Isn’t it funny how things keep ending upon Pluto!

You’re running into the problem of the sorites: a bunch of things that add up, little by little, so that a very small difference pushes something from one (somewhat arbitrary) category to another. The canonical example is the category of “bald men.” I have a pretty full head of hair, at lest compared to most men my age; I’m not bald. If I were to lose one hair, I still wouldn’t be bal; lose two, and, well, that’s only a different oce one hair. Flash forward until I’ve alost all but one of my remaining hairs, and sure enough I’ll be bald. But you’ll be hard-pressed to find the time in between at which removing one hair moved me from “not bald” to “bald.”

I talked with Dan about this myself, a couple of years ago, with the idea that there were inflections or discontinuities in the distributions of sizes that might give us a hint. The problem there is that, at least at that time and based on an over-lunch conversation, there really aren’t; the distribution of object size versus number of objects seems pretty continuous.

Of course, if that’s true, it’s unlikely that any definition will be able to crisply determine whether an object is, or isn’t, a planet. No matter what, it’s likely that we’ll find there are objects that seem “planet-like” but which are just outside our definition.

You can create indexes showing big gaps in just about anything; you just have to taylor carefully what goes into them. [...] And the mass ratio is everything.

That isn’t what the paper says. It establishes a natural gap. (Yes, using mass ratio as I said, to establish clearing/mass dominance. And a time scale, again naturally characteristic.)

The neighborhood being dinamically decided is yet another absolute nonsense. Orbits shift, and they shift to the point that we just can’t know for sure at present where the planets orbited in the past.

That wasn’t the process I (and, IIRC, the paper; yes, it was a while since I read it) described. My very first comment mentioned the planetary formation process from the planetary disk.

As for your observation that some bodies may change status, it is exactly what I mentioned earlier in the analogy with biology. There it makes sense, and if we regard the planetary formation process as basis for defining planetary bodies it may make sense here too.

[But if we try to make the analogy exact, IAU is actually trying to establish an analogue to a 'species', in which case they will have to look for a closely related population. That, I assume would include all inner system originated bodies; planets, most asteroids and most moons.]

And the checking time is the age of the Solar System, which is why the index details the current state of affairs, not what happened in the past, nor any possible future state.

As the current state is contingent on the historical path, I don’t see how one can argue that it isn’t a result of a dynamic process. [Btw, again this is very analogous to biology, because species are historically contingent on the dynamic process of evolution. But I'm not arguing that is necessarily a good thing.]

Thank you, SteveoR, for your very comprehensive list of why the IAU vote was a travesty and why Pluto, as well as Ceres, Eris, MakeMake, and other dwarf planets should be considered a subclass of planets. With the IAU having made such a mess, I hardly trust them as the “experts” in this field. In fact, most IAU members are not planetary scientists, and most planetary scientists are not members of the IAU. That leads to the inevitable conclusion that planetary scientists should form their own organization to deal with defining the objects they study.

I was at the Great Planet Debate, and Stern makes a compelling case for hydrostatic equilibrium as the one criteria that unites both “major” or “classical” planets and dwarf planets and separates them from asteroids or small solar system bodies. Objects at the mass where they can attain hydrostatic equilibrium are fundamentally different than inert asteroids–they are differentiated geologically, the same way the terrestrial planets are. Yes, they are dynamically different, in that being smaller, they don’t dominate their neighborhoods. That’s why they are classified as dwarf planets, but it is not reason to state that they are not planets at all.

Phil, Pluto DOES fit the bill for being a planet. You argue that science shouldn’t be driven by public opinion but don’t mention that neither should it be driven by fiat or decree of a small insular body whose members have their own personal agendas. I believe that when New Horizons gets to Pluto, we will have sufficient data to illustrate Pluto’s planetary features that there will be no need to rely on a decree because “someone says so.” The same is true for Ceres regarding Dawn. In the meantime, the public is fully capable of understanding that the issue is a subject of ongoing debate with two differing schools of thought and that there is no one “right” answer, at least until 2015.

For the lower limit it should be a fixed, specific number, based on parameters that the majority can agree are important. It will be arbitrary, but not excessively so if the parameters are chosen carefully, and all definitions are arbitrary at their extremes, and we accept that exceptions may occur (and in fact provide some of the most interesting science).

I would humbly propose using the roundness by self-gravity as a basis, but remove the uncertainties associated with composition and how round is round enough by basing the definition on a hypothetical planet with fixed properties. For example, we could use the minimum mass required for a body of 100% pure hydrogen, with no rotation, and no outside forces of any kind acting upon it, to shape itself into a perfect sphere. This would make the definition of planet fall in line with the definitions for stars, and would reflect the reality that objects in the universe fall along a continuum of size and mass.

(I chose hydrogen because it is the simplest and most common element, but one could choose other elements, such as iron, (endpoint of exothermic fusion), as well)

Anything greater than this minimum mass, regardless of actual shape, size, or location, can then be called a planet. We could avoid the potentially embarassing circumstance of one day discovering an object that should be big enough to be spherical, but isn’t. (say a Jupiter sized gas giant that is oblong because of rapid spin, or distorted because it is orbiting a neutron star, or an earth-sized object that isn’t spherical because it was involved in a big collision recently and hadn’t had time to become spherical again, but we don’t know these details yet because we’ve just observed the object and don’t know anything beyond shape and mass)

We could have separate subclasses for large moons and ejected planets.

No, it simply means you don’t understand what was meant by ‘orbital clearing’. As I said, the people who wrote this imprecise (for the lay public) language knew precisely what was meant because both Soter and Stern & Levison (and others) had defined it in at least two ways: as a characteristic time scale and as a mass ratio. Read Soter’s paper that I linked above. He also provides the caveat that his suggested characteristics would not be useful in an immature solar system.

And I am not taking definite sides (because nature doesn’t care a wit about “sides”), except to suggest strongly that our choice of labels on our boxes should “reflect natural relationships, not to obscure them.”

I have no difficulty using the word “planet” as long as it has a qualifier to reflect the fact that some objects belong to separate populations (by origin, by history, by dynamical importance, by…) than others. And no matter how this matter is settled, nature simply does not care (and will happily provide exceptions) — what matters is that our choice of language minimizes any obscuration and maximizes our understanding of our understanding of the phenomenon.

“we define an überplanet as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals …And we define an unterplanet as one that has not been able to do so,…”
and a little further…
“our Solar System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.”

Those were Alan Stern’s words! (in the 2002 paper with Harold Levison, with the purpose of defining what is meant by “planet”). That’s the point I was trying to make!

StevoR said:
“Whats this Soter saying here – that Pluto is a gypsy sub-planet & Neptune from the superior planetary master race or some such tripe?! Come on!

This seems to be just a pluto-haters rant, nothing much else is clear about Soter -whoever he is. Give me Alan Stern’s words instead anyday! ”

Sigh – these aren’t a scientifically useful arguments.

Thanks,
Tony

Oh, I know that the index shows a big gap, blah, blah, blah. You can create indexes showing big gaps in just about anything; you just have to taylor carefully what goes into them. If you wanted to exclude Mercury from planetary status, all you had to do was creating an index of atmospheric pressure versus body size, and there, you would’ve kicked Mercury from the group of planets. I’ll grant you that this one is a bit more significant than that, but is just as biased towards inner system bodies as the atmospheric index would be towards outer system bodies. It is, however, and as I’ve said numerous times, useful to separate two distinct groups of planets in the current state of our system, however arbitrary it might be. But does not work for any other and there are billions and billions of planets around other stars.

And the mass ratio is everything. It’s at the very base of the index. Did you read the paper? Apparently not.

The neighborhood being dinamically decided is yet another absolute nonsense. Orbits shift, and they shift to the point that we just can’t know for sure at present where the planets orbited in the past. If the neighborhood was indeed dinamically decided, throughout the whole of the system’s evolution, then neither Uranus nor Neptune would be planets, because it seems they’ve swapped positions somewhere in the past. If that theory is correct. We don’t know. But if it is, then a truly dynamically decided neighborhood would have to include both… unless it’s constantly shifting together with the body in question, in which case you may well get stuck in situations where, due to variations in eccentricity (which happen in a much larger scale than variations in semimajor axis) a given body is a planet during a few hundreds or thousands of years, stops being a planet for another few hundreds or thousands of years, becomes a planet again for a few hundreds or thousands of years, etc. Sheer nonsense.

That’s probably why the paper doesn’t say that. Go read. What it says is that the neighborhood is what goes from perihelion to aphelion. Current apsis, nothing dynamical about it. Why? Because they said so. And the checking time is the age of the Solar System, which is why the index details the current state of affairs, not what happened in the past, nor any possible future state. Why? Because they said so.

Not arbitrary, you say? Yeah, right.

The point, however, is that the hydrostatic equilibrium criterion only demands the relatively small arbitrary decision about how much hydrostatic equilibrium is enough, wereas the orbital clearing criterion has not one, not even two as I wrongly wrote above, but three arbitrary points:

Well, if you had actually read the paper you would have found that dkary is correct, the clearing criterion is a lot less arbitrary, as there is a huge mass ratio difference between objects that have cleared their orbits or not.

[More specifically, as you brought it up: IIRC the mass ratio doesn't matter within broad limits ("how clear is clear enough"), the neighborhood is dynamically decided (by the clearing process) and so is the checking time.]

IAU’s definition is a lot closer to looking at the actual formation process, and so more analogous to how biologists have solved their scientific problem of species recognition. But in fact they aren’t fully analogous – what IAU is trying to do is to establish a specific population of “planets”, a specific ‘species’. By looking at the process I would assume that would analogously mean incorporating all originally inner system bodies; planets, asteroids and most moons.

I’m also glad I understood you mostly right – afraid I’m geting quite zombiefied at this hour!

Correction :

“Looking at the HST image of Ceres, I don’t think toomany coild argue oit looks like planet too.

is meant to read :

Looking at the HST image of Ceres, I don’t think too many folks could argue that it does look like planet too.

Argh! Typos – if only we could edit here! Sigh.

“”… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”

Source : Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

Emphasis added -apologies if this turns up here twice. Web issues? Server issues? Use of & issues? – StevoR (Hoping this works this time.)

Looking at the HST image of Ceres, I don’t think toomany coild argue oit looks like planet too.
_____________________

Planets? The more the merrier ..

There are, after all, now over 300 found found orbiting other stars outside our solar system.

So why not add a few more to ours!

Regarding the emphasised bit, Juno, despite having been the 3rd discovered, is in fact only the 13th largest (or thereabouts) asteroid. At 236 km diameter it’s way too small to be rounded due to self-gravity, so it’s clearly in the asteroid class. Astraea is even (much) smaller, not even 150 km diameter. They are both unequivocally asteroids.

StevoR:

Your article is misleadingly titled. All I see is mostly a list of objections to the reasoning and process the IAU used to define what a planet is. At best, if your arguments hold water, that gets us back to square one, still trying to decide whether Pluto should be classified as a planet. Ho hum.

Well the thing is that the IAU definition was specifically drawn up to exclude Pluto.

So showing that the IAU definition is ridiculous and downright wrong -which I think it was – shows they shouldn’t have adopted it but rather a beter definition that does include Pluto as a planet.

So I don’t think my post was “misleading” because I was clearly (well I thought clearly) putting forward the pro-Pluto side of the debate.

Now the big question is :

Do you think my arguments hold water?

I do – obviously or I wouldn’t have raised them!

But do you agree with me?

If not, why not & where am I wrong?

(Please tell me even if you think I’m wrong. I’m open to civilised debate anytime. Well except when I’m going to bed like ..er ..now.)

If my arguments hold water as I think, then I reckon we’re a lot further on than just square one.

I think we’re close, at least, to saying : “Yes whatever definition we do have for “planet” has to include Pluto. Because the only one that doesn’t -that was specifically formulated deny Pluto’s planetary status – was downright dumb.

Okay folks I’m human I’m tired, I make mistakes, I hope y’all get the gist
anyhow.

Jorge said on August 15th, 2008 at 8:55 am :

“dkary, I know. That’s why I wrote: “Also, don’t believe in anyone that tries to tell you that there are non-arbitrary definitions. There aren’t. You’ll have to swallow some arbitraryness whatever the definition ends up being.”

The point, however, is that the hydrostatic equilibrium criterion only demands the relatively small arbitrary decision about how much hydrostatic equilibrium is enough, wereas the orbital clearing criterion has not one, not even two as I wrongly wrote above, but three arbitrary points: how clear is clear enough, where does the neighborhood starts and ends and how long must we wait until checking if the neighborhood is clear or not. 3-1. In the Low Arbitrary Games, hydrostatic equilibrium wins a clear victory.”

(Emphasis added.)

Sorry, call me thick if you like, but can we get that last bit in ordinary non-jargonese english please?

You mean if its round right? Or close enough to? That some object being round makes more sense as a criteria and is less arbitrary and results in less contradictory and absurd results than the “orbital clearnace” stupidity? Would that be a fair distillation down into the language of Plain or have I misunderstood you? (If so then sorry, late in the night – or morn for me again now.)

Oh & the three candidates bit I put in bold above –

“… which leaves 1 definite dwarf planet – Ceres – and 3 candidates – Vesta, Pallas and Hygea.”

What about Juno? Wasn’t that one of the original four asteroids found and one of the largest? Isn’t it another candidate? If so then as far as I’m concerned its “welcome to the planetary club Juno!”

If not, oh well, good on you Juno for trying!

Either way I’m curious – where does Juno (& for that matter Astraea?) fit in this? Anyone?

I don’t want to kick Mercury out of the club, but let Pluto back in. Oh and Eris and MakeMake too. I love routing for the small guy.”

Me too.

Well said. I couldn’t agree more. (emphasis mine.)

If there’s a sliding scale on any planetary continuum then Pluto and Mercury are at least equal if not Pluto being ahead when it comes to planetary status. & not meaning to diss Mercury at all but Pluto is a much more interesting and dynamic planet too.

If Mercury was where Pluto is then it wouldn’t be counted as a planet by the IAU definition fools either. Which shows again how silly they are – to use an analogy its like saying that a person is not a classed as a real person just because they live in another country.

—–

Hmm.. My earlier post from a few minutes ago seems to have disappeared? Whats going on there I don’t think I swore anywhere in it or was too offensive? Was I?

“… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”

Source : Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

SR (me) : Umm . Can we get a trabnslation to plain english please?

SS (Spaceman Spiff – & should be quotes in italics too):
And the fact that comets and asteroids cross “planetary” orbits is a red-herring argument.

What!! The criteria is suposed to be “orbital clearing” & yet somehow you’re saying it doesn’t matter that orbits aren’t clear? That comets, asteroids even potentially other stars and planets can come past and make an orbit NOT clear? That orbits didn’t use to be clear in the past and might NOT be again at some stage in future history.

Our solar system will encounter a star called Gliese 710 sometime in the distant future which may affect planetary orbits as will the death of the Sun as a red giant when it loses mass. Suppose in the latter case Jupiter’s orbit suddenly crosses Saturns – does that stop either of them being planets? According to the IAU baloney criteria it would – & how silly is that!

No sorry the fact that orbits are crossed – by anthing other than radiation and solar wind means that they’re NOT clear to anyone spekaing the language of reasonable.

Otherwise then what exactly is meant by “clear” and for how far around the orbit it needs to be ‘cleared’ needs to be well ..cleared up!
Which it really just can’t be. Its a fatal flaw in a stupid and unnecessary third criterion. This “orbital clearance” nonsense just doesn’t make sense. Period.

SS : Those who drafted the language were aware of both Soter’s work and of a paper Alan Stern wrote with Harold Levison in 2002 in advising NASA on how to define “planets” in which was said:

“we define an überplanet as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals …And we define an unterplanet as one that has not been able to do so,…”
and a little further
“our Solar System clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.” (kinda makes one wonder…)

SR : Uberplanets and unterplanets? Mein Gott! Vot ist happenink here are ve liffink in Germany!?

Whats this Soter saying here – that Pluto is a gypsy sub-planet & Neptune from the superior planetary master race or some such tripe?! Come on!

This seems to be just a pluto-haters rant, nothing much else is clear about Soter -whoever he is. Give me Alan Stern’s words instead anyday!

SS : Soter’s arguments don’t solve all problems. But in my opinion, something physically profound separates the inner 8 larger bodies from those in the asteroid belt or Kuiper belt. In particular, Figure 2 shows clear separation between the “isolation zone” of fully accreted “planets” and the “swarm zones” of the other condensed objects.
(Emphasis added.)

SR : Well in my opinion your opinion is wrong. In my opinion a planet is aplanet regardless of whether it skims the surface of a star like a HotJupiter or orbits in the far reaches of the solar system where there;’s alot more roomfor otherobjevcts like Plutodoes. I see NO reason why you
can’t have planets inside asteroid belts or cometary “swarms” or whatever if they fit the other criteria – roundness, non-nuclear fusing for energy, etc ..

If you imagine otherwise I reckon your imagination is
lacking. (& Einstein no less rated imagination over a lot else.)

I think the problem arose because orbital dynamicists had aproblem with a planet – Pluto – they disliked because it seemed a misfit. Ironically once they’ve found a few bodies that are similiar (& lets remember only 1,
Eris, is larger & that not by much at all.) now its nolonger amisfit by the leading member of its planetary sub-class they want to scrap it. For no good reason that reasonable folk (incl. many in the general public)
can see.

Sorry but if that reflects badly on orbital dynamicists then the fault lies with them and the only way for them to correct it & regain our respect
is for them to come to their senses and accept that : Yes Pluto really is a planet.

To paraphrase Dr Seus : A planets a planet no matter where it is & no matter how how small!

Incidentally, its not just Pluto, I’d be happy to term Eris, Ceres , Sedna & others planets as well. I see no reason why we should limit the number of planets just so there are less names to remember. I mean for pity’s sake! If there are twenty or even a hundred planets in our solar system then so be it – thats how many there are. And Pluto is one of them.

What you are saying is basically that there’s a murky area between diameters of 340 and 430 km. Agreed. In this size range, objects will have to be classified one by one, after further examination their properties, and some are likely to remain planet candidates for a long time.

However, this also means that all the other objects, which are the vastest of the vast majority, will be classifiable immediately, most on detection (most detections of new objects have implicit a lower and an upper mass limit, whatever the method of detection might be), others after the crudest estimates of mass.

Adding to this, it mustn’t be forgotten that exactly the same mass estimates needed to determine planethood through the hydrostatic equilibrium criterion will also be needed to determine planethood through the orbital clearing criterion. If anything, this one is even more dependent on fragile mass estimates, because it relies on estimating the masses of objects that are smaller than the object under consideration.

It leaves objects in murky areas for much, much longer, because, given the chaotic nature or gravitational interactions, planetary systems are not deterministic, i.e., you can’t know for sure which young planets will survive and which will be tossed out of the system, into the star or smash head on with other young planets. The best we can do is come up with probabilities, and those… you may know that simulation studies of our own, neatly ordered and middle aged solar system have shown that there is a low, but significant, probability that Mercury’s orbit becomes unstable, which would mean that it might magically cease to be a planet overnight by the dynamical criterion, which in fact already happened with Neptune and Uranus. And this is a complete nonsense.

And gets worse when we look away from the Solar System, into the stuff that rotates around nearby stars. By the physical criterion, we are able to say with absolute certainty that every single planetary object we’ve found so far is indeed a planet. Even PSR 1257+12 A (provided a pulsar is considered worthy of hosting planets), whose mass is similar to that of Ganymede. By the dinamic criterion, however, we can’t, because we don’t know practically anything about their “neighborhoods”, and won’t for centuries, especially as the mass of the new extrasolar planets becomes smaller.

You can hardly murk things more than this, quite frankly.

But don’t go Calvin on me . It’s just an opinion (hopefully an informed one).

” .. “playpuses” : Is that “platypuses” you mean, or am I missing something?”

Yes I’m afraid you are. To be horrendously pedantic, the plural form of platypus is actually platypi!

August 13th, 2008 at 9:20 am – “What works for biology works for astronomy, too.” I’m not so sure about that. Biology works to produce species that are separate (at least with animals and plants, maybe not with bacteria so much.) But astronomy? Where’s the borderline between F and G stars?”

The borderline is small & subtle but its in the stars spectrum and a matter of which lines are where in the “barcode” of its elements.

As I understand it, (& correct me if I’m wrong, please) astronomers take a spectrograph of a stars’ light captured on a special machine (a rainbow is a naturally occurring solar spectrograph -I think ..) This spectrum is then studied and the position of various lines and bands reveals certain elements in the star which tells us what type it is.

The temperature and mass of a star are revealed in the abundance of elemnets – which ones have the strongest lines.

These types are for normal hydrogen fusing main-sequence stars and
from hottest, bluest and most massive through to coolest, reddest and least massive : O, B, A, F, G (our Sun), K, M.

But there’s a complication in that abnormal helium & other element fusing
giant and supergiant stars also exist and tehse also fall into all these spectral classes plus a few extra ones too like W (Wolf-Rayet, super-hot, highly evolved stars with super strong stellar winds) R, N, S, C (rarely used classes for cooler giant stars with unusual chemical ratios.)

So astronomers also use leters to distinguish whether a star of any spectral
type is normal or strange in that second way. Normal hydrogen fusing main
sequence stars get V, giants get II & III, sub-giants (stars turning into giants get IV, supergiants get I & hypergiants like Eta Carinae get 0.

Oh and each spectral class is also sub-divided into 10 classes from 0 to 9 too based on how its precise spectrum looks which depends on exactly how massive and hot it is.

Hence our Sun is G2 V but Capella a yellow giant gets G5 III.

Now to finally answer the question :

a star of class F is a smidgin hotter, a trifle more massive and has a spectrum that a teensy bit different in exactly how its spectral lines look.

The borderline is between F9 to G0. Its subtle alright but its there.

As far as Pluto goes, well I guess its a good thing Lucas set Star Wars in a galaxy far, far away so there’s no issue there vs what K9 said with
Dr Who!
_______________________________
Bit of a long post I’m afraid & sorry if anyone’s already answered it better.

If folks want more info. I suggest Jim Kaler’s website or
Wikipedia …

Based on the criterion of what is “round” that I think everyone has in mind, if you set the crushing pressure K (plasticity limit) of the material to greater than the gravitational pressure (that from hydrostatic equilibrium) at the center of a constant density (rho) “spherical” object, one finds that the object’s diameter must exceed some value:

D > 170 km * (K/MPa)^1/2 * (rho/g cm^-3)^-1, where K is measured in mega-Pascals and the density rho is measured in units of water’s density.

Both K and rho depend on the type of material (ice, rock, iron, …), and here are some further comments….

1) K ~ 40 MPa, ρ ≈ 2.5 g cm-3 for rocky compositions: D > 430 km

2) icy compositions are less dense, but also have much smaller K, and so reach ’roundness’ at smaller radii (D > 340 km)

3) ice/rock, rock/iron mixes give slightly different results

4) depends also on melting history of object, which depends on mass and composition (and potentially even external tidal forces)

5) depends on rotation, as well as collision history

6) measuring shape difficult to do in practice (“shape” can be guestimated indirectly from measurements of object’s albedo and some idea of its composition based on same).

7) And how round is “nearly round”?

I am not suggesting a solution, but I am suggesting that one doesn’t lie in simply using a “roundness” criterion.