Ceres’ demotion turns out to have been wrong. The telescopes of 19th century astronomers could not resolve it into a disk, so they demoted it along with the other asteroids. In the 1990s, the Hubble telescope imaged Ceres and confirmed it to be spherical, meaning it has reached the threshold where it is pulled into a round shape by its own gravity, a state known as hydrostatic equilibrium. This is what distinguishes small planets from shapeless asteroids and KBOs. Any KBOs that are large enough to be in hydrostatic equilibrium are both KBOs and small planets. One does not preclude the other.

There have been proposals from the 19th century to today to classify the spherical moons of planets as “satellite planets” or “secondary planets.” These objects are geologically differentiated and have all the characteristics of planets except that they orbit other planets instead of stars directly.

Brown dwarfs are classed at the lowest end of star categories (actually, a new class of star was created to accommodate them) because the key point is that they once conducted hydrogen fusion even if they are not doing it now. According to Dr. Alan Stern, a planet should be an object that never underwent fusion.

Memorizing planet names makes little sense. Once upon a time, we knew little more about the planets than their names, so memorizing made sense. Today, we have a wealth of knowledge from 50 years of planetary exploration, and teaching that is much more valuable than having kids memorize names. We don’t ask kids to memorize the names of all the rivers or mountains on Earth; what we do ask is that they know what rivers and mountains are and the characteristics of different types of each.

I strongly second the statement that argument from authority is a logical fallacy. The IAU has authority only if enough people believe it does. Since several hundred professional astronomers signed a formal petition rejecting the IAU demotion, it is clear that the IAU represents not the “gospel truth” but one side in an ongoing debate. Significantly, even Dr. Tyson has come to admit that this is still an open debate. Unfortunately, when the astronomers who signed the petition formally requested a reopening of this issue at the 2009 General Assembly, the IAU leadership adamantly refused, leading these astronomers to boycott. Shutting off debate itself shows an underlying insecurity on the part of the IAU leadership with their position.

And amateur astronomers should have a say in the definition of astronomical terms. An incredibly large number of discoveries, including recent impacts on Jupiter and numerous comets, have been made by amateurs. In fact, amateurs usually have a broader knowledge of astronomy whereas professionals specialize in one field. The majority of the 424 (out of a total membership of 10,000) IAU members who voted on the demotion are not even planetary scientists but other types of astronomers.

I’m a liberal, so I’m not for punishments; the closest I come is my Pluto Blog at http://laurele.livejournal.com, my own public presentations, and my upcoming book “The Little Planet That Would Not Die: Pluto’s Story.” I will not leave this Earth before I see that demotion go the way of the dinosaurs.

The main beef seems to be with the “gravitationally dominates” aspect, although I can’t quite see, from the arguments above, why this is a problem.

It feels to me as if the main reason that people object to the new definition is not so much in the technical detail as in the fact that it demotes Pluto from planet status. Thus, getting into the technical details of what volume of space any particular planet dominates, and what is meant by “gravitationally dominant” are not ends in themselves, but means to get Pluto back into the club.

If Pluto brings its entire family with it, that seems to be OK.

@ MTU (too many comments to numerate) -
Your defintion may or may not be “better” than the present one. Either way, I fully support the right of the IAU, as the leading international organisation of professional astronomers, to define what the terms used in their science mean.

If you tried to redefine for me (I’m a biochemist) what may be meant by the term “nucleic acid”, I’d probably tell you to take a running jump. Or just ignore you.

We, as amateur astronomers, do not “own” the terms used in astronomy. If professional astronomers (as represented by the IAU) feel that the term “planet” needs to be defined in a certain way, then that is their perogative.

You made reference to a “numbers game” – well, perhaps the majority of IAU members felt that having 25 or 30 planets (with the prospect of discovering many more in the Kuiper Belt in the coming decades) rendered the term useless.

Ultimately, what we have now is an improvement over what went before, because this is at least an attempt at a systematic defintion of the term. A lot of people don’t like it – but the reasons for this dislike seem to me more emotionally-based than to do with the utility of the term. You claim that the IAU defintion is illogical, but you have not shown this.

I grow weary of the sheer quantity of text you have put in this thread, so I’ll leave you to have the last word.

My dog is named Pluto, and everybody loves him too! He told me he’d bite Mr. Brown in the shin, unless Brown gives him a treat.

LOL.

That reminds me of my brother’s dog named Cerberus after the three-headed dog guarding the Greek underworld – ruled by Pluto. I’m puzzled as to why when Pluto’s smaller moons* were discovered one of them wasn’t named Cereberus after that – would ‘ve been apt. Guess there must already have been an asteroid or something with that name?

@74 & 75. Gus Snarp : Thanks.

—-

* Now named Nix and Hydra found by the HST in I think 2005.

if you call Pluto a planet, but don’t call any other KBOs planets, then you render the meaning of the word planet “a member of this list” which is pretty much useless.

Not what I’m calling for. I agree here – I want all dwarf planets considered planets, incl, Eris, Sedna, Ceres, etc .. as well.

I think the first definition they came up with at the Prague conference -which is similar if not identical to the one I’ve given – is superior to the revised anti-Pluto current def’n. I am not alone in this opinion. (Yes, I am an amateur astronomer not a professional one.)

why do you seem to think it is worse that what we had before the IAU issued the definition?

See my comment #70 here.

Pluto does not gravitationally dominate its orbital path. It dominates a small portion of it.

Okay, allow me a thought experiment here. Imagine a rogue Superjovian planet wanders into our solar system and disrupts the orbits of the outer planets. It swings Neptune off course and draws Jupiter outwards so that the four gas giants are thrown into chaotic orbits they no longer dominate. Would this in your view mean they were no longer “planets”? I don’t think it would. Now if this rogue Superjovian captured, say, Neptune into orbit around itself it would make Neptune a moon & not a planet – but if Neptune was just disturbed and shifted into a region it couldn’t gravitationally dominate – like frex the Oort Cloud – would it stop being a planet in your eyes?

Methinks orbital dominance is vastly over-rated!

@86. shoblock : Neat story. I’d call you on your bias there but I hardly have to, you’ve pretty much done so yourself!

I hold no ill will at all towards Mike Brown, I’m sure he’s a great bloke. I do disagree with him on Pluto’s status though.

As far as I am concerned, the IAU could choose to define planets by colour if they so wish

Thus removing the planetary status from all worlds, bar only Earth & Neptune.

That would clearly be silly and wrong no matter their authority. Would you expect people to say “oh well, they’re the IAU so we must agree with that definition however absurd it might be” or would they say instead, “hang on that’s just not right!?”

If those in authority are making bad decisions then they should be called on it & made to correct them. If they refuse to do so then they are showing that they shouldn’t be in charge and, if it becomes bad enough to be necessary they get replaced by other more reasonable people. That is, in my view, the situation here.

But you are not the IAU. You do not make the rules. They do.

Appeal to authority is a logical fallacy. Just because the IAU are an authority doesn’t automatically make them – or their definition right.

Imagine the IAU does as you suggest in comment #89 :

As far as I am concerned, the IAU could choose to define planets by colour if they so wish

Thus removing the planetary status from all worlds bar Earth & Neptune.

That would clearly be silly and wrong no matter he authority. If those in authority are making bad decisions then they should be called on it & made to correct them. That is, in my view, the situation here.

It seems to me that where we disagree is on whether the change made by the IAU is an improvement or not. I think it is, and you seem to disagree (correct me if I’m wrong). I’m not saying it couldn’t be better, but I still think it’s an improvement. Now, at least, planets are defined by qualitative distinctions from other solar system bodies, whereas, before, a “planet” was merely a member of a list of nine objects.

I don’t think that last sentence is accurate – the planets were more than just members of this list and we did have an idea of what they were. The definition may not have been a formal one but we did have a working idea.

I do indeed disagree with the IAU definition – it might perhaps be considered an improvement but it isn’t a good logical definition and it doe shave problems that mena we should, in my view – & many others opinions too – reject it and replace it with something better.

But you are not the IAU. You do not make the rules. They do.

Well, not really. All they have is moral suasion to back up their definition.

Similarly, you make a lot of assertions that are no more than that. At the meeting there were 3 or 4 options available. One of which was substantially equivalent to what MTU proposes. The main argument for the current definition wasn’t some rigorous objective idea. It was strictly, “well, if we keep Pluto in the club, we’re going to have to admit all these others and probably Ceres as well”. It was simply a numbers game. There was also some mumbling about a bit of “stick it to the Americans” as well. Iraq and all that, y’know.

I was actually kind of surprised by this notion that there wasn’t a definition of planet. Back before Voyager 2 took it’s close-ups of Neptune, I took Astronomy 10. Our textbook and the instructor (An actual astronomer with a PhD, albeit a Solar astronomer) defined 3 non-stellar objects: Comets, Planets and Minor Planets(asteroids and moons). Basically, what you’re arguing about is whether to slice the pizza evenly or between the pepperoni slices.

Personally, I think Mike Brown gets a kick out of the notoriety. I seem to remember him saying recently that he is not now nor has he ever been a member of the IAU. So ,he didn’t actually play any direct part in demoting Pluto. He wins either way. He’s either discovered several new planets (something nobody has ever done) or played a part in discovering and defining a whole new class of solar system object. Win. Win. And with a new book out on the subject, this basketball player just opened him up to a whole new audience.

The best argument I’ve heard against Pluto was Neil Tyson’s “If you moved Pluto into the inner solar system, it would grow a tail. That’s no way for a planet to behave.” But I think he said that before the hot jupiter was discovered. I like the idea of 3 classes of solar planets, myself. Rocky, Gassy and Icy. Ceres excluded because if it got any bigger, Jupiter would tear it apart again.

@67. Nigel Depledge Says:

MTU (46) said: “Planets can be stranger and make up a far broader category than we used to imagine. In this context the IAU’s narrow and overly exclusive definition makes little if any sense.”

I think you mean “Objects that orbit stars can be stranger . . .”

No, I meant planets just like I wrote.

But you are not the IAU. You do not make the rules. They do. And the definition of “planet” is what it is according to the existing IAU rules. So you truly didn’t mean “planet” – what you instead meant was “objects that orbit stars”. However, you have made it abundantly clear that you want “planet” to mean what you choose it to mean, just like Tweedledum (or is that Tweedledee?).

Pluto is odd because it is just like several other bodies we know about in the Kuiper Belt, and there may well be dozens more that are fairly similar to it. None of the eight planets has that distinction.

So by that logic if we know of more objects like Jupiter somewhere or Earth that would makes *them* less “planet-ly” to coin a phrase? Que?!?

The fact that the KBOs have orbits that often cross one another is a consideration. If we had a solar system with – say – half a dozen Earth-sized objects occupying the same or similar orbital paths, I’m sure that would be a consideration when defining what to call these things. But this is mere speculation. We have the solar system that we have, and it contains 8 objects that are qualitatively different from all of the other objects.

If a future session of the IAU adopts a definition of “planet” along the lines of your preference, what would you suggest that we call these 8 objects to distinguish them from all the other sun-orbiting gravitationally-spheroidal stuff drifting around?

I’ve given my preferred definition already in comment #45 but once more :

A planet is defined best as an astronomical object that is :

a) Never self luminous by nuclear fusion thus not a star or brown dwarf,

b) not directly orbiting another planet thus not a moon,

&

c) large enough to be round or if rapidly spinning oblate spheroidal rounded through its own gravity thus not a comet or asteroid.

Historically, the planets were originally defined based on the criteria of showing apparent moving across the sky. They were the “wanderers” or wandering stars as opposed to fixed stars – and they included our Sun and Moon! I agree that that definition needed updating.

Unfortunately, your preferred definition has never been the defintion of a planet.

So, although you disagree with the new definition, why do you seem to think it is worse that what we had before the IAU issued the definition?

Again, so what? How far does “vicinity” extend for? How do you define “vicnity” in this regard?

I don’t, but I’m sure the IAU does.

Just because we’ve got Venus and Mars and Mercury relatively near us in our vicinity does that rule Earth out of planethood?

No, of course not – because the inner planets are all gravitationally dominant within their orbital paths. Pluto does not share that distinction.

You may consider it an illogical distinction, but you cannot ignore the fact that it is a part of the current IAU definition of a planet. As far as I am concerned, the IAU could choose to define planets by colour if they so wish – the point is that there must be some systematic way of rendering a meaning unto the word “planet”, and anything that is systematic is better than what we had before. And the IAU is the body that should make the definition.

It makes no difference if there are other planets nearby – unless Pluto is orbiting one of them and becomes a moon then as far as I’m concerned if it meets the other reasonable criteria* – and it does – then its a planet.

—-

* For the logical reasons I’ve already noted I don’t count the undefinable “orbital clearence” part as such – it is unreasonable instead.

OK, well I don’t know if you’re a professional astronomer or a more-than-usually informed amateur. You seem to be disputing the IAU’s right to make the rules. Don’t hassle me over that one. I don’t consider myself to be in a position to judge the validity of the IAU rules (although, clearly, you do so consider yourself) – but I do know that having a clear and precise (or as precise as is possible) defintion of the word “planet” is better than having it meaning nothing more significant than “a member of this list”.

@69. Nigel Depledge :

That would be akin to saying that Ganymede ought to be a planet.

Ganymede orbits Jupiter making it a moon.
Pluto orbits our Sun making it a planet.
If Ganymede orbited our Sun (or another star) directly then I’d have no issue at all callig it a planet.

I think you missed my point.

I was trying to emphasise that we need to focus on qualitative differences for classification, not on merely quantitative ones.

As you rightly point out, Ganymede has the qualitative distinction of not orbiting the Sun directly. I was emphasising that singling out Pluto as distinct from other KBOs shifts the focus to quantitative differences (whether that quantity is size, number of years since discovery or whatever).

I think we are agreed that we would both be happy with calling any sun-orbiting gravitationally-spheroidal object a planet.

It seems to me that where we disagree is on whether the change made by the IAU is an improvement or not. I think it is, and you seem to disagree (correct me if I’m wrong). I’m not saying it couldn’t be better, but I still think it’s an improvement. Now, at least, planets are defined by qualitative distinctions from other solar system bodies, whereas, before, a “planet” was merely a member of a list of nine objects.

Once we narrow your Dunnock here down to the “animal” kingdom which it clearly is – as it equally clearly isn’t a bacteria or a fungus or a plant then we can go to the next step down the taxonomic order which is that its a bird not a reptile or an insect or a mammal.

Actually, the next step down from Kingdom (at least, in the simplest classification schemes) is Phylum. In the case of the Dunnock, it is in phylum Chordata. Notice that birds, retiles and mammals (all chordates) are all qualitatively different from insects (all arthropods) – they have an internal skeleton, whereas all insects have an exoskeleton (in at least some stages of their life cycle).

The next step down from that would be to decide in which class of phylum Chordata it belongs. That’s the stage at which you would distinguish reptiles, birds, fish, mammals and so on from one another.

Star, planet, asteroid here is equivalent to animal, vegetable, mineral.

No.

Stars are qualitatively different from everything else out there, because they carry out sustainable nuclear fusion in their cores.

Planets and asteroids are the same kind of things. Or do you call Mercury an asteroid because it has no atmosphere and no moons? There is no qualitative difference between planets and asteroids. Classification – at least in biology – is all about identifying qualitative differences between organisms, not merely quantitative differences.

Similarly, any classification of solar system bodies should operate on qualitative, not quantitative, differences. The only thing that distinguishes Pluto from other KBOs is that it was discovered several decades before any others. But as I (and at least one other commenter) have noted before, the “history” argument doesn’t work either.

Also note that Jupiter’s orbit isn’t clear – it has trojan asteroids. Sun-grazing comets have orbits that cross all planetary paths.

Jupiter’s trojans only follow that orbit because of Jupiter’s influence. Therefore, they are gravitationally dominated by Jupiter.

Asteroids that cross planetary orbits do indeed have their orbits altered by the planets whose paths they cross. Whereas those asteroids don’t alter the orbits of these planets by more than a tiny, tiny fraction. Which is the gravitationally dominant body?

But you wouldn’t deny a pygmy her human rights or status as a person because of that just like you shouldn’t deny Pluto its status as a planet. It is, quite frankly, unethical.

What, so you reckon Pluto has some fundamental right to be defined in a category with Mercury, Venus ….. Neptune, instead of with all the other KBOs?

How do you work that out?

And, if you call Pluto a planet, but don’t call any other KBOs planets, then you render the meaning of the word planet “a member of this list” which is pretty much useless.

As I said further up, I’d be equally happy with a definition that included all the gravitationally-spheroidal bodies that orbit the sun, but the definition of planet must mean something about the things it’s describing.

I certainly don’t see anything about Pluto that merits special treatment.

But Pluto does gravitationally dominate its region – what do you call Charon, Nix and Hydra? Moons! Moons that are gravitationally dominated by and have their orbital path’s controlled by the gravity of their primary planet. Its just a matter of how far a given planet’s domination (Hill) sphere of influence extends.

Pluto does not gravitationally dominate its orbital path. It dominates a small portion of it.

My next question is (and again, I’m not trying to be snarky–I actually want to know), how do you define gravitational dominance?

Good question, to which the short answer is: I don’t know. What I mean is, I don’t know how the IAU define it (since there are several plausible ways in which it could be defined). But you’ve made me think about something that I had not previously given much thought, so thanks!

I hope an astronomer here can answer that for you (or both of us!).

“Pluto has moons like other planets,
Pluto has a geologically differentiated geological structure like other planets,
Pluto has an atmosphere with complex weather and seasons like other planets,
Pluto might well also have rings as well like many other planets do!”

Mercury and Venus do not have moons. There are asteroids as small as 120 meters across that have moons.
The asteroid Vesta also is differentiated and geologically interesting.
Saturn’s moon Titan has an atmosphere. Mercury (for the most part) does not.
It’s possible that Saturn’s moon Rhea has rings. Mercury, Venus, Earth, and Mars do not.

“But Pluto does gravitationally dominate its region – what do you call Charon, Nix and Hydra?”

Again… there are asteroids under a kilometer in diameter with moons. That doesn’t make them planets. Having moons has nothing to do with being a planet.

The one thing I do agree with? It is rather odd that dwarf planets are not considered a kind of planet, just from a linguistic point of view… I think it’d be more logical to consider them a sub-category of planets. But that’s just my personal opinion… the eight-planet system IS scientifically justifiable. So is the ‘everything in hydrostatic equilibrium’ system. (I would not be totally opposed to a system that made gravitationally rounded moons planets, but that’s just me… the major moons get no respect. ) The nine-planet system is not scientifically justified.

@64. Nigel Depledge :

How do you define the boundary between a brown dwarf and a planet? If it is purely by the criterion of nuclear fusion, then surely you will have brown dwarfs becoming planets on a regular basis. And therefore you will have planets becoming moons at the same time. No, I think it best to leave brown dwarfs out of it. They’re weird.

True enough I guess. Brown dwarfs would be defined by being able to fuse of deuterium in my book.

The rest of it would probably work, though. Now you just need to lobby enough IAU members to get the next vote in your favour.

Thanks I’m trying.

I hope this happens.