and this too

He says

but i think it should be whatever blah blah

yes i agree

@110. Nigel Depledge :

4. We know of several KBOs that are almost as large as Pluto;

So what? We know of several rock dwarfs that are almost as large as Earth but that doesn’t stop Earth being a planet and we know of several gas giants in around the same region of space whose diameter approaches Jupiter’s but we’re not saying that axiomatically makes Jove a dwarf planet are we!?

That fails the qualifying for axiom on two groudns – it is, in fact, an irrelevant non-sequiteur and it is far from self evident as a logical starting assumption.

5. We have no idea what else we might find in the Kuiper Belt;

Same applies as above. Again so what, again an irrelevant non-sequiteur and not a self-evident logical assumption.

We may or may not find other larger planets in the Edgeworth Kuiper cometary belt. If we do they deserve to join the list of known planets. But then maybe we won’t. Either way that isnt self-evidently relevant to defining “planet.”

6. It is reasonable to suppose that any undetected objects orbiting the sun (in near-circular orbits) closer than the Kuiper Belt must be pretty small.

Again, so .. ? Also what if we find a Hot Jupiter in a very short circular orbit that never or very rarelytransist the Sun.

Now this is unlikely I’ll grant you – but its not impossible or unreasonable in the sense that it is not a reasonable concept.

Such a hypothetical solar Hot Jupiter could be in such a tight orbit that it never strays far enough from our Sun to be detected but is just far enough away that it isn’t noticed in SOHO or SDO images. It could have formed from another protoplanetary disk* at another angle that formed around our daytime star much earlier then migrated all the way into its just out-of sight orbit whilst the rest of that disk disappeared or fell into the one that formed our ecliptic planets – Sedna and “Buffy” could even be outer survivors of this second rightangle sprotoplanetary disk!

Highly, improbale and amazing that we wouldn’t have unambigously detected sucha world. Indeed but it isn’t impossible or unreasonable. Improbable and unreasonable are not entirely identical notions remember!

* Stars with more than one protoplanetary disk have in fact been found although perhaps none yet quite as extreme as that hypothetical Solar hot Jove forming scenario!

given the axioms that:

So what’s an axiom – lets check with that fount of allknowlegde wikipedia (click on my name for link to source.) :

In traditional logic, an axiom or postulate is a proposition that is not proven or demonstrated but considered either to be self-evident or to define and delimit the realm of analysis. In other words, an axiom is a logical statement that is assumed to be true. .. (snip) .. Logical axioms are usually statements that are taken to be universally true..

Now then lets see if these are really axioms :

1. We have an obvious and natural discontinuity in our solar system;

Really?

Well, we do have the Sun and then the planets but I don’t think that’s what you mean is it?

We have Jupiter and that contains most of the solars ystems mass and then we have all the rest of the planets from Saturn to Ceres which, uh, don’t.

We have gas giant type planets, ice giant planets, rock dwarf planets and ice dwarf planets which have various similarities and differences.

We have planets close to the Star and further out and in the middle dividing roughly into rocky, gassy and icy (ice -rock mixes technically) categories but all being round by gravity, non-self luminous from nuclear fusion and not moons.

You have inner planets – Mercury and Venus without any moons and outer planets from there onwards with moons (except for Ceres or so we now think! )

So you have a whole range of possible “obvious and natural discontinuities” to choose from.

Which is important and relevant? Which is worth choosing as a defining feature? This is far from axiomatically clear!

Choosing that as an axiom would be to making a false assumption and creating a false axiom that does not stand up to scrutiny and questioning. You would say, I guess that the divide between the realm of ice dwarf planets and others is all important and most relevant – but I totally disagree and say the common features the ice dwarf planets have is more important than their differences just as the very large differences between the rock dwarfs and gas giants does not lead to the conclusion that Earth or Jupiter are other than planets.

2. We know of many exoplanets, but know very little about any of them;

That rather belittles the huge amount of study and knowledge that exoplanet hunters like Sara Seager, Debra Fischer and Geoff Marcy have obtained for us through their intellectual sweat and hard laour doesn’t it?

Its also wrong – we do indeed know quite a lot about exoplanets and our knowledge is improving all the time.

That “axiom” of yours too falls into the false assumption – and far from axiomatically obvious – category.

3. We have no idea when we might accrue detailed information about a representative cross-section of exoplanets (remember that our current knowledge of exoplanets is dramatically skewed towards those that are easiest to detect);

See above. Look at teh kepler, Corot and other sites exoplanet~wise. We’re getting a lot of results and information really rather quickly and, in any case, that isn’t axiomatically relevant to the question of how to define planet.

This is not an analogy.

It is an analogy. You may not consider it an apt one perhaps – but I do.

The IAU is not a dictator – it is a collection of professionals.

A collection of professionals that is, in my view, in this instance behaving badly and dictatorially in trying to impose a ridiculously bad definition on everyone else. Collections of professionals can act badly and be wrong too y’know.

“A flawed process that isn’t representative, that doesn’t hear equally from the various sides is just NOT going to result in a fair and reasonable definition and does need to be reviewed properly as opposed to unquestioningly defended.” -MTU
You don’t know this. It might result in a definition that is fair and reasonable. Or, at least, as fair and reasonable a definition as it is possible to achieve with our present level of knowledge.

Is that a possibilty? Perhaps it is. But is it *likely* – I doubt it. Is the way of arriving at such a definition dubious and in need of reviewing including setting aside the definition reached that way and reassessing it in a proper debate where all sides get a fair go? Too right it is!

That’s why there are retrials on dodgy court cases y’know! If the judges are biased and certain key evidence and witnesses and even defence lawyers get omitted or prevented from making their case then a convicted criminal can rightly appeakl for teh original verdict to be quashed and a retrial or pardon ordered. That’s analogous to the Pluto case here.

This implies that those IAU members who feel that they were not adequately consulted either accept the definition and are just getting on with their work, or are a small enough minority that they themselves are not representative of the IAU as a whole (otherwise, they could have got the definition changed subsequently).

.. Or just that those with power in the IAU have a vested interest in trying to maintain the status quo and are simply unwilling to admit they made a mistake even when it is very obvious to many that they have.

You have claimed it is illogical in several ways, but your arguments fail. Your argumentation is illogical,

On the contrary my argumentation is very logical and my arguments are good ones whether you are willing to accept that or not.

.. and it has convinced me that the IAU have done the best job that anyone could,

Well that saddens and baffles me because I fail to see how you can have reached that erroneous conclusion.

given the axioms that:

That word ‘axiom” I do not think it means what you think it means!

For technical uise among professional astronomers, yes.

So, what about everyone else? Or are we to have two definitions of planet one strict and absurd one for the IAU and another for the public that is broader and actually makes more logical sense? Hmm ..

You have not made any kind of case that the IAU’s decision to define the term “planet” is unethical. Instead you have suggested that the way in which the definition was introduced was unethical.

See the above comment and point (9) comment #29. If something is as undemocratic, unfair and generally dubious as the IAU definition was then trying to foist this controversial and unpopular definition upon everyone else is surely verging on unethical – okay not major league physically hurting people unethical but unethical nevertheless.

Picking on the smallest planets and denying they even *are* planets also strikes me as unethical. In the same way that you don’t discriminate against dwarf people and deny them their personhood and humanity it seems to me you also should avoid discriminating against small planets and denying them their planethood and full planetary status.

No, it doesn’t. It’s [the undemocratic , flawed process - ed.] a completely separate issue.

No its not a separate issue and I’ll note that again you dodged the question and refused to give a straight answer.

I believe that the only reason you cite the way in which the change was introduced as an argument against that change is because you disagree with the change, not because you have some fundamental objection to a change that is introduced in such a way.

The fallacy of attributing motivations and strawmanning on your part. I asked before and you refused to answer so I’ll ask again for you to look at it from the other angle:

If the IAU had (correctly) defined Pluto – and Eris, Ceres, Sedna etc .. as proper planets would *you* be arguing against that now?

Turn it around – if they had used the same approach and tactics to introduce a change that you favour, would you as vehemently argue against it? I very much doubt it.

If I thought the process at arriving at a even a good definition was flawed – unfair and undemocratic – then I’d certainly admit that I thought the process, the *method*, of arriving at the definition was flawed and undemocratic.

Wouldn’t you agree that is the case here; that the way the decision was arrived at – at the last minute with many of the key actors such as Alan Stern absent and with a minority of a minority making a decision that effects everybody – at the very least is dubious and raises a lot of questions over its validity?

Look again at and mull over more, please, the facts as noted in point (9) comment #29 :

For instance, of the 10,000 IAU members only 2,500 attended the 2006 Prague meeting that demoted Pluto and rejected the other planetary candidates, Eris, Charon and Ceres from planetary status. Furthermore, of those 2,500 only the merest handful – just 424 actually got to vote making therefore a very unrepresentative decision. Among those to excluded from voting and arguing their case in that last minute meeting were some highly relevant and articulate people – notably Pluto expert Alan S. Stern.

Would a debating team be considered legitimate winners of a debate if they maliciously prevented the other side from showing up and participating at all?

The present definition has been in place for nearly 6 years now. I am convinced that if enough IAU members disagreed with it, they would have moved to change it.

Slavery and various forms of racial, sexual and other discrimination were in place for many decades indeed centuries too. Did that at any stage make those things right?

Same applies here – the length of time a bad idea or definition lasts doesn’t make it any less bad.

The IAU’s refusal to change their bad definition shows that they are too proud and not willing to accept the reality and because they are the self-appointed authorities here doesn’t make them right in this case.

****

PS. Have to head off now. I’ll return to this again once home.

I totally disagree – if Pluto *clearly* fell into the “Other Stuff” category then it would’nt have ever been considered a planet – but it has been, correctly, considered one since its discovery in 1930 up to today when many people still , correctly, reject the IAU’s ruling and we wouldn’t be having this debate at all.

You keep saying that the boundary between dwarf planets and classical planets is clear but your repeating this assertion doesn’t make it true and I’ve already pointed out how it is in fact NOT at all clear.

If Pluto orbited closer to the Sun – say where Mars is – and thus had a clear orbit it would unquestionably be considered a full planet.

If Mercury, Earth or even Jupiter orbited out in the Cometary Cloud they couldn’t clear their orbits and so would be labelled mere dwarfs.

Clearly then the difference between dwarf planets and IAU-classical ones is merely what part of the solar system they inhabit and this is NOT good or reasonable grounds for distinguishing them.

I am baffled by the fact that you appear unable to grasp this basic truth.

If “many” professional astronomers objected to the definition, it would have been changed

Change takes time and especually when the current old guard have impose dit wrongfullyand thus have a vested interest inmaintaining it. It may well happen that it is changed to restore Pluto and add the other ice dwarfs- the sooner the better! Indeed aren’t you the one arguing that the IAU definition will indeed eventually have to be changed to allow for exoplanets?

,You *still* have not shown this. Stop using childish name-calling in place of argumentation.

It isn’t name-calling it is an accurate analogy. Calling something that is ridiculous “ridiculous” is entirely justified by the facts. Calling a definition that is bad “bad” and explaining how it is bad is NOT name-calling. I have made my case to you here as best I can – if you refuse to accept my logic as has been presented repeatedly for you here then that is your problem not mine. You calling me a “name-caller” and “childish” for pointing out the metaphorical emperors nudity is wrong and a false allegation. I expect better from you.

Why should professional astronomers listen to your whining?

Because I’m right. :p

Also it isn’t “whining” but legitimate criticism and there’s no law or reason to exempt the IAU or indeed any scientific body from legitimite rational criticism.

Yes it is. It is an analogy of an organistation making a definition that is ridiculous

You *still* have not shown this. Stop using childish name-calling in place of argumentation.

and the IAU definition is in my view

But you don’t matter, do you? Why should professional astronomers listen to your whining?

and many others ridiculous.

No. Not many. A few.

If “many” professional astronomers objected to the definition, it would have been changed.

It is saying that an object that clearly is a planet

No, it is not “clearly” a planet. What Pluto “clearly” is is the largest known KBO.

Stop assuming your goal can be used as a part of your argument.

– Pluto – is something else. Just as saying that something that is clearly a protein is something else would be ridiculous.

But what is “clearly” a protein?

We loosely classify phosphorylated proteins, glycosylated proteins and myristoylated proteins as proteins, in the same category as proteins that undergo no post-translational modification. Nothing is “clearly” a protein, except under common useage of the term, which is actually pretty vague. There is no exact boundary between a “mere” polypeptide and a protein. Insulin, for instance, has 52 amino-acid residues. Sometimes it is considered to be a very small protein, and others it is considered to be a peptide. There is no clear boundary.

We don’t have the same issue in our solar system. We have a clear boundary, and Pluto quite naturally falls into the “other stuff” category.

@105. Nigel Depledge :

1) Don’t *you* have any misgivings over the undemocratic, unfair way this Prague IAU definition was imposed ?

Irrelevant.

Really? I think it is very relevant and I think you’re trying to evade an answer. You even admit you haven’t looked into this. I think the way this definition was arrived at casts a very big shadow of doubt over its validity.

No, it doesn’t. It’s a completely separate issue.

I believe that the only reason you cite the way in which the change was introduced as an argument against that change is because you disagree with the change, not because you have some fundamental objection to a change that is introduced in such a way. Turn it around – if they had used the same approach and tactics to introduce a change that you favour, would you as vehemently argue against it? I very much doubt it.

The present definition has been in place for nearly 6 years now. I am convinced that if enough IAU members disagreed with it, they would have moved to change it.

You keep saying the IAU has the right to define planet as it chooses,

For technical uise among professional astronomers, yes.

that theprocess is entirely separate fromthe result but, to use an analogy, a dictator may have a legal right under his nations laws to execute politiical dissidents, violate human rights and even destroy cities in his own nation but that isn’t considered valid ethically or reasonable conduct by anyone outside of the dictator and his most blinkered followers.

This is not an analogy.

The IAU is not a dictator – it is a collection of professionals. Collectively, they have the right to define the terms they use in their profession (with no implication that the general public must also use such terms).

If a dictator has given himself the “legal” right to do all sorts of unethical things, this would be analogous to a body of professinals giving themselves the right to do some similarly unethical things. You have not made any kind of case that the IAU’s decision to define the term “planet” is unethical. Instead you have suggested that the way in which the definition was introduced was unethical.

A flawed process that isn’t representative, that doesn’t hear equally from the various sides is just NOT going to result in a fair and reasonable definition and does need to be reviewed properly as opposed to unquestioningly defended.

You don’t know this.

It might result in a definition that is fair and reasonable. Or, at least, as fair and reasonable a definition as it is possible to achieve with our present level of knowledge.

Not especially that the IAU definition is not set in stone, and that no subsequent meeting (to my knowledge) has moved to change the defintion. This implies that those IAU members who feel that they were not adequately consulted either accept the definition and are just getting on with their work, or are a small enough minority that they themselves are not representative of the IAU as a whole (otherwise, they could have got the definition changed subsequently).

The definition does indeed need to be reviewed rationally, and not unquestioningly attacked.

You have claimed it is illogical in several ways, but your arguments fail. Your argumentation is illogical, and it has convinced me that the IAU have done the best job that anyone could, given the axioms that:
1. We have an obvious and natural discontinuity in our solar system;
2. We know of many exoplanets, but know very little about any of them;
3. We have no idea when we might accrue detailed information about a representative cross-section of exoplanets (remember that our current knowledge of exoplanets is dramatically skewed towards those that are easiest to detect);
4. We know of several KBOs that are almost as large as Pluto;
5. We have no idea what else we might find in the Kuiper Belt;
6. It is reasonable to suppose that any undetected objects orbiting the sun (in near-circular orbits) closer than the Kuiper Belt must be pretty small.

Acknowledge what? If the IAU definition is so inevitably going to change – and Ithink italready needs changing – then as I’ve already said , let’s make it better right now.

How?

Your preferred definition has at least as many flaws as the IAU one, and perhaps one of those flaws is so immense that it would render the system unworkable – that you insist on including exoplanets about which we know almost nothing be included in the definition.

What is wrong with waiting until we actually know what we’re talking about?

Let’s have a better definition that applies to exoplanets and one that inlcudes ice dwarf type planets justas it includes gas giant, gas dwarf and rock dwarf type ones. “This factor” as you call it seems to argue more for my side of this issue than yours!

No. Just because you refuse to acknowledge that our ignorance about exoplanets matters does not change the fact that it does.

Are they really? I’ve already answered the “uniqueness issue by saying that uniqueness isn’t a criterion and that there’s almost certainly nothing ‘unique’ to any planet in our solar system.

Obviously, there are plenty of similarities to point out, but each of the 8 main planets is unique in several ways. Certainly, each of these 8 objects is unique within the region of its orbit. And perhaps this is enough.

We already know of many worlds analogous to Jupiter and Neptune – albiet most Neptunes found lie much closer to their sun than ours does.

Well, we assume from their masses that they are anaolgous, but we don’t know.

That is part of why exoplanets should not yet be included in a definition.

Now, okay, we havern’t foudn another Earth or Mars analogue yet but we fully expect to do so one day and have no good reason to rule such exoplanets out.

And what do we do in the meantime?

I can quite easily see that the IAU definition is a good, workable stop-gap to cover the period in which we know that many exoplanets exist but we have very little definitie information abotu them.

Pluto’s only uniqueness stems from its position as the largest known KBO and that it has the largest moon (proportionally) of any known object, and these are trivial things.

Are they really? I’ve already answered the “uniqueness issue by saying that uniqueness isn’t a criterion and that there’s almost certainly nothing ‘unique’ to any planet in our solar system. We already know of many worlds analogous to Jupiter and Neptune – albiet most Neptunes found lie much closer to their sun than ours does. Now, okay, we havern’t foudn another Earth or Mars analogue yet but we fully expect to do so one day and have no good reason to rule such exoplanets out.

While I might not have chosen to restrict the definition of “planet” to our solar system, I can see a perfectly good reason for doing so, and I can see that the definition is almost certain to change as new information comes in and new discoveries are made, be they in our Kuiper Belt or in other planetary systems around other stars. This factor by itself neutralises one of the main points you have raised, yet you do not acknowledge this.

Acknowledge what? If the IAU definition is so inevitably going to change – and Ithink italready needs changing – then as I’ve already said , let’s make it better right now. Let’s have a better definition that applies to exoplanets and one that inlcudes ice dwarf type planets justas it includes gas giant, gas dwarf and rock dwarf type ones. “This factor” as you call it seems to argue more for my side of this issue than yours!

@105. Nigel Depledge :

1) Don’t *you* have any misgivings over the undemocratic, unfair way this Prague IAU definition was imposed ?
Irrelevant.

Really? I think it is very relevant and I think you’re trying to evade an answer. You even admit you haven’t looked into this. I think the way this definition was arrived at casts a very big shadow of doubt over its validity.

You keep saying the IAU has the right to define planet as it chooses, that theprocess is entirely separate fromthe result but, to use an analogy, a dictator may have a legal right under his nations laws to execute politiical dissidents, violate human rights and even destroy cities in his own nation but that isn’t considered valid ethically or reasonable conduct by anyone outside of the dictator and his most blinkered followers.

A flawed process that isn’t representative, that doesn’t hear equally from the various sides is just NOT going to result in a fair and reasonable definition and does need to be reviewed properly as opposed to unquestioningly defended.

This is not a parallel.

Yes it is. It is an analogy of an organistation making a definition that is ridiculous and the IAU definition is in my view and many others ridiculous. It is saying that an object that clearly is a planet – Pluto – is something else. Just as saying that something that is clearly a protein is something else would be ridiculous.

So, to answer your question – if the IAU defined “planet” in such a way as to render all 8 of the existing planets not planets any more, I would indeed oppose such stupidity, in the same way that I would oppose a move to redefine “protein” as “virus”.

Actually if you apply the IAU criteria strictly and fairly that is exactly what it does. No planets orbit is clear as comets and asteroids cross all worlds paths from Pluto’s to Mercury’s even tody, our planets probably swapped orbits in the past and a rogue planet entering our solarsystem from deep space could theoretically cross our paths and make all our planets ludicrously into dwarfs by its mere presence. In addition the ability to clear an orbit depends pretty much entirely on where a planet orbits – a world close in on asmall orbit can clear that easily even if its smaller than Ceres whereas even Jupiter couldn’t clear out the Ooort cloud if it happened to wander out there.

Far from making the definition “more precise” as you suggest, what the orbital clearence (OC) criteria really does is make it much*less so* and raise a tonne of questions over what is meant by “clear” and so on as I’ve already mentioned right from the start. A precise definition is one thing but Occam’s razor says a definition that requires a answering a whole new set of questions more precisely as the OC criterion does is just NOT logically acceptable and must be rejected. Roundness can be easily determined – literally at a glance – and depends only on the object itself whilst this is not the case for OC which depends on location, the amount of material in as planetary system, the orbital trajectories of other bodies, et cetera.

@106. Nigel Depledge :

Yes, you have, mostly. What you have not addressed – or have ignored – is that most of your reasons do not withstand scrutiny.

You make that claim but cannot supported. Everything you claim I’ve not addressed or ignored I have already countered and answered for you. I have pointed out multiple times here where My reasons here *do* withstand scrutiny whether you accept that personally or not.

You have claimed that the gravitational clearance criterion is silly or illogical, and you cite hypothetical scenarios to illustrate your point but you have failed to demonstrate the relevance of those scenarios.

See comment # 100 – & # 99 & #68 and, well, think about it. I have indeed shown how the OC is illogical and sillyand how it fials teh Rcutio test and backed that up with many relevant examples. Your refusal to acknowledge this, Nigel, does no credit.

Your objections seem to me to be fundamentally emotional in nature.

Yes, because using logic methodology such as Occam’s Razor, Reductio ad absurdum and calling for consistency in matching how we use the term “dwarf star” with how we use the term “dwarf planet” is fundamentally just emotional. Yeah ri-ight.

Why may I ask Nigel do you seem so emotionally attached to defending the IAU’s absurd and terminally logically flawed definition?

What do you have against Pluto and the other ice dwarf planets that you wish to regard them as less than proper planets in their own right?

I really think I’ve done that in the planet~loads of comments I’ve written here since. I’ve explained what I think & why.

Yes, you have, mostly.

What you have not addressed – or have ignored – is that most of your reasons do not withstand scrutiny.

You have claimed that the gravitational clearance criterion is silly or illogical, and you cite hypothetical scenarios to illustrate your point (“If Earth were in the Kuiper Belt . . . ” or “If Ceres orbited a different star by itself . . . ” etc.) but you have failed to demonstrate the relevance of those scenarios. You have claimed that any definition of planet must include exoplanets but you don’t really have a rational case – you simply claim it is ridiculous for it not to include exoplanets. Bear in mind, of course, that the IAU has no intention of imposing their definition on common useage – the new definition is a precise definition for technical use.

You have objected to the gravitational clearance criterion on the grounds that it is vague (how clear is clear, and what volume does a body’s orbital region encompass), but you ignore that the gravitational roundness criterion is similarly vague – so, being logical, one must either object equally to both for this reason or object to neither. To object to one on the basis of its vagueness and not the other is illogical.

You cite public recognition as a criticism of the IAU definition, but you ignore that any precise definition of “planet” would either include objects of which the public at large is not cognisant, or exclude at least one object that has been considered to be a planet for several decades. Unless the definition is simply “a member of this group of nine objects”, which really would be pointless (although probably workable).

You seem deliberately to ignore the hefty dose of pragmatism that is – to me – obvious in the IAU definition (it is clearly imperfect, and it contains aspects that are adequate when applied to our solar system but easily shown inadequate by hypotheticals – but it was not devised to include other solar systems) . You also ignore the natural and obvious discontinuity that exists among objects that orbit our sun (and which the IAU definition recognises).

You have cited the way in which the change was introduced as an argument against the change, but do not acknowledge that this is a separate issue from the change itself.

You have cited Pluto’s superficial resemblance to some of the planets (atmosphere, weather, seasons, possession of moons, likelihood of active geology) with no acknowledgement that these characteristics are not exclusively planetary. An analogous claim would be that anything with 2 eyes, 2 legs and 2 wings is a bird, when such characteristics also encompass bats.

And so on. See most of my posts above for why I object to the arguments you propose.

I’m not sure what more you want or what I need to explain further on this. Clearly we still disagree but I don’t think you can say I haven’t tried or given my reasons!

Well, this is true enough, but you are moving the goalposts.

Your objections seem to me to be fundamentally emotional in nature. You certainly have used a fair amount of rhetoric in expounding them (such as your unjustified derisive references to the IAU definition). What I have not seen you make is an objective, rational case.

The closest you get to a rational objection is that the gravitational clearance criterion is vague, and I have already explained why this is not a good argument against the IAU definition.

Incidentally, I don’t think you ever did answer my earlier questions to you & would appreciate you doing so :

No, I had too little time available. And you started answering my responses to your early-fifties comments before I had even got as far as the late fifties, never mind getting through the 60s.

1) Don’t *you* have any misgivings over the undemocratic, unfair way this Prague IAU definition was imposed ?

Irrelevant.

The way a change has been made is a separate issue from whether the change is beneficial or not.

To be frank, I have not investigated to my satisfaction whether the accusations hold water, so I don’t feel sufficiently well informed on that point to comment. But I do know that I don’t need to know it to comment on whether or not the change is for the better or for the worse.

If anything, your rants against the IAU over Pluto have convinced me that the IAU definition is reasonable. I won’t say it’s perfect, and it has obvious limitations – but those limitations could easily have been deliberately incorporated – but it is useful and workable, despite your protestations to the contrary.

2) If this had been a decision / definition that you were unhappy with would you not point out the poor procedure and flaws that occurred in making it yourself?

I don’t know. Maybe. I would like to think that I can separate in my mind the changes themselves from the way in which they were made.

&

3) If the Biochemical Society was to decide that ‘proteins’ now meant viruses instead would you really be happy to go along with such stupidity? Honestly?

This is not a parallel. The IAU has not defined “planet” as “something that sustains nuclear fusion in its core”, they have merely introduced precision where previously none existed.

As it happens, “protein” already has a reasonably precise definition, but it could be more precise. For example, if my colleagues in the Biochemical Society were to collectively decide that a protein needed a minimum of 100 amino-acid residues (condensed in linear sequence) to count as a protein and that a linear polymer of 99 amino acids was a mere polypeptide, I’m sure there would be some arguments against the move, but I would at least try to recognise the reasons for the change. The IAU’s definition of “planet” is much more along these lines than the example you give.

So, to answer your question – if the IAU defined “planet” in such a way as to render all 8 of the existing planets not planets any more, I would indeed oppose such stupidity, in the same way that I would oppose a move to redefine “protein” as “virus”. However, if the Biochemical Society were to attempt to make the term “protein” more precise, then I would probably support such a move.

We can keep going around here but I don’t think we’re going to agree.

It seems that way.

You contend that the IAU definition is silly (etc.) but your arguments do not sway me.

You seem to feel it necessary for the technical definition of “planet” to be universally applicable, whereas I see a need for the application of pragmatism (i.e. accommodating what we do and don’t know about such objects).

You seem wedded to the idea that whatever definition is used, Pluto must remain a planet. I have no idea why you are so wedded to definitions that include Pluto as a planet. AFAICT, it is merely the biggest known KBO of many. And we can’t even be sure that it really is the biggest, because there are bound to be many more KBOs that we have yet to detect. Sure, Pluto bears some superficial resemblances to some of the other planets, but it is nowhere near as significant an object in terms of the influence it exerts within its own region of space. (And, yes, you can quibble about the terminology and about the subjectiveness of such factors, but that ignores the point).

I have no idea why you even refuse to address one of my main points, that the IAU definition recognises that Mercury, Venus, Earth, Mars, Jupiter, Saturn, Ouranos and Neptune are each unique in a variety of ways, but that Pluto isn’t. Pluto’s only uniqueness stems from its position as the largest known KBO and that it has the largest moon (proportionally) of any known object, and these are trivial things.

While I might not have chosen to restrict the definition of “planet” to our solar system, I can see a perfectly good reason for doing so, and I can see that the definition is almost certain to change as new information comes in and new discoveries are made, be they in our Kuiper Belt or in other planetary systems around other stars. This factor by itself neutralises one of the main points you have raised, yet you do not acknowledge this.

What makes you think you have any right to dictate to professional astronomers the language with which they work?

Well for starters, professional astronomers disagree about this themselves. It isn’t just me who thinks the IAU definition is idiotic – it is also Alan Stern and a number of others.

Secondly, I’m passionate about astronomy, its an area I’ve followed and particpated in for most of my life and so I think I *do* have some idea what I’m talking about here.

Thirdly, there’s just plain common sense that a decision as silly as the IAU’s anti-Pluto one is NOT immune from criticism however authoritative the group making it may be. As I put it in comment #70 :

..to refer back to my earlier anology – if the MAAOW decided to call pogosticks “wheelchairs” and vice versa wouldn’t you say the way they did that was wrong and the change unacceptable?
Just because you are an organisation of experts in a particular field does NOT mean that there are no limits on your power and does NOT mean that you can change everybody’s language in a way that is plain silly.
The Geologists Association can’t change the word “rock” to mean air.
The Zoological Institute cannot change the word animal to exclude birds from being animals.
Similarly, the IAU cannot change the word “planet” to exclude Pluto.

Incidentally, I don’t think you ever did answer my earlier questions to you & would appreciate you doing so :

1) Don’t *you* have any misgivings over the undemocratic, unfair way this Prague IAU definition was imposed ?

2) If this had been a decision / definition that you were unhappy with would you not point out the poor procedure and flaws that occurred in making it yourself?

&

3) If the Biochemical Society was to decide that ‘proteins’ now meant viruses instead would you really be happy to go along with such stupidity? Honestly?

Plus I’ll ask you again to answer my final question from comment #93 :

What would it take to convince you that Pluto – and the other ice dwarfs – should be counted as planets?

Remember too, please that all this started with you asking in comment # 24 here :

MTU, please explain, using reason rather than emotional appeals, why you still consider Pluto to be a planet as opposed to merely the largest known KBO.

I really think I’ve done that in the planet~loads of comments I’ve written here since. I’ve explained what I think & why. I’m not sure what more you want or what I need to explain further on this. Clearly we still disagree but I don’t think you can say I haven’t tried or given my reasons!

How can one come up with a useful defintion of a thing if you have 8 examples about which you know a great deal, and 600-odd examples about which you know
next to nothing?

I think its unfair and, indeed, wrong to say we know “next to nothing” about exoplanets. In fact we know a remarkably large amount even though we haven’t yet sent spaceprobes to them – which will, natch take centuries or aeons unless we find a way around the lightspeed limit someday.

Also as it happens, there is no precise dividing line between a star cluster and a galaxy, so an official definition of “galaxy” might not be too far away.

If so, then let’s hope they do a much better job than they did with defining planet!

My point there was that it is silly to define any astronomical category in a way that limits it only to our own solar system. Defining “planets” in a way that limits them to our solar system is equally silly as defining Galaxy to mean only our Milky Way. Other stars will have a range of objects orbiting them – asteroids and comets and planets. There’s more than enough evidence showing that. Saying that we can’t call the planets of other stars – and planets belonging to no stars – “planets” by definition is just beyond dumb.

Public perception does not really bear on the matter. If it did, your preferred definition would be just as unsatisfactory as you perceive the IAU definition to be (because your preferred definiton would include objects that most members of the public have never heard of and might never be able to pronounce – yes, Quaoar, I’m looking at you).

But pronouncing Quaoar is half the fun! “Kwah-wah” is I believe the correct way.

Why aren’t the public hearing about these things? Part of it is the media being lousy and having the wrong priorities sure but I also think that maybe we’d hear more and have more interest if it was anounced that astronomers are discovering new planets in our solar system which many of these Trans-Neptunian Objects I think rightly are. I think downgrading them to mere dwarfs, saying in essence these things aren’t planets when they’re in fact members of a class of small planets is not helping astronomy’s cause with the general public.

It would be if your analogy were valid, but it is not.

Well, you don’t think its valid but I do.

What is ridiculous about confining a precise definition to stuff you know something about?

Thing is, again, I think we do know enough about planets. Certainly enough to say that exoplanets should count – and so should ice dwarfs like Pluto. Yes we know more about some planets than others but I think we do know what mass range and what characteristics generally make something a planet or not.

Some of these objects are merely examples of collections of similar objects, while others appear to be unique.

Unique in what sense though? Jupiter and Saturn are not unique in terms of being big gassy planets with much in common and also their own unique features. Same applies to Earth, Mars and, yes, Pluto too. Uniqueness is NOT part of the IAU definition or anybody elses far as I know anyhow.

the IAU has chosen a particular path, i.e. to exclude objects about which we have too little knowledge, and to ensure that a “planet” is a significant object within the context in which it is found.

Significance and context is in the eye of the beholder. How significant is significant? Where do we draw the line?

Earth is found in a region containing a number of similar rocky planets, ditto Jupiter in a regional context containing gas giants and Pluto in a region with similar icy and rocky worlds. You seem to think the number of similar worlds is important, that because Pluto-type planets are more numerous this should rule them out of being planets at all but I don’t.

Pluto has significant features in common with other planets – moons, an atmosphere, core-mantle-crust internal structure, weather, perhaps rings, etc .. You don’t think these indicate that it should be considered a planet but I do.

We can keep going around here but I don’t think we’re going to agree.

How many times now have I pointe dout to you that the very fact that the IAU’s miserable excuse for a definition of planet is limited to our own solar system only is a terminal flaw in the definition on its own?

So, you would ignore the vast difference between the amount we know about our solar system, and the paucity of our knowledge about any other solar system?

This is significant. How can one come up with a useful defintion of a thing if you have 8 examples about which you know a great deal, and 600-odd examples about which you know
next to nothing? I hesitate to speculate about what kind of situation you might end up with, but it seems crazy to me to try to produce a precise defintion of something where knowledge is so limited. Thus, we have a definition that applies only to our solar system.

I am sure that, at some later point, the IAU will come up with a definition that might encompass exoplanets, but it would be folly to attempt this without quite a bit more knowledge about these things.

How about galaxies, Nigel Depledge,

‘S okay, you can call me Nigel.

shall we make our Milky Way the only galaxy that exists in the cosmos by an equally silly act of linguistic idiocy?

If our galaxy were the only one about which we had any detailed knowledge, then maybe. As it happens, we know as much about some of our near neighbours as we do about our own galaxy because a substantial portion of our galaxy is hard to see. Also as it happens, galaxies are classified mainly by morphology, so your point is purely academic. Also as it happens, there is no precise dividing line between a star cluster and a galaxy, so an official definition of “galaxy” might not be too far away.

Why don’t you get this!?

Why do you think it makes sense to try and precisely define objects about which we know next to nothing?

It seems to me perfectly reasonable to leave exoplanets until we know a lot more about them, and to restrict the definition of “planet” to cover the only solar system about which we have detailed knowledge.

A planet is a planet regardless of whether it orbits our sun – in common parlence and imagination and reality – if not in the IAU technical terminology (non)sense of the word.

And here we come to a key point.

Public perception does not really bear on the matter. If it did, your preferred definition would be just as unsatisfactory as you perceive the IAU definition to be (because your preferred definiton would include objects that most members of the public have never heard of and might never be able to pronounce – yes, Quaoar, I’m looking at you).

The IAU have arrived at a technical definition of planet for use in discussions of astronomical research. Do you for a moment expect the general public (by which I exclude amateur astronomers) to give two hoots what the IAU definition is? Similarly, if the IAU had used your preferred definition, such that Eris, Ceres, Makemake and several other objects would suddenly count as planets, do you think that the general public would suddenly feel any urge to learn about “the 14 planets” (or however many it might be)?

So, rather like many other words in science, “planet” now has a specific and precise definition for use in science, and some other, looser definitions for use in everyday speech.

I do not see this as an issue. It seems to me that you do.

Your comments make it seem that you believe the IAU should bow to popular public – non-technical – opinion. Yet you must recognise that, alongside terms like energy, power, radiation, theory, berry, nut and probably many others, technical scientific useage is different from uninformed public useage. I have said it before, and I’ll say it again – it is fitting that the IAU defines the terms they need to use in their profession. I’ve never claimed that the general public must use said terms, but it seems that you are insisting that the IAU use the public terms and not use precise definitions.

It is as silly as saying only our pet cat can be a cat or only the fish in our fishpond are actually defined as fish.

It would be if your analogy were valid, but it is not. Rather, cats are only cats if they are cats we know something about (such as, foe example, their possession of catlike characteristics). An animal that is probably rather like a cat, but about which we have no precise information, needs a different term. The same goes for fish. If you saw a pod of dolphins from a distance, and that rather distant and hazy view was all you had, would you call them fish, or would you conclude that you don’t know enough about them to define what they are that precisely?

To say that only our solar system has “definable planets” is just so ridiculous I cannot believe you type the words with a straight face.

Why?

What is ridiculous about confining a precise definition to stuff you know something about?

If the word ‘planet’ has any meaning at all it MUST include exoplanets and planets of stars other than our Sun as well as those that happen to orbit our daytime star.

Again, why? You have blustered a great deal, but you have not really made a case. We know that our solar system contains several different types of object. Some of these objects are merely examples of collections of similar objects, while others appear to be unique. To arrive at a useful definition, it seems that the IAU has chosen a particular path, i.e. to exclude objects about which we have too little knowledge, and to ensure that a “planet” is a significant object within the context in which it is found.

I am sure that, to the general public, your insistence is accurate enough, but why must the IAU use a definition that includes exoplanets?

What makes you thik you have any right to dictate to professional astronomers the language with which they work?

MTU (88) said : So are you actually denying the whole Big Splash theory then? The currently accepted scientific consensus on how the Pluto-Charon system formed? The Nice model of how our solar system became as it is today and much more? Honestly? Erm . . . what? How does emphasising that the IAU definition works with what is, as opposed to what might have been – or, indeed, what was likely to have been aeons ago – deny any theory about the formation of any solar system objects?

Because the Moon formed when a mars-sized planet and the Earth collided which in the IAU definition makes them both non-planets at least at the time they were on collision course. Which is plain silly. It is inconsistent and fails the Reductio ad absurdum test of logic to have a world stop being a planet when it is on collision course then become one again when it isn’t. Which was my point (4) on that original list of reasons.

Just as it fails the same Reductio Test of logic to have a world whether it is Pluto or Jupiter or Earth be a planet when it orbits close to the Sun having only a small area of space to clear but then cease to be a proper planet when it orbits further out and thus has more space to clear.

To be frank, I am not up to date on the current accepted theory for the formation of the Pluto-Charon system. In what way is it relevant?.

The exact same way as Earth’s Moon being formed in the Big Splash is. It is an example of colliding planets – and also presents an example of a binary planet as well. Both things I’ve already noted for you and have linked at comment #59. I’ve linked it to my name again here as well.

Really are trying to wind me up by ignoring everything I’ve said already or something?

Pluto is still the biggest of many chunks of icy stuff, rather than a unique entity.

But then Pluto doesn’t *have* to be unique to be a planet. Uniqueness isn’t demanded as a *defining* trait of planets any more than a planet’s exact temperature is. Pluto isn’t unique -agreed but then nor is Jupiter nor is Mars nor is 51 Pegasi b. Jupiter is just one of many Jupiter-like worlds out there in our cosmos. Ditto Mars for mars-like worlds. Ditto Pluto for Pluto-like ones. It just so happens we already know of the existence of many more worlds like Pluto – and even more Jupiter-like ones than we do than we know of worlds like Mars which I’m sure are out there to discover.

@98. Nigel Depledge:

The arguments you make do not hold water,

Well they’re not intended to hold fluid of any type whether dihydrogen oxide or otherwise but rather to prove my case on this logical proposition that ice dwarfs including Pluto are indeed planets and this I still believe they do.

Your failure to recognise that is your issue not mine.

I’ve raised 12 points in my original as-requested comment here. All of them are real and valid. The lack of counting exoplanets, the failure to permit planets to collide or exist in binary or trojan form and the failure of the IAU definition to pass the Reductio ad absurdum test are, I’d say, amongst the strongest points I make.

You have yet to point out any flaw and demonstrate that it is “fatal”.

On the contrary I have done so repeatedly and at great length and depth. That you are not satisified as to that is a mystery that Icannot fathom or it seems cure.

Your strongest objection to the gravitational clearance criterion (at least, I see this as the only real objection you have raised),

Actually, I’ve raised 12 points in my original as-requested comment here. All of them are real and valid. The lack of counting exoplanets, teh fialure to permit planets to collide or exist inbinary or trojan form and the failure of the IAU definitionto pass tehreductioo ad absurdum tets are I’d say amongst the strongest points I make.

.. that it is vague and ambiguous, is equally an objection to the gravitational roundness criterion, that you accept without question.

This again after beingdebunked already? Sigh. No it isn’t equal at all. Roundness is easy to deteremine, and a logical obvious intrinisc and consistent aspect that enables us to define ‘planet’ as I have explained for everyone in comments # 77 & 87 & 73 & #56 :

I [MTU} don’t think the two qualities are equal – I don’t find roundness is quite as arbitrary as a “cleared orbit” appears. Roundness is tangible, easy to calculate and it’s pretty obvious and immediately apparent. Roundness is the first thing people think of as a defining trait for what is a planet. If it looks round or gravitationally rounded by mass then it pretty much is. It just doesn’t raise as many questions as “orbital clearance” and it is intrinsic to the object rather than depending on the objects spatial and temporal relationships to other objects. I stand by that – again. Roundness is a lot clearer and raises less issues than “orbital clearing does.

Have you taken nothing that I’ve said in?

@94. Nigel Depledge :

Pluto is distinctly different from any of the 8 IAU planets because it is one member of a large collection of objects that orbit in the same region. It only makes sense that Pluto should be classed in a different category from those 8 planets.

The rocky planets inhabit the same region – from roughly 0 to 2 AU where they are all crammed in together.

Similarly the gas giants inhabit their own “gas giant belt” from 5 AU out to 30 AU.

Its true that there are far fewer of these larger objects but theprinciple remains the same – the fact that there’s more than one Pluto type planet out there should NOT disqualify it from being a planet any more than teh fact that Saturn exists should disqualify Jupiter or tehfact taht Mars exists should disqualify Earth from planethood.

Pluto – and Eris and Sedna have more in common with the other planets than they do with stars, asteroids and so on. See comment # 74 where I noted indicative features that you would expect a planet may have – and sure enough Pluto does :

My point is Pluto has the features and items you’d expect many planets to have. Moons – its got a quartet of them. Atmosphere – yep. Seasons and weather, yep. Rings? Maybe. Pluto then passes the duck test having many of the traits you’d expect to find make a planet. So if you are to disqualify it from planethood then you’re going to need something truly significant – and I don’t think a clear orbit is significant enough.

This also applies to varying extents to other ice dwarf type planets such as Haumea, Quaoar and Eris which, again, either have or almostcertanly have the sort of features – eg. moons, geologically differentiated internal strcutures (core-mantle-crust) you’d expect planets to have.

@ 95. Nigel Depledge :

As has already been pointed out to you, the IAU is free to change the definition in light of new evidence, ..

And, as I have already responded, there is already more than enough reason to state that the IAU definition should be changed rightnow and , infact should never have been adopted inthe first place.

The orbital clearance criterion only fails in your hypothetical scenarios – and it is obvious that this is your intent in constructing them. None of these scenarios is relevant to the solar system as it exists with our present knowledge, ..

Oh for pity’s sake! Didn’t you read all the real examples from our solar system that I provided in comment #68 :

..didn’t you see the examples which were specific to our solar system in comments #58 & #60 with links in #59?

Colliding planet examples from our solar system :

1. Earth & “Theia” leading to our Moon’s formation.

2. Pluto & another ice dwarf explaining Charon’s origin plus more.

3. Mercury’s Caloris basin & explainationfor its core:mantle ratio

4. Ouranos being tipped on its side -one well-known theory anyhow &

5. A huge collison is also theorised as explaining the disparity between Mars two hemisphere -the flater northern with the Vastitas Borealis ex-ocean and the older heavily cratered Southern hemisphere.

Binary planets in our solar system :

I. Pluto & Charon

II. Earth & Moon.

III. Various ice dwarf examples – incidentally a twin planet situation may be required to explain how Neptune captured Triton with Triton’s companion escaping and Trition being left in its unusual retrograde orbit typical of captured moons.

How many more times do I have to point this out to you?

No these examples are NOT purely hypothetical any mor ethan our Moon is and they are based on our present scientific understanding of how our Moon got there, how Charon, Nix, Hydra and P4 got there, why Mercury and Mars are as we see them today, etc .. Blink twice and nod your head if you can hear me, Nigel? Nigel? Can you hear me? Are you conscious?! (Sigh)

When a discovery is made that matches one of your scenarios, then I am sure the IAU will change the definition to match the level of knowledge that pertains at that time.

We’re already at that point – and I don’t assume the IAU won’t change their presently untenable definition; quite the reverse as that is exactly what I’m asking them to do!

Oh & is that a tacit admission on your part that the IAU definition will be changed to become better and, if so, why keep defending a definition you know is in need of replacement?

After all, we humans don’t have any other system of classification that is expected to be universally applicable for all time (although possibly the Periodic Table is the closest we have to such a thing).

How about the times tables – & other mathematical ones? How about teh broadest divisions of animal-mineral-vegetable

And as I’ve pointed out they are not only “free to” but they really need to do so because their current definition is fatally flawed.

You have yet to point out any flaw and demonstrate that it is “fatal”.

Your strongest objection to the gravitational clearance criterion (at least, I see this as the only real objection you have raised), that it is vague and ambiguous, is equally an objection to the gravitational roundness criterion, that you accept without question.

So, although you have claimed they need to change the definition now, you still have not come up with a serious rational argument for why they should do so. As I have explained over and over again. The arguments you make do not hold water, except for the one about the vagueness of the gravitational clearance criterion. But if this objection is upheld, then we lose the gravitational roundness criterion also, and then where would we be?

Ifs and buts don’t matter. The IAU definition deals with the solar system as it is, not as it might have been.

So are you actually denying the whole Big Splash theory then? The currently accepted scientific consensus on how the Pluto-Charon system formed? The Nice model of how our solar system became as it is today and much more? Honestly?

Erm . . . what?

How does emphasising that the IAU definition works with what is, as opposed to what might have been – or, indeed, what was likely to have been aeons ago – deny any theory about the formation of any solar system objects?

To be frank, I am not up to date on the current accepted theory for the formation of the Pluto-Charon system. In what way is it relevant? Pluto is still the biggest of many chunks of icy stuff, rather than a unique entity.

Exoplanets are completely irrelevant, for very good reasons. You keep bringing exoplanets into the discussion, but you do not address the reasons why they are irrelevant, and you have made no attempt (that I have seen thus far) to show them to be relevant to a definition that was clearly intended to apply only to our solar system.

How many times now have I pointed out to you that the very fact that the IAU’s miserable excuse for a definition of planet is limited to our own solar system only is a terminal flaw in the definition on its own?

How about galaxies, Nigel Depledge, shall we make our Milky Way the only galaxy that exists in the cosmos by an equally silly act of linguistic idiocy? Why don’t you get this!?

A planet is a planet regardless of whether it orbits our sun – in common parlence and imagination and reality – if not in the IAU technical terminology (non)sense of the word.

It is as silly as saying only our pet cat can be a cat or only the fish in our fishpond are actually defined as fish. To say that only our solar system has “definable planets” is just so ridiculous I cannot believe you type the words with a straight face.

If the word ‘planet’ has any meaning at all it MUST include exoplanets and planets of stars other than our Sun as well as those that happen to orbit our daytime star.

@94. Nigel Depledge :

Why should it not? Surely the whole point of defining a term is that the term should collect similar objects together and distinguish them from different things?

Indeed. Pluto is a planet because :

a) It is round because of its own gravity thus not an asteroid.
b) It is not & never has been self-luminous from nuclear fusion thus not a star

&

c) It does not directly orbit another planet and hence is not a moon.

It is thus defined by its simarities to other bodies that also fulfill those three criteria (eg. Earth, Jupiter, Gliese 581 b) and its differences from bodies (eg. stars, moons,& comets) that do not.

Orbital clearance is NOT a good defining criteria for the logical, scientific and semantic reasons that I have pointed out here already repeatedly.

Please don’t be deiberately obtuse here, Nigel, it really doesn’t suit you & I find it hard to imagine you really fail to grasp this reality and the implications this argument has for the IAU’s rubbish definition.

The implication of the argument is clear enough – that if your hypothetical scenario comes to pass, then the IAU definition breaks down.

So what? As has already been pointed out to you, the IAU is free to change the definition in light of new evidence, just as it was a sequence of new discoveries (especially Eris, IIUC) that prompted them to institute a precise definition in the first place.

Do you honestly believe that the IAU would refuse to change the definition if new discoveries were to render it unworkable?

My point is that it works for our solar system as it exists now – to the best of our knowledge.

What that shows and why we should care about it is that the orbital clearence criterion simply fails. We wouldn’t apply it to Earth or Jupiter or Neptune – so we shouldn’t apply it to Pluto either.

No, the orbital clearance criterion, complete with its intrinsic vagueness, works at a pragmatic level, in the same way that the gravitational roundness criterion – complete with its intrinsic vagueness – works. The orbital clearance criterion only fails in your hypothetical scenarios – and it is obvious that this is your intent in constructing them. None of these scenarios is relevant to the solar system as it exists with our present knowledge, and you seem to be tacitly assuming that the IAU would never change the definition in the light of new discoveries, which is unsupportable.

When a discovery is made that matches one of your scenarios, then I am sure the IAU will change the definition to match the level of knowledge that pertains at that time.

Therefore, your hypothetical scenarios do not constitute a rational criticism of the IAU definition in today’s solar system.

It shows that the IAU got it wrong.

Only if you insist on any definition being universally applicable for all time, which even you might agree is unreasonable. After all, we humans don’t have any other system of classification that is expected to be universally applicable for all time (although possibly the Periodic Table is the closest we have to such a thing).

It shows that orbital clearence as a qualifying criterion for defining what is a planet isn’t a workable or tenable notion. Period.

No, it does not. It shows that the orbital clearance criterion might not be workable as new discoveries are made in the depths of the Kuiper Belt, but it does not show that the definition fails now, unless you consider the gravitational roundness criterion to be equally faulty (i.e. they both contain a certain amount of vagueness and ambiguity and exist for the sake of pragmatism).

Being different shouldn’t stop it being counted as a planet

Eh?

Why should it not?

Surely the whole point of defining a term is that the term should collect similar objects together and distinguish them from different things?

and repeating the erronoeus assertion that it isn’t doesn’t make it become true.

Erm, well, I agree that repeating an assertion does not make it true, but that is what you are doing.

Pluto is distinctly different from any of the 8 IAU planets because it is one member of a large collection of objects that orbit in the same region. It only makes sense that Pluto should be classed in a different category from those 8 planets.

Just as the IAU can’t make a planet become a comet just by calling it one.

Well, no, but this is not relevant, because they have not done this. What they have done, which you might see if you detach yourself from your emotional attachment to Pluto, is recognise a natural difference that exists in our solar system by categorising Pluto alongside similar objects, and not leaving it in a category of objects to which it bears only the most superficial resemblance.