How Would You Define a Planet? 410
It doesn't come easy asks: "The argument over the definition of a planet continues. So far, two definitions are favored but without much consensus so far: base the definition of a planet simply on an object's size. Pluto would be near the lower limit and the newly discovered Kuiper Belt objects could also qualify, giving us 10 or 11 planets so far; or define the single dominant body in its immediate neighborhood as the only qualifying object for planetary status. If no one body dominated (such as the millions of individual asteroids in the asteroid belt) then none would qualify for planetary status. In this case Pluto would be disqualified (Neptune would be the dominant body in Pluto's region of space), and the newly discovered Kuiper Belt objects would also fail to qualify. The International Astronomical Union (IAU) working group charged with pinning down the definition of a planet may vote on the proposals within the next two weeks (or they may decide to start all over again with something new). Maybe Slashdot readers can give them some help. How would you define a planet?"
anything with a roman god name (Score:4, Funny)
Re:anything with a roman god name (Score:2, Funny)
KFG
Re:anything with a roman god name (Score:2)
Re:anything with a roman god name (Score:4, Informative)
I am fauna, but not terra. The child, but not the mother. I come from, but do not share identity.
In any case, the current official name of the earth is Earth, which is Germanic.
http://seds.lpl.arizona.edu/nineplanets/nineplane
KFG
Re:anything with a roman god name (Score:2)
Re:anything with a roman god name (Score:2)
Gotta be more specific.... (Score:5, Funny)
Anything you can blow up with the Death Star!
Re:Gotta be more specific.... (Score:4, Insightful)
What about twin planets, say Romulus and Remus? They both appear to be planets, they do support life, which one would be conidered the domanant planetary body in its region of space? As far as Pluto and everything else in the Kuiper belt, we won't be able to agree on it until infinite improbability drive is invented or V'ger 6 is launched to map the area.
Shape and orbit (Score:5, Insightful)
Re:Shape and orbit (Score:5, Insightful)
1. We will have to define round. This is a gray scale, and picking what "round" is will create controversy too. For example, how rough can the surface be? How oval can it be (even the earth isn't a sphere).
2. What about a baseball orbiting the sun? You need some sort of size requirement. The more liquidy a substance, the more easily it will become round at smaller sizes.
I don't mean to put down your definition - I actually like it - just pointing out that nothing is obvious in this debate.
Re:Shape and orbit (Score:3, Insightful)
Re:Shape and orbit (Score:4, Insightful)
Yes, exact definitions are subjective (and impossible). The problem in the first place was general definitions. We have generally defined planets as a large object orbiting a star. But this has only led us into problems and "scientifically splitting hairs."
I guess the lesson is if we can't define a planet, it doesn't really matter what a planet is. After all, "planet" is just a label. There realistically isn't a whole lot in common with Jupiter and the Earth, so why place them into the same category?
Re:Shape and orbit (Score:2, Insightful)
Re:Shape and orbit (Score:3, Interesting)
A better idea that I've heard being discussed would be to abandon the term "planet" altogether and instead label objects accor
Re:Shape and orbit (Score:3, Interesting)
Re:Shape and orbit (Score:2, Insightful)
Re:Shape and orbit (Score:2, Insightful)
Re:Shape and orbit (Score:2)
Re:Shape and orbit (Score:2)
I'm just wondering how they consider Neptune the dominant object in Pluto's section of space. Check out these facts...
Why does it need to orbit a star? (Score:3)
Quick definition (Score:5, Funny)
Oataox or whatever the hell? The guy who came up with that needs to be kicked out of the Astronomy club.
All Roman (Score:2)
I believe they are all Roman gods, but probably of Greek origin.
Sigh. (Score:4, Informative)
Saturn and Uranus were titans - beings that came before the gods. Neptune was named in modern times, but they kept the roman naming tradition, same with Pluto, Roman god of the dead.
And then they proceeded to waste all the other greek and roman names on every rock, brick and crater they could find which is why we're reduced to naming moons after Shakespearean characters.
The naming of Charon was a slick trick - the discoverer specified that the name be spelled like the name of the mythical figure, but that the name be pronounced "Sharon" - which happened to be the guys wife.
Re:All Roman (Score:3, Informative)
I'm partial to... (Score:2)
By mass & composition (Score:4, Insightful)
Re:By mass & composition (Score:5, Funny)
Re:By mass & composition (Score:2)
C//
caveat (Score:2, Insightful)
Re:caveat (Score:4, Interesting)
Personally I think it is silly to disqualify something because it has "neighbors" or orbits in a "belt".
My $.02:
Any object that revolves around a star, and is not a star, has enough mass to be roughly spherical (say +/- 1% of a perfect sphere) due to its gravity is a PLANET.
Objects that are roughly spherical that revolve around planets are MOONS (regardless of size.) If two objects revolve around each other and their center of gravity lies outside the radius of either partner then it is a binary planet.
Objects that revolve around a star that are not roughly spherical are MINOR OBJECTS. This leads us to a bit of a problem because under my definition there would be no distinction between our friends the comets and asteroids.
I imagine there could be a further classification, based on the shape of the orbit - so we can continue to have "comets" and "asteroids". However I do not like using the orbit shape in any definition.
Objects that revolve around a planet taht are not roughly spherical are SATELLITES.
By the way the reason WHY i do not like using the shape of the orbit or something like the vicinity of other objects in the definition is simple. Those characteristics can be changed. Orbits gradually change over time, especially early in the life cycle of the solar system.
Granted objects are "captured" by planets and stars, and "ejected" in the same way. However, given enough time (and any external influences) don't orbits tend to become circular? So just because comets have highly elliptical orbits now does not mean they will be elliptical forever. The orbital shape is a TRANSITION characteristic. It is not inherent to the object.
Oh well I have rambled on way to long about this....I am ponderig the much larger question - "Why do I care?"
Simplest is best (Score:3, Insightful)
The public will be happy to learn of more planets -- it feels like progress. It'll be hard to convince the public we lost a planet somehow. That sounds like an unimportant consideration, but I don't want us giving the Creationists more ammo for their arguments that Science is fickle. "They used to think there were nine planets, but then they found they were WRONG!"
It's not like any serious science rests on this definition anyway.
--Greg
Re:Simplest is best (Score:2)
The only issue, I can imagine, is the eccentric orbit that Pluto has. but everything else holds water.
Re:Simplest is best (Score:2)
The only trouble with that is that our own Moon is bigger than pluto. Our satellite has a radius of 1738 km versus 1195 km for the Kuiperplanet.
Compare NASA's moon factsheet [nasa.gov] with their pluto factsheet [nasa.gov]
Thank God fior the Uncyclopedia (Score:4, Funny)
Uncyclopedia: Planet [uncyclopedia.org]
Heh (Score:4, Funny)
If it's smaller
Wretched, isn't it?
Why bother? (Score:3, Insightful)
Re:Why bother? (Score:2, Interesting)
We could define "Small Orbital Bodies" and "Large Orbital Bodies" while still utilizing other terms, such as "brown dwar
Gravity (Score:2, Insightful)
By that definition, (Score:4, Insightful)
Re:Gravity (Score:2)
Probably with something about it's mass not reguarly (or within an orbit) changing more than a certain %.
Re:Gravity (Score:5, Insightful)
Apart from the Platonists in the audience, intelligent people realize that concepts are made things, artefacts created by humans to facilitate certain types of interaction with the world. Now, the world is a particular way, and that puts constraints on the sorts of concepts that are useful to us, but it doesn't determine a single set of concepts that will do the job. Therefore, concepts vary from person to person, and one person's pornography is another person's erotica, and so on.
Concepts, like all tools, are judged to be better or worse according to use. Some of the uses of "planet" are political--every astronomer monkey wants to be the discoverer of a "plant", because that will attract and impress other monkeys of the complementary sexual orientation. This is just part of our hertiage as monkey's, and we may as well admit it. Other uses are scientific--planetologists already divide planets into sub-categories like "gas giant" and "terrestrial planet", because quite different physical processes dominate these bodies, and distinguishing them allows us to focus our attention more fully on one set of processes or the other. For beings of definitely limited brain power, this is extremely useful.
Historically the term "planet" mixed several completely unrelated things: size, distance from Earth, and being in orbit around the Sun. Planets were "wandering stars", and it just happens that the only things that fell into that category were large bodies far from Earth that orbited the Sun. Things like the Moon, which is close, wasn't a planet because it had a visible disk, which stars do not. But this is entirely accidental--if one of the inner planets had had a moon visible from Earth with the naked eye it is likely that the concept of planet would already be more various than it already is.
I think it better to create a bunch of new terms that acknowledge the rich division of bodies we can now see, rather than get hung up on the historical term "planet". The things we care about include at least three axes: size, composition and orbit. Trying to assign a single word to a particular region of a three-dimensional space (which probably isn't even simply connected!) is a silly waste of time, driven purely by monkey psychology, and has no scientific value. In fact, it may even have negative consequences for science, because getting hung up on historical terms may also help keep people hung up on historical concepts.
So my vote would be to expunge the term "planet" from the astronomical lexicon entirely. It's the only way to be sure.
Re:Gravity (Score:2)
Re:Gravity (Score:2)
You are correct of course that everything has gravity (well, everything that has mass does) However, for a heavenly body to maintain it's spherical shape, it must have enough mass to generate the gravity to give it that shape. The definitions that astronomers have used for Planets has been wholy arbatrary whereas using gravity as your defining principle gives you a fixed point of reference. There are several
A consistant solar system centric orbit. (Score:2)
Who Cares? (Score:2, Insightful)
Tell those bitches to stop with the silly arguments and get back to the telescopes. When they have a valid scientific reason to differentiate a planet from a hunk of rock that just happens to orbit the sun, then we can start arguing about definitions with some kind of actual reason for it.
Re:Who Cares? (Score:2)
Of course I have no idea what the true motivations are of these people, and I'm probably just being silly, but the whole thing is so stupid on the face of it that it smacks of childish motivations. If you're going to be arbitrary about it anyway, why not arbit
Doesn't matter to me (Score:2)
I suggested... (Score:4, Funny)
I suggested this on www.randi.org [randi.org] a few weeks ago. In Pluto's case have astrologers draw up two parallel charts. One with Pluto as a planet, the other without. After a few weeks we can compare what happened in the world to the astrology charts and that'll settle it.
"The planets don't lie" as I said there. ;)
Re:I suggested... (Score:2)
Good idea too.
magnetic field (Score:2)
If it has a sustained atmosphere, then that's a plus, too (c.f., Mercury doesn't have one, however).
Re:magnetic field (Score:2, Informative)
Re:magnetic field (Score:2)
The main reason that Mars lost its atmosphere (or water) is that Mars is less massive, which allows particles (gases) moving at a tail of Maxiwellian velocity distribution. Having a magnetic field help trap charged particles, though I'm not sure how significant that process is in terms of planetary evolution.
In any case, just having a magnetic field wouldn't do.
Too many people (Score:2)
Like this (Score:5, Informative)
Whatever Wikipedia says. (Score:5, Funny)
The definative answer: (Score:2, Interesting)
Disqualifying Pluto (Score:4, Interesting)
What exactly is the definition of a region of space?
How much larger must an object be than its neighbors in order to be considered the dominant object of its neighbors? Twice as large? Four times?
Define More Than Simple 'Planets' (Score:2)
What would actually be useful (Score:2)
How about, big enough that its gravity could retain an atmosphere?
The Kirk Test (Score:5, Funny)
Re:The Kirk Test (Score:2)
Okay, we're all curious now: What are you calling Earth these days?
Re:The Kirk Test (Score:2)
Re:The Kirk Test (Score:2)
eg the 'sky' is 25M above or too hot / cold / electrified. Ok so you might be able to touch it, but it would be bad for your health
Something noticable that orbits a star (Score:2)
If there are cluster of entities in the same slot, asteroids.
Brown is right. (Score:2)
Flash back to 1915: hmmm...now that Einstein has published his theory of relativity, should we reconsider our definition of the term "luminiferous aether?" No, the term needs to go away completely, because it's become clear that it serves no useful scientific purpose.
Why is this so hard? (Score:3, Insightful)
Enough mass so that its gravity forces it into a spherical or an ellipsoid shape.
This defintion does make large astroids like Ceres a planet. Personally, I don't necessarily have a problem with this, but I don't really care. If you want to remove these you can add:
Must be a "free standing" object (i.e. not in a belt)
If you're dead set against Pluto, you can add:
Orbital inclination must be close to the orbital plane.
I not be an astronomer or an astrophysicist, but I really don't see what's so hard about defining a planet. Whatever the Powers That Be(tm) decide, it should be based on physics and not legislation. (e.g. "mass in excess of x metric tons")
Easy (Score:2)
If there's a Sailor Senshi... (Score:2)
And don't go talking to me about Sailor Moon. The Moon isn't a planet, because Sailor Moon is really Princess Serenity.
Morans.
Has anyone asked... (Score:3, Funny)
...bigger than a baby's arm? (Score:2)
Gravity? Christmas trees?
Who cares... (Score:2)
(There are monkeyboys in the facility.)
Why go binary? (Score:2)
We can't even agree within the limited sample size of this solar system on the definition of "planet." What happens when we get really good at finding objects that orbit other stars and have hundreds, thousands, millions, billions of potentially planet-like objects to classify. I'm sure wi
howabout... (Score:5, Interesting)
therefore, mercury would NOT be a planet (more like a moon of the sun)
and titan, even though it orbits saturn, WOULD be a planet
i think that makes most the most sense: what an object orbits shouldn't matter, it's composition should be the largest consideration
some other nomenclature can address what it orbits ("a moon of the sun" or "a planet of saturn")
it should be considered either
REGARDLESS of what it orbits
size, rotation on axis, gravity, moons (Score:2)
Size matters as well. How big is big enough? Well I would say it has to do with its gravitational force on other celestrial bodies.
Also, is it a orbiting another body? i.e. is it a moon? Our moon spins on an axis, exerts gravitational force on our ocean, but is a satellite of Earth.
Gravity of the situation (Score:2)
Spherical shape nor mass alone should be a factor. If you're going to go JUST by spherical shape and mass then the Earth's moon could be called a planet.
But obviously, we do need to take into consideration the object's mass, or every rock in the asteroid be
Argh! (Score:2)
Is it big?
If not, call it planetoid. Ta-da!
(Or am I being too simplistic?)
In a word, yes. (Score:2)
So, are *you* planning to memorize the names of an extra 40-50 planets over the next several decades?
Re:In a word, yes. (Score:2)
I'm voting for geology (Score:2)
Second, I think geological activity sufficient to create a distinct core, mantle, and crust would define the solid planets. The gas giants will obviously have some strata or some differentiation, so until they produce fusion, they count.
How does that sound?
I would say... (Score:2)
Wow. That's wrong on several levels. (Score:2)
Second, even guessing at what you meant, the definition is still ridiculous. Mercury doesn't have an atmosphere - at least no more than our moon has. On the other hand, there are lots of small irregular bodies that sport atmospheres at least some of the time - comets for example.
So, by your definition, we'd lose Mercury and gain hundreds of celestial chew toys.
Easy - second option; grandfather clause (Score:2)
Everyone's happy.
Pond or Lake? (Score:2)
Are you sure it's a river? (Score:2)
Re:Are you sure it's a river? (Score:2)
Generally streams are larger. (Score:2)
In my area, we've got a Perkiomen Creek that is quite wide (and serious enough to have cut a nice valley) but the Manatawny River barely qualifies as "wet" if you ask me.
Oh, and we tend to call them "criks" in Pennsylvania, too.
Easy, Slashdot-friendly method (Score:2)
Anything requiring 25% to 50% is a moon (Wait, that's no moon!)
Anything less than 25% is a rock.
(Those of other interests can substitute "Xindi Superweapon" for "Death Star" in above definition)
Simple... The Deathstar Test (Score:2, Funny)
The sensible thing is to realize that (Score:3, Insightful)
This leaves us free to give the things we discover designations that reflect their structure or their position.
I *do* think we will eternally regret wasting so many perfectly good names on moonlets and asteroids.
My definition (Score:2)
Any object with it's own orbital path around a star, with it's own atmosphere as faint or distinct as it might be.
By that definition (Score:2)
I define a planet EXACTLY... (Score:2)
A non-luminous celestial *BODY* (not bodies) illuminated by the light from a star, such as our sun, around which it revolves.
Comets, I think would not fit this definition, nor asteroids, simply because their orbit does not follow the regular path of orbit that the other planets do (minus Neptune/Pluto, which have switched orbits, making Neptune once again the closer 'planet' to
If the people think it's a planet... (Score:2)
Let CowboyNeal Decide (Score:2)
If you want additional caution, there is always the duck test.
Bottom line (Score:3, Insightful)
The bottom line here is that most professional astronomers don't care about these objective definitions. When astronomers are doing research, none of it hangs precariously on the definition of planet or asteroid or something else. They specify what they mean -- Jupiter, or the major gas giants, or the Earth-crossing asteroids, or transneptunian objects, or plutinos.
These names (planet, asteroid, comet, etc.) are just arbitrary labels invented by people, after all. They have no special significance, and they never have. After all, planet comes from the Greek word for "wanderer," a reference to the fact that planets appeared to be stars in the sky that moved. Asteroid means "star-like," a reference to the fact (as astronomical observations improved) they appeared to be moving objects that didn't have observable disks like the other planets (because they are too small).
The IAU, the international organization responsible for such names, has never given them any objective definitions. Why? Because they don't need any. Sorting out terminology like this is almost completely ancillary to getting actual astronomy and astrophysics done. The very reason that those interested in establishing definitions can't agree on objective definitions underlines the point: because they are totally arbitrary and not very important.
Almost all of the furor about redefining terms, recategorizing objects, demoting planets and promoting asteroids, has come from amateurs and the popular media. Don't you think that if professional astronomers thought that this was such a crucial issue that they wouldn't have taken care of it handily? They haven't because it's not nearly as important as amateurs seem to think. That amateurs and the popular media are seemingly fixated on such trivialities indicates strongly to me that they're missing something: namely that these classifications have no external significance.
The map is not the territory. Give it a rest, already. I know, why doesn't everyone concentrate their energies on doing actual astronomy?
Re:Planets should be bigger than Pluto (Score:2, Insightful)
Re:Dude... (Score:5, Funny)
BOOS: Big Objects Orbiting Star.
BOOBOOS: Big Objects Orbiting Big Objects Orbiting Star.
LOOS: Little Objects Orbiting Star.
FOSC: Floating Outer Space Crap.
Planet: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
Oh, and since I know you'll ask the difference between a BOOS and a LOOS is that a BOOS is large enough that it's own gravity keeps it roughly spherical.
Re:Dude... (Score:5, Funny)
Big Objects Orbiting Binary Stars...
Re:Dude... (Score:3, Funny)
Re:Planet criteria... (Score:2)