What are your thoughts about this. Are you in favor of categorizing celestial objects according to their physical characteristics, or would you rather keep everything just the way it is -- no change.

If Pluto is not actually a planet (in that it has characteristics strongly similar to Trans-Neptunian Objects (TNOs) but no physical characteristics which would be scientifically comparable to the planets in the Solar System, or even to the Solar System moons, should the scientifically correct answer be to place it in its actual category along with other similar objects in the Solar System, rather than force it into a category where it does not physically belong (such as planets)?

Now, be reasonable about this, because it looks as if Pluto (and Charon) will be considered as planet and moon, as well as being designated the largest TNOs in the Kuiper Belt.

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] [img]/phpBB/images/smiles/icon_frown.gif[/img]

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

Here is the criteria I posted to the old BABB, for consideration:

A planet is a body that meets the
following criteria:
1) It is not a star or brown dwarf
2) Primary gravitational influence is a star or brown dwarf
3) It is approximately spherical
4) Center of gravity of two-body system is not inside another planet
5) Larger than 200km radius

Number 3 is still vague, but we could decide on an actual parameter, excluding effects of rotation of course.

I will follow the thinking of Brian Marsden and the International Astronomical Union (IAU).

Pluto does not have a consistent orbit with relation to the other planets in the Solar System. It is not made of material consistent with any of the planets in the Solar System. Its orbit and its composition are very similar to the other Trans-Neptunian Objects (TNOs).

Perhaps you have made up your ideas on this matter. Perhaps the astronomers with the IAU have their ideas. I go with their thinking, because they are selected from the best of the international astronomers. It is their job to make such distinctions. I have trust in them.

Some of our asteroids are spherical. Does this also make them planets or minor planets? They orbit the Sun in the Asteroid Belt, many of them in a consistent motion to the other planets in the Solar System. Pluto orbits the Sun in the outskirts of the Kuiper Belt. Does this make objects in the Kuiper Belt which are spherical out to be true planets? Designations are simply made in order to distinguish objects as to their characteristics. You left out of your list several characteristics which would be different for Pluto with respect to the other planets in the Solar System. Pluto's orbit is much different and Pluto does not seem to have developed in the same manner as the other planets. Pluto's (and Charon's) composition is much different.

I simply thought that this subject would be a good one for people who are interested in astronomy of the Solar System. I respect Brian Marsden and the other members of the IAU and will go with their decisions.

ljbrs [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_rolleyes.gif[/img]

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

Last I heard, Brian Marsden's position was that Pluto should have a dual classification. A planet - since that is it's established classification. First and (so far) largest TNO.
I have no problem with this.

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

T. Anderson

That is exactly right! I do not expect that Pluto will ever be completely demoted. Originally, the IAU was going to switch Pluto's category to Trans-Neptunian Object, but changed its mind when the public uproar became deafening. Also, Clyde Tombaugh was still alive at the time of this proposed status reevaluation, and everybody backed off, perhaps out of deference to him, perhaps out of their respect for the public's (and astronomers') dislike of change (after having memorized the nine planets in their childhood).

Anyway, I think that the decision of the IAU to accept both categories makes study of the object, both as a Trans-Neptunian Object (TNO), and as our ninth planet, a much better decision. I think that the dual category is the best solution, keeping everybody happy.

Now, let us hope that there will be a mission to Pluto (and beyond) to study this difficult, and enticing, object.

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]

_________________
*Nothing is more damaging to a new truth than an old error.* Goethe

<font size=-1>[ This Message was edited by: ljbrs on 2001-12-29 15:18 ]</font>

I think TNO is the worst form of compromise. It is a compromise simply to be one. It's not a good category of 'space thing.'

As we learn more about other star systems, we will certainly find more planets. There are almost certainly other moons. There are probably comets and asteroids, and something like a Kuiper Belt and Oort Cloud around most other systems.

Nowhere else in the universe will there be other trans-Neptunian objects, because nowhere else is a planet named Neptune. Even if we change it to 'planetoids that have orbits partly inside the outermost planet,' what's the point? It's a pretty unimportant property to make a whole category out of it.

The distinction between planet and planetoid is gray - in nature, not just in astronomers' arguments. I have no problem with the decision to keep it as planet, since it was arbitrarily designated as such originally.

TNO is not a compromise. Pluto (and Charon) are more like the objects in the Kuiper Belt than like the objects one designates as planets. For one thing, Pluto's orbit seems to coincide with the TNOs and not at all with the planets. Pluto's composition is icy, which is more like Kuiper Belt objects. If scientists must give up carefully studied characteristics to differentiate objects, then there can be no categories at all. The defining categories are not for the casual observer of astronomy; they are for the professional astronomers themselves. Without categories, classification in astronomy would result in chaos. For instance, it would be foolish to categorize a black dwarf star (the final stage of a white dwarf star) as a black hole simply because neither black dwarf stars nor black holes can be easily observed. The black hole would be defined as having only three characteristics: Mass, charge, and spin. The same could not be part of the definition of a black dwarf star whose only similarity would be a difficulty in its being seen.

Pluto's and Charon's icy compositions and their odd orbit (not like any of the other Solar System planets) make it unusual. However, Pluto has a great similarity to the Kuiper Belt Objects. The Astronomers are simply giving in to majority rule (as far as the public is concerned) in permitting both categories. However, in order to study objects with Kuiper Belt characteristics, they need to include Pluto and Charon in that category for the purpose of science. The ordinary person need not become bothered about the details. Since Pluto and Charon will continue to be a planet/moon pair, why should the public care about technicalities? The public gets their classification and the Kuiper Belt astronomers will get their combined classification. Never the twain should need to meet. Categorization is found in science. Those who are not scientists, including myself, should not bother their heads about the nomenclature.

I, personally, have no problem with it. Changes in science have never bothered me. I always love it when a theory is reformulated because of revisions due to observation. Science is exciting because it changes.

Pluto and Charon are going to continue to be classified as a planet and moon. I do not see the problem at all.

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

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

Frankly speaking, I do think that Pluto should be classified as a TNO
object only. The "dual" nature seems something of mystical to me... [img]/phpBB/images/smiles/icon_biggrin.gif[/img]

If this has not happened (so far), it is because Pluto is the only "planet"
discovered by an American astronomer in the USA . From the human point of view,
it is understandable that people in the USA are reluctant to "remove" Pluto from
its previous status, so the compromise is acceptable, for one or two
generations.

The question is: what are teachers really teaching in the schools about the
solar system? What about astronomy books and astronomy media? Are they
accurately following the IAU recommendations or are they following the "ordinary
people spirit"?

Probably in 2102 (I will always be in the memberlist of BABB, and you?) nobody
will remember that Pluto was once considered a planet, also because in the
meantime many important objects will be discovered in the Kuiper Belt, and even
beyond!

This Sky and Telescope article says that Marsden, director of the Minor Planet Center, has been trying to demote Pluto for over twenty years, without success. He justifies the reclassification because it "might eliminate its inadvertent rediscovery by observers who occasionally mistake it for a faint new object." Anybody who "forgets" about Pluto is a little careless, don't you think?

Accusing the other side of personal motives is unfair. That opens up a can of worms--both sides are susceptible, and it's unfair. For instance, is Brian Marsden trying to "take over" Pluto, as director of the Center? What was his response when it was suggested that Pluto be classified as trans-Newtonian object TN/1? He opposed the creation of a new classification system--which doesn't seem to fit with the explanation that we are trying to categorize these objects appropriately. TNOs are as like the objects of the asteroid belt as they are planets.

I suggested those criteria for planets for consideration because we don't have any. As the Sky and Telescope article mentions, Pluto satisfies almost any criteria for planethood. Is there anything wrong with the criteria I mentioned? Would Ceres really be considered a planet also?

There are a few asteroids that satisfy all criteria, except #3. 1 Ceres seems distinctly football shaped (though that Sky and Telescope article, in its last paragraph, seems to say otherwise), 4 Vesta has a noticeable non-sphericity, and although I didn't find a photo of 2 Pallas, its dimensions are listed as 570 x 525 x 482km. 10 Hygiea has a radius of 215, but I didn't find any axial information -- so it might be a planet under this definition, but I doubt it. Those are the only asteroids with a radius greater than 200km.

TNO Varuna may have a radius of about 450km, but other TNOs discovered appear to be around 200km or less. I only really added rule 5 to make sure very small objects weren't considered--otherwise, a baseball thrown out the window of a planetary probe would satisfy all the other criteria!
_________________
rocks

<font size=-1>[Fixed format, added comment on rule 5]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2001-12-31 09:37 ]</font>

Well, then, I could live with it being designated a Kuiper belt object. Trans-Neptunian object is simply a bad term. IIRC, the term 'TNO' was introduced to create a new category for Pluto, and not as another name for Kuiper Belt objects.

In truth, I think, career astronomers already consider Pluto the largest known Kuiper belt object. The public at large considers it a planet. That's the way I think it's going to stay.

As for myself, I think it's quite probable that we may decide to demote Pluto eventually. But for now I say we should keep the current standings. I think there's just not enough data to be sure whether Pluto can be considered a TNO or not, because we don't know enough about either Pluto or TNO's to be able to compare them accurately.

What we really need is a mission out there to find out what Pluto really is like. More sightings of TNO's and their make-up would help. If we discover a few more very large TNO's and also find that Pluto is basically identical to them, then there'll be more of a basis for a demotion.

It would also help if we had a clear-cut definition of a planet, such as the one given above. But also, should composition be a factor? Is similarity (or lack thereof) to other known objects really a factor in determining planethood?

At this time there's no difficulty that I can see with keeping planetary status and at the same time studying Pluto as if it were a TNO. When we know more, we can make more clear-cut decisions. What concerns me more is why there's so much trouble getting a mission out there to begin with. It takes so long to get something out there that we have to get cracking. Let's hurry up and get a probe on it's way!


PS:

<font size=+2>***Happy New Year to everyone!***</font>

(It's 01:30am Jan. 1, 2002 where I am, so I can be the first to say so! [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_biggrin.gif[/img])

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

Unfortunately, NASA hemmed and hawed and missed the last window to launch a mission that could get there before the atmosphere freezes out.

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

T. Anderson

From the OP:
It depends on what physical characteristics one uses. As my list shows, Pluto does have physical characteristics that are scientifically comparable to the (other) planets in the solar system.

Is the list reasonable or not? Why not?

I don't think it was NASA per se. I'm sure the folks at NASA really want to send something up there as soon as possible. The problem seems to be more the bean-counters who won't allocate enough to fund a decent mission on time.

That and the current debate seems to stem on which is more important, Pluto or Europa. Everyone seems so keen on the idea that there might be LIFE up there that they don't really stop and analyze the situation carefully. I mean, we can get a mission to Jupiter quite easily. Mission after mission has been heading there for years. But it's quite difficult to get anything out far enough to visit Pluto. What, it's something like 8 years travel time?

Europa ain't going anywhere. I say we should focus on getting some data on all the major planetary objects before going back to explore more carefully what we have already visited. I'm also thinking about Mercury, which has had only one fly-by visit and is only half mapped. We may be missing something incredible simply because we aren't taking the opportunity to look.

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

An excellent point about whether composition should be an important part of determining planet status.

The definition of planet, in my mind, should be geared toward classifying things we haven't found yet - when we come to another object in another star system, how should we designate it?

If it's as big as Jupiter, but is essentially a giant ball of methane (forgive my ignorance - Pluto is mostly methane, right?), will we deny it planetary status?

Similarly, are the giants they are finding now, which have highly elliptical orbits, not planets because their orbits are so eccentric?

If it's as big as Jupiter, but is in the middle of that system's Kuiper belt (an impossible situation, granted, but for the sake of argument...) would it not be a planet?

If it's as big as Jupiter, but was captured by the system, rather than created by the leftover rubble when the star was born...?

etc.

For me, the answer to all the questions I've posted is 'still counts as a planet.' I agree with the criteria above - not a star nor brown dwarf, orbits a star or brown dwarf, bigger than anything that's not a planet.

So the debate is really the cutoff mass. Asimov's suggestion that it has to be big enough so that its gravity pulls it into something pretty close to a sphere, works for me - for solid planets, anyhow.

Pluto is not being denied planetary status. It has two designations, because it fits both designations.

Planets tend to be those which formed around the star (Sun) out of the primordial matter from which the Sun, itself, was formed. Objects which are thrown into the Solar System (such as comets) tend to have a separate category.

I think that the astronomers in the IAU are interested in studying Pluto in both ways. I personally like a science which is able to change with new information. I, myself, do not get bothered with change. So, I can live with this easily. However, just ignore the second category, because Pluto is, first of all, a planet, and only secondly, a TNO (almost as an afterthought).

HAPPY NEW YEAR!

And HAPPY NEW YEAR! to

BAD ASTRONOMER PHIL PLAIT and to this wonderful site!

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

<font size=-1>[ This Message was edited by: ljbrs on 2001-12-31 22:53 ]</font>

OH, YES... I HOPE NONE OF YOU HAVE VENTURED OUT ON "AMATEUR NIGHT"!

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

<font size=-1>[ This Message was edited by: ljbrs on 2001-12-31 22:59 ]</font>

Does it have two designations? The FAQ on the IAU.org website says that the press release of 01/1999 "expresses the position of the IAU regarding the status of Pluto. The IAU considers the discussion closed with this statement and does not intend to reopen it in the foreseeable future."

The press release says that "The Small Bodies Names Committee of the Division has, however, decided against assigning any Minor Planet number to Pluto."

But "Ways to classify planets by physical characteristics are also under consideration," so I think the discussion of such characteristics, in this thread, is appropriate.

And Happy New Year!

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

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-01-01 05:20 ]</font>

How about this. Instead of a dual classification system like we've been discussing for Pluto, we create a multi-layered system. We could classify objects something like this:

A: Large (Planetary) objects
Those that qualify under the planetary rules above

1) Large rocky bodies (Earth, Mercury)
2) Large gaseous bodies (Jupiter, Neptune)
3) Large icy bodies (Pluto, other large TNO's if found)
4) Other categories if found necessary (Hot Jupiters?)

B: Small (Planetoid) objects
Those that don't fall under the rules above.

1) Moons (Titan, Charon, The Moon)
2) Small rocky bodies (Asteroids)
3) Small icy bodies (Comets, Kuiper Belt Objects)
4) Other categories as necessary


Of course, these are just off the top of my head. It would take real experts to fix up a working system. Other sub-categories could be made and cross-categorization may also be possible. But this has the advantage of defining planets by both composition and orbital mechanics, while still leaving them open for inclusion with other objects. Pluto can retain its planetary status, but also still be included or considered as an icy Kuiper Belt object when the need arises.

Really, it's just a formal reworking of what we're doing already. Well, whaddayathink?



_________________
David Hall
"Dave... my mind is going... I can feel it... I can feel it."

<font size=-1>[ This Message was edited by: David Hall on 2002-01-01 09:41 ]</font>