Slashdot Mirror
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."
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.
How do you pronounce Quaoar?
And the l33t shall inherit the 34r7h.
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.
Feed the need: Digitaladdiction.net
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
okay, well a planet should fall under these circumstances:
1.) in a regular orbit around a star
2.) a generally round shape
3.) has regular rotation around a axis whist it orbits around its star
so basically that should do it, a planet does not need to have a atmosphere, but it should also be large enough for it's own gravity to make it a spherical shape. #3 is another thing that is overlooked often, and it also will keep asteroids and comets from being defined as planets.
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
Then I must ask, though it's been mentioned here - Does that mean the Moon is (or will be) a planet? In a few billion years, the Moon and Earth will be a binary system, and the only thing signifying the Earth as the planet is the fact that it will be the larger one.
(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?)
"In the end they will lay their freedom at our feet and say to us, 'Make us your slaves, but feed us.'" -Dostoevsky
That would include black holes, the odd rock about the size of a silly putty egg drifting on its own through "empty" space, a comet, a large gas cloud and flecks of paint that came off an Apollo mission.
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
1. Should revovle around the star & should not have 20% more eccentric orbit(to a circle)
2. Should be 5% bigger than the parent star
3. Should have volcanic activity.
If EITHER one is not satisfied..it is a big space-rock
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"?
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.
but what do i know, i'm just a model.
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.
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?
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.
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.
Pluto, if stripped of its rank as it should, shouldn't be called an asteroid, but a comet. It is made of the same composition as one but it just cannot get close enough to the center of the Sol system to have a tail. Instead of the smallest planet it could be known as the largest comet, but who knows what that Oort cloud is composed of...
- Danny
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.
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