An excellent posting, Jim, well thought-out and to the point. Again, however, the bulk of the statement revolves around the solar-system (and human history-I can't quite work that into the equation) as a basis for continuing debate. And again I have to call attention to the fact that one planetary system (that is known in detail, we must discount those around other stars, detected by indirect means) is a poor statistical sample, when held against the remainder of the universe. It's just way too early to say for sure what is, and what isn't a planet, and it only really matters to those who need labels, for whatever reason. --- Jim Cobb <james@cobb.name> wrote:
On May 11, 2004, at 9:54 AM, Joe Bauman wrote:
[...]
As Carl Sagan said, "Extraordinary claims require extraordinary evidence." In the is/isn't a planet debate, it's an extraordinary claim to say Pluto should be removed from the pantheon. It's also an extraordinary claim to add a planet.
Some historical perspective may be pertinent. Imagine how extraordinary it must have been to add Herschel's planet (aka Uranus) after the pantheon of planets had been established for thousands of years. Yet it was correct to add it, for calculation of its orbit indicated that it was indeed a planet.
The first asteroids, discovered about twenty years later, were initially considered planets. The orbit of Ceres fit the Bode-Titius "law," and thus occasioned little surprise. But then the subsequent discoveries fit in the same place in that scheme. When so many asteroids were found, and mass calculations showed them to be small--Ceres has 1/5000 the mass of the earth--astronomers reconsidered and downgraded them.
Yes Pluto has a long tenure as a "planet." But seventy years is vanishingly small compared to the tenure of the five known from antiquity. And for more than half of that tenure there has been discomfort with this classification. Pluto's biggest claim to planetary status did not pan out--it did not prove to be the source of Neptune's orbital perturbations. When Charon was discovered, and the size of Pluto could finally be inferred, astronomers were shocked how minor it was compared to the other planets. The mass of the earth is 6.0 x 10^24 kg. Mercury, the smallest planet other than Pluto, has mass .33 x 10^24 kg. Then a long step down is Pluto, with a mass of 0.015 x 10^24 kg. This is 1/400 the mass of the earth, and about 12.5 times the mass of Ceres. Pluto is closer to Ceres in mass than to the earth. Another way to look at this is Mercury has 22 times the mass of Pluto, and Pluto has 12.5 the mass of Ceres. Again, Pluto is closer to the minor planets than it is to the majors.
Instead of having gimmick votes, astronomers should use a system that proves most convenient for the study of the subject, and hopefully will be helpful as they catalog exoplanets. I don't know what that system would be.
The study of astronomy will continue to surprise us, and classifications will need to be updated. Half a century ago galaxies were called "spiral nebulae" because the Milky Way was thought to be the universe. Freezing a classification scheme regardless of new discoveries is what produced astrology, not astronomy.
Regards, Jim ---- Jim Cobb james@cobb.name
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