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Is Pluto a planet?


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Is Pluto a planet?  

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  1. 1. Is Pluto a Planet?

    • Yes
      23
    • No
      43


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Add me to the "we should have dozens of planets" caucus.

Ceres was a planet before Pluto was.

A planet should be defined by its shape and size, not by where it happens to be. By current definition a rogue gas giant could not be a planet because it's impossible to clear a hyperbolic orbit.

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20 hours ago, RCgothic said:

I think the *has cleared its orbit* criteria is rubbish.  If you transported Pluto to the inner solar system it would be a planet.

That doesn't seem like a good definition to me.

I agree. If you transported Mercury to the outer solar system it would cease to be a planet.

Then again if you transported Earth to an orbit of Jupiter it would also cease to be a planet and I expect we are all okay with that.

Pluto, Eris, Makemake, Haumea, and other TNOs should not be planets because their orbits are dominated by the influence of Neptune. Ceres and Hektor should not be considered planets because their orbits are dominated by the influence of Jupiter (and, in the latter case, because Hektor is not large enough to be gravitationally rounded). I think we should define planets based on their relationship to other solar system bodies, not based on a function of their size and semi-major orbital axis.

Spherical bodies in orbits with high eccentricity or high inclination like Sedna and Biden could be classified as "irregular planets".

9 hours ago, K^2 said:

P.S. I have one final argument. If we say that Moon/Luna is a planet, humanity is technically an interplanetary civilization.

I can get behind this.

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5 hours ago, Bill Phil said:

I'd call it a planet. I believe we should use properties of the body in question and not external properties. Essentially, "clearing an orbit" doesn't really make sense for defining something.

Trouble with using "properties of the body in question" is that we end up with a bunch of planets orbiting Jupiter and Saturn.

But I agree that "clearing an orbit" is a hackneyed test, because it has the same problem. If you apply Jean-Luc Margot's planetary discriminant Π to a moon like Ganymede, it receives a score of 3.2 making it indisputably a planet (though less of a planet than Mars and much less of a planet than Mercury). Even Titan receives a Π score of 1.4, so it is a planet too.

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20 minutes ago, sevenperforce said:

Trouble with using "properties of the body in question" is that we end up with a bunch of planets orbiting Jupiter and Saturn.

But I agree that "clearing an orbit" is a hackneyed test, because it has the same problem. If you apply Jean-Luc Margot's planetary discriminant Π to a moon like Ganymede, it receives a score of 3.2 making it indisputably a planet (though less of a planet than Mars and much less of a planet than Mercury). Even Titan receives a Π score of 1.4, so it is a planet too.

I don't have a problem with calling the Galilean moons planets. Though they would generally be referred to as moons (since they are moons) but I don't see why planet wouldn't be acceptable if it's physically accurate.

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7 hours ago, RCgothic said:

By current definition a rogue gas giant could not be a planet because it's impossible to clear a hyperbolic orbit.

That's not a problem, it's a rogue planet. That's a perfect name for it, just like "dwarf planet" suits a planet that is not gravitationally significant in its region. The only issue is the lack of a mathematical definition for that gravitational criterion.

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the only fundamental difference between a moon and a planet is ultimately what it orbits around. 

so lets take my proposed planet hierarchy:

in orbit of a stellar object:

giant planets - high gravitational influence across multiple orbits or the whole solar system. jupiter for example. 
standard planets - gravitational influence over their own orbit only. earth for example.
dwarf planets - not gravitationally dominant over its own orbit. ceres.
planetoids - large objects that dont meet hydrostatic equilibrium requirement. eros.
asteroids - small rocks in space

in orbit of a sub-stellar object (anything listed above):

giant moons - high gravitational influence over the moon system. the moon, ganymede.
standard moons - dominant over local orbit only. includes shepherd moons in ring systems. tethys, mimas. 
moonoids/dwarf moons - captured asteroids, shepherd moons and other objects in ring systems. also anything that isnt in hydrostatic equilibrium. phobos.

i couldnt really think of a major difference between dwarf moons and moonoids. so they got merged. i wanted to separate it between round and not round, but there is a lot of crossover and thought it was kind of arbitrary. moons in shared orbits tend to not be round. 

any of these can be considered binary if the barycenter is outside of the gravitationally dominants body. in this case pluto and charon would be a binary dwarf planet. charon would not be considered a moon in this case.

im not sure how i would define moons of a binary planet system or deal with trinary planet systems (if they in fact exist). i figure any third object would need to contribute significantly to the barycenter to promote it to a trinary. say you place a truncated cone such the biggest object is at the big end and the small object is at the small end, if the 3rd body puts the barycenter inside that cone, then it is a moon, otherwise its a sibling planet. not sure how you would determine if an object in a trinary object system is a moon or not. but they have not been observed and we could buck that problem down the line until we do. pluto-charon moons therefore remain moons. 

binary+ moons would in theory be possible, but since one has not been observed to the best of my knowledge, its only theoretical. sub-moons might also be a thing, but they usually aren't in stable orbits and could be called transient objects. if we do find evidence of stable sub moons then we could figure those out at that time.

Edited by Nuke
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2 hours ago, sevenperforce said:

Pluto, Eris, Makemake, Haumea, and other TNOs should not be planets because their orbits are dominated by the influence of Neptune.

Then there is no planet in the Solar System but Jupiter.

Also, what if Jupiter and Saturn were a double system? They would stop being planets?

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56 minutes ago, kerbiloid said:

I can add another shovel.

We are talking about the Solar System.

But what if a planet is orbiting in a double star system?
What if it has cleared its orbit only around one of the star components.
Is it a planet?

Then it's a half-dwarf

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22 hours ago, Bill Phil said:

I don't have a problem with calling the Galilean moons planets. Though they would generally be referred to as moons (since they are moons) but I don't see why planet wouldn't be acceptable if it's physically accurate.

I'd call them worlds but not planets.

  • Star. Any body large enough to sustain nuclear fusion.
  • Stellar Remnant. Any body which was formerly a star.
  • World. Any gravitationally-rounded body with a chemically solid surface.
  • Planet. Any gravitationally-rounded body which is not a star or stellar remnant, provided it does not orbit a barycentre inside another planet.
    • Rogue Planet. A planet which does not orbit a star or stellar remnant.
    • Giant Planet. A planet too large to have a solid surface.
    • Planet Binary. Two planets which orbit a common barycentre.
    • Major Planet. A planet or a planet binary which is not rogue and dominates all objects which share its stellar orbit (sharing its orbit means crossing its orbit but not the orbit of another major planet).
    • Irregular Planet. A planet or a planet binary which is not rogue and has an eccentricity greater than 1/3 or an inclination more than 45 degrees from the invariable plane of the system.
    • Dwarf Planet.  A planet or a planet binary which is neither a rogue planet, nor a major planet, nor an irregular planet.
  • Moon. Any body which orbits a barycentre inside a planet.
    • Major moon. A moon that is also a world.
    • Dwarf moon. A moon that is not a world.
  • Comet. Any body which passes close enough to a star to undergo an outgassing cycle.
  • Asteroid. Any body, other than a comet or a moon, which is too small to be a planet.
    • Trojan asteroid. An asteroid which is gravitationally bound to a Lagrange point of a planet.
    • Irregular asteroid. An asteroid which crosses the orbit of a major planet.
    • Rogue asteroid. An asteroid which does not orbit a star or stellar remnant.
    • Regular asteroid. An asteroid which is not a trojan, irregular, or rogue asteroid.

That should just about do it.

Also the word "planet" no longer feels real.

Edited by sevenperforce
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34 minutes ago, sevenperforce said:

I'd call them worlds but not planets.

  • Star. Any body large enough to sustain nuclear fusion.
  • Stellar Remnant. Any body which was formerly a star.
  • World. Any gravitationally-rounded body with a chemically solid surface.
  • Planet. Any gravitationally-rounded body which is not a star, provided it does not orbit a barycentre inside another planet.
    • Rogue Planet. A planet which does not orbit a star or stellar remnant.
    • Giant Planet. A planet too large to have a solid surface.
    • Planet Binary. Two planets which orbit a common barycentre.
    • Major Planet. A planet or a planet binary which is not rogue and dominates all objects which share its stellar orbit (sharing its orbit means crossing its orbit but not the orbit of another major planet).
    • Irregular Planet. A planet or a planet binary which is not rogue and has an eccentricity greater than 1/3 or an inclination more than 45 degrees from the invariable plane of the system.
    • Dwarf Planet.  A planet or a planet binary which is neither a rogue planet, nor a major planet, nor an irregular planet.
  • Moon. Any body which orbits a barycentre inside a planet.
    • Major moon. A moon that is also a world.
    • Dwarf moon. A moon that is not a world.
  • Comet. Any body which passes close enough to a star to undergo an outgassing cycle.
  • Asteroid. Any body, other than a comet or a moon, which is too small to be a planet.
    • Trojan asteroid. An asteroid which is gravitationally bound to a Lagrange point of a planet.
    • Irregular asteroid. An asteroid which crosses the orbit of a major planet.
    • Rogue asteroid. An asteroid which does not orbit a star or stellar remnant.
    • Regular asteroid. An asteroid which is not a trojan, irregular, or rogue asteroid.

That should just about do it.

Also the word "planet" no longer feels real.

Well 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.

As for Pluto-Charon, I'd call it a double planet, along with other bodies orbiting the Sun that are non-fusing and in hydrostatic equilibrium being planets. 

My problem with the IAU definition is that dwarf-planet is a distinct class from planet, and not a sub-class. But it should be other way around - major planets being ones that dominate their orbit and dwarf planets being ones that don't, with both falling under the planet category. The IAU definition is just weird. I feel as if they just really wanted to make Pluto not a planet, and didn't much care for the whole thing being sensible. 

Edited by Bill Phil
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14 hours ago, Deddly said:

I believe that makes Earth a dwarf planet

No. Earth only has to clear its orbit of asteroids to be considered a planet. Same goes for Moon. IAU is perfectly happy with the idea of binary planets. They just use barycenter definition, which I find to be faulty. Primarily, because moons shift in orbit due to tidal interaction. Case in point, Moon is drifting away from Earth, and barycenter of the Moon will end up outside of Earth's surface. So an Earth-Moon system is bound to become a binary planet by IAU definition. And it's not one of these weird, "Oh, something might happen to knock an object out of its orbit, changing its category." 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.

The tug-of-war definition is much better, because you're pretty much guaranteed that it's going to stay consistent for the majority of the system's life. You can still end up with edge cases where orbital drift is going to change the tug-of-war balance, but majority of these are going to be for moons drifting inward, and in these cases the two will eventually become a single planet, so a change of status is unavoidable there.

It's also consistent with origin of the body. Moons that form from planetary proto-disc are going to be tightly bound to the planet they've formed with, and so will be moons by tug-of-war definition. Our Moon has formed from an impact. It's effectively a lithobraking capture. It originated as a planet and ended up in orbit that a naturally formed moon is extremely unlikely to occupy. If it did not have energy and angular momentum during impact to take that high of an orbit, it probably would have merged with Earth producing just some moonlets in lower orbit from the debris. In other words, it's pretty unlikely that a capture that would otherwise be qualified as a planet would end up in a low enough orbit to be a moon by tug-of-war definition.

It doesn't matter a whole lot for Sol. It literally is just a matter of whether we want to rename the Moon, because it won't make a difference to any other celestial body I'm aware of. But consistent system of definitions is important for extrasolar bodies. We are discovering more and more exoplanets out there. Soon we will be discovering exomoons. And estimating planet sizes is tough. Figuring out if potential exomoon's barycenter is within or without the parent exoplanet might be difficult or impossible. And in many cases, the orbits might be elliptical enough for it to vary! The tug-of-war definition is unambiguous and measurable. If we can estimate an orbit a potential exomoon takes around its primary, we can estimate the gravitational pull. And that means we can say with high degree of confidence, or at least, way better than with current definition, whether we're looking at a planet-moon system or a binary planet.

And if we are going to be studying the star systems out there to get better understanding of how they form and evolve, we do need a consistent nomenclature that we can actually apply. And applying it to every star system out there except for Sol would be a little silly. Changing moon definition to be based on relative gravitational pulls of the star and planet is going to provide the least inconsistency. And by that definition what you see in the sky is no moon.

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I propose:

 

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.

 

An asteroid is not gravitationally rounded.

An asteroid orbiting a planet or planetoid is a natural satellite.

 

A planetoid is a transitional form that is somewhat rounded by gravity. 

A minor moon is a planetoid that orbits a planet.

 

Therefore the moon is a major moon that is a planet. Later in its life cycle it will be a binary planet with Earth.

Edited by RCgothic
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45 minutes ago, RCgothic said:

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

This is the current IAU definition, and I've outlined the problems with barycenter definitions just above.

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1 hour 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.

Triton was probably a dwarf planet long ago. Phobos will one day become a ring instead of a moon. The sun will eventually become a red giant instead of a yellow dwarf. Since solar systems change over time, I don't see any problem with changing classifications for objects as they change.

As for binary planets, the one with the smaller mass might be called the moon. Charon is usually called a moon of Pluto when it is identified in isolation. However, when referring to the whole system "Pluto" or "Pluto-Charon" are appropriate. How will we call Rask and Rusk?

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1 minute ago, cubinator said:

How will we call Rask and Rusk?

That's an easy one. Together they are binary planetary system called Risk. I don't care if it's not official, there is objectively no other option.

And yes, point taken on systems evolving, but it's a difference between a significant qualitative change and, "Well, it drifted out, so it's not a moon anymore." 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.

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22 minutes ago, K^2 said:

That's an easy one. Together they are binary planetary system called Risk. I don't care if it's not official, there is objectively no other option.

And yes, point taken on systems evolving, but it's a difference between a significant qualitative change and, "Well, it drifted out, so it's not a moon anymore." 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.

there is also rosk, resk, and the seldom used but very endearing rysk.

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. 

besides a moon is just one bad gravitational interaction from becoming an asteroid, a dwarf planet, or a rogue planet. an object might be multiple things throughout its life. 

Edited by Nuke
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On 10/12/2020 at 3:21 PM, RCgothic said:

No, Pluto is a dwarf planet, or planetoid.

That said, I think the *has cleared its orbit* criteria is rubbish.  If you transported Pluto to the inner solar system it would be a planet.

That doesn't seem like a good definition to me.

That logic works identically for arguing Titan or Europa or Luna should be planets

46 minutes ago, Nuke said:

there is also rosk, resk, and the seldom used but very endearing rysk.

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. 

besides a moon is just one bad gravitational interaction from becoming an asteroid, a dwarf planet, or a rogue planet. an object might be multiple things throughout its life. 

1 hour ago, K^2 said:

That's an easy one. Together they are binary planetary system called Risk. I don't care if it's not official, there is objectively no other option.

And yes, point taken on systems evolving, but it's a difference between a significant qualitative change and, "Well, it drifted out, so it's not a moon anymore." 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.

 

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

 

Edited by sh1pman
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7 minutes 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. Gas giants are just gas giants. “Planet” should be phased out as archaic.

 

categories exist because they're useful. they have a meaning and, by knowing something is in a given category, you know something useful about it. "Planet" has a useful scientific meaning when defined. Saying that it should be gotten rid of in favor of a completely different definition with no justification of why it's obsolete is just, well, stupid. Especially when for most scenarios I can think of, knowing even its vaguest orbital properties is more useful than knowing if it's gaseous or not. And with your logic, why does it even matter if it's rounded or not? A rocky unrounded planet could be more massive than an icy one in equilibrium, and you can do and talk about all of the same things with them as you can a rounded body. That distinction certainly feels more arbitrary than "orbits around another big honking object". So really, "worlds" should just refer to any non-gaseous body, and then I suppose we can use a term for whether they orbit another world and whether they're rounded... how about "moon" and "planet". We're back at the start, and then may as well split off dwarf planets due to the reasons that have been spit out a hundred times. It's really not that hard to remember, unless four terms is too many to keep in mind.

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The definition by the IAU is fairly arbitrary. And what does “clear its orbit” mean? And why does nearly no one call it a double planet — for which there is a reasonably undisputed definition — when it’s such a clear case?

”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.”

Funny argument for scientists called “astronomers,” after “stars.” Will that be the next definition we’re going to tighten? There are billions of those in the galaxy, and following the same logic we can’t have that.

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