Quote from: llanitedave on 06/16/2015 08:40 pmThe 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.
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.
Quote from: ChrisWilson68 on 06/17/2015 03:49 amQuote from: llanitedave on 06/16/2015 08:40 pmThe 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.
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?
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.
Quote from: Star One on 06/21/2015 11:29 pmHeavens 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.
Quote from: Blackstar on 06/22/2015 12:36 amQuote from: Star One on 06/21/2015 11:29 pmHeavens 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.
Quote from: Star One on 06/21/2015 11:29 pmHeavens 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.
Who knows, maybe a century from now people will be talking about the 150 planets in our solar system... or the seven planets.
Quote from: Blackstar on 06/22/2015 12:36 amQuote from: Star One on 06/21/2015 11:29 pmHeavens 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_moonsIt would be fairly easy to make a spreadsheet / other timeline with that info.
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?
Quote from: Nomadd 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?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.
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.
Quote from: Star One on 06/25/2015 03:43 pmI 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?!