Author Topic: Pluto-Planet debate discussions  (Read 52036 times)

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #480 on: 02/25/2017 09:03 PM »
All these issues may be avoided by readopting the old word: planetoid.

You'd just get the same debate, except you'd be using the word 'planetoid' instead of the compound noun 'dwarf planet'.

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'Dwarf Planet' is a horrible term.

It's hard to think of anybody who does like the term; but that's committees for you!

Interestingly, at least my impression is that professional astronomers don't care much (though of course there are some that do).

Or they don't seem to care much - it's a lot less hassle to keep your head down rather than publicly take a stance one way or another! But, it does also depend on their area of professional interest. Those working on galactic clusters, the distribution of dark matter or super-massive black holes etc. tend to be on the uninterested wing, whereas those working on matters within the solar system tend to be more interested. Some of the really interested ones are those who have personal as well as professional interests at stake!

Offline Bynaus

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Re: Pluto-Planet debate discussions
« Reply #481 on: 02/26/2017 06:36 AM »
All these issues may be avoided by readopting the old word: planetoid.

Like a planet, NOT a planet, but something different, though similar.

Big moons are planetoids, as are some asteroids. Pluto is on the edge of being a planet, but is certainly a planetoid. 'Dwarf Planet' is a horrible term.

I agree with all that, except that Pluto is not on the edge on being a planet, far from it. If you look at things like Stern & Levison's planetary discriminant or Margot's criterium (a few pages back in this thread), it's just very clear that it is far from the edge.

Planetoid is a better term than dwarf planet because a dwarf planet is not a planet, which is confusing. A planetoid is just that: something that has the shape of a planet - and planets are spherical*.

*your mileage may vary

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #482 on: 02/26/2017 11:04 AM »
Interestingly, at least my impression is that professional astronomers don't care much (though of course there are some that do).

When you consider that astronomers traditionally divide matter in the universe into Hydrogen, Helium and "metals"; and assemblies of matter into stars, gas, and "dust"; you realise that the vast majority of them aren't going to have a lot of interest in a naming system for one arbitrary type of "dust". Especially given the over-reaction of people who should know better.

All these issues may be avoided by readopting the old word: planetoid.

The people who have created a fuss over the "demotion" of Pluto hate the word "planetoid" (and Plutoid) as well. (Mention it to Stern and he starts going on about haemorrhoids.)

Personally, I'd drop the term dwarf planet and not replace it with anything. It isn't necessary or useful to have a term for roundish objects that aren't planets, there's no natural divide between dwarfs and non-dwarfs. So formally they are all just "Small Solar System Bodies", informally "asteroids", usually grouped by their location (main-belt, KBO, SDO, etc). Do we care that Ceres is a "dwarf planet" but Vesta/Juno aren't? Does it matter whether the largest handful of TNOs are given a different name from the remaining more than a thousand TNOs so far discovered?

Offline as58

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Re: Pluto-Planet debate discussions
« Reply #483 on: 02/28/2017 09:46 AM »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #484 on: 03/06/2017 07:35 AM »
There was an editorial in Nature a few days ago: http://www.nature.com/news/pluto-could-be-staging-a-comeback-and-it-s-not-alone-1.21516

Editors lurrv the controversy; it attracts readers. :)

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Also a proposal for reclassification: http://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf

The principle problem I see with their proposed definition of 'planet' ("a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters") is how they propose to ascertain this for the bodies commonly referred to as exoplanets? How do you apply this definition outside the Solar System? I foresee resistance to this idea among astronomers who study exoplanets (of whom there are many, nowadays!).

The authors note that "110 planets is more than students should be expected to memorize" and recommend that "students should learn only a few (9? 12? 25?) planets of interest". (I wonder which 9 they had in mind! :) ) The precise number seems a bit arbitrary.

They also recognise that "Understanding the natural organization of the Solar System is much more informative than rote memorization. Teaching the zones of the Solar System from the Sun outward and the types of planets and small bodies in each is perhaps the best approach: The zone closest to the Sun consists of rocky planets; the middle zone consists of gaseous, rocky, and icy planets; and the third zone consists of icy planets." Here they implicitly recognise that there are important distinctions within and between these bodies and that requires terminology to distinguish them. There are a number of such possible classification schemes and ultimately it's an arbitrary choice, as is which group of such objects should have the terminology 'planet' attached to them.

You could attach the word 'planet' to the bodies as proposed and have separate terminology to describe the eight solar-orbiting, gravitationally dominant objects; or, you could attach the word 'planet' to the latter objects and have separate terminology to describe the former ('worlds' or 'planemos', perhaps?). Which choice to take has nothing to do with science and comes down to personal preference and interest.

The final sentence is "This definition highlights to the general public and policymakers the many fascinating worlds in our Solar System that remain unexplored and are worthy of our exploration, along with the necessary budgets." (My italics.) Now we're getting closer to the real, underlying reasons for this proposal! :)

Offline redliox

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Re: Pluto-Planet debate discussions
« Reply #485 on: 03/06/2017 08:09 AM »
We need to find out if there are any other bodies akin to Pluto and Eris in size.  Apart from those two (and Triton by Neptune if you wish to count), we haven't found anything else that crosses over 1,000 km in radius.  Neither Makemake or Haumea, the next largest Kuiper bodies, cross this line as do the majority of moons (such as Rhea, Enceladus, Titania) or likewise Ceres.  Given that hydrostatic equilibrium is dependent on the material (and thus may vary the definition) and 'orbital clearance' might not suffice (the dinosaurs would say Earth did a poor job), a somewhat arbitrary yet astute line like a 1,000 km radius might suffice if it segregates a notable difference. 
« Last Edit: 03/06/2017 08:10 AM by redliox »
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Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #486 on: 03/08/2017 08:09 AM »
and 'orbital clearance' might not suffice (the dinosaurs would say Earth did a poor job)

Only if they didn't understand the concept.

We need to find out if there are any other bodies akin to Pluto and Eris in size.  Apart from those two (and Triton by Neptune if you wish to count), we haven't found anything else that crosses over 1,000 km in radius

2007OR10 is around 1200-1500km depending on assumptions about it's albedo.

Sedna's diameter is given as 995 +/- 80km. Straddling your line. The gap between Planets and Non-planets under the gravitational dynamical-dominance concept is orders of magnitude. Not half of one percent.

You'll have to draw your arbitrary line at 2000km, since the whole point of these new definitional games is to get Pluto back as a planet, for some reason.

a somewhat arbitrary yet astute line like a 1,000 km radius might suffice if it segregates a notable difference.

For what purpose? What are you dividing? The purpose of dividing planets from non-planets is because they "clear the neighbourhood", they are the regionally dominant gravitational force affecting the formation and evolution of that star system. The line is naturally occurring and scientifically important. Dividing sub-planetary objects by size serves no useful purpose.

What's the point of dividing one group of arbitrary rocks from other only slightly smaller arbitrary rocks? As near as I can see, the next natural and scientific division is internal differentiation, which requires individual study of the object, so isn't suited as a broad classification system.

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #487 on: 03/17/2017 07:19 PM »
Scientists make the case to restore Pluto's planet status

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Runyon and his co-authors argue for a definition of "planet" that focuses on the intrinsic qualities of the body itself, rather than external factors such as its orbit or other objects around it. In a short paragraph, they define a planet as "a sub-stellar mass body that has never undergone nuclear fusion" and that has enough gravitational heft to maintain a roughly round shape, even if it bulges at the equator because of a three-way squeeze of forces created by its gravity and the influence of both the sun and a nearby larger planet.

This definition differs from the three-element IAU definition in that it makes no reference to the celestial body's surroundings. That portion—which required that a planet and its satellites move alone through their orbit—excluded Pluto. Otherwise, Pluto fit the IAU definition: it orbits the sun and it is massive enough that the forces of gravity have made it round.

Stern has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter, and Neptune, which share their orbits with asteroids.

The new geophysical definition omits stars, black holes, asteroids, and meteorites, but it includes everything else in our solar system. It would expand the number of planets from eight to approximately 110.

That expansion is part of the appeal of the new definition, says Runyon. He says he would like to see the public more engaged in solar system exploration. As the very word "planet" seems to carry a "psychological weight," he figures that more planets could encourage that public interest.

The new definition, which does not require approval from a central governing body, is also more useful to planetary scientists. Most of them are closely affiliated with geology and other geosciences, thus making the new geophysical definition more useful than the IAU's astronomical definition.

http://hub.jhu.edu/2017/03/16/make-pluto-a-planet-again/

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #488 on: 03/18/2017 01:31 AM »
Stern has argued in the past that the IAU definition also excludes Earth, Mars, Jupiter, and Neptune, which share their orbits with asteroids.

In spite of knowing that such arguments are factually wrong and deeply misrepresent the IAU definition.

Stern himself found a clear mathematical distinction between the eight planets and all other objects in the solar system, based on their ability to gravitationally dominate their neighbourhood. He and the IAU simply chose different names for objects that do and those that don't. That's it. That's the only difference with his own findings. He didn't get his way on the name.

The new definition [...] is also more useful to planetary scientists.

Not really. Stern's definition, proffered by student Runyon, has no scientific utility. It is not useful for the study of exo-planets, nor of the formation of the Solar Systems (or other planetary systems). Additionally the line between objects in hydrostatic equilibrium and not is trivial, changeable, and hard to determine; and not of value when it comes to broad classification of objects. There's no reason to lump Ceres in with large KBOs like Eris, Pluto, Makemake, etc. Nor in separating Vesta from Ceres, but grouping it with the thousand smaller KBOs discovered so far. It's not a useful distinction.

OTOH, the "clearing the neighbourhood" measure of gravitational dominance represents a real physical phenomena that many scientists (including Stern, judging by his earlier paper) believe plays a role in the formation of planetary systems. It is a clear unambiguous division, and can already be measured for a large number exo-planets (something that hydrostatic equilibrium cannot), allowing statistical studies to feed into and test the theories of formation of planetary systems.

As the very word "planet" seems to carry a "psychological weight"

Which Stern has done everything in his power to emphasise and encourage.

Which seems to do a disservice to those geoscientists who study objects in the Solar System below Stern's preferred cut-off. After all, no-one who voted for the IAU re-definition was saying that Pluto and the newly discovered KBOs are unworthy of study. Just as Ceres/Vesta/etc weren't unworthy of study because they also haven't been considered "Planets". Nor Titan, nor Europa, nor any other interesting object in the Solar System that doesn't meet the definition of "planet".

Whereas, by putting such enormous emphasis on the "psychological weight" of the word planet, and relating it to public support for research, Stern seems to dismiss and diminish the value of those who study smaller objects. He considered his own work to be diminished by being grouped with "them".

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #489 on: 03/19/2017 02:33 PM »
A few responses to thoughts shared in recent posts:

1.  Orbital clearing -  The concept is pretty easy to understand if it is well explained.   I think Soter (2006) did a good job of exploring the topic.   The problem is that the IAU definition does not specify what is meant by "clearing".  Because of that lack of specificity it is very easy for people to misinterpret the meaning of "clearing".   A simplified way to explain it to people - the way I usually explain it to those that don't understand -  is that the sum of the masses of all bodies sharing a planet's orbital zone is much less than the mass of the planet itself.   For dwarf planets the sum of of the masses of all bodies sharing the orbital zone exceeds the mass of the dwarf planet.   Thus a dwarf planet resides in a "belt" containing numerous generally similar and mostly smaller bodies.   

Before I share any further comments let me say that I understand that many of you on this forum like the IAU definition so in this discussion the comments I am about to share likely will not be viewed favorably.  But hear me out because I think the IAU definition has very important aspects to it.  I've always felt that it is possible to structure a taxonomy that satisfies the interests of both the dynamical perspective and the geophysical perspective because both perspectives are important.

Also - for the record - I don't have a specific desire to see Pluto specifically restored to its status as a planet.  When the IAU originally made its ruling I was on board.  However, my views have evolved and now I think a good definition of "planet" can include Pluto ... but Pluto being restored as a planet is not the specific goal.  It is a side consequence of a system that would "in theory" acknowledge the concerns of both dynamicists and geophysicists.


2.  Exoplanets.   It has been suggested that the geophysical definition of a planet proposed by Stern's group has no application to exoplanets.  This is not accurate.   The derived masses of the exoplanets and physics tell us that the exoplanet's discovered to date will be spherical bodies - and therefore meet the geophysical definition of "planet".   Equation of state studies for planetary materials align with observations of small bodies within the Solar System and indicate that for icy bodies a minimum radius around 200 km (or a mass of ~3 x 1019 kg is needed for an ice-rich body to assume a nearly spherical hydrostatic equilibrium shape. 

3.  Usefulness of definitions.   This is a matter of opinion on all sides.  The dynamicists think the IAU definition is useful.  The planetary geophysicists do not.   The IAU definition is a dynamical definition with a geophysical element (spherical shape).  Stern's geophysical definition does nothing for dynamicists.   And therein lies the problem.  The IAU definition will never be satisfactory to planetary geophysicists.   And the IAU definition is going to become inadequate as the scope of exoplanet dynamics of continues to expand.  Eventually dynamicists are going to want to create different dynamical classes to account for the variety of dynamical circumstances seen in exoplanets.   

So my proposed solution is to adopt a geophysical definition similar to what Stern's group has proposed as a definition for "planet".  And then define dynamical classes of planets, composition classes of planets, a mass scale.

Yes - in this definition all IAU planets and dwarf planets are "planets" as are all spherical moons, exoplanets, rogue planets. 

Dynamical classes address the concerns of dynamicists in the IAU definition:

Principal planet: The planets that have cleared their orbit and are dynamically dominant.  This is the 8 IAU planets.

Belt planet: Planets that have not cleared their orbit.  This is the IAU dwarf planets.

Moon:  A planet orbiting a larger planet.  This includes all spherical moons.

Rogue planets: Planets not orbiting any star or brown dwarf.

Satellites: sub-planetary mass bodies in orbit around a Principal planet, Belt planet, or Moon. 

Small sub-planetary bodies: asteroids, comets, KBO's ... ect. that fall below the minimum planet mass.


These dynamical classes address the needs of dynamicists.  Planets that have cleared their orbit are the "Principal Planets".    Planets that have not cleared their orbit are "Belt planets".   Note that as data on exoplanet and Trans-Neptunian orbits evolves dynamicists can develop sub-categories for the dynamical classes - perhaps involving orbital resonance, different orbital classes within the trans-Neptunain region and so on. 

The distinction between "Moons" and "satellites" is a simple enough one to understand.  Currently the two terms "moon" and "satellite" are used interchangeably.  Just like dynamicists recognize a distinction between the "planets" and "dwarf planets", geophysicists see an important physical distinction between spherical moons and non-spherical moons.   Yet all of these bodies from the 1 km debris through large spherical bodies such as Ganymede are lumped into the single category sometime referred to as "moons" and other times referred to as "satellites".   So my proposal is to use the term "moon" for spherical objects orbiting a planet and use the term "satellite" for smaller bodies orbiting a planet.  So then "moon" would simply mean - "planet orbiting a larger planet".   The advantage of this is that while we now place these bodies under the umbrella of the "planet" definition, we would still call them "moons" so no confusion would be created among laypersons.   

Composition classes of planets can also be defined.  While the Solar System has generated the terms "Terrestrial", "Gas Giant" and "Ice Giant" these terms are inadequate to the growing understanding of exoplanets.  The classes "Rock", "Ice", and "Gas" make a more useful starting point.  I have a more detail on that part of my proposal that I won't bother sharing right now. 

The Kepler 11 exoplanet system is a great illustration of how our current nomenclature based upon the Solar System is not up to the task.   Dynamically this system has 5 planets within 0.25 AU of the star and 6 planets within 0.5 AU.  These planets have masses ranging from ~2 Earth masses to ~17 Earth masses.  These planets may in some cases be borderline resonant with others but the current data is not clear on that.  Dynamically, are these bodies actually "planets" or are they "dwarf planets" or do we need a new dynamical class? 

Composition of most of these planets is thought to be ~95+% rock with the remaining mass in an H/He envelope that is <5% of the mass, but at least 35% of the planetary radius.  There are no analog's for these planets in are Solar System.  They are not gas giants.  They are not terrestrial. 

But the system I'm proposing can handle all of this.   It takes some consideration from a different perspective.  But it cleans up the issues that dynamicists and geophysicists are wrestling with while serving as a foundation that can adapt to future developments.


« Last Edit: 03/19/2017 02:40 PM by DRussell »

Offline Star One

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Pluto-Planet debate discussions
« Reply #490 on: 03/19/2017 02:59 PM »
I can't help feeling all of this is not exactly helping Stern win friends and influence people. In that I haven't seen it win anyone round to his way of thinking that wasn't already on his side so to speak.
« Last Edit: 03/19/2017 03:00 PM by Star One »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #491 on: 03/21/2017 04:18 AM »
1.  Orbital clearing -  The concept is pretty easy to understand if it is well explained.   I think Soter (2006) did a good job of exploring the topic.   The problem is that the IAU definition does not specify what is meant by "clearing".  Because of that lack of specificity it is very easy for people to misinterpret the meaning of "clearing".   A simplified way to explain it to people - the way I usually explain it to those that don't understand -  is that the sum of the masses of all bodies sharing a planet's orbital zone is much less than the mass of the planet itself.   For dwarf planets the sum of of the masses of all bodies sharing the orbital zone exceeds the mass of the dwarf planet.   Thus a dwarf planet resides in a "belt" containing numerous generally similar and mostly smaller bodies.

Soter defined a planet as an object whose mass was more than 100 times that of all other objects in its orbital zone. There are a couple of problems with this. First, '100 times' is arbitrary. Second, it's practically impossible to measure the mass of all other objects in the case of exoplanet candidates.

Most astronomers who agree with the results of the IAU definition - though not necessarily its terminology! - prefer to use the concept of gravitational dominance. One advantage of this is that you only need to consider whether an object will clear its neighborhood during some specified span of time (usually the main-sequence lifetime of the host star), not whether it has actually yet done so. (Ironically, 'clearing the neighborhood comes from Alan Stern!)

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I've always felt that it is possible to structure a taxonomy that satisfies the interests of both the dynamical perspective and the geophysical perspective because both perspectives are important.

I think supporters of the IAU definition would not only agree that both dynamical and geophysical perspectives are important but that the IAU definition itself satisfies both interests.

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2.  Exoplanets.   It has been suggested that the geophysical definition of a planet proposed by Stern's group has no application to exoplanets.  This is not accurate.   The derived masses of the exoplanets and physics tell us that the exoplanet's discovered to date will be spherical bodies - and therefore meet the geophysical definition of "planet".   Equation of state studies for planetary materials align with observations of small bodies within the Solar System and indicate that for icy bodies a minimum radius around 200 km (or a mass of ~3 x 1019 kg is needed for an ice-rich body to assume a nearly spherical hydrostatic equilibrium shape.

Similarly, Margot proposes that the requirement for a planet to be in hydrostatic equilibrium be dropped, because an object that is gravitationally dominant is almost certain to be so. But, importantly, his resulting definition relies solely on measurable quantities, not on an inference from such quantities. He leaves open the possibility that an object may be discovered that is gravitationally dominant but not in hydrostatic equilibrium. The reason he proposed dropping it is that it is practically impossible to measure for exoplanet candidates. Even in the Solar system, the IAU requires hydrostatic equilbrium be measured, not inferred from size and constituency.

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3.  Usefulness of definitions.   This is a matter of opinion on all sides.  The dynamicists think the IAU definition is useful.  The planetary geophysicists do not.   The IAU definition is a dynamical definition with a geophysical element (spherical shape).  Stern's geophysical definition does nothing for dynamicists.   And therein lies the problem.  The IAU definition will never be satisfactory to planetary geophysicists.

Perhaps not, but it has nothing to do with the 'usefulness' of the IAU definition. I haven't seen any scientific difficulties caused by the IAU definition; anything that inhibits the development of their science. After all, there are words that collectively describe planets and dwarf planets - planemos or worlds, for instance - and they can always make up their own.

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And the IAU definition is going to become inadequate as the scope of exoplanet dynamics of continues to expand.  Eventually dynamicists are going to want to create different dynamical classes to account for the variety of dynamical circumstances seen in exoplanets.

In which case, they'll invent the appropriate terminology. But I have yet to see an example where the IAU definition inhibits this.

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So my proposed solution is to adopt a geophysical definition similar to what Stern's group has proposed as a definition for "planet".  And then define dynamical classes of planets, composition classes of planets, a mass scale.

This is just shifting labels about. As Mike Brown has pointed out, the choice of which class or sub-class of objects to label with the word 'planet' is ultimately aesthetic and not scientific. It's trivial to reproduce your entire classification scheme, but with modified labelling, including the modification whereby your 'principal planet' becomes 'planet'!

This is why Stern has had to resort to non-scientific arguments such as making objects sound important enough to attract funding! Fundamentally, there is no scientific reason to prefer the IAU definition of planet over one Stern would prefer. But it was felt that a choice had to be made, and a choice was made.

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The distinction between "Moons" and "satellites" is a simple enough one to understand.  Currently the two terms "moon" and "satellite" are used interchangeably.  Just like dynamicists recognize a distinction between the "planets" and "dwarf planets", geophysicists see an important physical distinction between spherical moons and non-spherical moons.   Yet all of these bodies from the 1 km debris through large spherical bodies such as Ganymede are lumped into the single category sometime referred to as "moons" and other times referred to as "satellites".  So my proposal is to use the term "moon" for spherical objects orbiting a planet and use the term "satellite" for smaller bodies orbiting a planet.

I actually agree with this, but it has nothing to do with the definition of planet itself.

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The Kepler 11 exoplanet system is a great illustration of how our current nomenclature based upon the Solar System is not up to the task.   Dynamically this system has 5 planets within 0.25 AU of the star and 6 planets within 0.5 AU.  These planets have masses ranging from ~2 Earth masses to ~17 Earth masses.  These planets may in some cases be borderline resonant with others but the current data is not clear on that.  Dynamically, are these bodies actually "planets" or are they "dwarf planets" or do we need a new dynamical class?

If by 'dynamically' you mean are they gravitationally dominant in their orbital zone, then by Margot's formulation, they are and they are therefore planets (if you assume they're in hydrostatic equilibrium!). If you think they need a new dynamical class then define one and give it a label.

I can't help feeling all of this is not exactly helping Stern win friends and influence people. In that I haven't seen it win anyone round to his way of thinking that wasn't already on his side so to speak.

No. He appears to be much more of an abrasive personality than, say, the more emollient Mike Brown.

Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #492 on: Today at 12:59 PM »
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Soter defined a planet as an object whose mass was more than 100 times that of all other objects in its orbital zone. There are a couple of problems with this. First, '100 times' is arbitrary. Second, it's practically impossible to measure the mass of all other objects in the case of exoplanet candidates.

Most astronomers who agree with the results of the IAU definition - though not necessarily its terminology! - prefer to use the concept of gravitational dominance. One advantage of this is that you only need to consider whether an object will clear its neighborhood during some specified span of time (usually the main-sequence lifetime of the host star), not whether it has actually yet done so. (Ironically, 'clearing the neighborhood comes from Alan Stern!)

Yes - Soter admitted that 100x was arbitrary and that it could be 1000x or only 10x.   I agree that "gravitational dominance" is the better terminology.   Keep in mind that there is a need to be able to explain it to students and laypersons.   The way I explain it (more mass vs. less mass compared with bodies sharing the objects orbital zone) captures one expected result of gravitational dominance that is in my experience pretty easily understood.  "Cleared orbit" is actually a pretty terrible way to described it because it is so open to misinterpretation. 

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I think supporters of the IAU definition would not only agree that both dynamical and geophysical perspectives are important but that the IAU definition itself satisfies both interests.

That is true.  I'm sure they do feel that way.  Unfortunately, planetary geophysicists do not seem to feel that way.   And from their point of view that is easy to understand.   Margot's excellent dynamical formulation drops the requirement that a planet be spherical.  This strictly dynamical definition then has no clearly recognized physical characteristic intrinsic to the body that identifies it as a planet.  Instead the definition is based entirely upon dynamical context and two different bodies with the exact same physical characteristics may or may not be a planet depending upon the dynamical context.   

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Similarly, Margot proposes that the requirement for a planet to be in hydrostatic equilibrium be dropped, because an object that is gravitationally dominant is almost certain to be so. But, importantly, his resulting definition relies solely on measurable quantities, not on an inference from such quantities. He leaves open the possibility that an object may be discovered that is gravitationally dominant but not in hydrostatic equilibrium. The reason he proposed dropping it is that it is practically impossible to measure for exoplanet candidates. Even in the Solar system, the IAU requires hydrostatic equilbrium be measured, not inferred from size and constituency.

It is an interesting contrast.  Margot's dynamical definition relies only on easily measureable quantitities but results in a definition where planets have no consistent intrinsic characteristic.  The geophysical definition is based upon an intrinsic characteristic that is perhaps harder to consistently measure and composition dependent at the lower mass boundary.

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Perhaps not, but it has nothing to do with the 'usefulness' of the IAU definition. I haven't seen any scientific difficulties caused by the IAU definition; anything that inhibits the development of their science. After all, there are words that collectively describe planets and dwarf planets - planemos or worlds, for instance - and they can always make up their own.

The difficulty primarily is that it is dictating to planetary geophysicists that they should communicate within their discipline using terminology that they disagree with and for their part claim is not useful.

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This is just shifting labels about. As Mike Brown has pointed out, the choice of which class or sub-class of objects to label with the word 'planet' is ultimately aesthetic and not scientific. It's trivial to reproduce your entire classification scheme, but with modified labelling, including the modification whereby your 'principal planet' becomes 'planet'!

This is why Stern has had to resort to non-scientific arguments such as making objects sound important enough to attract funding! Fundamentally, there is no scientific reason to prefer the IAU definition of planet over one Stern would prefer. But it was felt that a choice had to be made, and a choice was made.

You have two different research specialties in conflict those that research planetary dynamics and those that research planetary geophysics.  If this controversy is about something that is merely aesthetic and trivial and there is no scientific reason to prefer one definition over the other then why not seek a compromise position that satisfies the needs of both the dynamical and geophysical camps?  I certainly understand that the limited geophysical definition recently suggested by Runyon et a. is meaningless from a dynamical perspective. 

My proposal has been that the concerns of both groups can be addressed if "planet" is defined geophysically and then dynamical classes are defined to address dynamical concerns.  The dynamical classes I describe are - as you say - a shifting of labels.  So those classes still recognize the important distinctions the dynamicists require.    Whether the problem is considered aesthetic or scientific in nature it would benefit the astronomy community as a whole if a solution was identified that addresses the taxonomic concerns of both camps. 

But it seems to me that the way this is going to play out will simply be that planetary geophysicists are going to ignore the IAU's definition and there will be a "Geophysical definition of planet" leaving the IAU's as a "Dynamical definition of planet".

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I actually agree with this, but it has nothing to do with the definition of planet itself.

It has to do with the geophysical definition of planet.  From the geophysical perspective moons are a class of planet.  But then two terms are needed to distinguish the spherical bodies orbiting larger planets form the non-spherical bodies orbiting planets.  Using the terms "moon" and "satellite" for the two classes addresses the issue for the geophysicist.   
« Last Edit: Today at 01:00 PM by DRussell »

Online llanitedave

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Re: Pluto-Planet debate discussions
« Reply #493 on: Today at 09:39 PM »
Margot's dynamical definition relies only on easily measureable quantitities but results in a definition where planets have no consistent intrinsic characteristic. 


This, to me, is key.  I can think of no other scientific definition of any class of objects where intrinsic characteristics of the objects themselves are completely absent.  What, then, are we really defining?
"I've just abducted an alien -- now what?"

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