[X] My Assistant

[nprev]

Again, this is the age-old battle between the human tendency to put things into neat categories and Nature's inclination to have things exist along a continuum of parameters. Any distinction we make for planets therefore must be essentially arbitrary. For example, what do the terrestrial planets & the gas giants really have in common? Not very much, from casual examination.

Given that, I have to buy off on the "Mercury Standard": if it's smaller than Mercury and/or doesn't orbit the Sun independently, then it ain't a planet. Rationale: Mercury is the smallest body that has been universally accepted as a planet since antiquity.

We have to draw a conceded arbitrary line somewhere, and perhaps since the very term "planet" was derived from some of the oldest recorded astronomical observations in Western culture we should use the smallest member of the Mercury-Jupiter continuum to define the threshold for planethood.

My two-one-hundredths of a (badly inflated) dollar!

[akuo]

As far as I know, Pluto has been assigned a minor planet number (sorry, not minor planet, "small solar system thingy" or whatever the term was) in IAU Circular 8747 together with Eris and Dysnomia (Xena & Gabrielle).

This should mean that (1) Ceres is still the number one "minor planet" and still an asteroid. Unless there is a further disjoint within the sets of minor planets and asteroids. Funnily, it looks like dwarf planets are also a subset of minor plan... Small Solar System Objects.

[Jyril]

Dwarf planets are not SSSBs. That is clear from the definition. And not all minor planets are SSSBs.

[tuvas]

Dwarf planets are not SSSBs. That is clear from the definition. And not all minor planets are SSSBs.

Name 4 minor planets that aren't SSSBs, I only know of 3.

[Jyril]

What do you mean? There are only 3 non-SSSB minor planets i.e. dwarf planets for now.

2003 EL61 and 2005 FY9 will be most likely promoted into the dwarf planet status soon. Sedna, Orcus, Quaoar, Varuna, Ixion, and some others will follow.

[JRehling]

Again, this is the age-old battle between the human tendency to put things into neat categories and Nature's inclination to have things exist along a continuum of parameters. Any distinction we make for planets therefore must be essentially arbitrary.

Which is a compelling reason for not trying to enforce a formal definition.

Neptune's quite different from the KBOs. It's easy to imagine needing a term to distinguish the two.

What do Mercury and Neptune on the one hand have in common vs. Ceres on the other hand such that it makes more sense to put Mercury with Neptune than with Ceres? Scientifically, nothing. Historically, just a pinch.

There is no formal definition of "river" as opposed to "stream". There is a formal distinction between baryons and leptons. Without a reason to suppose that "planet" is a category like baryon, people are treating it like one. It's not -- it's like "river".

What we have are two (or more sides) dueling over who can be less wrong by proposing a less-bad formal definition for something that, like "river", doesn't need or deserve one.

The stuckness seems to come from the fact that just thirty years ago, it looked like there might be a significant gulf between planets and asteroids. When the facts have disproven that, the response should not be to focus on Ceres and Pluto, but to question what kind of category "planet" is -- like "river" or like "baryon". And call me gruff, but if someone dodges that question, I think they're dodging taking this issue seriously.

[nprev]

Couldn't agree more, JR.

It's hard by definition for objective scientific methodology to cope with subjective categories/concepts like "planet", "animal", "plant", etc. that predate modern taxonomical philosophy, and the IAU's obvious agony, indecision, and confusion is a direct result.

IMHO as I previously stated, an arbitrary line must be drawn, and not everybody will like it, but it must be done merely to put the whole mess behind us! It's painful to watch the IAU strain to produce all these sub-planetary sub-categories, and for what?

[Mariner9]

As an aside, I've found a useful comeback for the next time one of your friends or colleagues asks "Why can't those darn people define what a 'planet' is? I should think it should be clear by now!"

Just give them a terrestial example, like I did to a friend. "Well, a pebble, a boulder, and a mountain are all basically made out of the same thing... why do they have different words for them?"

If that doesn't stop them in their tracks, keep going with "And by the way, when does a pile of dirt and rock go from being a pile to a mound, and then into a hill? And exactly what is the difference between a hill and a mountain anyway? And don't try to tell me mountains have rocky tops, have you taken a good look in New England lately? And why is there a mountain range called the Blue Hills?"

So far I haven't had to go much past "pebble, boulder and mountain'

[JRehling]

And exactly what is the difference between a hill and a mountain anyway? And don't try to tell me mountains have rocky tops, have you taken a good look in New England lately?

The US state of Maryland all by itself has numerous named peaks which completely confound the terms, all located within a short distance of each other.

As a couple of the more extreme examples, Conneway Hill is 3193 feet high while Hearthstone Mountain is 2021 feet high. Just in Maryland, there are dozens of "hills" higher than that "mountain" and over a dozen "mountains" shorter than that "hill". You can only imagine what extreme examples could be found if you included the rest of the [English speaking] world.

The big difference here is that there is general agreement that the two terms serve a role and that "mountain" is the higher one, and that for any of the above entities, at least one of the two terms ought to apply. Moreover, there is no pretense of a natural distinction between the two terms. In that way, mountains and hills make a poor analogy, because it did once seem that there was a natural low end of the planet size range. The thing now is to watch how people react to the lack of such a distinction -- something that was always known to be true of hills/mountains.

[Guest_Kevin Heider_*]

What do Mercury and Neptune on the one hand have in common vs. Ceres on the other hand such that it makes more sense to put Mercury with Neptune than with Ceres?

Mercury may have been much more like Neptune and Uranus if Mercury was a Chthonian Planet. Pluto and Ceres certainly had much more mundane pasts.

just thirty years ago, it looked like there might be a significant gulf between planets and asteroids. When the facts have disproven that, the response should not be to focus on Ceres and Pluto, but to question what kind of category "planet" is, ie: "river vs stream"

This is true. Forty years ago, Mercury was the smallest planet and Ceres was a "mostly ignored" space rock.

-- Kevin Heider

[JRehling]

Mercury may have been much more like Neptune and Uranus if Mercury was a Chthonian Planet
-- Kevin Heider

Mercury couldn't have been a [pre]Chthonian planet. You would have to put a giant planet much closer to Sol than Mercury is to boil off the hydrogen before the early heavy bombardment began.

The planet category underwent "feature creep" -- the one thing that Mercury and *Jupiter* have in common is that they are easily visible to the naked eye. When Uranus was discovered, size became the criterion: planet then became a category not of astronomical observation but of astronomical reality. (Actually, Earth broke that seal first.) Mercury can't be kicked out of the club because it's a charter member, but that's only if you allow for the historical trappings of the category. But if you stick to visual magnitude as a criterion, then Neptune is kicked out. So Mercury and Neptune have been joined under one term precisely because the "planet" category is a hodgepodge. Since it's a hodgepodge, the effort to formalize it is essentially doomed.

If subsequent discoveries play out just right, Mercury will be on the firing line next. Note that Mercury, which comes within about 0.24 AU of Venus, is presumed to have cleared its orbit. If 0.24 AU is the standard, I think we're going to find plenty of things in or past the KB that don't have anything as large as Venus within 0.24 AU of them. And if I were to bet, some of them will be bigger than Mercury.

[nprev]

Again, I think that the Mercury Standard is perfectly acceptable precisely because of its historical roots.

Let's face it: We're never, ever gonna have a truly empirical, rigorous definition of "planet", so the term may as well be linked to its colloquial qualities rather then all these nit-picking characteristics proposed by the IAU.

By that standard, we have eight planets...but I too feel that we'll find objects Mercury-sized or better in the coming years, and the whole debate will begin anew...

Hmm...Remember when Pluto was thought to be Earth-sized? If that had been true, we wouldn't have ever had this debate. TNOs would have become TPOs, and no question of their planetary status would ever have been raised. Just goes to show how perceptually-influenced this issue really is.

[akuo]

"Mercury-standard" is in no way acceptable. That puny rock ball is smaller than some moons.

I recommend the planetary status of Mercury to be removed. For any Mercury fans out there, think of it in this way: you may be losing a planet but you will be gaining a Vulcanoid.

[nprev]

...and Ganymede, Callisto, Titan and Triton were known to the ancient naked-eye observers that coined the term "planet"?

If they were, then I'd find your argument persuasive. However, the subjectivity of the very concept of a planet seems to preclude considering the large satellites of gas giants (esp. given their miniscule masses with respect to their primaries) as planets ahead of Mercury. Furthermore, Mercury frequently exceeds the brightness of Sirius when observed from the Earth, and no other putative "dwarf planet" can do likewise.

And as long as I'm touting Mercury, we shouldn't forget that it has a large and apparently convective iron-nickel core with a significant planetary magnetic field. This shows some taxonomical affinity with Earth and the gas giants (Venus & Mars being two rather conspicuous omissions).

[tedstryk]

"Mercury-standard" is in no way acceptable. That puny rock ball is smaller than some moons.

I recommend the planetary status of Mercury to be removed. For any Mercury fans out there, think of it in this way: you may be losing a planet but you will be gaining a Vulcanoid.

Mercury is also about as massive as Mars, far more massive than any moon. The planet/moon distinction has to do with what a world orbits, not size. So I think you will just have to deal with what you consider unacceptable.