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Is {pluto|sedna} A Planet?
Dr. Zowie writes "NASA's announcement last week of Sedna's discovery reignited the debate
over whether Pluto is a planet. Dr. Alan Stern a noted planetary scientist and leader of the New Horizons mission to Pluto, pours on some gasoline with this
article in which he skewers the various arguments against Pluto-as-planet. Choice quotes include 'You wouldn't deny a chihauhau a place among dogs because it is too small,' and 'if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?'"
Does that mean every comet/asteroid that orbits the sun is technically a planet? If you throw out the size requirement, what other criteria remain for designating something a planet?
The Doormat
If you're not outraged, then you're not paying attention.
How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not. Pluto, BTW, Sedna, and many of the largest moons can all do this.
I also think, for the record, that if something as large as Luna, or Titan, or Europa were out floating in space orbiting the sun and not another planet, they would be considered planets too.
A problem with this is that there really is no clear-cut differentiation between "planet" and "planetoid". There's no qualifying size-- it's more subjective than anything. Almost like different species: we all differ genetically, yet a species is a generally-recognized "set".
One agreed-upon qualification is being formed round by its own gravity. I'm not sure if that applies to Sedna.
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And just what makes me an authority on this? I've taken more Astronomy classes in University than the average bear.
So here is my reasoning from an old assinment of mine:
A) Is Pluto a planet? Many measurable criteria signify that Pluto is a planet, but it is not a major planet. It is too small to be a major planet, so it is a minor planet or a giant comet. The only reason some astronomers still accept Pluto as one of the major planets, is because an American astronomer discovered Pluto in 1930, and they feel that changing its status to "minor" will minimize Pluto's significance in the solar system. Obviously books will need to be changed to reflect its new status, and many feel it would just be simpler to let it continue to be seen as a major planet, despite the facts saying otherwise.
It might make sense to consider placing Pluto into different categories, such as minor planet and comet. "Dual status already exists for some comets and minor planets, which are given formal numbers and names in both kinds of catalogues." [Green] The various categories we have for collections of matter in our solar system are many. The main categories are star, giant gaseous planets, smaller rocky planets orbiting the sun inside the "asteroid belt", satellites orbiting both major and minor planets, trojans, comets, trans-Neptunian objects (TNOs) and Kuiper belt objects. Meteoroids, and bits of dust, gas, and sub-atomic particles round out the other matter in our solar system.
A large asteroid named Ceres was first discovered in 1801, and was first presumed to be a comet. Then it was classified as a planet. That means astronomers, hypothesized that Ceres was a planet, they tested their hypothesis, and upon inspection of the available data years later they concluded that Ceres part of a new family of minor planets that was just being discovered. We now know of other TNOs, and Pluto doesn't look all too different from them, so we could adjust our view to place it as one of those other 100+ objects.
We can teach school children valuable lessons about science and astronomy if we teach them the history of the classification of Pluto, but stop calling it the ninth major planet. Pluto would not be called a major planet if it were discovered today, so it is a bad lesson in science to ignore data in favour of political concerns. People who say Pluto should remain a planet because for 70 years we have called it so, do not know the history of astronomy. They either don't know or don't care that many celestial bodies have been reclassified as new scientific data is gathered. Outdated models are thrown away in favour of newer, and more accurate models. Pluto no longer fits the major planet model that we use for the 8 major planets, so with our new data we should find Pluto a new category.
B)
Pluto was classified as a planet, when the data available to astronomers indicated it was one. Now the technology has allowed us to gather more accurate data about Pluto's characteristics, we should re-evaluate it's current categorization. People have had to re-evaluate "scientific facts" for millennia. Classifying the Earth as the center of the universe made sense several hundred years ago, but now we know more data that shows it cannot be the center.
From what we know about the physical characteristics about Pluto, I say it is a special minor planet. It seems odd to classify it as a kind of a comet, since I've seen no evidence that it leaves a trail of debris, and we don't know if the core is rocky, or ice like.
Saskboy's blog is good. 9 out of 10 dentists agree.
Grow bigger from space dust?? Are you sniffing space dust?
The sun will die out or explode before that planet increases it's mass in any significant way from space dust. Although large, I would have to say it's *not* a planet because of it's highly elipical and off-centered orbit, suggesting it was caught by the Sun's gravity and was not created along with the Sun, which is just my own definition of a planet. Pluto might be, hard to tell.
Alan Stern said, "wouldn't deny a chihauhau a place among dogs because it is too small"
yeah, no kidding. But if the definition of a dog included, "must not weigh less than 30lbs" then yes, a chihauhau would most certainly not be dog.
I know there is no such definitive critereon for planets, but jeeze...a simple webster's definition includes the phrase "...large heavenly bodies..." (emph mine). Any reasonable defintion of large would probably exclude pluto, just as any reasonable definition of "large dog" would most certainly exclude the lowly chihauhau
So what do we settle upon for criteria? Size is actually rather an arbitrary and vague boundary at both ends.
The fact that it orbits the Sun, specifically? The Sun's nothing special, we've found plenty of other stars that have planets. And if the Sun snuffed it tomorrow, would the Earth cease to be a planet? Would Ganymede be a planet if it were let loose for a stroll on its own away from Jupiter?
What about moons? Venus and Mercury don't have them, and those two rocks around Mars don't count.
It can't be geological activity, because Mercury is dormant and Io, a moon, beats everything we've yet seen for volcanic eruptions.
I think that having a discernable stata and a core of different composition than the crust sounds like a good rule of thumb, because then you're not just talking about a lump of rock that happens to be round, like Ceres. Now we just need to see what Pluto, Quaoar, Sedna have got in that department.
Okay, both are spheroids. Both have atmospheres. Both orbit our sun. Both even have satellites of their own. The only reasons one COULD say that they're not are that they're small, and they're way far out there. Both of those 'arguments' are pretty pathetic, IMO.
In short, there are more reasons for them to BE classes as planets than for them NOT to be.
On a related note, 'Sedna' is a really good name for an HMO, but a really _horrible_ name for a planet! *booo* Hell, even 'Planet 10' is a better name than Sedna!
Without wanting to wade into the tomato dispute itself, I'd like to state that evolutionary classification schemes are vastly more important than "common sense" to many.
What is common sense anyway? Something that most people believe? What the hell good is THAT?
At the very least, tradition is NOT a good reason to believe something when presented with conflicting evidence.
Funny, I didn't care about the tomato thing at all until you brought it up.
Pluto has a moon. Find me a minor planet that has a moon.
Cucumbers. Zucchini and all forms of Squash. Green beans...
Perhaps he didn't mention it because all objects meeting his "gravity rules" requirement happen to have stable orbits.
Damn my short-term memory loss. A week or so ago I read an article where an astronomer said that Pluto is not a planet because of it's interaction with the rest of the solar system. Pluto's orbit doesn't lie in the ecliptic and intersects Neptune's at two points (2D projection). Also, Pluto's orbit is slightly less circular than the rest of the planets.
This doesn't really disprove Pluto's status as a planet, but it does raise the question of Pluto being a planet in our solar system.
I also agree that there should be a Little Prince standard to classifying planets. Of course that is just placing our arbitrary viewpoint on science. Of course the whole argument is moot as long as we have to sit on our planet to explore the universe.
Dogs, coyotes, and wolves can interbreed and make perfectly fertile offspring -- that's a real problem for the preservation of wolves and coyotes. There's a quite a bit of tradition involved in deciding what is a species and what isn't. Greeks and Romans saw wolves as something other than wild dogs, and thus we do too. And of course, the vast majority of organisms on Earth are asexual, making the whole issue of "fertile offspring" moot. Logically, all decisions should simply be based on percent identity of DNA, but then the question becomes what percent should be the cutoff.
There may be another Earth-sized planet that was ejected by Neptune and that in turn shifted the orbit of Sedna. Why don't we see that planet? Because it may be in the aphelion, perhaps light week away. Not only it is far away from us, but it's also in the darkness, being far away from the Sun.
Or maybe the Sun approached another star in the past, which changed the orbits of the outermost Kuiper Belt Objects. Finally, maybe it was our Sun that snatched Sedna from another star.
This is exactly the same as the continent-versus-island debate in geography. Why isn't the United Kingdom considered a continent by anyone except the Brits? Why is one single monolithic land mass with a dotted line down the middle considered two continents, Europe and Asia? How is Australia not a continent?
Everything you said was right,
Though the earths "moon" should be considered a second plant in our bi-planetary system. It is large enough that the center of gravity for our orbit is well off from the center of the earth.
(just another pet peeve of mine when it comes to astronomy)
I live in a giant bucket.
Second, the object must be round. This criteria excludes most asteroids.
Third, the body must be large enough that its own gravitational forces can account for it shape. This criteria excludes any objects which might happen to be round but can't really be called planets, such as small round rocks or asteroids.
Fourth, there must not be any similarly sized objects in the same orbit unless the gravity of one significantly affects the orbit of the others. This requirement excludes comets and all remaining asteroids.
Fifth, the object cannot be in orbit around another object that otherwise qualifies as a planet. Objects which orbit eachother may qualify as a double planet if all other criteria are met.
File under 'M' for 'Manic ranting'
You wouldn't deny a chihauhau a place among dogs because it is too small,' and 'if your brain was so completely full of names of people that it just couldn't take any more, would anyone new who you met after that, therefore not be a person?'"
Mmmmmm... Somebody who likes the sound of his own voice way too much.
As far as planet vs planetoid goes, I'd think the difference relies on how much influence is imparted on what body of mass. For instance, the majority of influence imparted on asteroids comes from the planets and stars they revolve around, whereas the planets principle influence is the sun.
So which influnces these celetial bodies more? The sun or other planets around it? Does the body influence other celetial bodies a great deal? Does it have it's own bodies trapped in orbit around it? If this body careened through the solar system close to a planet (say, earth), how much influence would it impart on us??
I'm leaning more toward planet, especially in the case of Pluto. Sedna, I'm not so sure about given the lack of hard data, but I'm pretty sure a near pass from Pluto would seriously screw things up here. Besides, all this crap is relative anyway. I'm sure if you had a huge enough planet, Earth could be considered a moon or something.
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The definition that makes Pluto not a planet is not based completely on arbitary size and so would probably not demote Mercury as well. The most prevalent definition of a planet (which was not stated in the article as far as I could tell, at least not completely) is any gravitationaly round object that is more massive than the rest of the mass in a similar orbit COMBINED. Mercury would be safe with this definition, while Pluto would be quickly tossed out. I actually like this better as the term "planetoid" now means something different than either "planet", "asteroid" or "satelite." (it would become a synonym of one of these otherwise) I propose the following definitions:
Planetoid: Any object that becomes round by its own gravity but does not sustain fussion.
Moon: Any planetoid that orbits another planetoid (let's face it, it's a generic term and nothing will ever change that). BTW: This would demote a lot of "moons" to mere satelites.
Planet: Any planetoid that is more massive than the the rest of the matter in its orbit combined.
common sense: noun
What those who are ignorant of the subject matter think; usually wrong.
Fruit (froot) [n]--the ripened reproductive body of a seed plant
Try reading a dictionary that was written in 1800 or earlier. And try looking up the definition of tomato rather than the definition of fruit. You are reading a definition that is the result of the taxonomists having won and in the process completely missing the fact that there was ever a legitimate dispute. The farmers classified the tomato as a vegetable because it has a thin skin and is perishable. That is why the supreme court rulled the way it did, the tax law in question was written when the previous description was in force.
The OED definitions of 'vegetable' go from the incredibly broad (any plant) to the more specific (any plant that is eaten for food). Curiously I did not actually find the scientific definition cited in my copy (2nd ed). But that might be because the entry is two pages long and I overlooked.
The point I was making is that the whole idea of rigid taxonomy is a Victorian invention. The idea of fixed immutable categories that are determined by rigid application of science is very much a late 19th century view.
That is the point at which the Oxford English Dictionary, the Encyclopaedia Britanica, Principia Mathematica are all being written. Then by 1930 it has all slammed to an abrupt halt as relativity, Goedel's incompleteness theorem and quantum mechanics have all become mainstream. Suddenly the logical positivist view of the universe is no longer universal.
There is some logic to a taxonomy of fruits and veg based on genetics, there is equal logic to a taxonomy based on how well it keeps. The farmers lost out to the scientists here because the scientists got to write the taxonomy.
When it comes to the definition of 'planet' there is no real scientific basis for the taxonomy. Planets have simply been defined to be obejects orbiting a star that are not orbiting anything else (another planet) and are large enough to form a sphere under their own gravity. This gets subjective when the term 'sphere' is debated. Clearly the earth and the other planets are only roughly spherical, how much tolerance is there?
It is a silly dispute as are most taxonomic disputes, Pluto and Seldane are planets if people chose to call them such. Witgenstein was right.
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Personally I think his arguments are completely bunk - he just argues that just because something is small doesn't mean it shouldn't be considered a planet. Well, I hate to burst his bubble, but there are tons of small things out there that we don't call planets precisely because they are smaller: Asteroids! The human and dog analogy is completely inappropriate here - it's more like the difference between a boulder and a pebble. At what point does a rock become to small (or even big I suppose) to be considered a boulder? Too big to be considered a pebble? While not many people care about the exact, qualitative distinction between boulders and pebbles, the difference between asteroids and planets has all sorts of ramifications for cosmological classification systems. You have to draw the line somewhere.
My personal vote goes to the system that would make Pluto (and therefore Sedna, Ceres, et al) NOT a planet, but have Pluto grandfathered in solely for historical reasons.
I bet the folks who live on Jupiter think our solar system only has 4 planets. After all, Earth and Venus aren't so much bigger or different than Pluto or Sedna. Certainly, Earth is closer in size to Pluto than to Jupiter.
People argue so much over where to draw the line between Planet and non-Planet, but everyone seems to take for granted that Earth is a member of the former class.
Bigots.
Paranoid
Bwaahahahahaa.
There was a good presentation at today's blackboard lunch at the Institute for Theoretical Physics in Santa Barbara today. The first 15 minutes or so are a great summary of why Sedna is important for our understanding of the solar system.
Well, since we are throwing out just about every possible way to classify a planet, or in this case, a way to keep Pluto and exclude Sedna, why not use the old method that led to the discovery of Neptune and Pluto?
If the body in question exerts a measurable gravitational force on another planet, then it's a planet. As I understand, Sedna is much too distant to exert any kind of force on the orbit of Pluto. This also can discount Ceres and other large planetoid objects in the asteroid belt, since most of their gravitational exertion can only really be measured on other objects in the belt.
If we keep promoting solar objects with a mass greater than a Volkswagon to planetary status, then I guess all those numbers in Drake's equation just skyrocketed...
"Luck is what others call skill when they have none." --Phelan Kell
Let's be honest. The New Horizons mission is being launched for completeness. Once it completes its flyby, all of the nine planets will have been explored by a visiting spacecraft.
Classifying Sedna and Quaoar and all that other stuff out there as planets will require more missions. Demoting Pluto would elimiate the need for New Horizons. So lets make sure New Horizons hits space before doing anything else.
The first asteroid, Ceres, was predicted before it was discovered. According to the Titus-Bode series, there is a gap between Mars and Jupiter where there must be a planet. So, astronomers looked and surprise surprise, there's Ceres. But it didn't take too long to discover Vesta and Juno and Pallas and all of Ceres' other friends. Its those friends, immediately discovered, which caused Ceres to be demoted.
Pluto was predicted to explain otherwise unexplained perturbations in the orbits of Neptune and Uranus. So, astronomers looked and looked and looked and eventually found Pluto in the predicted position. Then they stopped looking. If they had kept it up, they would have discovered the rest of the Kuiper belt, and Pluto would have suffered the same fate as Ceres.
The proposed Gravity Rule would cause the answer to the question "How many planets are there in the solar system?" to change from "9" to "we don't know." We can be reasonably confident based on the tracks of the Pioneers and Voyagers that there aren't any other large masses out in the outer solar system. We will never be sure we have discovered all the round things out there.
Besides, there are objects which should be round, but aren't, like Hyperion, and things which have no business being round, but are, like Comet Wild/2. How would the Gravity Rule treat those?
Lots of technical and environmental problems are solved by the application of vast amounts of nuclear power
So if we accept that a planet is anything that has been rounded by its own gravity, we have to make assumptions about the density of the matter that it's made of. Stars that went through a supernova and no longer have enough mass to undergo fusion may be round and of comparable size to known planets, but many times denser. They clearly are not in fact planets but the author's criterion would have it that way. Of course we can say, "They aren't planets because they used to be stars" but he threw out the entire argument-from-origins criterion as being too difficult to establish.
Conveniently, he did not make a biological analogy for the argument-from-origins -- that is because biologically, it is the most sensible argument.
What he really did was shift the question from "how big do you have to be to be a planet" to "how dense do you have to be to be a planet." Now we have to establish an upper limit to planetary density, which is hard since it's hard to see small, dense objects.
Corn - Seeds lining a core, you eat the seeds - Vegetable Watermellon - Sometimes, and originally carried seeds, you eat the pulp in which the seeds reside - Fruit Socially though, Vegetables are things that are usually baked, or put into things that are cooked. Fruits that have qualities that work in those conditions are usually referred to as vegetables (Tomatos, Eggplants, Cucumbers/Zucchinis, Chilis, etc.) Fruits are usually considered VERY sweet (considering tomatos and sweet chilis are on the blander side of sweet), and are usually not cooked. Eating without anything else (Mellons, Oranges, Grapes), but occationally put on top of pancakes or other cooked batter foods, or put in pies... Mmmm pies. Really it's very subjective though, and while a Tomato, Cucumber, or Eggplant is technically a fruit, it's used so much as a vegetable, that for cooking reference, it's best categorized as a vegetable. ...mmmm.... pie.
We've been debating this here: vote totals so far:
Sedna is:
tenth planet 17 votes - 29 %
the eleventh planet 14 votes - 24 %
the 42nd planet 9 votes - 15 %
not a planet! 17 votes - 29 %
Energy: time to change the picture.
Asteroids are also called planetoids, which just flips the above comparison on its head -- they're like planets, but they're not exactly like planets. The really amusing thing about this double terminology is the way it confuses Star Trek writers.
Then there's the word planet, from a Greek word that translates literally as "wanderer". All the objects in the sky that move with respect to the stars were originally considered planets. Not including the asteroids, because you can't see an asteroid without a telescope which hadn't been invented yet. But what about the Sun and Moon? These were considered planets too. But not the Earth, because everybody knew that the Earth didn't move. Hey, motion is define in reference to the Earth, how could the Earth move? What is that Copernicus dude taking, anyway?
Incidentally, that's why there are seven days to the week. Each planet that you can see without a telescope (and thus that is actually considered to exist) is dominated by a deity, and each deity has their own special day: Saturn Day, Sun Day, Moon Day, Mars Day, Mercury Day, Jupiter Day, and Venus Day. Most of the names we use in English come from Norse gods that medieval scholars thought were cognate with familiar Roman gods; their logic was a little stretched, but nobody cared, since the Norse religion was already dead, and hadn't involved planet worship anyway.
But I digress. The important point it that all these names are historical relics -- there's no way to be really precise with them. The cover issues we no longer care about, and don't cover issues we do. If you want to be more precise than anybody is in real life, you refer to rocky body, gaseous bodies, and Kuiper objects. But in real life you use familiar terms, because they're, well, familiar. If there are confusions and ambiguities, you take a moment to clear them up ("for the purposes of this discussion, any large body that orbits the sun is a planet; also Greenland is an island, not a continent"), and then you move on to stuff that really matters.
When we see a white dwarf star orbiting around a red giant we don't quibble about whether the dwarf is a planet or star or star-oid or whatever. We classify it according to its intrinsic properties.
It would be useful to classify smaller bodies in the same way, regardless of their orbital situation:
1) Gas giants.
2) Bodies made of heavier elements large enough to have vulcanism and tectonic activity.
3) Smaller solid bodies large enough to be rounded by gravity.
4) Even smaller solid bodies, sub-classified into rocky and iron.
5)Dirty snowballs.
Of course there will not be an exact boundary between these classes. For example, vulcanisn in "planets" is fueled by long-lived radioactives but in Jupiter's "moons" by tidal energy.
Possibly there could be intermediate objects between 1) and 2) or between 1) and 5) or 3) and 5). Concievably there could be bodies made of water or solid methane or blue cheese but these would be unlikely to form naturally.
If you read the case summary (linked in a previous post) you find how a judge looked at some dictionaries and asked some fruit and vegetable vendors for their opinions. He did not ask any scientists. I think if a modern judge were to hear this case he'd give rather more weight to scientists specialising in thse plants. After all, we all know that judges can make some pretty dumb determinations when it comes to science or technlogy.
The EU reclasified carots as fruit.
This was so that someone could make and sell "Carot Jam".
For it to be clasified as jam, it has to be made from fruit.
So if she weighs the same as a duck, she is made of wood and therefor a witch.
I believe the word "planet" has origins in Greek, and means "wanderer". Hence, the "planets" were called such because they appeared as wandering stars by human eyes on earth. Certainly anything in orbit around our Sun, out to about a light year, will have enough "wandering" associated with it (by parallax due to Earth's revolution about the Sun) to fit this classification. Unfortunately, that simply isn't a very useful way to classify things since we already have some sort of common notion about what "planet" means, even though it still a little vague.
Orbital inclination might be a good start to finding a not-so arbitrary way to classify things as "planets" or maybe at least as "primary planets". Maybe everything with an orbit inclined less than 10 degrees from the ecliptic can be a planet. Other than the specific degree-cuttoff, this criteria is not arbitrary, since the ecliptic plane exists due to the rotation of the Sun. Things orbiting far off of the ecliptic are more likely to be things which were thrown out of the system at the onset of fusion, so maybe they shouldn't be called "planets", or maybe they can be "secondary planets". (Pluto's orbit is inclined *17 degrees*, the next most inclined is Mercury at *7 degrees*)
Perhaps we can add a mass criterion based on the size of the Earth, which is admittedly totally arbitrary, but when coupled with orbital inclination, it's much less so. Maybe nothing with less than 1% the mass of the Earth should be classified as a planet. That excludes asteroids from accidentally being "planets" under the orbital inclination criterion, but we can still have Mercury, which has about 5.5% the mass of the Earth. (Pluto has about 0.2% the mass of the Earth)
Finally, to account for moons, we could add that for multiple bodies orbiting one another, and together orbiting the Sun, in the case that both are more than 1% Earth-mass, the one containing the center of mass is the planet and the other(s) the moon(s). If the center of mass is outside both or all (if there are many), then it's a double- or triple- planet system.
How does that sound?
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