Well, before I answer your final question, I think the 'approximately spherical' should be refined to 'approximately spherical because of its own gravity.' That gets rid of the baseball, even without #5 (> 200 km), and would get rid of a really, really big balloon.

Getting around two planet-sized things locked in orbit? What about this:
One of your criteria (I think) was - primary gravitational influence is a star or brown dwarf. What about, "any gravitationally influential body, other than a star or brown dwarf, cannot have the center of gravity between the two bodies inside the other body."

I'm not sure if that makes our own moon a planet, but it would Charon. Hmm. Something in that direction though.

Also, the criterion about the gravtitational influence should allow for a planet orbiting a twin star system, or star-brown dwarf combo.

Thanks to ToSeek for the recent news about the Pluto mission.

I tend to agree with Angry Raisin's list of planet definers, although I'd increase the diameter to 2000km. This includes Pluto but excludes the asteroids and the KBOs/TNOs.

Frankly, I like having Pluto as a planet. No special scientific reason behind it, although GoW's list would provide some.

My feeling is that anyone who wants to demote Pluto from planethood status must be willing to foot the bill to catch up to the Pioneer probes and "correct" the plaques.

Yeah, but I bet the pilot would be greatful he didn't try to fly off and chase the charlies on a meatloaf. [img]/phpBB/images/smiles/icon_biggrin.gif[/img] That'd make a Katchupy mess on his G-suit. [img]/phpBB/images/smiles/icon_frown.gif[/img]

The problem with using "spherical because of gravity" is, no one is sure just how big that has to be.

Actually, that is the criteria number four from my list! The moon/Earth center of mass is inside the Earth, the Charon/Pluto center of mass is not inside Pluto.

I was wondering about going back to the original, Greek definition of planets as wanderers (planetes) in the sky. Could visibility be a criterion? Is Pluto visible from Neptune? Are any of the asteroids visible from Mars?

(Actually, I'm deciding that this isn't a very good criterion, but I'd still be interested in answers to my questions.)

__________________
Everything I need to know I learned through Googling.

I've got no business even being in this discussion, but reading all the other posts about how one distinguishes what's out there in the ether I've come up with a set of daffynitions that works for me, albeit I'm willing to accept that it might be too naive for real working astronomers:

A planetary object is any orbiting body whose primary is a star (or possibly brown dwarf, I'm open-minded about that); a moon is any orbiting body whose primary is a planetary object. A planet is a planetary object which does not share its general orbit (note: not specific orbit, since each asteroid has its own peculiar orbit) with other planetary objects; a planetoid is a planetary object which shares a general orbital area with other planetary objects (i.e., other planetoids)--moons don't count for this distinction (i.e., a planet can have moons without them making it a planetoid).

Thus the denizens of the Asteroid Belt and the Kuyper Belt both classify as planetoids, and the only distinction is "inner belt" v. "outer belt". Pluto/Charon are planetoids by this definition.

The reason I say I don't really belong in this discussion is that I don't know/understand enough about the whole zoo of other "thingies" floating around out there. Are there other "Trans-Neptunian" bodies besides Pluto/Charon and the Kuyper Belt? P/C doesn't come anywhere close to the Kuyper as I understand it, so if the only usefulness of the TNO designation is to group P/C with the KB then I'm agin it, as Granny would say.

I also realise that my classification doesn't do real good with non-belted asteroids, such as cross the Earth's orbit or Jupiter's Trojans. Come to that, a strict application of my definition to Jupiter would make it a planetoid, too, due to those very same Trojans, so I guess some sort of size distinction is necessary as well (i.e., one huge planetary object sharing its orbit with a bunch of smaller critters is a planet and the small junk are planetoids, whereas they're all planetoids if they're all close to the same size [within one order of magnitude, say]).

Well, as I say it's just an amateur's opinion. (I'm deprecating it mainly because the more I think it through myself, the less impressed with it I am.)

The (still orbiting the eggnog) Curtmudgeon

I thought about the Jupiter Trojans as a counterexample before I got to that part of your post, but I'm still wondering about our own moon. What do you mean by "primary"? If you mean, the object that has the strongest gravitational pull upon it, then the moon's primary is the Sun. Then, by your definition, the Earth and moon would both be planetary objects.

And don't hog the eggnog. I want some too.

I wonder if it might be useful to incorporate the idea of "sweeping" an area clear of small objects in the definition of a planet. After all, except for the asteroid belt, the inner solar system is relatively clear of small objects (okay, sure, there are NEAs and such, and the occasional comet, but they're pretty rare).

Also, does our definition have to consider the early stages of a planet's formation? Does a blob of molten rock undergoing heavy bombardment still qualify as a planet, or do we need a separate classification?

Certainly if our Moon were on its own, it would clearly qualify as a planet, as would some of the moons of Jupiter and Saturn.

I'm seeing a potential major motion picture here -- The Astronomer Who Went Up A Planet But Came Down A TNO. What do you think? Anybody wanna volunteer to play Tombaugh? [img]/phpBB/images/smiles/icon_smile.gif[/img]

Seriously, though, I'm wondering what prompted TPTB to consider reclassifying Pluto in the first place. From the discussions on this board, there is obviously no clear-cut official definition of a "planet." Was the reclassification originally proposed simply because they felt Pluto didn't "fit" with the other eight planets, rather then that it didn't meet some established objective criteria?

If the former, I think it can be dismissed on its face. Now that we're starting to detect and catalog bodies orbiting other stars, we shouldn't let the characteristics of the eight (or nine) major bodies orbiting our own star determine our universal nomenclature, should we?

So, until somebody comes up with valid arbitrary criteria for determining "planethood" that everybody can accept, things should just stay the same, shouldn't they?

Granted, some will dismiss for personal reasons any criteria which excludes Pluto, but it seems that others would probably dismiss criteria which doesn't just because of the subjective feeling that Pluto shouldn't be included . . .

Just my two cents . . .

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

No, I meant that under those criteria, both would be planetary objects, but not planets.

SeanF
According to the Sky and Telescope article, Marsden has been trying to change the classification of Pluto for twenty years. I'm still not certain why.

I think the companions orbiting the various asteroids, like Ida, are called moons. So, I don't think a moon has to orbit a planet.
http://antwrp.gsfc.nasa.gov/apod/ap990807.html

If everything out there is a planet (or a star), then science will go out the window, and we all will be back to the mysticism of the ancients. Witness the problem of going from a flat Earth to an Earth-centered universe, then to a Sun-centered universe (and finally to a universe without a center). There are folks still alive who believe in an Earth-centered universe (perhaps even in a flat Earth), and there are some people who still believe that the Sun is the center of it all. People HATE CHANGE. We now are faced with an accelerating universe and there are folks who get abdominal cramps over that one, as well. Whatever scientists do will not change the facts of the universe. There will be future scientists who come along to add some previously missed data and that will ruin a lot of people's lives in their having to cope with change.

It does not matter what scientists call anything. The whole idea of classification is to facilitate the study of nature and of nature's laws. If everything is deemed to be a planet, then science must be thrown out the window and we will be back living like the ancients with all of their superstitions.

I grew up in a family with a history of science and got used to scientific change as a given. To me, scientific change is absolutely exhilarating!

ljbrs [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
"There is in the universe neither center nor circumference." Giordano Bruno Born 1548. Torched 1600.

I don't think anyone is suggesting that we call everything a planet, we're just trying to consider what would be good criteria for the definition of planet. That seems to be the crux of the issue.

Classification is basic to science, there's no doubt about that, but there are a lot of issues to discuss. Just look at the clad wars over in biology.

I AGREE.

However, this *Pluto/Charon* business seemed to have been traumatic for many stargazers, et al., in recent years, and I think that it ought to be a marvelous subject for discussion here. Also, as you might note, I tend to be pretty stubborn about some things. This is not meant to mean that I am correct, but only that I am exceedingly stubborn. I am capable of change, but never quote me on that.

Perhaps if the astronomers had been (in the past) more particular in their designations, this problem would never have occurred to bother so many people. Astronomy is perhaps the most ancient of the sciences, so many bad habits have been produced by ancient astronomers and their descendants.

One possible criterion in categorizing a planet of the Sun might be that such planet would need to have developed together in the plane of the other major planets (in the thought-to-have-been primordial disk of our young Sun). I think that there is a need for careful designations. Moons should not be considered as planets. Late-coming additions to the planetary designations might have a hyphenated title to show this new classification.

Classifications of astronomical objects pertain to science and not to popularity polls. When an astronomical object is found to have been mistakenly named as one kind of object but is later found actually to have been another kind of object, scientists should stick to scientific methods and reclassify the object. Perhaps there needs to be a reclassification of all objects for scientific purposes only. Those same objects should be able (by the public) to be called whatever the public would wish to call them. Of course, this would most likely end in confusion (mixed with intense anger and fisticuffs) for everyone concerned.

Science evolves as it grows. I do not think that astronomy could be called a science if arbitrary rules prevailed in the science of astronomy itself which would never be followed (for instance) in physics. But physics is a relative newcomer (once considered a *natural philosophy*), particularly when considering some of its most recent discoveries.

Perhaps we should leave it up to the astronomers to make these decisions and go along with them if they make good sense. Tradition has no place in science. Remember the earth-centered universe and the problems involved where burning at the stake was (tragically) a probable punishment for challenging tradition (i.e., as in the case of Giordano Bruno). Astronomy has both serious and casual adherents. Professional astronomers cannot possibly please them all. The problem here, I think, is that there are so many astronomy lovers, casual, amateur, and professional (along with astrophysicists and cosmologists who are often off in a world of their own). Change is catastrophic to some people where astronomy is more like a religion than a science. I find change exhiliarating -- always have and always will. However, most people (understandably) seem to abhor change.

Now, I am not here specifically criticizing anyone. I just think that perhaps it is time for astronomers to be permitted scientifically to develop theories concerning these matters. Not being a scientist, myself, I shall leave that up to the scientists. I also think that controversy can be enlightening for all. Perhaps astronomers should keep all changes *under wraps* to avoid frightening everybody else. But *the truth will out*, eventually.

ljbrs [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]


P.S. I stubbornly refuse (incorrectly, despite the cries of horror by the punctuation police) to put commas and periods inside of parentheses, unless such commas and periods are a part of the quotation. [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]


<font size=-1>[ This Message was edited by: ljbrs on 2002-01-06 16:31 ]</font>

Well, so far, it appears that they've decided to not reclassify Pluto. Some of the links I read show that there is disagreement even amongst astronomers--but the bottom line is that the reclassification did not happen.

Pluto was once considered to have much more mass than it appears to have now. It hasn't shrunk, we just know more about it. If it had as much mass as it was once thought, we probably wouldn't be having this discussion--the orbit plane would not matter. Of course, the reason that it hasn't been reclassified is because it still has an order of magnitude more mass than the next closest object of the proposed type.

Things could get interesting if we ever find a Kuiper belt object bigger than Pluto....

Exactly. But until that time...

Until a couple years ago, Pluto was closer to the Sun than Uranus, for twenty years. That's as close as it gets. It's mean distance from the Sun is 5.9 billion km, but it was closer than 4.5 billion km. In a hundred years, it'll get out to 7.3 billion km, and be two magnitudes dimmer. What else could be out there? It may take a while to find out.

But the Sun isn't the body which the Moon directly orbits--it only orbits the Sun because the Earth does and it's orbitting the Earth. No, I wasn't thinking in terms of gravity so much as "geo"metry, if you'll excuse the planet-centric term here. I realise that orbits are elliptical not circular, and the "primary" isn't at the centre of the orbit but does (as I understand it) occupy one focus of the ellipse.

We can be eggnog-planetoids in the same orbit. Anyone else in the Eggnoid Belt? [img]/phpBB/images/smiles/icon_lol.gif[/img]

The (found a local place with great eggnog ice cream, too [img]/phpBB/images/smiles/icon_smile.gif[/img] ) Curtmudgeon

The moon does orbit the Sun directly. The path of the moon around the Sun, if you were to draw it on a graph paper, is always c