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.

[mmcirvin.livejournal.com]

I guess Herschel's name "Georgium Sidus" didn't catch on.

[mmcirvin.livejournal.com]

...Hmm, Wikipedia points out that there were obvious political problems with that name outside Britain. It also says that the name "Uranus" was actually catching on by the late 1820s, though it was far from universal.

[reverendluke.livejournal.com]

Yeah, it wasn't real popular in France for some reason.

[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.

[ice-hesitant.livejournal.com]

Wouldn't blowing up all those asteroids merely end up creating even more asteroids? It's a bit of a cutting-off-the-hydra's-heads solution, if I may say so.;)

[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?

[mmcirvin.livejournal.com]

The thing that bothers me about it is something other people pointed out: it's possible under this system for a body to oscillate between planet and moon status.

[mmcirvin.livejournal.com]

...So I think I sort of like the hydrostatic-ellipsoid part of the definition (though it needs to be more precise: how big a mountain will you tolerate? I get the impression that Pallas and Vesta are probably right on the edge), but I don't like the moon-barycenter part of the definition at all. I say if there's a bound multiple body consisting of potential planets, the biggest one is the only one that can be a planet, and the others are moons. It's just as simple, takes no more astronomical data, and is probably no more prone to tough calls.

[mmcirvin.livejournal.com]

...oh, and we'll say "biggest" by mass, just to make it easy on the astronomers.

[reverendluke.livejournal.com]

Yeah, I've been thinking the same thing. The whole barycenter-based distinction seems needlessly complex, and if not more prone to tough calls, it's certainly more prone to weirdness.

[mmcirvin.livejournal.com]

I first heard of the IAU proposal from a picture of the new planet lineup on the front page of a newspaper. My initial reaction was "2003 UB313 is a planet? Fine. Ceres is a planet? That's really surprising, but I guess I'm OK with it. Charon is a planet? They're on crack!"

[factitiouslj.livejournal.com]

Does this make Hydra and Nix moons of both Pluto and Charon?

[mmcirvin.livejournal.com]

If they insist that the barycenter rule distinguishes moons from non-moons, then I suppose they would have to be. We just got through learning that Pluto has three moons; now Pluto and Charon would be a double planet that share two collective moons.

This is all about words, but, still, the actual complexity of the situation doesn't seem to justify the complexity of the nomenclature.

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.