Author Topic: Pluto-Planet debate discussions  (Read 61452 times)

Online llanitedave

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Re: Pluto-Planet debate discussions
« Reply #20 on: 06/17/2015 08:17 PM »
The problem I have with the current IAU definition has nothing to do with Pluto per se, it's just a poor definition.  It classifies an object based on characteristics that are not intrinsic to that object.

Under this definition, whether an object is a planet or not depends largely on the happenstance of its age and orbital position.  Were Earth in the Kuiper belt, it would not fit the IAU's definition of "Planet".  Further, "clearing out the zone" takes time, and it takes longer in the outer solar system than the inner.  So there is some bright day when an object becomes a planet, which is not the same day that an object in a different orbit becomes a planet, even if they are otherwise identical.

Bottom line, this is not a "genetic" classification, it is an interim classification of political convenience.  It adds nothing to the knowledge base, nor does it help us when it comes to classifying bodies in other star systems.  That's why I've been promoting this definition every chance I get.  It removes all the weaknesses of the current, hastily applied system and provides a real basis for a natural and extensible planetary taxonomy.

Abstract:
A mass-based definition for planets is proposed with dynamical circumstances and compositional characteristics used to define types of planets. Dynamical planet classes include Principal planets, Belt planets, Moons, and Rogue planets. Compositional classes include rock, ice, and gas planets with refined classes when sufficient data is available. The dynamical and compositional definitions are combined with a six class planetary mass scale into a taxonomy that can be used to classify both Solar System and extrasolar planets. This taxonomy can be applied to spherical sub-stellar mass bodies with masses ranging from 3.75 x 10^19 kg to 8.08 x 10^28 kg.

I think the IAU definition is pretty bad too, but I don't agree that there's any particular reason to ignore a body's orbit and only look at "intrinsic" properties.

A chunk of iron in orbit around the Sun is an asteroid but sitting on the surface of the Earth it's just a chunk of iron.  I see no reason to reject that definition of asteroid.  Similarly, I don't see any reason to require that we count moons as planets, as this proposed definition would.

In looking for a good definition of planet, we shouldn't be looking to change the well-established meaning of the word, just to establish clear and consistent rules for the cases where the definition is not already well-established.  That's one of the big problems with the IAU definition, and it's one of the big ones with this proposed re-definition of the word "planet" to include moons.
I don't see much issue with the classification of moons into a subcategory of planetary bodies, if they meet the other criteria.  In a scheme that's supposed to be extendable to other planetary systems, we're very likely to encounter double-planet scenarios, where both co-orbiting bodies are very close in mass, composition, and size to one another.  In gradations of double-planethood, where do we draw the line as to whether the companion body is a planet or merely a satellite?  The taxonomy presented above gives us that line clearly.  In a double-planet system, both bodies are planets, as the least massive of the pair becomes smaller relative to the primary, then it is subclassified as a Moon, yet it remains a planet as it meets the other criteria.  It's actually a conceptually simple and elegant system, and as long as the debate is playing out, I think it deserves to be considered.
« Last Edit: 06/17/2015 08:18 PM by llanitedave »
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Offline ChrisWilson68

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Re: Pluto-Planet debate discussions
« Reply #21 on: 06/17/2015 08:48 PM »
The problem I have with the current IAU definition has nothing to do with Pluto per se, it's just a poor definition.  It classifies an object based on characteristics that are not intrinsic to that object.

Under this definition, whether an object is a planet or not depends largely on the happenstance of its age and orbital position.  Were Earth in the Kuiper belt, it would not fit the IAU's definition of "Planet".  Further, "clearing out the zone" takes time, and it takes longer in the outer solar system than the inner.  So there is some bright day when an object becomes a planet, which is not the same day that an object in a different orbit becomes a planet, even if they are otherwise identical.

Bottom line, this is not a "genetic" classification, it is an interim classification of political convenience.  It adds nothing to the knowledge base, nor does it help us when it comes to classifying bodies in other star systems.  That's why I've been promoting this definition every chance I get.  It removes all the weaknesses of the current, hastily applied system and provides a real basis for a natural and extensible planetary taxonomy.

Abstract:
A mass-based definition for planets is proposed with dynamical circumstances and compositional characteristics used to define types of planets. Dynamical planet classes include Principal planets, Belt planets, Moons, and Rogue planets. Compositional classes include rock, ice, and gas planets with refined classes when sufficient data is available. The dynamical and compositional definitions are combined with a six class planetary mass scale into a taxonomy that can be used to classify both Solar System and extrasolar planets. This taxonomy can be applied to spherical sub-stellar mass bodies with masses ranging from 3.75 x 10^19 kg to 8.08 x 10^28 kg.

I think the IAU definition is pretty bad too, but I don't agree that there's any particular reason to ignore a body's orbit and only look at "intrinsic" properties.

A chunk of iron in orbit around the Sun is an asteroid but sitting on the surface of the Earth it's just a chunk of iron.  I see no reason to reject that definition of asteroid.  Similarly, I don't see any reason to require that we count moons as planets, as this proposed definition would.

In looking for a good definition of planet, we shouldn't be looking to change the well-established meaning of the word, just to establish clear and consistent rules for the cases where the definition is not already well-established.  That's one of the big problems with the IAU definition, and it's one of the big ones with this proposed re-definition of the word "planet" to include moons.
I don't see much issue with the classification of moons into a subcategory of planetary bodies, if they meet the other criteria.  In a scheme that's supposed to be extendable to other planetary systems, we're very likely to encounter double-planet scenarios, where both co-orbiting bodies are very close in mass, composition, and size to one another.  In gradations of double-planethood, where do we draw the line as to whether the companion body is a planet or merely a satellite?  The taxonomy presented above gives us that line clearly.  In a double-planet system, both bodies are planets, as the least massive of the pair becomes smaller relative to the primary, then it is subclassified as a Moon, yet it remains a planet as it meets the other criteria.  It's actually a conceptually simple and elegant system, and as long as the debate is playing out, I think it deserves to be considered.

That would be fine if we had never seen a planet or moon before and we were just making up the words to use for them for the first time.  But that's not the case.

The words "planet" and "moon" have been used for centuries -- in English with those words and in other languages with other words.  For certain bodies, whether they are a planet or a moon has been established for hundreds of years, and not just among astronomers but among the public at large.

We now need to extend those definitions to new bodies we are discovering.  It's reasonable to debate how best to extend them.

What doesn't seem reasonable to me is to decide we're going to change the well-established parts of the definitions and tell billions of people they're wrong about whether that giant white disk they see in the night sky is a planet.  It's also a fool's errand because many people won't accept that.  Then we'll just get a huge amount of confusion if some people adopt your new definition and the rest don't.

Offline meekGee

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Re: Pluto-Planet debate discussions
« Reply #22 on: 06/17/2015 10:07 PM »
The problem I have with the current IAU definition has nothing to do with Pluto per se, it's just a poor definition.  It classifies an object based on characteristics that are not intrinsic to that object.

Under this definition, whether an object is a planet or not depends largely on the happenstance of its age and orbital position.  Were Earth in the Kuiper belt, it would not fit the IAU's definition of "Planet".  Further, "clearing out the zone" takes time, and it takes longer in the outer solar system than the inner.  So there is some bright day when an object becomes a planet, which is not the same day that an object in a different orbit becomes a planet, even if they are otherwise identical.

Bottom line, this is not a "genetic" classification, it is an interim classification of political convenience.  It adds nothing to the knowledge base, nor does it help us when it comes to classifying bodies in other star systems.  That's why I've been promoting this definition every chance I get.  It removes all the weaknesses of the current, hastily applied system and provides a real basis for a natural and extensible planetary taxonomy.

Abstract:
A mass-based definition for planets is proposed with dynamical circumstances and compositional characteristics used to define types of planets. Dynamical planet classes include Principal planets, Belt planets, Moons, and Rogue planets. Compositional classes include rock, ice, and gas planets with refined classes when sufficient data is available. The dynamical and compositional definitions are combined with a six class planetary mass scale into a taxonomy that can be used to classify both Solar System and extrasolar planets. This taxonomy can be applied to spherical sub-stellar mass bodies with masses ranging from 3.75 x 10^19 kg to 8.08 x 10^28 kg.

I think the IAU definition is pretty bad too, but I don't agree that there's any particular reason to ignore a body's orbit and only look at "intrinsic" properties.

A chunk of iron in orbit around the Sun is an asteroid but sitting on the surface of the Earth it's just a chunk of iron.  I see no reason to reject that definition of asteroid.  Similarly, I don't see any reason to require that we count moons as planets, as this proposed definition would.

In looking for a good definition of planet, we shouldn't be looking to change the well-established meaning of the word, just to establish clear and consistent rules for the cases where the definition is not already well-established.  That's one of the big problems with the IAU definition, and it's one of the big ones with this proposed re-definition of the word "planet" to include moons.

Exactly.  Context means a LOT.  And not just current context, but history as well.

----

The point is - the old definition that lumped Pluto with the rest of the planets but ignore the rest of the Kuiper Belt is not "problematic", or "ill defined"...  It is simply 100% wrong.  "There are 9 Planets" is a false statement no matter what your definition is.

So moving forward, one needs to recognize Pluto, Erin and the other KBOs as a subclass, and then either include or exclude the bunch of them, as a whole, from the "planet" category.
« Last Edit: 06/17/2015 10:07 PM by meekGee »
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Online llanitedave

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Re: Pluto-Planet debate discussions
« Reply #23 on: 06/18/2015 01:19 AM »

That would be fine if we had never seen a planet or moon before and we were just making up the words to use for them for the first time.  But that's not the case.

The words "planet" and "moon" have been used for centuries -- in English with those words and in other languages with other words.  For certain bodies, whether they are a planet or a moon has been established for hundreds of years, and not just among astronomers but among the public at large.

We now need to extend those definitions to new bodies we are discovering.  It's reasonable to debate how best to extend them.

What doesn't seem reasonable to me is to decide we're going to change the well-established parts of the definitions and tell billions of people they're wrong about whether that giant white disk they see in the night sky is a planet.  It's also a fool's errand because many people won't accept that.  Then we'll just get a huge amount of confusion if some people adopt your new definition and the rest don't.

Understood.  Although that white disk in the sky, under the proposed system, would still be called "The Moon".  And it would still be classified as "The Earth's Moon."  It would be a moon.  Which would now be a subclass of "planet."

The IAU, BYW, already seems to have changed "well-established parts of the definitions", even though it left the previously classified planets intact.  It just redefined "planet" so that only those 8 bodies would qualify.  That's not a scientific definition by any stretch.

And while I'm flattered, in a way, that you refer to the proposed definition as mine, I can't claim it as such.  I like it, I think it's the best I've seen, and I promote it, but it was not my idea.  David G. Russell is not even an acquaintance, an amateur researcher who has done a careful job in putting together a system that is logical and extensible. I've communicated with him only to the extent of expressing support for his idea.  The extensible part was what sold me, because when I first learned that the IAU was going to work on a formal definition of "planet" back in 2006, I was extremely enthusiastic, thinking we'd get a tool that would help us in exoplanet research.  We really did -- and do -- need a universal taxonomy.  When we got little more than a reclassification of Pluto, my disappointment was deep.

What Mr. Russell has done is bypass the politics and get back to first principles, and I think his work has given superior results.
« Last Edit: 06/18/2015 01:20 AM by llanitedave »
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Offline the_other_Doug

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Re: Pluto-Planet debate discussions
« Reply #24 on: 06/18/2015 02:25 AM »
How about a very, very, very, very failed star?

Where do we draw the line and rename Jupiter as a failed star then re-label the solar system as a binary system?

Just to toss it in, the proper term for a failed star is a brown dwarf (a starlike body too small to be able to sustain hydrogen fusion), and Jupiter is quite a bit smaller than a brown dwarf -- although I admit, there is a pretty spirited debate out there now as to where a gas giant leaves off and a brown dwarf starts up.  Myself, I think it's where the body can sustain any kind of fusion at its core -- probably starting with deuterium fusion.  That's at about 13 Jupiter masses, according to the latest theories I've looked at.
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Offline Superstring

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Re: Pluto-Planet debate discussions
« Reply #25 on: 06/18/2015 01:53 PM »
Eventually, we will need a rigorous taxonomy for exoplanets, similar to what Russell's paper proposes. For the time being, however, it strikes me as a bit too complicated to explain to the general public.

Stars, planets, moons, asteroids, and comets are all -- whether we like it or not -- universally accepted terms for space objects. Moreover, the following three statements are centuries-old maxims: the Sun is a star, the Earth is a planet, and the Moon is a moon. Any new definition should not radically alter what these terms mean, nor deviate from these maxims.

With that in mind, here's my proposed classification scheme. It's rough and flexible, and there is room for expanding as we learn more about exosolar systems.

PLANET: A gravitationally rounded object that does not (and never did) sustain fusion, and orbits a star

Our solar system can be divided into 3 broad categories of planets:

TERRESTRIAL PLANETS: Rocky, dense, and dominate their orbits close in to the Sun (Mercury, Venus, Earth, Mars)

GIANT PLANETS: Gaseous, massive, and dominate their orbits (Jupiter, Saturn, Uranus, Neptune)

DWARF PLANETS: Mostly icy, smaller, and tend to orbit in populations and in dynamic resonances (Ceres, Pluto, Makemake, Haumea, Quaoar, 2007OR10, Orcus, Eris, Sedna, and probably dozens/hundreds more)

Regarding the other types of objects in our solar system and likely others:

ASTEROIDS: Small objects that have not been gravitationally rounded, orbit a star, can be rocky or icy but do not grow a tail, many of which orbit in populations (Main Belt, Kuiper Belt)

COMETS: Small objects that have not been gravitationally rounded, grow a tail at some point in their orbits, and tend to take hyperbolic or parabolic orbits around a star

MOONS: Gravitationally rounded objects that orbit a planet (Luna, Io, Europa, Ganymede, Callisto, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus, Miranda, Ariel, Umbriel, Titania, Oberon, Triton)

MOONLETS: Small objects that have not been gravitationally rounded and orbit a planet/asteroid/comet (Phobos, Deimos, Dactyl, Amalthea, Janus, Hyperion, Phoebe, Nix, Hydra, hundreds more)


With this simple scheme, there is room to further group planets by composition (rock terrestrials, gas giants, ice giants, ice dwarfs, etc). One could even call moons by those terms -- Titan could be called an ice terrestrial, Enceladus an ice dwarf. These classifications are also applicable to exosolar systems, and additional terms can be used (superterrestrial planets, supergiant planets, etc). Planets that have been ejected from their system are technically not planets, but I'm fine with the popular term "rogue planet," just as I am with "escaped inmate."
« Last Edit: 06/18/2015 01:57 PM by Superstring »

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #26 on: 06/21/2015 11:29 PM »
Heavens knows what the IAU would do if they ever do find a further large earth sized or super-earth sized object out at about 200AU than has been perturbing the orbits of those trans-neptunian objects.

Offline Blackstar

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Re: Pluto-Planet debate discussions
« Reply #27 on: 06/22/2015 12:36 AM »
Heavens knows what the IAU would do if they ever do find a further large earth sized or super-earth sized object out at about 200AU than has been perturbing the orbits of those trans-neptunian objects.

I'll confess that I think that the current definition is sloppy and rather arbitrary. But if new data comes along, then we can fully expect the IAU to revisit their definition. After all, it was new data that prompted the last revision.

There must be a timeline somewhere that shows the number of known planets over time. That number has gone up and down based both on discoveries and changes in definition/new data. Who knows, maybe a century from now people will be talking about the 150 planets in our solar system... or the seven planets.

Offline Mr. Scott

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Re: Pluto-Planet debate discussions
« Reply #28 on: 06/22/2015 01:04 AM »
I don't understand why there is such a focus on how small Pluto is.  Pluto is a planet (which in Greek means 'wanderer').

If somebody started talking about how small Texas is, first you would be given a weird look.  Second, the next sentence would start with the word "Son..."

Size matters, and people don't mess with Texas!  If all previous measurements are accurate (about Texas), pro-Dwarfers should realize Pluto is significantly bigger than Texas.

Absolutely do not mess with Pluto!!!

« Last Edit: 06/22/2015 05:36 AM by Mr. Scott »

Offline redliox

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Re: Pluto-Planet debate discussions
« Reply #29 on: 06/22/2015 08:47 AM »
Heavens knows what the IAU would do if they ever do find a further large earth sized or super-earth sized object out at about 200AU than has been perturbing the orbits of those trans-neptunian objects.

I'll confess that I think that the current definition is sloppy and rather arbitrary. But if new data comes along, then we can fully expect the IAU to revisit their definition. After all, it was new data that prompted the last revision.

The only thing I can agree with the IAU regarding Pluto (and Eris likewise) is the need to set a definite minimum limit for planethood.  I certainly agree that if Pluto gets readmitted, Eris fairly deserves admission too.  There is a modest trend to note thus far with Kuiper objects: nothing quite the same size as Eris or Pluto has been found, and the next largest contender, Makemake (I think at least), is roughly 1500 km wide versus ~2250 for Eris and Pluto.  Every other Kuiper (or Oort if you count Sedna as a member from there) object is decidedly below the 2K kilometer diameter.  With the "gravitational influence clearing neighborhood" classification being a weak definition, I think a definition based on diameter could reasonably state 2000 kilometers as the minimum of planethood (or if you truly want to take a cheap shot in Pluto's favor, say 2200 so 'ol Pluto still skates past the line).

As far as orbits and other factors go...look to exoplanets: they've already found planets that orbit or spin backwards or in resonance orbits with neighbors.  There are BOUND to be NUMEROUS EXCEPTIONS to what is seemingly standard for the Solar System (*cough* hotjupiter *cough* blindsidedtheorists *cough*).  For us, Pluto and Eris...or perhaps the super-earth allegedly lurking between the Kuiper Belt and Oort Cloud...are our modest exception.
« Last Edit: 06/22/2015 08:48 AM by redliox »
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Offline Owlon

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Re: Pluto-Planet debate discussions
« Reply #30 on: 06/23/2015 08:32 AM »
Heavens knows what the IAU would do if they ever do find a further large earth sized or super-earth sized object out at about 200AU than has been perturbing the orbits of those trans-neptunian objects.

I'll confess that I think that the current definition is sloppy and rather arbitrary. But if new data comes along, then we can fully expect the IAU to revisit their definition. After all, it was new data that prompted the last revision.

The only thing I can agree with the IAU regarding Pluto (and Eris likewise) is the need to set a definite minimum limit for planethood.  I certainly agree that if Pluto gets readmitted, Eris fairly deserves admission too.  There is a modest trend to note thus far with Kuiper objects: nothing quite the same size as Eris or Pluto has been found, and the next largest contender, Makemake (I think at least), is roughly 1500 km wide versus ~2250 for Eris and Pluto.  Every other Kuiper (or Oort if you count Sedna as a member from there) object is decidedly below the 2K kilometer diameter.  With the "gravitational influence clearing neighborhood" classification being a weak definition, I think a definition based on diameter could reasonably state 2000 kilometers as the minimum of planethood (or if you truly want to take a cheap shot in Pluto's favor, say 2200 so 'ol Pluto still skates past the line).

As far as orbits and other factors go...look to exoplanets: they've already found planets that orbit or spin backwards or in resonance orbits with neighbors.  There are BOUND to be NUMEROUS EXCEPTIONS to what is seemingly standard for the Solar System (*cough* hotjupiter *cough* blindsidedtheorists *cough*).  For us, Pluto and Eris...or perhaps the super-earth allegedly lurking between the Kuiper Belt and Oort Cloud...are our modest exception.

I think if you're going to make a size-based cutoff on the definition of a planet, it should definitely relate to hydrostatic equilibrium and not a largely arbitrary diameter. To me, there are four major categories in regards to the size of celestial objects: irregular bodies like comets and asteroids; solid bodies in hydrostatic equilibrium like Earth, Mars, Ceres, etc; bodies with a massive atmosphere and no clear surface like Jupiter, Saturn, Uranus, and Neptune; and bodies large enough to trigger nuclear fusion--brown dwarfs and all larger stars. Each of these, of course, has numerous subcategories and blurry lines.

I would personally lean towards three broad types of planets: massive surfaceless planets corresponding to gas and ice giants like Jupiter and Neptune; rocky or icy planets like Earth that are small enough to have a definite surface; and small (or, you know... "dwarf") planets that are large enough to achieve hydrostatic equilibrium but not massive enough to gravitationally dominate their local region of a system. If I had to narrow it down to two categories, I would lump terrestrial planets in with the dwarf planets and leave gas giants as their own separate category.
« Last Edit: 06/23/2015 08:33 AM by Owlon »

Offline MattMason

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Re: Pluto-Planet debate discussions
« Reply #31 on: 06/23/2015 04:58 PM »
It's an oldie but it sums up my opinions about this issue.

Uh, anyone interested in that space probe thingie that's going to Pluto? Just curious. :)
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Re: Pluto-Planet debate discussions
« Reply #32 on: 06/23/2015 05:59 PM »
Heavens knows what the IAU would do if they ever do find a further large earth sized or super-earth sized object out at about 200AU than has been perturbing the orbits of those trans-neptunian objects.

I'll confess that I think that the current definition is sloppy and rather arbitrary. But if new data comes along, then we can fully expect the IAU to revisit their definition. After all, it was new data that prompted the last revision.

There must be a timeline somewhere that shows the number of known planets over time. That number has gone up and down based both on discoveries and changes in definition/new data. Who knows, maybe a century from now people will be talking about the 150 planets in our solar system... or the seven planets.

Known since prehistoric times: 5 planets (Mercury, Venus, Mars, Jupiter, Saturn) and one Moon (ours).

Skip to Galileo, Kepler, et al. (1600s) and the introduction of the telescope. Discovery of 4 Moons around Jupiter, realization that Earth is also a planet.

Saturn's moon Titan was discovered in 1655.

Uranus, sighted several times prior, discovered to be a planet in 1781.

ceres, discovered in 1801, was considered to be a planet for nearly 50 years until a dozen or so more asteroids were discovered.

Neptune, also sighted several times piror (possibly even by Galileo), discovered to be a planet in 1846.

Things pile up rapidly after this.

*note*
I found a much better and more comprehensive list on wikipedia.

https://en.wikipedia.org/wiki/Timeline_of_discovery_of_Solar_System_planets_and_their_moons

It would be fairly easy to make a spreadsheet / other timeline with that info.
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Offline ChrisWilson68

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Re: Pluto-Planet debate discussions
« Reply #33 on: 06/23/2015 06:19 PM »
Who knows, maybe a century from now people will be talking about the 150 planets in our solar system... or the seven planets.

Or the people of Sedna will be at war to be recognized as a planet so they can be admitted as full members to the League of Planets.

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Re: Pluto-Planet debate discussions
« Reply #34 on: 06/23/2015 06:25 PM »
Heavens knows what the IAU would do if they ever do find a further large earth sized or super-earth sized object out at about 200AU than has been perturbing the orbits of those trans-neptunian objects.

I'll confess that I think that the current definition is sloppy and rather arbitrary. But if new data comes along, then we can fully expect the IAU to revisit their definition. After all, it was new data that prompted the last revision.

There must be a timeline somewhere that shows the number of known planets over time. That number has gone up and down based both on discoveries and changes in definition/new data. Who knows, maybe a century from now people will be talking about the 150 planets in our solar system... or the seven planets.

Known since prehistoric times: 5 planets (Mercury, Venus, Mars, Jupiter, Saturn) and one Moon (ours).

Skip to Galileo, Kepler, et al. (1600s) and the introduction of the telescope. Discovery of 4 Moons around Jupiter, realization that Earth is also a planet.

Saturn's moon Titan was discovered in 1655.

Uranus, sighted several times prior, discovered to be a planet in 1781.

ceres, discovered in 1801, was considered to be a planet for nearly 50 years until a dozen or so more asteroids were discovered.

Neptune, also sighted several times piror (possibly even by Galileo), discovered to be a planet in 1846.

Things pile up rapidly after this.

*note*
I found a much better and more comprehensive list on wikipedia.

https://en.wikipedia.org/wiki/Timeline_of_discovery_of_Solar_System_planets_and_their_moons

It would be fairly easy to make a spreadsheet / other timeline with that info.

The IAU decision was fixed to keep the number of planets down to the "historical" planets. Pluto was demoted the same way as Ceres, so from a historic point of view, that was the correct decision if you are going down that route.

Personally, I prefer the hydrostatic equilibrium definition for planets. If it is round, it is a planet. If it is too small to be round, it is an asteroid/comet/rock/dust. If it is orbiting a planet, it is a moon.

Offline Nomadd

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Re: Pluto-Planet debate discussions
« Reply #35 on: 06/23/2015 07:50 PM »
 I don't see any of these standards defining Charon. What qualification does Pluto meet that makes it a planet or a dwarf that Charon doesn't?

Offline NovaSilisko

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Re: Pluto-Planet debate discussions
« Reply #36 on: 06/23/2015 08:29 PM »
I don't see any of these standards defining Charon. What qualification does Pluto meet that makes it a planet or a dwarf that Charon doesn't?

There really aren't any. IIRC, the IAU pretty much just arbitrarily declared that Charon would be considered a moon of Pluto despite how significant of a fraction of Pluto's mass it is and the fact the barycenter is above Pluto's surface. They seem to have taken no interest in any of the double planet/binary dwarf planet concepts.

Offline baldusi

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Re: Pluto-Planet debate discussions
« Reply #37 on: 06/23/2015 10:14 PM »
I don't see any of these standards defining Charon. What qualification does Pluto meet that makes it a planet or a dwarf that Charon doesn't?

There really aren't any. IIRC, the IAU pretty much just arbitrarily declared that Charon would be considered a moon of Pluto despite how significant of a fraction of Pluto's mass it is and the fact the barycenter is above Pluto's surface. They seem to have taken no interest in any of the double planet/binary dwarf planet concepts.
For now. After New Horizons and Dawn, and may be after JWST, they will have a ridiculous amount of extra data to debate what to call a planet.

Offline Star One

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Re: Pluto-Planet debate discussions
« Reply #38 on: 06/25/2015 03:43 PM »
I personally think the debate will recede if further planets in between the Keiper belt and Oort Cloud are discovered as it kind of becomes irrelevant then.

Offline Blackstar

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Re: Pluto-Planet debate discussions
« Reply #39 on: 06/25/2015 09:44 PM »
I personally think the debate will recede if further planets in between the Keiper belt and Oort Cloud are discovered as it kind of becomes irrelevant then.

Er, define "planets" to be discovered there.

Mike Brown has said--I think somewhat tongue-in-cheek--that he would not be surprised if a Mars-sized object is found way out in the Kuiper Belt. I think it is certainly possible to find something as big as, if not bigger than Pluto. But if something significantly larger than Pluto is found, it's going to really force a reevaluation of the definition of "planet."

But wouldn't that be f'ing great?!

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