Pseudo-history and the definition of a planet

[mmcirvin]

This Planetary Society article about the proposed redefinition of "planet" says:

In the past, no term in astronomy had seemed as clear cut as a "planet." There were nine, six of which had been known for several millennia, and while no formal definition was available, none seemed necessary. Planets seemed intuitively obvious, and if more were discovered somewhere in the universe, both professionals and lay people confidently assumed that they will know one when they see one.

factitiouslj points out that, at least in 1828, the definition was different:

The book First Steps to Astronomy and Geography, from 1828, listed the planets as "Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel." Apparently the name "Uranus" hadn't caught on yet.

So at least one textbook was listing four asteroids (three of which may be planets under the proposed IAU definition) as planets, 27 years after astronomers started finding asteroids and before Neptune was discovered. I suppose the demotion happened once they started to realize how many there were. If the definition of a planet was ever considered intuitively obvious, it was an obviousness that was subject to change.

Comments

[james-nicoll.livejournal.com]

If I recall correctly, there was a long period after 4 Vesta was found in which no more asteroids turned up. In fact, I think it was from 1807 to 1845. In 1828, four worlds sharing similar orbits was a bit odd but nothing like the "one million rocks larger than one kilometer" was supposed.

[reverendluke.livejournal.com]

The demotion of Vesta, Juno, Ceres, and Pallas came about after Neptune was discovered, and it became apparent that they were smaller and rather unlike the other eight planets. Also, it is rather odd that Uranus wasn't officially named until about that time, either. It's alternately referred to as "Herschel" or "Neptune" in most of the literature from the early 19th century.

[urbeatle.livejournal.com]

Originally, of course, a planet was anything that "wandered" against the background of stars. Comets weren't included, because the ancients thought each was a one-time occurence. When astronomers started to realize comets orbitted just like planets, they still made a distinction because the comet's tail made it seem distinct from planets.

It was the discovery of all those asteroids that messed everyone up. I say we blow them up.

[eb-oesch.livejournal.com]

As I understand it, the definition assumed for orbiting a body to determine planetary status is inconsistent. The center of mass of the sun-Jupiter system is well outside the Sun's radius. By the definition used to say that Charon does not "orbit" Pluto, Jupiter does not "orbit" the sun. Am I missing something here?

So, as for the hydrostatic-equilibrium part of the definition...

[mmcirvin.livejournal.com]

What about Vesta? (http://en.wikipedia.org/wiki/4_Vesta) It looks to me now as if the best shape models for Pallas are probably too lumpy for it to qualify, but Vesta's an odd duck: a shape that would be close to a classic oblate spheroid were it not for a colossal, deep impact crater with a tall rim and central peak, centered near the south pole and covering most of the southern hemisphere. The crater is so big that it alters the appearance of the asteroid's shape seen from near the equatorial plane. But it's really just a more extreme case of something like Mimas.