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

[Spaceman Spiff]

I prefer a definition synthesized from those proposed by Stern & Levinson, Buie, Basri, and Brown. That is:

planet: an object that orbits a star or stars and is large enough to be shaped primarily by gravity ("rounded") but not large enough to have ever undergone fusion in its interior

Hmm, yes, read that somewhere. It's no good though. Again, it's impractical when it comes to deciding whether a newly discover point-like object or extrasolar 'planet' is a planet. How can you tell it's shape? How can you tell if the heat is from fusion, primordial (Brown Dwarves!), tidal or recent collision?

Is Ceres rounded or not?

Spiff.

[adamnieman]

The fact is, there is a wide variety of objects in orbit around the Sun. This makes theoretical distinctions between categories of objects difficult and contingent. Rather than trying to shoe-horn a theoretical distinction between planets and asteroids into their definition, I advocate doing it as we always have: How do we decide what to call a planet and what to call an asteroid? Planets are bigger than asteroids!

The cut-off point is arbitrary and Pluto is as good a size as anything. So if it's Pluto-sized or larger, it's a planet; if it's smaller than Pluto, it's an asteroid. Really big asteroids can be called 'planetoids', really small ones can be called 'meteroids'.

This definition makes no ontological distinction between planets and asteroids (essentially, planets are asteroids and asteroids are planets) which keeps the solar system more straightforward conceptually.

Adam

[Spaceman Spiff]

Yes, but...

The division line being set to let Pluto in as a planet and not smaller TNOs is also vulnerable to quibbles. There's a real possibility that there are dozens, hundreds of undiscovered TNOs larger than Pluto, or Mercury, maybe even as large as Mars. In twenty years time, who's going to insist on naming all of the 256 planets?

A reason why I'd like to get the number of planets down to eight is that it's better for schoolchildren who learn astronomy to recite something achievable such as listing eight names in order rather than 256. If they add to teh eight "... and there's Pluto, and Ceres is the largest asteroid, and there's one called Pallas ... " that's extra brownie points. But when there's 256 planets, there's just no such fun and opportunity for a sense of achievement. "Think of the Children!" .

Spiff.

[selden]

Pluto is still called a planet for historical and social reasons, not for technical ones. The IAU argued about this for years and finally decided it wasn't worth the pain involved in changing what it's called. There's no point in arguing about it here: they aren't going to change how Pluto is classified in the popular literature. Anyone who spends any time studying astronomy quickly learns about the different types of bodies orbiting the sun.

[ajtribick]

If they find something the size of Mars in the Kuiper Belt then things will get interesting.

Maybe the minor/major (or ??ber/unter) planet distinction is a good one - reserve the term "planet" for those objects large enough that their shapes are determined primarily by gravity (so they are spherical), then divide by dynamical considerations - Pluto remains a planet, but it is an unterplanet. Though I wonder if this system wouldn't make Mu Arae's gas giants unterplanets...

I guess this is what happens when you dive in with a classification system too early in the game... you're nice ABCDEF system ends up as OBAFGK...

[Spaceman Spiff]

Pluto is still called a planet for historical and social reasons, not for technical ones.

Selden, yes, the IAU doesn't have a rule or a definition, just a list. But, don't you want to see the astrologers shown up? They'd blame astronomers naturally, but why didn't they foresee this problem???

Spiff.

[Spaceman Spiff]

I guess this is what happens when you dive in with a classification system too early in the game... you're nice ABCDEF system ends up as OBAFGK...

Quite! One should be able to wait until enough data is gathered to study how they relate to each other. Using orbits and masses lets you do that more quickly that surface shape or features, but some people are just so impatient! "Is it a planet or what? Tell me now! I have to write something for the readers!! You're an expert!!! Why won't you tell me!!!!"*

Spiff.

* OK, I exaggerate the hysteria...

[Ryan McReynolds]

Hmm, yes, read that somewhere. It's no good though. Again, it's impractical when it comes to deciding whether a newly discover point-like object or extrasolar 'planet' is a planet. How can you tell it's shape? How can you tell if the heat is from fusion, primordial (Brown Dwarves!), tidal or recent collision?

But you can tell all of these things, by estimating mass. Relatively simple math will tell you if the object's gravity is sufficient to overcome mechanical forces and allow it to flow into an equillibrium ellipsoid--to "round" it. Likewise, either the object is massive enough that the conditions for deuterium fusion can occur or it isn't.

But for the sake of argument, let's pretend that it takes a bit (or a lot) of study to determine if something is a planet or not. So what? Why is it crucial to classify an object immediately upon its discovery? And why can't the classification simply be changed if it turns out the initial speculation was wrong? This is science, not dogma: it evolves. There's no compelling reason that an object has to be permenantly assigned a classification before studying it. You discover an extrasolar planet and later find out it's a little more massive than you thought and probably fused deuterium. Big deal, you start calling it a brown dwarf instead.

Is Ceres rounded or not?

Of course it is. So are Vesta, Pallas, Varuna, Pluto, Sedna, Quaoar, 2003 UB313, etc. They are all minor planets in this classification scheme. There could be thousands of minor planets found in the Kuiper Belt and Oort Cloud. Using gravitational roundness as a criterion basically says that they were right to call Ceres a planet when they first discovered it, because it is not fundamentally different from the other planets. Despite the confusion of late, planets have always been round objects that shine with reflected sunlight, and in my scheme they still are. No other scheme seems to preserve this classical definition.

[MKruer]

Here is my definition of a "planet"

A Planet is a body that is geological active, or more precisely has a ?€?warm?€

[MKruer]

Yes, but...

The division line being set to let Pluto in as a planet and not smaller TNOs is also vulnerable to quibbles. There's a real possibility that there are dozens, hundreds of undiscovered TNOs larger than Pluto, or Mercury, maybe even as large as Mars. In twenty years time, who's going to insist on naming all of the 256 planets?

A reason why I'd like to get the number of planets down to eight is that it's better for schoolchildren who learn astronomy to recite something achievable such as listing eight names in order rather than 256. If they add to teh eight "... and there's Pluto, and Ceres is the largest asteroid, and there's one called Pallas ... " that's extra brownie points. But when there's 256 planets, there's just no such fun and opportunity for a sense of achievement. "Think of the Children!" :).

Spiff.

This is such a lame reason not to classify a new body as a planet. Oh no I have to remember another few names. Hell most people don?€™t know the names of all the planets anyway why would adding more ?€?planets?€

[Scorpiove]

Mkruer: A small rocky body would be classified as a planet sooner then a ball of ice, but at what point should a ball of ice be considered a planet? If you can answer that question precisely then there is no reason for my definition, problem is that any other definition would be more arbitrary then.

I would like to see what you think about planets made mostly of carbon instead of silicate. I'm not against anyone in this discussion, I just saw you mention ice and rock so I had to throw in carbon .

http://www.novacelestia.com/space_art_extrasolar_planets/carbon_planets.html

I myself like the "kuiper belt planet" idea.

[MKruer]

Mkruer: A small rocky body would be classified as a planet sooner then a ball of ice, but at what point should a ball of ice be considered a planet? If you can answer that question precisely then there is no reason for my definition, problem is that any other definition would be more arbitrary then.

I would like to see what you think about planets made mostly of carbon instead of silicate. I'm not against anyone in this discussion, I just saw you mention ice and rock so I had to throw in carbon :).

http://www.novacelestia.com/space_art_extrasolar_planets/carbon_planets.html

I myself like the "kuiper belt planet" idea. :)

Well carbon is heaver then ice, yet light then silicone so it would fit some where in between. Another reason why I chose the thermal probe tries rather then the size or mass it because a planet made with 50% ice and 50% rock would be vastly different then one that was 10% ice and 90% rock or via versa, however chance are the 90% rock one would be considered a planet while the 90% one would not even if there were the same mass. People consider rock being more a planet then the same mass of ice being a planet for some reason.

The idea to call Pluto a ?€?kuiper belt planet?€

[MKruer]

If they find something the size of Mars in the Kuiper Belt then things will get interesting.

Maybe the minor/major (or ??ber/unter) planet distinction is a good one - reserve the term "planet" for those objects large enough that their shapes are determined primarily by gravity (so they are spherical), then divide by dynamical considerations - Pluto remains a planet, but it is an unterplanet. Though I wonder if this system wouldn't make Mu Arae's gas giants unterplanets...

You will never have a truly an absolute classification, meaning that there will always be a exception. However you can get the classification down to a level that finding an object that does not fit the standards would be one in a million or one in a billion. Right now or current definition or lack there of one, does not even stand for one it ten.

I guess this is what happens when you dive in with a classification system too early in the game... you're nice ABCDEF system ends up as OBAFGK...

That is not what happened to the spectra chart the OBAFGK if I recall correctly the Latin names for the colors in the spectrum. The problem with the OBAFGK is that it has been discovered that some stars have temperature that should classify them as say, type O, but spectra reflects them to be in type B. For the most part this work for 99% of all the stars out there. I tried to create a system based upon the number of element and the quantity of element but I think a billion dimensional chart is a little over kill.

[Scorpiove]

[quote="MKruer"]The idea to call Pluto a ?€?kuiper belt planet?€

[MKruer]

Don?€™t get me wrong that there would need to be additional breakdowns such as Asteroidal Class, Planetesimal Class, Terrestrial Class, Jovian Class, but my entire argument it that it should be based upon present or historical thermal core characteristic rather then by some arbitrarily diameter or mass. Again the reasons why are because the composition can be radical different.

For example what if Pluto was made out of mostly rock (it still might be), but had the same mass as it does now. Most would say it was a inner planet that was flung out of the solar system, just because it was rock. Now you find a ball of ice 3 times larger, well this is not a planet because its not made of rock. Where is the dividing line?

I will admit that my idea has problems, manly that you can not instantly classify an object, but it is still radically better then this is not and this is on a case by case basis. My rules would not change.

[Scorpiove]

Sorry MKruer, that wasn't directed at you, I was just throwing out random ideas. I'm not against your ideas at all.

[Spaceman Spiff]

But you can tell all of these things, by estimating mass. Relatively simple math will tell you if the object's gravity is sufficient to overcome mechanical forces and allow it to flow into an equillibrium ellipsoid--to "round" it.

I don't think this is clear cut. Take Enceladus and Puck, each about 400km in diameter. One is so round it's as round as round can be (and I note the continuing lack of a precise demarcation of ellipsoidal a, b, and c variance). The other (Puck) is, to quote Carl Sagan, "lumpy". Same size, same mass, same composition, why different? One is heated so flowed better, the other is so cold and the ice is so tough that it didn't become spherical.

OK, these are not planets because there are moons, but again, it they were orbiting the Sun, one would be found round and the other not. And I can imagine that one day examples of this could be found with rocky bodies too. Roundness ain't going to save the day.

And yet you do bring in mass as a factor to sphericity yourself. Like I say, mass is more fundamental than shape or size. It is that which should be used on the orbit-mass scattergram to see what the populations of bodies are, and which go in the 'planet' group.

Why is it crucial to classify an object immediately upon its discovery?

Quite, that's what chaos syndrome pointed out and I agree with him. But really the issue is forced more by press relations than science, isn't it: "So, doc, is it a new planet or isn't it???"

Is Ceres rounded or not?

Of course it is. So are Vesta, Pallas, Varuna, Pluto, Sedna, Quaoar, 2003 UB313, etc. They are all minor planets in this classification scheme.

So are they major planets? Because that's what the issue with Pluto is: is it the ninth major planet and is 2003 UB313 the tenth major planet?

Changing to 'minor planet' is avoiding the argument.

Spiff.

[Ryan McReynolds]

I suspect that the mass/composition levels that lead to gravitational roundness are probably pretty similar to those that would lead to significant interior heating. Perhaps these two definitions are essentially the same, but approaching them from different angles. Though gravitational roundness is much easier to determine than core temperature, much less historical core temperature . . .

My objection to using the presence of a heated core is more that it is a significant departure from historical and intuitive definition of planets: big round celestial objects that shine with reflected sunlight. The "gravity-fusion" definition is essentially the same old definition that has always been used, but scientifically specific about what qualifies as "round" and "shining;" that is, gravitational superiority over mechanical force and deuterium fusion. These are fundamental, non-arbitrary features of all presently accepted planets that asteroids, comets, rocks, brown dwarfs, and stars lack. They are independent of unproven theories of formation history and of unpredictable cosmological circumstance. Since all nine accepted planets have these features, it makes sense to suggest that any other object with the same features is therefore also a planet.

I do vascillate between the major/minor planet definition and calling the eight major planets simply "planets" and the multitude of minor planets "planetoids"--objects that would be planets if they dynamically cleared their orbits. But since there are possibly Mars- and even Earth-sized objects in the Kuiper Belt and Oort Cloud, calling them "planetoids" just seems inappropriate. But that is admittedly aesthetic.

[Spaceman Spiff]

Planet: A celestial body large enough to generate a minimum heat of 1000K at is core through its own radiogenic heating, gravitational (primordial) heating or tidal heating, at any point during its historical past with a minimum duration of 100million years.

OK, here you've tightened up precision - good - but the problem is that you've tightened up precision on parameters that are derived not observed. Therefore these are so open to debate and review that even though I agree with chaos syndrome that it would be nice to have time to settle the issue, we'd be criticised for the impracticality of it.

It's like the astronomer who wanted to differentiate between Brown Dwarves and Superjupiters by their different formation mechanisms. Everyone else* asked, "how can we decide?," and the guy hasn't got an answer.

On top of that I never said that the magnetic field was required to classify any body as a planet.

No, but I'm not sure you ever said it wasn't . Anyway, I understand it that you're holding finding any (residual) magnetic field as evidence of e.g., a molten core (that has been) hotter than 1,000K for 100 million years.

For Mars the evidence of a molten core is tenuous.

<sarcasm>So Mars is no longer a planet?</sarcasm>

Hey! Good use of humous tags there!

No, I wasn't saying Mars is no longer a planet. I was pointing out that there is not an adbundance of evidence for a Martian molten core. For example, this reference ( http://www.anl.gov/Media_Center/News/20 ... 26mars.htm ) soon after yours states at one point: "The data support previous models of the martian core. ", which as d.m.falk mentions consider mars to have a solid core, but goes on to discuss inference of a molten core. Ironically (or double ironically!), it says sulphur impurities allow molten iron cores at reduced temperatures, so the core temperature issue is complicated by this discovery.

So there are complications. The global magnetic field of Mercury could have been induced; Venus appears geologically asleep (that is, we still haven't detected the kind of core you require), and impurities could upset the required temperature to molten iron.

... what about rouge planets, inner planets that were for one reason or another flung out of a system and are now wondering the void or planets that for one reason or another have a highly eccentric orbit, like some of the extrasolar planets that are 5 times more massive then Jupiter, but highly elliptical orbits?

Doh! I forgot about rogue planets (spelling! sir, otherwise I don't care about the planet's colour. ). I think tracing back the orbit to formation might allow us to recover an original orbit. For the collection of eccentric extrasolar orbits, population identitfication in orbit-mass scattergrams takes care of that.

Finally the reason why I came up with the idea for classify planets based upon it thermal core properties is because it is mass, size, orbit, and composition independent. A small rocky body would be classified as a planet sooner then a ball of ice, but at what point should a ball of ice be considered a planet? If you can answer that question precisely then there is no reason for my definition, problem is that any other definition would be more arbitrary then

"... mass, size, orbit, and composition independent", but technically requires spacecraft probe data to derive the necessary parameters. but I can avoid the pitfall of your question of limit by the scattergram method. Even so, why should a 13,000km diameter 'rock ball' be sooner considered a planet than a 13,000km diameter 'ice ball'?

Spiff.

* Another convenient exaggeration by me...

[Spaceman Spiff]

[quote="MKruer"]
This is such a lame reason not to classify a new body as a planet. Oh no I have to remember another few names. Hell most people don?€™t know the names of all the planets anyway why would adding more ?€?planets?€