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

[james-nicoll.livejournal.com]

The current definition of planet does not seem to be able to distinguish between stars, brown dwarfs and the objects we previously called planets.

[mmcirvin.livejournal.com]

I think they're more concerned about our solar system at the moment, where ambiguities mostly exist on the small end; but as they take into account all those extrasolar planet discoveries they're going to have to deal with the large end too.

[doctroid.livejournal.com]

Quoting the linked article, "a celestial body that is in orbit around the sun..." So stars and brown dwarfs are out (unless they're undiscovered sun-orbiters), as are, apparently, all those so-called extrasolar planets people have been discovering.

[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.

[muffyjo]

http://www.cnn.com/2006/TECH/space/08/24/pluto.ap/index.html also talks about that.

[muffyjo]

It gets to be a dwarf planet instead. Along with ceres and xena.

[reverendluke.livejournal.com]

If our solar system ends up with a (dwarf) planet that is offically named "Xena", I'm leaving.

A planet named "Xenu" on the other hand, would be perfectly acceptable.

[muffyjo]

Xena is the most common name to date. It looks like it's sticking. At least a nickname.

[doctroid.livejournal.com]

It was unclear how Pluto's demotion might affect the mission of NASA's New Horizons spacecraft, which earlier this year began a 961/27-year journey to the oddball object to unearth more of its secrets.

Quick! Call it back!

Sheesh.

[mmcirvin.livejournal.com]

Yeah, I thought that sentence was pretty stupid too.

To my mind, Pluto's identification as a large Kuiper Belt object among many actually makes it a more interesting target for exploration, not less. It's not an oddball footnote; it's characteristic of a whole population of poorly-explored bodies.

[doctroid.livejournal.com]

The IAU website is slashdotted, but according to Wikipedia, the text of the resolution as approved is as follows:

The IAU...resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:

A "planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian objects. All other objects, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

The first sentence isn't in Wikipedia's statement of the final resolution, but is in the final draft, and I suspect is really in the final resolution (or at least in the text accompanying the final resolution.) It explicitly punts on the applicability of the definition to extrasolar objects. Wisely so.

Looking at the full text, I can't see how Pluto's crossing of Neptune's orbit is relevant. Maybe the thinking is that Pluto didn't clean up its region of the solar system because Neptune did (and Neptune's failure to get rid of Pluto is too small an omission to worry about.)

[mmcirvin.livejournal.com]

I think the article's claim about the reason for Pluto's disqualification is wrong. I think what the IAU is trying to get at is that the existence of many comparable plutinos and other Kuiper Belt objects disqualifies Pluto (but then, as I said, you'd also have to make sure the definition doesn't disqualify any giant planet with Trojan asteroids).

[aderack.livejournal.com]

Whoa, big stuff is afoot.

I Have No Science Here, I Just Want To Say

[infrogmation.livejournal.com]

"TOFKATPP" The Object Formerly Known As The Planet Pluto

Re: I Have No Science Here, I Just Want To Say

[partiallyclips.livejournal.com]

"MVEMJSUNP" "My Very Educated Meeting Just Said, "Um, No Pluto."

Re: I Have No Science Here, I Just Want To Say

[infrogmation.livejournal.com]

My Very Educated Meeting Committee Just Said Um, No Pluto, So X...

[ultraman.livejournal.com]

Why would they choose the name "dwarf planet" for the class of objects which are not planets?

"Dwarf" as a modifier usually doesn't undermine the attribute it modifies, but apparently it does in this case.

Dwarf stars are still stars, right?

[doctroid.livejournal.com]

Apparently they're trying to make the Plutophiles think Pluto is still a planet while at the same time trying to make the anti-Pluto contingent think it isn't.

It's not working.

[mmcirvin.livejournal.com]

Well, it's the same pattern used previously with the term "minor planet" applied to asteroids.

BULLSHIT MR HAN MAN!

[timchuma.livejournal.com]

Everyone will just ignore this ruling and keep calling Pluto a planet anyway. The books won't be updated for decades. Does anyone even use astronomy textbooks now?

What about Uranus?

[timchuma.livejournal.com]

Did they wipe that too?

[paracelsvs.livejournal.com]

Why didn't the earlier suggestions just dump the overly complicated and confusing nonsense about barycenters, and just say "NO SATELLITES ALLOWED" like the new one? That would have gotten rid of the Pluton-Charon double planet, which I am sure was way too silly for too many people to allow it to stand.

[mmcirvin.livejournal.com]

Personally, I would have liked that best: let in all the little guys that are round but aren't moons, and just be sensible about what is a moon. Let Pluto and Ceres and 2003 UB313 and Sedna and dozens or hundreds of other Kuiper Belt objects be planets, but don't mess around with the relatively obvious classification of satellites.

(I suppose if you squint sideways at the "dwarf planet" designation, the adopted definition looks something like this. But it sounds as if "dwarf planets" are no more supposed to be proper planets than "minor planets" before them.)

Ultimately, though, I think what most of the IAU wanted was a physically motivated definition that gave something close to the accepted nine-planet lineup. And the only thing you can really sensibly do given those two criteria is demote Pluto. In and of itself, that's logical. What dissatisfies me is that the wording of the definition as adopted is so vague; it could have been better.

Won't anybody think of the children?

[mmcirvin.livejournal.com]

...As for the popular reception, I think what the reaction all boils down to is a simple principle: By and large, people want their children to be taught in school the same things they were taught in school. This applies broadly across political lines, and it doesn't really matter what relation those things bear to reality. This is also why there's all the fuss over history education standards that don't focus on a Parade of Great American Heroes, evolution, the Pledge of Allegiance, etc.