The Astronomy Thread

[Mr Bojangles]

hi hi

Speaking of grandfathering things in, we're still going to have to explain to children hundreds of years down the line why the solar system has a completely different classification system for planets than any other system in the universe.

Wouldn't we just apply our classification system to other solar systems? Or are you saying that you know that we're using a classification system that other sapient life doesn't? In the future, no less.

Are you some sort of time-traveling alien, icekatze??

[Razor One]

I dunno. If we grandfather it in, then we have to keep explaining it to children hundreds of years down the line, including those born on other worlds and around other stars, and we Terrans would end up looking like sentimental nitwits in the process.

At which point we explain the story of how Pluto was discovered using the power of mathematics, science and observation, how it was named, and the important astronomers who contributed to its discovery. The reason we call it a planet? The same reason we call certain hills on Earth mountains and why it varies from country to country, there is no strict definition. It's consideration as a planet comes as part and parcel of being the first object of its kind to be discovered and in recognition of the historical and cultural impact it had and of the imaginations it captured, it's something that the other KBO's, as momentous as their ongoing discoveries are, simply don't really share withe the foremost member of their population.

This would of course apply just as well to our descendants living in far flung star systems centuries down the road too. If they feel strongly enough that a decently sized object in their system should be called a planet for whatever reason, then perhaps they too can share the story of why and how it came to be designated as such.

[icekatze]

hi hi

The official definition of "planet" used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets. As of April 2011, the only defining statement issued by the IAU that pertains to exoplanets is a working definition issued in 2001 and modified in 2003. That definition contains the following criteria:

Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.

Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.

Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

Categorizing something does not inherently add information. Science is about observing, not in deciding what the universe is, either by dictate or vote. People still haven't come to a consensus about whether Europe and Asia are separate continents, or if Australia is a continent or a large island, and scientifically, it doesn't matter.

They should have asked linguists to come up with definitions, because linguists at least can understand that words are allowed to have multiple definitions.

[Namaphry]

Pluto is what it is. Twentieth century cultural baggage doesn't make it more significant than Eris, Haumea, or Makemake, not any more than centuries of revisionism makes Christoforo Columbo more significant than all the explorers of the Americas that came before and after him. There's a lot of sentiment invested, but it's not from my generation.

'It was important to us, so it should be important to you' is a really bad attitude to have toward education, it can be used to justify almost anything. When our understanding changes, we need to review where we afford our reverence. Otherwise, we're resisting the change, trying to hold on to the way we thought things were rather than grasping the way things are.

Of course, there's a lot of subjectivity involved in discerning 'the way things are' and we still don't know everything. Still, it's pretty obvious that we were missing information when Pluto was discovered, and for many years afterward. 'The Search for Planet X' was based on an inaccurate estimate of the mass of Neptune. Nothing like that ever fit into the cosmic scheme. Pluto was searched for and discovered with overinflated expectations, and yet most people never realized it didn't meet them, or that the premise itself was false.

It reminds me of a story I heard, that an overambitious explorer called the island north of Montreal 'Lachine' because he thought he'd found China. This isn't quite true. He was overambitious, and searching for a route to China, but his land was called 'Lachine' as a joke at his expense, when he failed.

For Pluto, I'd say we're missing an admission that it's not 'Planet X', and that there never was one. But if we can call an island on the St. Lawrence River 'Lachine', I can understand calling an ice dwarf in a trans-Neptunian orbit 'the 9th planet'. It just seems a bit mean, from this perspective.

[icekatze]

hi hi

Twentieth century cultural baggage doesn't make any of the planets significant, but significance itself is completely subjective to begin with.

Why is the Earth even considered a planet? Planet means "wanderer," and was derived from the points of light that wandered through the night sky, rather than staying in place like stars. But the Earth doesn't wander through the night sky when people look up at night. Talk about cultural baggage, trying to impose astronomical significance on an object just because it's the place where someone lives, or because it isn't the center of the universe anymore.

What sort of cultural baggage do you suppose the term "trans-neptunian" orbit carries with it? Why does being further out than neptune have any significance? Why not call all the objects sunward of Jupiter "sub-jovian" objects, and all the objects beyond Jupiter, "trans-jovian" objects? After all, the only objects in the solar system are the Sun, Jupiter and assorted debris.

The major difference between Pluto and China is that China is inhabited by people that can say that any other place is not actually where they live. If there had been no China to begin with, there would be nothing stopping someone from naming a newly discovered land China.

[Namaphry]

Earth is defined as a planet because it's like all the other planets of the Solar system, that seems straightforward enough to me. They're all massive objects that formed from the sun's protoplanetary disk. Mercury is a bit of an outlier, since it was so close to the sun that all the lighter materials were blown away. It's mostly a lump of iron. It's still larger than any moon in the Solar system except two of Jupiter's, and still more massive than those. Jupiter is another outlier, for that matter, since it's the one planet that formed at our sun's snow line radius, which is probably why it's so large in comparison.

I'm apparently mistaken about the definition of 'trans-Neptunian object', I thought it meant any object that actually crosses Neptune's orbit, so that sometimes it's closer and sometimes it's further away. This is more significant, since Neptune is very big, and nothing large enough to disturb its orbit exists beyond it, despite what we used to think. There was never a real 'Planet X', despite anything we found while searching for one.

I've been careful not to call Pluto 'debris', considering that the entire galaxy could be called 'debris'. Luna is a pretty big chunk of debris itself, and this is in a more practical sense, as its material was apparently torn from the Earth by a collision, rather than having formed through accretion, at length, in a more-or-less stable orbit, like our eight planets mostly seem to have done. We don't have much information about how Pluto formed, but considering it has five moons despite being so small, I'd guess it had a very hard time of it.

The main reason why planets don't form so far from their system primary is because at such great distances, orbital periods are so long that protoplanets don't have time to collect enough matter and clear their orbits before the star ignites. It's not a great neighbourhood, that goes without saying.

The major difference between Pluto and China is that China is inhabited by people that can say that any other place is not actually where they live. If there had been no China to begin with, there would be nothing stopping someone from naming a newly discovered land China.

I was comparing LaSalle's search for a shortcut to China through North America to the search for Planet X. He didn't find this shortcut because there never was one, and we never found Planet X for that same reason. The difference is that despite the story I'd heard, LaSalle never made the mistake of jumping to the conclusion that Lachine was actually China. But people really did make the mistake of jumping to the conclusion that Pluto was 'Planet X'.

Even through we determined fairly quickly that it was far too small to be responsible for the perceived discrepancies, nothing changed for the rest of the century, not until we had evidence of other objects in its class did we review the assumption that had already been disproven by Voyager, in 1989. The context of Pluto's discovery was poisoned by false premises, and yet we're still carrying on, trying to make consequence out of the inconsequential. That's the sort of baggage I'm talking about.

[icekatze]

hi hi

I believe you have entirely misunderstood my point. Why does any "planet," have consequence? Can you detect the consequence with a microscope? Is it perhaps derived from some component of atomic structure?

Regardless of the misconceptions surrounding Pluto's discovery, it is what it is.

So, Earth is like all the other planets of the Solar System? Earth doesn't have a massive crushing atmosphere of hydrogen gas like Jupiter. It doesn't have numerous rings and moons like Saturn. If I were to look at all the various objects' characteristics of objects in the solar system, I would wager that Mars would show more properties in common with Pluto than it does with Jupiter. (And Mars hasn't cleared it's neighborhood of Amor Asteroids.) In terms of being massive or not, the four gas giants comprise about 98.9% of all the known mass orbiting the sun. (445.6 Earth's vs ~5.26) Pluto is closer in mass to Mercury than Earth is to Saturn. How can Earth even be considered in the same category?

Also, planets can form a whole lot further away from their parent star than Pluto. Pluto has a semi-major axis of 39.5 AU. GU Piscium b has a semi-major axis of ~2000 AU, and is a gas giant.

Basically, the term "planet," is not a particularly proscriptive term to begin with. If scientists want to come up with some jargon terms that are precise, they had better come up with some new words.

[Mr.Tucker]

Definitions get upgraded as the science behind them is refined and expanded. The reason we haven't been forced to categorise a gas giant in a resonant orbit with another body as a dwarf planet is because we haven't found any. When we do, we'll probably give a better definition. It's only usefull for the general education of the public, astronomers make very little distinction between big bodies and small bodies of the Solar System.

[Namaphry]

Definitions have consequence for our understanding. Putting this under a microscope would mean looking at how it influences behavior. People who consider Pluto a planet regard planets and other objects like Pluto differently from those who don't. I haven't put anyone under a microscope to figure out how it influences their behavior, but I see no reason to believe that we're better off defining it as such; it only came to be this way because we went searching for a ninth planet based on a misestimation of the mass of Neptune.

Pluto's not what we thought it was, because it wasn't what we went looking for, because what we went looking for doesn't exist. Nothing out there is big enough to affect the orbit of Neptune, and yet that's how we defined 'the ninth planet'. If it really existed, it would have much more in common with the other planets.

Astrographically, Earth has more in common with the other planets (excluding Pluto) that it does with Pluto. They're all much more massive, even Mercury. Mercury is closer in mass to Pluto, but it's closer in composition to the Earth's core. Most of all, the eight planets are surprisingly orderly, neatly herded over billions of years into roughly circular orbits on the plane of the ecliptic. Pluto is a small ball of ice on a ballistic trajectory!

I know 'planet' is a descriptive term. I know there are gas giants that are almost right next to their stars and that are far, far away from them, but I'm given to understand that they don't form at such distances. A stellar system that doesn't have our 'conventional' gas giant arrangement won't be orderly and any definitions based on the order we have here are very unlikely to apply. They should work just fine for other star systems that do.

Objecting to planetary orbits not being spic-and-span is nitpicking. It's not like clearing orbits requires a Dyson Sphere-powered autoclave. I'd say a protoplanet has managed to clear its orbit if there's not enough crossing or crowding it massive enough to potentially destroy the main body and/or knock it out of orbit. Given the way objects beyond Neptune's orbit move, I'd say they definitely never met that criteria. The rest of the Solar System doesn't have to worry about this, even the worst events that could happen to terrestrial planets, short of stellar events, only tear them apart or turn them inside-out, common enough events during planetary formation.

I'm curious how exactly gas giants manage to fling themselves into seemingly impossible positions around other stars. I hope it's just due to interactions with their stellar neighbours, but I get the feeling it's more messy than that.

[Arioch]

It's only usefull for the general education of the public, astronomers make very little distinction between big bodies and small bodies of the Solar System.

I think this is the larger point: the definition has almost nothing to do with science. And it's confusing to the public, because they mistakenly assume that it does. Telling the public that Pluto is no longer a planet does more harm than good as far as general education is concerned.

[icekatze]

hi hi

Pluto was discovered in 1930. Pluto was conclusively determined to be too small to have perturbed the gas giants in 1978. All of this happened before I was born. Why should I, the new generation, care about the cultural baggage of decades long past? All this stuff about people's mistaken search for Planet X has nothing to do with what Pluto is anymore. I don't have any emotional investment in the search for Planet X, it was not for my generation.

...but I see no reason to believe that we're better off defining it as such.

Do you have any evidence that shows that we are not better off defining it as such? Otherwise, that sounds an awful lot like an argument from silence. I mean, if someone doesn't want to call Pluto a planet, that's fine, but it is a stretch to say that someone else is scientifically wrong for calling it a planet.

Earth is more massive than Mars, why does that not exclude Mars? The Earth's core is nowhere near the same composition as Jupiter's core, so why are they considered comparable? Mars and Pluto both have solid, stratified cores, unlike Earth which has a molten, moving core.

If any planet can be said to be in a ballistic orbit, it is Jupiter, because it has the fewest number of outside influences on its trajectory, on account of being the single most massive object. Pluto, on the other hand, is in a very orderly resonance orbit with Neptune.

If anyone is curious as to how gas giants fling themselves into positions around stars, they may be interested to know that the gas giants in our own solar system have moved into different positions over time. This is a process known as Planetary Migration, and using Nice Model (which brute forced the n body problem) we can see that the gas giants originally started in significantly different positions then they are currently in. Forming in nearly circular orbits between 5.5 and 17 AUs. Saturn, Neptune, and Uranus move into higher orbits because of their small size, which disturbs the orbits of planetesimals and sends them into Jupiter's orbit. Meanwhile Jupiter moves into a lower orbit because it has enough mass to eject objects from the solar system entirely. Then, once Jupiter and Saturn crossed their mean 1:2 resonance, Saturn was ejected further out and disturbed the orbits of everything in the entire solar system, including Uranus and Neptune (which switch places in almost half of the models), and may have caused the Late Heavy Bombardment.

((Models also predict a lengthening of Mercury's orbit which could lead to it being ejected from the solar system, and in a small percentage of the models, causing collisions between any of the inner solar system planets, including Earth and Venus, or Earth and Mars, in about 3.3 billion years. Gregory Laughlin, University of California, 2009))

The universe is a big place. As we discover more about exoplanetary systems, we're learning that our own solar system may not be the gold standard for which to base all of our understanding on.

[Namaphry]

Pluto was discovered in 1930. Pluto was conclusively determined to be too small to have perturbed the gas giants in 1978. All of this happened before I was born. Why should I, the new generation, care about the cultural baggage of decades long past? All this stuff about people's mistaken search for Planet X has nothing to do with what Pluto is anymore. I don't have any emotional investment in the search for Planet X, it was not for my generation.

No kidding, this is my point exactly. Pluto was defined as a planet when I went to school because people had made an error in my great-grandparents' time. Even though it was learned that there was never a 'Planet X' before I went to preschool, it was still in our elementary school's learning materials, still commonly showed up in childrens' media, and we were still taught that Pluto was just as significant and in the same class as the (other) eight planets. If we kept on top of this, I'd think it should have been when my parents were in high school that teachers went back to teaching about 'the eight planets' just like they had since the dawn of public education. The 'Planet X' idea had too much momentum by then, and we're still reeling from it.

Do you have any evidence that shows that we are not better off defining it as such?

This sounds a lot like an argument from ignorance. The burden of proof is on those making the extraordinary claim, and in this case, it's that Pluto deserves special treatment to distinguish it from Eris, Haumea, Makemake, and other similar objects, in favour of including it with far more significant rocky planets and gas giants.

I mean, if someone doesn't want to call Pluto a planet, that's fine, but it is a stretch to say that someone else is scientifically wrong for calling it a planet.

I'm not sure what 'scientifically wrong' means here. As commonly used, it's not really a scientific definition, and when it's given one, they tend to be problematic (as seen earlier in the thread.) It's more about what's semantically right and wrong... which is why I disappeared for a few days before posting this, arguing semantics is not fun. I only bother because I remember how difficult it was to reconcile Pluto's planet status with everything else I was taught in school--I don't want that to happen to other students. If they're going to learn about Pluto at all, it'd be much more coherent to group it with similar objects, like Eris, Haumea, and Makemake.

Earth is more massive than Mars, why does that not exclude Mars? The Earth's core is nowhere near the same composition as Jupiter's core, so why are they considered comparable? Mars and Pluto both have solid, stratified cores, unlike Earth which has a molten, moving core.

The Solar System is diverse... but it's still useful to divide it into parts. The inner planets are so dramatically different from each other they can each be called their own 'type' of planet, and we can compare exoplanets to them using terms like 'Earthlike' (possessing liquid water), 'Martian' (cold, dry rocks), or 'Venusian' (hellish pressure cookers, even more unpleasant than deep space). The state of a planet's core isn't terribly important for such high-level discussion. I didn't learn about magnetism until five years after learning astronomy, and geology wasn't even taught at my high school.

If any planet can be said to be in a ballistic orbit, it is Jupiter, because it has the fewest number of outside influences on its trajectory, on account of being the single most massive object. Pluto, on the other hand, is in a very orderly resonance orbit with Neptune.

What I meant was, I'm pretty sure all the (other) planets settled into their current configuration without actually being smacked around, and none of them are can be called 'stray'. Pluto's current orbit looks to be the result of some great catastrophe of colliding protoplanets, with an outcome where it only still exists because it fell into a resonant orbit by sheer luck. This is probably true of Haumea, too--it's spinning incredibly fast because it got hit incredibly hard. It might be sheer luck that the rest of the Solar System inside the Kuiper Belt hasn't suffered this kind of fate, but for the foreseeable future, it still makes a clear distinction.

I'm already convinced that there is no 'gold standard' for star system models. Red dwarfs might have trouble with photosynthetic life due to their emission spectra, and large stars don't last long enough for life to form (or even planets, in extreme cases!), among their many other issues. But there's a lot of room inbetween, and there's far more layers of complexity than just star type to consider before it's possible to estimate how likely there's somewhere well-suited for life as we know it, to say nothing of other biochemistries. But aside from that, it's mostly just ice, rocks, and gas, which seem like more of a matter for industrialists and economists than scientists and explorers. Some day they'll teach astrogeology in business schools, that's scary.

I think this is the larger point: the definition has almost nothing to do with science. And it's confusing to the public, because they mistakenly assume that it does. Telling the public that Pluto is no longer a planet does more harm than good as far as general education is concerned.

I was confused from the moment I was taught that Pluto was 'the ninth planet' until the day I learned it was a Kuiper Belt Object, one of several. I really liked learning about them--Haumea is especially interesting, it's an oblong!--though I'm not sure how important it is to teach in schools. I'm honestly not expecting us to colonize anything beyond the moons of Saturn within the next century, Uranus and Neptune are too far and don't seem to have much to offer. But leaving them out would leave holes in our understanding of other planets, while leaving out Pluto doesn't raise any questions, except when resistance to change comes in.

It might be confusing to say Pluto's not a planet, since most people don't know why it ever was considered one, but it was plenty confusing to be taught it was one to begin with. That won't stop until teachers have their story straight. Either Pluto's not included, or the other 'plutoid' minor planets are all included as their own group, so that we have the 'inner, or rocky planets', 'the middle, or gas giant planets', and 'outer, or icy planets'. Valid, but a lot more effort than taking the scattered ice balls out of elementary school education entirely. Though, I imagine a lot of kids would call Haumea their favourite planet, after seeing a picture of it.

[Razor One]

Disregarding Pluto for a moment, Kepler 452b may well be the most Earthlike planet around a sun-like star confirmed to exist.

NASA Page.

No guarantees on there being lush tropical environments filled with blue skinned elves who live in a warrior society in an expansionist phase as yet and probably in the wrong direction for that anyway, but we can always hope.

[icekatze]

hi hi

Uhg, NASA's page breaks by browser. Still, I wonder if anyone is ever going to design a probe that could realistically reach another system, even if it is not within our own lifetimes. I think it would still be pretty excellent.

((Namaphry, there is a significant difference between saying something there is no evidence to support a stance, and saying that a stance is untrue. A difference that was especially relevant to the burden of proof within the discussion. As for the rest of the sources I cited, that you ignored, I don't have anything else to add.))

[Namaphry]

Namaphry, there is a significant difference between saying something there is no evidence to support a stance, and saying that a stance is untrue.

It doesn't make any sense to say that the stance is 'untrue'. The claim 'Pluto is a planet' is even more unfalsifiable than 'bellbottoms are cool'. It's not an objective statement. You can add '...by IAU definitions', but then people try to say that by those same definitions Mars or whatever doesn't count as one, either. The question isn't whether Pluto is or isn't, it's whether it should be considered one or not.

As for the rest of the sources I cited, that you ignored, I don't have anything else to add.

You didn't cite anything relevant. Aside from the Amor Asteroids--a nitpick at the IAU definitions, which is nothing new, and beside the point--you just brought up a data model that suggests that the orbits of the gas giants were disrupted to some extent during the evolution of the Solar System, which, while interesting, doesn't counter the one point I made that was relevant to that. It's still not comparable. Just for one point in favour of 'it's chaos out there but not in here', all three of the minor planets I mentioned cross Pluto's orbit, not one of the eight planets do this with any body of significant size except Neptune with Pluto.

Yet, these arguments are 'it doesn't make sense', which isn't the main reason I'm objecting. The main reason is because it's not helpful to define Pluto as a planet. It's helpful to define a tomato as a vegetable in cooking even though it's a fruit, because it tastes like a vegetable, not a fruit. Pluto doesn't have that sort of distinction, it neither looks nor behaves like the rest of the 'Planet' group, while simultaneously looking and behaving like the members of another distinctly known group with its own characteristics.

I should hope that's apparent enough that I never have to say anything else about it.

***

I had someone in a Traveller game mention Kepler 452b with great excitement that turned to defensiveness when he said it was our 'first likely Earthlike planet', the GM asked 'In what ways Earthlike?' and I pointed out it looks like it's about as Earthlike as the least Earthlike planets with atmosphere in the Traveller franchise. The first article I found was just as misleading. It may be the most Earthlike planet we've found yet, but it's much, much more massive than Earth, likely massive enough to retain atmospheric helium. Between the surface gravity and the air pressure, it would be pretty hostile even if it had the ideal hydrographics, biochemistry, and oxygen levels for human life. But, thick clouds (providing 100% coverage) and powerful storms are pretty likely, too. So, calling it 'the first likely Earthlike planet' seems like a pretty big jump to make.

The good news is, there are still lots of smaller worlds with lower albedos closer by that we haven't detected yet which could be much more habitable than Kepler 452b (and, theoretically, even more habitable than our own planet!) So, there's a lot to hope for, we just have to be careful about getting too excited about these new finds. Also, Kepler 452b may well have its own native life that is adapted for its environment, even if our type of life wouldn't stand a chance. Even if not, its likely to be geologically and meteorologically fascinating. So, if you're more interested in exotic worlds to study and speculating on what strange inhabitants they might have than in new homes for humanity, it's pretty big news after all.

[icekatze]

hi hi

I am having trouble reconciling that idea that, "the inner planets are so dramatically different from each other they can each be called their own 'type' of planet," with the notion that Pluto must be put in a separate category because it, "neither looks nor behaves like the rest of the 'Planet' group."

Instead, I'm going to post some information in the abstract and hope that some of it is informative.

• Our best contemporary science actually seems to indicate that the various planets formed in circular orbits and were thrown out of their original orbits by as much as 10 AU, during the Jupiter Saturn 1:2 MMR event. This is the exact same event that caused the resonant orbits that many of the objects beyond Neptune have. This event likely took less that 50 million years, and did not involve any physical contact between any of the planets, or Pluto. In a sense, the only planets that cleared out their orbits were Jupiter and Saturn, because they cleared out their orbits, and everyone else's orbits at the same time. (99% of all planetesimals in the solar system)
• The objects beyond Neptune's orbit are estimated to have been stable there for the majority of the solar system's life span, and the way objects beyond Neptune's orbit move strongly suggests that most Trans-Neptunian Objects have a common origin.
• A comet the size of shoemaker levy, couldn't physically move fast enough to knock Pluto out of its orbit through an impact, although if it was moving at unreasonable relativistic speeds, it could possibly shatter Pluto.
• The Ecliptic plane is defined by Earth's orbit, and thus Earth necessarily has a 0º inclination. Earth's inclination to the Sun's equator is 7.155º. Pluto's inclination to the Sun's equator is 11.88º, and Mercury's is only 3.38º.

[Namaphry]

There's two reasons why the inner planets are useful examples of 'planet types'. They're dramatically different in terms of black body temperature and mass. The consequences of these differences, having the effects they do on atmospheric composition and density, are very relevant to human life. It doesn't make much difference to us whether a gas giant is composed primarily of hydrogen or methane. We can classify them by size pretty easily, but it doesn't change their Maximum Molecular Weight Retained, which is still always 'hydrogen', by definition. A planet that's too big or too small won't have an atmosphere we can use; neither will one that's too close to its star, due to greenhouse effects. And it's much easier to deal with too little atmosphere than too much, and too much heat and light than too little, so we afford a great deal of relevance to distinctions that are superficial in cosmic terms, for practical reasons.

Pluto and other trans-Neptunian objects are so cold that their atmosphere is frozen solidly onto their surfaces for most of their local year, and they're too small to retain the few molecules that could remain gaseous year-round. They are totally alien worlds, places where none of our considerations matter. They may still be better places to live than Venus--that isn't saying much. Yet, they're still less familiar. They don't have iron cores like the terrestrial planets, they don't even have much rock. They're icy, right down to their cores, and are distinguished in composition only by what kinds of ice they're composed of. Worlds that are relevant to us are mostly distinguished by what sorts of atmospheres they have, or failing that, their available water ice and mineral resources. Any sophonts who thought in terms of what kind of frozen air they'd like to live and work around would be as alien to us as the Kuiper Belt Objects are.

That covers 'looks'. As for 'behaves'--Pluto is a tiny object in a 3:2 resonance with Neptune. Its orbit is wildly different from anything else of comparable size except Haumea and Makemake. As a result, the three are classed together as Kuiper Belt Objects, along with many smaller objects in similar orbits, though Pluto is a 'plutino' and the other two are 'classical Kuiper Belt objects' on the basis of whether they have an orbital resonance with Neptune or not. Eris is not a KBO, its orbit is far more elliptical, but it's also closer in size to Pluto than anything else known. This group forms up much more neatly and practically than an organization that disincludes Pluto from its neighbours on the basis that, at best, it's bigger than an asteroid and close to Neptune's orbit, or at worst, because it was discovered under false pretenses, and was greatly exaggerated.

A final note on the plane of the ecliptic: The reason for using this based on Earth's own orbital inclination is convenience. The non-Earth-centric version, the invariable plane, is difficult to calculate precisely, but a bit more accurate, which can help if you're struggling to see how little deviance there is among the planets. This graphic, conversely, should give a good idea how much there is among Kuiper Belt Objects in comparison. Of particular note is Orcus, a plutino that is almost a perfect mirror of Pluto in its orbit, and is in the same resonance.

The innermost planets are the farthest off the plane if you only go by their degree of inclination, This might be because the sun isn't perfectly round, so Mercury is compelled to orbit between the invariable plane and the Sun's equator in part due to tidal attraction. It could also be due to a collision, but if so, the effect isn't that severe. In any case, Mercury's orbit is so small that it doesn't stray as far from the plane as even Jupiter does. It is very tightly bound to the Sun, to the point where it can't even rotate independently.

[icekatze]

hi hi

And yet, none of the issues of core composition, atmosphere, inclination, or usefulness to human life were part of the IAU definition. Instead they chose a seemingly ad hoc criteria that is conspicuously not a measurable quality of an object itself, but an weakly inferred story about an object's past history.

I find claims about Pluto's core to be especially extraordinary, since scientists do not yet have a solid understanding about the nature of Pluto's core. New Horizons only passed by Pluto a few days ago, and scientists have not yet had much of a chance to analyze the information sent back. But currently, the best guess that scientists have is that Pluto is not mostly ice. Current estimates, which may be revised by New Horizons, indicate that Pluto's core is mostly rocky, and may support a layer of subsurface liquid.

I also find claims about the similarities in the cores of the terrestrial planets to be extraordinary, especially considering that, "...all other deductions about the interior of Venus are based on models of Earth-like planets with internal temperatures and pressures adjusted for the slightly different radius and possible compositional differences. One of these models has led to the hypothesis that the core of Venus may be completely solid or 'frozen' today"

It is not surprising that the gas giants would line up along the invariable plane, since their orbits were heavily influenced by interactions with each other, and that they comprise 98% of the invariable plane's effect. However, the Sun's equator is a noteworthy measurement because it is very nearly the plane that the solar system's protoplanetary nebula formed in. Deviation from the Sun's equator illustrates how much a body has moved from the accretion disk over time.

To get a good idea of just how little there is in the Kuiper Belt, you can dig up Kuiper's original paper (1951) or any number of following studies which estimate that the mass of the objects in the 30-50 AU range is two orders of magnitude smaller than it should be. Over 30 Earth masses of material are missing, which the Nice model accurately reproduces.

[Razor One]

Let's take a step back for a second and indulge in a tangent for a moment. I will by degrees come back to the subject of Pluto.

This is Mount Scott in Oklahoma. This is Mount Davidson in California. As you may note, they are both mountains.

Except that they're not.

There is no universally accepted definition of a mountain. Elevation, volume, relief, steepness, spacing and continuity have been used as criteria for defining a mountain.[2] In the Oxford English Dictionary a mountain is defined as "a natural elevation of the earth surface rising more or less abruptly from the surrounding level and attaining an altitude which, relatively to the adjacent elevation, is impressive or notable."[2]

Whether a landform is called a mountain may depend on local usage. The highest point in San Francisco, California, is called Mount Davidson, notwithstanding its height of 300 m (980 ft), which makes it twenty feet short of the minimum for a mountain by American designations.[citation needed] Similarly, Mount Scott outside Lawton, Oklahoma is only 251 m (823 ft) from its base to its highest point. Whittow's Dictionary of Physical Geography[3] states "Some authorities regard eminences above 600 m (2,000 ft) as mountains, those below being referred to as hills."

In the United Kingdom and the Irish Republic, a mountain is usually defined as any summit at least 2,000 feet (or 610 metres) high,[4][5][6][7][8] whilst the official United Kingdom government's definition of a mountain, for the purposes of access, is a summit of 600 metres or higher.[9] In addition, some definitions also include a topographical prominence requirement, typically 100 or 500 feet (30 or 152 m).[10] For a while, the US defined a mountain as being 1,000 feet (300 m) or taller. Any similar landform lower than this height was considered a hill. However, today, the United States Geological Survey (USGS) concludes that these terms do not have technical definitions in the US.[11]

The UN Environmental Programme's definition of "mountainous environment" includes any of the following:[12]

Elevation of at least 2,500 m (8,200 ft);
Elevation of at least 1,500 m (4,900 ft), with a slope greater than 2 degrees;
Elevation of at least 1,000 m (3,300 ft), with a slope greater than 5 degrees;
Elevation of at least 300 m (980 ft), with a 300 m (980 ft) elevation range within 7 km (4.3 mi).

Using these definitions, mountains cover 33% of Eurasia, 19% of South America, 24% of North America, and 14% of Africa.[13] As a whole, 24% of the Earth's land mass is mountainous.[14]

By every measure, both Mount Davidson and Mount Scott fail at being mountains, and yet they have retained being called mountains.

And then you've got Cavanal Hill which is by the above definitively a mountain but gets called a hill. This is not an isolated case either, Crag Hill is a mountain that gets called a hill too, and I could likely dredge up more if I were so inclined.

Now, we could go and try to get a universally accepted definition of a mountain and go about renaming everything, or we could accept that trying to define what is and is not a mountain is a meaningless exercise in pedantry that contributes nothing to the study of geology and use the only yardstick that counts when the definition is arbitrary; human opinion. Yes, you're going to get contradictions, as I displayed up above, but this does not harm us in any way. Geologists do not bemoan (publicly at least) that a mountain under 300 meters is called a mountain, or that hills over 300 meters aren't being called mountains as they should. It does not in any way meaningfully detract from public learning and appreciation for mountains. The worst you might get is an argument over what people consider to be a mountain and what is not.

Planets are much the same in my book. The only key difference here is size and the ludicrously small sample size that we can study up close (not counting Exoplanets here since we can't study them up close and in detail, yet). With such a puny sample size we really can't go about trying to define a universe worth of planets, since that would be horrendously unscientific. With such a small grouping, we can afford to take an individualistic approach to each planet in our solar system and leave a broader reaching and more concise definition once we've got 1,000 or so objects (minimum necessary for statistical significance) studied that fit the loose definition of a planet so we can come to a more concise definition. Most people can memorise 7 +/- 2 items, and 9 planets still fits in the upper bound, Pluto was the first KBO discovered and can have honorary planet status as the KBO's official big bro, elder, rep, whatever.

You're never going to get a concise definition that satisfies everyone on this front. Biology and Geology suffer from this problem as well. If you want concise, talk to physicists and mathematicians. Until they have a well studied and statistically significant sample size and a more broad consensus (232 members deciding out of 10,000 hardly constitutes a majority), the IAU is little better at this than Linnaeus.

[GeoModder]

In terms of being massive or not, the four gas giants comprise about 98.9% of all the known mass orbiting the sun. (445.6 Earth's vs ~5.26) Pluto is closer in mass to Mercury than Earth is to Saturn. How can Earth even be considered in the same category?

Also, planets can form a whole lot further away from their parent star than Pluto. Pluto has a semi-major axis of 39.5 AU. GU Piscium b has a semi-major axis of ~2000 AU, and is a gas giant.

~5.26 Earths? Where did you find the extra planets? AFAIK Earth, Venus, Mercury and Mars alone barely mass 2 Earths.
GU Piscium b may as well be a brown dwarf at its estimated mass, so its formation history could be different.