Pluto’s identity crisis

What, precisely, is a planet?

The concept is thousands of years old, but astronomers still can't agree on a precise definition. The word 'œplanet' comes from the Greek word for 'œwanderer,' and referred to the five bright objects ancient peoples could see gradually moving against the unchanging backdrop of stars: Mercury, Venus, Mars, Saturn, and Jupiter. In the 18th and 19th centuries, astronomers spotted two more wandering bodies, Uranus and Neptune. Finally, in 1930, Pluto was detected at the distant edge of the solar system, and the family of planets, including the Earth, was complete at nine'”or so it seemed. Each of them circles the sun in an elliptical orbit, produces no light of its own (shining instead with the sun's reflected brightness), and is believed to have formed in the same cloud of gas and dust that gave birth to the sun. 'œEverybody knew what was a planet and what wasn't,' said California Institute of Technology astronomer Michael Brown. 'œBut with the discovery of new objects, it has become somewhat confusing.'

What are these new objects?

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Last year, astronomers discovered a distant body orbiting the sun about 8 billion miles away'”about twice as far away as Pluto. Between 800 and 1,100 miles in diameter, it's about the same size as Pluto, appears to be made of mostly ice and some dust, like Pluto, and circles the sun in a highly eccentric orbit'”also like Pluto. Its discoverers called their find Sedna, and said that if Pluto is a planet, then Sedna certainly is, too.

So there are 10 planets?

Actually, no. This July, a different group of astronomers announced that it had found another planet-like object, a billion miles beyond Sedna. It appears to be one and a half times bigger than Pluto, and is so far away that the sun appears in its sky no larger than a particularly bright star. Critics immediately questioned whether the new object'”unofficially dubbed Xena'”is a planet. Instead, they said, it's just a particularly large member of a collection of thousands of asteroid-like objects at the distant edge of the solar system, known as the Kuiper Belt. Cal Tech's Brown, co-discoverer of Xena, doesn't deny his find is in the Kuiper Belt, but says, 'œOur feeling is that Pluto has been called a planet for so long that the world is comfortable with that, and it seems to be a logical extension that anything orbiting the sun that's bigger than Pluto has to be a planet, too.'

Is Pluto truly a planet?

It would appear not. 'œThere is no question that if Pluto were discovered today,' said astrophysicist Neil de Grasse Tyson, director of New York's Hayden Planetarium, 'œit would not be classified as a planet.' When Pluto was discovered by American astronomer Clyde Tombaugh in 1930, he announced that the distant, icy world was about as large as Earth, and his find was heralded by the American press as one of the great scientific breakthroughs of modern times. Astronomers now say that Pluto is, in fact, only one-sixth the size of Earth, and has nothing in common with the other planets. Pluto's funky orbit'”tilted 17 degrees out of the plane of the other planets'”its remote location, and its tiny size all put it in the category of a Kuiper Belt object. Several leading astronomers, including Brian Marsden of the International Astronomical Union, have been lobbying to downgrade Pluto's status. He points out that the four 'œterrestrial' planets closest to the sun'”Mercury, Venus, Earth, and Mars'”all have distinct similarities, as do the four 'œgas giants''”Saturn, Jupiter, Uranus, and Neptune. Pluto belongs to neither group. Saying there are nine planets, Marsden says, 'œgives a misleading impression to the public and to schoolchildren.'

Why not reclassify Pluto, then?

It's been tried. When the Hayden Planetarium reopened after an extensive renovation in 2000, the model of the solar system showed eight planets'”with Pluto missing. Patrons were outraged. 'œI still have folders of hate mail from third-graders,' said Tyson, the Hayden's director. Pluto, it turns out, is the most popular planet in the public imagination'”the cold, lonely runt of the litter, far removed from the sun's warmth, scrappily tailing after its larger brothers and sisters. Some American astronomers are protective of Pluto, too, based on their friendship and respect for Tombaugh, who died in 1997 at the age of 90. 'œThis isn't about science, or things,' said astronomer David Levy. 'œIt's about people.' We've all grown up believing there were nine planets, Levy argues, and kids are fond of Pluto'”so why not leave it that way?

So how many planets are there?