An alternative proposal now means Pluto might be demoted after all, and the number of planets in our solar system not increased but reduced, to just eight...

This is starting to sound a bit like a Monty Python sketch, dontcha think? I can see the vote next Thursday... an astronomer staggers into the hall carrying an enormous spherical object. Another shouts over at him...

"What's that you've got there?"

"Pluto. It's a planet..."

"No it's not - it's a dwarf".

"A dwarf? Nonsense. You can clearly see it's a planet..."

"I can see no such thing. It's too small. Weird orbit, too. No, it's a dwarf, or a Pluton. Either way it's an ex-planet. A deceased planet. A planet that never was in the first place - "

"A Pluton? Are you out of your bleeding mind? LOOK at it!" (astronomer bangs planet on table to emphasise its mass) "What about all those innocent little kiddies! Are you going to be the one to tell them their rhyme is wrong? It's a planet!"

"Not according to my list here..."

"Right! If it's NOT a planet then what is it?"

"Calm down! I didn't expect the Spanish Inquisition - "

(Figures in red burst in from side door)

"Ha! NO-ONE expects the Spanish Inquisition...!!!"

"This *is* the IAU, is it not?"

"Yes, sir, it is."

"And... you really don't have any definitions of what's a planet, do you?"

"Ummm... no, sir. We don't. Sorry."

"I see. You do realize, I'm going to have to shoot you, now."



-the other Doug

Wow--backlash!

http://www.space.com/scienceastronomy/0608...w_proposal.html

Apparently Charon-as-planet crossed the line.

Perhaps Christine Lavin was prescient:

"Scorpios look up in dismay
because Pluto rules their sign;
Is now reading their daily Horoscope
just a futile waste of time?"

hmm, so what exactly does "by far the largest object in its local population" mean exactly? How is that defined? Without a precise understanding of what that means, you could expand or shrink the region to what ever fits your fancy. Titan is the largest object by far in its local population, is big enough to be roundish, isn't a star. Wow, it must be a planet. Yeah, this one is going to open up a whole new can of worms.

I would have thought "Titan's local population" would be Saturn and the rest of its moons... so Saturn would be the largest object in that population..?

Or, as I said, chaos.

Here's how they define "local population":

[1] The local population is the collection of objects that cross or close approach the orbit of the body in consideration.

I suspect that, historically, this notion of planetary uniqueness in a "locality" has something to do with the Ptolemaic conception of the crystalline spheres in which planets were embedded; these spheres had to be nested within each other because obviously they couldn't interpenetrate.

I don't know what a "close approach" is technically, but I suspect that Sedna doesn't "closely approach" any other object currently known. It isn't just the largest object in its local population; it is (as far as we know) currently the only body in its local population. As we detect more objects deeper into space, that is likely to change, but that points up a problem in the definition: it allows objects to be classified as planets on first discovery (if applied retroactively to Ceres in 1801, it would have unambiguously allowed Ceres to be classified as a planet -- as of course it was) but then allows the objects' planetary status to be yanked if it turns out there are more of the same kind.

In terms of general classification this is not a problem, but if planetary status is linked to nomenclature it is a problem, because it means that newly discovered objects may be stuck with names that are (retroactively) inappropriate. As may become the case with (UNNUMBERED) Pluto. It would be nice to have a way of fixing the identity of an object soon after discovery in a way that could not be altered by subsequent discoveries. It would also be nice to not upset current classifications too much, at least not ones that have been in place for decades or centuries. For this reason I would have supported a thoroughly arbitrary 2000km diameter or 10^22kg mass boundary, either of which would have retained current classifications untouched. But this kind of thinking has been dismissed as "unscientific".

But it depends on how you define "local population". I could define it as those objects that orbit in the outer part of the Saturn system. In which case, Titan is the largest object. the definition makes no stipulation that a "planet" must orbit a star. This would thus open the door for at least 16 objects (plus 1-3 borderline cases) currently classified as "moons".

I think it actually does.

"A Planet is a celestial body that (a) has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and ( b ) is in orbit around a star and is neither a star nor the satellite of a planet."

Doug

In the official proposal, yes it does stipulate that. But in the alternative definition linked to by vexgizmo, there is no such stipulation:



Thanks to that little omission, I would support this. I'd take the new planets Io, Titan, Enceladus, Europa (yes, even Europa), and Triton over Pluto ;)

Maybe they should use Justice Potter Stewart's definition "I know it when I see it"

Ironically, the satellites of our Solar system are (probably adventitiously) far more amenable to taxonomic classification than the planetary bodies. They can be split into three categories, which might be called Major Moons, Mesomoons, and Minor moons, or more whimsically Satellites, Selenoids, and Moonlets.

Category 1 is a set of objects that cluster around or just below the lower end of what are currently called "planets": the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton -- these are all large and, for the most part, interesting bodies.

Category 2 is a set including objects smaller than this (after a quite large gap) down to the minimum "round" sizes: Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Charon -- these are all icy satellites which -- with the notable exception of Enceladus -- are mostly interesting for their craters, though Iapetus and Miranda have pretty interesting topography.

Category 3: everything smaller.

I realize this is all after-the-fact justification on my part, and quite useless to inject into the controversy at this point, but what about "large enough to retain an atmosphere" (whether it actually has one or not) as a pseudo-natural basis for a dividing line? Since we seem to have established that Enceladus' atmosphere is really a localized mist due to the southern polar geysers, that pretty much lets off anything smaller than Pluto.

Poor Proteus; bigger than Mimas, but forgotten again.

Well, that's what it gets for being a slacker. Not having the common decency of pulling itself into a roundish shape. If it did, people wouldn't forget it.

Aren't a lot of the asteroids now believed to be "rubble piles"? Seems like there is a difference between a solid body that is sphereical, and a pile of boulders that might settle into a sphereical shape.

Just chiming in, I'm in the camp that says Pluto is a Kupier Belt object.
I saw it once suggested to create a separate class: Classic Planets. That would include the 9 we now know. Pluto would have a side note, in that it is likely a Kupier Belt object, but was mistaken as a planet due to the comparatively inferior technology at the time of its discovery.
But after that, well, that's about it. Anything beyond the orbit of Neptune seems like it should be classified as a Kupier Belt Object.

But who knows, in a few hundred years, when we discover an object made/coated entirely of whatever covers the dark side of Iapetus, is the size of Titan, and is maybe 75AU from the sun, and has a regular orbit, maybe then we'll have this same argument all over again.
It seems space just keeps getting weirder and weirder, constantly challenging our terminology and classification systems. Even here on Earth, new animals are still being discovered which defy classification.

Simply getting the public to know about the 9 classic planets seems enough. Maybe add in that there are other sizable chunks of matter way the heck out there, but if they could just know that Mercury, Venus, Earth, and Mars are the rocky ones, while Jupiter and Saturn are almost entirely gas, and Uranus and Neptune are icy rocks with thick, dense atmospheres, with little Pluto on the outskirts, well, that's probably more than most know right now. Save the 53 planets, or however many it may be, for the more intensive science/astronomy classes. Otherwise, people will hear "53 Planets" and just figure "Oh well, it's too complicated for me to bother with at all."


Interesting sidenote on the definition of a planet - the object must be "round". How about Saturn? From NASA's website:
"The planet's diameter is 13,000 kilometers miles (8,000 miles) larger at the equator than between the poles."
Earth's diameter is right around that much. How round is "round?"
(I'm not seriously debating Saturn's planetness, just pointing out how abstract wording might cause more problems.)