12 Planets?

[Umbran]

Other than history, pretty soon we'll see there's nothing that distinct about it compared to dozens, or hundreds, or possibly thousands of other rocky ice balls out beyond Neptune.

Well, there'll be something other than history to make Pluto distinct - it is large enough to have gravitational effects that can be detected, and both large and close enough to see with a decent telescope.

The proposal is, if I recall correctly, to have a separate category for the iceballs - "plutons". Not that having the number of "planets" be small is really a meaningful goal, but they can be excluded from some discussions as a separate class of object.

# Must a "round" object.
# Must be at least 800 km in diameter.
# Must be at least 1/12,000th the mass of Earth.

I think, more technically, the body has to have come to a round shape under the effects of it's own gravity - the size and mass limitations are there mostly because they describe the typical point at which this happens.

 

[ssampier]

I suggest we name the 2003 UB313 planet "Xenu" instead.

Oh, great. :\

All we need is a bunch of geekish colonists to call themselves Gygaxians.

Well, he was one of the few "things" left after the Universe collapsed on itself.

 

[occam]

How is it that different from the current situation?

Drop Pluto, and make a one-time change. Add Pluto and an ever-expanding number of other small things... It's completely different, that's the point.

Oh, come now, that's historical cussedness talking too... "We've only ever had 9 planets! We don't want more! In fact, let's get rid of one!"



But that's the whole point... We're learning that there's a lot more floating around our solar system than we ever thought.

Sure, but does that mean they're all "planets"? We don't feel compelled to call comets "planets", why does Pluto need to be one, other that "historical cussedness"? Actually, with this proposal, if there were a big enough comet, it would be a "planet".

 

[occam]

Well, there'll be something other than history to make Pluto distinct - it is large enough to have gravitational effects that can be detected, and both large and close enough to see with a decent telescope.

That's an accident of placement. Just because Pluto's closer doesn't make it fundamentally different than all the other KBOs out there.

I think, more technically, the body has to have come to a round shape under the effects of it's own gravity - the size and mass limitations are there mostly because they describe the typical point at which this happens.

But "roundness" is a completely meaningless distinction. It doesn't help categorize objects according to how they were formed, orbital characteristics, rotational characteristics, composition, ... i.e. things that actually matter. Should Ceres really be in a whole different class of objects than other asteroids? Should Pluto (and Charon) really be classified differently than the uncounted horde of slightly smaller objects in the Kuiper Belt?

 

[Umbran]

That's an accident of placement. Just because Pluto's closer doesn't make it fundamentally different than all the other KBOs out there.

Who said that classifying a thing as a "planet" meant it was fundamentally different from all other classes? Quite the contrary - most planets also fit into other classes. Some planets are gas giants. Others are rocky worlds. Why not have a dirty iceball planet?

Accidents of placement should not be overlooked - the Jovian moons are going to remain classified as satellites, though in size and structure many of them are very planet-like. The difference is only accident of placement.

But, to handle exactly your problem, the official category of "plutons" is part of the new definition.

But "roundness" is a completely meaningless distinction. It doesn't help categorize objects according to how they were formed, orbital characteristics, rotational characteristics, composition, ... i.e. things that actually matter.

But "planet" never officially referred to any of those things anyway, so I don't see the problem. A couple of things you mention - like rotational characteristics and composition - I don't feel have anything to do with the characterization as a planet. When Earth, tiny resonant and eccentric Mercury, and sideways gas giant Uranus are all covered by the definition "planet", the definition has to be rather broad.

That an object has enough mass that gravitation overcomes rigid-body forces so it can come to a form of hydrostatic equilibrium (and thus be round) says some significant things about the object.

Should Ceres really be in a whole different class of objects than other asteroids?

I think Ceres is a very good illustration of the fact that any definition clear enough to be useful will have annoying edge cases. The Universe is only occasionally digital - it will create a continuum of objects no matter how finely we decide to split hairs between distinct classes. No matter what the definition, we can find or imagine a case that will be troublesome. So, I'm not too concerned about it.

 

[occam]

Who said that classifying a thing as a "planet" meant it was fundamentally different from all other classes?

What's the point of a classification other than to draw essential distinctions? The distinction of roundedness isn't scientifically interesting, in and of itself.

Quite the contrary - most planets also fit into other classes. Some planets are gas giants. Others are rocky worlds. Why not have a dirty iceball planet?

No, they don't. Planets are not asteroids, nor comets, nor meteors, nor stars, nor moons. These are all orthogonal categories, that relate to details of formation, composition, etc. That the category of "planets" may also be subcategorized into gas giants and rocky planets doesn't change that.

The proposed definition of "planet" only means "big, orbiting a star, but not a star itself". How is that useful? You still need a way to refer to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune as different from everything else in the Solar System, including Ceres and any large KBOs (including Pluto and Charon). What do you call them, then? "Planets that aren't also asteroids or KBOs or large examples of some other things?" Or do you just call them planets, with the unspoken caveat that you really don't mean to include Ceres?

Accidents of placement should not be overlooked - the Jovian moons are going to remain classified as satellites, though in size and structure many of them are very planet-like. The difference is only accident of placement, not one of just placement.

The large Jovian moons, and Titan, and Triton and other large moons (including Earth's) formed differently than the planets around which they orbit. It's an important distinction.

A couple of things you mention - like rotational characteristics and composition - I don't feel have anything to do with the characterization as a planet. When Earth, tiny resonant and eccentric Mercury, and sideways gas giant Uranus are all covered by the definition "planet", the definition has to be rather broad.

Limiting the discussion to objects in the Solar System*: Planets orbit the Sun. They all orbit within the ecliptic (not counting Pluto), and in the same direction. They all have nearly circular orbits (not counting Pluto). They all have substantial atmospheres (not counting Pluto, or Mercury, which has extenuating circumstances). They all probably formed in a similar manner (possibly not counting Pluto). Taken together, those characteristics help define a useful category of objects (which can be further subcategorized).

* No doubt, future discoveries in other star systems will muddy the waters, but we're not quite there yet.

That an object has enough mass that gravitation overcomes rigid-body forces so it can come to a form of hydrostatic equilibrium (and thus be round) says some significant things about the object.

No, it doesn't, other than that it happens to be big (or more specifically, massive). By itself, it doesn't provide any real insight into the processes of formation, so limiting the definition of planethood to this one thing (OK, two things, with the star-orbiting criterion) doesn't further any discussion. You still need to make further distinctions, so what's the point, other than to allow Pluto to retain its legacy status?

I think Ceres is a very good illustration of the fact that any definition clear enough to be useful will have annoying edge cases.

Pluto is the edge case! Heck, it's not really even an edge case; it's been an oddball inclusion from the start. Drop it, and there's no further confusion (in this star system, at least).

 

[Pbartender]

What's the point of a classification other than to draw essential distinctions? The distinction of roundedness isn't scientifically interesting, in and of itself.

You are greatly mistaken. Being naturally round is an rather extraordinary phenominon, no matter how it happens.

The proposed definition of "planet" only means "big, orbiting a star, but not a star itself". How is that useful?

Just as useful as calling a star, essentially, "a self-luminous gaseous spheroidal celestial body of great mass which produces energy by means of nuclear fusion reactions"...

Or an animal "any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials, in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation".

How are those useful?

You must start with a broad definition, then categorize by type... Rocky dwarfs, Gas giants, Icy dwarfs, etc...

No, they don't. Planets are not asteroids, nor comets, nor meteors, nor stars, nor moons. These are all orthogonal categories, that relate to details of formation, composition, etc. That the category of "planets" may also be subcategorized into gas giants and rocky planets doesn't change that.

So what? All these guys are doing are stipulating the rules of the various orthogonal categories and then re-evaluating current know bodies in the solar system, and re-categorizing them. Why is that such a problem?

You still need a way to refer to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune as different from everything else in the Solar System, including Ceres and any large KBOs (including Pluto and Charon).

Why? There's no reason for it.

What do you call them, then? "Planets that aren't also asteroids or KBOs or large examples of some other things?" Or do you just call them planets, with the unspoken caveat that you really don't mean to include Ceres?

Likely, people will simply continue calling them the "major planets", though that won't technically be an official classification.

The large Jovian moons, and Titan, and Triton and other large moons (including Earth's) formed differently than the planets around which they orbit. It's an important distinction.

Perhaps, but it's not normally a very good way to classify something... Maybe in geology, but certainly not in astronomy, a subject in which it is generally very difficult to determine the origin of any object you are looking at.

Consider stars, for instance... They are primarily classified by what they ARE (size, mass, temperature, spectrum, elemental composition, etc...), not by what they used to be, or by how they came to be.

Limiting the discussion to objects in the Solar System*: Planets orbit the Sun. They all orbit within the ecliptic (not counting Pluto), and in the same direction. They all have nearly circular orbits (not counting Pluto). They all have substantial atmospheres (not counting Pluto, or Mercury, which has extenuating circumstances). They all probably formed in a similar manner (possibly not counting Pluto). Taken together, those characteristics help define a useful category of objects (which can be further subcategorized).

Again, so what? There's more than one way to classify any given group of things.

Just because you've only ever eaten red apples, doesn't mean that a green aple isn't an apple.

By itself, it doesn't provide any real insight into the processes of formation, so limiting the definition of planethood to this one thing (OK, two things, with the star-orbiting criterion) doesn't further any discussion. You still need to make further distinctions, so what's the point, other than to allow Pluto to retain its legacy status?

So you propose to replace an easily measured and concrete qualification with a something that can, at best, be only theoretically guessed at? Because, the simple fact is, we don't really have a very good idea of how the planets formed, or whether they formed differently from all the other debris floating about the solar system.

And what would you do with the moon, which has an especially unique theory to explain its formation?

 

[occam]

The proposed definition of "planet" only means "big, orbiting a star, but not a star itself". How is that useful?

Just as useful as calling a star, essentially, "a self-luminous gaseous spheroidal celestial body of great mass which produces energy by means of nuclear fusion reactions"...

Or an animal "any of a kingdom (Animalia) of living things including many-celled organisms and often many of the single-celled ones that typically differ from plants in having cells without cellulose walls, in lacking chlorophyll and the capacity for photosynthesis, in requiring more complex food materials, in being organized to a greater degree of complexity, and in having the capacity for spontaneous movement and rapid motor responses to stimulation".

How are those useful?

Those are functional definitions, not based on one simple surface feature. They provide useful information about the subjects being studied, and separate them from other things that are fundamentally different and that developed in different ways.

Ceres is no different from other asteroids because it's round. There's nothing fundamentally different between Pluto and legions of other KBOs. Why should they be classified differently? It's a useless distinction to make.

All these guys are doing are stipulating the rules of the various orthogonal categories and then re-evaluating current know bodies in the solar system, and re-categorizing them. Why is that such a problem?

Because the proposed definition of "planet" is NOT orthogonal. It includes "true" planets, as well as asteroids and KBOs. Why do that?

You still need a way to refer to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune as different from everything else in the Solar System, including Ceres and any large KBOs (including Pluto and Charon).

Why? There's no reason for it.

From what we can currently hypothesize about the formation of bodies in the Solar System, the eight objects known as "planets" (not counting Pluto) all probably formed similarly. You can discuss theories of planet formation that apply to all eight. If you include Ceres and KBOs, you can't do that. So you still need a way to distinguish those eight objects. What do you call them if you don't call them just "planets"?

Perhaps, but it's not normally a very good way to classify something... Maybe in geology, but certainly not in astronomy, a subject in which it is generally very difficult to determine the origin of any object you are looking at.

Uh, so you're saying it's not worth classifying astronomical objects by supposed origin?

Consider stars, for instance... They are primarily classified by what they ARE (size, mass, temperature, spectrum, elemental composition, etc...), not by what they used to be, or by how they came to be.

They're classified by both. It just so happens that stars with common measurable characteristics have (or are presumed to have) similar origins.

Again, so what? There's more than one way to classify any given group of things.

Not every way is a useful one.

The characteristics common to "true" planets that I listed are useful. They refer to fundamental differences and advance scientific discussion by prompting questions about why these objects have these common characteristics.

I'll turn this question around: Why should the proposed definition be used to define a "planet"? How does the consideration of simple roundedness advance scientific understanding? Why should slightly smaller asteroids and KBOs that aren't completely round be excluded? How are they fundamentally different from round things, in ways that point to scientifically interesting distinctions in composition, development, etc.?

Anyway, it's now looking like the IAU is getting its wits about it, and will vote out Pluto in a day or two. One way or another, my daughter's bedside poster will probably soon be out-of-date. ;-)

 

[Umbran]

What's the point of a classification other than to draw essential distinctions?

There are many possible points to classification, and which one you're trying to serve can determine which distinctions you ought to be dealing with, essential or otherwise.

The proposed definition of "planet" only means "big, orbiting a star, but not a star itself". How is that useful?

Historically, the term "planet" has had nothing to do with details of formation, or composition. A term for "thing that orbits the sun that we can see reasonably easily" has served astronomers since the times of the Greeks, at least. What we have here is merely a small refinement on that.

You still need a way to refer to Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune as different from everything else in the Solar System, including Ceres and any large KBOs (including Pluto and Charon). What do you call them, then?

"Rocky world", "gas giant", "pluton", or "asteroid" as appropriate. The terms for three of these thigns already existed, and the fourth is up for adoption now. If you're aiming to make essential distinctions, then lumping gas giants and rocky worlds together is rather against your goal, as they are essentially different, in both composition and details of formation.

The large Jovian moons, and Titan, and Triton and other large moons (including Earth's) formed differently than the planets around which they orbit. It's an important distinction.

Some would argue that Titan, and several other large moons, may have formed more like a planet than Luna. And neither have much similarity to the origins of Phobos. So, the term "moon" or "satellite" really has little to do with formation. It's a matter of orbit, and that's about all.

Limiting the discussion to objects in the Solar System*: Planets orbit the Sun. They all orbit within the ecliptic (not counting Pluto), and in the same direction.

We already have several examples of extrasolar planets that orbit with largish inclinations, so that's already falling through as a distinction.

They all have nearly circular orbits (not counting Pluto).

Mercury's eccentricity is nearly that of Pluto. Most known extrasolar planets appear to have high eccentricities. So I'd say goodbye to that criterion as well if I were you.

They all have substantial atmospheres (not counting Pluto, or Mercury, which has extenuating circumstances).

That depends upon what you call "substantial". Mars has only a wisp, less than that of the smaller, colder body of Titan. I think Mars and Mercury long ago removed atmospheres from the general definition of "planet" in the eyes of the scientific community.

They all probably formed in a similar manner (possibly not counting Pluto).

Possibly, possibly not. Since we don't know, we really shouldn't be talking about excluding it yet on this basis, now should we?

By itself, it doesn't provide any real insight into the processes of formation, so limiting the definition of planethood to this one thing (OK, two things, with the star-orbiting criterion) doesn't further any discussion.

It does - but it only deals with very basic discussion, which is reasonable for a very basic classification. There are already terms for the finer distinctions, so there's no need to try to cram them into "planet".

 

[Pbartender]

Those are functional definitions, not based on one simple surface feature. They provide useful information about the subjects being studied, and separate them from other things that are fundamentally different and that developed in different ways.

As does the proposed definition of a planet. To use the IAU's own words, "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 a satellite of a planet."

That's a pretty fundimental definition there.

Ceres is no different from other asteroids because it's round.

Yes, it is. Ceres is different because is "has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape". Other asteroids don't. Therefore, Ceres is fundimentally different than other asteroids.

There's nothing fundamentally different between Pluto and legions of other KBOs. Why should they be classified differently?

They aren't, necessarily. There's a short of of about a dozen other KBOs that could qualify for planethood jsut as easily as Pluto, if the resolution passes.

Because the proposed definition of "planet" is NOT orthogonal. It includes "true" planets, as well as asteroids and KBOs. Why do that?

You're looking at it wrong. The proposed definition includes a few objects that we used to consider asteroids and KBOs, and would be promoted to planets, if it passed.

From what we can currently hypothesize about the formation of bodies in the Solar System, the eight objects known as "planets" (not counting Pluto) all probably formed similarly. You can discuss theories of planet formation that apply to all eight. If you include Ceres and KBOs, you can't do that. So you still need a way to distinguish those eight objects. What do you call them if you don't call them just "planets"?

But it's all just thories, and that's the point. What happens if in 20 years we discover that the theories aren't quite right, and planets, asteroids or KBOs didn't form in similar manners or even in the way we thought? We'd have to reclassify everything, since we based or original classification on something that we weren't certain of to begin with.

Uh, so you're saying it's not worth classifying astronomical objects by supposed origin?

Yes... Precisely because you have to include the word "supposed" in that sentence.

They're classified by both. It just so happens that stars with common measurable characteristics have (or are presumed to have) similar origins.

Yep... and that's a lucky coincidence.

Not every way is a useful one.

Nope... And in astronomy, supposed origin is not a very useful one.

The characteristics common to "true" planets that I listed are useful. They refer to fundamental differences and advance scientific discussion by prompting questions about why these objects have these common characteristics.

I'll turn this question around: Why should the proposed definition be used to define a "planet"? How does the consideration of simple roundedness advance scientific understanding? Why should slightly smaller asteroids and KBOs that aren't completely round be excluded? How are they fundamentally different from round things, in ways that point to scientifically interesting distinctions in composition, development, etc.?

There's your problem. Categories aren't meant to advance discussions and prompt questions. They are simply there as an organizational tool. Something that can be used to definitively distinguish one set from another.

You can still discuss and ask questions regardless of whether or not a method of classification encourages that or not... A classification method will never prevent such discussion.

Consider the means of disguishing planets from other objects...

With regards to planets, the first stipulation writes itself: A planet must orbit a star.

After that, you need to set a minimum and maximum size. Again, the upper limit write itself... We already have definitions for stars, so if the object qualifies as a star, it's too big and is disqualified from being a planet. The lower limit is the tricky point. When you take a good look at planets and asteroids, the one fundemental break point in size is when an object has enough mass to make itself round... So that's what they used.

Finally, there are a lot of moons that could technically fit this description, so you add a moon stipulation... the common center of gravity.