Thoughts On The Proposed Planet Definition

by Jimmy Akin

in Science

Earlier I said I’d offer my own thoughts on the proposed IAU definition of what a planet is, so here goes . . .

I am largely . . . pleased.

The basic reason that I’m pleased is that the number of planets is going up. What could be better than new planets? In fact, if the definition sticks, the largest expansion of the number of known planets in human history may occur in our lifetimes! Yee-haw!

It would be a real downer, in fact, if they had gone with a definition that stripped Pluto of its status as a planet. That would have been a disappointment. It would have created a feeling that there was an eighty-year mistake that was being undone, and since the definition of "planet" is largely arbitrary (as is the case for most words), why go through the hassle of trying to convince everyone in the world that Pluto is not a planet when a definition could be crafted that could easily accomodate the idea?

I mean–I know that some people (such as canonist Ed Peters, and more power to him) have been gleefully dancing on Pluto’s grave for some time–but the idea of Pluto is a planet is just too deeply embedded in our culture to try to get everyone to stop referring to it as a planet. Think about the practicalities of doing that. Ick. It’d be much easier just to accomodate the definition of "planet" so that Pluto counts.

Put another way: It’s easier to get people used to the idea of accepting new planets than declassifying ones they grew up with.

So I think the IAU’s committee made the right decision in keeping Pluto as a planet.

This still leaves open the question of what kind of definition they would use.

One definition that I would have been okay with would be to simply draw an arbitrary line and say "Pluto is the smallest planet by definition. Any thing with a larger radius or mass than Pluto is a planet. Anything that has a smaller radius and mass than Pluto is something else."

I’d be okay with that–and on that formulation we’d only get one new planet (Xena)–but it’s scientifically inelegant. It just draws an arbitrary line instead of basing the definition on a natural kind.

A natural kind (as the term is here being used) is a distinct type of thing that you find in nature. For example, lions and ants and daisies and geodes and geysers and rainbows are natural kinds. They aren’t all living, and they are categories that have fuzzy boundaries, but they are things that you find in the universe that are significantly similar to each other to form a kind and sufficiently distinct from other things that humans are inclined to come up with a unique word for them.

I’d much rather see the definition for "planet" be based on the kind of object that people have traditionally called a planet than simply drawing an arbitrary line.

One reason for this is that the arbitrary line that could have been drawn for Pluto is quite close to the kind of line that would suggest itself if we based the definition of planets off of natural kinds.

One thing that all the traditional planets have in common is that they are at least roughly spherical (i.e., they’re sphereoids), and this is no accident: It’s because they all have a certain mass, which compresses them into a sphereoidal shape, rather than letting the structural properties of the material they’re made out of determine their shape (as with many asteroids, which are basically chunks of rock that aren’t spherical at all or at least aren’t spherical due to gravity).

This mass-based definition also coheres with our intuition that a planet should be a body of a certain size, rather than any ol’ fleck of rock we find in the solar system.

If we go with a natural kind-based definition, the obvious lower threshhold for what counts as a planet is the massive-enough-to-be-a-sphereoid level. That’s still a fuzzy line that leaves room for further clarification (just how sphereoidal does it have to be?), but at least it’s not completely arbitrary.

The problem with proposing this as a lower threshhold is that a lot of objects in the solar system meet this test, and in coming years we’re probably going to find many more. Personally, I find the idea of lots of new planets cool, but it’s also quite an adjustment for many people, and so I’m impressed by the IAU’s willingness to go with the more scientifically elegant definition rather than an arbitrary definition based on Pluto’s size that would be more restrictive of the number of new planets.

What I’ve said above covers the lower threshhold of what counts as a planet under a natural kinds definition, but that still leaves the question of what the upper threshhold would be. This is something the IAU’s proposed definition doesn’t deal with, but I think there is an obvious natural kinds-based line to be drawn there as well: If an object becomes so massive that–at some point during its life cycle–it undergoes nuclear fusion then it is no longer a planet but a star (or a dead star if it’s nuclear fuel is spent and fusion has stopped).

My preferred natural kinds-based definition of a planet is thus:

An object is a planet if and only if:

1) It is massive enough that its shape is dictated by its gravity rather than by structural factors (i.e., it’s massive enough to be a sphereoid) and

2) It is not so massive that nuclear fusion naturally occurs in it at some point.

Unfortunately, the IAU didn’t go all the way to my preferred natural kinds definition. It didn’t treat the second criterion explicitly (though it did distinguish planets from stars), and it went beyond my definition by adding what I consider to be an inelegant, arbitrary, and . . . frankly . . . stupid criterion–one based on where an object is.

Specifically, the IAU’s proposed second criterion was:

(b) is in orbit around a star, and is neither a star nor a satellite of a planet.

This is just dumb, and I suspect it won’t survive long term.

One reason is that not all planets are in solar systems. There are bound to be objects that are otherwise identical to planets that have been flung off from solar systems, and to refuse to call intersolar planets "planets" just because they aren’t orbiting around stars is dumb. If we had a close encounter with something that knocked one of the classical planets out of our solar system, we wouldn’t say it should be declassified as a planet just because it isn’t orbiting the sun any more.

The other bit of this criterion that I don’t like is that to count as a planet an object must not be "a satellite of a planet."

A satellite–as they’re using the term–means any object that is non-massive enough that the barycenter it orbits is within another object.

Now, in case it’s been a while since you had physics or astronomy or an equivalent course, a barycenter is a point that two or more objects are orbiting. Y’see (forgive me if I oversimplify a bit), whenever two or more objects are in a stable orbital system (or subsystem), the masses of the objects are all pulling on each other in a way that they orbit a single point.

This point is not simply the center of the largest object, so when the Moon "orbits" the Earth, it isn’t swinging around the center of the Earth. It’s swinging around a point that is part way between the center of the Earth and the center of the Moon. That point is known as the barycenter, and–because of the relative masses of the Earth and the Moon and their distance from each other, the barycenter of the Earth-Moon system is inside the Earth.

But it doesn’t have to be that way. If you had two equally massive objects orbiting–if there was Earth and Counter-Earth, let’s say–then the barycenter would not be inside either of them but between them.

That’s the case with Pluto and its former moon, Charon. Pluto and Charon are equal enough in mass that the barycenter of their system isn’t inside Pluto but between the two bodies and, since Charon is big enough to be a sphereoid under its own gravity, it would get counted as a planet in the new definition.

Good for Charon, but I think it’s dumb to base whether or not something is a planet on something as arbitrary whether the barycenter it’s orbiting is above or below the crust of a neighboring body. Based on that criterion, any object, no matter how much it looks like a planet–even one as massive as Jupiter–would cease to be a planet if it were pushed into orbit around a sufficiently massive neighbor.

That gets us away from a natural kinds definition, and I don’t like that. Basing whether something is a planet on what its neighbors are like is just scientifically inelegant. Planethood should be intrinsic to the planet itself, not conditional on the other members of its orbital system.

Now, I know darn well why the IAU included this condition. There’s a very specific reason: It’s to keep us from having to classify the Moon as a planet. The Moon is larger than Pluto and, if it wasn’t orbiting the Earth-Moon barycenter it would be classified as a planet. In fact, the Moon is larger than all three of the new planets–Ceres, Charon, and Xena.

Furthermore, the Moon is slightly smaller than Mercury and other moons–like Ganemede and Titan–are bigger (in radius if not mass) than Mercury, whose status as a planet very few are willing to challenge.

The IAU’s committees, though, felt that they had to include some kind of location-based criterion in their definition just to keep the Moon from being classified as a planet.

I think that’s dumb. It’s scientifically inelegant as it gets us away from a natural kinds definition.

Put another way: What a celestial body is is more important than where the celestial body is.

I’d much rather bite the bullet and say, "Guess what, folk! We’re living in a twin-planet system and always have been: The Moon is our twin planet!"

I think that would be cool, as well as more scientifically elegant.

But that’s my opinion, and others are free to hold whatever ones they want.

After all, the term "planet" is of human construction and humans together should decide what it means. I’m just advocating the most non-arbitrary definition I can think of (big enough to be a sphereoid, small enough it doesn’t fuse).

We should know within a week what course the IAU finally takes, and I’m hoping that they’ll adopt at least something like the proposed defintion (though I’d love it even more if they adopted mine instead).

I’m just jazzed about getting new planets in my lifetime.

What’s cooler than that?

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{ 60 comments }

Rosemarie August 17, 2006 at 6:14 am

+J.M.J+
Yeah, but now I won’t be able to teach my kids “My Very Easy Mother Just Served Us Nine Plums”! :-)
In Jesu et Maria,

SDG August 17, 2006 at 7:33 am

“My Very Easy Mother”?!! Yikes! What about “Very Educated” or “Very Excitable”?
Wikipedia offers several variations including

My Very Educated Mother Just Served Us Nine Pizzas
My Very Easy Mnemonic Just Summed Up Nine Planets

And, from the Colbert Report, on Pluto-skepticism:

My Very Educated Mother Just Said “Uh-oh, No Pluto”

And then very recently (Wikipedia says “today”!) from CNN:

My Very Educated Mother Can’t (Ceres) Just Serve Us Nine Pizzas with Chovies (Charon) X-cluded (Xena / 2003 UB313)

though surely we can do better than that. :-)

Rosemarie August 17, 2006 at 7:48 am

+J.M.J+
I dunno, they taught us “easy mother”, though “educated mother” does sound better.
Hmmm: “My Very Educated Mother Can’t Just Serve Us Nine Politically Correct Xylophones” works, but makes absolutely no sense whatsoever. :-)
In Jesu et Maria,

Bear August 17, 2006 at 7:56 am

I just hope they come up with a real name for 2003 UB313 soon, before the “Xena” nickname starts to stick in people’s heads. I’d hate to have a planet named after a t.v. character.

bill912 August 17, 2006 at 8:12 am

I like “Forbidden Planet”, myself.
Or, perhaps, “Vulcan”, although it can’t be very hot there.

Anonymous August 17, 2006 at 8:28 am

“My very educated mother just said, Uhoh no Pluto!!”
http://www.youtube.com/watch?v=93TNjAeIJlU

IA_ August 17, 2006 at 8:29 am

“My very educated mother just said, Uhoh no Pluto!!”
http://www.youtube.com/watch?v=93TNjAeIJlU

SusanM. August 17, 2006 at 8:36 am

I love the idea of a bunch of new planets. We will have to figure out a way to put the new ones on my 11-yr-old sons planet mobile.
How about…
“My Very Educated Mother Can Just Shoot Up-to Nine People’s X-rays”?

francis 03 August 17, 2006 at 8:44 am

More planets would rock. (Or gas, or ice, as the case may be.) I agree that orbiting a star should not be a criterion. I also think, however, that keeping the term “moon” (as distinct from “planet”) would be useful in a future space age, in the same way that our current distinction between continents and islands is useful now– of course they’re both land. Islands just tend to be smaller than and in some cases subsidiary to continents ecologically, economically, and politically. Yeah, there’ll be some fuzz at the edges (see Australia/Charon), but that seems necessary. Just when an orbiting body gets big enough that it ceases to be a moon and becomes a double planet is a tough question, but the barycenter solution seems pretty practical and still rather elegant to me– it takes into account the sizes of both the bodies, and it corresponds to a real natural characteristic of their relationship.
Anyway, I imagine that future spacers will find our definition of “planet” to be not entirely practical. Such a society might be more interested in knowing what places they can travel to, land a spaceship on, and mine or build colonies on. In that case they might coin a new term such as “world” to describe any body composed mainly of solids that has a certain minimum gravitation.

Monica August 17, 2006 at 9:00 am

I would not adjust well to having no Pluto. I am still having enormous difficulties with the Saber Tooth CAT instead of TIGER as I was taught. I will call it a saber tooth tiger till my dying day regardless of what the docents at La Brea Tar Pits try to tell me. Unless, of course, the government sets up sensitivity training classes to help me adjust!

SDG August 17, 2006 at 9:24 am

I am still having enormous difficulties with the Saber Tooth CAT instead of TIGER as I was taught.

I know what you mean. I get around that one by just saying “sabretooth.” However, I am partial to the revisionist “sea stars” over the traditional “starfish” (they ain’t fish).

Augustine August 17, 2006 at 9:49 am

I’m glad that Pluto is still a planet, but although it’s conceivable to have binary planets, it sure sounds weird to call Charon a planet. Yet, I can live with that.
On errant planets, if and when we find one, whose existence is theoretically possible, I think that they’ll deserve a denomination of their own. IMO, it’s not unlike white or brown-dwarf stars: they’re still starts, but with a different name.

Jared Weber August 17, 2006 at 10:01 am

Dude, I always memorized it by “Mary’s violet eyes make John stay up nights playing.”

Ken Crawford August 17, 2006 at 10:34 am

Jimmy, this has to be about the most illogical post you’ve ever done! (Luckily that’s because you generally have such a high standard that it doesn’t take much ill-logic to qualify for that title.)
To simplify your post I’ll give an executive summary of what you said (visualize power-point):
-IAU is defining what a plant is and Jimmy mostly likes the definition.
-Jimmy likes it because it doesn’t significantly alter what people think of when they think of plants (example: not stripping Pluto of its status as planet)
-Jimmy also likes it because it’s based on a natural metric/phenominon and not something arbitrary.
(Mid executive summary note: you were fine up until here.)
-What Jimmy doesn’t like about the new definition is the inclusion of an orbiting a star requirement and a no barycenter inside another planet requirement.
-Jimmy sees both of these requirements as arbitrary.
-Jimmy thinks being a planet has nothing to do with where it is but solely what it is.
I hope that was a reasonably good executive summary becuase my point was not to setup a straw man but just to simplify as to be able to respond without violating Da Rulz regarding brevity.
Your second to last point (as defined by my executive summary) is just plain false. Whether something is orbiting a star is a VERY natural thing and a very reasonable natural metric. Whether one should use that natural metric to define a planet is a different question (addresed later in this comment) but to call it arbitrary is not accurate. Similarly, the barycenter point of two objects is a natural metric. While defining the edge for whether one of the two is subordinate to the other has some arbitraryness to it, picking the edge of a planet is a fairly natural solution (the other being the 50% point).
So overall I think it is a false criticism to call these criteria arbitrary.
Beyond that, your 2nd point and your last point (as defined by my executive summary) contradict each other in my opinion. You ask any second grader what a planet is and they’ll very likely state something about orbiting around a sun. Whether you agree or not, the laymans definition of a planet is actuall MORE about where it is than what it is (although both are required). If you desire continuity as opposed to forcing everyone to re-educate themselves about what what a planet is, keeping a planet separate from a moon is an important aspect as well as the requirement to orbit a star.
All said I think the IAU has done a brilliant job defining a planet and assuming it is approved, it will stand the test of time.

Rosemarie August 17, 2006 at 10:39 am

+J.M.J+
>>>However, I am partial to the revisionist “sea stars” over the traditional “starfish” (they ain’t fish).
They’ve also renamed jellyfish “sea jellies”.
In Jesu et Maria,

J.R. Stoodley August 17, 2006 at 10:55 am

“Starfish” is fine with me, as long as it isn’t star fish. There are planty of things with a word included in their name that doesn’t describe them, like Douglas-fir (not a fir), ground-pine (not a pine or even a seed plant), groundhog, etc. The people in the animal world should just copy the botanists and change spaces to hyphens making it sapretooth-tiger, Tasmanian-tiger, guinea-pig, etc.
I personally would not have much of a problem with dropping Pluto off the list if it made sense. So what if people kept calling it a planit for a while? People still call fungi plants, despite the fact that they are nothing of the sort biologically at least. That doesn’t mean our taxonomic systems need to go back to the 19th century (or 1970’s high schools).
Still, Jimmy’s arguments do sort of convince me Pluto should be a planit. Maybe.
I’m not crazy about his idea that the Moon should be considered a planet though. Planets were supposed to be wandering stars. We found out the structure of the fixed stars and the rather different structure of the planets but for a while there still remained something of the old identity of the planets. The discovery of Pluto, which you can not see with the naked eye or even a modest telescope, hurt that idea a little further, but it still remained somewhat.
Now though between science fiction and planets outside the solar system the old concept is finally being replaced completely by the other worlds version. If the venerible old Moon were to be suddenly grouped among them, who previously was the counterpart even in Genesis of the Sun (increasingly thought of as a star) then the transition would be rather complete.
Maybe that would make more scientific sense, like separating fungi from plants, but I for one will morn the loss. Maybe both losses.

J.R. Stoodley August 17, 2006 at 11:02 am

Sea jellies? I am developing some rather unchristian attitudes toward these marine zoologists. I say we put some foresters over them to keep them from getting out of line like this.

Rosemarie August 17, 2006 at 11:03 am

+J.M.J+
Ya think the komodo dragon will be next in line for a name change? :-)
In Jesu et Maria,

J.R. Stoodley August 17, 2006 at 11:08 am

Komodo dragon? You mean Komodo lizard.
Crayfish? Water cray.
Whale shark? Great shark.
Horseshoe crab? Sea horseshoe.
Sea horse?…

J.R. Stoodley August 17, 2006 at 11:13 am

Then again, “sea jellies” arn’t really made of jelly.
And horseshoe crabs not only are not crabs but are not horseshoes either.
And getting back to astronomy, “sea stars” are not actually stars.
Time for another round of revision.

John F. Kennedy August 17, 2006 at 11:37 am

I detest the Xena name. I would go for “Planet X” or the “Mysterous Planet”
Krypton would also be interesting.
Seriously, I do like the historical use of Greek and Roman god names. I would like to continue it and NOT introduce any other mythological gods into the mix. Let’s keep them a happy family. But I might consider Thor or Odin. Is there a god of Ice or cold?
I do not know how these traditional names are translated into other languages? Anyone have any information on this?
John F. Kennedy

john F. Kennedy August 17, 2006 at 11:39 am

I detest the Xena name. I would go for “Planet X” or the “Mysterous Planet”
Krypton would also be interesting.
Seriously, I do like the historical use of Greek and Roman god names. I would like to continue it and NOT introduce any other mythological gods into the mix. Let’s keep them a happy family. But I might consider Thor or Odin. Is there a god of Ice or cold?
I do not know how these traditional names are translated into other languages? Anyone have any information on this?
John F. Kennedy

john F. Kennedy August 17, 2006 at 11:40 am

oops

CaeliDS August 17, 2006 at 11:42 am

Should we call this Planned Planethood?

Rosemarie August 17, 2006 at 12:15 pm

+J.M.J+
Let’s rename the tasmanian devil next. I mean, it’s not a *real* devil, now is it?
And don’t get me started on sea lions…. :-)
In Jesu et Maria,

Some Day August 17, 2006 at 12:32 pm

Cool, but soo Byzantine.
I think the where is more important, because if our moon was not next to us but next to Pluto or Mercury, it would be a planet. But interesting anywhow.

A.M. August 17, 2006 at 12:45 pm

What’s wrong with constructing a definition of “planet” in such a way as to exclude the Moon? If you think Pluto’s status as a planet is too entrenched in culture to change it now, imagine how tough it would be to get people to quit thinking of the Moon as a moon!

J.R. Stoodley August 17, 2006 at 12:45 pm

Some Day,
What do you mean by so Byzantine? You’ve said that before. Do you mean overly intelectual or something like that?

J.R. Stoodley August 17, 2006 at 12:46 pm

Some Day,
What do you mean by so Byzantine? You’ve said that before. Do you mean overly intelectual or something like that?

Some Day August 17, 2006 at 1:12 pm

“Byzantine”refers to arguements that in relation to real problems, are silly or unimportant.
It refers to the Siege of Constantinople, where they are about to be destroyed, with a horde outside the walls, and yet they were argueing if Adam had a belly button and what gender are Angels. Silly at the momment.
—-
I meant it here in a cheery manner.
But most of the times its a term used as a scolding to working on pointless things and giving it great importance, at least when there are greater problems and dangers.

SDG August 17, 2006 at 1:25 pm

“Byzantine”refers to arguements that in relation to real problems, are silly or unimportant.
It refers to the Siege of Constantinople, where they are about to be destroyed, with a horde outside the walls, and yet they were argueing if Adam had a belly button and what gender are Angels. Silly at the momment.

I assumed you meant “byzantine” in the sense of “highly complicated; intricate and involved,” as attested in definition 5b at Dictionary.com. FWIW, Dictionary.com doesn’t attest the usage indicated above.

Paul Hoffer August 17, 2006 at 1:25 pm

Who cares whether there are nine planets or a hundred planets? What I want to know is why do we have to name the planets and asteroids and other celestial bodies out there after pagan gods and goddesses that never existed in the first place? Why couldn’t we name the planets after virtues or something?
Seriously, though, who is going to write more movements for Holst’s “The Planets”?

Paul Hoffer August 17, 2006 at 1:28 pm

One more thing~we don’t deserve any more planets. We can’t take care of the ones we have.

J.R. Stoodley August 17, 2006 at 1:37 pm

Kennedy,
In Norse mythology the Jotun are connected with cold and winter. Especially the “ice-giants.” They are often called giants in English or even trolls (since they were one of the origins of the Medieval Scandinavial trolls) and they are connected to the Anglo-Saxon Ents (giants), and likely the Finnish jatti (giants) and Tibetan Yeti (hairy giants). Yet I am of the opinion that evil-gods or glutton-gods or maneating-gods would be a better translation.
You would have to chose one of them, maybe that guy Thor killed dressed like a bride, just cause it’s a funny story and makes him one of the more famous Jotun. I forget his name.
In any case though I am torn between an ethnic/cultural desire to see the Norse gods exalted and a sense that it is best to indeed keep the planets one “family” had make them all Roman gods. The name “Xena” definitly needs to go, though.

J.R. Stoodley August 17, 2006 at 1:38 pm

Paul Hoffer,
You’re no fun. Don’t take away my gods in the sky.

Marc Lewandowski August 17, 2006 at 2:00 pm

The other bit of this criterion that I don’t like is that to count as a planet an object must not be “a satellite of a planet.”
[…]
But it doesn’t have to be that way. If you had two equally massive objects orbiting–if there was Earth and Counter-Earth, let’s say–then the barycenter would not be inside either of them but between them.
That gets us away from a natural kinds definition, and I don’t like that. Basing whether something is a planet on what its neighbors are like is just scientifically inelegant. Planethood should be intrinsic to the planet itself, not conditional on the other members of its orbital system.
Now, I know darn well why the IAU included this condition. There’s a very specific reason: It’s to keep us from having to classify the Moon as a planet.
I think that’s dumb. It’s scientifically inelegant as it gets us away from a natural kinds definition.
Jimmy, this is just seriously wrong. Maybe you need to take a break from the SF, or something, I don’t know…
The other bit of this criterion that I don’t like is that to count as a planet an object must not be “a satellite of a planet.”
“Not your father’s moon any more…”
But it doesn’t have to be that way. If you had two equally massive objects orbiting–if there was Earth and Counter-Earth, let’s say–then the barycenter would not be inside either of them but between them.
… and they’d both be planets. So?
That gets us away from a natural kinds definition, and I don’t like that. Basing whether something is a planet on what its neighbors are like is just scientifically inelegant. Planethood should be intrinsic to the planet itself, not conditional on the other members of its orbital system.
ZZzzzt! A moon’s planet isn’t just it’s “neighbor”. They aren’t on the level of “peers”. They have a specific interaction which makes one the satellite of the other. A planet has moons. Not other planets. That’s just confusing.
Now, I know darn well why the IAU included this condition. There’s a very specific reason: It’s to keep us from having to classify the Moon as a planet.
Had it occurred to you that maybe it’s because the Moon is a moon? That the goal was keep moons in general from being reclassified as planets?
I think that’s dumb. It’s scientifically inelegant as it gets us away from a natural kinds definition.
Only if the definition of “sattelite” is arbitrary.
Based on that criterion, any object, no matter how much it looks like a planet–even one as massive as Jupiter–would cease to be a planet if it were pushed into orbit around a sufficiently massive neighbor.
1: Of course. Because then it would be a satellite, therefore a moon of another planet. 2: Then again, the only objects massive enough to have an object the size of Jupiter for a satellite are stars, so I think the point may be moot.
Put another way: What a celestial body is is more important than where the celestial body is.
This isn’t about “where,” but specifically how it is interacting with its surroundings.
I’d much rather bite the bullet and say, “Guess what, folk! We’re living in a twin-planet system and always have been: The Moon is our twin planet!”
Okay, so all planetary satellites large enough to be spheroidal are … planets? Seriously? Honestly, you walk into a crowded room populated with normal human beings and say that out loud, you’ll have more holes in you than a sign that says “Abilene: 58 km”. Give up the Moon as moon? No, but thanks for playing.
“Blue planet… I saw you standing alone…”

Marc Lewandowski August 17, 2006 at 2:01 pm

That was much longer and crankier than it needed to be. It says something that this actually ired me so.

Anonymous August 17, 2006 at 2:10 pm

i hate every once in a while visitors who write long stuff

Maureen August 17, 2006 at 2:11 pm

Not all moons are planets, but some planets are moons.
It’s the feudal satellite relationship that makes something a moon, and the mass that makes one a planet.
Seems pretty simple to me.
Maureen, who thinks that if somebody else can say Byzantine, she can say feudal. Besides, words mean exactly what I want them to mean, neither more nor less.

Ry August 17, 2006 at 2:11 pm

Basing whether something is a planet on what its neighbors are like is just scientifically inelegant. Planethood should be intrinsic to the planet itself, not conditional on the other members of its orbital system.
Look at it this way, the IAU is defining a planet according to what it does (orbit a sun), not where it is in relation to other bodies.

Rosemarie August 17, 2006 at 2:16 pm

+J.M.J+
>>>In any case though I am torn between an ethnic/cultural desire to see the Norse gods exalted and a sense that it is best to indeed keep the planets one “family” had make them all Roman gods.
Not to nitpick, but wasn’t Uranus (Ouranos) technically a Greek divinity? They just used the Latin spelling of his name (The Roman equivalent of Ouranos was evidently Coelus, Roman god of the sky.)
Since we already have a Greek divinity amidst the Romans, maybe a Norse divinity would also be acceptable – unless they only want to keep the names within Greco-Roman mythology.
>>>The name “Xena” definitly needs to go, though.
Yeah, but Hercules might work…. :-)
In Jesu et Maria,

JohnH August 17, 2006 at 2:40 pm

So, in several million years, when the barycenter of the moon moves outside the earth (the moon’s orbit is slowly growing – it’s moving further away), the moon will become a planet?

francis 03 August 17, 2006 at 3:29 pm

I propose a new system of nomenclature: each orbit that contains at least one body large enough to have become spheroidal under the influence of gravity is called a “planetary system.” Most systems will be singletons, with just one planet, but others will contain binary planets or more (i.e., two spheroidal bodies orbiting each other, or one orbiting the other), and others will be just systems– i.e., the Kuiper belt, which might well include two small planets in the same orbit around the sun, but not oribiting each other. This way the Galilean satellites would be planets, but they would also be moons of Jupiter and part of the “Jovian system”– just as all the planets are part of the solar system. So we would have maybe 20-30 planets, but only 10 or 12 planetary systems.
Come to think of it, maybe this isn’t really a new system of nomenclature, but just a refocusing of the popular usages of these terms. Either way, I’m rather out of my depth here, but this IS fun to think about.

Rosemarie August 17, 2006 at 4:18 pm

+J.M.J+
>>>What I want to know is why do we have to name the planets and asteroids and other celestial bodies out there after pagan gods and goddesses that never existed in the first place? Why couldn’t we name the planets after virtues or something?
I remember reading somewhere that a medieval Christian astronomer tried to rename the twelve signs of the zodiac for the Twelve Apostles (Aries became St. Peter, etc.). Evidently, it never caught on.
If we rename the planets to remove the pagan names, we should by the same token rename the days of the week as well. After all, the seven days of the week were originally named after the seven planets: Sun, Moon, Mars, Mercury, Jove, Venus and Saturn. A remnant of this remains in most romance languages (except Portuguese) and even in English (though our Tuesday through Friday are named after Norse divinities).
The Byzantines, however, did throw off the pagan day names in favor of Judeo-Christian ones. Sunday became the Lord’s Day (Kyriake), Friday the Preparation Day (Paraskeve) and Saturday the Sabbath (Sabbaton) while Monday thru Thursday were simply numbered second through fifth (Deutera, Trite, Tetarte and Pempte, in case anyone was wondering).
Then we have the first eight or so months of the year, also pagan in origin. So they have remained throughout two millenia of Christianity. Even the Greek names retain some pagan influences in this case, I believe. For some reason, Christendom as a whole never quite saw the need to completely “Christianize” the calendar. Though some smaller groups tried, like the Quakers, I think.
In Jesu et Maria,

Puzzled August 17, 2006 at 4:49 pm

Very screwy of the IAU. Pluto is a KBO, not a planet.
It is a big ‘duh’.
And with the new criteria, an inch-wide ball of ice would be a planet.
So now we also have the Moon, Vesta and Juno, Io, Europa, Ganymede, Callisto, Titan, Rhea, Enceladus, Iapatus, Mimas, Chiron and Triton, Santa Claus and several other KBOs they for some reason didn’t mention, as planets.
So, kids just won’t be taught the planets at all.

Puzzled August 17, 2006 at 4:53 pm

BTW, there are probably hundreds if not thousands of Plutinos – KBOs that are spherical in shape and orbit the Sun.
The IAU is being goofy due to political pressure, I guess, and I contributed to a nomenclature paper to them once upon a time.

Bear August 17, 2006 at 5:34 pm

What I want to know is why do we have to name the planets and asteroids and other celestial bodies out there after pagan gods and goddesses that never existed in the first place? Why couldn’t we name the planets after virtues or something?
Most of the planets were already named after pagan gods long before Christianity ever came along. Now it’s tradition, so why not stick with it?

Bear August 17, 2006 at 5:37 pm

Puzzled,
And with the new criteria, an inch-wide ball of ice would be a planet.
No, because the new IAU definition says a planet by definition “has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape”

SDG August 17, 2006 at 6:03 pm

Something I haven’t seen anyone address yet: AFAIK, the first nine planets INCLUDING Ceres but EXCLUDING Pluto actually have something important in common: They are all “native” to our solar system. They share a common evolutionary origin with the sun, as evidenced by the way their orbits all occupy the same plane and move in the same direction, sort of like the rings of Saturn, but around the sun.
OTOH, the KBOs, including Pluto and Charon, are AFAIK not native to the solar system, and have no such common history with the sun. They just happened to be passing by and got snared by the sun’s gravity, as indicated by their eccentric orbits.
I’m not saying they shouldn’t be considered planets, at least in some sense, just as we might speak of “rogue planets” in deep space not orbiting a sun. But certainly there is a natural distinction to be made between our own planets and those from outside the solar system.
Ceres, OTOH, seems to be a real planet, sharing along with the rest of the asteroid belt a common origin with the sun and the other planets. So I would be in favor of a nine-planet system that includes Ceres but excludes Pluto and the KBOs. Though it’s fine with me if kids want to learn the KBOs too. :-)

Puzzled August 17, 2006 at 7:35 pm

Bear, but it would hold together electrostatically,and then freeze, and be held together by electroweak.
Jimmy, more than 150 real planets have been found in your lifetime, nearly all since 1996.

Sonetka August 17, 2006 at 8:18 pm

I can’t help wondering – whatever happened to Sedna? I remember a few years back there being a brief splash about a new planet being discovered and the name Sedna was being mooted – is this the Xena planet or was it something else?
Sedna was an excellent choice for a frozen planet, even if it broke the Greco-Roman string; if I remember my childhood reading of “Folktales From Around The World” correctly, Sedna was an Inuit goddess who was originally a mortal but whose father tried to sacrifice her to the ocean for some reason or other. She hung onto the edge of the canoe, trying to survive, and her father cut her fingers off – the fingers became whales and narwhals and various cold-water creatures, and Sedna sank to the bottom of the sea, where she continues to live and is the mother of all coldwater marine life. (I could be remembering details incorrectly, but I think that was the gist). For cold and creepy, you don’t get much better than that – though if they wanted to go with a Norse god, that would be appropriate also. I don’t actually mind the idea of breaking the Greco-Roman canon; they didn’t really have much in the way of cold-weather gods.

JGC August 18, 2006 at 12:08 am

In one of his science essays in Galaxy Science Fiction back in the 1960s, Isaac Asimov quipped that the Earth is a double planet because it is *pair-shaped*. The same would apply to Pluto-Charon.

J.R. Stoodley August 18, 2006 at 1:10 am

I am no fan (any more) of Isaac Asimov, but it may interest some of you that great discovery at the the last book in his combined Robot/Foundation series comes about from the sudden realization that the Moon is actually a planet.
When will the Asmovian horrors stop?

john F. Kennedy August 18, 2006 at 7:17 am

JR Stoodley;
What book was that in?

BenYachov(Jim Scott 4th) August 18, 2006 at 9:53 am

12 Planets??? HA!!!!!! I say HA!!!!!!!!
There should be THIRTEEN Planets!
Long live SEDNA the other Red Planet!!!!
http://www.gps.caltech.edu/~mbrown/sedna/
James Scott future Emperor of SEDNA!
(now if I could ONLY find a way to get there to claim my throne?).

Some Day August 18, 2006 at 5:12 pm

This is retarded.
I LEARNED MY NINE PLANETS IN ELEMENTARY AND I AM NOT GOING TO LEARN THEM AGAIN.
Retarded…

J.R. Stoodley August 18, 2006 at 11:59 pm

Rosemarie,
After all, the seven days of the week were originally named after the seven planets: Sun, Moon, Mars, Mercury, Jove, Venus and Saturn. A remnant of this remains in most romance languages (except Portuguese) and even in English (though our Tuesday through Friday are named after Norse divinities).
Actually, Tuesday through Friday are all named after Anglo-Saxon gods, though these are pretty much the same as the Norse gods, with dialect differences.
Tuesday is named after the god named, depending on your tribe, Tiw Tiu, Tio, or Tig (Norse eqivalent:Tyr). He was the ancient Proto-Germanic king of the gods, derived from the Indo-European king of the gods Dyeus, who also became Zeus, Jupater (from Dis Pater), the Irish Dagda, the Vedic (Hindu) Dyaus Pitar, and other Indo-European king-gods, and the words Theos, Deus, Baltic Dievas, Slavic Div, Hindu Deva, all meaning god, and the Zoroastrian daevas (demon). Tyr was still considered the bravest of the gods by the Norse and associated with justice.
Wednesday is named after Woden (Norse equivalent: Odin), probably originally a wind god and/or god of the dead, who eventually replaced Tiw (Tyr, etc.) as the king of the gods for most Germanic tribes. He was associated with wisdom, song, magic, nobility, valor, etc. and to him the souls of the great warriors went after death. He also wandered over the earth in the form of an old man (likely a reflection of his former identity).
Thursday is named after Thunor (Norse equivalent: Thor) the god of thunder. Son of Woden. A great hero-god, arguably the most popular god among the common people. He is the source of the modern word “thunder.”
Friday is named after Frige (Norse equivalent: Freya) the goddes of nature, beauty, and sexuality and love outside of marriage. Also a great warrior (battle-maiden) whose symbol (the boar) warriors would put on their helmets for protection.
Saturday is comes from the Roman Saturn and Sunday and Monday are of course named after the sun and moon.
I think it would be a terrible robbery to deprive our culture of these linguistic ties to our past. Few people people today are tempted to believe in these gods so what is the harm. Besides, I greatly perfer pre-Christian culture to “post-Christian” culture. Pre-Christians are ready for evangelization, but those who consider themselves “post-Christian”?

J.R. Stoodley August 19, 2006 at 12:12 am

Kennedy,
The book I refered to is Foundation and Earth, the one where Trevize and Pelorat set out from Gaia to find Earth and the mysterious force suspected of living there. My account of the ending may or may not be considered exactly accutate, but I do not want to give away the details so I had to simplify. I would not recommend people read any of Asimov’s books, but I suppose I will let people decide for themselves and not ruin the ending for them.
oh, and I realize I mispelled “Jupiter” above. I was thinking too much of its origin, plus my spelling has always been bad.

patrick August 19, 2006 at 2:35 am

In Japanese, Sunday and Monday were named after the sun and the moon while Tuesday to Saturday were named after the Five Elements. Same goes for the planets: Sun,Moon,Mars,Mercury,Jupiter,Venus and Saturn (Fire, Water, Wood, Metal[Gold],Earth) in that order. The other recently discovered ones were named: Star of the King of Heaven (Uranus), Star of the King of the Sea (Neptune) and Star of the King of the Dead (Pluto), as you can see, the names of the recently discovered planets were titles from the Greek gods. I wonder what will the others be named.

Rosemarie August 24, 2006 at 3:22 pm

+J.M.J+
Well, they’ve officially demoted Pluto. Looks like we’ll now have to teach our kids “My Very Easy Mother Just Served Us Noodles.” Oh well.
In Jesu et Maria,

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