Author Topic: Pluto-Planet debate discussions  (Read 76632 times)

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #540 on: 03/31/2017 02:34 PM »
I keep hearing that the dynamical definition is so useful for exoplanetary systems.
But clearing the orbit takes time, so in the early solar system Earth wasn't a planet.

Not what "cleared its neighbourhood" means.

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

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

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

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

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

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

Online meekGee

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Re: Pluto-Planet debate discussions
« Reply #541 on: 04/02/2017 12:53 AM »
The problem with classifying due to orbital neighbors is that it is open to too much opinion and leeway and isn't in any way a scientific way of looking at the matter.

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

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

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



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

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

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

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

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

This started, IIRC, way back with the debate on which mission to choose, when New Horizons was one of the candidates.
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Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #542 on: 04/03/2017 01:46 PM »
Quote
Stern is clearly trolling, and IMO is doing so since he realized he can "energize" support for his programs.  There is no other reasonable explanation to him misunderstanding this.

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


Offline DRussell

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Re: Pluto-Planet debate discussions
« Reply #543 on: 04/03/2017 01:59 PM »
Quote
IMO, extrinsic classification of even sub-planetary objects is more scientifically useful. KBO, SDO, main-belt asteroid, Trojan, etc. 

Why?

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

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

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

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

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

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

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

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

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

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

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

Paul451,

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

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

Online meekGee

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Re: Pluto-Planet debate discussions
« Reply #544 on: 04/03/2017 02:37 PM »
Quote
Stern is clearly trolling, and IMO is doing so since he realized he can "energize" support for his programs.  There is no other reasonable explanation to him misunderstanding this.

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

Second, the timing. I remember it starting back then.

But of course this is just an opinion - that goes without saying.
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Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #545 on: 04/03/2017 05:01 PM »
The geophysical definition is more useful to the geophysicist.

As I tried to show in my comment, I don't think any of the proffered geophys definitions are useful. Whether roundness or arbitrary mass cut-offs. It either doesn't correlate to the differences that matter, such as composition or history, or doesn't correlate tightly enough with an actual distinction, such as internal differentiation.

Quote
Stern is clearly trolling, and IMO is doing so since he realized he can "energize" support for his programs.  There is no other reasonable explanation to him misunderstanding this.
Can you expand upon this?

We have to be careful, Stern is apparently a member of the NSF, and under site rules there's a line that you can't cross when criticising other site-members.

What makes you think he isn't just expressing his opinion regarding which definition of planet is more useful from a geophysical perspective? I haven't really seen any indication he's feigning a misunderstanding of the IAU definition.

Do you think, based on his prior paper, that he is actually confused about the meaning of "cleared its neighbourhood" enough to be genuine when saying that Earth or Jupiter don't meet the IAU criteria? Do you think he misunderstands the IAU voting method enough to justify his claims of "less than five percent" of astronomers hijacking the debate? Do you think he didn't know of the role of his own faction in trying to ram his preferred definition through the first round (and being defeated by over 90%)? Do you think he doesn't understand enough on the history of science and of scientific committees to justify his acting like a vote on standardising nomenclature is somehow extraordinary and unprecedented? Do you think his denigration of his colleagues is appropriate behaviour for a scientist at all, let alone one in the public spotlight as his is? (For eg, likening astronomers to "podiatrists" doing "brain surgery" when categorising the solar system.)

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #546 on: 04/03/2017 10:02 PM »
This debate is not a matter of the "right" vs. the "wrong" way to define "planet".

More accurate to say 'which group of celestial objects to assign the label 'planet' to'. There is no right and wrong way to decide that - at least scientifically - but a decision had to be made, and it was!

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It truly is a matter of two different points of view about what is the more useful approach.

I don't think usefulness had or has anything to do with it. I have yet to see any classification scheme claimed to be more 'useful' that cannot simply be re-labelled to have the same eight celestial objects be the only planets in the solar system.

'Usefulness' is just another rationalisation made to justify personal preferences motivated by other reasons.

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The IAU definition is more useful to the dynamicist. The geophysical definition is more useful to the geophysicist.

Neither makes any difference at all. What is more or less useful is a classification scheme with an appropriate structure. The precise labels are neither here nor there.

Quote
Neither definition would inhibit the other point of view from conducting their research ... although it does impact the language used to communicate in their field.

True.

Quote
And then there is pedagogy.  From an instructional point of view I prefer the geophysical definition with dynamical classes and composition classes.

Another rationalisation to produce the desired result.

You can create a classification scheme by first dividing the relevant set of celestial objects dynamically and then sub-dividing them geophysically, or you can first divide them geophysically and sub-divide them dynamically. And there's many different ways to do both. Then you can decide which labels to give which group of objects. I have yet to see any evidence that either way of producing a classification scheme is intrinsically preferable from an instructional point of view.

Offline Star One

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Pluto-Planet debate discussions
« Reply #547 on: 04/28/2017 08:58 PM »
Distant Dwarf Planet DeeDee Stirs Up the Pluto Planethood Debate

Quote
"What it's really showing is the diversity of planets in our solar system,” said Runyon of the DeeDee news, “and giving us a better understanding of planets in the rest of the galaxy.”

"DeeDee is almost certainly made out of ices — water ices, methane, and carbon dioxide — which is similar to what Pluto is made of," he added. "These are very soft materials, compared with rocky silicate. It's more easily pulled into a sphere than rock or metal."

Adding more fodder to the debate over the definition, when the New Horizons spacecraft flew by Pluto in 2015, it unveiled a world of surprising complexity, ranging from mountainous areas to vast nitrogen-ice lakes.

"We call Pluto a 'dwarf' planet, but it's just an adjective for 'planet,’” Runyon said. “It's still a planet, and that's where we take umbrage with the IAU.”

"Astronomers aren't experts in planetary science, and they basically passed a bunch of B.S. off on the public back in 2006 with a planet classification so flawed that it rules the Earth out as a planet, too," Stern remarked in 2016. "A week later, hundreds of planetary scientists, more people than at the IAU vote, signed a petition that rejects the new definition. If you go to planetary science meetings and hear technical talks on Pluto, you will hear experts calling it a planet every day."

http://www.livescience.com/58842-dwarf-planet-deedee-stirs-up-pluto-planethood-debate.html
« Last Edit: 04/28/2017 08:59 PM by Star One »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #548 on: 04/29/2017 01:48 AM »
Distant Dwarf Planet DeeDee Stirs Up the Pluto Planethood Debate

A more accurate title would be 'Usual suspects use DeeDee to try to stir up the Pluto planethood debate with the aid of a clickbait-seeking website'! :)

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #549 on: 04/29/2017 08:43 AM »
Distant Dwarf Planet DeeDee Stirs Up the Pluto Planethood Debate

A more accurate title would be 'Usual suspects use DeeDee to try to stir up the Pluto planethood debate with the aid of a clickbait-seeking website'! :)

Well I did find the link on Alan Stern's Twitter feed, make of that what you will.

Offline K-P

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Re: Pluto-Planet debate discussions
« Reply #550 on: 05/07/2017 06:28 PM »
Stern really has some issues...

http://nasawatch.com/archives/2017/05/planet-9-sour-g.html#comments

I sincerely hope Brown stays above this BS and continues to do exactly what he has done so far:
making observations to put more nails in that Planet Pluto -coffin.

Online IanThePineapple

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Re: Pluto-Planet debate discussions
« Reply #551 on: 05/07/2017 06:36 PM »
Calm down guys
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Offline Nomadd

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Re: Pluto-Planet debate discussions
« Reply #552 on: 05/07/2017 06:45 PM »
 I'm still waiting for anyone to explain why Pluto and Charon aren't a binary, dwarf or not.

Offline baldusi

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Re: Pluto-Planet debate discussions
« Reply #553 on: 05/07/2017 07:31 PM »
I'm still waiting for anyone to explain why Pluto and Charon aren't a binary, dwarf or not.

From my limited understanding, they didn't wanted to get into the issues of defining binary planets or dwarf planets or what not. My guess is that if Planet 9 is discovered to be a real thing, they will put that and the binary issues all on the table. Meanwhile, they will coast until they have much better information.

Offline mikelepage

Re: Pluto-Planet debate discussions
« Reply #554 on: 05/08/2017 02:23 PM »
I'm still waiting for anyone to explain why Pluto and Charon aren't a binary, dwarf or not.

From my limited understanding, they didn't wanted to get into the issues of defining binary planets or dwarf planets or what not. My guess is that if Planet 9 is discovered to be a real thing, they will put that and the binary issues all on the table. Meanwhile, they will coast until they have much better information.

Suppose for the sake of argument that (Batygin's / Brown's) Planet 9 turns out to be a binary, with a "moon" the size of Earth.  If the orbit of said moon was elliptical enough, tidal heating might even lead to constant Io-like volcanos that would keep it warm enough to support liquid water (on the surface), with a Titan-like stratosphere tall enough to maintain decent surface pressure.  One could walk outside on a planetary surface in Earth-like gravity, perhaps needing only warm clothing and a gas mask.

Now suppose that scenario isn't, in fact, Planet 9, but an ice-giant/Earth-mass binary free-floating among the stars.  Something that could support non-photosynthetic life as we know it, minus a sun.  (Moot point: for all intents and purposes, Planet 9 lacks a sun anyway).  As far as I know, there could be millions or more of these in the galaxy.

Kinda renders planet/not-a-planet debates a bit meaningless doesn't it?

How about biggest>smallest:
Gas giant
Ice giant
Differentiated globe,
Undifferentiated globe,
Small body

Keep whether something is a moon/binary/primary as a separate descriptor.


Offline K-P

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Re: Pluto-Planet debate discussions
« Reply #555 on: 05/08/2017 02:43 PM »
"Your definitions are just fine...

as long as Pluto remains a planet...!"

As I would expect some people to say.
:)

As a side note, I would rather be exploring with my team a first ever object of some "category" than the last one of some other... but hey, what do I know about PR and fame for the history books.


Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #556 on: 05/09/2017 11:34 PM »
I'm still waiting for anyone to explain why Pluto and Charon aren't a binary, dwarf or not.

From my limited understanding, they didn't wanted to get into the issues of defining binary planets or dwarf planets or what not.

Dwarf planets are defined. The problem with binary dwarf planets (or planets, for that matter) is whether or not the individual objects orbit the Sun, as the definition requires. (Interestingly, the IAU General Assembly specifically resolved that Pluto is a dwarf planet!)

Quote
My guess is that if Planet 9 is discovered to be a real thing, they will put that and the binary issues all on the table. Meanwhile, they will coast until they have much better information.

It depends what they find. If it smoothly fits in to the current classification scheme then nothing will happen. The IAU will probably not revisit this question until either it can rubber-stamp a scientific consensus on a change or somebody discovers something that requires a clarification (what we might call 'an Eris event'!).

Kinda renders planet/not-a-planet debates a bit meaningless doesn't it?

Not really, as the debate centres around the 'clearing the neighbourhood' criteria.

As a side note, I would rather be exploring with my team a first ever object of some "category" than the last one of some other... but hey, what do I know about PR and fame for the history books.

I'd agree, but it's still better to explore the last one - and thereby 'complete the set' - than to explore one of the ones inbetween!

This was part of the PR for New Horizons. Unfortunately, en route Eris was discovered and we all know what happened next - they couldn't even claim the first dwarf planet exploration, as Dawn reached Ceres first!

Offline baldusi

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Re: Pluto-Planet debate discussions
« Reply #557 on: 05/10/2017 01:43 PM »
I'm still waiting for anyone to explain why Pluto and Charon aren't a binary, dwarf or not.

From my limited understanding, they didn't wanted to get into the issues of defining binary planets or dwarf planets or what not.

Dwarf planets are defined. The problem with binary dwarf planets (or planets, for that matter) is whether or not the individual objects orbit the Sun, as the definition requires. (Interestingly, the IAU General Assembly specifically resolved that Pluto is a dwarf planet!)

I didn't used Oxford comma, I meant binary planet, binary dwarf planet or binary whatnot (except binary star). Of course that dwarf planet is defined.

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #558 on: 06/23/2017 02:32 AM »
Cross-posting from the Dwarf planet discovery hints at a hidden Super Earth in solar system thread to deal with the planet definition aspects:

Planet ten or nine depending on which way you count all these proposed planets we seem to have missed. Though whether this is another planet gets caught up in how we define planets now.

The curious case of the warped Kuiper Belt

https://astronomynow.com/2017/06/22/the-curious-case-of-the-warped-kuiper-belt/

Here's the original press release.

https://uanews.arizona.edu/story/ua-scientists-and-curious-case-warped-kuiper-belt
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Because a planet, by definition, has to have cleared its orbit of minor planets such as KBOs, the authors refer to the hypothetical mass as a planetary mass object.

It actually has to have "cleared the neighbourhood around its orbit", but either way this is not defined (except by necessary implication from the fact that the IAU has declared some bodies to be planets and others not to be so!). But if it's not a planet, then it's a dwarf planet, so why not say so? Perhaps in the paper itself they're unsure as to its status?

Soter proposed a planet would be an object 100 times more massive than everything else in its orbital zone. The entire Kuiper belt is estimated to mass 40-100% that of Mars, but the vast majority of this would not be in the hypothesised object's orbital zone. However, Eris, which is in this zone, was itself previously estimated to have a mass 0.1 times that of everything else in its orbital zone, implying 'everything else' has a mass about 0.3 that of Mars. All of which goes to show that Soter's test is not so easy to apply in practice!

The Stern-Levinson parameter for distinguishing between 'überplanets' ("dynamically important enough to have cleared its neighboring planetesimals") and 'unterplanets' would make a Mars-sized body an 'überplanet' up to about 146 AU, suggesting this body would be an überplanet, or a planet, as the IAU would term it. However, Margot's planetary discriminant would make a Mars-sized body a planet up to about 53 AU suggesting this body would be a dwarf planet! So, if this proposed body was demonstrated to exist, the IAU would probably be forced to have to define 'cleared the neighbourhood around its orbit' more precisely!

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #559 on: 06/23/2017 04:08 AM »
Cross-posting from the Dwarf planet discovery hints at a hidden Super Earth in solar system thread to deal with the planet definition aspects:
[...]
The Stern-Levinson parameter for distinguishing between 'überplanets' ("dynamically important enough to have cleared its neighboring planetesimals") and 'unterplanets' would make a Mars-sized body an 'überplanet' up to about 146 AU, suggesting this body would be an überplanet, or a planet, as the IAU would term it. However, Margot's planetary discriminant would make a Mars-sized body a planet up to about 53 AU suggesting this body would be a dwarf planet! So, if this proposed body was demonstrated to exist, the IAU would probably be forced to have to define 'cleared the neighbourhood around its orbit' more precisely!

Not necessarily. Given the huge gap between planets and non-planets in all the planetary-determinant methods suggested so far, it's unlikely that a post-Pluto 9th planet will be anywhere near the line. If it is a "super-Earth", there's certainly no issue.

And after all, if it is real, then the only reason it was discovered was because it gravitationally dominated its region, which is the whole point of differentiating between planets and non-planets. Hence if a Mars-sized mass has had the effect on the other KBOs that has been observed, then yes, those like Margot can revise their planetary-determinants to reflect that. (It would be a very useful finding for that reason alone.) Which is why the official definition of an IAU-planet shouldn't pick a specific determinant yet. (Although they should replace Stern's misleading "cleared the neighbourhood" phrase.)

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