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Glenn E. wrote:that seems to have been deliberately created to exclude Pluto.

What is the evidence of this deliberate exclusion? A bright line was found, a logical and natural bright line, and used. Eight things lay on one side of it, not nine. That’s not deliberate exclusion.

With the discovery of lots of large-ish KBOs, a definition was needed.
• That definition could have been the current definition of a dwarf planet: directly orbiting sun, big enough to be rounded by gravity. That would make Pluto and may other KBOs into planets. Sure. Would you have preferred that? (I don’t strongly object.)
• Or it could be the sensible definition involving gravitational domination.
• Or, perhaps my favourite, it could be the previous “plus the nine objects that were, prior to 2006, called planets”.
• Or a better bright line. What?

jdaw1 wrote:

Glenn E. wrote:that seems to have been deliberately created to exclude Pluto.

What is the evidence of this deliberate exclusion? A bright line was found, a logical and natural bright line, and used. Eight things lay on one side of it, not nine. That’s not deliberate exclusion.

IAU: "(1) A "planet"1 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."

Random Astrophysicist: "Uh... you do realize that the Earth is no longer a planet if you use that definition, right?"

IAU: "Oh crap! Um... I know, let's change it to say '(c) dominates the neighborhood around its orbit.' There! All fixed!"

Random Astrophysicist: "..."

Does cleared mean cleared like I clear the kitchen (earth has massively out-performed), or like my mother-in-law does (it’s a tie)?

jdaw1 wrote:Does cleared mean cleared like I clear the kitchen (earth has massively out-performed), or like my mother-in-law does (it’s a tie)?

One must assume that since the IAU felt the need to clarify their definition from "cleared" to "dominated" that Mars, Earth, Jupiter, Neptune, and Pluto have all not "cleared" their orbits.

And for the record, I do not accept this as a given:

jdaw1 wrote:With the discovery of lots of large-ish KBOs, a definition was needed.

Why? Why does the discovery of lots of large-ish KBOs prompt a need for a new definition? Can they not be studied unless we know whether or not they are planets? Would the be studied differently if they were, say, comets or mere asteroids?

Glenn E. wrote:the IAU felt the need to clarify their definition from "cleared" to "dominated"

That was me, not the IAU. By cleared the IAU seems to mean that it is at least a dozen times chunkier than all other things in that orbit.

Glenn E. wrote:Why? Why does the discovery of lots of large-ish KBOs prompt a need for a new definition? Can they not be studied unless we know whether or not they are planets? Would the be studied differently if they were, say, comets or mere asteroids?

Because, I guess, folks asked whether they were planets, and the initial response — “vell, you know, planets are just these planety things” — was somehow thought unsatisfactory. So they wanted a bright-line as-clear-as-possible as-natural-as-possible definition. They didn’t start with “This is our chance to knife that Pluto thing in the back”; they started with a hunt for a seemingly natural definition.

IIRC, the original Stern–Levison paper used different words. It called ‘unterplanets’ things that the IAU calls dwarf planets; it called ‘überplanets’ things that the IAU calls planets. Same bright line; different labels.

jdaw1 wrote:

Glenn E. wrote:the IAU felt the need to clarify their definition from "cleared" to "dominated"

That was me, not the IAU. By cleared the IAU seems to mean that it is at least a dozen times chunkier than all other things in that orbit.

I believe I've seen that distinction being made prior to this conversation, so no I don't think it's just you.

jdaw1 wrote:

Glenn E. wrote:Why? Why does the discovery of lots of large-ish KBOs prompt a need for a new definition? Can they not be studied unless we know whether or not they are planets? Would the be studied differently if they were, say, comets or mere asteroids?

Because, I guess, folks asked whether they were planets, and the initial response — “vell, you know, planets are just these planety things” — was somehow thought unsatisfactory. So they wanted a bright-line as-clear-as-possible as-natural-as-possible definition. They didn’t start with “This is our chance to knife that Pluto thing in the back”; they started with a hunt for a seemingly natural definition.

They're KBOs. Pluto happens to be a KBO that we also call a planet. Eris is a scattered disk object; we haven't decided whether or not to call it a planet yet. Planet has no scientific meaning; it's just a label.

Aside: still haven't answer the question of what we're supposed to call all those objects orbiting other stars. Can't call them planets anymore because the IAU screwed up the definition, as often happens when doing something that really isn't necessary.

jdaw1 wrote:IIRC, the original Stern–Levison paper used different words. It called ‘planets’ things that the IAU calls dwarf planets; it called ‘Über-Planets’ things that the IAU calls planets. Same bright line; different labels.

Unterplanets and Überplanets.

Personally, I'd have left "planet" as a broad category that includes them all, with 8 "major" planets and a few (at that time) "minor" planets.

Win-win. Pluto is still a planet, but it, Eris, Ceres, Haumea, Makemake, etc can be distinguished by the Stern-Levison Parameter for those who care.

Planet → Major planet;
Dwarf planet → Minor planet.

That would have worked excellent for me. It maintains the physically meaningful Stern–Levison distinction, and keeps you happy. Win-win.

Hurray!!

Does that mean Pluto is a planet again?

How are we going to break the news to the world?

jdaw1 wrote:Planet → Major planet;
Dwarf planet → Minor planet.

That would have worked excellent for me. It maintains the physically meaningful Stern–Levison distinction, and keeps you happy. Win-win.

Yes. My problem with the IAU's decision is multifold, but one component is that a "dwarf planet" is not considered a sub-class of a "planet." With the original Stern-Levison distinction, they were all still "planets" with further sub-classes.

I think it is also notable that Stern himself does not support the IAU's definition.

The problem has shifted. It seemed previously to be the reasoning and formula behind the distinction. Whereas I thought the bright line to be in a well-judged place. Now it seems the problem is with the chosen names. Fine: names schmames.

Glenn is right. Particularly as he has the astronomical correspondent on side.

But please, carry on.

jdaw1 wrote:The problem has shifted. It seemed previously to be the reasoning and formula behind the distinction. Whereas I thought the bright line to be in a well-judged place. Now it seems the problem is with the chosen names. Fine: names schmames.

I think you are completely missing the point of Glenn's argument. There is no need for a line, bright or otherwise.

Someone decided to draw a line and then convinced some people that the line had meaning. Now people worship the line and believe it to be true whilst others think it to be just an invention that suits the needs of the inventor and his followers.

This is not the first time this has happened, but that doesn't make the invention necessary.

Question: ignoring their distances from the Sun and relative differences in the vastness of their orbits, does Pluto have more in common with Mercury than it does with Jupiter?

DRT wrote:Question: ignoring their distances from the Sun and relative differences in the vastness of their orbits, does Pluto have more in common with Mercury than it does with Jupiter?

Why ignore that manifest and long-lived fact?

jdaw1 wrote:The problem has shifted.

Not really. I have simply revealed yet another aspect of the problem. I still have problems with the formula, as well, such as the fact that it includes distance from the Sun. This means that there could be a Jupiter-sized ball of rock somewhere out there that would be considered a dwarf planet. Jupiter. A dwarf. Right.

The elephant is still in the room. According to the IAU, what do we call objects that orbit other stars? We keep discovering them. If it's so important to be able to refer to objects in the Kuiper Belt correctly, surely it is also important to be able to refer to objects orbiting other stars correctly, too.

And this is the actual crux of the issue. The entire thing came up because of arguments about what to name newly discovered objects. The IAU has naming conventions for newly discovered objects, and it is apparently critically important that we only name objects after the appropriate IAU-designated mythological group. Otherwise the universe will implode. Or aliens won't be able to find our address. Or something.

This required a vote of 9000+ astronomers? (Of which reportedly on 424 actually voted?)

Glenn E. wrote:This means that there could be a Jupiter-sized ball of rock somewhere out there that would be considered a dwarf planet. Jupiter. A dwarf. Right.

Correct. If Jupiter were more than 98 light years from the sun (yes, that far) it would indeed be a dwarf planet. Except that such a Jupiter would be outside the sun’s Hill radius, and so not in orbit around the sun at all. Obviously Jupiter’s cutoff distance is largest, but even Uranus would need to be ≥102,000 A.U. from the sun, so, again, outside the sun’s Hill radius. For a lump of rocky as piddly as the Earth the cutoff is 2,870 AU, which is still not small.

Glenn E. wrote:The elephant is still in the room. According to the IAU, what do we call objects that orbit other stars? We keep discovering them. If it's so important to be able to refer to objects in the Kuiper Belt correctly, surely it is also important to be able to refer to objects orbiting other stars correctly, too.

Entirely agree. Needs doing. same definition — albeit different words — please. Major, minor. Except that we can’t yet detect mid-size ‘planets’.

Glenn E. wrote:Otherwise the universe will implode. Or aliens won't be able to find our address. Or something.

If you want a Dark Stuff versus MoND dispute, start a different thread. I like MoND, so want somebody competent to explain why it just ain’t so.

Glenn E. wrote:This required a vote of 9000+ astronomers? (Of which reportedly on 424 actually voted?)

Lordy! And now you want me to rant about electoral systems? Really?

jdaw1 wrote:

DRT wrote:Question: ignoring their distances from the Sun and relative differences in the vastness of their orbits, does Pluto have more in common with Mercury than it does with Jupiter?

Why ignore that manifest and long-lived fact?

Because they are irrelevant to what the objects are. All of these objects formed through the coalescence of material orbiting a star. Rather than avoiding the question you could perhaps try to answer it. Which is more like Pluto: Jupiter or Mercury?

DRT wrote:Because they are irrelevant to what the objects are.

These things are not stationary lumps of rock under a deity’s microscope. Their movement is essential to the not-falling-into-the-sun trick. (Good trick if you can do it. Highly recommended.) The movement is part of what they are, where they are, how they formed, and the rest. So “irrelevant to what the objects are” is, at best, contentious.

I have often found in life that when someone repeatedly avoids answering a direct question it is because they know their argument is built on sand.

So, jumping to the next part of the argument, given that Mercury and Pluto are much more similar objects than either is to Jupiter (and both by an enormous margin), why would one not be a planet just because it is farther away from the Sun and therefore denied the opportunity to "clear its orbit"? It is a distinction that has less meaning the farther away an abject is from the Sun therefore the bright line is arbitrary and not related to the fundamental basis of what these objects actually are.

Glenn E. wrote:This required a vote of 9000+ astronomers? (Of which reportedly on 424 actually voted?)

Which gives the argument as much legitimacy and sense as a London Tube strike.

DRT wrote:Which is more like Pluto: Jupiter or Mercury?

Body	Λ	Log[Λ]	Distance Pluto
Mercury	1.93E+03	3.29	5.82
Jupiter	1.30E+09	9.11	11.64
Pluto	2.95E-03	-2.53	0

So Mercury is more like Pluto than is Jupiter, says the Stern-Levison parameter Λ.

DRT wrote:

Glenn E. wrote:This required a vote of 9000+ astronomers? (Of which reportedly on 424 actually voted?)

Which gives the argument as much legitimacy and sense as a London Tube strike.

Different questions. Is the Stern-Levison much more sensible than you Luddites seem to claim? Yes, definitely.

Was the process behind the decision entirely satisfactory? Oh, is that the time? And my glass is empty. Oh! Must deal with these things. {Wipes sweat from brow.}

DRT wrote:I have often found in life that when someone repeatedly avoids answering a direct question it is because they know their argument is built on sand.

The question explicitly makes an assertion — location irrelevant — which ain’t necessarily so.

DRT wrote:So, jumping to the next part of the argument, given that Mercury and Pluto are much more similar objects

Except that the one that is the planet is 25 times chunkier, butt up against the sun and almost tidally locked to it, and has no moons; whereas the non-planet is 25 times less massive and so far from the sun that most things that are gases at Port-drinking temperatures are instead lumps of solid ice, has a moon a very large chunk of its own mass and smaller moons too (this being possible because further from the sun), and has an orbit controlled by a planet named after a watery god.

So, not quite the same. Mostly because of location.

And, while we’re on the Mercury-Pluto comparison, a lump of rock as massive as Mercury but in Pluto’s orbit would be a planet, albeit with not much spare.

'The Sky at Night' has a Pluto special on now for the UK based side of the argument.

Your argument appears to be boiling down to size.

There are many examples in nature where size does not affect the consideration of whether or not two things are from the same class:

Is a Chihuahua a dog even though it might be 100 times less massive than a very large dog?

Is a Bonsai tree a tree?

Is Warwick Davis a person?

Is Ben Nevis a mountain?

Why would size be important to what can be considered a planet? Surely it would make more sense to define what properties a planet should have and then categories objects as planets if they have those properties? Being geologically active and not just a dead lump of rock covered in impact craters seems like a much better test than "small and far away".

I strongly suspect that if the pictures we have seen in the past few days had been available at the time of the vote Pluto would still be a planet.

LGTrotter wrote:'The Sky at Night' has a Pluto special on now for the UK based side of the argument.

I just watched it. It didn't change my view but did raise the question of whether or not Charon is also a Planet.