Just as earthlings were getting their first good glimpse of Pluto, some were coming to a shocking conclusion: It is not the solar system's ninth planet.
In fact, the lump of rock and ice 3.7 billion miles from the sun might not be a planet at all, some astronomers say."It's just too small," says Larry Esposito of the University of Colorado's Laboratory for Atmospheric and Space Physics.
There's no cutoff point at which an asteroid becomes a planet, Esposito concedes. But, he adds, "I think it has to be bigger than Earth's moon."
The Hubble Space Telescope sent the first clear pictures of Pluto to Earth on Feb. 29. They showed an inhospitable world about two-fifths the size of the Earth's moon. Pluto is orbited by its own moon, a chunk of rock about the size of Colorado.
The notion that Pluto is not a planet might surprise people who have been taught that nine bodies orbit the sun. But some astronomers have not been calling it a planet for awhile, says Ted Snow, director of Colorado's Center for Astrophysics and Space Astronomy and the author of three college astronomy textbooks.
It's not just size, Snow says. It's that other bodies similar to Pluto exist in the same part of space.
"Most modern astronomers would describe Pluto as the largest example of a class of outer-solar-system bodies, which are primitive bodies that have not evolved much," Snow said. "It's a bit more like one of the satellites of one of the outer planets."
Pluto seemed to be unique when it was discovered in 1930 because the quantity of space debris at the outer edge of the solar system was unknown, scientists say.
Scientists had predicted a ninth planet because irregularities in Neptune's orbit suggested gravitational influence from another body, University of Denver astronomer Robert Stencel says. "Given its larger size, it (Pluto) may have been the easiest (piece of space debris) to find," Stencel adds. But it turns out that other pieces also affect Neptune.
Astronomers say they're not very concerned about whether Pluto should be called a planet.
`It's a distinction most astronomers don't worry about much," says Snow. "Most of us don't lose any sleep over this question."
One person who takes the issue seriously is Clyde Tombaugh, the astronomer who discovered Pluto in 1930. A woman who answered the phone at Tombaugh's New Mexico home late last week said the 90-year-old scientist was upset by reports that some scientists had demoted Pluto and had taken to his bed.
In any case, scientists say new theories don't detract from Tombaugh's contribution at a time when nothing was known about the outer edge of the solar system.
But University of Colorado astronomer Fran Bagenal said many people agreed with her position - stated on the Pluto Home Page on the Internet, which she set up - that the tiny world is, indeed, a planet.
"It's an emotional issue, rather than a scientific issue; people want to stick up for the little planet," says Bagenal.
"I think there's something about Pluto - something about its name, its association with cartoon characters and Hollywood - it has a comical element to it, whereas Jupiter does not. Jupiter is a pretty serious planet," she adds.
But there also is a strong scientific rationale for calling it a planet, Bagenal adds. Pluto has sufficient gravity to organize itself into a sphere, like Earth, she says.
"I think it's an important scientific distinction between an object that is big enough that its own gravity pulls it together and makes it round, and something which is not big enough to pull itself together - it just looks like a potato."
Some large asteroids are also round and should also be called planets, Bagenal says.
Whether Pluto is a planet could be decided - once and for all - by the International Astronomical Union. After all, it can't be decided by referendum, says Esposito. "There are no people on Pluto to express an opinion."
(Berny Morson is a staff writer at the Rocky Mountain News in Denver.)