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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?'"
Planet or not, it's out there and it's circling the sun. It's large enough to attract space dust and rocks in its vicinity. It will eventually grow larger and then there will be no doubt that it is a planet.
But really, who cares? Is this a big deal?
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Th proposal in the article is that every body that is rounded by it's own gravity (apparently this happens at a few hundred kilometres) should be considered a planet. Actually sounds a reasonable definition to me.
The Atlantic Monthly had an article about the Pluto situation years ago. The problem, though, is that "kids love Pluto." Scientists have tried to change names before (such as the dinosaur example). It'll be interesting to see what the public says about Pluto's demotion (if it occurs).
If it's a sphere, it's a planet. If it's irregular shaped, it's not.
According to plenty of legal definitions tomatos are in fact vegetables and not fruit. The tomato=fruit idea was introduced long after the classification as a vegetable as well established.
The reason for the reclassification of tomatos by the biologists was that they started to buy into the evolutionary classification schemes. So the taxonomy was redefined to fit the new theory.
Same thing happened with the Dinosaur formerly known as Brontosaurous. A bunch of jumped up greybeards with nothing to do decided that Brontosaurous and Apatasaurous were the same beast and that their idiotic rules were more important than common sense.
In this case there does not seem to be any particularly important theoretical issues. A planet and a planetoid behave in exactly the same way. The distinction between the two will inevitably be arbitrary at some point. Its like getting hung up on the definition of continent. Exactly why is Europe a continent but India is merely a 'sub continent' despite being much larger and a much more distinct geographical area? There is no real justification, except that Europe has to be a continent by the original definition, The fact that it is contiguous with Asia is conveniently ignored. India would have been considered a continent if they had not already reached the magic number of seven.
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Honestly, who cares.
:P
It's not like there's some legal reason to have the definition of a planet rigidly defined, it doesn't effect the money anyone gets, it doesn't influence political boundaries, and it won't get anyone out of jail.. so who cares?
If it's big, it's a planet. If it's not big, it's an asteroid. If it's not big and made of ice, it's a comet.
Might as well debate which text editor is bettor or whether we should be putting GNU in front of Linux.. it's such a silly thing to discuss it baffles me this shows up in the news so often.
He wants a Boolean criterion (yes/no) for planethood, but then criticizes a 'minimum mass' limit as being arbitrary. It is not possible to impose a Boolean criterion onto parameters that vary continuously without there being an arbitrary boundary somewhere.
Other than that, a pretty good discussion. His suggestion will still require an arbitrary boundary (how round is round?) but it is not totally arbitrary.
His rule has a problem that it turns into planets objects that we had previously decided were not planets. It has the advantage of being less arbitrary than the alternatives. Whether the advantage outweighs the disadvantage is a matter of taste.
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I think part of the problem is the fact that memorizing the 9 planets are all most people really know about our solar system, and so they tend to be fairly sentimental about it. I think a much more accurate and interesting approach to teaching kids would be to start of by brainstorming all the different types of objects in space - galaxies, solar systems, stars, moons, astroids, comets, nebulas. Then instead of memorizing just the planets memorize all the different regions of our solar system and what makes them special. Start with the sun, then you get to the inner planets, then astroid belt made mostly of rock, then giant gass planets, then the Kupier Belt full of icy objects and finally the Oort cloud. Then lastly you describe the interesting features of each area, including the planets and what makes them unique.
This journey approach would be far more interesting to the kids and by the time you got to the point of describing pluto and charon, they would have an understanding of how diverse (for lack of non PC word) matter in space is and would be less concerned about sticking a specific catagory on it, and just be excited that it was yet another unique and interesting thing.
It's the difference between decribing the cool terrain, people and features in country as opposed to just memorizing the state capitals. The former is far more interesting, and informative, and kids will eat it up.
How about the simple argument that planets are gravitationally strong enough to pull themselves into nearly spherical objects, whereas asteroids are not.
:)
:) But clearly a planet-sized object orbiting another planet is a moon. Again, this definition makes perfect sense.
:)
I like this definition a lot. While it does leave some wiggle room as to what exactly constitutes "spherical", it is still based on a physical property of the object related to its mass. This makes it better than any arbitrary size/mass requirement (e.g. "Anything as big or bigger than Pluto").
Pluto, BTW, Sedna, and many of the largest moons can all do this.
I'm going to be extremely unhappy with any definition that demotes Pluto. Also, anything that makes Pluto not a planet is going to be close to making Mercury not a planet, and that's just not acceptable.
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.
Titan is bigger than Mercury, so a Sun-orbiting Titan not being considered a planet is unacceptable.
I'm not an astronomer (but I play one on occasional weekends), but of all the definitions I've heard, "big enough to be spherical and orbiting a star" is the simplest and most logical.
And for the record -- if there was some comet out in the Ort cloud with an incredibly eccentric orbit around the sun that was the size of Titan, that'd be a planet too. IMHO.
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"Both even have satellites of their own."
Ehhh...
To be really picky about Newton's third law, moons don't orbit the planet itself, but instead both tend to revolve around a point between the two centers of mass (ie. the center of mass of the planet-moon system) because of mutual gravitational attraction. For example, the reason we're able to find (disgustingly massive) extrasolar planets is that the planets pull on its parent star enough for the star's motion to be visible from here.
I don't know off the top of my head whether the mass ratio between the earth and the moon is enough to pull the center of mass of the earth-moon system outside of the earth, but I do know the center of mass of Pluto-Charon is well outside of Pluto.
So that might throw a wrench into the works of a "it has a moon so it's a planet" idea.
That was in 1893. A fruit importer filed a lawsuit since to recover duties levied on fruits (but not vegetables). It had nothing to do with school lunch or nutritional guidelines.
Do you even lift?
These aren't the 'roids you're looking for.
I think there needs to be the addition of an atmosphere to be considered a planet. Really it's just a round rock without one. It pretty much classifies moons as planets without that qualifier.
First -- how much atmosphere? Every sizable rock will be able to hold onto at least a few gas molecules.
Second -- the Sun has stripped Mercury of its atmosphere. However, if Mercury were orbiting at the distance of Mars, it would have been able to retain quite a bit more air. Your definition is biased against close planets.
Third -- our atmosphere came largely from outgassing. A planet with a different composition (say, similar to the moon), or less active tectonics, might have dramatically less of an atmosphere.
So, you have now tied the definition of what is a planet to a complicated interplay between its size, composition, geology, distance from the sun, and who-knows-what-else factors.
Can't we just say "You need to be this big to be called a planet" and leave it at that?
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...kinda threw me for a loop. Look, chihuahuas are classified as dogs due to their genetic composition. They're sorted down to a certain genus and species and are most certainly close enough (in their genetic makeup) to other animals we call dogs to be considered one. Size does not matter.
:P
If we classifed planets using the closest possible method, say material composition ("This rock is 70% nickel, 30% iron... whatever, you get what I mean?"), it would be too damn hard to classify an object as a planet properly. Compare Jupiter's chemical composition to Earth's. Doesn't work. So instead, I believe planets should be classified by relative size, orbital pattern, and possibly the rate at which they orbit.
It's easy to say that in order to be a planet, the object must be determined to be orbiting a solar system's sun. You could also possibly define a certain speed of orbit to further clarify things, though it would be harder to settle on. The first (relative size) isn't as easy to define as the others though, for example say a rock out there orbiting the sun is, oh, 5km in diameter. We decide to call it a planet. It's fairly big, it's orbiting the major heavenly body (sun) in the solar system in question. Then suddenly from around the planet comes another rock with a slightly larger orbit and different orbital trajectory. It's big too, but only 4.9km in diameter. Or 4km. Or 3km. Size is hard to include in the definition of a planet. But it does matter.
But no matter what you decide, with the knowledge we have of the universe today, it's going to be hard to settle these little conflicts over what's a planet, and what's just an asteroid, and what's just a little rock that happens to be orbiting the sun. Maybe a committee should come up with a set of standards for defining and classifying heavenly bodies?
I dunno. I just kinda jumped on the chihuahua thing and wanted to contest that argument.
Is Ceres the fifth planet from the sun, then? It is shaped into a (rough?) sphere by gravity.
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Perhaps eccentricity of the orbit could be a qualifier of planetary status. Planets have relativly circular orbits compared to things like hailey's commet. Combine this with some minimum size requirement (say, half the difference between the size of Sedena the largest known asteriod/comet known) and you should be able to classify things as planets or not.
Ceres is round. Vesta is nearly so. Do they get promoted to planet status?
If we call Jupiter a gas giant, can we call Pluto a ice midgit?
When looking at the vast differences in other planets, is there yet a significant scientific reason to classify Pluto as something different.
(That is the question.)
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Unless you want to say that a vegtable is any sort of large plant product that doesn't classify as a fruit, I'd say that the Tomatoe can reasonably classify as both.
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exist to illustrate similarrities and differences. It is less useful to argue whether this definition of a planet or that definition of a planet should rule but rather we should be discussing what is a useful classification system.
Pluto has more in common with a whole class of objects which spend most of their time out past the orbit of Neptune. Sedna is another such large object but there are hundreds more identified.
That Neptune and Pluto's orbits cross is, I think, a major blot on our current classification.
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None of the steroids or comets would be planets, nor specs of dust nor billiard balls orbiting the sun. This seems like a much more reasonable criterion than "it's bigger than 2000km."
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Its like pond and lake. The definition of a pond is a body of water smaller than a lake and bigger than etc.. The definition of a lake is a body of water larger than a pond and etc.. Their are lakes larger than seas and some smaller than ponds. Their are puddles in the rainforest classified as lakes. What's a lake? What's a pond? Spent an entire hour in seventh grade science class tryng to come up with a good definition and we couldn't come up with one. The teacher said he asked a few phd's and a few professors and they didn't know. He spent weeks trying to come up with one himself and he couldn't. Its an almost entirely subjective label.
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Oh, and if anyone come up with a grand unified theory, they shouldn't publish it, because just think of all the physics texts they'd have to update.
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The guy seems to have a very absolutist view of the world.
It's part of the schtick. I'm not even sure Cecil Adams is a real person.
The point is that there are some subjects where you can have right and wrong, 'the earth is flat' being one of them. But when it comes down to definitions there may not be an ultimate 'truth'.
Well, in the case of the tomato, it's a matter of which side you're coming at it from. To a botanist, a tomato is a fruit. To a chef, it's a vegetable.
I really don't think there was ever a brontosaurus. I mean, they put the wrong head on the skeleton. That's not really a matter of opinion.
Pluto I would call a minor planet. Sedna I might call a minor planet. But you're right, the line isn't bright.
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Like Daddy always said: if you can't dazzle 'em with brilliance, baffle 'em with bullshit.
But even in your own statement you show the problem: Terms like "large" are relative. They're comparative.
A "large dog" is likely to be much smaller than a "large boat," is likely to be much smaller than a "large house," is likely to be much smaller than a "large planet," is likely...
In fact I threw in all those "likely" statements precisely because the definition of "large" is so arbitrary. Is 1,000,000 a large number? It sure is if compared to .000001. It's not so impressive compared to 999,999 and it's small compared to 1,000,000,000,000.
In order for size to be a valid criteria you would have to pick specific size over which an object must be to be considered a planet. If we're talking diameter, Pluto is roughly half the size of other objects we call planets. Then again, the moon of Calisto is roughly the same size as the planet Mercury. If we're talking mass, then it's roughly 25 times less massive than Mercury--but then again Mercury is 15 times less massive than Venus, 18 times less massive than Earth and over 300 times less massive than Neptune. What is our cutoff point? And more importantly, how did we arrive at it? If we're going to assign it arbitrarily right now then we can decide for ourselves whether or not it would be a "reasonable definition" to include objects similar to Pluto and the debate begins anew.
(For those interested, the numbers at http://www.nineplanets.org/datamax.html were used for comparisons and my calculations are probably wrong. :P)
The article has an elegant rule - the boundary is when the object's gravity can overcome the mechanical forces that hold an object in a specific shape. This usually produces a sphere (except, of course, in your pedantic case) but the actual shape is irrelevant.
Cool - another point to debate. What is the transition point from 'planet-moon' to 'bi-planetary' ?
Basing it on the center of gravity seems like a good idea, but 'well off from the center' is a little bit fuzzy. We could pick a number - say, 50% of the larger planet's radius - in which case the Earth-Moon system meets the criterion, since the center point is about 75% of the Earth's radius away from the Earth's center (some references).
But now we've done the same thing the original article was complaining about - we picked an arbitrary value, just, well, because.
It's seems like a physical point would work a bit better - say, the surface of the larger planet. Then the definition becomes a bit easier: if the center of gravity is in space, it is a dual-planet system. Otherwise, it's a planet-moon.
How you categorize a center of gravity within an atmosphere is left as an exercise ...
What is common sense anyway? Something that most people believe?
Common Sense is that thing that is distinctly lacking in today's society, the thing that makes people like me able to drive a car and remember to leave my hands on the wheel instead of taking them off because the manual doesn't explicitly state that they should both be used at once.
Its that thing that means I don't go throwing hot coffee over myself, no matter HOW hot it might not be. The same way that I don't run my fingers down knife blades to see how sharp they are, or jam my fingers in power points to see if they are live.
Common Sense is what stops me from needing instructions like "do not iron clothes while on body" or any of those other messed up warning labels that appear on products and foods "Open mouth, insert muffin, eat" is one of them.
Common Sense is what disappears first when a society turns into a legal battlefield, and the most important thing in any child's upbringing.
You ask what Common Sense is, I tell you to use your own to find the answer. Should you have none, go find someone with two arms, two legs, no visible scars, someone that hasn't been to the emergency room for 5 years or the lawyer's office for 10 and ask them why they are so much more than you are. They will tell you, and they will probably save your life by it.
This was not a flame, this is a soapbox post to all those people who want to sue over stuff that wouldn't have happened if they used a little brain in their day to day lives.
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You might be disqualifying Mercury from planethood, which would be odd, considering it's one of the original 5.