.

"The more we learned about Pluto, the odder it became," said Tyson, who has been accused of "living in another universe"--and worse--by astronomers angered that he shunned the official nomenclature that still classifies Pluto as a planet.

Tyson said demoting planets has historical precedent. In 1801, Italian astronomer Giuseppe Piazzi saw an object floating between Mars and Jupiter, named it Ceres and called it a planet.

It was an asteroid. Within 40 years, hundreds more were discovered in a region we now refer to as the asteroid belt. The 580-mile-wide Ceres was unceremoniously stripped of its planetary title. Asteroids are now called minor planets.

Now that hundreds of Pluto-like objects have been found in the Kuiper Belt, Tyson says Pluto should go "from the puniest of planets to the king of Kuiper Belt."

"Everyone was angry with us that we kicked Pluto out," he said. "But you have to recognize the long-term folly of this."

Among the angriest was Alan Stern, who studies outer solar system objects at the Southwest Research Institute in Boulder, Colo. "By Neil Tyson's definition, when a cowboy herds his cows, we should reclassify him as a cow because that's what he's next to," Stern said.

A decade ago, Stern wrote a paper proposing one "simple" way to define planets. Unlike asteroids or other small celestial objects, a planet must be large enough for gravity to pull it together uniformly into a sphere. With smaller bodies, such as asteroids, gravity is relatively weak and usually chemistry holds them together instead. As a result, they often have odd shapes.

Stars, too, are round, but they are not considered planets. That is because a star is so large that massive gravitational forces trigger thermonuclear reactions that generate light and heat.

Stern said he likes his definition of a planet "because it fits with what kids think. If it's a planet, it's going to be round. It's not going to be shaped like a washing machine."

Stern concedes that his definition could eventually yield 900, rather than nine, planets in our solar system, but says that's fine. "Why are we so worried it should be an exclusive club?" asked Stern, who said the debate is "more like one between religious fanatics than scientists."

Some of the wittiest thinking on the matter comes courtesy of Louis Friedman, director of the Pasadena-based Planetary Society. "For planets, I have to paraphrase the Supreme Court discussion on pornography," he said. "I don't know what it is, but I know it when I see it."

Pluto is not the only object causing planetary headaches. On the other end of the scale are the slew of extrasolar planets discovered in the last five years. Last week, a Swiss team announced the discovery of 11 more, bringing the total known to 67.

The known extrasolar planets are generally bigger than Jupiter, the largest planet in our solar system, because it is difficult to detect smaller planets at such distances. Planets don't give off light of their own and are detected by the gravitational pull they exert on stars they orbit.

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