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How Would You Define a Planet?
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?"
The obvious conditions are round shape and orbits the sun. Size is somewhat subjective although to have a round shape it would have to be above a certain mass.
Personally, I think a good definition of a major planet is one that is massive enough that, given its composition, it assumes a sphere-like shape.
cb
Oooh! What does this button do!?
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
Words like "planet" are meant to "carve nature at its joints". Problems arise when historically there appeared to be joints (planets moved differently than stars in the sky) but, we are learning now that there are no useful joints here. Why bother defining the word planet at all? Is it really that useful to astronomers? And why, say, want Mercury (a small rocky body with no atmospere) to be grouped in a category with Jupiter (a large, mostly gaseous body with an atmosphere) instead of with asteroids (small, rocky bodies with no atmospere)?
No, Xena (one of the new trans-neptunian objects) is quite a bit larger than Pluto, so that would be 10 now.
--The universe will not be altered by forum threads, even those which are very wry. --Tycho Brahe (Penny Arcade)
wouldn't stars be "planets" as well?
Orbits a star or stars.
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")
When borderline cases arise, concepts proliferate.
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.
Blasphemy is a human right. Blasphemophobia kills.
I would add the condition that it must orbit a star, (to exclude moons) and not be a star itself (to exclude binary or multiple stars). And not be part of a belt of similar objects (to exclude Ceres, Juno, Pluto/Charon, and Sedna which are all spherical).
The days of the digital watch are numbered.
"planet" is a social term, not a technical one and freeze the list of planets at 9.
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.
Clear, Dark Skies
That would further expand the definiton of a planet as there isn't much you can't blow up with the Death Star, the rebel cruisers in Return of the Jedi and the targeted rebel base in A New Hope on the moon of Yavin. It may even be able to destroy a star, since it got from Aldaraan to Yavin in what was at most a few I'm guess it has hyperdrive and can outrun the shockwave created by an exploding/collapsing star.
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.
F7 doesn't work, ignore spelling and grammar
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?
If you can land on it and score with an alien chick, it's a planet.
By that definition, most slashdotters live on a big blue asteroid.
Table-ized A.I.