Planets again

[mmcirvin]

...And now the AP says the IAU has gone in the less expansive direction, distinguishing between dwarf planets and real planets and demoting Pluto. I can't tell exactly from the article but it sounds as if there's a qualifier that excludes objects obviously part of a population of similar bodies in similar orbits.

(The article claims that Pluto is disqualified because its orbit "overlaps" Neptune's, but that can't be right without further detail, or it would disqualify Neptune too! I would think that Pluto is disqualified because it's one of a whole population of similar bodies in similar orbits, some of which are of comparable or even greater size.)

Comments

[doctroid.livejournal.com]

Hmm, I recall reading a day or so some proposed language along the lines of "... dominates the region around its orbit ..." or something vaguely like that. "Dominates" is the word I remember. With that language you could disqualify Pluto for crossing Neptune's orbit without disqualifying Neptune. But now the phrase seems to be "has cleared the neighborhood around its orbit", and, as you say, if that applies to Pluto, it applies likewise to Neptune.

[mmcirvin.livejournal.com]

Well, if you interpret "neighborhood" in this context as referring to phase space or to classes or orbits, you could maybe plausibly interpret it as allowing Neptune but not Pluto. There are a lot of "plutinos" in Pluto-like orbits (that are resonant with Neptune in the same way as Pluto) but anything in a Neptune-like orbit is orbiting Neptune.

...unless it's something like a Neptune Trojan (do any such things exist?) Presumably they don't mean to disqualify Jupiter as a planet on the grounds that there are Trojan asteroids in Jupiter-like orbits.

[mmcirvin.livejournal.com]

...there are in fact several known Neptune Trojans (http://en.wikipedia.org/wiki/Neptune#Trojan_asteroids).

[james-nicoll.livejournal.com]

Actually, Pluto and Neptune maintain a healthy separation and get no closer to each other than we to, hrm, Uranus.

[eb-oesch.livejournal.com]

That would have been an elegant solution, I think. On that basis, one could make a strong case that the presence of Trojan satellites speaks in favor of planetary status, because a body has to be about 25 times bigger than its satellites in order for the relationship to be stable, and being able to corral objects into pseudo-orbits about your L4/L5 points from such great distances is pretty telling. Similarly, Pluto's 3:2 harmony with Neptune could be seen as evidence of Neptune's dominance, and Cruithne's (or the Moon's) relationship with Earth would be more of the same. It might be hard to explain what "dominance" means in a single sentence written at a second-grade level, but a slightly deeper discussion of what dominance can mean would probably be wonderfully didactic, and even if it's got some level of intractable vagueness, at least it seems like an accurate description of the 8 planets.

Also, maybe you've seen this BBC discussion of the controversy (http://news.bbc.co.uk/2/hi/science/nature/5283956.stm) (from Slashdot)?

Also also, I dislike the decision that planets orbit the Sun. If you're going to define a planet to be those eight and nothing else, then why bother formulating a general definition, and if you create a general definition, then why not apply it to all star systems?

[mmcirvin.livejournal.com]

Probably they were worried that something about their definition might not apply to other star systems, and decided to kick that vote down the road a way once there's more data. As James noticed, the decision seems to deal mostly with the ambiguities at the small end of the planet definition, since the planet/star distinction is quite clear in our own solar system; but it is not at all clear when one considers the 200+ extrasolar planets known. A definition that applies to extrasolar planets would have to deal with the separate question of what constitutes a star or brown dwarf, and whether any of these are planets. And that's probably a whole other round of infighting.

[mmcirvin.livejournal.com]

...I think it's amusing that Mike Brown seems to have changed his opinion on this issue every time somebody asks him.