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IAU Standard for Planets is Not Supported in Research Literature

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#1 BillP

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Posted 25 August 2019 - 08:50 PM

https://www.ucf.edu/...lanet-research/

 

The title of this post is a statement from University of Central Florida's planetary scientist Philip Metzger.  It is a sentiment also supported by the principal investigator of the NASA New Horizons spacecraft, planetary scientist Alan Stern. 

 

As Alan Stern says, "... if you put Earth where Pluto is, it would be excluded!"

 

As Philip Metzger says. "The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research."  &  "We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition..."  &  "It’s a sloppy definition.  They [IAU] didn’t say what they meant by clearing their orbit.  If you take that literally, then there are no planets, because no planet clears its orbit."

 

And the IAU actually made a false claim as justification for their new classification!  Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said the IAU’s definition was erroneous since the literature review showed that clearing the orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets.

 

As we knew all along...Pluto remains a planet and is "The" 9th planet in our solar system.   Just sayin' smile.gif


Edited by BillP, 25 August 2019 - 08:51 PM.

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#2 vtornado

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Posted 25 August 2019 - 09:09 PM

So Pluto's back in????



#3 bobzeq25

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Posted 25 August 2019 - 09:41 PM

https://www.ucf.edu/...lanet-research/

 

The title of this post is a statement from University of Central Florida's planetary scientist Philip Metzger.  It is a sentiment also supported by the principal investigator of the NASA New Horizons spacecraft, planetary scientist Alan Stern. 

 

As Alan Stern says, "... if you put Earth where Pluto is, it would be excluded!"

 

As Philip Metzger says. "The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research."  &  "We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition..."  &  "It’s a sloppy definition.  They [IAU] didn’t say what they meant by clearing their orbit.  If you take that literally, then there are no planets, because no planet clears its orbit."

 

And the IAU actually made a false claim as justification for their new classification!  Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said the IAU’s definition was erroneous since the literature review showed that clearing the orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets.

 

As we knew all along...Pluto remains a planet and is "The" 9th planet in our solar system.   Just sayin' smile.gif

The one thing you cannot have is 9 planets.  It's either 8 or many.  Pluto is not that special among Kuiper Belt Objects.

 

Ceres was a "planet" until they saw the asteroid belt.  It remains, by a decent margin, the biggest asteroid.  There's no way of distinguishing this case from the case of Pluto.

 

The chance of this changing is very small.  Despite a few vocal opponents.  The IAU may have been inartful in _how_ they did this, the idea that they _shouldn't_ have done it is wrong.  You can draw a line between many and 8.  You can't draw it between many and 9.


Edited by bobzeq25, 25 August 2019 - 09:45 PM.

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#4 llanitedave

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Posted 26 August 2019 - 02:24 AM

The one thing you cannot have is 9 planets.  It's either 8 or many.  Pluto is not that special among Kuiper Belt Objects.

 

Ceres was a "planet" until they saw the asteroid belt.  It remains, by a decent margin, the biggest asteroid.  There's no way of distinguishing this case from the case of Pluto.

 

The chance of this changing is very small.  Despite a few vocal opponents.  The IAU may have been inartful in _how_ they did this, the idea that they _shouldn't_ have done it is wrong.  You can draw a line between many and 8.  You can't draw it between many and 9.

There are not eight, nor nine.  There are billions.  The Sun is only one of many planetary hosts.

 

There was a period early in the history of the solar system where Jupiter, Saturn, Uranus, and Neptune caught each other in resonances that made big changes to their orbits.  The Nice model envisions a period of time when the giant planets had not only not yet cleared their own orbits, but they were scattering objects back and forth in multiple directions, resulting in the Late Heavy Bombardment of about 3.9 billion years ago.  It seems to me that if you accept the IAU's definition, the our four giant planets were not planets at all for more than half a billion years after they formed!

 

A planet can get caught in an unfortunate resonance with another, larger planet long after it has supposedly cleared it's own orbit, and get flung into a much large orbit where clearing is not possible.  Does that mean it suddenly changes from being a planet to a non-planet?

 

The bottom line is that the IAU's definition of "planet" was not created by planetary scientists.


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#5 bobzeq25

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Posted 26 August 2019 - 10:40 AM

There are not eight, nor nine.  There are billions.  The Sun is only one of many planetary hosts.

 

There was a period early in the history of the solar system where Jupiter, Saturn, Uranus, and Neptune caught each other in resonances that made big changes to their orbits.  The Nice model envisions a period of time when the giant planets had not only not yet cleared their own orbits, but they were scattering objects back and forth in multiple directions, resulting in the Late Heavy Bombardment of about 3.9 billion years ago.  It seems to me that if you accept the IAU's definition, the our four giant planets were not planets at all for more than half a billion years after they formed!

 

A planet can get caught in an unfortunate resonance with another, larger planet long after it has supposedly cleared it's own orbit, and get flung into a much large orbit where clearing is not possible.  Does that mean it suddenly changes from being a planet to a non-planet?

 

The bottom line is that the IAU's definition of "planet" was not created by planetary scientists.

I totally agree.   But...

 

The authority of the IAU to name and characterize things is well established.  Specialists like planetary scientists may disagree with the IAU on a specific decision, that's life.  There are virtues to having a more generalist authority.

 

My real disagreement is with wanting nine, and only nine planets in the solar system.  That just doesn't work technically, no matter how you look at it.  It's a sentimental position.

 

Even the planetary scientists gave up on nine.  The last proposal I saw includes many things as planets, for the purpose of getting Pluto included.  That's not going _anywhere_.


Edited by bobzeq25, 26 August 2019 - 10:46 AM.


#6 llanitedave

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Posted 26 August 2019 - 02:53 PM

I totally agree.   But...

 

The authority of the IAU to name and characterize things is well established.  Specialists like planetary scientists may disagree with the IAU on a specific decision, that's life.  There are virtues to having a more generalist authority.

 

My real disagreement is with wanting nine, and only nine planets in the solar system.  That just doesn't work technically, no matter how you look at it.  It's a sentimental position.

 

Even the planetary scientists gave up on nine.  The last proposal I saw includes many things as planets, for the purpose of getting Pluto included.  That's not going _anywhere_.

The best proposal I've seen includes many things as planets, because it makes sense.  It's not that there are definitions designed merely to include Pluto, it's that the IAU's definition was shoehorned specifically to exclude Pluto, and in doing so became scientifically meaningless.

 

Which is why those who actually produce papers on planetary science have ignored it.



#7 russell23

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Posted 26 August 2019 - 07:21 PM

Dave is exactly right.  The IAU definition was more about non-scientific and political things than about developing a useful system of definitions.  Despite the protestations that advocates of the IAU definitions make that the definitions are useful, the real non-scientific motivations eventually are revealed:

 

(1) If Pluto, Eris and other spherical bodies are included as Solar System planets there will be too many planets for school children to learn.

 

    Comment:   Seriously?   We are going to base a definition on the difficulty that might be encountered by memorizing an extensive list.  Maybe we should cut back the number of constellations too?  Or how about the number of named stars?

 

(2) Pluto should not be a planet because it is too small.

 

   Comment:  This could be scientific except that those that argue Pluto is too small don't actually provide an intrinsic physical criteria for what constitutes "too small".  We are told that it is smaller than the Moon or this many times smaller than that planet none of which constitute scientific arguments. 

 

(3) Pluto should not be a planet because it was discovered by an American.

 

  Comment:   I actually did not think this could possibly be an actual motivation by some but as I've read stuff over the years I've come to realize that some people were motivated by this aspect of Pluto.  That is politics not science.

 

From a geophysical perspective the IAU definitions may most charitably be described as "incomplete".   The problem is that an important geophysical state occurs when a body acquires sufficient mass to take an a spheroidal shape.  This shape is rarely (if ever) a "sphere", but the key aspect is that the body has acquired sufficient mass to self-gravitate into a spheroidal shape.  When this happens the body will experience internal differentiation (separation into layers of differing composition) and geochemical/compositional/mineralogical evolution from the primitive raw material composition.

 

There are four general dynamical classes for spherical sub-stellar bodies formed in a proto-planetary disk:

 

1.  Orbit a star in a "cleared" which results in the body having more mass than the combined mass of all other bodies crossing the orbit. (IAU planets)

 

2.  Orbit a star in an orbit that is not cleared which results in the body have less mass than the combined mass of all other bodies that cross the orbit.  (IAU dwarf planets)

 

3.  Orbit a larger spherical body (spherical moons).  

 

4.  Have been ejected from their orbit and do not orbit any star. (rogue planets)

 

The above is what I mean when I say that the IAU definitions are "incomplete".    Spherical sub-stellar bodies formed in a proto-planetary class are a distinct geophysical class of object and the IAU only provided partial definitions.   All of these bodies should be considered "planets".  Then as described above they can have 4 dynamical classes:

 

1.  Principal planets (IAU planets)

2.  Belt planets (IAU dwarf planets)

3.  Satellite planets (spherical satellites)

4.  Rogue planets

 

If the IAU had simply done the above there would be minimal controversy.   For those that have the unscientific concern about school children learning the names of all the planets they can focus their instruction on the "principal planets".  For those that think Pluto is too small to be a planet, Pluto is not in the same planet class as the Principal planets. 

 

And then the spherical satellites would have a class to distinguish these important bodies that are receiving a lot of attention for geophysical and astrobiology reasons.

 

When I originally proposed this system a few years ago I had suggested that instead of using the terms "moon" and "satellite" interchangeably that each term be give a different meaning.  Specifically, use the term "moon" to apply to the spherical bodies orbiting principal planets and belt planets.  Use the term "satellite" to apply to the non-spherical bodies orbiting planets and belt planets.  But now I think that "satellite planets" makes more sense since the terms "moon" and "satellite" already have so much historical use.


Edited by russell23, 26 August 2019 - 07:24 PM.

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#8 DaveC2042

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Posted 26 August 2019 - 09:59 PM

I must say I'm bemused by the passion in this debate.

 

From where I stand it seems pretty simple.

  • Pluto has some fundamentally different characteristics from the other 8 'planets';
  • It shares those characteristics with a large number of objects which have never been considered planets;
  • If you think planet should have a definition, and the definition should be sensible, then you have to draw the line somewhere;
  • It doesn't make sense to include Pluto but not all those other objects - you should exclude Pluto or include all the others;
  • The '8' really deserve a category, base on their shared characteristics, so it's out for Pluto.

But Pluto is still there, and it's still the same thing.  The IAU can call it a planet, or a minor planet, or a dwarf plant, or a Kuiper Belt Object, or whatever - Pluto is exactly the same.

 

Personally, I think the IAU decision is sensible.  But if they changed it tomorrow to make Pluto a planet, and nothing else, I really wouldn't care.

 

Why all the outrage?


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#9 llanitedave

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Posted 26 August 2019 - 11:06 PM

I must say I'm bemused by the passion in this debate.

 

From where I stand it seems pretty simple.

  • Pluto has some fundamentally different characteristics from the other 8 'planets';
  • It shares those characteristics with a large number of objects which have never been considered planets;
  • If you think planet should have a definition, and the definition should be sensible, then you have to draw the line somewhere;
  • It doesn't make sense to include Pluto but not all those other objects - you should exclude Pluto or include all the others;
  • The '8' really deserve a category, base on their shared characteristics, so it's out for Pluto.

But Pluto is still there, and it's still the same thing.  The IAU can call it a planet, or a minor planet, or a dwarf plant, or a Kuiper Belt Object, or whatever - Pluto is exactly the same.

 

Personally, I think the IAU decision is sensible.  But if they changed it tomorrow to make Pluto a planet, and nothing else, I really wouldn't care.

 

Why all the outrage?

The "fundamental differences" between Pluto and any other planet are really no greater and no more fundamental than the differences between Earth and Jupiter, among others.  And while I agree that the line should be drawn somewhere, that somewhere should be defined by properties intrinsic to the object being defined, not extrinsic to it.  Basing the definition on where the planet happens to reside is the equivalent of basing the definition of "human being" upon whether or not the individual in question lives in Europe or Africa.

 

Why the outrage?  I can't speak for anyone else, but mine has nothing to do with Pluto per se.  When I first learned that the IAU was considering a formal definition of "planet", I was overjoyed.  I believed it was to address the plethora of new exoplanet discoveries being made, and to give some structure to the question of when something can be considered a star, a planet, or a brown dwarf.  I had no idea it was merely a backlash against Pluto -- I didn't even know that solar system bodies were an issue.  And as it turns out, the questions that they should have been addressing were completely ignored.

 

So MY outrage is due to feeling betrayed by a scientific body making a purely political and deeply unscientific decision.  It would be akin to the IPCC suddenly ruling out of the blue that CO2 is not a greenhouse gas after all, after some major donations from the fossil fuel industry.


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#10 bobzeq25

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Posted 26 August 2019 - 11:43 PM

The "fundamental differences" between Pluto and any other planet are really no greater and no more fundamental than the differences between Earth and Jupiter, among others.  And while I agree that the line should be drawn somewhere, that somewhere should be defined by properties intrinsic to the object being defined, not extrinsic to it.  Basing the definition on where the planet happens to reside is the equivalent of basing the definition of "human being" upon whether or not the individual in question lives in Europe or Africa.

 

Why the outrage?  I can't speak for anyone else, but mine has nothing to do with Pluto per se.  When I first learned that the IAU was considering a formal definition of "planet", I was overjoyed.  I believed it was to address the plethora of new exoplanet discoveries being made, and to give some structure to the question of when something can be considered a star, a planet, or a brown dwarf.  I had no idea it was merely a backlash against Pluto -- I didn't even know that solar system bodies were an issue.  And as it turns out, the questions that they should have been addressing were completely ignored.

 

So MY outrage is due to feeling betrayed by a scientific body making a purely political and deeply unscientific decision.  It would be akin to the IPCC suddenly ruling out of the blue that CO2 is not a greenhouse gas after all, after some major donations from the fossil fuel industry.

You do realize that a large number of scientists disagree with you?  Whatever the words, I think the substance of the decision was scientific.  Ceres was a planet until it was discovered that it was just one member of the class of asteroids.  Now the data shows that Pluto is just one member of the class of Kuiper Belt Objects.  There is a clear distinction between the 8 planets and asteroids or Kuiper Belt Objects.

 

I see no political gain had here by anyone.  One can make a better case for political loss.

 

Mike Brown has written a wonderful book.  The title is apt.  "How I killed Pluto (and why it deserved it)."



#11 DaveC2042

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Posted 27 August 2019 - 12:15 AM

The "fundamental differences" between Pluto and any other planet are really no greater and no more fundamental than the differences between Earth and Jupiter, among others.  And while I agree that the line should be drawn somewhere, that somewhere should be defined by properties intrinsic to the object being defined, not extrinsic to it.  Basing the definition on where the planet happens to reside is the equivalent of basing the definition of "human being" upon whether or not the individual in question lives in Europe or Africa.

 

Why the outrage?  I can't speak for anyone else, but mine has nothing to do with Pluto per se.  When I first learned that the IAU was considering a formal definition of "planet", I was overjoyed.  I believed it was to address the plethora of new exoplanet discoveries being made, and to give some structure to the question of when something can be considered a star, a planet, or a brown dwarf.  I had no idea it was merely a backlash against Pluto -- I didn't even know that solar system bodies were an issue.  And as it turns out, the questions that they should have been addressing were completely ignored.

 

So MY outrage is due to feeling betrayed by a scientific body making a purely political and deeply unscientific decision.  It would be akin to the IPCC suddenly ruling out of the blue that CO2 is not a greenhouse gas after all, after some major donations from the fossil fuel industry.

I have to say this doesn't clear it up for me.

 

I don't see where you get the idea the decision was 'political', or a 'backlash against Pluto'.  Their reasoning isn't completely bulletproof, but it seems fairly straightforward and a reasonable take on the facts.

 

On the other side, I don't understand why their creating a definition would initially cause joy.

 

And I don't see the IPCC analogy - that would have vast real-world economic and political consequences, and would be unarguably scientifically wrong.

 

All I see is some scientists making a definition for administrative reasons.

 

I'm not trying to be rude - we seem to agree on a whole bunch of things.  I'm just genuinely stumped as to why it all matters so much.



#12 Mister T

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Posted 27 August 2019 - 05:50 AM

I must say I'm bemused by the passion in this debate.

 

From where I stand it seems pretty simple.

  • Pluto has some fundamentally different characteristics from the other 8 'planets';
  • It shares those characteristics with a large number of objects which have never been considered planets;
  • If you think planet should have a definition, and the definition should be sensible, then you have to draw the line somewhere;
  • It doesn't make sense to include Pluto but not all those other objects - you should exclude Pluto or include all the others;
  • The '8' really deserve a category, base on their shared characteristics, so it's out for Pluto.

But Pluto is still there, and it's still the same thing.  The IAU can call it a planet, or a minor planet, or a dwarf plant, or a Kuiper Belt Object, or whatever - Pluto is exactly the same.

 

Personally, I think the IAU decision is sensible.  But if they changed it tomorrow to make Pluto a planet, and nothing else, I really wouldn't care.

 

Why all the outrage?

Because the "internet" needs the attention.

you can't just have a rational discussion about a disagreement over semantics.

you have to act like the future of mankind is at stake.

Saving humanity from Mom's basement....



#13 bobzeq25

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Posted 27 August 2019 - 10:04 AM

The short number of posts in this thread is meaningful.  Also the small number of views.  Most have moved on.  A notable exception is the admirable guy who headed up the New Horizons mission.   He's spent most of his professional career on Pluto, and nobly struggled to get the mission to Pluto approved, for years.  Then spent 10 years waiting (and working) while New Horizons flew to Pluto.  It's pretty understandable.

 

I enjoyed his book in general.  The multi-page diatribe about the IAU decison - not so much.

 

https://www.amazon.c...n/dp/1250098963


Edited by bobzeq25, 27 August 2019 - 10:11 AM.


#14 russell23

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Posted 27 August 2019 - 10:23 PM

https://www.sciencea...uto-is-a-planet



#15 llanitedave

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Posted 28 August 2019 - 02:40 AM

You do realize that a large number of scientists disagree with you?

Of course a large number do. Otherwise the vote would never have been taken. But, a large number also agree with me. To quote from the link posted by Russell23, (and the information is available from many other sources):

It's not just that only 424 of around 9,000 IAU members voted on the resolution, nor that hundreds of planetary scientists immediately petitioned against it.

The decision was made by a small minority of astronomers, very few of whom were actually planetary scientists, very few of whom were actually involved in the structure and geology of planets in general.

 

Whatever the words, I think the substance of the decision was scientific.  Ceres was a planet until it was discovered that it was just one member of the class of asteroids.  Now the data shows that Pluto is just one member of the class of Kuiper Belt Objects.

And any other planet that you can point to is a member of a class of vast numbers of similar objects distributed across the Universe. Saying something is a member of a class is not an argument for excluding them from a more comprehensive class.

 

There is a clear distinction between the 8 planets and asteroids or Kuiper Belt Objects.

There's a clear distinction between Earth and Jupiter. In many ways Ceres is more similar to Earth or Mars than those two planets are to Jupiter. That's if you're talking about intrinsic characteristics. When you accept the dynamicist argument, you're talking EX-trinsic characteristics, which can change arbitrarily. They should not be part of the basic definition, but subcategories.

 

I see no political gain had here by anyone.  One can make a better case for political loss.

I do agree with you here. The uproar has led to no useful outcomes.

 

Mike Brown has written a wonderful book.  The title is apt.  "How I killed Pluto (and why it deserved it)."

I'm glad you enjoyed it. Brown has his arguments, and so does Alan Stern. I can easily concede that neither is completely objective, but the fact that Brown is in the middle of promoting his own "Planet 9" does make me step back a bit.
 
 

I have to say this doesn't clear it up for me.
 
I don't see where you get the idea the decision was 'political', or a 'backlash against Pluto'.  Their reasoning isn't completely bulletproof, but it seems fairly straightforward and a reasonable take on the facts.
 
On the other side, I don't understand why their creating a definition would initially cause joy.
 
And I don't see the IPCC analogy - that would have vast real-world economic and political consequences, and would be unarguably scientifically wrong.
 
All I see is some scientists making a definition for administrative reasons.
 
I'm not trying to be rude - we seem to agree on a whole bunch of things.  I'm just genuinely stumped as to why it all matters so much.

Politics has been part of the status of Pluto from the beginning.  In fact, if you really wanted to dispute my own position, you could argue that part of the original designation of Pluto as a planet was due to promotion from Lowell observatory.  And you'd be right.  And there had always been some international tension about Pluto being an "American" planet.

 

A good definition would create joy for me personally, because I think that to organize our investigations of the natural world, we require an intellectually useful taxonomy.  Taxonomy is as important in astronomical objects as it is in biological ones.  We have a decent taxonomy of stars, but not of planets.  In 2006, I believed that's what the IAU was attempting to create.  I was disappointed.


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#16 DaveC2042

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Posted 28 August 2019 - 04:08 AM

Of course a large number do. Otherwise the vote would never have been taken. But, a large number also agree with me. To quote from the link posted by Russell23, (and the information is available from many other sources):
The decision was made by a small minority of astronomers, very few of whom were actually planetary scientists, very few of whom were actually involved in the structure and geology of planets in general.


And any other planet that you can point to is a member of a class of vast numbers of similar objects distributed across the Universe. Saying something is a member of a class is not an argument for excluding them from a more comprehensive class.


There's a clear distinction between Earth and Jupiter. In many ways Ceres is more similar to Earth or Mars than those two planets are to Jupiter. That's if you're talking about intrinsic characteristics. When you accept the dynamicist argument, you're talking EX-trinsic characteristics, which can change arbitrarily. They should not be part of the basic definition, but subcategories.


I do agree with you here. The uproar has led to no useful outcomes.


I'm glad you enjoyed it. Brown has his arguments, and so does Alan Stern. I can easily concede that neither is completely objective, but the fact that Brown is in the middle of promoting his own "Planet 9" does make me step back a bit.


Politics has been part of the status of Pluto from the beginning. In fact, if you really wanted to dispute my own position, you could argue that part of the original designation of Pluto as a planet was due to promotion from Lowell observatory. And you'd be right. And there had always been some international tension about Pluto being an "American" planet.

A good definition would create joy for me personally, because I think that to organize our investigations of the natural world, we require an intellectually useful taxonomy. Taxonomy is as important in astronomical objects as it is in biological ones. We have a decent taxonomy of stars, but not of planets. In 2006, I believed that's what the IAU was attempting to create. I was disappointed.


Fair enough. I agree taxonomy matters by the way - it's just not high on the list of things I enjoy.

#17 russell23

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Posted 28 August 2019 - 09:04 AM

A great example of why taxonomy matters can be illustrated with this example:  Are objects orbiting brown dwarfs "moons" or "planets"?

 

The answer to this question is much more complicated and controversial than it might seem, but it is very much related to the need to have useful definitions for sub-stellar bodies. 

 

The official IAU definition for a brown dwarf is a sub-stellar body that exceeds the deuterium burning (DB) limit no matter the mechanism of formation.  Well that seems ok ... right?  Sure - definitions are not "right" or "wrong" they are "useful" to varying degrees.  The DB limit is convenient because it is only based upon mass, but it is not useful to answer the question of what we should call objects orbiting brown dwarfs.

 

The concept of Brown dwarfs was proposed in the 1960's by Kumar, who determined that the star formation process (gas collapse) could produce bodies too small to begin core hydrogen fusion.  He referred to these objects as "black dwarfs" and later on they became "brown dwarfs". 

 

When I was growing up the the 1980's Jupiter was sometimes referred to as a "failed star".   However, this is not the case.  Jupiter did not form by a stellar gas collapse process.   Instead it formed within a protoplanetary disk.  The preferred theory today is the "core accretion" theory in which Jupiter began by building a rock/ice core that upon reaching a mass somewhere between 5-10 Earth masses began to accrete its massive hydrogen envelope.   There are other proposed mechanisms and stages of the mechanism such as disk instability and pebble accretion.  The important thing is that a body formed in a protoplanetary disk will have heavy element (elements more massive than Helium) enrichment relative to the parent star.  And such is the case with Jupiter and Saturn.  Both have heavy element mass percentages well above the Sun's.

 

The IAU's adoption of 13 Jupiter masses to distinguish brown dwarfs from giant planets is problematic for a number of reasons:

 

1.  Observations have shown that gas collapse mechanisms can form objects as small as ~4 Jupiter masses.

2.  It is possible for core accretion or disk instability to form objects in a protoplanetary disk that exceed the 13 Jupiter masses

3.  Plots of Mass-radius relationships (used to characterize exoplanet characteristics) show no feature that would indicate there is any significance to deuterium burning.

4.  Deuterium burning is a short lived stage that has negligible impact on object formation and evolution.

5.  As noted above, objects that form by gas collapse do not have the heavy metal enrichment and core that objects formed in a protoplanetary disk have.  Gas collapse products have a hydrogen core.

 

Items 1&2 result in an "overlapping mass regime" for brown dwarfs and giant planets.  What this means is that there is no mass boundary (including DB) that provides a way to distinguish a giant planet from a brown dwarf.  Between 4 and at least 10 to possibly 42 Jupiter masses the body may have formed by gas collapse or it may have formed in a protoplanetary disk. 

 

Items 3&4 illustrate that DB is not a meaningful aspect of giant planets and brown dwarfs - so it does not make a useful criteria that creates more understanding in our classification system.

 

Item 5 is important because it illustrates that a brown dwarf and a giant planet - even if the same mass - do not have the same structure, history, and composition.

 

So what is the solution?  If we are trying to have definitions that are meaningful then this should be the definition for a brown dwarf:

 

a gaseous, sub-stellar mass body, formed by star-like gas collapse mechanisms, with insufficient mass to enable core hydrogen fusion.

 

The mass range for brown dwarfs would then be 4 to ~60 Jupiter masses.  But some objects in the lower end of that mass range would be giant planets. 

 

And the original question:  Are objects orbiting brown dwarfs moons or planets?   The answer is that since brown dwarfs form like stars the planetary mass bodies orbiting brown dwarfs are planets ... or dwarf planets if they do not dominate their orbit.  

 

As can be seen - to get to that answer requires a lot of consideration of science observation and theory.   And so it is with the definition of "planet".  The IAU did have a good reason to look at the problem, but the solution they came to was not as useful as it needs to be.


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#18 llanitedave

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Posted 28 August 2019 - 09:17 AM

Fair enough. I agree taxonomy matters by the way - it's just not high on the list of things I enjoy.

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#19 corax

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Posted 29 August 2019 - 12:15 AM

Taxonomy is important, but it's much more about how people organize things to think about and discuss than it is about how things actually occur in nature. Nature does what it does, with no regard for the tidy little labeled boxes we create for it. This is true even in biology: while animals obey the classical taxonomic rules fairly well, plants do much less so, and microorganisms are pretty much out the window. We tend to come up with rules that apply fairly well to the bulk of examples we deal with day to day, and then don't worry too much about edge cases for which the rules apply less well. When we have to deal with them at all, we make it clear in the context of the discussion that we are applying a special set of rules for the particular edge case at hand.

 

It's striking to me that so few planetary scientists weighed in on the Pluto question in the first place. Maybe they're used to dealing with the inherent ambiguities of their field, recognize Pluto as an edge case, and (apart from obvious partisans like Mike Brown or Alan Stern) just couldn't be bothered with it?

 

On a less serious note:

 

One of these things is not like the others
One of these things just doesn't belong...

 

Planet  Ecc.      Inc. (° to invariable plane)
Mer     0.205     6.34
Ven     0.007     2.19
Ear     0.017     1.57
Mar     0.093     1.67
Jup     0.049     0.32
Sat     0.057     0.93
Ura     0.046     1.02
Nep     0.009     0.72
Plu     0.249     15.6

 

I know that eccentricity and inclination weren't even part of the IAU definition, but Pluto's have annoyed me since I first saw models of the solar system at age seven. Good riddance! (And watch your step, Mercury, you're looking pretty shaky too.)

 



#20 DaveC2042

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Posted 29 August 2019 - 01:47 AM

Taxonomy is important, but it's much more about how people organize things to think about and discuss than it is about how things actually occur in nature. Nature does what it does, with no regard for the tidy little labeled boxes we create for it. This is true even in biology: while animals obey the classical taxonomic rules fairly well, plants do much less so, and microorganisms are pretty much out the window. We tend to come up with rules that apply fairly well to the bulk of examples we deal with day to day, and then don't worry too much about edge cases for which the rules apply less well. When we have to deal with them at all, we make it clear in the context of the discussion that we are applying a special set of rules for the particular edge case at hand.

 

It's striking to me that so few planetary scientists weighed in on the Pluto question in the first place. Maybe they're used to dealing with the inherent ambiguities of their field, recognize Pluto as an edge case, and (apart from obvious partisans like Mike Brown or Alan Stern) just couldn't be bothered with it?

 

On a less serious note:

 

One of these things is not like the others
One of these things just doesn't belong...

 

Planet  Ecc.      Inc. (° to invariable plane)
Mer     0.205     6.34
Ven     0.007     2.19
Ear     0.017     1.57
Mar     0.093     1.67
Jup     0.049     0.32
Sat     0.057     0.93
Ura     0.046     1.02
Nep     0.009     0.72
Plu     0.249     15.6

 

I know that eccentricity and inclination weren't even part of the IAU definition, but Pluto's have annoyed me since I first saw models of the solar system at age seven. Good riddance! (And watch your step, Mercury, you're looking pretty shaky too.)

The big thing about biology is that there is a unique 'correct' taxonomy, being the evolutionary 'tree diagram' of all life.  While we may disagree about specifics such as where the branch points are in the past, and whether two populations do or don't constitute distinct species, these are really just difficulties of interpreting evidence and terminology.  The correct tree does actually exist, independently of whether we know it, and regardless of our difficulty defining a species.

 

By comparison, most (all?) other taxonomies are not fundamentally 'correct', but really just useful ways of ordering things for a given purpose.

 

So for example, humans are apes, bats are not birds, and 'reptile' (as commonly understood) is not a meaningful category, and these represent unambiguous facts.

 

Sure, there can be sensible and non-sensible definitions of 'planet, but I can't see how there can be a single 'correct' definition that doesn't leave a grey area or fail to work in some context.



#21 russell23

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Posted 29 August 2019 - 08:24 AM

You do realize that a large number of scientists disagree with you?  Whatever the words, I think the substance of the decision was scientific.  Ceres was a planet until it was discovered that it was just one member of the class of asteroids.  Now the data shows that Pluto is just one member of the class of Kuiper Belt Objects.  There is a clear distinction between the 8 planets and asteroids or Kuiper Belt Objects.

 

I see no political gain had here by anyone.  One can make a better case for political loss.

 

Mike Brown has written a wonderful book.  The title is apt.  "How I killed Pluto (and why it deserved it)."

Not exactly.  Reality is more interesting and instructive:

 

https://arxiv.org/abs/1805.04115



#22 llanitedave

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Posted 29 August 2019 - 08:56 AM

Taxonomy is important, but it's much more about how people organize things to think about and discuss than it is about how things actually occur in nature. Nature does what it does, with no regard for the tidy little labeled boxes we create for it. This is true even in biology: while animals obey the classical taxonomic rules fairly well, plants do much less so, and microorganisms are pretty much out the window. We tend to come up with rules that apply fairly well to the bulk of examples we deal with day to day, and then don't worry too much about edge cases for which the rules apply less well. When we have to deal with them at all, we make it clear in the context of the discussion that we are applying a special set of rules for the particular edge case at hand.

 

It's striking to me that so few planetary scientists weighed in on the Pluto question in the first place. Maybe they're used to dealing with the inherent ambiguities of their field, recognize Pluto as an edge case, and (apart from obvious partisans like Mike Brown or Alan Stern) just couldn't be bothered with it?

 

On a less serious note:

 

One of these things is not like the others
One of these things just doesn't belong...

 

Planet  Ecc.      Inc. (° to invariable plane)
Mer     0.205     6.34
Ven     0.007     2.19
Ear     0.017     1.57
Mar     0.093     1.67
Jup     0.049     0.32
Sat     0.057     0.93
Ura     0.046     1.02
Nep     0.009     0.72
Plu     0.249     15.6

 

I know that eccentricity and inclination weren't even part of the IAU definition, but Pluto's have annoyed me since I first saw models of the solar system at age seven. Good riddance! (And watch your step, Mercury, you're looking pretty shaky too.)

There are Jupiter-sized bodies in other systems with orbits that look like comets compared to Pluto.  Are they not planets on that account?



#23 llanitedave

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Posted 29 August 2019 - 09:07 AM

The big thing about biology is that there is a unique 'correct' taxonomy, being the evolutionary 'tree diagram' of all life.  While we may disagree about specifics such as where the branch points are in the past, and whether two populations do or don't constitute distinct species, these are really just difficulties of interpreting evidence and terminology.  The correct tree does actually exist, independently of whether we know it, and regardless of our difficulty defining a species.

 

By comparison, most (all?) other taxonomies are not fundamentally 'correct', but really just useful ways of ordering things for a given purpose.

 

So for example, humans are apes, bats are not birds, and 'reptile' (as commonly understood) is not a meaningful category, and these represent unambiguous facts.

 

Sure, there can be sensible and non-sensible definitions of 'planet, but I can't see how there can be a single 'correct' definition that doesn't leave a grey area or fail to work in some context.

If I find a fossil skull and identify it as Homo antecessor and you respond that no, it is Homo erectus, which one of us is correct?  This is a gray area partly due to the traditions of our taxonomy, partly due to ignorance as to where these populations actually relate to the evolutionary tree, and partly because they may simply be minor variations of a single species, and not really different at all.

 

Even the "tree of life" can also be compared to a "braided stream."  Lineages don't just diverge, they sometimes merge.  Recall the Neanderthal and Denisovan DNA that has found its way into some (but not all) of the modern human population.  So there are gray areas even there.

 

Any taxonomy that we can construct will have ambiguities and edge cases.  Pluto is a valid edge case in some respects, not others.  But this is why for me, the IAU ruling was not about Pluto, or any other individual body.  It's a bad taxonomic system, pure and simple.  I think the reason it is a bad taxonomic system because the main motivation for the decision was a knee-jerk desire to demote Pluto.  We can do better.


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#24 bobzeq25

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Posted 29 August 2019 - 11:04 AM

Not exactly.  Reality is more interesting and instructive:

 

https://arxiv.org/abs/1805.04115

Alan Stern and his buddy's reality.  

 

I'm not technically opposed to hundreds of planets (which is latest tack Stern, etc. are taking, having recognized that nine is _never_ again going to fly).  Just opposed to nine, which makes the least technical sense of any alternative.

 

Hundreds is a concept that indeed might be best arbitrated by the IAU (the traditional arbiter of these disputes), and a vote.  There are implications far beyond planetary physics.

 

So let's have another vote in the IAU.  A two year process.  Proposals for 8, 9 (I'd love to see Alan Stern write that one, and try to justify it as "planetary science"), and many are introduced at one IAU meeting.  Discussed for a year by whoever wants to.  Have symposiums.  Then the next year there's a vote at the beginning of the IAU meeting.  I don't think there'll be any problem gathering a large group for the vote.  I'd take a large bet on the outcome.  And would bet the house that it won't be 9.

 

Minor point.  Personally, I don't think the IAU was set on specifically demoting Pluto.  I think they were trying to deal with the difficult issue that distinguishing Pluto from other very similar Kuiper Belt Objects was not science (and it's not planetary science, either), no matter how many people were emotionally embedded.   They may not have done the best job of handling it, but to attribute that motivation (that they were out to get Pluto) to them is, I think, wrong.

 

In particular Mike Brown had a lot to gain by keeping these larger Kuiper Belt Objects as planets.  He'd have been the greatest discoverer of planets there ever was, or ever would be.  No doubt would have made a _lot_ more money, had a lot more fame.  His heirs could have made his house a public museum with a bronze plaque.  <grin>


Edited by bobzeq25, 29 August 2019 - 11:28 AM.


#25 DaveC2042

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Posted 29 August 2019 - 05:48 PM

If I find a fossil skull and identify it as Homo antecessor and you respond that no, it is Homo erectus, which one of us is correct? This is a gray area partly due to the traditions of our taxonomy, partly due to ignorance as to where these populations actually relate to the evolutionary tree, and partly because they may simply be minor variations of a single species, and not really different at all.

Even the "tree of life" can also be compared to a "braided stream." Lineages don't just diverge, they sometimes merge. Recall the Neanderthal and Denisovan DNA that has found its way into some (but not all) of the modern human population. So there are gray areas even there.

Any taxonomy that we can construct will have ambiguities and edge cases. Pluto is a valid edge case in some respects, not others. But this is why for me, the IAU ruling was not about Pluto, or any other individual body. It's a bad taxonomic system, pure and simple. I think the reason it is a bad taxonomic system because the main motivation for the decision was a knee-jerk desire to demote Pluto. We can do better.


The issues you raise about biology are definitely real, but I feel they are really shortcomings in our knowledge about past events, and in our attempts to impose species labels near branch points, where they don't belong.

To the second point, the tree structure doesn't just tell you the correct taxon for a lifeform. It also shows you where taxons merge, and insistence on discrete names doesn't work

So in the Neanderthal/Denisovan situation, the 'correct' answer is that you can't discretely name them at that point, as they are not yet discrete species.

Just because there is a correct taxonomy doesn't mean it's going to be easy to identify, understand or apply.

Ring species are a good example of this (good explanation in Wikipedia). Applying names to them coherently is impossible. But the problem isn't the reality of the evolutionary relationships, which are perfectly clear. The problem is a linguistic one where we insist on naming things as different species or the same species, while reality is very clearly telling us neither is the case.

I think this is fundamentally different from what is going on when we name astrophysical objects.


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