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Defining "Planet"
beardoc writes "The Sydney Morning Herald is running a story today about a controversial proposal to define what size a planet might be - depending on what the final definition of how big a planet is, we could end up losing Pluto (at 2300 kilometres) to the status of "asteroid" or gaining three more planets - Quaoar, Varuna and Ceres."
... agreed to be "Marlon Brando"
If someone bothered to name a Roman god after it, it's a planet. Pluto, Mars, Jupiter--all friendly planets.
Alpha Centauri? Bah--probably a reflection off that Hubble thingy.
We recently had heard in the office over one of the Yellow Machine that's made by Anthology Solutions.
Maybe my neighbour can be defined as the "first living planet"?
Wouldn't it make sense to take into account whether the planet could feasibly sustain life too? I mean could a 700km round body in space support an atmosphere?
(stolen from DaBum) I am dyslexia of borg - your ass will be laminated.
The fellows at www.lunarembassy.com have announced(on the Conan O'Brian show, no less) their interest in selling the entire planet of Pluto for about $250k. If Pluto gets downgraded to a mere asteroid or Oort-object or what not, will that lower it's real-estate value?
Planet: n. Any object orbiting a star, not orbiting a planet, and having a radius greater than the radius of Pluto minus one millimeter.
I went to an astronomy talk at the University of Toronto a few years ago. The presenter defined a planet as any celestial body that doesn't radiate light. That explicitly includes asteroids and moons. Why is it necessary to make the distinction between planet and asteroid?
The whole point of the article is to arbitrarily define the distinction which just proves how stupid it is.
Jason
ProfQuotes
they say bodies larger than 700km go from being potatoe shaped to round. why not set a defined width above this 'minimum', and anything larger be called a planet? twice the minimum sounds plausible, and that means Pluto would still be defined as planet.
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Because the traditional designation of what made up a planet was anything that we could find that orbits the sun. We didn't include comets because they looked different from planets and we didn't include asteroids because we couldn't resolve them. Now that we continue to find many large objects that are really little different from Pluto it has suddenly become important to have a real definition of which are planets and which are just big asteroids.
Also, as we find bodies orbiting other stars, the traditional designations for planets is obviously useless.
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Because the traditional designations are spotty at best, and certainly not definitive enough.
Currently, a planet is defined to be a body larger than an asteroid and orbiting a star. There's no distinction between planet and asteroid, except "oh that looks big enough.. i guess it's a planet."
Why do they always need to complicate things. I thought size doesnt matter.
I've left to find myself. If you happen to see me, please, keep me there until I return.
a) this is science, not tradition, scientific terms need an absolute definition.
b) traditionally, you only had the naked-eye planets: Mercury, venus, Mars, Jupiter, Saturn. What do you call the other gas giants? Not to mention, Mercury was thought to be two planets by some (the morning and evening star).
c) my opinion, just set it so that Pluto-size is the cut-off. Anything smaller isn't one. However, in a few centuries when we can detect "planets" in other solar systems this would seem a bit heliocentric, so I can see the Basri's point (in the FA: "Basri's definition, a planet must orbit a star, not another planet, and it must be round. That means it must be 700 kilometres in diameter, when gravity moulds it into a sphere, or bigger.").
it seems silly to go about redefining something like what constitutes a planet. what possible scientific value could this have? why can't we be content to keep the traditional designations of what make up a planet?
that's exactly the problem; there_are_no universal definitions for "planet." the most common definition is "any celestial body that orbits a star". I think we can all see the problem with that definition; we would have to classify even the least massive meteors (probably numbering in the millions in our solar system alone) as planets.
People keep trying to wage a debate about this, but no matter what technical hand-waving is going on in the press, the International Astronomical Union is committed to the traditional status of all nine planets, and isn't likely to change that opinion.
--brian
As a representative one of the nine planets, I find this proposal deeply troubling, especially since there are not any other representatives from the other eight. Once a planet is classified as an "asteroid" or "floating piece of shit with gravity", it not only loses its prestige, but also, it cannot apply for federal grants, and hence, usually suffers a major economic blow. Laugh you may, but I've seen planets go from a heavenly body to a drunk spinning horizontal and finally distingrate into an asteroid belt in no time. We must support our planets because if we don't, then who will?
Is there a corresponding Sailor Senshi? If so, it's a planet. Ironically, this means that the Earth is not a planet, but the Moon is. Go figure.
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The moon is actually very large, especially in comparison to the size of the Earth (Earth = 6371, Moon ~1750, in comparison Pluto ~ 1130). Current thought is that the Moon formed by impact by an approximately Mars sized body early in planetary formation.
While the proposed definition says that a Planet must "orbit the sun and not another planet", I think that if this definition is accepted, we should be considered a "binary planet system" or something similar.
Anyway, just my 2 cents.
If I had no sense of humor, I would long ago have committed suicide. -Ghandi
With the discovery over the past few decades of the Oort Cloud and Kupier Belt, it seems obvious that there are tens, if not hundreds/thousands of Pluto-sized objects out there. Obviously, we're not going to name all 10,000 of these rocks "planets." But then again, Pluto has a special place in history as the last "great" planet discovery on the level of Uranus and Neptune, so purists wouldn't want to ruin that by demoting it.
:-) Therefore, I say we define a planet however they want, but keep Pluto for historical significance. I knew med school would start paying off soon :-)
My solution? Define "Planet" as something bigger than Pluto, maybe with Mercury as the smallest, or whatever. But keep Pluto as a planet (as an exception ot the rule) for historical purposes. But, you may be thinking, "that's so stupid! Why give something a name if that name is now invalid?" The answer? We do it all the time. Here's an example...
Take a look at ANY diet softdrink/diet product with Nutrasweet. It warns you that this product contains "Phenylalanine" and should not be taken by "Pheylketonurics." Take a look at that word. It's called "Phenyl-keton-uria" (PKU) because years ago, people with this disease were diagnosed when "Phenylketones" were detected in their Urine. However, no one diagnoses PKU via a urine test anymore, they use another method. So should we change the name of the disease? Of course not. But due to historical significance, we keep it. Unlike the Indian/Native American designation, "Planet Pluto" should not offend anyone
This is just like taxonomy... some scientists like to lump similar creatures into one family or genus, while others like to split them up into different categories based on minor differences.
Looks like astronomers do it too.
Different discipline, same problem.
Even if a man chops off your hand with a sword, you still have two nice, sharp bones to stick in his eyes.
Indian astrology studies planets as those heavenly bodies that affect life-forms on the Earth in a 'major' way. Thus the Sun and the Moon are also planets as per the Indian definition. Two planets (Raagu & Kethu) are also defined - these do not denote physical planets, rather, the clock-wise and the anti-clockwise 'spin' of the Earth.
The system also defines 27 stars (the nearest ones from the Earth) and a 60-year cycle.
Under this system:
It is possible to accurately determine 'events' such as eclipses, birth & death, progeny, well-being, etc.
There is no need for 'leap-year' correction, since a year can be 'born' at mid-day, mid-night or anytime in between.
If you keep throwing chairs, one day you'll break windows....
This is sort of handled(here):
Since the COM is inside the Earth, I think it's fair to say that the Moon orbits the Earth (and not vice versa).
The Mongrel Dogs Who Teach
This superficial naming convention makes absolutely no difference at all. It has no effect on anything.
It would be like if you changed the biological classification system so that bears were no longer Mammals. What difference does this make to the bears? None. What difference does this make in how we relate to bears? None.
It is simply an arbitrary naming convention. As are all naming conventions.
It reminds me of an old Zen saying that I am likely paraphrasing miserably:
"Before Zen, a mountain is a mountain. While one is practicing Zen, a mountain is no longer a mountain. After Zen, a mountain is once again a mountain."
Justin Dubs
Alone in it's orbit, it's moons orbits it
isn't too far out
Sorry about the writing. Robot fingers, you know? Cliff Steele in DOOM PATROL #23
Why is there so much mention of radius and size and such. It's just so petty. I would think in this enlightened age we'd all know:
It's not the size that matters, it's how you orbit!
*Dodging tomatoes should be a sport*
Hogwash. Scientific terms need a definition in terms of a scientific theory. So an "absolute" definition like "anything spherical with a radius smaller than the sun and bigger than the moon" or something like that, although suitably absolute (notwithstanding changes in the radii of the sun and moon), obviously has no connection to a scientific theory of planets. But there's a bigger point here too: absolute definitions make for crappy science. We can't legislate the way the world is via definition; good science should seek to describe the world. Imagine, for a moment, we were having this discussion not about planets but about marmots. We wouldn't want to specify maximum and minimum sizes in some definition of marmots because it would be silly to disqualify something as a marmot purely on the basis of its size, regardless of other factors (say, its marmot parents). And analogously, it would be silly to disqualify something that otherwise fit into our theory of planets perfectly on the basis of its size. Or to be forced to include something as a planet on the basis of its size, despite the fact that it has no other place in a scientific model of planets.
Does jupiter have volcanic activity?
Just use the Star Trek planet classifications... Come on, it's time to make use of sci fi in astronomy for once. :-) Hmm, btw, I wonder what the heck the copyright at the top of the page is about? Courtesy JPL? Errr...
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Please define "is".
Seems to me that the only problem with the whole sphere thing is that objects of higher densities could be spherical at a much lower diameter, neutron stars for example.
I have taken several classes on the universe and our solar system, and everything I've heard makes me believe that Pluto should not even be considered a planet, due to its extremely small size and different composition that the rest of the outer planets.
I think you mean: Should have 5% of the size of the parent star, not 5% bigger...
But on another note, Pluto is not an asteroid nor a planet, it is a comet. Its got a tail, albeit a small one, but then again Pluto is friggin 14th magnitude.
Bill
-Bill
Any Definition for "planet" will be arbitrary. Is a little ball of snow and ice on a highly elongated orbit a planet? No. It's a comet. Is a gas giant that generates more heat internally than it receives from the star it orbits a planet? Maybe, maybe not. OK, perhaps that's not arbitrary. If the thing gives more heat then it gets, then perhaps you could classify it as a brown dwarf, but what if the star it orbits flares up? Then does it suddenly become a planet because it starts receiving more heat?
I think the only thing we can conclude is that the definitions for "planet", "moon", "ring material", "asteroid", "comet" and "brown dwarf" are all arbitrary. It's all a matter of perspective.
So, here are my definitions:
Planet -- orbits a star, is big enough so that gravitational pull forces it to appear round or smoothly eliptical to the naked eye.
Asteroid -- orbits a star, If it's not round due to gravity, it's definitely an asteroid. Problem--this makes Ceres a planet.
Moon -- orbits a planet, unless it's not round then it's just a "captured asteroid". Problem--this makes Deimos and Phobos non-moons.
Ring material -- If the human eye perceives the planet as having rings, then any ojbect within the region containing the perceived rings is "ring material" regardless of how big it is or how it's shaped.
Comet -- any item that forms a tail when passing close to the star.
Brown dwarf -- Gives off more heat then it gets.
Really, when you get right down to it, all of these things are just "stuff that's not space". Choosing to call them "planet" or "comet" makes as much sense as choosing to call one city Cincinnati and another Buffalo. Somebody's gotta name the thing. Now, people have been living in Buffalo a long time, and they've been calling Pluto a planet a long time too. Whaddya say we make a deal? Get Buffalo to change its name to Cincinnati, and we can stop calling Pluto a planet. Now, what do I call a single hydrogen atom on a hyperbolic trajectory with Jupiter?
For all intensive purposes, "whom" is no longer a word. That begs the question, "who cares"?
Just as you spell it.
There are two rules for success:
1. Never tell everything you know.
But doesn't it [Ceres] have a satellite? -- and -- What would we qualifty that as, because a satellite must orbit a planet.
It doesn't appear that Ceres has any satellites. But, there are 31 asteroids that do! That doesn't make them planets though...they're just small asteroids with really small moons.
Can anyone remind me what that sequence of numbers is called that vaguely predicts the distances of planets from the Sun?
Yep, its the Titius-Bode Law. Ceres does fit into this. But the reason we don't have a planet in between Mars and Jupiter is because "many astronomers think the asteroid belt is where a planet tried to form, but was pulled apart before it could solidify, caught between the strong opposing tugs of Jupiter and the sun's gravity." Quote taken from here.
Why does a planet _have_ to be a shpere...How perfect a sphere?
Well.... Ceres's shape is too distorted. Its shape is not spherical enough to be like regular planets. And, to get really technical, no planet is really a sphere. Due to rotation, all planets have a slightly distorted shape.
planet n.
1.) An object orbiting a star that is smaller than Cowboyneal's ego but larger than his mother. 2.) Cowboyneal's mother. 3.) Any large piece of rock, such as a fundie's brain.
Repeal the DMCA!
Badly?
If the crew of Enterprise would use the teleporter to reach the SURFACE of it, then it's a planet, if they are teleporting to a chamber inside it, it's most likely an asteroid or something.
"Your superior intellect is no match for our puny weapons!"
This is because the moon is so massive and close to earth compared to all other planetary moons in the solar system.
We need to define what a "moon" is, and I would suggest a definition based around the relative gravitational forces on the body of sun and primary. The sun is about 300000 earth masses and is about 400 times as far from the moon as the earth is - so a rough calculation suggests that the sun-moon gravity is about twice that between the moon and the earth. On this basis, the moon seems to be a satellite of the sun rather than the earth, and the earth-moon system is a dual planet. Despite the size of the inner moons of Jupiter, their paths are almost totally controlled by Jupiter's gravity and they are moons.
I can't find the reference, but I think Isaac Asimov may have made this point at greater length in a magazine article.
Panurge has posted for the last time. Thanks for the positive moderations.
How about this: A mass with an self-sustained atmosphere of measurable magnitude.
I swear, the reason we're not in flying cars powered by cold fusion is because the world's best and brightest are too busy arguing over stupid things like the definition of a planet. Maybe I need to rethink my concepts of "best" and "brightest."
scientific terms need an absolute definition
A rigorous definition may be necessary but not sufficient for the "usefulness" of a scientific term. In this case, it doesn't seem like the proposed definitions are useful.
If we determined (for example) that bodies above a certain mass had some other properties of interest, or that stellar systems with a certain number of bodies above a certain mass had some interesting properties, then it would be useful to define "planets" as being bodies of at least that mass. The class of such bodies would be a regular subject of analysis, and it's easier to say "planets" than to say "non-stellar gravitationally stellar-bound bodies of Werkeltroff-Schmeltergruber-Minayevich mass or greater."
See, in the ordinary course of developing a scientific lexicon, we discover scientifically useful concepts, and then define terms for those concepts in order to provide economy of expression.
In this case, however, it seems that we have a term that already exists in the popular lexicon, but no related scientific concept with a compelling need for the term.
So why bother? Why not just allow the term to continue its peaceful existence in the popular lexicon, without attaching an arbitrary definition to it? Are we trying to serve some purpose other than allowing slashdot geeks another way to point out where the popular press gets something wrong? If we can identify a purpose for a definition, I the definition will probably follow naturually from the identified purpose; if not, then the whole discussion is silly, at best.
How do we solve that? We say what we mean in a particular context and then use the word as a shorthand. "In this paper, we will use the term 'planet' to refer to extrasolar bodies with diameters over 700km and masses less than 13 times the mass of Jupiter." "In this paper, we will be talking about the traditional nine planets of the solar system, Mercury, Venus, ..." Etc.
Terms like "planet" would actually be less useful if they did have a precise definition, because than each of those papers would have to use a much more awkward circumlocution when referring to bodies that don't meet the definition precisely.
Quaoar - http://www.gps.caltech.edu/~chad/quaoar/ Varuna - http://www.ifa.hawaii.edu/faculty/jewitt/varuna.ht ml
Ceres - http://www.mallorcaweb.net/masm/Ceres1.htm
cat
Problem: There are likely many Pluto-sized objects
in the Kuiper Belt. How about making the
threshold dependent on radius / solar-distance?
Pluto lucks out by being close enough to get
counted first.
-I like my women like I like my tea: green-
I like Basri's definition; it makes the most sense so far. But I wonder if one should take into account the shape of the orbit of the object. A highly irregular orbit (as in, "not moving in (or close enough to) the same orbital plane as the majority of objects orbiting the star") might make an object an asteroid. But this definition might not make sense anywhere else but the solar system; also, it might throw out Pluto (but just because we call it "Planet" now doesn't mean we have to make sure the definition fits it).
So for now the characteristics used to define a planet include (1) directly orbiting a star, and (2) having a size large enough to allow gravity to shape it spherical. Maybe there will be (3) its orbit has a certain nature, or (4) its distance from the star is not larger than x, or (5) its density is between x and y (because maybe there might be non-round objects more than 700 km in diameter which are simply not dense enough); (5) might be avoided by defining (2) as above through the shape and not a diameter number.
I haven't seen many posts here trying to define characteristics -- I'd like to see some ideas here, even if it might mean that Pluto isn't a planet anymore, or we suddenly have twelve planets in our system.
If Pluto was discovered today, it'd just be considered an asteroid or Kuipier belt object
The fact that its called a planet is just due to when it was discovered.
Not that it really matters...
The International Astronomical Union released a statement (a little dated) that they would not consider changing the status of Pluto. It can be found here.
The IAU is the body that would make such an official decision and it seems they don't want to change it.
Because new objects have been and will be discovered in our solar system, and others, that some might think of as "planets", and so the question of where actually to draw the line has arisen (actually many years ago, this is just the latest proposal).
Anyway, I won't continue this thread. Feel free to make more condescending comments for your fans.
it's a space station!
I think you mean: Should have 5% of the size of the parent star, not 5% bigger...
It doesn't work any way you slice it. He grossly underestimated how big stars are.
Even at 5%, it wouldn't look good for Earth's status as a planet. We aren't even 1% the Sun's size, and we don't have even a tenth of one percent of the Sun's mass... and our Sun is rather small as far as stars go.
sysadmins who name their servers after planets.
Fine for your first 2 or 3 servers, but...
Now if we could just lose the rest of Disney, our freedom might be safe.
-Miko
Miko O'Sullivan
scripsit minus_273:
If they weren't doing photosynthesis, however, why on earth (er, on planet?) would they look anything like terrestrial plants? There is a practically infinite variety of forms to choose from, and non-photosynthetic plants could just as well look like mushrooms, or brown algae, or whatever -- but there'd be no reason to grow up if they weren't trying to reach light.
In principio creauit Linus Linucem.
Look down.
Look up at the Sun.
See? Earth is definitely much larger than the Sun.
The earth and moon are a double-planetary system. If you calculate the gravitational pull of the earth on the moon vs the pull of the sun on the moon, the sun's pull is always grearter. That means that the moon's orbit is always concave toward the sun. The earth does a lot to perturb the moon's orbit, but it's not strong enough that the moon can be said to orbit the earth as the earth orbits the sun.
The moons of jupiter and saturn, for example, move in paths that are always concave toward their respective planets. Earth's artificial satellites and so forrth can be sait to orbit the earth, but the moon does not.
Liberty uber alles.
Jupiter would probably need about 10x the mass it currently has to start fusing. It would probably be a red dwarf if this was the case -- relatively cool compared to Sol, but super-long lived.
Bodies like Jupiter and Saturn are sometimes referred to as 'Brown Dwarves'.
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No, but Uranus occasionally erupts! *ba boom*
:p
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The moon fits point 'a', point 'b' and point c, but does not fit point 'd' or point 'e'; therefore is not a planet. The moon orbits the Earth (a planet.) The Earth-Moon unit (no relation to the Zappa's) orbit the sun (which is a star.) The moon itself doesn't orbit the sun, rather it orbits the sun along with us. Ergo, it's a satellite (and not the Toshiba kind, either.)
I remember readin that if Jupiter had just a bit mor mass, it would be a star itself, and that would certainly make things interesting. Perhaps if we start shooting out trash at jupiter....
All those AOL disks ought to be enough to ignite it.
Table-ized A.I.
The poster should have pointed you guys to this Berkeleyan article where the whole debate is fleshed-out...
... Planemos, KPOs, 'super-Plutos' [berkeley.edu]
An orb by any other name
I may be wrong, but gas giants (like jupiter, saturn, neptune, etc) become brown dwarves at 10-13x Jupiter's mass, and the point at which they become full stars (well, red dwarves) is some unknown mass above that. Really- last I checked, the exact mass wasn't known.
So, to recap. Stars are objects massive enough to maintain nuclear fusion in their cores and are hot enough on their surface to radiate visible light. Brown dwarves are objects not as massive as stars, but massive enough to maintain fusion in their cores but aren't hot enough to radiate visible light. Planets are objects not as massive as brown dwarves, but are.... uhhhh.... hmm....