I Suggest a revised definition of "planet"...

[Scorpiove]

I blame Neptune.... If it didn't migrate out it wouldn't have ruined the orbits of the KBOs and they would have formed into the last planet! hehe

[Spaceman Spiff]

I think any solar system object with a diameter of more than 1000km ...

Hmm, yes, very metric of you! I wonder if someone would argue for imperial units (1,000 miles) with radius rather than diameter somehow, then both Pluto and 2003 UB313 wouldn't make planet status.

This planet definition by size limit reminds me of a mountain-hill definition conundrum. Searching a dictionary to check whether there was supposed to be a limit of 1,000 feet for height, I found these (circular) definitions:

Mountain: mound of earth larger than a hill.

Hill: mound of earth smaller than a mountain.

I blame Neptune....

Quite right, and we can blame Jupiter for the Asteroid Belt.

Spiff.

[ajtribick]

Personally I blame the Population III stars which had the gall to spew out all those heavy elements which started this mess in the first place...

[Scorpiove]

I think the problem is when people think of planet they think of "world" and vice versa. I would just be happy calling the larger KBOs "significant worlds". Because planet or not you would still get that "worldly feeling" if you were to be standing on it, heck you get that feeling just thinking about its size. Heck if you landed on Ceres it might feel that way too. Hopefully Nasa's New Horizon mission gets some great pictures of Pluto and Charon, and whatever else KBOs that Nasa decides to visit.

Edit: you can add the larger moons of the solar system to that "worldly feeling" too ;

[Electra]

I blame Neptune.... If it didn't migrate out it wouldn't have ruined the orbits of the KBOs and they would have formed into the last planet! hehe

LOL!
This thread is great, I'll have to save it.

[Spaceman Spiff]

Oddly enough... Farewell Pluto? ( http://news.bbc.co.uk/1/hi/magazine/4737647.stm ).

Looks like demotion of Pluto matches well with the majority of comments.

Spiff.

[d.m.falk]

What I don't like about this is delimiting based entirely on non-scientific bases.

I strongly prefer the mass-accretion definition of a planet.
(Note that I said mass, not gravity.)

d.m.f.

[Ynjevi]

Like it been has said before, it's time to scrab the old simple major/minor planet system and replace it with more complex taxonomical system.

The current system is way too simple; for example, although Pluto is much smaller than Mercury, it is considerably more Mercury-like than Jupiter.

The giant planets have two very different subgroups, the gas giants (Jupiter, Saturn) and ice giants (Uranus, Neptune). The four terrestrial planets are quite alike each other. Largest main belt asteroids resemble terrestrials, although they are much smaller. Still, they are very different from small asteroids having much more complex structure and geological histories. Pluto & other giant TNOs form a group of their own (although if their compositions and structures vary a lot, we probably wouldn't know it because of current limited knowledge). They're not just some giant comet nuclei.

[d.m.falk]

Particularly since Sedna has not shown any significant icy content in the reflective spectrographs that have been analysed.. And you're right- There are at least 3 classes of planets that we know of in our Solar syste- Rocky, condensed-gas core (proto-stellar) giants and solid-core gas giants. We're just going to have to accept that we're not going to have a SMALL number of planets- Certainly not for the size of our planetary system.

Delimiting based on radius or diameter is both unscientific and ridiculous. Just as the second is not measured as being 1/86400th day, but of a specific length based on the number of oscillations of cesium, planetary limits should be based on a mathematical limit of the minimum mass to accrete a spheroid.

I expect nothing less.

d.m.f.

[Michael Kilderry]

I think any solar system object with a diameter of more than 1000km ...

Hmm, yes, very metric of you! I wonder if someone would argue for imperial units (1,000 miles) with radius rather than diameter somehow, then both Pluto and 2003 UB313 wouldn't make planet status.

Might be best to use metric units though if we are going to use diameters and radius to decide whether something is a planet or not, as metric units are simpler and are becoming more common than the old imperial units.

Here's another classification suggestion, based on the idea of breaking up the planetary groups a bit:

Jovian, 128000 km in diameter and up (not exceeding 13 Jupiter masses): Jupiter

Saturnian, 64000-128000 km in diameter: Saturn

Neptunian, 32000-64000 km in diameter: Uranus and Neptune

Panthalassic, 16000-32000 km in diameter: (No example in our solar system has been discovered)

Earth-Sized, 8000-16000 km in diameter: Earth and Venus

Marsian, 4000-8000 km in diameter: Mercury and Mars

Plutonian, 2000-4000 km in diameter: Pluto and 2003 UB313

Quaoaric, 1000-2000 km in diameter: Sedna, Orcus, Quaoar and others...

Ceresian, 500-1000 km in diameter: Ceres, Pallas, Vesta and others...

Unterceresian, 250-500 km in diameter: Minor planets, similar in size to moons; Mimas, Hyperion, Miranda, Proteus or Nereid.

Unterunterceresian , 125-250 km in diameter: Minor planets, similar in size to moons; Himalia, Janus, Phoebe, Puck, Larissa

Microplanets, less than 125 km in diameter.

With the above classification method, the children learning astronomy at school can recite the names of the twelve planetary size groups in order from smallest to largest (or largest to smallest) instead of reciting the names of the 8,9,10 or 1638329846395694236092460 known planets of the solar system in order from the sun.

- Michael

[Scorpiove]

The problem with that is that its not very discriptive of what type of planet you have. For example.... if you have a planet like mercury that is the size of mars. Would you call that a Marsian planet? As our solar system goes there are gonna be many many types of planets. But as the galaxy and universe are concerned I can suspect there may be sever hundred types of planets. One of my favorites to think about are the Super hot giants that are so hot they may have silicate clouds.

[Spaceman Spiff]

Might be best to use metric units though if we are going to use diameters and radius to decide whether something is a planet or not, as metric units are simpler and are becoming more common than the old imperial units.

But, if the metre is based on the Earth's size, then can you say the 1,000km limit is any less arbitrary that 1,000 miles?

Your list, while so colourful, is 'sizist'! At least, a planet 10 times the mass of Jupiter would actually be somewhat smaller than Jupiter... And why call Mercury a Marsian (Martian?) world, why not call Mars a Hermian world? Ah, details...

At least you have logarithmic scale...

Spiff.

[Michael Kilderry]

But, if the metre is based on the Earth's size, then can you say the 1,000km limit is any less arbitrary that 1,000 miles?

The 1,000 km limit sounded like a good idea to me because it approximates to a 10th of the Earth's size, so everything is sort of rounded off that way. I'm not quite sure what you mean by the above statement.

Your list, while so colourful, is 'sizist'! At least, a planet 10 times the mass of Jupiter would actually be somewhat smaller than Jupiter...

How much smaller? Smaller than Saturn? I thought they would be about the same size as Jupiter. My classification system would of course be "sizist" since size is what it's based on.

And why call Mercury a Marsian (Martian?) world, why not call Mars a Hermian world? Ah, details...

Same reason why they call the new smaller exoplanetary discoveries Neptune-sized planets rather than Uranus-sized planets (if I'm not mistaken), Mars is more popular and well known than Mercury. But if you think this is unfair, we can call them "Mercur-martian" worlds. (I must have missed the "Martian spelling mistake in the last post).

- Michael

[Juan Marino]

I suggest a boring and new arbitrary definition :

ASTEROIDS/PLANETS (mercury,venus,earth,mars)/solid moons (hard elements) ---> Class A

according to its size 1-9 (totally arbitrary).
according to its chemical composition.
according to its orbit E (Eccentric) or N, or moon type M

Example:

A.1.I.M

----------------------------------------------------------

GAS GIANTS ----> Class B

according to its size 1-3.
according to its chemical composition (or temperature?).
according to its orbit E or N.


Example:

B.2.II.E

----------------------------------------------------------

COMETS/Pluto and other KBO?€™s/ice moons---> Class C

according to its size 1-9 (totally arbitrary).
according to its chemical composition.
according to its orbit E or N; or moon type M

Example:

C.6.IV.E

[Spaceman Spiff]

But, if the metre is based on the Earth's size, then can you say the 1,000km limit is any less arbitrary that 1,000 miles?

The 1,000 km limit sounded like a good idea to me because it approximates to a 10th of the Earth's size, so everything is sort of rounded off that way. I'm not quite sure what you mean by the above statement.

I'm trying to raise awareness of the arbitrariness in categorising things by adopting some 'round' number as a standard, in a teasy way. Please forgive my tone below, I'm just trying to lead you through the problem step by step.

I mean this: what is significant about the metre in relation to the size of planets or with major/minor planet formation? And what is 'round' about the number 1,000?

One could say, well the metre is based upon the size of one planet: Earth. So the metre is more pertinent than the yard and that relates to planet size nicely, and metres beats yards or miles, doesn't it? Even if the metre is based upon the size of the Earth, 1,000km (or 1,000,000 metres) is still not as free of contrivance as one might think. 1,000 looks round in base 10, but it looks round in base 12, 16 or 2 too. So, what is so natural about base 10?

See, if history followed one of those alternative quantum outcomes (or whatever it does), it could have been that the French succeeded in converting the metric system to base 12 (which the British Imperial system almost does do), and the metre would have been one 35,831,808th of the Earth's pole-equator dimension, or 0.279 present meters, and a 1,000km limit in base 12 would be 2,985.984km in base 10, and would actually be 833.3km using present metres. Taking 1,000km then would make Ceres a planet.

So then, there's nothing special about 1,000,000 of our modern metres. Yet, choosing 1,000km as nice and round while at the same time letting Pluto into the (major) planet club, but rejecting Ceres, is to me 'sizist'! You favour one but not the other based upon an arbitrary size limit.

Your list, while so colourful, is 'sizist'! At least, a planet 10 times the mass of Jupiter would actually be somewhat smaller than Jupiter...

How much smaller? Smaller than Saturn? I thought they would be about the same size as Jupiter. My classification system would of course be "sizist" since size is what it's based on.

And, yes, smaller than Saturn - degenerative pressure squashes them. In fact, there can be quite a range of sizes for given mass. Generally, there's more to categorising planets by size alone. I think it's orbits that matter first, then mass. Size even comes merely as a consequence of composition - such as our Rock Giant we discussed recently.

And why call Mercury a Marsian (Martian?) world, why not call Mars a Hermian world? Ah, details...

Same reason why they call the new smaller exoplanetary discoveries Neptune-sized planets rather than Uranus-sized planets (if I'm not mistaken), Mars is more popular and well known than Mercury. But if you think this is unfair, we can call them "Mercur-martian" worlds. (I must have missed the "Martian spelling mistake in the last post).

Ah, I see, Typo. But I think you're onto a tough job with this classification scheme. If someone was describing a planet to me, and they said 'Martian', I'd just think of Mars, and not even Mercury. But if they instead said it's 0.2 Earth masses and 8,000km diameter, that's so much more informative.

Spiff.

[Planet X]

I wonder if someone would argue for imperial units (1,000 miles) with radius rather than diameter somehow, then both Pluto and 2003 UB313 wouldn't make planet status.

Spiff.

Well, maybe not. On Mike Brown's site, he has different values for 2003 UB313's size based on certain albedo figures. The lowest albedo figure of 38% gives a diameter of 3550 km, or 2235 miles. If this proves true, then 2003 UB313 would attain planet status, even if a 1000 mile radius were to be eventually implimented. At any rate, I have a far simpler way to tell planets apart from asteroids/KBO's. Just so everyone knows right now, I DO NOT view asteroids and minor planets as the same thing. So, here goes our solar system's breakdown as I see it:

Giant Planet: 49000-143000 km in diameter: Jupiter, Saturn, Uranus, and Neptune

Terrestrial Planet: 6000-12800 km in diameter: Venus, Earth, and Mars

Subterrestrial Planet: 2000-6000 km in diameter: Mercury, Pluto, and 2003 UB313 (so far)

Asteroid/KBO: fist sized-2000 km in diameter: (All asteroids and KBO's)

Of course, this does not take into account subclasses and other classes of planets NOT in our solar system. See Michael Kilderry's post above for that stuff. Later!

J P

Note: edited on 08/07/05 to change the name "Minor Planet" to "Subterrestrial Planet."

[symaski62]

http://science.nasa.gov/headlines/y2005 ... lanetx.htm

http://www.spaceweather.com/swpod2005/0 ... allen1.jpg
http://www.spaceweather.com/swpod2005/0 ... immons.jpg
http://mak-newt.com/visitors/MPEC2005-O ... index.html

[Planet X]

Hey, I should also point out that Brian Marsden of the Minor Planet Center (MPC) thinks that only bodies Mars sized or larger should be called planets. This means that if he had his way, our solar system would be down to just 7 planets! What a prick! Later!

J P

[Michael Kilderry]

So then, there's nothing special about 1,000,000 of our modern metres. Yet, choosing 1,000km as nice and round while at the same time letting Pluto into the (major) planet club, but rejecting Ceres, is to me 'sizist'! You favour one but not the other based upon an arbitrary size limit.

Well it's all just arbitrary in the end, isn't it? The term 'planet' is just a label for some of the many objects in our universe.

The point of the 1,000 km diameter limit is to keep things nice and simple and make it easier for everyone, rather than having some complicated system of what gets classified as a planet or not, like internal structure, which may take a while to investigate properly for certain objects like those of the Kuiper Belt.

I think favouring Pluto over Ceres is logical, at least for historical reasons. Pluto was known as a planet for a long time before all these TNO's stepped into our telescopes, Ceres on the other hand, has not been considered a planet for a very long time.

And if we were to consider Ceres a planet, then it would be arbitrary not to consider all the TNO's at least as large as this asteroid into the circle of planets as well, and then it would be arbitrary not to consider anything that's just a little bit smaller, as why should Ceres be the cut off mark for planet sizes? What's so special about it? This is why it's good to keep things simple with the 1000 km cut-off mark, after all, no planetary classification system is going to be perfect and to everyone's taste.

And also, neither Pluto or Ceres have been studied up close. Pluto could be quite planet like and Ceres could be rather asteroidal in appearance. We may as well be arbitrary at classifying planets at this point in time because we don't know enough to set a proper limit.

But don't get me wrong, I have nothing against asteroid Ceres, but I do against Varuna, every time I tried to search for info on KBO's, Varuna was all that ever came up, what shameless overpromotion!

And the 1000km size limit just barely stops Varuna from being classified as a major planet, that's the REAL reason I chose the 1000 km cut off mark. [/joking]

Hey, I should also point out that Brian Marsden of the Minor Planet Center (MPC) thinks that only bodies Mars sized or larger should be called planets. This means that if he had his way, our solar system would be down to just 7 planets! What a prick! Later!

Brian Marsden (anyone notice he's got the word "Mars" in his name?) must be thinking along the lines of nothing being classified as a planet unless it is at least half the size of the Earth. This would make Mars-sized the cut-off mark for planethood.

I think it's funny how it's got to the point where not only Pluto's planetary status is being debated, but so is Mercury's! Mars, you're next in line....

- Michael

[Spaceman Spiff]

Just so everyone knows right now, I DO NOT view asteroids and minor planets as the same thing. So, here goes our solar system's breakdown as I see it:

Giant Planet: 49000-143000 km in diameter: Jupiter, Saturn, Uranus, and Neptune

Terrestrial Planet: 6000-12800 km in diameter: Venus, Earth, and Mars

Minor Planet: 2000-6000 km in diameter: Mercury, Pluto, and 2003 UB313 (so far)

Asteroid/KBO: fist sized-2000 km in diameter: (All asteroids and KBO's)

Hey, I should also point out that Brian Marsden of the Minor Planet Center (MPC) thinks that only bodies Mars sized or larger should be called planets. This means that if he had his way, our solar system would be down to just 7 planets!

So you propose that Mercury is considered a minor planet, but are unhappy with Brian Marsden after you claim he thinks and is wrong to think Mercury and Pluto are too small to be considered major planets? Hmm... You do realise that the term minor planet does have a particular meaning in planetary science? That 'Asteroids' and 'Kuiper Belt Objects' are two specific types of minor planets?

If you move Mercury, but not Pluto, into 'terrestrial planets', and label the rest 'minor planets', I'd go along with that.

Spiff.