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Is Pluto a planet?


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Is Pluto a planet?  

66 members have voted

  1. 1. Is Pluto a Planet?

    • Yes
      23
    • No
      43


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12 minutes ago, Kerbart said:

”We don’t know how many Pluto sized objects there are in the Kuyper belt. Clearly we can’t call those planets if there are hundreds, thousands, perhaps even dozens of those.”

Did someone say that? Was his first name "Straw"? :D

I do not see any reason to think the number of planets in our solar system (or "let's get Pluto!") had anything to do with the definition. Clearly there are plenty of planets outside our solar system, likely more than there are stars, and no one in the IAU has deigned to call them "not planets."

14 minutes ago, Kerbart said:

And what does “clear its orbit” mean?

If you want math, it means:

393f8d0fbedbe9b5b36bb565afb9c492be9d64e4     or     8beb43a6cfe829f564543079add96b603188749d     or     5acee82ef78c953a3a61de38c24b236d7e8d077d

For definitions see https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

(aside)

I don't particularly care if Pluto is a planet, a world, or a flambily-doodle. What I do care about is that "But Pluto's so CUTE can't we make it a planet just because I feel things!" is not a good enough reason to put what the IAU has done in the trash.

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Actually,

planet
/ˈplanɪt/
 
Middle English: from Old French planete, from late Latin planeta, planetes, from Greek planētēs ‘wanderer, planet’, from planan ‘wander’.

 

A "planet" is a star which moves across the sky, unlike other stars.
That's why it's called "planetes", "a wanderer".

There are five of them: Mercury, Venus, Mars, Jupiter, Saturn.
The Sun and the Mun are moving, too, but they are not "planetes", because they give us light from the sky.
The Earth is not a planet, because it's a World. A whole world. We stand on it, and it's covered by the sky, across which the planets are moving.

So, following the most classic definition of "planet", we should call "planet" any moving visible object in the sky, except the stars, the Sun, the Moon, and the comets (because they have tails and preced important events on the Earth).

Telescopes are appropriate, too, like everything what allows us to make proper astrological predictions.
So, the new planets (some call it Uranus, Neptune, and sometimes Pluto) are good, too. As well, several hundred thousands minor planets discovered nowadays (though, it's hard to list their astrological meanings).

***

So, the IAU definition of "planet" has nothing common with what the "planet" is by definition.  It's just taken from head.

While the classic definition clearly follows clear astronomical properties of the "planet", and the "planet" as "a big ball of rock" follows clear physical principles, the IAU for some reason mixes physical and orbital characteristics without any explanation.
So, if Mars gets captured by Jupiter and becomes its satellite, it's not a planet anymore? But if the Jupiter catches Saturn, it's a double planet?

And what if a planet is orbiting around a double star lesser component, which orbits around the greater one? Is it a planet?
And what if the lesser component is a brown dwarf? If we treat brown dwarves as stars, it's probably a planet. But if they are supergiant planets, it's not a planet?

What is "rogue planet"? Unless it's thrown away from the galaxy, it's orbiting around the star (we call it Sgr A*), it's not a rogue.
But as it clearly hasn't cleared its orbit from the galactic arms and other junk, it's not a planet anymore?

So, any usage of orbital characterstics in the definition of "planet" is a full nonsense, just complicating the picture with no visible gain.

As obviously we can't use the historical definition based on astronomy, the "planet", "wanderer" should depend only on physical characteristics of the compact non-star object, moving around anything it likes.
 

Edited by kerbiloid
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6 hours ago, Nuke said:

changing barycenters doesnt seem like a problem if you treat each orbit as circular, but with equivalent net energy as the actual orbit. calculate what the barycenter would be in that situation. that should solve the problem of eccentricity. 

That makes the definition even more arbitrary than it already was.

Tug-of-war is straight forward. Does Moon primarily orbit Earth or Sun? In this case, the Sun. Our Moon's trajectory around the Sun is convex. Unlike every moon in Solar System, it never accelerates away from the Sun, because Sun's gravity is the dominant force on the Moon. Earth's effect is secondary, and qualitatively, Earth's and Moon's trajectories around the Sun look the same. Whereas all the moons of other planets follow a complicated curve with twists as their parent planet pulls them to accelerate away from the Sun.

If you list all the ways in which Moon is like any other planet of Sol, it's going to be a fairly extensive list. The list of similarities to moons is, "There exists another body, such that the barycenter of the two happens to fall inside the later." That's the only thing they have in common. It's entirely artificial and makes for bad categorization.

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18 hours ago, Bill Phil said:

I would only call them planets in certain contexts. They're moons as well, I just think that planet would make more sense to describe certain properties like hydrostatic equilibrium in a non-fusing body. This is because there are processes and properties that can be found on these moons that are similar to what happens on (what are commonly accepted to be) planets, like cryovolcanism. Basically they're planets in the context of planetary science, but moons in the context of astronomy.

That's why making "world" a term in planetary science rather than in astronomy solves the problem. When you talk about visiting new worlds, landing on new worlds, building colonies on new worlds, etc., you're talking about gravitationally-rounded bodies in hydrostatic equilibrium with solid surfaces. Doesn't matter where they are.

Then planet can describe a body's position in the solar system. As the term was originally intended -- planete, describing the wandering motion of the planets known to the Greeks.

18 hours ago, Bill Phil said:

As for Pluto-Charon, I'd call it a double planet

"Double planet" or "planet binary" are equally okay, I suppose. In the lexicon I proposed, the Pluto-Charon system is a dwarf planet binary.

18 hours ago, Bill Phil said:

My problem with the IAU definition is that dwarf-planet is a distinct class from planet, and not a sub-class.

Yes, I agree. I think they wanted to define MVEMJSUN as "planets" when they should have made them a sub-class of planet. It makes much more sense to say "We have major planets and dwarf planets" than to say "We have planets and then we have dwarf planets that are not planets but could be planets if they were in a different place."

17 hours ago, K^2 said:

This is in progress, and I don't think a question of whether Moon is a planet or a moon should be a matter of timing.

Timing is fine; we can only describe things as we observe them. Conversely, I don't think a question of whether Moon is a planet or a moon should be a matter of the system's orbital distance. 

17 hours ago, RCgothic said:

A planet is a gravitationally rounded body.

major moon is a planet that orbits a barycentre inside a non-stellar primary.

binary planet is a pair of planets orbiting a barycentre that is in free space at least some of the time.

So Jupiter and Sol are a binary planet?

15 hours ago, K^2 said:

Worst part is that Moon's orbit is elliptical. So we know that there will be a period in Earth's history, even if we're not around, when you'd have to check your calendar to know if the Moon is a moon today or a planet, because it would depend on whether it's near its apogee or perigee.

Easily dealt with by saying that if the barycentre is ever in free space, it is a binary.

17 hours ago, RCgothic said:

major moon is a planet that orbits a barycentre inside a non-stellar primary.

I don't love saying that planets can be moons. Moons can be worlds but moons should not be planets.

13 hours ago, sh1pman said:

+1 for the idea of World instead of Planet. “Planet” is so confusing it feels arbitrary and unnecessary. Any gravitationally rounded body with a definable solid or liquid surface is a world. Earth, Moon, Mars, Io, Ceres, Pluto are worlds. It shouldn’t matter what they orbit and where, only what kind of object they are.

Thank you. This is my view.

13 hours ago, sh1pman said:

 Gas giants are just gas giants. “Planet” should be phased out as archaic.

Language is descriptive, not prescriptive. We should choose language which is as consistent as possible but which also describes common usage as closely as possible. I think "planet" and "moon" should describe the role that bodies play in orbital dynamics, because that's what aligns most closely with common usage.

When you say "Astronomers have discovered a new moon!" a layperson's first question is usually, "Oh, what planet is it around?" Similarly, if you say "Astronomers have discovered a new planet!" then common lay questions would be "Oh, where is it?" or "Does it have any moons?" The lay usage of "moon" and "planet" align well with treating these terms as part of a hierarchy.

So we should use the term planet, but planets should not be moons.

11 hours ago, Kerbart said:

The definition by the IAU is fairly arbitrary. And what does “clear its orbit” mean?

The proposed planetary discriminants feel like an exercise in extreme ad hoc filtering. "Let's come up with an equation which DEFINITELY excludes Ceres and Pluto but DEFINITELY includes Mars and Mercury." Because reasons.

That's why I prefer "dominates its orbit". Virtually everything that orbits closer to Earth than to Mars or Venus does so in some degree of orbital resonance with Earth. Virtually everything that orbits between Mars and Saturn does so in orbital resonance with Jupiter. What we know about solar system formation suggests that stable solar systems depend on resonance structures, so it makes sense to use this (not some ad hoc planetary discriminant) as the measure.

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11 hours ago, Superfluous J said:

If you want math, it means:

393f8d0fbedbe9b5b36bb565afb9c492be9d64e4     or     8beb43a6cfe829f564543079add96b603188749d     or     5acee82ef78c953a3a61de38c24b236d7e8d077d

For definitions see https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

See also:

Spoiler

curve_fitting.png

I, too, can invent maths that include or exclude portions of a dataset. You could change up the variables such that Pluto is definitely a planet and Mercury is definitely not. It's all ad hoc.

But if you talk about "dominating the orbit" then you are getting somewhere.

There are a number of ways to define domination of an orbit. For example, you could put it this way:

A solar system body dominates its orbit if the total mass of smaller bodies with which it is in resonance is greater than the total mass of non-resonating bodies which cross its orbit.

This means that if a solar system body has a large number of trojans and resonant bodies, it "dominates its orbit" even if there is a lot of other crap that crosses its orbit. Mercury dominates its orbit because it exerts resonant influence over Venus trojans, for example.

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35 minutes ago, sevenperforce said:

That's why making "world" a term in planetary science rather than in astronomy solves the problem. When you talk about visiting new worlds, landing on new worlds, building colonies on new worlds, etc., you're talking about gravitationally-rounded bodies in hydrostatic equilibrium with solid surfaces. Doesn't matter where they are.

Then planet can describe a body's position in the solar system. As the term was originally intended -- planete, describing the wandering motion of the planets known to the Greeks.

"Double planet" or "planet binary" are equally okay, I suppose. In the lexicon I proposed, the Pluto-Charon system is a dwarf planet binary.

Yes, I agree. I think they wanted to define MVEMJSUN as "planets" when they should have made them a sub-class of planet. It makes much more sense to say "We have major planets and dwarf planets" than to say "We have planets and then we have dwarf planets that are not planets but could be planets if they were in a different place."

There are many fields that conflict in their terminology. It's not really a problem. And in this case using the term planet to describe objects that are physically similar and have similar processes would be beneficial. In astronomy the definition could be anything, though I do believe that major and dwarf planets should be a subset of planet. Some merging of astronomy and planetary science might call large moons like Ganymede "planet-like moons" since they're just that large.

Planetes described wandering stars. The original intent was for anything of that sort. Under that intention then asteroids constitute planets as well since even with reasonably powerful telescopes they look like stars.

It's not too big of a deal, and having objects like Pluto (and Ceres, plus a lot more) be counted as planets (though given the modifier minor or dwarf depending on their significance in the system) isn't a problem either.

 

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1 hour ago, sevenperforce said:

See also:

  Hide contents

curve_fitting.png

I, too, can invent maths that include or exclude portions of a dataset. You could change up the variables such that Pluto is definitely a planet and Mercury is definitely not. It's all ad hoc.

But if you talk about "dominating the orbit" then you are getting somewhere.

There are a number of ways to define domination of an orbit. For example, you could put it this way:

A solar system body dominates its orbit if the total mass of smaller bodies with which it is in resonance is greater than the total mass of non-resonating bodies which cross its orbit.

This means that if a solar system body has a large number of trojans and resonant bodies, it "dominates its orbit" even if there is a lot of other crap that crosses its orbit. Mercury dominates its orbit because it exerts resonant influence over Venus trojans, for example.

gravitational influence seems to be the line of thinking in the iau definitions.

hydrostatic equilibrium - enough gravity to be round

planet - enough gravity to be round and to kick other things out of its orbit

you could extend that to include giant planets, as those have enough gravity to be a huge influence in the entire solar system. that would be better and give you 3 distinct sub-categories for planets. all three would be planets though. i think flat out demoting pluto was the easy way out. 

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2 minutes ago, Nuke said:

gravitational influence seems to be the line of thinking in the iau definitions.

hydrostatic equilibrium - enough gravity to be round

planet - enough gravity to be round and to kick other things out of its orbit

Yes, I agree. I just think their "discriminant" is ad hoc. Again, if you apply the discriminant to Ganymede or Titan then they become planets too.

2 minutes ago, Nuke said:

you could extend that to include giant planets, as those have enough gravity to be a huge influence in the entire solar system. that would be better and give you 3 distinct sub-categories for planets. all three would be planets though. i think flat out demoting pluto was the easy way out. 

Well, actually Uranus ends up in a weird category. Jupiter and Saturn work together to influence the orbits of all the inner planets and Neptune controls the orbits of a bunch of TNOs but Uranus exerts very little influence despite being essentially just as big as Neptune.

There are a lot of ways to draw arbitrary lines. Apart from the sun, Jupiter is twice as heavy as every other object in the solar system put together. On that basis, you could say that our solar system is actually a quasi-stellar binary with one star, one protostar, and three giant planets. This would make Ganymede and Mercury both terrestrial planets orbiting close to stellar primaries. 

1 hour ago, Bill Phil said:

There are many fields that conflict in their terminology. It's not really a problem. And in this case using the term planet to describe objects that are physically similar and have similar processes would be beneficial. In astronomy the definition could be anything, though I do believe that major and dwarf planets should be a subset of planet. Some merging of astronomy and planetary science might call large moons like Ganymede "planet-like moons" since they're just that large.

I maintain that we should keep our terminology as close to common usage as possible, while also striving for consistency.

The whole hubbub about Pluto really stems from challenges in public usage. Laypersons have a pretty clear idea of what "planet" and "moon" mean, and so changing the definition just seems weird. We shouldn't pick definitions that confuse the public even further.

Think about dropping your terminology into lay conversations and evaluate whether it would make things less confusing or more confusing. For example, my terminology:

  • "It's been 48 years since man walked on the surface of another world."
  • "So far, we have only been able to land spacecraft on two other planets and two moons."
  • "The year I was born, we completed flybys of every major planet in our solar system."
  • "With the arrival of Dawn at Ceres, we have now placed orbiters around every planet out to Saturn and every world within the inner solar system."
  • "Look how close Jupiter and Saturn are in the sky. You might think you're only staring at two planets, but you're also staring at 161 moons, including 11 worlds humans could one day set foot on."
  • "Earth is the only major terrestrial planet orbited by another world. The moons of the planet Mars are the size of asteroids, and the rest of the moons in our solar system orbit gas giants or distant dwarf planets."

You could make any of those statements to a layperson and they'd be able to follow along without a bunch of explanations or yammering about "planetary discriminants" or "clearing the orbit". Yet every usage is consistent.

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16 minutes ago, sevenperforce said:

if you apply the discriminant to Ganymede or Titan then they become planets too.

Other than that pesky "orbiting a star" part.

If Jupiter and Saturn didn't exist, and Ganymede and Titan were in their place, then Ganymede and Titan would be planets. Seems totally reasonable to me.

A system like that probably exists somewhere in the Galaxy, and those worlds would be planets.

Actually, IMO "moons that would be planets if they replaced their parent and all their parent's other moon's weren't there either" is an important enough distinction to warrant another new term.

Moon-Planets? Planet-Moons?

I suggest we refer to Ganymede and Titan as Ploons.

Edited by Superfluous J
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14 hours ago, Superfluous J said:

What I do care about is that "But Pluto's so CUTE can't we make it a planet just because I feel things!" is not a good enough reason to put what the IAU has done in the trash.

I'm not sure where the obsession comes from to lay words in our mouth that we think Pluto is cute. I'm sure you can provide some quotes to back up that claim; please do. If it helps you to deal with the shock that someone has a different opinion than yours: for most of my life, Pluto has been a planet. It's round, it's "out there" by itself (not in a asteroid belt like some of the bodies in the asteroid belt that are larger than it), it was there before the IAU came up with its definition, and I don't understand why they had to come up with a definition seemingly made to exclude it as a planet. That was a choice, not a scientific driven motivation.

Suppose the IGU would announce that based on their new criteria, Europe and Australia would no longer be consider continents. Would your reply to those who disagree be "stop saying Europe is so CUTE?"

My opinion might differ from the IAU, but please point to the part of OP's post (“Is Pluto a planet?  According to the IAU, no, but what do you think?”) that did not ask for an opinion.

 

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3 minutes ago, Kerbart said:

I'm not sure where the obsession comes from to lay words in our mouth that we think Pluto is cute. I'm sure you can provide some quotes to back up that claim; please do.

https://www.google.com/search?q=pluto+planet+cartoon&tbm=isch&ved=2ahUKEwi5t9vb7LTsAhUHOawKHehLCFUQ2-cCegQIABAA&oq=pluto+planet+cartoon&gs_lcp=CgNpbWcQAzICCAAyAggAMgIIADICCAAyBggAEAUQHjIGCAAQBRAeMgYIABAFEB4yBggAEAUQHjIGCAAQCBAeMgYIABAIEB5Q5h1YlDBgyjFoAHAAeACAAWeIAaoFkgEDNy4xmAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=s1WHX_mRMYfysAXol6GoBQ&bih=978&biw=1920&client=firefox-b-1-d&safe=off

From the 2nd line.

pluto-not-a-planet.jpg

If you're talking about you in particular, or someone else in particular, I can only say I didn't intend to put words in your mouth. I was explaining my own main issue with the "Pluto is a planet" crowd.

5 minutes ago, Kerbart said:

Suppose the IGU would announce that based on their new criteria, Europe and Australia would no longer be consider continents. Would your reply to those who disagree be "stop saying Europe is so CUTE?"

Actually from any definition of Continent other than "we drew the lines this way" Europe isn't a continent. But that's a different story. Australia's a harder sell but it definitely falls either in "Large island" or "Small continent." And yes, if people drew comics like the above with Europe crying and Africa taking North America aside to tell them to not play with Europe, I'd tell them to stop because they were being silly.

I also don't have any problem with people having a problem with the IAU's decision based on the merits of the decision. I don't agree with you on that one and yes, I don't have a problem with that either. I don't agree with a lot of people on a lot of things.

I guess maybe if we went with "What benefit to the language would we get calling Pluto a planet, instead of a dwarf planet?" instead of "should Pluto be a planet?" we'd get further. I don't see any benefit AT ALL to calling Pluto a planet but not all other dwarf planets. I also see no benefit to calling all dwarf planets, planets. So from where I stand, the language is better now.

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33 minutes ago, Superfluous J said:

Other than that pesky "orbiting a star" part.

Technically they do orbit a star.

34 minutes ago, Superfluous J said:

Actually, IMO "moons that would be planets if they replaced their parent and all their parent's other moon's weren't there either" is an important enough distinction to warrant another new term.

Moon-Planets? Planet-Moons?

What about "planets that would be moons if they were placed in orbit around the largest planet in their system"?

11 minutes ago, Kerbart said:

for most of my life, Pluto has been a planet. It's round, it's "out there" by itself (not in a asteroid belt like some of the bodies in the asteroid belt that are larger than it)

Nothing in the asteroid belt is larger than Pluto. Pluto masses more than everything in the asteroid belt put together.

But more importantly, it's not "out there" by itself. It's out there with at least 892 objects in the same region of space. 482 of those actually share its exact same orbital period, and several of those are also "round" in the commonly-understood sense.

6 minutes ago, Superfluous J said:

I also see no benefit to calling all dwarf planets, planets.

That seems like saying "I see no benefit to calling all miniature horses, horses."

Miniature horses are horses. It's right there in the name.

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17 minutes ago, sevenperforce said:

Technically they do orbit a star.

Where is that defined in that way?

17 minutes ago, sevenperforce said:

What about "planets that would be moons if they were placed in orbit around the largest planet in their system"?

I don't see a use in the language for that. But if say Jupiter was where Earth was, and Earth was orbiting it (and the sun too apparently) then there is a useful distinction there. As is there a useful distinction between Ganymede and - say - Amalthea

25 minutes ago, sevenperforce said:

That seems like saying "I see no benefit to calling all miniature horses, horses."

Miniature horses are horses. It's right there in the name.

That's a great example of how poor use of language can cause confusion. I don't actually know if miniature horses are horses or not. I assume they are. I'm not so sure of hedgehogs and wombats though. I don't think they're hogs or bats.

Sadly I wasn't consulted on any of these names, up to and including Dwarf Planet. I prefer Plutoid. You know exactly what that is in context.

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1 hour ago, sevenperforce said:

That seems like saying "I see no benefit to calling all miniature horses, horses."

Miniature horses are horses. It's right there in the name.

this isn't how loveing language works. This is the worst argument and everybody who makes it should feel embarassed

 

And filter, believe me, this is me being polite 

Edited by NFUN
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5 hours ago, sevenperforce said:

Easily dealt with by saying that if the barycentre is ever in free space, it is a binary.

You keep making the definition more and more complex. "A body is a moon if and only if there exists another body qualifying as planet or dwarf planet, such that the barycenter of the two is on a quasistable trajectory entirely contained within the surface boundary of the later." This sounds like a theorem from convex geometry. When your definitions for categories start to sound like that, you have a bad taxonomy. All just to try and squeeze the Moon into a definition of the moon. It's not helpful.

Compare to a clean taxonomy. An object is a planet if it meets current criteria of a planet and is pulled by the primary star(s) stronger than by any other object. Done.

Most importantly, as I've indicated earlier, the above is something we can easily measure for exoplanets and exomoons once we detect them. In contrast, measuring sizes of exoplanets is very, very hard. And in many cases, estimating barycenter precisely enough to know whether it falls within an exoplanet or not is going to be impossible.

Think about it, the first extrasolar planetary systems we are going to detect are going to involve rather large objects. Perfect candidates for being double planets. Under current definition with barycenter we won't be able to classify them as moon or double planet, because we simply won't be able to tell with enough precision where barycenter falls. If we go with tug-of-war definition, we won't need to. We'll be able to catalogue them as double exoplanets or exomoons as appropriate.

The case with the Moon is the same as with Pluto. Taxonomy should be useful for studying the world around us, not just give things convenient labels everyone is used to. For all the reasons IAU pulled the trigger and made Pluto not a planet, they should update definitions for moons and make Moon a planet.

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2 hours ago, Superfluous J said:

I don't see a use in the language for that. But if say Jupiter was where Earth was, and Earth was orbiting it (and the sun too apparently) then there is a useful distinction there. As is there a useful distinction between Ganymede and - say - Amalthea

I've seen planemo (planetary mass object) tossed around for anything that would otherwise be considered a planet but isn't due to not orbiting a star. Could also be construed as a portmanteau of planet and moon.

50 minutes ago, NFUN said:

this isn't how loveing language works. This is the worst argument and everybody who makes it should feel embarassed

 

And filter, believe me, this is me being polite 

For that example, sure. But when it comes to planets all we're doing is using external factors to distinguish between planet and dwarf planet. The actual physical processes on the bodies involved though have enough similarities that it's questionable to make dwarf planet a completely separate category of object.

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3 hours ago, Bill Phil said:

I've seen planemo (planetary mass object) tossed around for anything that would otherwise be considered a planet but isn't due to not orbiting a star. Could also be construed as a portmanteau of planet and moon.

... .

NEXT UP: Argument over distinguishing between a Chonky Gaseous Planemo and a Slender Brown Dwarf. Is temperature everything when you're cold as hell?  Does size really matter?  And what is all of this about wind speed and unladen Swallows? 

 

 

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Every moon orbits the Sun (in/on/by/?...) epicycloid trajectory.

And no IAU definition prohibits that. There is no word "elliptic" there.

'Oumuamua orbits the Sun (in/on/by/?...) hyperbolic trajectory.
(Yes, it's not recurrent, but so what?)
And there is no celestial body in Solar System, whose orbit is cleaner.
If it was enough massive to get hydrostatically shaped, by IAU definition it should be a solsystem planet, too.

Jupiter is not a planet, as it hasn't cleaned its orbit from Trojans. It's a dwarf planet.

Upd.
Btw. Jupiter.
We actually don't know if it's hydrostatically shaped. We can see just its atmosphere, but not the surface, and as we could read several months ago, it's core is fuzzy.
So, it's probably a dwarf planet even twice.

Edited by kerbiloid
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