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On the classifications of planets

Kitsune said:
The trouble is that Earth has asteroids in our orbit. Jupiter, Saturn, and other bodies do as well. They just have done a better job than Pluto and other trans-Neptunium bodies.
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Aren't those asteroids in Lagrange points, rather than outright orbiting the sun though?

I think the orbit clearing should be "cleared orbit of other possible planet candidates", not "cleaned up all the space dust", since the latest is never going to happen. So you would be disqualified if another sun orbiting, rotation shaped object is sharing your orbit, but not if asteroids are trailing around.

Kitsune said:
I think a better classification is this
1. Giant Planets - Jupiter, Saturn, Uranus, and Neptune
2. Terran Planets - Earth, Venus, and Mars
3. Dwarf Planets - Basically everything else of reasonable size.
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Figure out rules outputting this consistently is the problem. As is, it isn't a classification, just a wishlist.
 
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For practical reasons I think/believe the 'cleared orbit' part has been functionally dropped or never used by astronomers.

Why?

Exoplanets.

Copied from Wikipedia.

Exoplanet - Wikipedia
"As of 1 October 2018, a total of 3,851 confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s."

In Exoplanet - Wikipedia they call them exoplanets and planets interchangeably. There is no discussion of clearing orbits.

With the technology we have today we can barely image the exoplanets.

As it has been pointed out in this discussion, with today's technology ( and with out a lot of word play ;)) we would not be able to 'prove' that any of our solar system's acknowledged planets have 'cleared' their own orbits.


It seems to me that 'clearing their orbits' as part of the definition of a planet is unworkable and not useful. A step too far as it were.
 
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Frogs321 said:
It seems to me that 'clearing their orbits' as part of the definition of a planet is unworkable and not useful. A step to far as it were.
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The problem is that what works for exoplanets do not work for our own, because we can detect a lot more in the solar system than around other stars.

So we end up with everything vaguely round and sun orbiting being a planet, which is quite a lot.
 
The only thing in that post that I was suggesting that was not useful was the 'clearing the orbit' part of the planet definition.

The physical description of a astronomical object can do a lot of sorting with out including the 'clearing the orbit' part. :)
 
Frogs321 said:
The only thing in that post that I was suggesting that was not useful was the 'clearing the orbit' part of the planet definition.

The physical description of a astronomical object can do a lot of sorting with out including the 'clearing the orbit' part. :)
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No. By itself, simply orbiting the sun and being in hydrostatic equilibrium leaves us with a large amount of bodies most people don't care about and do not want to hear of as planets.
 
Most bodies with enough influence to be detected from light-years away are probably fine to call planets. I could be completely mistaken though; it would fascinating to work out which bodies in our solar system could be detected at interstellar distances with our current technology
 
Frogs321 said:
For practical reasons I think/believe the 'cleared orbit' part has been functionally dropped or never used by astronomers.

Why?

Exoplanets.

Copied from Wikipedia.

Exoplanet - Wikipedia
"As of 1 October 2018, a total of 3,851 confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late 1980s."

In Exoplanet - Wikipedia they call them exoplanets and planets interchangeably. There is no discussion of clearing orbits.

With the technology we have today we can barely image the exoplanets.

As it has been pointed out in this discussion, with today's technology ( and with out a lot of word play ;)) we would not be able to 'prove' that any of our solar system's acknowledged planets have 'cleared' their own orbits.


It seems to me that 'clearing their orbits' as part of the definition of a planet is unworkable and not useful. A step too far as it were.
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That's a pretty weak attempt at a semantic "gotcha", and a moment of thought would have revealed to you that your "it seems to me" is working from incorrect assumptions.

"Cleared the neighbourhood" in an astrophysical sense means that it is the dominant body of its scale in its orbit - everything either orbits it (satellites), or is otherwise controlled by its gravity (Trojans and the like). Another term for it is "dynamical dominance".

How exactly do you think we're finding exoplanets? In most cases, it's either by gravitational influences on the star or by the occlusion of the star as the planet passes in front of it.

Both of these require the planet to be pretty massive. Which is why we've found so many more massive gas giant planets than the paltry few Earth-scale planets we've found. It's much, much easier to find very big planets, ie, the things with the least chance that they haven't gobbled up or dominated everything smaller in their orbit.
 
Nyvis said:
No. By itself, simply orbiting the sun and being in hydrostatic equilibrium leaves us with a large amount of bodies most people don't care about and do not want to hear of as planets.
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And why does this matter? I don't believe science is about being a popularity contest and sorting the universe in a way people like. It's about describing and explaining reality as it is.

So I guess you could set up arbitrary rules to just get roughly the planets you want, but ultimately it would be a bit arbitrary. And the clearing the orbit one always was clearly a bit arbitrary. If one just removes that rule, one immediately starts getting a more physically useful delineation of objects.
 
Gravitas Hunt said:
Most bodies with enough influence to be detected from light-years away are probably fine to call planets. I could be completely mistaken though; it would fascinating to work out which bodies in our solar system could be detected at interstellar distances with our current technology
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We mainly detect gas giants and super Earths. Both of which we should probably assume to be planets, even within the current definition relying on orbit clearing.

We may have detected the first exo moon. If so, it would be a gas giant one, and definitely not a planet by most definitions, despite being much bigger than Earth.

As for detectable bodies in the solar system? Mostly Jupiter, Saturn, Neptune and Uranus, unless you are very close or very lucky.

Quickshot0 said:
And why does this matter? I don't believe science is about being a popularity contest and sorting the universe in a way people like. It's about describing and explaining reality as it is.
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This is not about science, this is about semantics. Science doesn't care one bit if you call something a planet or a dwarf planet. Popularization of science does. And it should try to meet people's expectations rather than force them to change when it can afford to.

Quickshot0 said:
So I guess you could set up arbitrary rules to just get roughly the planets you want, but ultimately it would be a bit arbitrary. And the clearing the orbit one always was clearly a bit arbitrary. If one just removes that rule, one immediately starts getting a more physically useful delineation of objects.
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Every rule is arbitrary because "planet" is an arbitrary concept, like most pieces of nonscientific language. What we are trying to do is give it a meaning people who use the term can agree on. Yes, the definition is more precise if you remove that rule, but it is also completely alien to anyone already using that word. It would be fine if you were defining a new, science exclusive word, but this isn't the case. The only result you will obtain is having science use a term completely mismatched with popular understanding, and this will just create more barriers to understanding and popularization.
 
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While you have a bit of a point, I feel you are pushing the point a bit to far.

For one planets is a concept that has changed quite a bit over time as people learnt more about the heavens, showing they've been quite willing to make rather large changes to it over time, so long as it improved ones understanding on what things in the heavens actually were.

For a second, while definitions are arbitrary in a sense, it's preferred if they're at least some what logical break points. That way you are at least describing something somewhat real instead of making it truly completely arbitrary.


So while one prefers that objects in the end still cover the original meaning so far as possible, this has never been a good excuse to not modify the matter if a better understanding could be had from it. We once called many of the things in the sea fishes, yet in the end we carved entire groups off the 'fish' group because living in the sea was less meaningful then 'came from a certain branch of life'; and thus how now aquatic mammals are still thus mammal. Just like how ice was once a more limited concept that we expanded to cover many many many ices.


As such I fail to see the issue with expanding the term planet in a logical way that actually agrees with many a common persons intuition on such super large objects. It certainly doesn't match common intuition that planets lose the status planet, just because due to planetary dynamics two of the gas giants decided to start swapping place. (A phenomenon that does happen at times) So sure, having a more broad category for planet might swamp out the original planets a bit, but so what? One can always give them a tag of being historical planets found in ancient times, or some other kind of subcategory. There is no need to maintain the list of ancient worlds exactly the same.
 
Quickshot0 said:
For one planets is a concept that has changed quite a bit over time as people learnt more about the heavens, showing they've been quite willing to make rather large changes to it over time, so long as it improved ones understanding on what things in the heavens actually were.
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For most of that time, it was mostly a scientific matter though. For the average person without a telescope or access to scientific books, it was "that slightly brighter spot in the sky".

Quickshot0 said:
For a second, while definitions are arbitrary in a sense, it's preferred if they're at least some what logical break points. That way you are at least describing something somewhat real instead of making it truly completely arbitrary.
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I don't think the cleared orbit rule is "completely arbitrary". That would be giving an exhaustive list of what is a planet, and that's not happening.


Quickshot0 said:
So while one prefers that objects in the end still cover the original meaning so far as possible, this has never been a good excuse to not modify the matter if a better understanding could be had from it. We once called many of the things in the sea fishes, yet in the end we carved entire groups off the 'fish' group because living in the sea was less meaningful then 'came from a certain branch of life'; and thus how now aquatic mammals are still thus mammal. Just like how ice was once a more limited concept that we expanded to cover many many many ices.
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The first example is more similar to removing Pluto from the list than anything. The second is acceptable because it doesn't change the current understanding of ice, it just reuses the word for new things. Ice doesn't suddenly include things that weren't ices, like liquid water. It just expands to include things that behave similarly enough but are parallel to it rather than in the same space.

Quickshot0 said:
As such I fail to see the issue with expanding the term planet in a logical way that actually agrees with many a common persons intuition on such super large objects. It certainly doesn't match common intuition that planets lose the status planet, just because due to planetary dynamics two of the gas giants decided to start swapping place. (A phenomenon that does happen at times) So sure, having a more broad category for planet might swamp out the original planets a bit, but so what? One can always give them a tag of being historical planets found in ancient times, or some other kind of subcategory. There is no need to maintain the list of ancient worlds exactly the same.
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Creating a tag to replace the definition you just broke is pointless work. Just create another tag for the ones you wanted to include as planet, and use that, rather than make everyone else change their definition for you.

As for the orbital change, people already answered that. the rule about clearing your orbit isn't about whether it is currently clear, it is about whether you will clear it. Those swapping gas giants will move to stabilize into clean orbits again at some point.


Let's be frank. I don't love that definition. But I don't think saying everything is a planet is the answer.
 
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Nyvis said:
Creating a tag to replace the definition you just broke is pointless work. Just create another tag for the ones you wanted to include as planet, and use that, rather than make everyone else change their definition for you.

As for the orbital change, people already answered that. the rule about clearing your orbit isn't about whether it is currently clear, it is about whether you will clear it. Those swapping gas giants will move to stabilize into clean orbits again at some point.


Let's be frank. I don't love that definition. But I don't think saying everything is a planet is the answer.
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I think you kind of misunderstood my point of view, which is that I'd prefer to do with planets as we did with ices. The new definition doesn't break the old one, it's just a super set of it. Thus the currently agreed planets are just like water ice for me. But I thus also consider there to many other variants.

And the orbital change one might not collapse in such a case, but it would still cause a crazy issue when for instance a gas giant captured say a super earth as a moon. Suddenly the super earth not being a planet would continue to be weird. And one can continue to imagine further issues with the definition, up to and including, if a gas giant is ejected in to deep space due to orbital interactions over a long time, now it suddenly isn't a planet any more.

Basically the orbital clearing idea has quite a series of problematic issues that crop up. I've been far from exhaustive, but I believe some astronomers have been more so and they had double digit numbers of potential problems.


Also the alternative idea was not to call everything a planet, just a certain mass range. Which would typically cover all objects that would be showing variety of changes due to their sheer mass. Like the obvious being round, but also differentiation of materials and at least at one time active geology. Things your random rocks in space do not have.

Personally I don't see the issue with it really, why not call all things ice-like an ice? That's what we did in the past with other such things, so why not now?
 
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Quickshot0 said:
I think you kind of misunderstood my point of view, which is that I'd prefer to do with planets as we did with ices. The new definition doesn't break the old one, it's just a super set of it. Thus the currently agreed planets are just like water ice for me. But I thus also consider there to many other variants.
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Again, this is not comparable, because it does break the old one. You are skewing the metaphor to fit your view. This is not saying all things ice-like is ice, this is saying liquid water is ice. You aren't extending the definition to things with parallel behaviour but different composition, you are outright dropping key parts of the definitions needed to keep it consistent around the initial objects it was used for.

Quickshot0 said:
And the orbital change one might not collapse in such a case, but it would still cause a crazy issue when for instance a gas giant captured say a super earth as a moon. Suddenly the super earth not being a planet would continue to be weird. And one can continue to imagine further issues with the definition, up to and including, if a gas giant is ejected in to deep space due to orbital interactions over a long time, now it suddenly isn't a planet any more.
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Wait, no, that's ridiculous. It's a moon. Of course it's not a planet. I get doubt of the cleared orbit rule because it is somewhat vague and arbitrary, but orbits the sun is pretty clear. And necessary. Moons do not behave like planets.

Basically, the definition you want is "body in hydrostatic equilibrium". But I really don't see the point, because those objects have little in common in their behaviour.
 
Nyvis said:
Basically, the definition you want is "body in hydrostatic equilibrium". But I really don't see the point, because those objects have little in common in their behaviour.
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I disagree, I find the opposite. Their behavior I think is defined by what they are in general. These large objects all have a behavior of becoming spherical, differentiating and having at least at some time an active geology. Historically speaking one could pretty much thus recognize a planet at a glance if one was close enough to it. Or even from a distance it was recognized based on mass. Which is how they in fact determined what they called a planet in the past. If it was to small/light then it was an asteroid, else a planet.

And this definition was useful as it clearly defined between reasonably distinct types of objects.


The behavior you meanwhile attribute as important to it is so far I can tell but an arbitrary one, it's just a location. It says very little about the object itself. In this for instance you dismissed how a planet became a moon saying it's a moon. But even random tiny rocks can be called a moon, despite being really different from a major terrestrial sized world. Also this doesn't clear away the consistency problem of if the object first orbited separately in its own cleared orbit and then later on becomes captured. (Such captures can happen) As in, you now have what was once a clear planet even by the definition you were following and now it has become a not-planet, moon. This is not the kind of consistent behavior most people expect from planets. Where just swapping a position can demote something to not planet being. Just like it doesn't feel very consistent that a world that once had its cleared orbit and was a planet gets ejected from the system eventually and now is not a planet? Also not expected naming.

One can even see this is not the expected naming scheme for things as for instance planets in deep space are still called planets, specifically 'rogue planets'.


Basically the clearing orbit definition is the definition that actually started eliminating things from being a planet, despite them being considered a planet before. It is the one that is declaring things to not be ice any more, despite commonly being considered ice before.

I thus emphatically disagree with your presentation of it being me who is dropping vital parts of something being a planet, I think I've captured reasonably well how planets have been considered in the past and thus there is nothing wrong with how I presented it.
 
A planet is a big round thing wot's out in SPACE. I subscribe to orkish definitions, under which a sufficiently large ball of garbage tossed into space is, technically, a planet.
 
Quickshot0 said:
I disagree, I find the opposite. Their behavior I think is defined by what they are in general. These large objects all have a behavior of becoming spherical, differentiating and having at least at some time an active geology. Historically speaking one could pretty much thus recognize a planet at a glance if one was close enough to it. Or even from a distance it was recognized based on mass. Which is how they in fact determined what they called a planet in the past. If it was to small/light then it was an asteroid, else a planet.
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Orbit is far from meaningless. It's the first thing we observed about planets, way before we could know about their composition or shape. When something is so small on astronomic scales it is basically just a dot, how it moves is the most important piece of information you have on it.

You seem to oscillate between attacking the whole definition, including something as simple as orbiting the sun, and pointing out small inconsistencies in the orbital clearing rule. I have no problem refining it or replacing it with something with a similar purpose. What I find ridiculous is the idea that we can make planets something they have never been, and expect the gap in language it would create between scientists and laymen to go smoothly.
 
Nyvis said:
Orbit is far from meaningless. It's the first thing we observed about planets, way before we could know about their composition or shape. When something is so small on astronomic scales it is basically just a dot, how it moves is the most important piece of information you have on it.

You seem to oscillate between attacking the whole definition, including something as simple as orbiting the sun, and pointing out small inconsistencies in the orbital clearing rule. I have no problem refining it or replacing it with something with a similar purpose. What I find ridiculous is the idea that we can make planets something they have never been, and expect the gap in language it would create between scientists and laymen to go smoothly.
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Certainly the first thing we observed of them was that they were wandering stars, yes. There is no denying that is true.

I'm more dubious of your claim on my position being something it's never been. To my knowledge it's roughly how planetary scientists have handled the matter for quite awhile and still do. That doesn't sound like never to me and there lots of the public take their directions in things like this from the experts, I'm kind of dubious it's not a usage the public knew of either.


In the end though I guess we find each others position ridiculous then. As we seem to fundamentally disagree on how planets have been viewed and thus should be viewed. I thus also don't agree there would be a gap between the layman or scientist either, rather my definition is easier I believe as the IAU definition includes it and adds the orbital clearing portion.* Thus it's less complicated and as such easier for them to grasp, most people can instinctively handle objects of a certain size range and shape having a particular name. So it seems to me this would lead to better agreement between everyone.




* And in a recent discussion with some one over planets they asked about that, as they'd partially forgotten and not quite understood that part of it
 
Quickshot0 said:
In the end though I guess we find each others position ridiculous then. As we seem to fundamentally disagree on how planets have been viewed and thus should be viewed. I thus also don't agree there would be a gap between the layman or scientist either, rather my definition is easier I believe as the IAU definition includes it and adds the orbital clearing portion.* Thus it's less complicated and as such easier for them to grasp, most people can instinctively handle objects of a certain size range and shape having a particular name. So it seems to me this would lead to better agreement between everyone.
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First, being simpler doesn't mean the change would be easy to accept. It would be a good argument if it was a new definition, rather than replacing an existing one. Second, what the definition was before the orbit clearing rule was added is meaningless because it wasn't properly applied. No non scientist talked about any of the small specks beyond Pluto being a planet. Pluto was only ever considered a planet because we had no idea how many roughly similar objects there was out there.
 
Nyvis said:
First, being simpler doesn't mean the change would be easy to accept. It would be a good argument if it was a new definition, rather than replacing an existing one. Second, what the definition was before the orbit clearing rule was added is meaningless because it wasn't properly applied. No non scientist talked about any of the small specks beyond Pluto being a planet. Pluto was only ever considered a planet because we had no idea how many roughly similar objects there was out there.
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Number of objects is a terrible argument to make for something to be a planet or not, kind of sounds like a very arbitrary way of deciding things. Rather then you know, on actual properties. If anything that sounds almost like an argument against this definition being a good idea.

Secondly, your argument seems in part to be that the definition will help reduce the difference between scientists use of what a planet is and the general public. But instead it does the opposite, because so far I understood recent research on it, planetary scientists still do not use it and never have. Which means instead that the gap in definition is now growing wider and wider. And the same term is now increasingly being used differently between both groups. Significant as planetary scientists make plenty of articles where they use planet in their way and not the IAU definition. And thus their definition unlike your claim most definitely does have reach in parts of the general public, as you can see in my disagreement with you here as well.


Thus we are now in a situation is which confusion is increasing. Due to what is basically an arbitrary rule on what is a planet or not, that no planetary scientist used before that and few if any after. And that had no existence so far I know before the IAU created that definition in contravention of the planetary scientists advice. And thus did not exist in such a form in the public mind before. (As it didn't exist) *

This does not strike me as a good outcome at all. It seems more like a situation where disagreement will continue to increase and confusion thus will become worse yet. As now the expert group and the definition group the public uses for guidance on the matter are now in open disagreement with each other.



*And thus previously the public obviously used an ad hoc point of view on what planets were, as they basically copied what the planetary scientists told them was one. As can be seen quite clearly with Pluto generally being seen as a planet before the IAU meeting.
 
Honestly, the only definitions I care about are the ones where Pluto still counts as a planet
 
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