For some people this means that Pluto was “demoted” because it is no longer defined as a planet. For others it means that the “integrity” of the word planet must be maintained at all cost. (I mean really, we’re not talking about the integrity of the definition of the word ‘marriage’ here, are we?) Was it really necessary to amend the constitution of the IAU (so to speak) just to keep an arbitrary definition of a word acceptable to people who aren’t open to new ideas (ie, hundreds of planets in a single system)? And why does anyone even care that Pluto isn’t a planet anymore? It’s not like its physical characteristics have changed.

What has led people to attach such religious significance to a word?

Simples:
The word “planet” should have some meaning that recognises the qualitative differences between “planets” and “not planets”, as opposed to being merely “a member of this list of objects”.

A lot of the objections, no matter how clearly or otherwise they may be phrased, seem to boil down to “Pluto was a planet when I learned about planets, so it’ll always be a planet to me,”.

Others have pointed out that Pluto has been an anomaly for at least the last 15 or so years, because – as far as we can tell – it’s pretty much the same as dozens of other KBOs. Somewhere there has to be a line between “planet” and “not planet”, otherwise the word “planet” has no meaning. The IAU drew that line in such a way that Pluto is no longer a planet, and neither is any large KBO we may discover in the future.

Earth wouldn’t be a planet in the far reaches of the outer solar system under the new definition. That makes no sense to me.

Are you suggesting that Earth would not gravitationally dominate the Kuiper Belt if it had formed there instead of nearer the sun?

If so, on what do you base this supposition?

BTW, unless you are a professional astronomer, I do not think it is your place to whine about the way in which the IAU has arrived at a definition of the term “planet”.

How to define a “Planet”

A good – simple, clear, logical and easy to determine – definition for planet is that a planet is an object which:

a) Has enough gravity to be in hydrostatic equilibrium (i.e. is round or if rapidly spinning a rotationally flattened spheroid.)
Thus a planet is not an asteroid or comet.

So, you’re quite happy for a handful of asteroids (those that have enough gravity to be round) to be renamed “planets” then, yes?

(Actually, I’d be quite happy for us to have 30 or 40 planets instead of just 8, I am merely trying to point out that your preferred definition is not as clear and simple as you claim.)

b) Is incapable of ever being self-luminous through shining by nuclear fusion. Thus is not a star or brown dwarf.

&

c) Is not directly orbiting another planet. Thus is not a moon.

So, does this mean you’d be happy for Charon to be a “planet” and for Pluto to be its “moon”? This definition does not address this question at all, and since the system’s barycentre is within neither body (AFAIK) there is no self-evident choice for the dominant body, and no giudance embedded in your definition.

What if you had a double planet system in which one were more massive but the other had a larger radius (say, if Neptune and Ouranos were a double-planet system instead of being separated by lots of empty space)? Then which one is the planet and which one the moon?

. . .
As well as on Pluto & its planethood directly here :

[url omitted]

where Croswell, an astronomer and one of my favourite astronomy authors, argues that :

…a better definition exists: a planet of our solar system is an object that orbits the Sun and has a diameter that equals or exceeds Pluto’s.

My own preferred definition of planet differs somewhat but would also include Pluto as well as the larger ice dwarfs.

Although I’d be fine with accepting Croswell’s one as a huge improvement over the IAU one anyday!

But Croswell’s (that you quote above) is just as arbitrary as the official IAU definition, so how is it any different, other than allowing Pluto to be called a “planet” again?

Ultimately, I don’t believe any of us amateur astronomers has any right to tell professional astronomers what they should call the objects they study. Professional astronomers who care deeply about this terminology should be lobbying other professional astronomers – they should not be trying to involve the lay public.

Being a professional biochemist, I’d be quite affronted if some amateurs tried to dictate to me and my colleagues how the professional organisation to which we belong should define and use (for example) the term “protein”.

- In My Humble Opinion Naturally.

This word you just used. I do not think it means what you think it means.

Personally, I prefer to think of “dwarf planets” as being merely small planets which don’t dominate their orbital zones. But, as someone else noted, it’s all a matter of semantics.

I’m not inclined to number the dwarfs in among the eight biggest planets. Why? Simply because a term like “the fifteenth planet” is going to be subject to frequent change as more dwarfs are discovered and recognized officially. Remember, there could easily be hundreds of them. I’m not sure it’ll make sense to number them at all, for the foreseeable future.

For memorization purposes, teachers could elect a “top ten” approach which would include Pluto and Eris and would resemble the solar system we grew up with. However, students would have to understand that Pluto and Eris have many, many smaller dwarf-planet siblings in the Kuiper Belt. And, we don’t want to forget about Ceres in the Asteroid Belt.

Perhaps teachers could make it the “top ten plus Ceres” list.

Or, of course, they could just focus on the “big eight” and ignore the dwarfs altogether. It would be their call.

The most flexible proposed definition of “planet” I’ve encountered is on Jorge Candeias’ website http://thousandplanets.wordpress.com . He would classify anything anywhere as a “planet” if it has reached hydrostatic equilibrium, but is too small to be a “star”. The large moons would be “secondary planets”. I’ve also seen the terms “satellite planets” and “moon planets” proposed elsewhere.

Under Jorge’s system, a “binary planet” would be, naturally, two planets orbiting a common center of gravity, no matter how similar or how different their relative sizes. This would include the Earth/Moon and Pluto/Charon systems.

The gas giants in our solar system would represent multiple-planet systems.

My prediction is that, once we have a better feeling for what’s out there in the cosmic neighborhood, a more flexible classification system will evolve on its own, with or without the IAU’s active participation. I think it’s already happening. And that’s fine with me.

“Also we know already that Fomalhaut b, 1RXS J160929.1-210524 b and the trio of exoplanets orbiting HR8799, to cite just a few examples, *are* located at distances equivalent to far out in our Edgeworth-Kuiper cometary zone ie. 100’s of Astronomical units.”
Already commented by CB.

“It seems likely that more such worlds are orbiting there – and these are superjovian planets.”
Ooch, so large. Great, then ratio of masses between these planets and mass of entire belt is zilion to one. Very strong argument for planetness of Pluto you have here.

“Nor I gather can even such massive gas giant worlds clear their orbital zones making them, effectively, superjovian “dwarf” planets!”
Taking aside “argument from incredulity” fallacy… do you suggest that “clearing orbit” means removing every little speck of dust? In this case no celestial body ever in universe is planet. So yes, it is ridiculous, but in bad (for you) way.

“Ridiculous? Yes, absolutely – that’s my point! ”
You make fool only from yourself. It seems concept of “clearing orbit” means for you various different things as is convienent for you. This is why I prefer “gravitational dominance”. With this term, is more obvious why moons or Trojans or sufficiently less massive space trash and dust are no problem for being planet.

“Planets around other stars have turned out to be much stranger and mind-bogglingly bizarre than we’d imagined beforehand.”
This is why IAU for time being constrained definition of planet to our solar system. We know them better and it will take long time to properly assess what is possible. This definition will be eventually changed.

By the way, no, Pluto will not be back ever.

“Ruling out exoplanetary possibilities without having *very* good reason to do so”
Being contrived example prepared specifically to somehow support planetness of Pluto is sufficiently good reason.

By the way, your lack of explanation why you still use retarded non sequitur arguments like “Pluto have moons, so Pluto is planet!” did not gone unnoticed.

Actually Fomalhaut b is orbiting inside — as in closer to its star than — a large belt of objects, while its own region appears clear,, and is thought to be responsible for maintaining this ring’s clean inner edge much like Jupiter shapes the inner asteroid belt. In other words, it seems likely that it has cleared its orbit despite its huge distance from the Fomalhaut.

But that presupposes the very idea you claim to use to prove your idea. That there, ladies & gentlemen, is a textbook case of the “Begging the question” logical fallacy.

Only with regards to the semantic argument of what should be called a planet.

If you take “planet” in that statement to mean the IAU definition of planet (or any definition which includes gravitational dominance of the orbit as a criterion) as I believe was intended, then the statement is simply saying that the KBOs are there because no object formed which was large enough to dominate the orbit and clear or absorb most of the other objects. Which is completely true and a salient observation about our solar system.

No planet formed? Bzzt. Wrong! The planet Pluto formed there as did the planets Eris, Makemake, Haumea, Sedna, “Buffy” (2004 XR190) and others. In fact, you could say that most of the planets actually formed in the cometary belt beyond Neptune and only a handful formed inside it. The “Big 8″ worlds are really the planetary oddities – the ice dwarf types are the average, typical class of planet!

Yes, if you define planet to include those objects.

And yes, it is an accurate observation to say that the majority of bodies in our solar system did not accrete enough mass to become gravitationally dominant and clear their orbit.

Which is exactly why the 8 bodies that have cleared their orbits stand out so clearly and obviously as distinct from the huge number of bodies that haven’t.

So whether you call those “planets” and the smaller round but non-dominant objects “dwarf planets”, or call everything round a “planet” and the 8 dominating planets “uberplanets”, or whatever, I don’t care.

But there is a salient distinction between them which it only makes sense to recognize and attempts to group Pluto and Earth in the same class of objects without distinguishing ignores this.

I like the hydrostatic equilibrium basis for a lower bound because it’s something that can be reasonably measured, rather than something vague like “cleared its orbital neighborhood” – which considering how many asteroids are in the Jupiter L-4 and L-5 points, I’d have to say Jupiter really fails. … how well has Mars performed clearing *ITS* orbital territory? Or Earth, for that matter?

Jupiter outmasses all of those asteroids by a ratio of over six hundred thousand to one. Mars has more than a hundred thousand times more mass than the other objects in its orbit. Earth has over one million times the mass of the rest of its orbit. Excepting the moon, which is only in earth’s orbit around the sun by virtue of being in orbit around earth.

Pluto, on the other hand, accounts for less than 10% of the mass in its orbit — not counting Charon, of course, for the same reason we don’t count Luna. Pluto has cleared Charon from its orbit via capturing it as a direct satellite — setting aside discussions of whether it should be considered a binary system since that wouldn’t change anything. Look at the Pluto-Charon system vs the rest of the belt and it is still a small fraction of the total mass in its orbit.

So yeah, “cleared it’s orbital neighborhood” is vague, and yet still perfectly sufficient to indicate the distinction between the 8 bodies who have obviously done it, and the bodies that obviously haven’t. There’s a huge difference between them, as in several orders of magnitude. There’s no point in being specific in that case, and in fact trying to draw an exact line through the gigantic empty space between things that have cleared their orbit and those that have not would be silly.

Like I said, I’m not really concerned with exactly what word we use to describe these things. I do believe that the IAU definition touches on a significant and salient distinction between objects in our solar system that should be recognized in any classification system.

You do realise that science starts with “making up” a hypothesis, right, MaDeR? You do remember that the first step of the scientific method is coming up with something that you then try to confirm experimentally – or in this case observationally – right?

I think you’ve hit the nail on the head there, actually. The point is, though, that our best hypothesis at the moment is that we won’t in fact find large objects stably in an uncleared belt. That could turn out to be wrong, in which case the IAU definition will have lost its value, but it’s our best science at the moment.

Compared to that, random what-if scenarios are just that, random what-if scenarios that, on current understanding, are unphysical. The neutrino and other predictions were made on the basis of theory, not in defiance of it.