Author Topic: Pluto-Planet debate discussions  (Read 77936 times)

Offline Bynaus

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Re: Pluto-Planet debate discussions
« Reply #560 on: 06/23/2017 05:01 AM »
Nevertheless, if we find a Mars-sized object out there, then regardless of its distance, it is going to be difficult to convince the public that it should NOT be a planet. Especially if we know of no other, similar object on a similar orbit. We all have a mental picture of what a planet should be, but perhaps gravitational dominance is not capturing this correctly - it is more about uniqueness, about being the single largest object in its neighborhood.

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #561 on: 06/23/2017 07:33 AM »
Talking of Pluto it interested me that Alan Stern re-tweeted this news but then I believe he has been advocating an object like this in the Kuiper belt for some time.
« Last Edit: 06/23/2017 07:34 AM by Star One »

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #562 on: 06/23/2017 07:39 AM »
We all have a mental picture of what a planet should be

We had a mental picture of what the celestial sphere was, what matter was, what the aether was, but we adapted. Once we thought light was made of particles, then we knew it was made of waves, then we got wavelike-particles and QM. We adapted to each change.

Once upon a time, "planet" meant wandering star (because we understood neither planets, nor stars). Then we thought it meant objects like Earth (read pre- and Golden Era SF). Then it meant the range of properties of the actual planets of the solar system, as we understood more about gas-giants and airless bodies. As we learn about exoplanets, our mental model needs to twist and expand again, especially when dealing with extremes like ultra low-density super-hot planets with metal atmospheres, etc. Meanwhile, as it expands in that direction, the IAU definition limits it again, but in different ways. We will adapt.

[Edit: I mean, we know that Jupiter is a planet, because we've been taught it is. Ask people if Jupiter is a planet, and they'll say yes. But if you could pull that "mental picture of what a planet should be" out of people's brains, would a large gas-giant really fit that picture? Stern plays on people's mental picture, but if we really used that picture to define "planet", then Titan would be a planet but Jupiter and Saturn would need to be called something else. Something between "planets" and brown-dwarfs.]
« Last Edit: 06/23/2017 07:46 AM by Paul451 »

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #563 on: 06/23/2017 07:45 AM »
Looking online it seems the public has spoken and this is already being called the tenth planet. Even though it's really planet nine and if they find Mike Brown's one that will be planet ten of course.

Being as Pluto seems to be remarkable active for its size heck knows what a Mars size object will have going on.
« Last Edit: 06/23/2017 07:47 AM by Star One »

Offline Bynaus

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Re: Pluto-Planet debate discussions
« Reply #564 on: 06/23/2017 06:09 PM »
Quote from: Paul451
We adapted to each change.

And we should. As you might remember, I don't agree with Stern (or his mental picture of a planet, which seems to be essentially a "sphere"), but again: it will be very difficult to convey to the public why a planet an object which has almost the size of the Earth should not be considered a planet. And I think I would side with the public here, despite the fact that I like the elegance of the Margot criterium for planethood (I adapt ;) ).

To me, a planet is a large (probably round), lone object which (is not a star and) orbits a star. Pluto is part of population of similar-sized objects, so is Ceres, and Titan is not orbiting a star - so for me (for my "mental picture"), it is evident that neither of the three objects should be a planet, but this new object (if it exists) fits the "mental picture" I just described quite well. It doesn't matter too much that it could not clear the neighborhood over the age of the solar system, but perhaps it shouldn't have to, perhaps this is just not the right (or best) way to come up with a "scientific rendition" of the "lone" condition.
« Last Edit: 06/23/2017 08:06 PM by Bynaus »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #565 on: 06/29/2017 04:14 AM »
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.

Any planetary-mass object will go below the line if far enough away. A Mars-sized body at the distance mooted is certainly near the line on Margot's proposal; though, as you say, a super-Earth would not be. But, until it's discovered there's not much point worrying about it! :)

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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.)

Yes, an option. I was too hasty in saying "the IAU would probably be forced to have to define 'cleared the neighbourhood around its orbit' more precisely". There are other alternatives - such as the one you mentioned.

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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.)

I agree with both of those. And while they're at it, they can replace the term 'dwarf planet' as well!

Nevertheless, if we find a Mars-sized object out there, then regardless of its distance, it is going to be difficult to convince the public that it should NOT be a planet. Especially if we know of no other, similar object on a similar orbit. We all have a mental picture of what a planet should be ...

Yes, but our mental pictures are primarily based on what we're taught. If people are taught differently in future, they'll have a different mental image. We all have a mental picture of what dinosaurs looked like; mine didn't include birds (but when young did include marine and flying reptiles!). Science changes; people are taught differently, and mental images shift.

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... but perhaps gravitational dominance is not capturing this correctly - it is more about uniqueness, about being the single largest object in its neighborhood.

If it wasn't the largest object in its neighbourhood, it wouldn't be gravitationally dominant; but being the largest object in the neighbourhood is insufficient to be gravitationally dominant (Ceres and Eris, for example). Gravitational dominance seems to be a vital criterion for a lot of astronomers; but perhaps it's not sufficient by itself?

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #566 on: 06/29/2017 02:36 PM »
Given the huge gap between planets and non-planets in all the planetary-[discriminant] methods suggested so far, it's unlikely that a post-Pluto 9th planet will be anywhere near the line.
Any planetary-mass object will go below the line if far enough away. A Mars-sized body at the distance mooted is certainly near the line on Margot's proposal

You miss my point. When using the various planetary-discriminant methods, there is a huge gap between the least-planety planet and the most-planety sub-planet. That suggests a physical phenomena in the planetary formation process which prevents objects remaining in the gap -- if an object gets close to the magic-line during the early formation period, it will go well beyond it. If that's the case, there won't be a Mars-sized object around 50AU. If there had been, it would have either swept up enough smaller objects to grow well beyond the magic-line via mass, or it would have interacted with enough matter to change its orbit significantly (dropping well inside 50AU, or being thrown well beyond.)

If we start finding objects within the gap (right mass at the right distance), then it means the gap in the rest of the solar system isn't as significant and isn't telling us anything fundamental about the formation of solar systems.

Aside:

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.)
I agree with both of those. And while they're at it, they can replace the term 'dwarf planet' as well!

I'd prefer if they eliminated the category entirely. I can't see it serving any useful purpose. "Roundness" doesn't really capture things like internal differentiation, or composition, or history of formation, or any other potentially valuable intrinsic property of interest to planetary scientists. There's also no clear line between dwarfs and sub-dwarfs, so there's no inherent boundary there. And the category seems to only exist as a second prize to Stern's faction by those in the definitions group who were trying to come up with a reasonable compromise. Since Stern's faction is not reasonable, what's the point in keeping it?

Merge "Dwarf Planets" with the rest of the asteroids and comets in the SSSB category. (Demote Pluto again! Bwahahaha!)
« Last Edit: 06/29/2017 02:37 PM by Paul451 »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #567 on: 07/02/2017 05:20 AM »
Given the huge gap between planets and non-planets in all the planetary-[discriminant] methods suggested so far, it's unlikely that a post-Pluto 9th planet will be anywhere near the line.
Any planetary-mass object will go below the line if far enough away. A Mars-sized body at the distance mooted is certainly near the line on Margot's proposal

You miss my point.

Yes, I did! :) I take all the blame. (Maybe. ;) )

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When using the various planetary-discriminant methods, there is a huge gap between the least-planety planet and the most-planety sub-planet. That suggests a physical phenomena in the planetary formation process which prevents objects remaining in the gap -- if an object gets close to the magic-line during the early formation period, it will go well beyond it. If that's the case, there won't be a Mars-sized object around 50AU. If there had been, it would have either swept up enough smaller objects to grow well beyond the magic-line via mass, or it would have interacted with enough matter to change its orbit significantly (dropping well inside 50AU, or being thrown well beyond.)

This is similar to the explanation of the so-called 'Fulton Gap' between super-Earths and mini-Neptunes (when a proto-planet accretes sufficient mass it rapidly accretes a sizable gaseous envelope, etc).

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If we start finding objects within the gap (right mass at the right distance), then it means the gap in the rest of the solar system isn't as significant and isn't telling us anything fundamental about the formation of solar systems.

But, as you say, the observed gap in the solar system is quite wide - Margot's planetary discriminant varies from a low for planets of 54 (Mars) to a high for dwarf planets (Ceres) of 0.04 - and we don't know where within this any gap due to a physical phenomenon actually lies. Margot set the boundary between planets and sub-planets when his discriminant equals 1, but perhaps the gap is actually around, say, 2 or 0.5?

Also, although there is an explanation for why the Fulton Gap arises, there are still objects within the gap, just relatively few of them. Similarly, one could expect that there would be the occasional object lying within any planet/sub-planet gap and we could just happen to have one such object in our solar system.

Unfortunately, resolving either of those two questions requires a sufficiently precise audit of other solar systems (which seems some way off!) or perhaps some theoretical underpinning backed up by computer simulations?

However, if we discover any object in the solar system with a mass between that of Mercury and Eris, it will kick-off an argument about whether it is a planet or not as the current IAU definition does not resolve the question!

Aside:

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And while they're at it, they can replace the term 'dwarf planet' as well!

I'd prefer if they eliminated the category entirely. I can't see it serving any useful purpose. "Roundness" doesn't really capture things like internal differentiation, or composition, or history of formation, or any other potentially valuable intrinsic property of interest to planetary scientists.

There are astronomers other than planetary scientists! :) Although, unlike dynamicists and the definition of planet, I can't off-hand think of any with an interest in this particular boundary!

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There's also no clear line between dwarfs and sub-dwarfs, so there's no inherent boundary there.

There's certainly a conceptual boundary (hydrostatic equilibrium); how physically relevant or scientifically useful it is remains to be seen.

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And the category seems to only exist as a second prize to Stern's faction by those in the definitions group who were trying to come up with a reasonable compromise. Since Stern's faction is not reasonable, what's the point in keeping it?

Probably because the fuss would be more bother than abolishing it is worth. I expect it will remain until altering it becomes necessary.

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Merge "Dwarf Planets" with the rest of the asteroids and comets in the SSSB category. (Demote Pluto again! Bwahahaha!)

I'm not sure if I do or do not want to be around Alan Stern when he discovered that! :)

Offline Paul451

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Re: Pluto-Planet debate discussions
« Reply #568 on: 07/02/2017 09:21 AM »
Also, although there is an explanation for why the Fulton Gap arises, there are still objects within the gap, just relatively few of them. Similarly, one could expect that there would be the occasional object lying within any planet/sub-planet gap and we could just happen to have one such object in our solar system.

If the Stern-Levinson gap relates to a real phenomena, then there will be a specific reason an object would end up in the gap. It makes that object uniquely interesting. (Eg, it must have migrated after major planetary migrations. But why? Outside influence like passing close to a neighbouring star?)

It's the same if Brown's 9th planet turns out not to exist, but that the disturbance in the force orbits is real. Could density waves be moving through the Kuiper Belt/scattered-disk and simulating the presence of a large planet. (Similar to the arms of a galaxy.) Could those density waves be echoes of the planetary migration period or of a close-pass with another star, or...? Etc etc. Weirdness is where you learn.

However, if we discover any object in the solar system with a mass between that of Mercury and Eris, it will kick-off an argument about whether it is a planet or not as the current IAU definition does not resolve the question!

If it is discovered because of its gravitational influence over its neighbourhood, then it is a planet by definition. Its mass, and how much its "cleared its neighbourhood" will feed back into refining the proposed planetary-determinant methods. But the logic is "This is a planet, therefore it helps us refine the method of calculation of planetness", rather than debating whether it's a planet.

If it's discovered in the same way as other non-planetary objects out there, Eris/etc, then if its in the SL-gap, it will be "interesting". But I don't think it will significantly influence the debate over the definition of planets -- whichever side of the line it falls.

There are astronomers other than planetary scientists!

However, usefulness to planetary scientists (as opposed to mere "astronomers") is the primary justification given for using HE as the top level classification.

There's also no clear line between dwarfs and sub-dwarfs, so there's no inherent boundary there.
There's certainly a conceptual boundary (hydrostatic equilibrium)

But it doesn't tell you anything useful. HE isn't the line between internal differentiation and lack thereof. You have to be well beyond that boundary before objects reach HE. Likewise, while it can tell you something about the internal composition (an icy object will be in HE at a vastly lower mass than a rocky object), we will generally assess HE from our knowledge/estimates of composition long before we can measure it directly. And trying to determine HE by measuring roundness ends up being misleading, since an object like Vesta can be "round" but no longer in HE, because it was in HE earlier in its formation; meaning that HE is quite difficult to determine, so it tends to come after other "classifications" like location, composition, history, temperature, etc.

It doesn't tell you anything useful enough to make it a top level classification system.

It's probably less useful (and much less practical) than division(s) based on simple mass or diameter, similar to the USGS silt/sand/pebble classification scheme.

And the category seems to only exist as a second prize to Stern's faction by those in the definitions group who were trying to come up with a reasonable compromise. Since Stern's faction is not reasonable, what's the point in keeping it?
Probably because the fuss would be more bother than abolishing it is worth. I expect it will remain until altering it becomes necessary.

Probably. But they will need to revisit the definitions when they expand the definition of "planet" to include exo-planets.

So I'm putting in an early request.

[edit: typo]
« Last Edit: 07/11/2017 09:10 PM by Paul451 »

Online Alpha_Centauri

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Re: Pluto-Planet debate discussions
« Reply #569 on: 07/02/2017 10:54 AM »
Looking online it seems the public has spoken and this is already being called the tenth planet. Even though it's really planet nine and if they find Mike Brown's one that will be planet ten of course.

Well we designate exoplanets b,c,d etc based on their order of discovery, not how far away they are from their host star.  So I guess it depends on which is found first!
« Last Edit: 07/02/2017 12:06 PM by Alpha_Centauri »

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #570 on: 07/02/2017 11:34 AM »
Looking online it seems the public has spoken and this is already being called the tenth planet. Even though it's really planet nine and if they find Mike Brown's one that will be planet ten of course.

Well, we designate exoplanets b,c,d etc based on their order of discovery, not how far away they are from their host star.  So I guess it depends on which is found first!

But would we be so vigorous in applying that our own solar system?

One thing is if we did find Brown's planet nine and it was a sub-Neptune object that would make our Solar System more typical as at the moment it seems more atypical. But I suppose that could just as easily be observational bias as to what planets we can and cannot observe.

Online Alpha_Centauri

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Re: Pluto-Planet debate discussions
« Reply #571 on: 07/02/2017 12:09 PM »
Yes sub-Neptunes are common on short periods, but we clearly don't have one of those. Whether they are more common beyond the snow line is TBD.
« Last Edit: 07/02/2017 12:10 PM by Alpha_Centauri »

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #572 on: 07/02/2017 12:13 PM »
Yes sub-Neptunes are common on short periods, but we clearly don't have one of those. Whether they are more common beyond the snow line is TBD.

If it exists isn't Brown's belief it formed closer into the Sun and that Jupiter & Saturn chucked it outwards to its present distant perch.

Offline Bynaus

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Re: Pluto-Planet debate discussions
« Reply #573 on: 07/04/2017 02:50 PM »
Yes sub-Neptunes are common on short periods, but we clearly don't have one of those. Whether they are more common beyond the snow line is TBD.

If it exists isn't Brown's belief it formed closer into the Sun and that Jupiter & Saturn chucked it outwards to its present distant perch.

Well, not THAT close to the Sun. The hot Neptunes are thought to have formed beyond the snow line (by most researchers, at least). Brown's idea is that P9 would have formed beyond the snow line, roughly in the region 5-15 AU where the gas and ice giants are all thought to have originated. Starting in this "birth zone", interaction with the gas gives you inward migration (leading to the formation of hot Neptunes and at least some of the hot Jupiters), and interaction with other planets and/or a planetesimal disk gives you outward migration (leading to Uranus and Neptun in their current orbits, and, perhaps, P9).

At least for red dwarfs, we know that Neptunes are common beyond the snow line (from both Kepler and Microlensing data).

Offline Star One

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Pluto-Planet debate discussions
« Reply #574 on: 07/11/2017 06:46 PM »
On the most recent episode of the Sky At Night I noticed this possible object was referred to as a dwarf planet even though it might be the size of Mars or larger. To me that's just ludicrous as if they do find something that big in the Kuiper belt I have a hard time imaging them being able to sell it as anything other than planet 10.

The issue is addressed in this article as well.

http://www.popsci.com/dont-get-too-hyped-about-planet-10-just-yet#page-3
« Last Edit: 07/11/2017 07:12 PM by Star One »

Offline CuddlyRocket

Re: Pluto-Planet debate discussions
« Reply #575 on: 12/15/2017 08:41 AM »
Artificial Intelligence, NASA Data Used to Discover Eighth Planet Circling Distant Star

https://www.nasa.gov/press-release/artificial-intelligence-nasa-data-used-to-discover-eighth-planet-circling-distant-star
(See the original post for the video.)

Interesting in that as far as I can see there's been no pushback from professional astronomers or journalists against the notion that the solar system has eight planets. (There may be some, but I haven't seen it!) Of course, it's in the interests of the scientists and the institutions they represent to present this as a record-equalling number of planets in one system, rather than as 'one off the record' - much better for raising one's profile! :) Similarly, people will be equally keen to present it as 'record-making' if and when someone discovers a ninth planet in the system (pace Planet Nine actually being confirmed in our system).

Still, it's clear which way the trend is going in the Pluto-Planet debate. No doubt there will always be some die-hards, but unless there's some major change in the science, I think the position is pretty settled.

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