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."

Comments

How about "Life sustaining?"

[beernutz]

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?

Re:How about "Life sustaining?"

[Anonymous Coward]

I mean could a 700km round body in space support an atmosphere?

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.

Why not set a defined width?

[Jailbrekr]

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.

Re:silliness

[1u3hr]

why can't we be content to keep the traditional designations of what make up a planet?

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.").

The IAU Says There's No Cause for Concern

[bziman]

... in their press release on the topic, nearly three years ago.

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

Re:Why is size an issue?

[kfg]

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

Re:3 parameters

[B3ryllium]

Does jupiter have volcanic activity?

Any Definition Will Be Arbitrary

[istartedi]

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?

Re:Planet

[n3k5]

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?

The system won't change from now until then, only some variables within it, e.g. the distance between the earth and the moon, the earth's rotational speed etc. You figured out correctly that two objects orbiting around each other at a constant distance actually orbit around a fixed point that lies on the line between their centres of gravity. The only thing missing from the puzzle (to answer your question) is this: It's the same for two objects orbiting around each other at a variable distance, just that the centre point is moving along the connecting axis. In fact, because the moon is closer to the earth than it will be in the future, it exerts a stronger force on it than it would in your 'geosynchronous' scenario, thus the earth is actually pulled out of its orbit around the sun more in the present.

Summary:

will the Moon still completely orbit the Earth?

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:Earth's moon

[panurge]

In fact, if you actually trace out the orbital paths, the moon does not "revolve" around the Earth. What actually happens is it is sometimes further from the sun and sometimes nearer, and it sometimes leads and sometimes lags the earth in orbit. This gives the appearance, from earth, of revolution, but from the point of view of an observer on a line perpendicular to the plane of the Ecliptic, it just looks like a wobble.

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.

Re:Planet

[umofomia]

The question is, what if it's eventually determined that Charon (presently considered Pluto's satellite) is as large or larger than Pluto? Can we have a binary planet?

Umm... Charon is already known to be smaller than Pluto. It's about half the size, though some people like to consider them a binary planet anyway since their sizes are similar.

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?)

No, the definition of planet says that the body must revolve around a star, not another planet. Bodies that revolve around a planet are moons.

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?