Author Topic: Pluto-Planet debate discussions  (Read 258286 times)

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #520 on: 03/26/2017 08:29 pm »
In the final analysis, when we send a probe to study a planet, we are not studying its orbit.  We care about its orbit mainly in the sense that it allows us to navigate to the planet through space  When we get there, we are studying the planets intrinsic physical characteristics, its geology, its atmosphere, its surface features, its interior, its magnetic field, its composition, its morphology, its evolution.  Those are the things we care about.

Who's the 'we'?

Yes, when a probe is sent to study a planet it will be to study the planet's intrinsic properties. That's because probes that are sent to planets are well suited to that kind of study; principally because they're designed that way. But it's not the only way of studying planets (and is no way of studying exoplanets at all).

And it's not as if such probes aren't used to study the orbits of planets. The Cassini probe has been used to accurately determine the orbit of Saturn, for instance. And such study was in turn used in considering the potential existence of Planet Nine, which proposal was based on a study of the dynamics of the solar system. (Is existence an intrinsic property?)

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Those are the things that make it a planet, and make it interesting.

Whether it's interesting or not depends on what you're interested in. But regardless, all that does it make is an object of interest. Whether it's a planet or not depends on your classification scheme and what label you give the particular class or sub-class the object is in according to that scheme. Being called a planet is not an intrinsic property of any celestial object.

That the IAU insisted on making it about Pluto is what led them into this error in the first place.

I doubt many people think it was the IAU that has made it "Pluto specific"...

It wasn't about Pluto at all; it was about Eris - was Eris a planet or not? The IAU would probably have preferred to stay out of it, and let the argument rage on without conclusion. Unfortunately, because of decisions made decades earlier, the IAU has two different systems for naming planets and other bodies. It had to choose one or the other. Plus, there was a lot of media and public interest in the answer to the question and a lot of pressure on the IAU to not duck the issue.

The perils of politics, PR and bureaucracy! :)

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #521 on: 03/27/2017 01:59 pm »
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I was clearly referring to the IAU definition of a "dwarf planet". Vesta isn't a "dwarf planet" because it is no longer capable of being in hydrostatic equilibrium. (Ditto Pallas).

However, Vesta is differentiated. Therefore the only justification you came up with for a geophys definition fails. Lack of "roundness" doesn't tell you that a body hasn't differentiated.

All I'm saying is that I would include Vesta and Pallas in the list of bodies classified as "planets" as identified in the geophysical definition of planet.   Stern & Levison (2002) actually did include both objects.

Vesta and Pallas are obviously on the lower end of the mass scale for spherical bodies.  As with any continuous sequence (mass in this case) as you approach the boundary (mass necessary to be spherical) you run into borderline examples. 

Neither differentiation nor hydrostatic equilibrium can be completely guaranteed amongst the population of bodies that are nearly spherical.   For example, Callisto is likely only a partially differentiated body despite having a mass larger than our moon.   Notice that Runyon et al. make no mention of hydrostatic equilibrium or differentiation in their definition.

However, the fact remains that most bodies that are in the nearly spherical class will have at least some differentiation of the interior along with chemical evolution from the raw material state. That is what makes "round" an geologically interesting set of bodies.

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Actually Eris is an SDO, not a KBO.

Yes.  I do know that.   And Sedna has what is likely another class of orbit. 

I chose the term "belt planet" as an alternative to "dwarf planet" in the dynamical classes I propose because the bodies in this dynamical class share their orbits with numerous other bodies.  Hence the asteroid belt and Kuiper belt.   The term "dwarf" does not convey that information in any respect.   The size of the body is related to the fact that these "dwarf planets" do not dominate their orbit, but as you say - the context is what actually defines the class.   "Belt planet" describes context better than "dwarf planet".

Stern prefers to call the Kuiper belt the "Kuiper disk". 

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #522 on: 03/27/2017 02:34 pm »
As with any continuous sequence (mass in this case) as you approach the boundary (mass necessary to be spherical) you run into borderline examples.

My point was that there aren't boundaries in a continuous sequence.

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #523 on: 03/27/2017 03:22 pm »
As with any continuous sequence (mass in this case) as you approach the boundary (mass necessary to be spherical) you run into borderline examples.

My point was that there aren't boundaries in a continuous sequence.

I'm talking about the lower mass boundary for the geophysical planet definition.  Obviously the mass sequence is continuous.  But there is a boundary region at lower masses where the population transitions from spherical bodies to non-spherical bodies.   The lowest mass "round" body in the Solar System is Mimas and its mass serves well enough as the lower mass limit for "planet" when the term is defined as Runyon et al. have done. 

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #524 on: 03/27/2017 03:30 pm »
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If "moon" is just a subclass of all substellar objects in the solar system, then so are "planets", by any definition. And no more or less valid than any other subclass, such as SDO or KBO or main-belt object. llanitedave was trying to suggest that we never classify things after their location or extrinsic properties, when it's actually very common in the solar system. And "moon" is the perfect example. A whole classification based solely on a single extrinsic property, a substellar object in the solar system being in orbit around some other substellar object, regardless of whether that "some other" is Jupiter or a small asteroid.

And in bold is not what illanitedave is suggesting.   Neither or us are suggesting that.   We are suggesting that there are different levels in a taxonomy and that at the top of the sub-stellar taxonomy "planet" should be the set of sub-stellar bodies that are round no matter their dynamical circumstances ... or composition.

Dynamics (orbital context or just "context" or "extrinsic"- whatever you prefer) would be used to identify dynamical sub-classes of planet just like rock, ice, and gas are composition subclasses of planets.   


Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #525 on: 03/27/2017 06:51 pm »
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It wasn't about Pluto at all; it was about Eris - was Eris a planet or not? The IAU would probably have preferred to stay out of it, and let the argument rage on without conclusion. Unfortunately, because of decisions made decades earlier, the IAU has two different systems for naming planets and other bodies. It had to choose one or the other.

Ok ... first, I'll agree that at this stage it is highly unlikely that the IAU is going to change what they came up with because that is not what happens once a body has voted on an official position.  There is just too much inertia to change ... and no such body every goes back and admits they could have done it better.   

I'll also agree that regardless of how we choose to name the various classes of objects that are being studied, the science moves forward. 

Those things agreed to, that doesn't mean a different and better way of classifying sub-stellar bodies cannot be devised.  So just hear me out on the following.

First, I've never said that the dynamical (or contextual, or whatever term is preferred) distinctions between "planets" and "dwarf planets" identified by the IAU are not important.  What I have argued is that just like we classify planets by composition types (rock, ice, gas), we can also classify planets by dynamical classes.

So all I've been proposing is that the taxonomy could have been better structured if dynamics was used to identify classes of planets - in part the very same classes the IAU identified - rather than to identify what is a planet. 

In my proposal a planet is defined any round sub-stellar body formed in a proto-planetary disk.  That includes all planets, dwarf planets, and spherical moons.  That also includes exoplanets and any free floating planets ejected from a planetary system.     

The dynamical classes I have described before:  "Principal planets" (IAU planets), "Belt planets" (IAU dwarf planets), "Moons" (spherical moons --> planet orbiting a larger planet), "Rogue planets" (not orbiting any star), and "satellite" (non-spherical bodies orbiting planets).

The composition classes for planets are at the foundation "rock", "ice", and "gas" and there is more to it than that but it is not important to go into that at this time. 

Now I think this system is especially useful for pedagogy. 

Go back in time and imagine the IAU went this sort of direction - defining planet in a more inclusive way as opposed to an exclusive way.  Instead of a distracting focus on why Pluto is no longer a planet, the focus for kids could have been an awe factor as to how many new planets there are.

The focus of instruction becomes understanding the structure and types of bodies within the Solar System:   

1. What is significant about planets being round?
2.  What types of orbits do planets have?  Dynamically dominant (Principal planets), dynamically not dominant (Belt planets), planets that orbit a bigger planet (moons), planets that don't orbit any star (rogue). 
3.  Where are the different types of planets (orbits, composition) found in the Solar System and what does this teach us about how the Solar System formed?
4.  What are the different types of materials planets are made of?

And there are more but I need to get on to something else.

I think from an educators point of view this is a better way to teach the Solar System to students.  And there is no reason it cannot be useful to researchers. 

And the concerns of the dynamical perspective are addressed - not ignored.  In fact, calling the IAU planets "Principal planets" would make pretty big impression on students in school - "You know how the Principal is the person in charge of the school?  Well, the Principal planets - like the Earth and Jupiter - are the big planets that dominate the orbits in the Solar System."



Offline launchwatcher

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Re: Pluto-Planet debate discussions
« Reply #526 on: 03/27/2017 07:04 pm »
Just noticed that Ed Lerner weighed in, suggesting alternate terminology that separates orbital dynamics from geophysics:
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IMO -- and I'm not the only person to express this opinion -- we have a perfectly fine label for any round sub-stellar body: world. I'd be for world to be the label for any sub-stellar body that's round. There would then be three classes of worlds: planets (orbit stars), moons (orbit planets), and free-floating (orbit neither stars nor planets [but likely orbit much larger constructs, like star clusters or a galaxy as a whole]). Less massive objects, never round, would still be called asteroids.

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #527 on: 03/29/2017 10:22 am »
As with any continuous sequence (mass in this case) as you approach the boundary (mass necessary to be spherical) you run into borderline examples.
My point was that there aren't boundaries in a continuous sequence.
I'm talking about the lower mass boundary for the geophysical planet definition.

There is no "mass boundary". Any intrinsic property depends not just on mass, but on the composition, age, closeness to the sun of the object, and varies wildly between objects of the same mass.

You can draw arbitrary lines on a continuum, just for the sake of nomenclature, if no other arrangement presents itself. But if the universe presents you with an existing wide separation of classes of objects, why reject it? Why choose a greatly inferior classification system?

The lowest mass "round" body in the Solar System is Mimas and its mass serves well enough as the lower mass limit for "planet"

Serves what? What scientific need is served by it?



Instead of a distracting focus on why Pluto is no longer a planet, the focus for kids could have been an awe factor as to how many new planets there are.

"Oh, somebody please think of the children!"

The focus of instruction becomes understanding the structure and types of bodies within the Solar System:   
1. What is significant about planets being round?
2.  What types of orbits do planets have?  Dynamically dominant (Principal planets), dynamically not dominant (Belt planets), planets that orbit a bigger planet (moons), planets that don't orbit any star (rogue).
[...]
And the concerns of the dynamical perspective are addressed - not ignored.  In fact, calling the IAU planets "Principal planets" would make pretty big impression on students in school - "You know how the Principal is the person in charge of the school?  Well, the Principal planets - like the Earth and Jupiter - are the big planets that dominate the orbits in the Solar System."

The people who fussed over the "demotion" of Pluto are not the sort of people who would have explained dynamical dominance, or hydrostatic equilibrium, or anything else. Those who didn't understand the concepts made no attempt to understand the IAU definition, and those who did understand it, not only made no attempt to explain the science behind it, but went out of their way to encourage public misunderstanding.

Your "won't somebody think of the children" plea assumes the people who opposed the IAU definition are acting honestly and with the broader interests of science at heart. But I see nothing to suggest that.

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #528 on: 03/29/2017 10:30 am »
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If "moon" is just a subclass of all substellar objects in the solar system, then so are "planets", by any definition. And no more or less valid than any other subclass, such as SDO or KBO or main-belt object. llanitedave was trying to suggest that we never classify things after their location or extrinsic properties, when it's actually very common in the solar system. And "moon" is the perfect example. A whole classification based solely on a single extrinsic property, a substellar object in the solar system being in orbit around some other substellar object, regardless of whether that "some other" is Jupiter or a small asteroid.
And in bold is not what illanitedave is suggesting.

I can think of no other scientific definition of any class of objects where intrinsic characteristics of the objects themselves are completely absent.

How does "moon" not meet llanitedave's requirement of a classification based purely on extrinsic/location/environmental properties?

"Moons" are implicitly a sub-category of the broader class of all substellar objects, but explicitly so are IAU-Planets. Both are dynamical subcategories of the broader intrinsic class (substellar masses).

Just as KBOs are a dynamical sub-category of the broader class of substellar non-planetary objects.

We are suggesting that  [...] at the top of the sub-stellar taxonomy "planet" should be the set of sub-stellar bodies that are round no matter their dynamical circumstances ... or composition.

Why?

IMO, extrinsic classification of even sub-planetary objects is more scientifically useful. KBO, SDO, main-belt asteroid, Trojan, etc.

What is the point in placing an arbitrary intrinsic property first? You separate objects of extrinsic similarity. For what reason? What is the benefit of grouping Ceres with Pluto, Pluto with Eris, instead of grouping them first with their neighbours and siblings. What scientific usefulness is served by such a grouping? What science has been harmed by the previous lack of such "roundness" classification?

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #529 on: 03/30/2017 10:22 am »
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There is no "mass boundary". Any intrinsic property depends not just on mass, but on the composition, age, closeness to the sun of the object, and varies wildly between objects of the same mass.

It does not vary wildly.  Lineweaver & Norman looked at it.  At 400 km radius and larger all bodies in the Solar System are "round".  At 198 km radius (Mimas) and larger ice-rich bodies in the Solar System and larger are "round".  The transition zone within which composition impacts whether or not the body is spherical is within this radius range.  Within the full mass range of "round" sub-stellar bodies the mass zone where composition difference can lead to differences in whether or not a body is round is small.

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You can draw arbitrary lines on a continuum, just for the sake of nomenclature, if no other arrangement presents itself. But if the universe presents you with an existing wide separation of classes of objects, why reject it? Why choose a greatly inferior classification system?

Your existing wide separation of classes exists in the dynamical classes of planets.   The dynamically dominant planets are "Principal Planets".  The planets that do not dynamically dominate their orbits are called "Belt planets".   Just like planets are divided into different composition classes.  They can also be divided into dynamical classes. 

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Serves what? What scientific need is served by it?

Serves as the boundary mass/radius dividing spherical bodies form those that are non-spherical.  In other words dividing planets from sub-planetary bodies.   The scientific need served is to identify a useful boundary for a geophysical definition of planet.

What need is served by adopting a definition of "planet" that excludes numerous bodies that are subjects of comparative planetology?

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  The people who fussed over the "demotion" of Pluto are not the sort of people who would have explained dynamical dominance, or hydrostatic equilibrium, or anything else. Those who didn't understand the concepts made no attempt to understand the IAU definition, and those who did understand it, not only made no attempt to explain the science behind it, but went out of their way to encourage public misunderstanding.

Your "won't somebody think of the children" plea assumes the people who opposed the IAU definition are acting honestly and with the broader interests of science at heart. But I see nothing to suggest that. 

Not a plea.   When the IAU definition was adopted all these arguments were made that the number had to be limited to 8 planets.  How could we expect school children to learn all those planet names if the others were accepted?   I was simply illustrating that you can turn that argument the other way. 

But what is it with some of you trying to imply that the opposition to the IAU planet definition is motivated by something other than science.  People can disagree on this topic from differing views on what is more scientifically useful.  And yes - pedagogy is part of science.  You have to communicate to the next generation of scientists.

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How does "moon" not meet llanitedave's requirement of a classification based purely on extrinsic/location/environmental properties?

"Moons" are implicitly a sub-category of the broader class of all substellar objects, but explicitly so are IAU-Planets. Both are dynamical subcategories of the broader intrinsic class (substellar masses).

Just as KBOs are a dynamical sub-category of the broader class of substellar non-planetary objects.

What I'm suggesting is that the first category of subdivision for sub-stellar bodies be mass with the lower mass boundary for planets at ~3 x 1019 kg and the upper mass boundary for planets at 60 Jupiter masses. 

The next level of classification is to break the spherical bodies (planets) into dynamical types (cleared orbit, not cleared orbit, moons, rogue).   The meaning of "moon" is not purely extrinsic.  It is first a body that has a mass falling in the mass range for spherical sub-stellar bodies. 

And what about non-spherical bodies orbiting planets?  Call them "satellites".  I've discussed that before.

And what about brown dwarfs?  I've discussed that before here as well.

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Why?

IMO, extrinsic classification of even sub-planetary objects is more scientifically useful. KBO, SDO, main-belt asteroid, Trojan, etc. 


Why?  IMO first identifying the sub-set of sub-stellar bodies that is spherical is more useful.  Then divide all the sub-stellar bodies into dynamical groups and composition groups.

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What is the point in placing an arbitrary intrinsic property first? You separate objects of extrinsic similarity. For what reason? What is the benefit of grouping Ceres with Pluto, Pluto with Eris, instead of grouping them first with their neighbours and siblings. What scientific usefulness is served by such a grouping? What science has been harmed by the previous lack of such "roundness" classification?

These questions can all be turned right back at you about the system that you prefer.  Why exclude all spherical bodies from the class "planet"?   

Why group Ceres with Pluto? It's called "comparative planetology".  It's something geophysicists do when they look at the intrinsic geophysical characteristics of these bodies.  And it turns out that common geophysical characteristics cross dynamical lines. 

Your dynamical classes are all still recognized and part of the taxonomy - just at a different level in the taxonomy.

« Last Edit: 03/30/2017 10:25 am by DRussell »

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #530 on: 03/30/2017 03:04 pm »
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IMO, extrinsic classification of even sub-planetary objects is more scientifically useful. KBO, SDO, main-belt asteroid, Trojan, etc. 

Why?

Because it's how they are actually grouped by researchers in order to do comparative science.

Even some of the intrinsic properties that are used in informal grouping, such as composition (icy/rocky), are often considered more interesting because of what it says about the extrinsic nature of such objects, such as their location of formation and what that says about dynamic systems. And vice-versa, there's a strong overlap between intrinsic and extrinsic properties. Being a KBO or a main belt asteroid is as much intrinsic as not.

Extrinsic/location/environmental classification as a top-level-sort doesn't ignore intrinsic properties, it better enables their analysis. Whereas having intrinsic properties as the top-level classification requires research on the individual objects in order to classify them, which will generally require a prior division into extrinsic groups in order to allow enough analysis to then put them in intrinsic categories.

For example, your desire to use mass limits as a divider: If an object lacks a visible moon, then in order to estimate its mass, you need to know its size and composition, which usually means understanding where it formed, which means grouping it with objects that formed in similar areas. You need to do an extrinsic grouping in order to even estimate your intrinsic classification. So why not have the extrinsic grouping at a higher level, since that's what researchers will actually do anyway?

It's the same for any measure you try to pick for sphericalness. Use hydrostatic equilibrium (as the IAU did for the nonsensical "dwarf" category) and you exclude objects that share major geophysical properties but fall on different sides of the line. Your own scheme is worse. There's no reason (neither dynamical nor geophysical) for objects just above and just below that mass limit to be grouped separately. It is not a natural grouping. Actual research will inevitably have to create other primary groupings that ignore your top-level classification anyway. Your system doesn't actually add anything useful.

I can see internal-differentiation being a useful divider. Trying to create an actual "line" is still too arbitrary to turn into a major classification system, but at least it is scientifically interesting property of the objects being studied. However, as I said earlier, because it depends so much on composition, historical-temperature, collision history, etc, it is a fairly unique trait of each object. Once you've been able to classify an object's degree of differentiation enough to put it in one of two groups, such crude super-categories are no longer useful to the study of that object and those like it.

OTOH, extrinsic groups remain useful, KBO/main-belt/trojan/moon's-of-Saturn/etc, even if the intrinsic properties of some members of the group are understood in greater detail. Indeed because some members of the group are understood.

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What is the point in placing an arbitrary intrinsic property first? You separate objects of extrinsic similarity. For what reason? What is the benefit of grouping Ceres with Pluto, Pluto with Eris, instead of grouping them first with their neighbours and siblings. What scientific usefulness is served by such a grouping? What science has been harmed by the previous lack of such "roundness" classification?
These questions can all be turned right back at you about the system that you prefer.  Why exclude all spherical bodies from the class "planet"?

Because they really fall into different groups. It's a thing which actually exists. A large separation between object-types that was found in nature, not an arbitrary line drawn in a continuum. The group inside the category plays a fundamentally different role in the history and structure of the entire solar system than the group outside the category.

An example: An apparent systematic bias in the orbits of TNOs has led some researchers to speculate that there is another planet somewhere out there.

If it exists, it will be considered a planet because it does that. Hence it belongs to a fundamentally different class of objects than the smaller TNOs like Eris, Pluto and Sedna, because they don't do that. Just as Plutinos are slave to Neptune's orbit, not the other way around. The difference is fundamental.

Offline tea monster

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Re: Pluto-Planet debate discussions
« Reply #531 on: 03/30/2017 05:50 pm »
The problem with classifying due to orbital neighbors is that it is open to too much opinion and leeway and isn't in any way a scientific way of looking at the matter.

Someone jokingly pointed out that Neptune isn't a planet according to the definition in use at the moment as it hasn't cleared Pluto out of it's orbit. This was probably suggested in jest, but it does show how poor the definition is. Jupiter could be demoted too as it hasn't cleared it's orbit of it's forward and aft trailing trojan debris. How much 'debris' is acceptable? How do you choose and where do you draw the line?

What happens if we find a 'Plutoid' out in the Oort cloud that is larger than Mercury and is perfectly round? What do we do if we find out that some of the planets that we are seeing around other stars are sat inside a debris belt?

Using a different field of study, we can definitely tell if a particular creature is, for example, a dog. The dog has certain points that differentiate it from other creatures. These points are easily determined and allow those who have never seen a particular specimen to determine if the creature they are looking at is, or isn't, a dog.

The dog is still a dog, no matter where it is. If it is in isolation, or if it is surrounded by sheep or cats, it is still a dog.  It doesn't change it's taxonomy depending on it's location or surroundings.

We need a similar taxonomy system for planets, based on their physical attributes.

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Re: Pluto-Planet debate discussions
« Reply #532 on: 03/30/2017 09:07 pm »
The problem with classifying due to orbital neighbors is that it is open to too much opinion and leeway and isn't in any way a scientific way of looking at the matter.

Someone jokingly pointed out that Neptune isn't a planet according to the definition in use at the moment as it hasn't cleared Pluto out of it's orbit. This was probably suggested in jest, but it does show how poor the definition is. Jupiter could be demoted too as it hasn't cleared it's orbit of it's forward and aft trailing trojan debris. How much 'debris' is acceptable? How do you choose and where do you draw the line?

What happens if we find a 'Plutoid' out in the Oort cloud that is larger than Mercury and is perfectly round? What do we do if we find out that some of the planets that we are seeing around other stars are sat inside a debris belt?

Using a different field of study, we can definitely tell if a particular creature is, for example, a dog. The dog has certain points that differentiate it from other creatures. These points are easily determined and allow those who have never seen a particular specimen to determine if the creature they are looking at is, or isn't, a dog.

The dog is still a dog, no matter where it is. If it is in isolation, or if it is surrounded by sheep or cats, it is still a dog.  It doesn't change it's taxonomy depending on it's location or surroundings.

We need a similar taxonomy system for planets, based on their physical attributes.

It has. Neptune is the gravitationally dominant body, and Pluto is in resonance with it.  That's the whole point.  Earth has a moon, Jupiter has trojans, but they (moon, trojans) are all dominated by their respective planets.

This is what Planets do - they dominate their orbital neighborhood.  This is what makes them unique.

Given its size, and the size of its orbit, Pluto (and other Pluto-like bodies) are incapable of doing so.

« Last Edit: 04/02/2017 12:50 am by meekGee »
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Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #533 on: 03/31/2017 01:51 am »
The problem with classifying due to orbital neighbors is that it is open to too much opinion and leeway and isn't in any way a scientific way of looking at the matter.

No there isn't. Read back through the thread, the various measures that have been proposed for dynamical dominance show a gap between planets and non-planets of orders of magnitude. There's no ambiguity, opinion, or leeway, it's based purely on the reality of the solar system. (See below)

Someone jokingly pointed out that Neptune isn't a planet according to the definition in use at the moment as it hasn't cleared Pluto out of it's orbit. This was probably suggested in jest, but it does show how poor the definition is.

If that someone was Alan Stern, then you should be aware that he co-wrote one of the early papers that defined the difference between the eight planets and the non-planets, in a paper called "Criteria for Planethood". He even used the language "clearing the neighbourhood" to describe the difference. So he when he now publicly "misunderstands" the IAU definition, try to image my contempt for him.



Jean-Luc Margot's paper. And the earlier Stern/Levison paper. Both give you a good understanding of the scale of the gap between planets and non-planets, even if you only skim the maths.

Using the handy wikipedia tables, Stern/Levison score and rank the planets thus:

NameRankValue  -    RankValue
Jupiter11,000,000,000  -140,000
Saturn2     50,000,000  -2  6,000
Uranus3          400,000  -5     400
Neptune4          300,000  -6     300
Venus5          160,000  -3     950
Earth6          150,000  -4     800
Mercury7             2,000  -7     130
Mars8             1,000  -8       50
IAU Planet  -                    1  --         1
Pluto9                    0.003  -10         0.03
Eris10                    0.002  -11         0.02
Ceres11                    0.001  -9         0.04
Haumea12                    0.0002  -12         0.008
Makemake13                    0.0002  -13         0.007
                 Stern/Levison, 2002              Margot, 2015

Note that even using different methods still results in the same clear distinction between planets (>1) and non-planets (<1), with nothing near the margin.

Offline Quagga

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Re: Pluto-Planet debate discussions
« Reply #534 on: 03/31/2017 07:52 am »
I keep hearing that the dynamical definition is so useful for exoplanetary systems.
But clearing the orbit takes time, so in the early solar system Earth wasn't a planet. In Margot's definition it's a planet if it is able to clear the orbit in the lifetime of its parent star (which depends on the mass of the star). So if the sun were a red dwarf Ceres might count as a planet (I haven't done the math), and non-spherical bodies in close orbits around red dwarfs might very well clear their neigbourhood in the lifetime of their parent stars.

I'm not saying we should drop dynamic classes, but it makes very much sense for comparative planetology to have a geophysical definition. Titan is comparable to Earth in many aspects, but how much does it have in common with Saturn's satellite Pan?

Offline RonM

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Re: Pluto-Planet debate discussions
« Reply #535 on: 03/31/2017 02:03 pm »
Let's look at this from an exoplanet discovery perspective.

The dynamics of a system will be the first thing we can determine from observation. Largest objects first and as our technology improves smaller objects. That puts the Margot or Stern/Levison numbers at the top of the list. It will be a very long time before we can find objects as small as Pluto or Ceres. Therefore, every object we see will be in hydrostatic equilibrium. It's a given.

Following the tradition from naming types of stars, then with exoplanets we can have planets as substellar objects that dominate their orbit (per Margot or Stern/Levison) and dwarf planets as substellar objects that do not dominate their orbits. The definition of the lower cutoff for dwarf planets can be an exercise for future generations once that actually becomes an issue.

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #536 on: 03/31/2017 02:34 pm »
I keep hearing that the dynamical definition is so useful for exoplanetary systems.
But clearing the orbit takes time, so in the early solar system Earth wasn't a planet.

Not what "cleared its neighbourhood" means.

The definition for the solar system assumes the current time, in a mature planetary system, so speaks in a past tense, an action already completed. But if you are talking about the early solar system, you need to use the future tense. Criticising the definition by applying it to a situation it was never meant to cover is not meaningful.

it makes very much sense for comparative planetology to have a geophysical definition. Titan is comparable to Earth in many aspects, but how much does it have in common with Saturn's satellite Pan?

How much does it have in common with Ceres? (Or with Saturn, for that matter.)

The "many aspects" in which "Titan is comparable to Earth" is not because they are both roundish. It's because they both have complex atmospheres, solid surfaces, and most importantly, an atmospheric component that can exist in a liquid state on the surface. Their similarity is not described by a crude binary classification scheme like Planet/Non-Planet, it requires a much more detailed understanding of both objects.

As I said in my previous comment: If you want to do "comparative planetology", you need to enough data to meaningfully compare. For example, internal structure and composition, or surface effects, etc. That either means you have studied the individual bodies in detail, or you are using a proxy in order to estimate that data (such as the location in solar system, distance from the star, location of formation, time since formation). In order to do the latter, you must first classify objects by their location/orbit. If you have detailed ground-truths from a specific body, you don't need crude classification binaries like Planet/Non-Planet. OTOH, you may want to use the studied body to characterise much less studied neighbouring objects that share the dynamic history of the target.

And that's what happens in practice. Objects in the solar system are divided by region (extrinsic classification) in order to estimate specific data for the objects (multiple intrinsic classifications) in order to do comparative studies (either comparisons within the extrinsic group, or between extrinsic groups.)

Online meekGee

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Re: Pluto-Planet debate discussions
« Reply #537 on: 04/02/2017 12:53 am »
The problem with classifying due to orbital neighbors is that it is open to too much opinion and leeway and isn't in any way a scientific way of looking at the matter.

No there isn't. Read back through the thread, the various measures that have been proposed for dynamical dominance show a gap between planets and non-planets of orders of magnitude. There's no ambiguity, opinion, or leeway, it's based purely on the reality of the solar system. (See below)

Someone jokingly pointed out that Neptune isn't a planet according to the definition in use at the moment as it hasn't cleared Pluto out of it's orbit. This was probably suggested in jest, but it does show how poor the definition is.

If that someone was Alan Stern, then you should be aware that he co-wrote one of the early papers that defined the difference between the eight planets and the non-planets, in a paper called "Criteria for Planethood". He even used the language "clearing the neighbourhood" to describe the difference. So he when he now publicly "misunderstands" the IAU definition, try to image my contempt for him.



Jean-Luc Margot's paper. And the earlier Stern/Levison paper. Both give you a good understanding of the scale of the gap between planets and non-planets, even if you only skim the maths.

Using the handy wikipedia tables, Stern/Levison score and rank the planets thus:

NameRankValue  -    RankValue
Jupiter11,000,000,000  -140,000
Saturn2     50,000,000  -2  6,000
Uranus3          400,000  -5     400
Neptune4          300,000  -6     300
Venus5          160,000  -3     950
Earth6          150,000  -4     800
Mercury7             2,000  -7     130
Mars8             1,000  -8       50
IAU Planet  -                    1  --         1
Pluto9                    0.003  -10         0.03
Eris10                    0.002  -11         0.02
Ceres11                    0.001  -9         0.04
Haumea12                    0.0002  -12         0.008
Makemake13                    0.0002  -13         0.007
                 Stern/Levison, 2002              Margot, 2015

Note that even using different methods still results in the same clear distinction between planets (>1) and non-planets (<1), with nothing near the margin.

Stern is clearly trolling, and IMO is doing so since he realized he can "energize" support for his programs.  There is no other reasonable explanation to him misunderstanding this. He's not some Joe Schmo on a CNN talk-back thread.

This started, IIRC, way back with the debate on which mission to choose, when New Horizons was one of the candidates.
ABCD - Always Be Counting Down

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #538 on: 04/03/2017 01:46 pm »
Quote
Stern is clearly trolling, and IMO is doing so since he realized he can "energize" support for his programs.  There is no other reasonable explanation to him misunderstanding this.

Can you expand upon this?  What makes you think he isn't just expressing his opinion regarding which definition of planet is more useful from a geophysical perspective?   I haven't really seen any indication he's feigning a misunderstanding of the IAU definition.  It reads to me that he disagrees that the IAU definition is the best approach.  In the 2002 Stern & Levison paper the dynamical definition was presented as a refinement to the geophysical definition.  It was never argued that the word "planet" should be defined dynamically in that paper.  He hasn't changed his view on that.


Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #539 on: 04/03/2017 01:59 pm »
Quote
IMO, extrinsic classification of even sub-planetary objects is more scientifically useful. KBO, SDO, main-belt asteroid, Trojan, etc. 

Why?

Because it's how they are actually grouped by researchers in order to do comparative science.

Even some of the intrinsic properties that are used in informal grouping, such as composition (icy/rocky), are often considered more interesting because of what it says about the extrinsic nature of such objects, such as their location of formation and what that says about dynamic systems. And vice-versa, there's a strong overlap between intrinsic and extrinsic properties. Being a KBO or a main belt asteroid is as much intrinsic as not.

Extrinsic/location/environmental classification as a top-level-sort doesn't ignore intrinsic properties, it better enables their analysis. Whereas having intrinsic properties as the top-level classification requires research on the individual objects in order to classify them, which will generally require a prior division into extrinsic groups in order to allow enough analysis to then put them in intrinsic categories.

For example, your desire to use mass limits as a divider: If an object lacks a visible moon, then in order to estimate its mass, you need to know its size and composition, which usually means understanding where it formed, which means grouping it with objects that formed in similar areas. You need to do an extrinsic grouping in order to even estimate your intrinsic classification. So why not have the extrinsic grouping at a higher level, since that's what researchers will actually do anyway?

It's the same for any measure you try to pick for sphericalness. Use hydrostatic equilibrium (as the IAU did for the nonsensical "dwarf" category) and you exclude objects that share major geophysical properties but fall on different sides of the line. Your own scheme is worse. There's no reason (neither dynamical nor geophysical) for objects just above and just below that mass limit to be grouped separately. It is not a natural grouping. Actual research will inevitably have to create other primary groupings that ignore your top-level classification anyway. Your system doesn't actually add anything useful.

I can see internal-differentiation being a useful divider. Trying to create an actual "line" is still too arbitrary to turn into a major classification system, but at least it is scientifically interesting property of the objects being studied. However, as I said earlier, because it depends so much on composition, historical-temperature, collision history, etc, it is a fairly unique trait of each object. Once you've been able to classify an object's degree of differentiation enough to put it in one of two groups, such crude super-categories are no longer useful to the study of that object and those like it.

OTOH, extrinsic groups remain useful, KBO/main-belt/trojan/moon's-of-Saturn/etc, even if the intrinsic properties of some members of the group are understood in greater detail. Indeed because some members of the group are understood.

Quote
What is the point in placing an arbitrary intrinsic property first? You separate objects of extrinsic similarity. For what reason? What is the benefit of grouping Ceres with Pluto, Pluto with Eris, instead of grouping them first with their neighbours and siblings. What scientific usefulness is served by such a grouping? What science has been harmed by the previous lack of such "roundness" classification?
These questions can all be turned right back at you about the system that you prefer.  Why exclude all spherical bodies from the class "planet"?

Because they really fall into different groups. It's a thing which actually exists. A large separation between object-types that was found in nature, not an arbitrary line drawn in a continuum. The group inside the category plays a fundamentally different role in the history and structure of the entire solar system than the group outside the category.

An example: An apparent systematic bias in the orbits of TNOs has led some researchers to speculate that there is another planet somewhere out there.

If it exists, it will be considered a planet because it does that. Hence it belongs to a fundamentally different class of objects than the smaller TNOs like Eris, Pluto and Sedna, because they don't do that. Just as Plutinos are slave to Neptune's orbit, not the other way around. The difference is fundamental.

Paul451,

I like everything you have to say here.   This debate is not a matter of the "right" vs. the "wrong" way to define "planet".  It truly is a matter of two different points of view about what is the more useful approach.    The IAU definition is more useful to the dynamicist.   The geophysical definition is more useful to the geophysicist.   Neither definition would inhibit the other point of view from conducting their research ... although it does impact the language used to communicate in their field.    And then there is pedagogy.  From an instructional point of view I prefer the geophysical definition with dynamical classes and composition classes. 

 
« Last Edit: 04/03/2017 02:00 pm by DRussell »

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