So Pluto is a planet!?

[Albert VDS]

I couldn't agree more, which is why I don't understand the need to produce a contorted definition that conforms to the historic use.

Planet means wanderer. I dont see how you think the IAU is trying to conform to that with their requirements for a planet.

[plainawesome]

I think Pluto should be a planet. If it was one it always will be one to me. Not just as a science thing, but it's also kind of personal. My dad actually met and shook hands with Clyde William Tombaugh (the guy who discovered Pluto). But all personal matters aside, Pluto should be a planet.

[JedTech]

I think we need to make the word "Planet" more of a generic term. And then we need to develop a taxonomy system to properly categorize and classify various celestial bodies. A celestial body should be classified by its: size, orbit, stability, and composition.

[Camacha]

By what standards exactly? Soter's planetary discriminant says that the difference between Mercury and Jupiter is a factor of 7. The difference between Mars and Ceres is a factor of 500000. According to the Stern-Levison parameter, the difference between Mercury and Jupiter is about the same as between Mars and Ceres.

In terms of "clearing the neighbourhood" the 8 planets are clearly within one league. In terms of scattering power there's a big gap clearly separating dwarf planets and the big eight, although it is true that the big eight are quite widely spread out themselves.

If we look at weight, size or composition, for example. People are so intent on this Soter's planetary discriminant, but I am arguing that is a rather random factor as it. If you want a definition of something to include certain objects and exclude others is is always possible to come up with some value or factor that suits your needs, that does not make it a good scientific requirement in my book. Since the definition has been disputed since day one by people who know their business, I appear not to be alone in this.

It rather looks like people have been hunting for some kind of definition that would only include the historic planets. Since Pluto was too much alike a lot of new objects that could not be included, but 8 is closer to the desired result than 400.

Mercury, Mars and Jupiter scooped up nearly everthying in their part of the solar system. That is the important bit. The fact that Jupiter scooped up much gas, and is able to hold it, doesn't differentiate it. If we have several vacuum cleaners, and I use mine to scoop dirt, and you use yours to scoop water, they are both still vacuum cleaners.

I am not sure why you are comparing planets to vacuum cleaners and beetles, but the remark I make above also goes for this rather colourful comparison. If you want to differentiate objects in a way that includes certain bodies there generally is some value you can come up with to facilitate that.

The definition is pretty objective. A definition being objective has nothing to do with agreement or matching your version. The definition "a sphere is a round thing" is subjective, "a sphere is an object of constant curvature" is objective. You could now argue that a flat plane is no sphere, but that's only so because you project your expectation into it and/or because there is another possible definition. The given definition itself is still objective.

I hear the word subjective a lot, but have not seen any real substantiation. Maybe I am overlooking something, but it seems to be a requirement that can be defined pretty well. Let's be clear we are talking about hydrostatic equilibrium.

- - - Updated - - -

I think we need to make the word "Planet" more of a generic term. And then we need to develop a taxonomy system to properly categorize and classify various celestial bodies. A celestial body should be classified by its: size, orbit, stability, and composition.

Size or mass?

Edited October 6, 2014 by Camacha

[Vanamonde]

With regard to several removed or edited posts, please keep in mind that your fellow forum members are not your enemies, and do not resort to insults. Also, there is no place on our forum for casting aspersions on entire nationalities.

[JedTech]

Size or mass?

By "size", I intended to convey a general thought that might include: size, mass, density.....

I am not venturing to compose the complete list of criteria that should be used in the classification of celestial bodies. Merely stating that I think a commonly understood Taxonomy system for celestial bodies would be beneficial.

[Camacha]

Merely stating that I think a commonly understood Taxonomy system for celestial bodies would be beneficial.

I feel hierarchy is relevant. The whole star > planet > moon system denominates a body's place in the system. To make sure not every piece of floating rock or debris is called a planet, only bodies with enough mass to be round (*to have reached hydrostatic equilibrium) are included. Round means planet, unround is called an asteroid (or a term that is a little more accurate).

I feel that is a pretty clear-cut and comprehensive system. There are a couple of things that should be looked at, like rogue planets, but that should not be too hard as the denominator rogue signifies the lack of a parent star.

Edited October 6, 2014 by Camacha

[metaphor]

For one thing, because the gravitational influence of all the stuff except the planets (and the Sun) is negligible. The 8 planets (and the Sun) are the objects that dominate the orbital mechanics of our solar system. Everything else is just debris.

Do you include Mercury in that?

Of course. Why do people get hung up on Mercury? Sure it's the smallest planet, but that's because it's the closest to the Sun. There just wasn't as much stuff there to coalesce into a single body when the solar system formed. The gases, ices, and much of the rocky material were vaporized and blown away by the nearby Sun, which is why Mercury is mostly made of metal. (That's also why the planets formed beyond the frost line at ~3 AU are much bigger than the closer-in planets, since there's a lot of water in the solar system but it couldn't coalesce closer in than the frost line.) The 8 planets actually all have a similar internal structure when you account for the temperature at which they formed, with farther-out planets being able to accrete substances with a lower boiling point. (Mars is kind of an outlier, but that can be explained by the Grand Tack model.)

But Mercury is still huge compared to any other non-planets orbiting the Sun. Mercury is more massive than the entire Kuiper belt for example, even though the Kuiper belt contains about 100,000 objects larger than 100 km in diameter. And Mercury is about 100 times more massive than the entire asteroid belt.

[Camacha]

But Mercury is still huge compared to any other non-planets orbiting the Sun. Mercury is more massive than the entire Kuiper belt for example, even though the Kuiper belt contains about 100,000 objects larger than 100 km in diameter. And Mercury is about 100 times more massive than the entire asteroid belt.

I am repeating myself, but if you look at the distribution of mass Mercury is closer to the dwarf planets than to the gas giants. Mercury differs two orders of magnitude from Eris and Pluto, while it differs four orders of magnitude from Jupiter. When we look at absolute differences the gap becomes even larger - 4,3 moons difference versus 25824 moons difference. (And yes, moons are the new banana.)

You are proving a point, I am afraid it is not the point you wanted to make though

Just for fun (and somewhat approximate):

Eris - 0,16 moon

Pluto - 0,18 moon

Moon - 1 moon

Mercury - 4,5 moons

Mars - 9 moons

Earth - 81 moons

Neptune - 1400 moons

Jupiter - 25828 moons

By what standards exactly?

Mass is a fairly obvious one, but far from the only one.

Edited October 7, 2014 by Camacha

[Capt.Joseph Kerbertson]

I grew up with Pluto being a planet... it's still a planet to me.

[No one]

@All the people who say "Pluto is still a planet to me"

What about Ceres?

Of course. Why do people get hung up on Mercury? Sure it's the smallest planet, but that's because it's the closest to the Sun. There just wasn't as much stuff there to coalesce into a single body when the solar system formed. The gases, ices, and much of the rocky material were vaporized and blown away by the nearby Sun, which is why Mercury is mostly made of metal. (That's also why the planets formed beyond the frost line at ~3 AU are much bigger than the closer-in planets, since there's a lot of water in the solar system but it couldn't coalesce closer in than the frost line.) The 8 planets actually all have a similar internal structure when you account for the temperature at which they formed, with farther-out planets being able to accrete substances with a lower boiling point. (Mars is kind of an outlier, but that can be explained by the Grand Tack model.)

But Mercury is still huge compared to any other non-planets orbiting the Sun. Mercury is more massive than the entire Kuiper belt for example, even though the Kuiper belt contains about 100,000 objects larger than 100 km in diameter. And Mercury is about 100 times more massive than the entire asteroid belt.

We don't know the mass of the entire Kuiper belt, some estimates put it at more than Mercury, some estimates put it at less.

Mercury is about 20 times the mass of Eris, and about 1/20 times the mass of Earth. When you look at it based on mass, Mercury seems a rather arbitrary place to draw the line.

Edited October 7, 2014 by No one

[Camacha]

I grew up with Pluto being a planet... it's still a planet to me.

That's fine, but fairly irrelevant

[m4v]

I am repeating myself, but if you look at the distribution of mass Mercury is closer to the dwarf planets than to the gas giants. Mercury differs two orders of magnitude from Eris and Pluto, while it differs four orders of magnitude from Jupiter. When we look at absolute differences the gap becomes even larger - 4,3 moons difference versus 25824 moons difference. (And yes, moons are the new banana.)

You are proving a point, I am afraid it is not the point you wanted to make though

So? there's rocky planets, gas giants and dwarf planets.

Also, plot of Stern-Levison parameter

[Camacha]

So? there's rocky planets, gas giants and dwarf planets.

You mean planets, planets and planets? Or maybe the last one should be *planets.

Also, plot of Stern-Levison parameter

http://mel.ess.ucla.edu/jlm/epo/planet/scattering_parameter.jpg

I admire your stamina, but the response stays the same.

[lajoswinkler]

This discussion reminds me of any generic religious discussion between a believer and nonbeliever. It's basically pointless.

[LLlAMnYP]

I admire your stamina, but the response stays the same.

Actually, his point is convincing. It's perfectly fine to categorize things according to a dimensionless parameter. >>1 or <<1. Especially convenient, when objects with a parameter on the order of 1 do not exist.

No one has quite correctly stated, that when you look at mass, Mercury seems quite an arbitrary place to draw the line. That is precisely why just mass alone is a bad parameter to separate planets and non-planets. Also, dimensionless parameters FTW.

[LLlAMnYP]

Let me make this somewhat clearer. I've taken the data from wikipedia about the eight planets and five dwarf planets.

First I've ordered them by mass and plotted that on a log-scale:

Then I've ordered them by the value of Soter's discriminant:

And finally by the Stern-Levison parameter:

All three parameters have physical meaning. We could use either of them.

So let's say, we take mass, as some of the folks here really would like to. Then we have thirteen planets, there's really no place to draw a specific line here. But oh wait... what about the other big rocks in the asteroid belt? Vesta, Pallas, and Hygeia are in the same league as Ceres and on the plot of masses would not stand out. Shall we promote them to planethood as well? Is it okay, that we would have at least four planets sharing a common orbit (without being in some sort of resonance) with a few hundred thousand smaller rocks?

The other two parameters are 1) dimensionless 2) have a clear gap around unity 3) separate the planets from the dwarf planets in an equal way (into the same two groups).

[LLlAMnYP]

I am repeating myself, but if you look at the distribution of mass Mercury is closer to the dwarf planets than to the gas giants. Mercury differs two orders of magnitude from Eris and Pluto, while it differs four orders of magnitude from Jupiter. When we look at absolute differences the gap becomes even larger - 4,3 moons difference versus 25824 moons difference. (And yes, moons are the new banana.)

I don't see, why you compare Mercury to its nearest neighbors of lesser mass, but then compare it to its most distant cousin of higher mass. For a fair comparison I would say that the second lightest planet to Mercury is Mars, at 2 masses of Mercury, while the heaviest object after Mercury is Eris at 1/20th of its mass.

Alternatively, you may compare the trio Ceres-Mercury-Jupiter (lightest (?) in hydrostatic equilibrium - lightest "proper" planet - heaviest planet). Then Ceres would be "only" 330 times lighter than Mercury, while Mercury would be a huge 5000 times lighter than Jupiter. That would be somewhat more convincing.

In any case, as I said, this just goes to show that mass isn't a great parameter.

[No one]

But one could say that Mars and Mercury are 1 order of magnitude away from Venus and Earth, and also 1 order of magnitude away from Pluto and Eris.

Why are we drawing the line at Mars and Mercury?

[LLlAMnYP]

Are you even reading what I wrote (two or three times now)?

Because we are not using solely mass to categorize them! It's obviously a bad criterion!

[metaphor]

But one could say that Mars and Mercury are 1 order of magnitude away from Venus and Earth, and also 1 order of magnitude away from Pluto and Eris.

Why are we drawing the line at Mars and Mercury?

Because we are not using mass as a criterion, we are using both mass and distance from the Sun (more specifically, M^2 / A^1.5, where M is mass and A is distance from the Sun). That combination of mass and orbital radius gives the average time for a body to "clear its orbit". If you compute that for each body in the solar system there is a clear dividing line between the 8 planets and anything else.

[Camacha]

Are you even reading what I wrote (two or three times now)?

Because we are not using solely mass to categorize them! It's obviously a bad criterion!

No one is saying mass should be the decisive factor, it is just used to illustrate that Soter's discriminant and the Stern-Levison parameter (which for most intents and purposes are the same thing) are an arbitrary factor to decide. Why should we use that as the decisive factor? It's rather striking the results line up almost exactly with our (somewhat recent) historic view of what planets are. We could pretty much engineer any value or factor that lines up with some idea we feel should be the result or even manipulate the same factor by either using a linear or logarithmic scale, but I am not sure that is how science should operate.

Hierarchy and hydrostatic equilibrium seem much more universal values and factors that are also applicable to other solar systems. That does mean we end up with a more comprehensive set of planets. Some people consider this bad for some reason, I view it as an advantage because it lines up a little better with what our solar system actually is. We could still teach kids about the 15 largest bodies or features of the solar system.

Edited October 8, 2014 by Camacha

[ZetaX]

He already explained that the parameter is well-suited to measure how much an object has cleaned it's neighbourhood. Are you disputing that, or are you disputing that cleaning the orbit is a good measure¿

The measure was definitely not just engineered to fit the data. And it works for many (every¿) solar systems.

[Camacha]

He already explained that the parameter is well-suited to measure how much an object has cleaned it's neighbourhood. Are you disputing that, or are you disputing that cleaning the orbit is a good measure¿

Disputing the values itself would be silly, although a discussion could be had about the value of either one. I am disputing that clearing the orbit is the factor(s) we should look at to make this distinction.

The measure was definitely not just engineered to fit the data. And it works for many (every¿) solar systems.

How do you know?

[LLlAMnYP]

Soter's discriminant, qualitatively is by definition a measure of how much the object has cleared its neighborhood, therefore it will be applicable to any star system. I'm not sure about the derivation of the Stern-Levison parameter, it is, essentially, a measure of the capability of an orbiting object to scatter other objects (within a certain large amount of time). It is tied to the Hubble time which, I accept, is somewhat arbitrary. There is, however, nothing arbitrary about scattering power alone, which would allow us to use that as a criterion of the capability of a planet to clear its neighborhood (yes, in any star system where Newtonian physics work).

Soter's discriminant and the Stern-Levison parameter are by no means "the same for all intents and purposes". Soter's discriminant actually quantifies the extent, to which the neighborhood is cleared, while Stern-Levison's parameter is merely a measure of the capability of an object to do so.

I certainly reject the notion of admitting Ceres (and possible some of the other large objects in the asteroid belt) to the planet's club. They share the same orbit with each other and a few hundred thousand other rocks. I equally reject including the Plutinos and other large KBOs. Again, they are merely several rocks (some estimates suggest a possibility of hundreds of object that have achieved hydrostatic equilibrium in the Kuiper Belt) in one big trans-neptunian asteroid field. I would reconsider my point of view, had we had no asteroid belt or Kuiper belt. In that case there would have been little reason not to include the dwarf planets in the planet's club. If we find out that other star systems do not generally have a cloud of debris around them, such as the Kuiper belt, it would also be a case to reconsider. However, in the absence of data, going with a prior that our solar system is not super-special seems more justified to me, than saying "no, our solar system is totally unique, this definition will totally not work for extrasolar planets".

Hydrostatic equilibrium alone isn't a sufficient criterion to separate planets from non-planets. Take Saturn's tiny moon Methone, for example. It is a mere 3km across, yet because of its composition is most likely in hydrostatic equilibrium. It's very likely, that we may find several further objects that would, by this measure, also be dwarf planets, even if they would be just tiny balls of fluff among a swarm of asteroids.

It is true, that the definition seeked by the IAU was such as to include the "classical" planets. There is little wrong with that. Indeed, before more recent developments in astronomy we could only detect the largest and most significant objects in our solar system and call them planets. With advances in measurement techniques came a need to formalize the definition. I do feel, that the inclusion of the "classical" eight is a must. Otherwise, in formalizing the definition we would have simultaneously completely redefined the concept of "a planet".

You are welcome to suggest a much better criterion than "clearing the neighborhood" if you wish to, but I beg you to go over this wikipedia article first, so as to avoid the failures of some preliminary drafts of the IAU definition. I would, actually, also accept a definition that might exclude Mars or Mercury from planets if you can justify that sufficiently.