Re: That planet definition

on 19 Aug 2006 00:29:41 GMT, pete <vincent@xxxxxxxxxxxxxxx> sez:
` on Fri, 18 Aug 2006 08:37:55 -0600, Joe Strout <joe@xxxxxxxxxx> sez:
` ` In article <ec4hu9$ef6$1@xxxxxxxxxxxxxx>,
` ` arromdee@xxxxxxxxxxxxxxx (Ken Arromdee) wrote:

` ` > This is absurd:
` ` > -- A serious attempt to divide planets into classes would also separate
` ` > the gas giants out from the rest of the planets. Dividing the planets
` ` > into 8 planets/plutons, but *not* dividing them into rocky planets/gas
` ` > giants/plutons, smells like the proposal is more an attempt to keep Pluto
` ` > away than to honestly classify planets.

` ` No, I think it's an attempt to keep Pluto a planet (pluton is a
` ` subcategory of planet), while simultaneously recognizing that it's
` ` really just a KBO, and there are likely to be thousands of other KBOs by
` ` the time we're all done.

` ` > -- The discussion--at least the public discussion (I'm not privy to IAU
` ` > meetings, of course) didn't address the barycenter issue at all. It seems to
` ` > have been dropped in out of the blue, perhaps as another attempt to emphasize
` ` > Pluto's difference from the big 8.

` ` No, this was necessary to properly fill out the definition. If you're
` ` going to say that a planet can't be a satellite of another planet -- a
` ` very sensible thing to say, IMHO -- then you have to define how you tell
` ` when that is the case. Often it's obvious, but in the case of
` ` Pluto/Charon, it's not. This is not the IAU's fault, it's just a fact
` ` of nature. How do you define a satellite? By relative mass? Volume?
` ` Or by the COG they're orbiting? The latter seems as good a criterion as
` ` any.

` ` > Consider that if a moon kept its orbit,
` ` > but the primary was made out of denser material, the moon might be a planet
` ` > with no change to its mass or orbit.

` ` Yes, and conversely, if the primary kept its density, but the other body
` ` moved closer to the primary, it might become a moon. Or if it kept its
` ` density and distance, but the primary were more massive, the other would
` ` become a moon. So you can hypothetically change the parameters and make
` ` a non-moon into a moon or vice versa -- so what?

` ` > And plenty of people have pointed out
` ` > that in billions of years the Earth's moon will move far enough away to
` ` > become a planet by this definition.

` ` In billions of years both the Earth and its moon will be incinerated.
` ` Again, so what?

` ` > (And they could just as well use the "orbit
` ` > around the sun is concave" definition instead of going by barycenter, which
` ` > would classify our moon as a planet right now.)

` ` Which demonstrates why that isn't a good definition. The goal is to
` ` have something which is precise, but also more or less matches the
` ` common meanings of the words as we've used them for centuries. The
` ` common meanings were not precise, so it's impossible to match them
` ` exactly, but I think they've done a great job of getting close.

` ` > -- While roundness is superficially appealing as a way to classify planets,
` ` > I don't recall any press releases about Kuiper Belt objects which state that
` ` > scientists have discovered their shape.

` ` You don't need to discover their shape; you only need to know the mass
` ` and some idea of the strength of material it's made of. Remember, a
` ` small object that happens to be round doesn't count; the definition says
` ` that it has to be big enough to be pulled into a spheroid by its own
` ` gravity.

` ` > Size is crudely estimated, but shape doesn't seem to be determined at
` ` > all (except by saying "if it's that big, it must be round").

` ` Exactly.

` ` > -- This proposal fails the "what would you do if you found a Kuiper Belt
` ` > object as big as the Earth?" test.

` ` Um, how does it fail that? What you'd do is give it a name and add it
` ` to the list of planets. Seems sensible to me.

` ` Best,
` ` - Joe

` These tables put it all somewhat into perspective...

` http://www.spacedaily.com/reports/The_IAU_Draft_Definition_Of_Planets_And_Plutons_999.html

Damn, copied the wrong textwindow, that should be

http://www.biocrawler.com/encyclopedia/List_of_solar_system_objects_by_radius


` http://www.biocrawler.com/encyclopedia/List_of_solar_system_objects_by_mass

` There does seem to be a lot of energy being expended on the status
` of a few objects quite a long way down the list, while objects
` which would quallify as quite respectable planets get relegated
` as mere moons due to accidents of association with rather large
` neighbours...


--
==========================================================================
vincent@triumf[munge].ca Pete Vincent
Disclaimer: all I know I learned from reading Usenet.
.

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