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[Chicago Astronomer - Astro Joe]

Orbiting recent findings is the question: Is it a planet? Or (gasp) only a comet?

The debate about Pluto's place in the solar system has simmered since the discovery of a new body of objects, in 1992, called the Kuiper Belt, and the 1997 death of Pluto's discoverer, Clyde Tombaugh.

Astrophysicist Neil deGrasse Tyson, known in the fairly small world of astrophysics as a bit of a renegade willing to "stir the pot," took a controversial plunge when he grouped Pluto with the Kuiper Belt objects in the planetarium's display upon its debut in 2000. Tyson said that keeping Pluto listed as the ninth planet is primarily an American concern, perhaps because it's the only one discovered by an American.

European astrophysicists, he said, don't have a problem "de-listing" it. "If you brought Pluto to where the Earth is, the heat from the sun would evaporate it and create a tail. Now, that's no behavior for a planet, to make a tail!" Tyson said. "It think it's a Plutocracy," he joked.

The entire story here at the South Bend Tribune: www.southbendtribune.com/stories/2005/05/08/living.20050508-sbt-MICH-F3-New_spin_on.sto
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Pluto is indeed an odd addition to the Solar System. With it's eccentric orbit and very small mass, it could be debris. But it's also rounded and has a moon of it's own.

So I am stumped for now...

[Centaur]

My vote is Kuiper Belt Object. The larger asteroids are spherical. Some of the asteroids have satellites.

When the first asteroid was discovered in 1801, it was designated a planet. And indeed, all asteroids and KBO's are minor planets. But after several other asteroids were found, 19th century astronomers realized that scientific research would be better served if they were classified as something other than major planets.

In 1930, research assistant Clyde Tombaugh was assigned the task of checking photos of a section of the sky in which Percival Lowell thought a Planet X might exist. Even though it was later proven that Lowell's calculations were in error, Tombaugh found a dot that changed positions. Serendipitously, Pluto was making one of its rare crossings of the ecliptic around that time.

Tombaugh was looking for a planet. He thought finding one would make him famous. Senior American astronomers wanted him to look for a planet. They thought such a discovery would be a boon to American science. So without a great deal of study and debate, it was designated a planet. This was entered in textbooks and taught to us by our fourth grade teachers. Disney even named a cartoon dog after Pluto. It became part of the popular culture. It then becomes hard to change the minds of a public that "knows" something to be true.

Since the hypothesized Planet X was supposedly 10 Earth Masses, its dimness made astronomers think Pluto was coal black. When it did not seem to gravitationally perturb planets, the estimate was reduced to 1 Earth Mass. Still, it had to be black. Later it was learned to be only 0.002 Earth Masses (about 1/6 the mass of our Moon) and quite reflective of light.

As has been noted, Pluto's orbit is quite eccentric (occasionally closer to the Sun than Neptune), and highly inclined to the ecliptic. Unlike the 8 major planets, no theory that accurately describes its long-term motion has been devised, since it is so greatly perturbed by the outer planets.

If Clyde Tombaugh had questioned his discovery, he may have come up with a better theory of the origin and makeup of the Solar System a half century earlier than others did so. Since he did not, I cannot honor him as an important scientist. If he had, we might now be referring to TBO's (Tombaugh Belt Objects), of which Pluto is currently the largest known example.

While it may seem to be merely semantics, a logical classification system aids scientists in developing theories about nature and origins. Let's set aside what our teachers told us, and accept that it is more rational to refer to Pluto as a KBO (a type of minor planet.)

[Maddad]

Huh! So much for Americans wanting to keep Pluto a planet because it's the only one discovered by an American. I didn't even know it!

I agree with Centaur that Pluto actually is a Kupier Belt object, a comet core, but I suspect that we'll always call it a planet. That's what we're used to, be it right or wrong.