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Everything I need to know I learned through Googling.

I get an error 403 with that link

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Well, there's a space in there, but it works with all three of my browsers. Try this (a page that includes the image).

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Everything I need to know I learned through Googling.

I also got the 403 error, but the new link shows me the image. Very silly. How many asteroids are there in the Solar System? How many have hit the Earth in the last 3 billion years? Aside from that, we aren't calling Pluto an asteroid. We're calling it a dwarf planet.

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It's not a matter of "round enough", as if roundness were somehow an important feature. It is the internal physics-- what force is balancing gravity? If the answer is molecular bonds, which are capable of exerting forces in arbitrary directions, then roundness need not appear-- any more than it does in a common boulder. If the answer is isotropic pressure, then you must reach the equipotential shape because such pressure forces can only exert forces perpendicular to the surface. So "roundness" is simply a shortcut for identifying the internal force balance, a crucial element for understanding the internal physics. I wouldn't be against instead using differentiation as the key internal physics issue, and that might limit us to somewhat larger objects than just the roundness constraint. For one thing, it rules out "piles of dust" that are weakly gravitationally bound.

But an object like Enceladus is essentially a giant drop of water with an ice coating. It's round because it's made of ice, when an object of similar mass made of rock would not be. I'm not sure, but tidal forces probably play a role in keeping it round as well. How could we determine which objects are round through isotropic pressure?

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

Enceladus is mostly rock. Or at least it is more rocky than Saturn's other icy satellites. The (tidally melted?) rock is believed to heat up the covering icy layer forming pockets of liquid under the south pole. The water then escapes as the observed geysirs.

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The BBC TV science series Horizon has revamped it's website.

This link is to a vote on whether Pluto is a planet and they have video clips of two people from oposite sides of the argument.

The against person seemed somewhat arrogant to me.

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Neil Tyson

He's the guy who brought the issue to the public's awareness when he removed Pluto from the list at the American Museum of Natural History. He starts out "We've known from the beginning it was an oddball."

The "beginning"? I can't imagine what he means by that, in that context.

It's too bad that this video of his interview wasn't widely shown a few months ago--Pluto would still be a planet.

I watched the video's...I think they are both wrong

Among the other things discussed, I think it should be fairly evident that planets orbit a star, and moons orbit a planet, with the exception of rogue planets that have been somehow ejected from their system.

The one guy thought all the moons should be planets...
The other guy was indeed arrogant...

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The relevant criteria is that the object 'has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape'. Not that it is 'nearly round', and not that it is round through isotropic pressure.

The phrase is perfectly adequate to deal with any object in the solar system about which we already have sufficient information. But it is not (as with many scientific definitions) entirely precise. If we come across a case that will require more precision, then the definition will be refined (like the current refining of the definition of lunar latitude and longitude).

One of the bits of imprecision relates to 'rigid body forces'. Does this mean such forces actually present in the object, or does it mean any conceivable rigid body forces that might be present in an object of such size and/or mass?

Then there is the practical question of determining things near the boundary. Should give a few postgrads a subject for there theses! But any definition always has measurement difficulties near the boundary (unless it's so wide it has no boundary).

Buie's idea of just basing the definition off a low and high mass (nothing else) IS the simplest and most scientific IMO...keep in mind it allows plenty sub-categories based on size and orbit. This was essentially my earlier scheme.

However, this may be TOO simple, and does not have cultural acceptance. One confusing transition will be explaining to people that large moons are actually planets. I don't think that'll fly. Also, there would be dozens if not hundreds of small planets beyond Neptune, all crossing each other's orbits and within the same belt. I'd personally be fine with that if we go by the above classification, but again, I don't think we need to do that when we can just make a whole different class for those objects.

So with that in mind, my most recent idea is to seperate large bodies into four diffierent categories: planets, planetoids, moons, and free floaters...based on orbital characteristics. (I don't like the term dwarf planet since it implies a difference based on size, which it doesn't). Since this coincides much more with culture's understanding of a moon, I think this works just as well, if not better.

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Actually, that is a fairly good definition of isotropic pressure. After all, once you overcome rigid forces, what do you think is left? But if you are saying that you feel this should be the sole definition, then I tend to agree. Although I think differentiation might actually be better, I wouldn't split hairs about any definition that looks only at the internal physics and not the motion of the object. That's why I absolutely think that large moons should be considered planets, and I think that will be even more patently obvious when we start landing on them.

I'm sure isotropic pressure occurs in other scenarios. The next thing after rigid body forces would be electron degeneracy pressure, but not really relevant when considering a lower limit for planets. The point is that the mass must be sufficient to overcome rigid-body forces if it has to - it wouldn't if the body solidified from liquid for instance, as that would already be round, much less mass being needed for self-gravity to make a liquid object round.

I'm not. I was just commenting on the fact that a lot of people seem to get hung up on 'nearly round', when it's not the criteria.

I would want a classification scheme that distinguishes objects both on relevant internal characteristics but also on where they are and how they formed. Which is the upper-level classification, and which the lower, I'm not fussed about.

I would still want a classification scheme that distinguishes between objects orbiting stars and objects orbiting planets (or other planets, as the case may be), because I think that's an important difference. Personally, I would restrict the word Moon to objects that meet a self-gravitationally-round criteria, using another term for smaller natural satellites. You would call them planets, but, although there are many logical classification schemes, I think it best not to conflict with pre-existing usage where possible.

No, the next thing is normal nondegenerate forms of pressure. Degeneracy pressure would come much later, and it is also a form of isotropic pressure.
Then we are in agreement that differentiation is a better criteria than force balance.
I agree, 'nearly round' sounds like an aesthetic consideration, rather than a physical one.

Well, that's what has to be decided-- it's a part of the classification scheme to decide which is the upper level. On the surface, there's no need to get "fussed" about any classification, we'd eventually figure out what the other person is talking about even if we had no classification schemes at all. But having schemes changes how we think about the universe, and so does the choice of the upper level scheme, and I argue the upper level scheme should always be based on the physics that is internal to the object, not the environment it happens to find itself in. For example, if the Earth is ever ejected from its orbit into interstellar space, and humanity continues to live on underground using fusion energy or some such thing, do you really want their astronomers to have to say "oops, we're not living on a planet any more" just because there's no more day and night?

I agree, but that is what I am arguing should be a lower level aspect of the classification, just as is whether a planet is a terrestrial planet or a gas giant planet. Making that part of the upper level scheme falls victim to all the pitfalls I've mentioned above, the biggest being, it artificially changes the way we think about the Earth and, say, Titan, when it shouldn't. The main difference between Earth and Titan has to do with distance to the Sun and the smaller size of Titan, which may alter the degree of complete differentiation. The orbit around Jupiter likely affects its icy outer layers, but the crust of any planet will be affected by its surroundings. If we call it a planet, then we can further classify it as a satellite planet in honor of its relation to Jupiter.

On the contrary, pre-existing usage in scientific circles is to call all these things planets. I was at a seminar the other day where an approach to Titan was described as "approaching the planet", and not an eye was batted. That's as it should be, and don't be surprised to find reference in the scientific literature to Titan's "planetary atmosphere" or "planetwide cratering". It is only in the popular literature that "planet" is reserved for the big 9, but now apparently the IAU wants to adopt the popular usage in the place of an elegant and useful scientific definition.

The problem is, without sending out some kind of seismic sounding probe, how could you tell whether an object such as, say, Ceres, is internally diffrentiated, as opposed to an object that isn't?

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There is a growing tendancy to think of Man as a rational, thinking being, which is absurd.- Marvin the Martian.

It's gotten to the point where careful investigation is needed just to tell parody from reality. I think that means reality is broken.- Noclevername.

Degeneracy pressure is a pressure whose form is isotropic. All isotropic pressure means is a pressure that is the same in all directions. This is the case for the internal pressure of a body for which gravity dominates, but is not true for all smaller objects. In such smaller solid bodies, rigid-body forces dominate. These forces are electromagnetic, and once you get these, then together with gravity there is nothing else in play until the onset of electron degeneracy.

No we are not. How you get from my pointing out that the criteria in the IAU definition is not 'nearly round' to my agreeing that differentiation is a better criteria than force balance is beyond me. Don't put words into my mouth.

Whether it is or is not, the point is that it is not the IAU criteria.

Which is an arguable point of view. But not one a lot of astronomers would agree with (certainly not of those at the last IAU Congress). I think people's stance on this may have something to do with what they are interested in, or study about, planets. If it's the objects themselves, you prefer to highlight their internal physics. If it's their relationship to other objects (like for orbital dynamicists), it's where it is, etc.

I don't think it's a scenario we need worry about.

That's the main difference if you're thinking about the objects themselves. If you're thinking about their relationship to others, the fact that one orbits the Sun and the other does not would be the main difference. Other people have different points of view.

You mean Saturn. Jupiter and Saturn are themselves satellite planets (of the Sun), but, yes, you could sub-divide the 'planets' like that. I'm not saying you cannot come up with a classification scheme on those lines. But I'm not the one you have to convince.

Which scientific circles? Given the strength of feeling displayed at the IAU, I doubt very much if this was anywhere near universal.

Yes, there's quite a lot of tolerance in seminars for obvious errors! Possibly a slip of the tongue. Or perhaps the speaker is of the same mind as you as to definition of a planet. But evidence as to a specific instance of a particular usage is not evidence that that usage is a general one.

I'll be surprised to find it from now on, as editors will stick to the IAU definition (though the odd example might be missed). You could say that before because there was no scientific definition of planet. There is now.

Er no, according to the IAU there are only eight planets. The IAU adopted a definition. As it's the IAU one, it's the scientific definition, and certainly those astronomers who voted for it thought so, presumably on partly utilitarian grounds. Elegance, like beauty, is in the eye of the beholder, but I haven't seen any alternative definition that is more elegant (though there are simpler ones).

Our club held a discussion on this matter last Friday night. A panel consisting of members for and against brought their arguments. We came to the following decision for the following reasoning.

The life sciences group things together but they also subdivide those groups. Humans, lions, whales and deer are all grouped as mammals and are distinguished from amphibians and reptiles..The plant kingdom has its heirarchy as well to help define and point to reasonings for those subdivisions of kingdoms and phyla.

We, therefore, need to group all the bodies that circumvent the sun into one group with subdivisions as the life sciences do. What those names are is undecided at the moment but it appears likely that all bodies that orbit the sun are going to be called planets..They could then be subdivided into terrestrial planets, gaseous planets, asteroidal planets, cometary planets, and Kuiper Belt planets or some other nomenclature. The reason some thought all will be called planets is to keep a consistency with the term "planetary nebula", which only holds its definition from gases that are ejected from a dying star yet are holding an orbit.

With this reasoning there are thousands of planets.

Anyway, that's our two cents.

Trillions (maybe even way beyond septillions) of planets around our Sun. Your definition has no lower size limit. Do you count a single Iron ion orbiting the Sun in between Jupiter and Saturn a planet? How about a dust grain composed of between a hundred and a thousand atoms? How about a five micron chip from an asteroid released by a collision?

If you choose this idea for a planet, you are changing the definition of the term very substantially.

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That is correct. But our original conclusion was not that the word "planet" would necessarily survive to describe all of the bodies in orbit around the sun. But we do need one. If we choose "planet" then we have allowed all those bodies whose electrical/elastic characterisitics that exceed their gravitational characteristics to become planets. I don't think that will happen but I do think we may need an inclusive term for all bodies. Perhaps you may be suggesting that we throw out "planetary nebula?"

Is true.

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Yes, that is a good point, at least sphericity is obvious. But we have the laws of physics, we could do the best we can for the boundary objects.

Although I agree with the idea of a rather inclusive definition for planet, including everything in orbit is going a bit overboard, because a classification should tell you something about the object so you know where to begin your thinking about it. And note that "planetary nebula" is a particularly unfortunate term to base an argument on-- it is a terrible misnomer, that comes not from the idea that the gas is still orbiting the star (it isn't), but rather that in a poor telescope such a nebula looks a bit like a planet!

All you are doing is outlining the reasons why planets have different internal physics than smaller objects, and elucidating why I like that type of definition.


Hang on, if all people care about is relationship to other objects (as for orbital dynamicists), then all that matters about an object is its mass! Not its composition, not its formation history, not its internal physics, nor atmosphere, nothing, just the mass. For orbital dynamicists, every object is basically just a point with one internal attribute-- mass. Why on Earth would we want to let orbital dynamicists define a planet?

Indeed I did mean Saturn, thanks for the correction. And yes, it is the IAU I must convince, and unfortunately they are not soliciting my opinion. Perhaps I'll get an opportunity to voice this point, or perhaps someone else will. In the mean time, I rather suspect that my definition of planet will continue to be used in conversation and unofficial texts, and perhaps will eventually come into broader acceptance.

The way I look at it, the "strength of feeling" is all based on mistakenly equating "planet" with "major planet", an equation that I would argue the "anti-Pluto" camp were making unconsciously. As soon as it is recognized that what the IAU was really voting on was whether or not Pluto should be considered as major a planet as the other 8, everything comes into perspective, and the emotions immediately begin to calm.

Not at all, it was quite an intelligent use of the term, and has been common parlance for a long time, and I predict will continue to be for some time to come. The statistics of my example are poor, being just one case, but if you keep your ears peeled for this phenomenon I predict you will find it over and over. Not in official publications, you are right those will likely be "censored", as it now stands, but in common parlance.

Actually, it is indeed the IAU definition, but that does not make it "the scientific definition". Scientific definitions emerge from a body of literature, not from fiat by a particular group. If astronomers in practice adopt the IAU definition, then it will in time become the scientific definition. I for one hope that will not be the case, although good planetary astronomy will continue one way or the other (and in a similar vein, don't be surprised if Pluto, and Titan, and Europa, continued to be classified under "planetary astronomy").

But if they are not deemed planets, courtesy of the IAU, will funding these studies become more difficult. I heard that in the days of Voyager one of the scientists had to fight to get them to even consider looking at the moons around Saturn and Jupiter.

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Well, that is exaggeration. However, one has to remember that next to nothing was known about the moons prior to the Voyager flybys and they all were considered more or less dead, cratered bodies. They really stole the show.

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That is definitely part of the argument for using an inclusive definition. Often, definitions subtley effect what we think of as "interesting". For example, how many times have you heard it referred to as "demoting" Pluto? That isn't healthy for science, we should be trying to foster interest, not limit it.

But the mission to Pluto is already launched, so that's a moot point.

That means the net effect is that a lot of otherwise-insignificant objects in the Kuiper Belt, plus Ceres, have been PROMOTED from being big iceballs to almost-planets.

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Ah, but the larger of those objects should be promoted to planets, in my view. I've heard the argument that "Kuiper belt object" is a term that infuses interest into these fellows, but nope, I don't really think the general populace gives a hoot about Kuiper belt objects.

San Jose Mercury: "Plutoed" chosen 2006 word of the year by dialect society
(subscription may be required -- or Google your own hit; it's Associated Press)

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