Why the Word 'Planet' Will Never Be Defined

eldavojohn writes "What makes a planet a planet? Slashdot covered the great debate about whether or not Pluto qualified and Space.com now has up an article explaining why we'll never have the term 'planet' defined to a point that everyone can agree on. Divisions in the scientific community currently stand over whether or not it has to be in orbit around a star, the dynamics of the body in question and apparently the country you come from plays a part in it too. Some feel the United States is the dominant deciding factor on the definition but the IAU has not turned to democratizing the definition yet." From the article: "In the broadest terms, a planet could be thought of as anything from an 800-kilometer-wide (500-mile-wide) round rock orbiting a dead star to a colossal gas ball floating alone in space."

Comments

democratic?

[SillyNickName4me]

Some feel the United States is the dominant deciding factor on the definition but the IAU has not turned to democratizing the definition yet."

Lets see now.. democratically deciding a definition? hmm...

At any rate, the USA being the dominant deciding factor might make some sense seeing how they also invest a lot into the actual science part of this, but if the IAU did turn to democratize the decision, then the USA can't be the deciding factor seeing how they are a mere 4% of the world population....

Re:democratic?

[vertinox]

Lets see now.. democratically deciding a definition? hmm...

To be fair, all definitions are democratically decided even if no one votes on them.

If tomorrow everyone on the earth decided to call what we use to call the color blue as the color red... Then tomorrow the sky would be red.

If tomorrow everyone decided that a yard (or meter or what have you) is not 3 feet but now four and we adjusted all our documentation and measurement tools to reflect this then it would be so.

Heck... We could even call the Antarctic hot and the Sahara cold as long as we all agreed that the term hot meant one would "burn" to death of hypothermia and you would "freeze" to death of heat exhaustion.

Really... Definitions themselves do not imply or detail facts.

Calling something a moon or a planet does not change its behavior or physical properties, but it does change how we as humans relate to said objects and property behaviors.

Of course we don't go around changing things willy nilly because it is hard to get everyone to agree all at the same time. Although... Come to think of it... Since we are not all speaking English on this world of ours, we might not be really agreeing as much as we think.

Sometimes terms in other languages used for the same object or property, doesn't have the same exact meaning as another languages word for the same thing.

Re:democratic?

[SillyNickName4me]

If I say that whatever everyone else calls blue is in fact called red, then I made a definition. The fact that noone other then me might use it doesn't change that. While the usefullness of a definition often depends on how well accepted it is within the target audience, definitions themselves do not depend on that.

Original Meaning

[scottennis]

From Wikipedia:

In ancient times, Grecian astronomers noted how certain lights moved across the sky in relation to the other stars. These objects were believed to orbit the Earth, which was considered to be stationary. The "wandering" lights were called planets, a Greek term meaning "wanderer".

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Why not just stick to this original definition? If it "wanders" among the stationary celestial lights and casts light visible to the naked eye, it's a planet.

Everything else can be labeled SAO "speculative astronomical object."

Re:Original Meaning

[Anonymous Coward]

Why not just stick to this original definition? If it "wanders" among the stationary celestial lights and casts light visible to the naked eye, it's a planet.

Your definition of planet includes commets, which the Greeks didn't count as planets. To answer your question, the problem with the original definition is it doesn't let people claim they discovered a planet. That's the whole reason silly things like Pluto are counted as planets, because finding another KBO isn't going to make your name go down in history.

What's wrong with this definition?

[Ant P.]

"Anything which is roughly spherical under its own gravity, in a more-or-less circular orbit around the sun, and its orbital path isn't shared with an object larger than itself."

Why define it?

[Dirtside]

The real question is, why do we need a precise, "official" definition of "planet"? Astronomers and other scientists aren't going to make scientific decisions based on it -- it's not like it matters whether Pluto is officially a planet according to the IAU when an astronomer decides to study Pluto. "Oh, the IAU says it's not a planet, therefore it's not interesting enough to study."

In general, the whole point of category words like "planet" is so that I can point at an object and say, "That's a planet," and you immediately have some basic information about it, because we agree on what "planet" means. But if we're scientists, studying it (or deciding whether to study it), then we need a whole lot more info. Gas giant? Small, terrestrial rock? Iceball? Distance from star? Eccentricity of orbit? Etc. "Planet" doesn't tell you any of that.

Ultimately, the main reason to specify an "official" definition of "planet" is for the sake of deciding whether and how we want to encourage space travel, exploration, astronomy, and related sciences. To give an extreme example, if the definition of "planet" included any solid body primarily orbiting a star, there'd be millions of planets in every star system, and saying that NASA's going to go explore a planet would be meaningless. The public wouldn't care and wouldn't go out of its way to support it.

At the other extreme, limiting the planets to rocky or gaseous bodies at least the size of Mercury, orbiting a star, and having a very low orbital eccentricity, means that when you discover a body that only misses ONE of those criteria, the definition seems arbitrary and people will just ignore it. Imagine if we find a trans-Neptunian object that's the size of Mars, and is a rocky, terrestrial body like Mars, but merely has an eccentric orbit? Very few laypeople would accept that that's not a planet, mostly because laypeople's perception of a stellar body is based on its physical characteristics, not its orbital ones. If Earth was somehow flung out into space, orbiting nothing, it'd stop being a planet? (Well, we'd all be dead, but that's another issue.)

Re:Why define it?

[nine-times]

The real question is, why do we need a precise, "official" definition of "planet"?

Yes, exactly. The word "planet", as used today, describes a specific collection of bodies in our own solar system. It was devised by ancient astronomers to describe the lights in the sky that didn't follow the normal pattern of stars. Stars go in circles, planets go back and forth. The word has taken on a new meaning as our understanding of celestial bodies has grown, but now it's generally used to indicate that one of the collection of 9 specific bodies. Just as "sun" is used to indicate our star specifically, "planets" indicate specific bodies orbiting our sun.

If we really need a more general definition that's more scientific than "largish body that orbits a star," for the sake of scientific accuracy, then come up with a new term. Take the word "far" for example. It's a general-use word, not a scientific one. We might use it when talking about scientific issues, like, "the nearest star to the sun is far away," but it doesn't have scientific accuracy. This doesn't mean that we need to define "far" as "greater than 1 light-year" and then try to force people to stop claiming that someplace on earth is "far". It means that, if you want a precise scientific term to indicate "greater than 1 light-year away", you need to come up with a new word which isn't "far".

Re:Why define it?

[aitala]

Why define it? Cause that's one of the things astronomers do... they define and classify objects. Its just gotten more complicated because of new objects in the Solar System and around other stars...

This is NOT a science issue

[oneiros27]

It's a problem with any discipline -- language is not exact.

What qualifies as meat? Does seafood count as meat? Not for Catholics.

What qualifies as a person? What about in utero? Maybe for manslaughter, but why not count that time for age restrictions?

What qualifies as blue? Is cyan blue enough? It depends on what you're using the category for.

Anyone who's tried to work on standardize terminology (eg, specialized thesauri, or even just a controlled vocabulary) will know that it is a long, exhasting process that takes years in some cases, and even then, is likely to change.

Planets are not a classical category, and will be subject to prototype effects.

Richard Feynman on Definitions

[j_f_chamblee]

Richard Feynman had something to say about this debate, though somewhat obliquely. The parentheticals below are my own.

"You can know the name of a bird (or a planet) in all the languages of the world, but when you're finished, you'll know absolutely nothing whatever about the bird (or planet)... So let's look at the bird ( or planet) and see what it's doing -- that's what counts. I learned very early the difference between knowing the name of something and knowing something."

The point here is that scientific knowledge (whether it be social, biological, or physical) is about explaining how things work (understanding processes) or why they are the way they are (understanding variation). Debate over essentialist categories like "planet," "species," "nation-state," etc. are, as one other person in this discussion has already mentioned, problems of language.

Interestingly, Wittgenstein might have a thing or two regarding this topic as well, especially in later work.

Re:I got one...

[geoffspear]

If the Earth suddenly stops orbiting the Sun, I can confidently say that no one will care about the defintion of "planet" anymore. But since I can't imagine a situation in which we suddenly stopped being in orbit around the Sun that doesn't involve the planet soon afterwards being sucked into the Sun and crushed, I'd say "yes".