First off, I think the new IAU planet definition is poorly worded, with too many loop holes.
This brings me to an important concept. If the term "Minor Planet" has officially been ditched as a reference to small bodies, I'd suggest that they revive the term to refer to bodies in the 2000-6000 km diameter range.

With that being said, I think it's time to spell out my own personal definitions of the various types of bodies in our solar system. Listed below are my classifications and what they mean, sort of based on the IAU definitions and sort of not.

1. Major Planet - a celestial body that: (a) is above 6000 km in diameter, (b) directly orbits the Sun and not another body, (c) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (d) has sufficient mass to both have a differentiated interior and retain an atmosphere in a vacuum and finally, (e) has definitely cleared the neighborhood around its orbit (with the legitimate exception of Trojan Bodies).

2. Minor Planet - a celestial body that: (a) is 2000-6000 km in diameter, (b) directly orbits the Sun and not another body, (c) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (d) has sufficient mass to both have a differentiated interior and retain an atmosphere in a vacuum, but (e) may or may not have cleared the neighborhood around its orbit (with the legitimate exception of Trojan Bodies).

3. Planetoid - a celestial dwarf body that: (a) directly orbits the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has insufficient mass to have a differentiated interior or retain an atmosphere in a vacuum, (d) has not cleared the neighborhood around its orbit, and (e) is not a satellite of another body.

(4) Planetesimal - a celestial dwarf body that: (a) directly orbits the sun, (b) has insufficient mass for its self-gravity to overcome rigid body forces so that it assumes a basically potato-like (irregular) shape, (c) has insufficient mass to have a differentiated interior or retain an atmosphere in a vacuum, (d) has not cleared the neighborhood around its orbit, and (e) is not a satellite of another body.

That's my take on the matter. I think this would be a good starting point to fine tune the new IAU planet definition. In this scheme, we would have 10 principal bodies orbiting the sun: 7 major and 3 minor planets. I also think the public would accept a scheme like this, as it would allow Pluto and Eris to be planets, yet not force the classification of planet to a million round bodies below 2000 km in diameter. Plus, as Mike Brown even said after discovering Eris, it would give future generations the possibility of actually finding a new planet. Besides, it's highly unlikely the number of bodies above 2000 km in diameter will skyrocket to ridiculous numbers anytime soon, even out to several hundred AU distant.

I'm planning on e-mailing this concept of mine to Alan Stern, who is against the new IAU definition of planet and plans to come up with a counter definition in August 2007. So, what are your thoughts on this matter? Later!

J P

Oh, there goes poor Mercury....

No, planet should be defined as a) orbits the Sun and b) diameter > 2000 km.

Short and sweet. Nice and simple. End of story.

I think we rushed the whole thing. The reason behind this IAU situation is because we have found some objects in deep space that call into question our classification of Pluto. We have a probe on the way, we've waited for many decades to visit Pluto and see what it really is. And now with that probe on the way, we decide to go ahead and re-classify Pluto before it arrives and provides us with the very information we need to make a definitive classification. What's up with that?

We've waited so many decades already. What's another eight years? I think we should just ignore the IAU, wait until New Horizons does its thing, and then revisit the issue, either on our own or via the IAU. It's just semantics anyway. We could classify Pluto as a mammal if we wanted to, it would still be made up of the same materials and would exhibit the same properties.

I agree that we need a better definition. The one we've been using up to now was quite dated. The new one is up-to-date, but not satisfying. I just think we're jumping the gun.

I agree. No planet should be defined by those two criteria.

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Please. Everyone was clamoring for the IAU to make a decision, they were getting mocked for taking so long, and now you want to ignore the decision they've made?

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

IAU has made a technical definition that might be used by astronomers, but it's not going to have much of an effect on the general public, except perhaps for the status of Pluto. For example most people are not going to be convinced that an earth type body that doesn't orbit the sun is not a planet. And I don't think they'd be convinced that if the earth somehow stopped orbiting the sun that it would no longer be a planet. After all, just because you take a fish out of the water doesn't mean it stops being a fish, does it?

I don't really have much to add to the discussion but I just have to welcome Planet X to the Board.

Since we've been discussing Planet X for years here it's so nice that it finally made an appearance.

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You say that as if everyone else has been waiting on tenterhooks for you to make your views known!

Your definitions are too arbitrary (why 2,000 and 6,000 km?) for most people - certainly most astronomers. Also, you not only redefine a term (minor planet) but completely change the meaning. You also use a term (planetesimal) that already has a specific meaning relating to planetary formation without thinking of a replacement.

Unless it's changed, it's what the general public will use. It's what they'll read in encyclopaedias and other factual books; what they'll be taught in school, even what the answers will be on quiz shows.

Well, as the IAU hasn't said that such a body is not a planet, I don't think there's much of a problem!

Not likely to happen in practice.

No, but it might if the particular species is re-classified as a non-fish!

I basically like the IAU definition, with the below exceptions:
1. there would be no "dwarf planets" only Mercury-Neptune 8 planets
2. Pluto and Eris would be Kuiper Belt Objects. This is clearly indicative of their kinship with each other, they must have formed at a similar time and location in solar system.
3. Ceres would remain an asteroid because it was formed in the same fashion as other asteroids in the asteroid belt.

Classification makes more sense to me when objects are grouped into groups of objects who share a common history/location of formation. The real planets do have this in common: they all swept their environments free of most of debris and are unique in this regard.

Currently, IAU has non-related objects like Ceres and Pluto grouped as "dwarf planets". they have no history in common, but do have history in common with the KBOs and Asteroid belt.

I would eliminate part (a)-- is it really necessary? Seems a bit arbitrary. But the main thing that I agree with here is, the very idea of having a designation called "Major planet". That's the crucial factor here-- the IAU basically uses the word "planet" when they actually mean "major planet". Please do carry on with your plans to email this idea, and stress the importance of having the "major planet" designation, regardless of its attributes. That pretty much solves the problems in one fell swoop, and allows for a much more inclusive definition of "planet", which is the key element.

Also, while you're at it, I recommend you also include a new category, called "satellite planet". I really hate this idea that a moon can't be a planet. When you look at Titan, and Io, etc., and study what is going on with these objects, it is very tempting to make comparisons to Earth. That's called "comparative planetology", but you wouldn't do it unless at some level, they were all planets! To borrow Ronald Brak's apt analogy, you wouldn't do "comparative fishology" between a halibut and a dog, but you certainly would between a halibut and a trout. It wouldn't matter that one was in the ocean and the other in streams, they are clearly both fish if you put them on the table and give them a good looking over. A planet by any other name... will just devalue obviously appropriate comparisons. This will become paramountly clear if and when the search for life on "other planets" begins in earnest in the next few decades.

The term minor planets was renamed to SSSB (Small Solar System Bodies) so that people would not consider them as a true subset of planets. I agree that it is strange that the term 'dwarf planets' (compound noun) is currently in use but the term can can be renamed later.

This removes Mercury as Major Planet even though Mercury has been known as a Planet (wanderer in the sky) for thousands of years. Current theory even suggests that Mercury may even have been a Cthonian Planet due to its high density and metal-rich core.

Just change the current IAU definition to say: "and (c) dominates the neighborhood around its orbit clearing it of comparable objects"

When people can't even agree on Planets vs 'Dwarf Planets' you want to create an arbitrary 3rd grouping?

Asteroid Vesta (580km in dia) has a differentiated interior.

Those are already called asteroids. The term "Planetesimal" is already in general use.

Even with the current IAU definition of a Planet there is a decent chance of finding a Planet beyond the Kuiper Belt.

And this is the part of the reason that objects under 2000km in diameter AND large enough to be spherical, BUT ARE NOT DOMINANT in their region, are considered 'dwarf planets'.

If you don't like calling Pluto a dwarf planet, call it a planetoid, protoplanet, or a 'minor planet' if you want to.

-- Kevin Heider

Personally, I have no problem at all with calling Pluto a dwarf planet or minor planet, as long as these are viewed as subclasses of planet. It seems very unfortunate to me to limit the definition of planet to not include subclasses over which "comparative planetology" is clearly of great value.

I am currently ok with calling Pluto a failed protoplanet, planetoid, or 'dwarf planet'. But perhaps I will change my mind after we visit Vesta (destroyed, yet differentiated), Ceres, and the Pluto system.

Planets should either be all inclusive (ie: spherical moons should be included) or they might as well have Planets be exclusive via requiring that they clear their orbits of comparable objects.

And since I don't think Luna and the large Jovian moons will ever be considered real Planets I think I am ok with Pluto not be considered a real Planet. We currently define Planets and Moons by 'where they are' as much as 'what they are'.

-- Kevin Heider

Yes, the decision they made was poor at best. Just because it was the IAU that made it, doesn't mean people should be required to follow it, especially if a majority of the people think it is flawed.

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I agree completely, there's no point in setting the moons aside.
The issue isn't if they are "real planets", but simply, are they planets? Why do we need the qualifier? They are not major planets, that much is clear.
And the fact that this is not done for any other primary astrophysical classification shows why it is a bad idea. Why don't we classify stars in binaries differently than single stars, even if the center of mass lies inside their companion? Because the physics isn't any different, and astronomy is anchored by physics.

If I were king...

"Planets" would be Mercury, Venus, Earth, Mars, Jupiter and Saturn.

Everything else would require a qualifier. Uranus, Neptune, Pluto, etc, would be "telescopic planets;" you could have "exo"-planets (orbiting other stars); "minor" planets, etc.

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^ Uranus can be viewed without a telescope.

I think it was handled perfectly and see no reason to worry about it any farther, it was obvious that pluto wasnt a planet like the other 8, and the change needed to be made. I dont see why people who normally have no real interest of knowledge of astronomy feel so strongly about this.

I agree completely.

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And what about the people who do have a real interest in astronomy, who think that an exclusive definition of planet is scientifically ill advised? Before I thought much about it, I thought 8 planets were fine, but then I thought on it some more, and realized that it should be 8 major planets. Highest-level classifications never include detailed aspects of the object, like its motion. For example, think on the definition of "comet" the way it used scientifically. Must it be close enough to the Sun to have a tail?

Actually, this has been done for centuries. Titan is a Moon and Pluto was a planet precisely because of differences in their orbital characteristics

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"Barbarism is the natural state of mankind. Civilization is unnatural. It is a whim of circumstance. And barbarism must always ultimately triumph" -- Conan

To most planetary astronomers, Titan has always been a planet, this is my point. The recent definition calls this into question. In any evernt, you must not choose your example from the issue at hand, but rather from the complement of astronomy.

i see no problem with the definitions as they currently are, mostly because i have failed to think of anything better myself and they do that sort of stuff for a living

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Planetary astronomers are not the only astronomers with an interest in the question, so they don't get to decide the question by themselves (as was obvious from the 2006 IAU Congress debate).

Anyone can come up with a definition scheme, and there are lots of different logically consistent schemes you can come up with, but the further your scheme is from the current position the less likely it will be to gain sufficient support (or indeed much support at all) within the astronomical community.

And it should have been left to them for sure, this is a big part of the problem. It would be like cosmologists defining stars, rather than stellar astronomers!
That's true, it's basically a political reality. I'm talking about what definition serves the best at what definitions are intended to do (organize thinking in useful ways). But political reality is another issue, and that's why I would never suggest re-ordering the ridiculous OBAFGKM stellar type list either! But one can't use the "it's tradition" argument on the new planet definition-- look at poor Pluto.