Defining "Planet" 470
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."
The definition of "planet" is universally ... (Score:5, Funny)
Re:3 parameters (Score:5, Interesting)
Re:3 parameters (Score:3, Funny)
Thanks, I'll be here all night
Re:3 parameters (Score:3, Informative)
Bodies like Jupiter and Saturn are sometimes referred to as 'Brown Dwarves'.
Re:3 parameters (Score:3, Funny)
Look down.
Look up at the Sun.
See? Earth is definitely much larger than the Sun.
A simple rule of thumb: (Score:5, Funny)
Alpha Centauri? Bah--probably a reflection off that Hubble thingy.
Re:A simple rule of thumb: (Score:2)
Re:A simple rule of thumb: (Score:5, Informative)
Re:A simple rule of thumb: (Score:2)
not to mention a Disney character
Re:A simple rule of thumb: (Score:5, Funny)
Re:A simple rule of thumb: (Score:3, Funny)
Re:A simple rule of thumb: (Score:5, Informative)
The most common Latin word for "Earth" is Terra, the name of the goddess of the Earth. That's right, Terra. She is I believe almost exactly analogous to Gaia.
Gaia is Greek; another Greek form of the name is "Ge." She is a major early goddess (early meaning pre-Olympian).
"Tellus" is Latin for "land" or "earth," including the concept of Earth as a planet. The name is used for a goddess; that -us ending is not the same one you know from "alumnus," but is feminine 3d declension, and forms its plural as "Tellures." I don't know how it relates to "Terra" or "Gaia" (most educated Romans knew Greek as a second language).
Quaoar, Ceres, and Varuna are all the names of gods or goddesses. Varuna is a Hindu god, of rain, I believe, and so a type of creator god; Quaoar, a native American creator god (IIRC); Ceres is the goddess of agriculture in Roman mythology (she is called Demeter in Greek; the long Homeric poem Hymn to Demeter is the centerpiece of her myth; her daughter Persephone might be familiar to SF fans).
Ceres is also the patron goddess of Sicily, and her discoverer was G. Piazzi, a Sicilian scientist. It was given such an important name (Ceres was a major goddess) because it was assumed, from the application of Bode's "Law," that there must be a planet between Jupiter and Mars, and when Ceres was found, it was at first trumpeted as a planet. However, when the asteroids named after Juno (=Hera, the queen of the Gods), Pallas (=Athena, the goddess of wisdom, warfare, etc.), and Vesta (~Hestia, the goddess of the hearth and home, more important to the Romans than to the Greeks - you've probably heard of the Vestal Virgins, the priestesses of Vesta who kept the eternal flame going in her temple and took an oath of chastity they were executed for violating) were all found in roughly similar orbits, they were reclassified as not "planets" but "asteroids."
Hmmz (Score:2, Funny)
How about "Life sustaining?" (Score:5, Interesting)
Re:How about "Life sustaining?" (Score:2)
Re:How about "Life sustaining?" (Score:2, Interesting)
Actually The moon is 2,476 Km, and it does not maintain an atmosphere. So I would doubt that a 700 Km body could sustain one. (And yes I understand just be cause the moon does not have one, that it may very well be capable and that it is just lacking one.)
Re:How about "Life sustaining?" (Score:2)
So you're proposing a two planet solar system? Just Earth and possibly Mars?
Re:How about "Life sustaining?" (Score:5, Interesting)
Well, sometimes Pluto has an atmosphere, and sometimes it doesn't. Only when it gets closer to the Sun in it's orbit does it "generate" an atmosphere from sublimation of ice. Later on it evaporates away be due to lack of gravity to hold it there. I doubt we would classify it as a part-time planet. BTW.. comets can have a "pseudo atmosphere" too.
Uh oh (Score:2, Funny)
Planet (Score:5, Funny)
Planet: n. Any object orbiting a star, not orbiting a planet, and having a radius greater than the radius of Pluto minus one millimeter.
Re:Planet (Score:2, Informative)
And there are a few moons larger than Pluto... would they become planets, even though they orbit a planet? (Or, converseley, does a planet have to orbit a star? Can it orbit other things?)
Re:Planet (Score:2)
No, because the parent's definition clearly states objects orbiting planets are not planets themselves - they must only orbit a star.
Re:Planet (Score:2, Interesting)
(Question - Does anyone know if, when that happens [the Moon rises to a high enough orbit to be "geosynchronous" and the same faces are towards each other always] will the two bodies orbit around a neutral point, which may or may not be above the surface of the earth, or will the Moon still completely orbit the Earth?)
Re:Planet (Score:4, Informative)
Re:Planet (Score:4, Interesting)
Summary: If that is supposed to mean if the moon orbits the centre of gravity of the earth: It doesn't do that now, but the approximation is getting better over time.
Re:Planet (Score:3, Insightful)
Well, if you're nit-picking about the original poster's definition, you should read the definition you're nit-picking about.
Re:Planet (Score:4, Interesting)
Now this raises another interesting question... what is the definition of a moon? Many moons we know of are round like our moon, but there are a few that are too small to become round (i.e. Mars' moons, Deimos and Phobos) yet we still consider them to be moons. But if even non-round satellites are considered moons, where do you cut the line? You wouldn't consider all the rocks in Jupiter/Saturn/Uranus/Neptune's rings to be moons, would you?
Re:Planets (the Indian definition) (Score:2, Informative)
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.
What is the mass/orbit of an unladen planet? (Score:3, Funny)
Tradition should rule (Score:5, Insightful)
As for planets and planetoids, etc., in other systems, go completely scientific. In time, if we do enough research on other planets, Pluto's definition may shift. Until then, don't just wake up one day and say, "OK, it's not a planet anymore!" I don't think it's prudent to start saying that Pluto isn't a planet until we've discovered and researched other planets.
Re:Tradition should rule (Score:2)
It wouldn't be prudent. Not at this juncture. Not gonna do it.
Re:Tradition should rule (Score:5, Insightful)
Re:Tradition should rule (Score:5, Informative)
Re:Tradition should rule (Score:4, Interesting)
There is no privileged point of view in the universe, so one can define the "center" as he bests wishes.
I see how it is! (Score:5, Funny)
Re:Tradition should rule (Score:5, Funny)
This should clear things up then (courtesy of the Miriam Webster online dictionary) ....
Pronunciation: 'pla-n&t /-"lIk/ adjective
Function: noun
Etymology: Middle English planete, from Old French, from Late Latin planeta, modification of Greek planEt-, planEs, literally, wanderer, from planasthai to wander -- more at FLOOR
Date: 13th century
1 a : any of the seven celestial bodies sun, moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars b (1) : any of the large bodies that revolve around the sun in the solar system (2) : a similar body associated with another star c : EARTH -- usually used with the
2 : a celestial body held to influence the fate of human beings
3 : a person or thing of great importance : LUMINARY
- planetlike
So in summary, Pluto and Uranus are not planets (1), but any asteroid on a collision course with Earth is (2). Also, Kofi Annan is a planet (3).
Why is size an issue? (Score:5, Insightful)
The whole point of the article is to arbitrarily define the distinction which just proves how stupid it is.
Jason
ProfQuotes [profquotes.com]
Re:Why is size an issue? (Score:5, Interesting)
Such a definition defies what *anyone* understands to be a planet.
While you are correct that the definition is going to be somewhat arbitrary, there is certainly an element of "knowing what it is when I see it" already involved.
Jupiter is a planet. A Coke can dropped out the garbage chute of a Vogon ship is not.
I think you'd have a hard time finding anyone who would disagree with the above.
In the same vein no one has ever come up with a clear definition of a human being either, but you're likely to know one when you see one with at least a certain level of accuracy.
Planets can't file civil rights suits though, so we get to define them, even though whatever that definition ends up being will also end up flawed.
I suppose the real question is whether having some sort of definition has a pragmatic *usefulness* in scientific communications, so that when one scientist is talking about planet the other one *knows* the object is question is *not* a giant gas cloud, paint chip or discarded Coke can.
The answer to that is, yes. Yes it does.
KFG
Why not set a defined width? (Score:5, Interesting)
Re:Why not set a defined width? (Score:5, Informative)
Diameters:
Pluto: 2274km
Charon: 1172km
Ganymede (orbits Jupiter): 5262km
Callisto (same): 4800km
Titan (orbits Saturn): 5150km
Triton (orbits Neptune): 2700km
Earth: 12756km
Moon: 3476km (Yes, our Moon is larger than Pluto)
Mars: 6794km
Deimos (orbits Mars): 12.6km
Phobos (same): 22km
(all figures courtesy http://seds.lpl.arizona.edu/nineplanets/nineplane
In other words, simple definitions based on size are inadequate. Also, since they're debating whether or not Pluto is a planet, the criteria that it orbits the sun may also be inadequate.
A planet is something which: orbits a star AND is round AND is larger than an arbitrary size AND.. what? The above criteria still allows for a lot of things to be planets that aren't.
We know so little about massive, non-solar bodies outside our solar system. Let's do a little more research on them before we start redefining things.
Re:Why not set a defined width? (Score:2)
Re:Why not set a defined width? (Score:4, Insightful)
I think you missed the most important comparison to support your claim:
Mercury: 4880 km
Ganymede and Titan are both larger than Mercury. This is important because there's no argument about Mercury's standing as a "real" planet.
will Titan be classified as a planet? (Score:2)
Maybe we need to define a planet as something relatively big, not orbiting something bigger than itself, and almost alone. e.g. Pluto is a planet because it's pretty much by itself and bigger than anything around it. Ceres is not a planet because it's got a lot of other stuff around it.
Even this exception would need an exception to handle things like Earth's moon and Pluto's moon.
Oh what a tangled web we weave...
Re:will Titan be classified as a planet? (Score:2)
Bah (Score:5, Funny)
Re:Bah (Score:2)
Sounds like a good idea (Score:2)
That seems like as good a measure as any. For something like this it's nice to have some sort of event like the forming of a sphere (or whatever you consider a sphere) to give a line in the sand rather than picking a nice sounding number.
On the other hand it didn't cover reason the other astronomers wanted to drop Pluto, is it missing some characteristics that the other 8 planets are? He also has a nice upper limit where a planet becomes a sun.
Still being the nitpicker I am one would have to wonder if they found a object as big as Pluto pulling a figure-8 around Jupiter and the sun would it be a planet or not?
Re:Sounds like a good idea (Score:2, Informative)
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.
The IAU Says There's No Cause for Concern (Score:5, Interesting)
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
Don't forget Luna. (Score:2)
There are those that claim that Luna (the moon) actually qualifies as a planet.
Rogue "planet"s ? (Score:2)
Re:Rogue "planet"s ? (Score:2)
Re:Rogue "planet"s ? (Score:3, Insightful)
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.
Somewhere along the line (Score:2)
It's not absolute size but importance in the neighbourhood, I would have thought....
Re:Somewhere along the line (Score:2)
Unless you count it's satellite, Charon, which is almost half as big as it is. [arizona.edu]
Re:Somewhere along the line (Score:2)
This is deeply troubling (Score:5, Funny)
Seems pretty simple (Score:4, Funny)
Re:Seems pretty simple (Score:2)
But it's clear that Pluto is a planet, if only because I wouldn't want the be the astronmer on the wrong end of Meiou Setsuna's "Dead Scream".
The Moon. (Score:3, Interesting)
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.
A good "compromise" (Score:5, Insightful)
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
Re:A good "compromise" (Score:4, Informative)
Let's rephrase that: there *might* be hundreds/thousands of Pluto-sized objects. But we certainly haven't found any yet!
- Pluto - 2300 km
- Quaoar - 1300km
- Varuna - 900km
- Ceres - 479km
- Chiron - ~175km
Note that Quaoar, the largest of the bunch, is half Pluto's size and barely larger than Pluto's moon, Charon.As long as Pluto is substantially larger than any other known transneptunian object, it doesn't seem like we would need to worry about planetary definitions.
Yours truly,
Jeffrey Boulier
Whats next? (Score:2)
planet vs asteroid (Score:2)
A planet is a planet and an asteroid is an asteroid. How hard is that to remember? Get with the program! Gesh.
Absolutely moot... (Score:5, Insightful)
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
Location Location Location (Score:5, Insightful)
Alone in it's orbit, it's moons orbits it
isn't too far out [go.com]
WARNING: Pun ahead! (Score:5, Funny)
It's not the size that matters, it's how you orbit!
*Dodging tomatoes should be a sport*
It has to be said...... (Score:2)
"Size matters not. Judge me by my size, do you?"
Sorry, my willpower is weaker than my taste for an perfectly timed movie quote. As for my opinion, I think what they are wanting here is a guideline for future exploration and habitation, or possibly to try and narrow down what would constitute studying under "planetary" effects like rotation, atmosphere, etc. and debris effects, like asteroids and comets. Still though, it seems kinda silly, but I can understand them wanting a more explicit point of reference. Still, it does sound kinda silly, after all, its a big universe, and remember, we're still learning it's rules, it doesn't play by ours. I'm sure theres a rock the size of a winnebago with its own atmosphere out there.....
Planets are fluid? (Score:2)
My $0.02
Use Star Trek's classification! (Score:5, Funny)
More to the point (Score:2, Funny)
Based on size? (Score:2)
If Ceres is a planet, then Paverotti could be one too
Any Definition Will Be Arbitrary (Score:5, Interesting)
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?
Bah, use the traditional rule... (Score:4, Funny)
My definition (Score:3, Insightful)
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."
language is rarely precise (Score:3, Insightful)
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.
The IAU word on the matter (Score:3, Informative)
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 [iau.org].
The IAU is the body that would make such an official decision and it seems they don't want to change it.
ObSW: That's not a planet... (Score:3)
More planets would be great for... (Score:4, Funny)
Fine for your first 2 or 3 servers, but...
Lose Pluto? (Score:5, Funny)
Now if we could just lose the rest of Disney, our freedom might be safe.
-Miko
what I'd use (Score:3, Insightful)
Re:Luna fulfills most of these... (Score:3, Insightful)
The Moon does not orbit the Earth (Score:3, Interesting)
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.
a better article... (Score:3, Interesting)
An orb by any other name
Re:silliness (Score:4, Informative)
Also, as we find bodies orbiting other stars, the traditional designations for planets is obviously useless.
Re:silliness (Score:2, Informative)
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."
Re:silliness (Score:5, Interesting)
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.").
Re:silliness (Score:2, Insightful)
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.
Re:silliness (Score:2)
Yes. Which is not contradictory with what I wrote.
And analogously, it would be silly to disqualify something that otherwise fit into our theory of planets perfectly on the basis of its size.
"Analogously" doesn't prove anything, it's just a way of illustrating what you believe.
And anyway, "size" WASN'T the determinant offered by the astronomer, but being large enough to be spherical due to gravity, which turns out to be about 700 km diameter.
Re:silliness (Score:4, Insightful)
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.
Re:silliness (Score:3, Interesting)
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.
Re:silliness (Score:3, Interesting)
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.
Re:silliness (Score:3, Interesting)
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.
Lumpers and Splitters (Score:4, Insightful)
Looks like astronomers do it too.
Different discipline, same problem.
Re:Earth's moon (Score:5, Informative)
This is sort of handled(here [infoplease.com]):
Since the COM is inside the Earth, I think it's fair to say that the Moon orbits the Earth (and not vice versa).
Re:Earth's moon (Score:2, Funny)
If we did that, then America would be accused of trying to be imperialistic, trying to run the cosmos, make-the-rules-for-everybody, or some crap like that
Re:Earth's moon (Score:5, Interesting)
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.
Maybe, ybe not (Score:2)
Whoa, too many things to clarify (Score:4, Informative)
It doesn't appear that Ceres has any satellites. But, there are 31 asteroids that do! [space.com] 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 [wolfram.com]. 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. [csmonitor.com]
Why does a planet _have_ to be a shpere...How perfect a sphere?
Well.... Ceres's shape is too distorted. [spaceflightnow.com] Its shape is not spherical enough to be like regular planets. And, to get really technical, no planet is really a sphere [regentsprep.org]. Due to rotation, all planets have a slightly distorted shape.
Re: How do you pronounce Quaoar? (Score:5, Funny)