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 ©2010 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 convex.

And, for the Sun and Jupiter for instance, the center of mass is outside of the Sun, so the Sun doesn't even contain the focus of Jupiter's orbit.

The August Sky and Telescope has two articles that discuss planet classification. The first is by Denise Kaisler, The Puzzles of Planethood, (p.32), and the second is by Alan Stern and Hal Levison, Toward A Planet Paradigm, (p.42).

Kaisler discusses mass and cosmogony as boundary markers for planethood, and points out that we already use mass to differentiate between stars and brown dwarfs. Some people believe that about 13 Jupiter masses should draw the line between brown dwarfs and planets. Others believe that cosmogony (the way the body formed) should be a factor.

University of California-Berkeley planet hunter Geoffrey W. Marcy is said to favor this definition: "A planet is an object that has a mass between that of Pluto and the deuterium-burning threshold and that forms in orbit around an object that can generate energy by nuclear reactions." She says Berkeley brown-dwarf specialist Gibor Basri favors: "A planet is a spherical non-fusor born in orbit around a fusor." A fusor is any body that fuses either hydrogen (stars) or deuterium (brown-dwarfs).

Kaisler points out that neither definition would apply to IPMOs (isolated planetary-mass objects), but I suppose the acronym takes care of that. [img]/phpBB/images/smiles/icon_smile.gif[/img]

Stern and Levinson say they intend to publish their work in a future volume of the Transactions of the International Astronomical Union. In their opinion, a definition of planet should have the following qualities: 1) be based upon easily observed characteristics, 2) be quantitative, 3) be robust to new discoveries, 4) classify uniquely, 5) be context independent, 6) be uniquely deterministic, 7) involve the fewest possible criteria. In addition, it is desired that it be backwards compatible.

They analyze the possibilities, and conclude that a planetary body must be defined by an upper limit on size (low enough that it at no time had slef-sustained fusion reaction) and a lower limit (massive enough that its shape is determined primarily by gravity).

They claim that these criteria would include 22 of our solar system bodies--the four giants, the four inner rocky ones, seven moons, Pluto and Charon, 2 of the known Kuiper Belt Objects, and 3 asteroids (Ceres, Vesta, and Pallas). However, one of my previous posts had links that disputed the sphericity of the asteroids.

Then, they define a planet as a planetary body bound in an orbit around a single or multiple-star system. So, our solar system would have about 14 planets, they say (which would include Pluto). They go on to make other distinctions (unbound planet, planetary-scale satellites, double planets).

I'd like to consider my definition with regards to their criteria. Mine fail their number 5, for one, as they go on to explain that it means "An object's classification should not depend upon the nature of other bodies in its vicinity." However, they violate that one themselves, when they talk about a planet as being in orbit around a star.

<font size=-1>[Got rid of [img]/phpBB/images/smiles/icon_cool.gif[/img] smilie]</font>

<font size=-1>[ Fixed url ]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2003-03-10 14:06 ]</font>

I found this a while ago
What pluto has to say about KBO

just in case you need a small diversion from serious disscusion [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
The universe is not only queerer than we imagine, it is queerer than we can imagine.
J. B. S. Haldane

I love it!

So, the definition of "planet" should also include attitude?

P: I'm a planet. Read my lips. Pla-net. You got a problem with that?

BS: I was just making a point.

P: I've got a point to make. Bite me.

__________________
Never attribute to malice what can be adequately explained by ignorance or stupidity.
Isaac Asimov

Seven moons?
1)the Moon
2)Io
3)Europa
4)Ganymeade
5)Callisto
6)Titan
7)Triton

I think they missed a few (in no particular order):
[img]/phpBB/images/smiles/icon_cool.gif[/img]Umbriel
9)Ariel
10)Miranda
11)Oberon
12)Titania

Each of them has a lot of similar characteristics with the aforementioned qualifications. Miranda holds its spherical shape as demonstrated by the folds in its surface indicating it had broken up and reformed over time. Another of the moons has evidence of ancient tectonic movement that occurred before the deep freeze set in.

On there own they'd be candidates on par with Pluto for planetary rights. And here's one for everyone to roll around in their mind too. Would a spherical body in orbit around a Brown Dwarf be a moon or a planet? And would that vary if the Dwarf were a stellar companion or a solo body? And what if, by some odd chance, that a lone brown dwarf where to turn up with a gas giant companion the size of Uranus or Neptune? Would that be a minor binary or a Star/planet? Keep in mind, large gas giants have been found up to 17Mj and Brown dwarves start around 30-40 Mj, its entirely possible that a co-orbital pair could be found. If memory serves, there have been a number of planet sized bodies found free floating in the Orion nebula. Such a pairing is outside possible. The definition of what a planet is or isn't (the core of the topic here) has a lot of room to grow. Why would not Pluto be a planet to the KBO's when the Earth and the other inner worlds are planets to the Asteroids? Where is the threshold? How can we attach such a narrow definition on a class of objects that have such awesome variation? Maybe we need to separate planets by type? Silicate Planets (Mercury, Venus, Earth, Mars), Gas Planets (Neptune, Uranus, Saturn, Jupiter and their extrasolar cousins), Sub-stellar planets (Brown Dwarves for those that can't fathom them as stars), and ice planets (Pluto and other massive KBO's)

We seem to be trying to package an awful lot into one word. Why not reclassify the lot of them if we want to do it to one? Create specific definitions for each type of planet, then the controversy over whether its a planet or not becomes a question of what its composition and size/shape relative to other objects of its class.

_________________
Never discount a possibility, always charge full price - HB Marketing Philosophy

<font size=-1>[ This Message was edited by: Doodler on 2002-10-07 17:16 ]</font>

Argh. I wondered where that post got off to when I tried to fix the original--I finally did it but I had no idea what had happened. Thanks for the heads up.

<font size=-1>[ This Message was edited by: kilopi on 2003-03-10 15:59 ]</font>

I *think* you're still missing a [/quote] in there somewhere to distinguish the old post from the new addition. [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Petition called Create more Planetary categories and keep Pluto

http://www.petitionpetition.com/cgi/...on.cgi?id=6891
Petition to

Alan Boss, boss@dtm.ciw.edu, Head of the 13-member IAU group charged
with defining the word planet.

Whereas...The definition of the word planet has undergone many
changes starting with the supposedly nonfixed stars and sometimes
including the sun and moon. All this when it was thought that the
earth was the center of the universe. Such was the case with the
asteroids where the original definition was the minor planets between
Mars and Jupiter and so the first asteroid discovered Ceres was
called for want of a better term a planet and was dropped as a planet
when it was seen that it was really a member of a hitherto unknown
class of objects. Pluto too is a member of a hitherto unknown class
of objects and so like Ceres before it being the first of its kind
discovered it was called for want of a better term a planet just as
other galaxies were called nebulas and considered a part of our
galaxy when people did not know what they were. None of these
definitions are wrong it is just that they have added too our
language and in some cases are preserved only as archaic definitions.
People are used to many definitions in dictionaries sometimes they
are even when abstruse enough counterintuitive. People are used to
even only some definitions being shown in some dictionaries. Surely
since people only need a scientific definition of a planet in order
to not get confused about what laws apply but are used to the
classifications of planets in our Solar System as being a matter of
conventional rules beyond that, as witnessed by the continual
acceptance of Pluto and the historical evolutions of the term planet
within our solar system there is certainly no harm in keeping Pluto
as included among the planets as people can justly feel as they do
that there is scientific justification for the inclusion of Pluto
even when going beyond the usual characteristics of a planet that one
would look for. The gravitation of the planet Neptune was not enough
to explain disturbances in the orbit of the planet Uranus and so
under the mistaken assumption that there was a planet X that could
explain such disturbances it was predicted where that planet would be
and so was discovered Pluto. In the same way as our definitions of
the term planet has evolved as newer definitions for objects have
arisen but the older definitions are employed at times as well so let
it be with Pluto and perhaps it will more than just occasionally be
employed but continue to be in plain usage. The various definitions
proposed for the term planet are just part of what people already
think of as various definitions that at times they use for the term
planet just scientifically spelled out and would never be taken as
more. There should therefore be more categories of planets. This will
increase scientific description and accuracy. Their should be a
category called "Planetary Asteroid" which would include Ceres
because it is planet shaped and could even encompass Pluto if Pluto
is an Asteroid. Their should be planets called "Wandering Planets"
and "Far Distant Solar System Planets." All the planets except for
Pluto should be standardized for the universe in terms of their
definitions.

I, the undersigned, therefore petition the IAU to broaden the
definition of the term planet in accordance with the above with this
broadening preserving the full planetary status of Pluto.

Sincerely,

I don't think that, was, exactly, how, Pluto, was, discovered.

Welcome to the BABB.

I think we should refer the planets, asteroids, comets, etc all as objects. Classification shouldn't be so organized-there isn't always a fine line between what's something and what's not.

"Yisrael Asper wrote:
The gravitation of the planet Neptune was not enough
to explain disturbances in the orbit of the planet Uranus and so
under the mistaken assumption that there was a planet X that could
explain such disturbances it was predicted where that planet would be
and so was discovered Pluto."

>I don't think that, was, exactly, how, Pluto, was, discovered.

>Welcome to the BABB.

Thanks for the welcome BABB. What is the part that you feel is not exactly how Pluto was discovered. All planets really have disturbances and maybe Uranus' disturbances were not big enough at all for a planet X really and Pluto isn't exactly where it was predicted it should be, nothing is. Still this minus the now known fact that Pluto is not Planet X is the story behind Pluto's discovery.