Author Topic: Pluto-Planet debate discussions  (Read 76824 times)

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #40 on: 06/25/2015 09:48 PM »
I personally think the debate will recede if further planets in between the Keiper belt and Oort Cloud are discovered as it kind of becomes irrelevant then.

Er, define "planets" to be discovered there.

Mike Brown has said--I think somewhat tongue-in-cheek--that he would not be surprised if a Mars-sized object is found way out in the Kuiper Belt. I think it is certainly possible to find something as big as, if not bigger than Pluto. But if something significantly larger than Pluto is found, it's going to really force a reevaluation of the definition of "planet."

But wouldn't that be f'ing great?!

I was thinking beyond the Kuiper belt.

http://forum.nasaspaceflight.com/index.php?topic=34329
« Last Edit: 06/25/2015 09:51 PM by Star One »

Offline whitelancer64

Re: Pluto-Planet debate discussions
« Reply #41 on: 06/25/2015 10:59 PM »
Never mind how small a planet can be, what's the largest that a rocky planet can be?
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Offline baldusi

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Re: Pluto-Planet debate discussions
« Reply #42 on: 06/25/2015 11:12 PM »
I personally think the debate will recede if further planets in between the Keiper belt and Oort Cloud are discovered as it kind of becomes irrelevant then.

Er, define "planets" to be discovered there.

Mike Brown has said--I think somewhat tongue-in-cheek--that he would not be surprised if a Mars-sized object is found way out in the Kuiper Belt. I think it is certainly possible to find something as big as, if not bigger than Pluto. But if something significantly larger than Pluto is found, it's going to really force a reevaluation of the definition of "planet."

But wouldn't that be f'ing great?!
Formed, migrated or captured?

Offline the_other_Doug

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Re: Pluto-Planet debate discussions
« Reply #43 on: 06/26/2015 01:12 AM »
Actually, IIRC, one of the lesser-known definitions of a planet in the current IAU codes is that a planet orbits the sun within the plane of the ecliptic.  (They must have given a plus-or-minus in there somehow, since I know the "eight planets" vary from that plane by, what, up to five degrees?)

Pluto, of course, has an orbit that's well... different.  Which leads me to the best reason I ever read as to why, under the new IAU rules, Pluto is not a planet:

"Because it is not so inclined."

:D
-Doug  (With my shield, not yet upon it)

Offline Nilof

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Re: Pluto-Planet debate discussions
« Reply #44 on: 06/26/2015 10:45 AM »
Actually, IIRC, one of the lesser-known definitions of a planet in the current IAU codes is that a planet orbits the sun within the plane of the ecliptic.  (They must have given a plus-or-minus in there somehow, since I know the "eight planets" vary from that plane by, what, up to five degrees?)

Pluto, of course, has an orbit that's well... different.  Which leads me to the best reason I ever read as to why, under the new IAU rules, Pluto is not a planet:

"Because it is not so inclined."

:D

That makes absolutely no sense to me. The upsiolon andromedae system would be a perfect counterexample of that as well: http://www.space.com/8476-weird-orbits-alien-planets-affect-chances-life.html

For two large planets in similar orbits, it is actually very easy for them to end up in orbits with a very high relative inclination.
« Last Edit: 06/26/2015 08:46 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline notsorandom

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Re: Pluto-Planet debate discussions
« Reply #45 on: 06/26/2015 01:28 PM »
This debate is literally a debate over semantics. Language is pretty much a crowd sourced deal. Words and meanings tend to be assigned by the speakers as a whole, not a small authoritative group. I think that is where the IAU is erring. The word planet is widely known and used. To most English speakers it means an object which orbits a star and is round. The argument was that this was too unspecific to be useful to science.

Yet the astronomical community is not hindered by the distinction between terrestrial worlds and gas giants. Calling both these objects planets even though they are radically different is not a problem. It is also based on a historical precedent, something which the IAU says has no place in science. Before a planet meant round thing that orbited the sun it meant point of light in the sky that moved. There was no way to tell the nature of the point of light back then. They were also quite wrong about what orbited what.

When the nature of Jupiter and Saturn was discovered, and Neptune and Uranus were found, the definition of a planet should have been changed. Jupiter and Saturn should have been kicked out of the club. That is what happened to Pluto after all. If Pluto were the only object of it's type it would still be called a planet. Concerns over how many planets we have to memorize are not scientific.

Offline meekGee

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Re: Pluto-Planet debate discussions
« Reply #46 on: 06/26/2015 03:24 PM »
The name of the class is immaterial.

The emphasis should be on the classification itself.  Pluto belongs to a different group of objects.  That's all there is to it.

There are rocky planets, gas giants, and kp objects aka dwarf planets.

There are also other types of objects.

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Offline mikelepage

Re: Pluto-Planet debate discussions
« Reply #47 on: 06/26/2015 03:55 PM »
What Mr. Russell has done is bypass the politics and get back to first principles, and I think his work has given superior results.

And for that I applaud his work - the best definition I've yet seen.  I just think a single concession to cultural reality is needed - don't try to redefine "moon" vs "satellite" categories.  Create a "planetary-mass moon" category, which applies to our moon along with all the other Jovian/Saturnian/Uranian and Neptunian moons that qualify.

People may be confused if you call them planets all of a sudden, but "planetary-mass moon" basically implies what the definition is saying - if it weren't in orbit around another, larger planet then it would qualify as a planet in its own right.  I don't think that's too big a leap.
« Last Edit: 06/26/2015 03:56 PM by mikelepage »

Offline the_other_Doug

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Re: Pluto-Planet debate discussions
« Reply #48 on: 06/26/2015 06:20 PM »
I still like the term "moon" or "satellite" for a body in orbit around a planet (i.e., in orbit around a body that is itself in orbit around a star).  I'd go for "dual planet" or "double planet system" if the barycenter of the orbit is outside of the sphere of the primary, as I believe it is between Charon and Pluto, but is not between Luna and Earth.

Also, as to Nilof's point, I know that when material is spread out into an orbit around a body, be it a planet or a star, the material fairly quickly organizes itself into an equatorial ring.  The orbital characteristics cause the "sports" in higher-inclination orbits to be pulled into the equatorial plane.  It breaks down a certain distance from the primary, but as far as Sol is concerned, the material that organized itself into orbit around our star back nearly five billion years ago organized itself into a pretty thin ring or disk of material that seems to have extended out at least as far as Neptune, and all of the eight currently recognized planets evolved into final orbits no more than a few degrees outside of this plane in inclination.
-Doug  (With my shield, not yet upon it)

Offline Nilof

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Re: Pluto-Planet debate discussions
« Reply #49 on: 06/26/2015 09:30 PM »
Also, as to Nilof's point, I know that when material is spread out into an orbit around a body, be it a planet or a star, the material fairly quickly organizes itself into an equatorial ring.  The orbital characteristics cause the "sports" in higher-inclination orbits to be pulled into the equatorial plane.  It breaks down a certain distance from the primary, but as far as Sol is concerned, the material that organized itself into orbit around our star back nearly five billion years ago organized itself into a pretty thin ring or disk of material that seems to have extended out at least as far as Neptune, and all of the eight currently recognized planets evolved into final orbits no more than a few degrees outside of this plane in inclination.

The fact that planets migrate as the disk becomes thinner tends to break the nice pattern of small relative inclinations as they can end up scattering each other during the migration. The major planets in our solar system happen to have survived with a low inclination, but there is nothing that says they had to survive, and even main belt asteroids are seriously inclined compared to the planets which is not compatible with the naive picture that everything is in the plane of the old disk.

In a large portion of other solar systems, the planets themselves do have strongly inclined orbits to each other. This is almost always the case for systems that have a hot Jupiter. Any definition of a planet should be applicable to other solar systems than our own.
« Last Edit: 06/26/2015 09:32 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline llanitedave

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Re: Pluto-Planet debate discussions
« Reply #50 on: 06/27/2015 01:53 AM »
Also, as to Nilof's point, I know that when material is spread out into an orbit around a body, be it a planet or a star, the material fairly quickly organizes itself into an equatorial ring.  The orbital characteristics cause the "sports" in higher-inclination orbits to be pulled into the equatorial plane.  It breaks down a certain distance from the primary, but as far as Sol is concerned, the material that organized itself into orbit around our star back nearly five billion years ago organized itself into a pretty thin ring or disk of material that seems to have extended out at least as far as Neptune, and all of the eight currently recognized planets evolved into final orbits no more than a few degrees outside of this plane in inclination.

The fact that planets migrate as the disk becomes thinner tends to break the nice pattern of small relative inclinations as they can end up scattering each other during the migration. The major planets in our solar system happen to have survived with a low inclination, but there is nothing that says they had to survive, and even main belt asteroids are seriously inclined compared to the planets which is not compatible with the naive picture that everything is in the plane of the old disk.

In a large portion of other solar systems, the planets themselves do have strongly inclined orbits to each other. This is almost always the case for systems that have a hot Jupiter. Any definition of a planet should be applicable to other solar systems than our own.

Agreed.  The planet formation process is chaotic, but the IAU seems to require that it end up orderly.  If it does, that's probably completely accidental, and IMO not sufficient to establish a taxonomy with.
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Offline ChrisWilson68

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Re: Pluto-Planet debate discussions
« Reply #51 on: 06/27/2015 02:11 AM »
Never before in history has there been so much debate about terminology.

Throughout history there's been far more debate on many more topics of terminology.

See, for example, the debate over the name of the country that was formerly part of Yugoslavia and wants to call itself Macedonia.  Or whether what happened to the Armenians in Turkey in the early part of the 20th century should be called genocide.

So it is important to debate this.

Back in Christopher Columbus's time, it seems that there was not enough debate about whether the Earth was flat or spherical.

Nobody seriously thought the Earth was flat in Columbus's time.  The debate was about how big it was.  Most people got the size pretty much right.  Columbus calculated it was much smaller and hence he'd be able to sail to China.  He was wrong, but he lucked out and there was land for him to find where he thought China should be.

Back in antiquity, there was probably a large majority that actually believed the Earth was spherical.  Then there was the minority that thought it was flat.  Obviously both were wrong and the Earth is more ellipsoidal.

I don't think it's correct to call those who thought of the Earth as spherical wrong.  Nobody ever thought it was a perfect sphere, just that the shape was roughly spherical.

So having a good debate about whether Pluto is a planet or a dwarf planet is important to help guide the future state of how the glossary section is written.

The debates aren't really comparable.  Debates about the shape of the Earth are debates about facts, not terminology.  With Pluto, not facts are up for debate, just the terminology various people choose to use.

If history is an indicator, Pluto will not be termed a planet or dwarf planet in the future.  I'm sure that the scientists of the future will look back into our days thinking of us as odd and say "why did scientists actually believe Pluto was a planet, or even a dwarf planet?". 

I think the future will define the object type from details on how the object was formed.  Asteroids seem to imply that a planet was destroyed and is simply debris from that process.  Planets seem to imply formation thru some form of hot magma condensing plus a large amount of bombardment.  Then there seems to be objects that have appeared from the Oort cloud.

If a rogue planet was captured from interstellar space, obviously that should be termed differently than something like a simple native planet formed in our solar system.

Size doesn't matter.

I don't know why you're saying "If history is any indicator" since nothing in the historical record provides evidence to support that.  You're just stating that you think people in the future will like the subjective choices that appeal to you.  But everyone else feels the same way about their own subjective choices.  Framing it in terms of what future historians will think adds nothing to the debate.

Offline the_other_Doug

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Re: Pluto-Planet debate discussions
« Reply #52 on: 06/27/2015 02:34 AM »
Also, as to Nilof's point, I know that when material is spread out into an orbit around a body, be it a planet or a star, the material fairly quickly organizes itself into an equatorial ring.  The orbital characteristics cause the "sports" in higher-inclination orbits to be pulled into the equatorial plane.  It breaks down a certain distance from the primary, but as far as Sol is concerned, the material that organized itself into orbit around our star back nearly five billion years ago organized itself into a pretty thin ring or disk of material that seems to have extended out at least as far as Neptune, and all of the eight currently recognized planets evolved into final orbits no more than a few degrees outside of this plane in inclination.

The fact that planets migrate as the disk becomes thinner tends to break the nice pattern of small relative inclinations as they can end up scattering each other during the migration. The major planets in our solar system happen to have survived with a low inclination, but there is nothing that says they had to survive, and even main belt asteroids are seriously inclined compared to the planets which is not compatible with the naive picture that everything is in the plane of the old disk.

In a large portion of other solar systems, the planets themselves do have strongly inclined orbits to each other. This is almost always the case for systems that have a hot Jupiter. Any definition of a planet should be applicable to other solar systems than our own.

Oh, I agree with you and with llanitedave, there.  I don't believe that a planet's inclination should define its planethood.

I think the idea was to try and come up with something that would define a planet as a body that coalesced out of the original planetary disk.  That would differentiate a planet from a body that has slowly coalesced from the sphere of debris that was located all around the very young Sol, like KBOs and Oort cloud objects.

It is a somewhat different accretion process out there in the Kuiper Belt and beyond than it was in the inner solar system, with less heat, apparently less abundance of radioactive (i.e., heat-inducing) elements, a lot more ice than went into in the inner-system bodies, and much longer timeframes between major accretion events.  And since there might be thousands of good-sized KBOs out there, I think the faction of the IAU that pushed the new rules through thought they were defining things not so much by where they are now, but by where they were formed.  As a side benefit, you don't get thousands of planets, most of which are small, icy KBOs.

I don't agree with that way of thinking.  My belief is the more the merrier.  If you're big enough to be gravitationally rounded and you orbit a star, and you're not a brown dwarf, then congratulations, you're a planet!  Doesn't matter if a migrating Gas Giant tossed you into a weird orbit, you're still a planet!  Heck -- in my mind, it doesn't even matter if that migrating destined-to-be-hot-Jupiter ejects you from the solar system entirely.  You're still a planet!  (A rogue planet, maybe, but still a planet...)

Now, you can subdivide your general category of "Planets" into descriptive sub-categories, and in fact we already do that.  For the eight IAU-recognized planets, we have four Rocky Planets, two Gas Giants and two Ice Giants (though some people still just categorize all four trans-Mars planets as Gas Giants).  You might want to classify anything smaller than a certain size as a dwarf planet, but you might want to categorize by other means, relating to distance, composition, inclination, or any number of other factors.

Since the KBOs that could qualify as planets likely have any number of family resemblances (or indeed may represent a number of families based on different characteristics), we may have different sub-categories of the planetary KBOs, like Dwarf Ice Planets, or Little Halo Planets (for those in polar orbits around Sol), or Teeny Stony Planets, or Squishy-Slushy Planets (like Ceres), or Wee Peat-Colored Planets...  The list of possible sub-categories, both useful and fanciful, can go on and on.

But all of them, regardless of sub-category, ought to be recognized as planets.  In my humble opinion.
-Doug  (With my shield, not yet upon it)

Offline mikelepage

Re: Pluto-Planet debate discussions
« Reply #53 on: 06/27/2015 09:35 AM »
Yet we draw the line when it comes to grains of sand.  Nobody would reasonably go running around a beach in the attempt to identify each grain, let alone classify the differences.

I have a geologist friend you should meet ;) Believe me, they've done it.

The utility of terminology is mainly in its ability to be as specific or vague as is needed in the context of whatever conversation you're having.  In general conversation, "moon", "planet" & "star" are just fine, but if the terminology used by the experts is based on arbitrary factors, it doesn't remain in use for long.

New Horizon's exploration of the Pluto system and Dawn's exploration of Ceres will no doubt teach us a lot more about "dwarf planets" and (I suspect) add further material to the argument that this is a poor label.  Give it a year or two and I reckon the IAU will be compelled to supply another definition.

As Mr Russell discusses in his definition paper linked above, you can have bodies in the size range of giant planets/brown dwarfs which form by different processes (gas collapse vs accretion in a planetary disk), so by using a different label, that immediately communicates certain things about the body you're talking about.

Likewise, I suspect they'll eventually have a different category for moons that are captured objects (as is suspected about Phobos/Deimos) versus moons that resulted from an impact and accretion around a planet (like our moon).  It tells you much about the expected composition of the body in question, but they are still moons (or "planetary-mass" moons as I suggested).
« Last Edit: 06/27/2015 09:36 AM by mikelepage »

Offline llanitedave

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Re: Pluto-Planet debate discussions
« Reply #54 on: 06/28/2015 04:06 AM »
Yet we draw the line when it comes to grains of sand.  Nobody would reasonably go running around a beach in the attempt to identify each grain, let alone classify the differences.

I have a geologist friend you should meet ;) Believe me, they've done it.

The utility of terminology is mainly in its ability to be as specific or vague as is needed in the context of whatever conversation you're having.  In general conversation, "moon", "planet" & "star" are just fine, but if the terminology used by the experts is based on arbitrary factors, it doesn't remain in use for long.

New Horizon's exploration of the Pluto system and Dawn's exploration of Ceres will no doubt teach us a lot more about "dwarf planets" and (I suspect) add further material to the argument that this is a poor label.  Give it a year or two and I reckon the IAU will be compelled to supply another definition.

As Mr Russell discusses in his definition paper linked above, you can have bodies in the size range of giant planets/brown dwarfs which form by different processes (gas collapse vs accretion in a planetary disk), so by using a different label, that immediately communicates certain things about the body you're talking about.

Likewise, I suspect they'll eventually have a different category for moons that are captured objects (as is suspected about Phobos/Deimos) versus moons that resulted from an impact and accretion around a planet (like our moon).  It tells you much about the expected composition of the body in question, but they are still moons (or "planetary-mass" moons as I suggested).

Yep, I've done it.  Some of those grains may be fossil fragments, some may be crystal fragments, and some may be rock fragments. The percentage of each may tell you whether there was a single source area, or whether the sand has a multi-source history.  If you can identify an index fossil in it, you can get the age of the host rock.

And not just modern beaches --  I know where a couple of nice paleo-beaches are that each have a distinctive sandstone signature.

Believe me, if it becomes possible to apply such fine distinctions to solar system bodies, it will be done.
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Offline Nomadd

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Re: Pluto-Planet debate discussions
« Reply #55 on: 06/28/2015 08:38 PM »
I still like the term "moon" or "satellite" for a body in orbit around a planet (i.e., in orbit around a body that is itself in orbit around a star).  I'd go for "dual planet" or "double planet system" if the barycenter of the orbit is outside of the sphere of the primary, as I believe it is between Charon and Pluto, but is not between Luna and Earth.

We need to schedule a party for 800 million years or so from now when the moon gets promoted to planet. Or will it be dwarf planet? Or 1/2 binary planet?

Offline KelvinZero

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Re: Pluto-Planet debate discussions
« Reply #56 on: 06/29/2015 08:11 AM »
Not directly related to the naming debate, but from this wiki link:

https://en.wikipedia.org/wiki/Dwarf_planet
"Estimates are that up to 200 dwarf planets may be found when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered"

..which I think is sort of awesome. The estimate comes from an Alan Stern article that did have some relevant points, but the link seems to be broken. His point from memory was that it was unnatural to have a single beurocracy in charge of classifying new objects, and that normally it just works by scientific consensus for objects such as distant stars.. and that the small number of officially accepted dwarf planets was misleading.

Offline Profwoot

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Re: Pluto-Planet debate discussions
« Reply #57 on: 06/29/2015 05:38 PM »
It's interesting to note the similarities in this taxonomic debate with those that happen in my field. My expertise is in human evolution, where the definition of a given taxon, and the concept of e.g., species in general, is very fuzzy, and very often changes in accordance with new evidence. Natural phenomena tend to vary continuously rather than discretely, so any categories created to organize them will by necessity be arbitrary.

The goal in biology is generally to distinguish groups with distinct evolutionary heritage, i.e., to have taxonomy match phylogeny (patterns of evolutionary descent) as closely as possible. In practice, this is very difficult. It seems that planetary science has a somewhat analogous goal of distinguishing bodies that formed within the original disc of the solar system from those that were captured later. This criterion doesn't help distinguish something like Ceres (assuming it was also an "original member" of the solar system; I might wrong on that), so additional criteria have been invented based on things like size, hydrostatic equilibrium, and ability to clear one's neighborhood, which may or may not reflect anything about a body's origin. Many other proposed criteria are similarly arbitrary.

To me, it's important to remember that taxonomic debates should have at their center the goal of organizing things in a way that maximizes our ability to understand something important about them, and that if no available criterion does that, then it ultimately doesn't matter how we choose to categorize them.

Offline Star One

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Offline TakeOff

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Re: Pluto-Planet debate discussions
« Reply #59 on: 07/08/2015 07:57 AM »
I can call Pluto a dwarf planet because it is a small planet, but hydrostatic equilibrium is a better criteria. The word "dwarf" refers to size, not to orbit or company. Just because most small planets also have eccentric and uncleared orbits, doesn't make that useful criteria for what meaning the adjective "dwarf" should have. It will certainly not be helpful for exoplanets (hard to see if they've cleared their orbits, for example).

The solution is to use planet för "wanderer", a fuzzy definition. Then use additional descriptors to specify those properties of the object which are of interest. Eccentric gas giant, accreting terrestrial double planet, hot dwarf planet with cleared and circularized orbit. And as for if the public would want many planets, well they don't seem to want 8! We could have 9 cultural planets regardless of any astronomical definition.
« Last Edit: 07/08/2015 08:01 AM by TakeOff »

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