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Fraser
2006-Aug-16, 04:27 PM
The International Astronomical Union, currently meeting in Prague, has announced a proposal that would boost the number of planets in the Solar System to 12. Under their new classification, the asteroid Ceres, Pluto's moon Charon, and the newly discovered UB313 (aka Xena) would join the traditional 9 planets we're familiar with. Any additional large bodies would also be described as planets. The IAU will make a final vote on this proposal on August 24.

Read the full blog entry (http://www.universetoday.com/2006/08/16/officials-propose-12-planets-in-the-solar-system/)

ToSeek
2006-Aug-16, 07:08 PM
Discussion well underway here. (http://www.bautforum.com/showthread.php?t=45759)

Ray Bingham
2006-Aug-17, 02:19 AM
Well at last the IAU has made a decision. They have deceided to make a decision on Aug 24.

Lets see what develops. What is proposed sounds somewhat reasonable except for the fact that it requires Pluto and Charon to be classified as a dual planet system while ignoring the Earth Luna system.

Ray bingham

Leafguy
2006-Aug-17, 05:19 AM
By making Charon a planet, wouldn't that theoretically make our moon a planet as well? I know there has been some consideration about Pluto and Sharon being a twin planet system because of Sharon's mass relative to Pluto's, and with Luna falling into this weight category of being 1/10 the mass of Earth, that should theortically make grounds to consider it a planet as well.

As far as the definition also goes, anything that can sustain its own gravity. There are several reported asteriods with satellites. They have certainly pulled other objects in with their own gravity, so that to should technically make them planets as well.

Steve Moore

hhEb09'1
2006-Aug-17, 05:33 AM
They have certainly pulled other objects in with their own gravity, so that to should technically make them planets as well.How do you know that they are just not slowly losing them, rather than pulling them in? :)

WaxRubiks
2006-Aug-17, 05:37 AM
how will "Planet Hollywood" be effected by this decision?

01101001
2006-Aug-17, 05:42 AM
By making Charon a planet, wouldn't that theoretically make our moon a planet as well?

Not according to the proposal. It's covered in an article (http://www.bautforum.com/showthread.php?p=805934#post805934) in discussion topic Pluto's a planet! (And Ceres and Charon and ... )

hhEb09'1
2006-Aug-17, 05:42 AM
how will "Planet Hollywood" be effected by this decision?going to be renamed Theirani

trinitree88
2006-Aug-17, 10:39 AM
Oooh! Oooh! Any system that names a planet after Xena the Warrior Princess, is fine by me.:D

kzb
2006-Aug-17, 11:48 AM
I really like the idea of "plutons"; "kuiper belt objects" doesn't exactly trip off the tongue. What I don't like is reclassifying Ceres as a planet, and classifying Charon as a planet when it is clearly a satellite. I think they should drop the double planet definition for pairs where the centre of gravity is outside both bodies. Also where does this leave the two small pluto satellites, are they now moons of a double planet?

Blob
2006-Aug-17, 01:05 PM
Hum,
i suspect that the Nix and hydras centre of gravity is outside both bodies - but they are not round.
So they would still remain moons of the double planet.

A weird aspect of Pluto/Charon system is that each planet would sometimes be the 11th planet and sometimes be the 12th.
And if their mutual orbit lay perpendicular to their collective orbit around the sun they would be the 11 and the 12th at the same time....

qeveren
2006-Aug-17, 03:31 PM
Actually, they'd sometimes become the 10th and 11th planets, as well, when they sneak inside the orbit of Neptune. :)

jhwegener
2006-Aug-17, 03:41 PM
question the idea that "planets" are one fundamental or even important category of objects. The "original" seven planets inluded sun and the moon. Our anestors could see with their eyes that above us are two fundamental different types of objects: The "fixed" stars, round a stationary center earth and the "wanderers". With telesopes it is all challenged. Perhaps astronomers should make a discussion about more adequate terms?

goddardrocketry
2006-Aug-17, 04:14 PM
I was looking at the picture i IAU chose to release to descibe our newly "understood" solar system. Look at the shadows. They are all bein cast in different directions. So much for constants in the universe. We not only have more planets to teach our kids about, we also have the evident changes in the physics of light to deal with. Dang-it! My kids will be so confused.

kzb
2006-Aug-17, 04:45 PM
What's the definition of "round" anyway? Just how far away from perfectly spherical does something have to be to cross the line?

nikolamilevski
2006-Aug-17, 05:10 PM
The proposed definitios says:

“A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”

Isn't then logical contradiction to include Charon among the planets when, as we know, it is satellite of a planet, or vice versa? If it is double-planet system, then why our Moon is not a planet (Earth-Moon double planet system)?

Blob
2006-Aug-17, 05:43 PM
What's the definition of "round" anyway?

Hum,
during GA-XXVII the IAU hope to define “nearly round”...

Doodler
2006-Aug-17, 07:04 PM
If it is double-planet system, then why our Moon is not a planet (Earth-Moon double planet system)?


The gravitational center of the Earth-Moon system is within the Earth. With Pluto and Charon, the center is between them.

nikolamilevski
2006-Aug-17, 09:35 PM
With Pluto and Charon, the center is between them.

Although these is true, in the most of encyclopedias Charon is said to be satellite of Pluton. For example here (http://en.wikipedia.org/wiki/Charon_(moon)). So, if we define Charon as satellite, could it be planet at the same time?

01101001
2006-Aug-17, 09:55 PM
Although these is true, in the most of encyclopedias Charon is said to be satellite of Pluton. For example here (http://en.wikipedia.org/wiki/Charon_(moon)). So, if we define Charon as satellite, could it be planet at the same time?

It's probably not good to cite Wikipedia in support of maintaining the status quo, as it changes swiftly. There have been lots of additions there in the last couple of days about the moon/planet issue and what Charon's status may soon be.

On the other hand, citing more staid, less flexible, encyclopedias isn't much of an argument either. Astronomy facts are amended all the time. New data is obtained. Definitions change to accommodate improved understanding.

If encyclopedias defined Charon as a moon of Pluto, then they'll just change, if necessary, to describe it as the lesser member of the Pluto-Charon dual-planet system. No big deal.

Can a satellite be a planet? I don 't know. I'm not sure there's a solid definition of satellite out there to help with that; it wouldn't bother me if it could -- in general, there's no reason a member of one class cannot also be a member of another class.

It certainly appears a satellite might become a planet, when definitions change. Will it remain a satellite? It depends on what a satellite is. Time for another IAU working group?

Brack the Barbarian
2006-Aug-17, 11:27 PM
Science thrives on controversy and this one is set to be a humdinger. The full draft of the proposition has only recently been released and already it is proving highly controversial and is being heartily condemned by some pretty hefty players. Whether or not it will get through remains a moot point and a fascinating diversion for us onlooker.

The trouble stems from the fact that there are really only two options.

1. To have only 8 'Planets' and exclude Pluto, everything else being called something else.

2. (my own favourite) Call everything that orbits the Sun a 'planet' - the word only means 'wanderer' anyway- and then distinguish between them when necessary - cometary planets, asteroidal planets etc. The capital P being reserved for the big 8

The IAU has gone for neither of these and chosen a clumsy third alternative that falls between both stools and pleases neither side.

hhEb09'1
2006-Aug-17, 11:34 PM
2. (my own favourite) Call everything that orbits the Sun a 'planet' - the word only means 'wanderer' anyway- and then distinguish between them when necessary - cometary planets, asteroidal planets etc. The capital P being reserved for the big 8

The IAU has gone for neither of these and chosen a clumsy third alternative that falls between both stools and pleases neither side.Unfortunately, the Sun is a wanderer as well--it was one of the original planets. :)

Brack the Barbarian
2006-Aug-17, 11:47 PM
Hum,
during GA-XXVII the IAU hope to define “nearly round”...
This gets to the clumsiest part of the proposal. We don't even know yet whether or not Vesta is 'round enough' to be called a planet. What if it turns out to be something like Miranda - would that be 'round enough'??

Brack the Barbarian
2006-Aug-17, 11:50 PM
Unfortunately, the Sun is a wanderer as well--it was one of the original planets. :) Yes I know but what has that got to do with it?? Planetes means wanderers in general not merely 'heavenly' bodies

RUF
2006-Aug-18, 12:21 AM
(a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

I like this definition. It should be worded to rule out Charon. Make the planet name "Pluto/Charon". We are going to find too many asteroids or KBOs that orbit about each other, and then they would all be planets instead of "dual-planets" or the like.

I also dislike "plutons." I like "planetoids" or "planetesimals"

gerrol
2006-Aug-18, 12:35 AM
I'm all for simpler and shorted names...but why plutons? This clashes with plutons used geology (igneous intrusives). Better to avoid potential ambiguities by looking at terminology in other disciplines. On the other hand, context and factors of scale are likely to reduce amiguity here.

Anyone for plutoids? plutesimals? I like RUF's suggestions of planetoids or planetesimals?

Gerrol

nikolamilevski
2006-Aug-18, 01:23 AM
It certainly appears a satellite might become a planet, when definitions change. Will it remain a satellite? It depends on what a satellite is.

Of course. But that mean that if we want to decide weather Charon is planet or not and in the same time to keep the definition propoused by IAU, first we must define what satellite is.

01101001
2006-Aug-18, 01:52 AM
Of course. But that mean that if we want to decide weather Charon is planet or not and in the same time to keep the definition propoused by IAU, first we must define what satellite is.

That part of satellite is defined in the proposal (http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html).


[2] For two or more objects comprising a multiple object system, the primary object is designated a planet if it independently satisfies the conditions above. A secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto-Charon a double planet.

It's difficult to word answers to your questions, given the possibilities. How about: If the resolution doesn't change and passes, then, according to the IAU, Charon will not be a satellite and will be a planet -- at least until the next change.

Leafguy
2006-Aug-18, 03:10 AM
Ruf,
Its true we may find plenty of Kuiper belt objects that will fit into the pluton category or twin planet. Speculations even suggest there are some as large as earth. I don't think for the next few years, that we will find alot, but given the series of telescopes they plan to launch and have in Space, that may very well lead to another discussion from the IAU in a few years.

homo_cosmosicus
2006-Aug-18, 07:19 AM
Of course. But that mean that if we want to decide weather Charon is planet or not and in the same time to keep the definition propoused by IAU, first we must define what satellite is.

Hi, IAU made proposal that satellite is object orbiting around planet,
which means that common center for planet and satellite is inside the planet
not in the space between planet and satellite.

This sounds logical, but brings us into interesting situation:

If Moon was around 520.000 km from the Earth it would be a planet.
Now, lets assume that we find in future, a planet with mass of 10 jupiter,
and around it a object with mass of, lets say, 1 jupiter...
Now, if the smaller object (1 jupiter mass) was close enought to the first
object with mass of 10 jupiters, then it would be considered a satellite,
althought its mass is 25000 times greater then Moon's mass.

homo_cosmosicus
2006-Aug-18, 07:50 AM
There is other thread about the same topic:

Pluto's a planet! (And Ceres and Charon and ... )
http://www.bautforum.com/showthread.php?t=45759

hhEb09'1
2006-Aug-18, 08:48 AM
Now, if the smaller object (1 jupiter mass) was close enought to the first
object with mass of 10 jupiters, then it would be considered a satellite,
althought its mass is 25000 times greater then Moon's mass.The core of the Earth has mountains and valleys and "weather"--and it is about the size of Mars. It is not going to be considered a planet either. :)

It's part of another planet. I think that is the logic behind the IAU proposal, with the barycenter concept.

kzb
2006-Aug-18, 12:04 PM
<<I like "planetoids" or "planetesimals">>

No you can't have those either, planetoid has been used as an alternative to asteroid and planetismals are what got together to form the planets. "Plutoid" might be OK, as it solves the ambiguity issue with the geological term "Pluton". Personally tho I wouldn't think this ambiguity can be a serious issue, they wouldn't be discussed in the same contexts.

I really don't think the defiinition of satellite is good at all. All this about where the baryonic centre is. Why not just stick with relative size? The biggest one is the planet (as long as it meets the other criteria) and the others are satellites, simple.

schlechtj
2006-Aug-18, 12:43 PM
The Earth Luna system is not a double planet by the new rules.

Mass of earth = 6x10^24kg
mass of moon = 7.36x10^22

ratio of earth to lunar mass = 81

Distance from earth to moon = 238857 miles

center of mass of the system = distance from earth to moon/mass ratio = 3000 miles

Since the radius of the earth is about 4000 miles, the center of mass is still within the earth so it wobbles but does not rotate around the moon, thus the system is not a double planet

Joel Schlecht

Leafguy
2006-Aug-18, 01:36 PM
I will have to agree with what the cosmic homo said :)
In that scenario if they find two multiple jupiter mass objects, the smaller one may be considered a satellite. Which brings me to another interesting question.
So lets say we do find these two objects, except the smaller jupiter mass object has satellites rotating around it as it rotates around the larger planet. Does it still count as a satellite? And secondly, would they consider it a moon if it has rings :)

schlechtj
2006-Aug-18, 01:47 PM
The most likely thing is they will wait untill they have half a dozen cases then they will vote on it. Most likely, they will propose something opposite to what every one was thinking in the first place.

schlechtj
2006-Aug-18, 01:52 PM
By the way, this is almost already the case as Titan and Gynamede are larger than both Mercury and Pluto And they have atmospheres where the planets dont.

Kootenaistar
2006-Aug-18, 09:45 PM
I, also, have poor feelings about the naming of "plutons" because many, like I, have interests in several sciences. No, I am not professional in any sort of way, but that is exactly where it could become confusing. Astronomy is related to almost all of the sciences, and re-using a defined name from one to name something totally different in another is like running muddy water over the road, for me at least. I do hope that the committee will see and rethink this.

WaxRubiks
2006-Aug-19, 03:56 AM
if the Earth was somehow thrown into the orbit of Jupiter then it would become a satellite. A planet should be a planet just because of its size not where it is or how it orbits IMO.

hhEb09'1
2006-Aug-19, 01:26 PM
if the Earth was somehow thrown into the orbit of Jupiter then it would become a satellite. A planet should be a planet just because of its size not where it is or how it orbits IMO.That's the issue that I was trying to address in this post above (http://www.bautforum.com/showthread.php?p=806893#post806893). So that the lunar progression that the BA described (erroneously) in his blog would be like calving off a new planet. :)

Dubb
2006-Aug-19, 04:04 PM
-

hiro
2006-Aug-21, 04:34 PM
If the object is bigger than Mercury and orbit around the sun, then it should be defined as a planet.

Sporally
2006-Aug-21, 09:23 PM
Sorry for jumping into this thread without reading the other threads, but i just need a quick answer to this and i won't bother you no more;)

I don't know if it has been discussed here, and maybe i already know the answer. Quaoar and Sedna, two objects orbiting Sol, they are not stars and they have the gravity to make themselves spherical. How come they won't be counted as planets?

I heard that other objects might be added to the list of planets later, but i presumed they were talking about objects not yet found. Now that i come to think of it they might have been talking about objects like Quaoar and Sedna. But if that's the case, how come they aren't added to the current list of 12 planets (in case that is the one which will win the vote) if they qualify in all ways? I've also heard someone say 24 planets - is that another definition or is it that there actually are 24 objects that have the criteria for being a planet but astronomers don't want to call them planets anyway or something?

Maybe i am just way of track:razz: Hope you don't mind my subject change, but this thread seemed somewhat fitted for this type of talking, hope you don't mind...:) Thx in advance for your help on this subject.

01101001
2006-Aug-21, 10:23 PM
Quaoar and Sedna, two objects orbiting Sol, they are not stars and they have the gravity to make themselves spherical. How come they won't be counted as planets?

The IAU proposal Q&A is linked in this article (http://www.bautforum.com/showthread.php?p=806023#post806023) (as well as more entries in this table).


Other objects that appear large enough so that their shape satisfies the definition of “planet” will be further considered on a case by case basis.

Table 2: Planet candidates as per 24 August 2006 to be given future consideration if “Resolution 5 for GA-XXVI” is passed.

Object Unofficial diameter estimate
[...]
(90377) Sedna 1200-1800 km
[...]
(50000) Quaoar ~1000 km
[...]
(10) Hygiea 500&#215;400&#215;350 km

I believe they'll just need to be more certain of their mass and equilibrium-ness.

schlechtj
2006-Aug-22, 05:08 AM
I mentioned above that the earth and the moon are not a double planet. However, the earths rotation is slowing due to tidal friction. As the earths day retards, the moon spirals out in order to keep the angular momentum of the system constant. This is gradualy moving the center of gravity of the earth moon system and will eventually create a double planet by the current proposal. Presumably they will vote on something more accurate by then...

WaxRubiks
2006-Aug-22, 05:31 AM
I used to think that the shape of the object in orbit should be the defining quality to make something a planet but I have changed this view. I now think it should purely be a matter of surface gravity that is for example that anything over 0.02g should be a planet regardless of shape. I mean what difference does the shape really mean, to argue the definition of planet based on shape is shapest(shock horror!). as has been pointed out in the BAblog some planets are easily shaped into a sphere whereas other, equally massive planets, are not.

JonClarke
2006-Aug-22, 07:22 AM
Shape is the result of specific and universal processes that reshape the entire interior of the body and have majpor implications for future evolution. In separates highly evolved bodies from primitive ones. It is a real criterion. A particular surface gravity has no real meaning.

Jon

WaxRubiks
2006-Aug-22, 07:50 AM
Shape is the result of specific and universal processes that reshape the entire interior of the body and have majpor implications for future evolution. In separates highly evolved bodies from primitive ones. It is a real criterion. A particular surface gravity has no real meaning.

Jon

what meaning or importance does shape have over surface gravity( or just gravity effects generally if you like).

Sporally
2006-Aug-22, 09:19 AM
I mentioned above that the earth and the moon are not a double planet. However, the earths rotation is slowing due to tidal friction. As the earths day retards, the moon spirals out in order to keep the angular momentum of the system constant. This is gradualy moving the center of gravity of the earth moon system and will eventually create a double planet by the current proposal. Presumably they will vote on something more accurate by then...

Have you calculated when that will happen? I haven't done that, but from what i have heard the moon moves away from Earth at 1cm per year at the current pace. But as the distance between Moon and Earth expands the gravity is decreased. Doubling the distance (which i would guess would double the distance from Earth to the center of gravity in case the gravity wasn't changed), would result in only 1/4 the gravity.

g = (G*M)/r^2

g = gravitationel pull
G = gravitational constant
M = Mass of Earth
r = radius between Earth and Moon

Furthermore, i believe the shape is an odd thing to decide what is a planet from. You could say, what is the definition of a spherical shape? None of the planets are entirely spherical, and what about Mars' Olympus Mons which might be considered a part of the planet, which increases the size at the equator while the pole to pole is smaller asusal. That makes the differences bigger. However i think the total mass of the object could be the definition instead of the shape, but i can agree that people, including me, want a somewhat spherical object to be a planet rather than a big object. So i guess for this reason the shape is the best to define the planets:shifty:

JonClarke
2006-Aug-22, 12:12 PM
what meaning or importance does shape have over surface gravity( or just gravity effects generally if you like).

The presence of sufficent mass for the establishment of hydrostatic equilibrum results in major reorganisation of the interior of the body. Bodies smaller than this threshold will be very different in terms of their internal structure than those above it. They will have undergone melting, differentation, formation of a core, mantle, and crust, and in some cases partial to complete loss of volatiles.

By contrast, an arbitary value of 0.02 G is just that, arbitary. It is no more significant than 0.1 G, or 0.3.

Jon

Ivan Viehoff
2006-Aug-22, 02:41 PM
I observe a two competing definitions are now proposed for the attention of the conference. One would have great many planets, and probably has little support. The other is to have just 8 planets, by including the "dominant in its orbit" rule.

Short of arbitrary radius rules one can hardly include Pluto but exclude Ceres, but Ceres' non-planet status has bothered few to date. That is because most of us have quietly known that Pluto isn't a planet either. I suspect the 8 planets proposal must have a good chance. But I suspect the IAU official proposal will get through because demoting Pluto would not be good publicity for the New Horizons probe, and because we can all be excited by hunting for new planets.

In any definition, "roundness" will require care, because Saturn is 10% off round.

Brack the Barbarian
2006-Aug-22, 05:13 PM
I'm all for simpler and shorted names...but why plutons? This clashes with plutons used geology (igneous intrusives). Better to avoid potential ambiguities by looking at terminology in other disciplines. On the other hand, context and factors of scale are likely to reduce amiguity here.

Anyone for plutoids? plutesimals? I like RUF's suggestions of planetoids or planetesimals?

Gerrol

Excellent Gerrol, I couldn't have put it better myself - I was toying with saying something similar. Plutons IS a daft word, already used elsewhere. My own favourite is Plutoids, drawing an obvious analogy with asteroids.

schlechtj
2006-Aug-23, 01:12 AM
Ok, rough calculation.....

According to my calculations above, the system is short of a double planet by about 1,000 miles, since the ratio of earth to moon mass is 81 times, the moon would have to move out about another 81,000 miles.

According to the Nasa lunar laser ranging project at http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html
the moon is moving away at the rate of 3.8 centemeters a year or 1/42,351 of a mile a year.

So, 81,000 miles / 1/42,351 miles/year = about 3.4 billion years.

Yes I know I didn't use calculus to figure out how the process will slow, but I hope no one sues me for a few measley million years. I'll put a penny in the bank to cover the litigation when the time comes.

Joel Schlecht

01101001
2006-Aug-23, 02:11 AM
[...]about 3.4 billion years.
Let the BA do the calculation for you. BA Blog: I made a massive mistake (http://www.badastronomy.com/bablog/2006/08/18/i-made-a-massive-mistake/)

But that neglects what others point out, for instance The Earth/moon double planet blogs (http://www.bautforum.com/showthread.php?t=45888): after tidal lock happens, the recession process stops and the Moon begins approaching Earth.

01101001
2006-Aug-23, 02:19 AM
I observe a two competing definitions are now proposed for the attention of the conference. One would have great many planets, and probably has little support. The other is to have just 8 planets, by including the "dominant in its orbit" rule.

My surmise (links here: article in topic Pluto's a planet! ... (http://www.bautforum.com/showthread.php?p=809945#post809945)), after straw polling Tuesday Aug. 22, is that the orbital-dominance proposal fell, a third compromise between the original and orbital-dominance fell, and that the original -- with the pluton re-use removed -- got about 50-50 support.

Edit: But, after watching fragments of the Aug. 22 meeting (link here (http://www.bautforum.com/showthread.php?p=810099#post810099)), I now believe it's more like:
Part 1, a compromise specifying hydrostatic-equilibrium roundness, with 8 classical planets, dominant, as well as dwarf planets, not dominant (e.g. Pluto), got 50-50 support; part 2, defining a class for TNO Pluto-like dwarf planets as plutons, plutoids, and other undecided names didn't get a lot of support; part 3, defining a secondary member of a system as a planet if it fit and the barycenter was outside the primary didn't get a lot of support.

hhEb09'1
2006-Aug-23, 04:43 AM
Yes I know I didn't use calculus to figure out how the process will slow, but I hope no one sues me for a few measley million years. I'll put a penny in the bank to cover the litigation when the time comes.You don't need to use calculus :)

The angular momentum of the earth/moon system will stay the same, until the moon's revolution about earth matches the rotation of the earth. That distance is less than that needed to move the barycenter out of the earth.

jkmccrann
2006-Aug-23, 08:14 AM
The presence of sufficent mass for the establishment of hydrostatic equilibrum results in major reorganisation of the interior of the body. Bodies smaller than this threshold will be very different in terms of their internal structure than those above it. They will have undergone melting, differentation, formation of a core, mantle, and crust, and in some cases partial to complete loss of volatiles.

By contrast, an arbitary value of 0.02 G is just that, arbitary. It is no more significant than 0.1 G, or 0.3.

Jon

As you say, settling on a value of 0.02G to provide the lower boundary for what may be classified as a planet seems like a completely arbitrary value to settle on - but as you say, which I've bolded, there is a threshold there somewhere, are you aware at what gravity level that kicks on - because that is not an arbritrary value.

As for whether the Moon one day, perhaps in billions of years becomes a planet under the proposed definition - it may be an interesting scientific discussion - but does anyone seriously use that as a reason to dismiss the proposal before the IAU? It seems fairly ludicrous to argue that because of a possible situation billions of years into the future - when its extremely unlikely we'll even still be around, that the current proposal must therefore be inherently unreliable or somehow - just not right.

As far as using barycentres to determine whether a body is a moon/satellite or a planet, I really can't think of a better way to go about it - and if the barycentre moves in different directions and into different places over time - then so be it - we don't live in a static universe and we should be flexible enough in our attempts to classify things to recognise that.

Grand_Lunar
2006-Aug-23, 03:37 PM
Perhaps we need different catagories for planets, aside from the two that exist now; gas giants and terrestrials. It'd be like what we have for galaxies.
I do believe someone else suggested this idea, but I forgot whom.

Anyway...
Rather than "plutons", why not "ice dwarves"? Or is that term taken?

If Ceres is upped in status to a planet, then I suppose one could provide a class known as "dwarf planets".

Rather than roundess, suppose the criteria would be mass and composition? That seems reasonable.

WaxRubiks
2006-Aug-23, 11:48 PM
As you say, settling on a value of 0.02G to provide the lower boundary for what may be classified as a planet seems like a completely arbitrary value to settle on - but as you say, which I've bolded, there is a threshold there somewhere, are you aware at what gravity level that kicks on - because that is not an arbritrary value.

well, gravity or mass as the criteria for 'planet' would overcome the fact that some objects form more easily into spheres than others of the same mass.
You could determine the mass for a 'planet' as being the mass that objects of composition x,y and z form into round objects then it would be less arbitrary and leave other objects, that are to tough to form into spheres, still in the catagory of planet regardless of their shape.

schlechtj
2006-Aug-24, 03:02 AM
I am verrrryyy steamed! I calculated how much momentum the earth will impart to the moon and how much further out it will be for tidal lock. I wrote out each step in my message but, it took me about 2 hours. Aparantly, if you take too long to post it will log you out and you will loose all your text.

Beleive me or not.... At tidal lock, the center of mass for the earth-moon system will be over 5,600 km above the surface of the earth. Way more than enough to make it a double planet.

Now, I got the time to double planethood calculation on the first try where the BA himself did not so that gives me a little bit of credibility. But, If many people want i can recreate my calculations. But PLEASE just take my word for it.

Joel Schlecht

01101001
2006-Aug-24, 03:21 AM
But PLEASE just take my word for it.

OK. Word taken. But, in the meantime, the double-planet issue appears to have spun itself out of existence. It's not in the final vote-upon resolutions I saw. Official IAU News posting of the final resolutions is due in about 3-4 hours.

JonClarke
2006-Aug-24, 03:47 AM
As you say, settling on a value of 0.02G to provide the lower boundary for what may be classified as a planet seems like a completely arbitrary value to settle on - but as you say, which I've bolded, there is a threshold there somewhere, are you aware at what gravity level that kicks on - because that is not an arbritrary value.

But it is an artibary value of no significance to planetary science. Where ad the miorphology definition is of great significance. Whether or not a body attains hydrostatic equilibrium is partly determined by mass, partly density, and partly by the level of heating, which itself is the result of internal (extinct and extant radioelements)and external factors (giant impacts, tidal flexure)


As for whether the Moon one day, perhaps in billions of years becomes a planet under the proposed definition - it may be an interesting scientific discussion - but does anyone seriously use that as a reason to dismiss the proposal before the IAU? It seems fairly ludicrous to argue that because of a possible situation billions of years into the future - when its extremely unlikely we'll even still be around, that the current proposal must therefore be inherently unreliable or somehow - just not right.

As far as using barycentres to determine whether a body is a moon/satellite or a planet, I really can't think of a better way to go about it - and if the barycentre moves in different directions and into different places over time - then so be it - we don't live in a static universe and we should be flexible enough in our attempts to classify things to recognise that.

This is the problem of using orbits as part of the deinition. You end up with objects that are planets in all but name (Titan, for example), objects that were once planets but are captured to become satellites, and satellites that are lost and become planets. You may also have bodies that are structurally plents of various origins that are relfloating in interstellar space.

Personally, I would like to see morphology as the primary definition: A planet is a non-fusing astronomical body that has attained hydrostatic equilibrium. A secondary level that classifies them according to composition (gassy, rocky, icy). A tertiary level that classifies them by orbit - primary satellite, unattached, etc.

Jon

Chuck
2006-Aug-24, 03:54 AM
http://cagle.com/news/Planets/images/matson.gif

01101001
2006-Aug-24, 07:00 AM
Final resolutions for final General Assembly vote, August 22:

The Final IAU Resolution on the definition of "planet" ready for voting (http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html)

Finally.

Maksutov
2006-Aug-24, 08:36 AM
What's the definition of "round" anyway? Just how far away from perfectly spherical does something have to be to cross the line?Here you go. (http://www.bautforum.com/showpost.php?p=806986&postcount=21)

Two problems (at least) with IAU definition.

1. "(nearly round) shape" is not a numerical value and is therefore subject to interpretation, which opens another can of worms.

2. "Round" is the wrong tolerance zone. It is two-dimensional. Sphericity is the correct tolerance zone as it controls three-dimensional shapes.

As can be seen, this means the IAU needs to change their form tolerance, plus they need a specific numerical value for how much the body in question is allowed to deviate from perfect sphericity. In the referenced specification this allowed deviation is the difference in radius between the inner and outer coaxial spheres which define the tolerance zone.

Y14.5 only defines the tolerance zones. It doesn't assign values to them. It's up to the IAU to assign a value. "(nearly round) shape" doesn't cut it.

Grand_Lunar
2006-Aug-24, 12:01 PM
Looks like they're dropping the term "plutons" and going with "plutonian objects."

Ivan Viehoff
2006-Aug-24, 02:14 PM
We seem to be heading for a fudge in which both sides can say they have won. By allowing Pluto, Ceres, etc to be "dwarf planets", then you can continue to assert that Pluto is a planet if you want to. But since the number of dwarf planets will remain uncertain and potentially large, "8" will be the only possible answer to a meaningful "how many" question.

The other increasingly meaningless "how many" question is "how many moons has...", as some of Saturn's new-found "moons" seems to be less than 3km across. Where will such nonsense end? I say that we need a new category of dwarf moons to sort this out...

hhEb09'1
2006-Aug-24, 03:30 PM
We seem to be heading for a fudge in which both sides can say they have won. By allowing Pluto, Ceres, etc to be "dwarf planets", then you can continue to assert that Pluto is a planet if you want to. But since the number of dwarf planets will remain uncertain and potentially large, "8" will be the only possible answer to a meaningful "how many" question.Ceres always has been a minor planet, right?

Ivan Viehoff
2006-Aug-24, 04:47 PM
I see the results are now published and we have 8 "planets", plus "dwarf planets", (the proposed "classical planets" terminology must have lost). And what were once proposed to be "plutons" are now, as they were before, "trans-neptunian objects".

A vote for common sense, and the creation of "dwarf planets" avoids too much egg on the face of Pluto supporters. The BBC still rubs it in their face - "Pluto loses status as planet" - so perhaps not.


Ceres always has been a minor planet, right?Minor planet or planetoid is anything bigger than a meteoroid, ie, more than about 10 metres across, not a lot of prestige in that. The new dwarf planet category is much more prestigious, being only about 12-15 candidates currently known.

CuddlySkyGazer
2006-Aug-25, 03:05 AM
Ceres always has been a minor planet, right?
No. When originally discovered it was classified as a planet. It and other similar objects gained the apellation of 'minor planets' because publishers of astronomical tables found they were making the pages look untidy and relegated them to an annex (which they gave that title)!

hhEb09'1
2006-Aug-25, 04:24 AM
It and other similar objects gained the apellation of 'minor planets' that's what I meant, that it has been a minor planet as long as the term has been used.

My point was, that calling Pluto a dwarf planet gives absolutely nothing towards planethood--anymore than calling Ceres a minor planet did, as is well recognized.

"was" to "has"

schlechtj
2006-Aug-26, 02:51 AM
I have a problem with the "cleared the neighbourhood around its orbit" part of the resoloution. This works fine for the solar system as we know it today but, could cause problems possibly in our solar system later and definatly in other systems.

We know that large objects can form far away from the parent star because we can observe brown dwarfs in other systems. The orbit of Sedna indicates that there may be another large body in our solar system. An Earth sized object at 1000 au, a Neptune sized object at 2000 au, or a Jupiter size object at 5000 au. An orbit of 5000 au would take over a third of a million years to complete one revolouton around the sun. At that rate, how long will it take to clear the neighborhood of other material? And even if it did, a few tens of thousands of years may be enough time for material to accumulate once sweapt away. It would seem very silly to call a Jupiter sized object a dwarf planet simply because it could not clear its orbit of debris.

In the early years of the solar system while the planets were sweeping up material in their orbit I soppose we could have called them protoplanets but still the hypothetical jupiter at 5000 au really would not be forming any more so this distinction does not seem to fit either.

Furthermore, "Classical Planet" does not work because that completely ignores planets orbiting other stars.

Personaly it makes more sense to drop the "cleared the neighbourhood around its orbit" part and have a few dozen planets rather than have to change the definiton later.

But, whats done is done until they have to change it again. Which they would not have to do if they left just that little bit out.

Joel

jkmccrann
2006-Aug-29, 09:26 AM
Furthermore, "Classical Planet" does not work because that completely ignores planets orbiting other stars.

Joel

I believe that the resolution calling for the 8 recognised planets to be referred to as `Classical Planets' was in fact defeated at the Congress so, although you are right in saying `Classical Planet' has nothing to do with planets orbiting other stars - its an irrelevant point because no one is referring to `Classical Planet' in any formal sense.

As others have pointed out, the term `Classical Planet' is already reserved, for certain Solar Bodies - and it doesn't include Uranus or Neptune!

hhEb09'1
2006-Aug-29, 01:46 PM
As others have pointed out, the term `Classical Planet' is already reserved, for certain Solar Bodies - and it doesn't include Uranus or Neptune!Does it include the sun and moon, and exclude earth?

antoniseb
2006-Aug-29, 02:25 PM
The Sun and Moon were Luminaries, not planets, but yes if you're discussing pre-Copernican astronomy it should exclude Earth.

hhEb09'1
2006-Aug-29, 02:32 PM
The Sun and Moon were Luminaries, not planets, but yes if you're discussing pre-Copernican astronomy it should exclude Earth.This wiki article (http://en.wikipedia.org/wiki/Seven_heavenly_objects) says " In some cultures, the Sun and Moon have also been counted as planets, to make the seven classical planets." Whch cultures did? Which didn't?

Our seven days of the week are basically (in some related cultures) named after the Sun, moon, Mars, Mercury, Jupiter, Venus, and Saturn.