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?"

Comments

Shape and orbit

[Belseth]

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.

Re:Shape and orbit

[rm999]

Thats an observational definition (all the planets are round, so that makes a good definition) but like all definitions of planets that have been so far this produces problems. A couple I can think of:

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

[Belseth]

A baseball isn't a naturally occuring object. There obviously would have to be some limitations set as to what defines a round body since perfectly round is impossible. The round shape more defines a given mass. Low mass objects can be fairly large and still not be round but above a certain mass the gravity tends to form round shapes. It has to be a definition of mass and orbit since even composition brings up issues. Half the planets in our system aren't rocky and everyone seems to agree gas giants are planets. Are large gas clouds planets? There needs to be a mass range given as a ceiling for gas giants as you enter brown dwarf territory at a certain point. Exact definitions are subjective but general ones should be easy enough. After mass and orbits are definated simply live with the result. scientifically splitting hairs is pointless. Condensed objects vary from asteroids and comets to suns and everything in between. Size, shape, composition and orbit are the defining factors but there will always be close but no cigar objects. The line between will always be arbitary. Say it's .25 earth mass but a new object is found that is .2499999999 earth mass, is it a planet? No based on the definition. The line between a large object orbiting the sun and a planet will always be arbitary.

Re:Shape and orbit

[rm999]

"Exact definitions are subjective but general ones should be easy enough"

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?

By mass & composition

[EngrBohn]

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.

Simplest is best

[Greg Hullender]

Use Pluto as the yardstick. Require a "planet" have at least the mass of Pluto and be in solar orbit -- any solar orbit, regardless of eccentricity or orientation.

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

Why bother?

[Ardeaem]

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)?

By that definition,

[Sturm]

wouldn't stars be "planets" as well?

Re:Gravity

[radtea]

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.

Re:Gotta be more specific....

[E8086]

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.