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On the classifications of planets

Quickshot0 said:
So I guess you could set up arbitrary rules to just get roughly the planets you want, but ultimately it would be a bit arbitrary. And the clearing the orbit one always was clearly a bit arbitrary. If one just removes that rule, one immediately starts getting a more physically useful delineation of objects.
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I'm not sure about that, going from isolated planets in the solar system to swarms of similar sized plutinos in the Kuiper Belt seems like a reasonably meaningful division to me. More generally I would expect any protoplanetary disk to have an denser inner segment that aggregates into distinct planets with well-separated orbits, and an outer fringe rarefied enough that you have populations of objects at similar orbital distances.

Quickshot0 said:
For a second, while definitions are arbitrary in a sense, it's preferred if they're at least some what logical break points. That way you are at least describing something somewhat real instead of making it truly completely arbitrary.
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The division above is a pretty informal description of a logical breakpoint. I'm not sure how exactly to translate it to a technical definition but it sounds like "clearing its orbit" is probably fairly close.

Edit: Among the dwarf planets Haumea is a particularly odd one. It's the only body I know of which is probably in hydrostatic equilibrium with a non-spheroidal shape.
 
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thuealing said:
I'm not sure about that, going from isolated planets in the solar system to swarms of similar sized plutinos in the Kuiper Belt seems like a reasonably meaningful division to me.
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Sure... but why is it so important that there are lots of a particular class of world somewhere? You could as easily split this apart in to the Oligarchs and those that aren't then, at least that would be referencing planetary formation science then. Then you'd be actually discussing formation history, rather then just where they happened up. After all, planets can get a bit mixed up in location over time.
 
Kitsune said:
There is the firm possibility of additional planets that are the size of Pluto or larger out there as well.
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And things known, like (225088) 2007 OR10, which are big enough to be planets by that definition ((225088) 2007 OR10 is bigger than Haumea by some estimates) but not yet on the official lists.

I for one eagerly look forward to the day schoolchildren have to memorize a list containing the name (225088) 2007 OR10, and seeing them try to work that into an acronym mnemonic.
 
I still don't know why they'd have to learn them. At a certain point you'd just split parts off as not that important to know, they're pretty far away and small anyway. Not like people learn all the stars around us either.
 
Quickshot0 said:
I still don't know why they'd have to learn them. At a certain point you'd just split parts off as not that important to know, they're pretty far away and small anyway. Not like people learn all the stars around us either.
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Perhaps you should firm up that "split off parts as not important" into some kind of technical definition? Maybe split it off where you start having bunches of similarly-sized planets in overlapping orbits? Sounds familiar

Edit: I think Eris is the best example of a dwarf planet, it's distinctly heavier than Pluto, if not quite as fluffy.
 
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Planets are things that:

1. Can be seen by the naked eye from Earth AND
2. Are not in fixed positions as the Earth rotates AND
3. Move across the sky in predictable patterns AND
4. Aren't the Sun

5. OR are the Earth

Therefore the list of planets is Mercury, Venus, Earth, the Moon, Mars, Jupiter, and Saturn.
 
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foamy said:
Planets are things that:

1. Can be seen by the naked eye from Earth
2. Are not in fixed positions as the Earth rotates
3. Move across the sky in predictable patterns
4. Earth

Therefore the list of planets is Mercury, Venus, Earth, the Moon, Mars, Jupiter, and Saturn.
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this is perfect, not a thing wrong with it :D
 
Kaiya said:
this is perfect, not a thing wrong with it :D
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Tweaked it slightly to more clearly represent what I meant, but hey, I figure antiquity got it pretty right! My one substitution is taking out the Sun for the Earth, as the Earth would meet every other qualification if we were not in fact standing on its surface, and the Sun is more coherently classified as a star for basically every purpose.
 
foamy said:
Tweaked it slightly to more clearly represent what I meant, but hey, I figure antiquity got it pretty right! My one substitution is taking out the Sun for the Earth, as the Earth would meet every other qualification if we were not in fact standing on its surface, and the Sun is more coherently classified as a star for basically every purpose.
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I posit that the Sun is the only true planet because it is the biggest. Earth is one of the Sun's moons. Fight me!
 
foamy said:
Therefore the list of planets is Mercury, Venus, Earth, the Moon, Mars, Jupiter, and Saturn.
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With my naked eyes the only "planet" under 1-4 is the moon :p

Otherwise, isn't ISS a planet too?

With good eyes Uranus can definitely be seen, it's just dim and slow enough that it wasn't recognized as a planet without the aid of telescopes. It's also nice to cut things off at the end of naming by mythological ancestry, at Jupiter-Saturn-Uranus. It's not polite to have planets named after an Olympian and a Titan without including a primordial deity too.
 
I find that I agree, like, empathize or sympathize with all the posts.

Let me ask...

When is a apple not a apple? A apple on it's tree is apple. Apple in a box is apple. Put it in storage, it is a apple. Cut it up and put it on a plate for a snack it is a apple, a cut up apple but it is a apple.

So.. Jupiter. A planet? A failed star? A brown dwarf star?

If Jupiter is a planet, then is it not a planet if it is a fee roaming or rogue planet? How about if it is in a comet like orbit? If it is in the Kuiper belt or the Oort Cloud? Is it still Jupiter? Is it still a planet?

There was a big (more or less big) story about how a 'comet' was found in the asteroid belt. Is that 'comet' really a asteroid or is it a comet that is in the asteroid belt?
 
Those are not all unclear.

Frogs321 said:
So.. Jupiter. A planet? A failed star? A brown dwarf star?
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No fusion rules out any kind of "star" except maybe "failed".

Frogs321 said:
If Jupiter is a planet, then is it not a planet if it is a fee roaming or rogue planet?
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That's one of the more confusing cases, but especially confusing if something of that small size formed on it's own without a parent star. (not sure if it's actually possible for such a small perturbation to collapse by itself, I think Jupiter is believed too small, but smaller than brown dwarf should be possible).

Frogs321 said:
If it is in the Kuiper belt or the Oort Cloud?
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Jupiter is probably around 1000x as massive as the whole Kuiper belt, so yes, still a planet even in that neighborhood.
If you imagine a belt with many Jupiter-sized planets I don't know, but that may not be possible in natural systems.

I'm not sure if it's actually accounted for in "cleared orbit", but I think a definition of planet that takes into account the neighborhood should probably also allow multiple planets sharing similar orbits if they are strongly interacting and only stable by precise coreography.
 
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Quickshot0 said:
While you have a bit of a point, I feel you are pushing the point a bit to far.

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

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


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


As such I fail to see the issue with expanding the term planet in a logical way that actually agrees with many a common persons intuition on such super large objects. It certainly doesn't match common intuition that planets lose the status planet, just because due to planetary dynamics two of the gas giants decided to start swapping place. (A phenomenon that does happen at times) So sure, having a more broad category for planet might swamp out the original planets a bit, but so what? One can always give them a tag of being historical planets found in ancient times, or some other kind of subcategory. There is no need to maintain the list of ancient worlds exactly the same.
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Scientifically dolphins are fish because any non-paraphyletic grouping of fish has to include all tetrapods. What changed is the the non-scientific meaning of the word was slightly modified to 'things in what that have gills' from 'animals that live in the water'. But more to the point scientific definitions get changed all the time because it makes thing easier to deal with the public or even to deal with the past literature. Triceratops should have been synonymized into Torosaurus but that would make things a horrible mess. The very concept of what is a dinosaur was turned on its head so the very definition of what is a dinosaur was changed to keep Sauropods and all the other Saurischia dinosaurs. The same thing happens in biology with the most common research animal Drosophila melanogaster needing to have its genus name changed and everyone going 'ehhh, lets not'. The names we give to things are to make it easier to communicate and there are plenty of times science changed things to keep confusion to a minimum.
 
Tasrill said:
Scientifically dolphins are fish because any non-paraphyletic grouping of fish has to include all tetrapods. What changed is the the non-scientific meaning of the word was slightly modified to 'things in what that have gills' from 'animals that live in the water'. But more to the point scientific definitions get changed all the time because it makes thing easier to deal with the public or even to deal with the past literature. Triceratops should have been synonymized into Torosaurus but that would make things a horrible mess. The very concept of what is a dinosaur was turned on its head so the very definition of what is a dinosaur was changed to keep Sauropods and all the other Saurischia dinosaurs. The same thing happens in biology with the most common research animal Drosophila melanogaster needing to have its genus name changed and everyone going 'ehhh, lets not'. The names we give to things are to make it easier to communicate and there are plenty of times science changed things to keep confusion to a minimum.
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Even then, fish are a a paraphyletic group. There are fish (lobbed fin fish) that are closer related to humans than ray finned fishes. Some scientists consider us to be fish, in fact.
 
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