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

The definition of "planet" is universally ...

[B3ryllium]

... agreed to be "Marlon Brando"

A simple rule of thumb:

[AEton]

If someone bothered to name a Roman god after it, it's a planet. Pluto, Mars, Jupiter--all friendly planets.

Alpha Centauri? Bah--probably a reflection off that Hubble thingy.

Re:A simple rule of thumb:

[forkboy]

Eros is the same deity as Cupid...one is Greek, the other Roman. Eros is actually the Greek name, whereas Cupid is the Roman. However, since at the time the moon Eros was named Cupid was already defined in our culture as a fat little fairy with wings, they went with Eros.

Re:A simple rule of thumb:

[Da Web Guru]

Well, I guess that someone needs to name a Roman god "Earth"...

Re:A simple rule of thumb:

[Anonymous Coward]

You mean Gaia? I bet you thought you were being clever, too.

Re:A simple rule of thumb:

[kalidasa]

The most common Latin word for "Earth" is Terra, the name of the goddess of the Earth. That's right, Terra. She is I believe almost exactly analogous to Gaia.

Gaia is Greek; another Greek form of the name is "Ge." She is a major early goddess (early meaning pre-Olympian).

"Tellus" is Latin for "land" or "earth," including the concept of Earth as a planet. The name is used for a goddess; that -us ending is not the same one you know from "alumnus," but is feminine 3d declension, and forms its plural as "Tellures." I don't know how it relates to "Terra" or "Gaia" (most educated Romans knew Greek as a second language).

Quaoar, Ceres, and Varuna are all the names of gods or goddesses. Varuna is a Hindu god, of rain, I believe, and so a type of creator god; Quaoar, a native American creator god (IIRC); Ceres is the goddess of agriculture in Roman mythology (she is called Demeter in Greek; the long Homeric poem Hymn to Demeter is the centerpiece of her myth; her daughter Persephone might be familiar to SF fans).

Ceres is also the patron goddess of Sicily, and her discoverer was G. Piazzi, a Sicilian scientist. It was given such an important name (Ceres was a major goddess) because it was assumed, from the application of Bode's "Law," that there must be a planet between Jupiter and Mars, and when Ceres was found, it was at first trumpeted as a planet. However, when the asteroids named after Juno (=Hera, the queen of the Gods), Pallas (=Athena, the goddess of wisdom, warfare, etc.), and Vesta (~Hestia, the goddess of the hearth and home, more important to the Romans than to the Greeks - you've probably heard of the Vestal Virgins, the priestesses of Vesta who kept the eternal flame going in her temple and took an oath of chastity they were executed for violating) were all found in roughly similar orbits, they were reclassified as not "planets" but "asteroids."

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.

Planet

[blair1q]

Planet: n. Any object orbiting a star, not orbiting a planet, and having a radius greater than the radius of Pluto minus one millimeter.

Re:Planet

[Marticus]

The distinction between a planet-moon and binary planet system is usually the common centre of mutual orbit. If it resides in one body, that body is the planet, and the other a moon, however if it lies between them, in space, then it is a binary planet system.

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: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?

Why is size an issue?

[Jason1729]

I went to an astronomy talk at the University of Toronto a few years ago. The presenter defined a planet as any celestial body that doesn't radiate light. That explicitly includes asteroids and moons. Why is it necessary to make the distinction between planet and asteroid?

The whole point of the article is to arbitrarily define the distinction which just proves how stupid it is.

Jason
ProfQuotes

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

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:Why not set a defined width?

[aleonard]

Because some moons are larger than Pluto... would they be considered planets?

Diameters:
Pluto: 2274km
Charon: 1172km
Ganymede (orbits Jupiter): 5262km
Callisto (same): 4800km
Titan (orbits Saturn): 5150km
Triton (orbits Neptune): 2700km

Earth: 12756km
Moon: 3476km (Yes, our Moon is larger than Pluto)

Mars: 6794km
Deimos (orbits Mars): 12.6km
Phobos (same): 22km

(all figures courtesy http://seds.lpl.arizona.edu/nineplanets/nineplanet s/nineplanets.html )

In other words, simple definitions based on size are inadequate. Also, since they're debating whether or not Pluto is a planet, the criteria that it orbits the sun may also be inadequate.

A planet is something which: orbits a star AND is round AND is larger than an arbitrary size AND.. what? The above criteria still allows for a lot of things to be planets that aren't.

We know so little about massive, non-solar bodies outside our solar system. Let's do a little more research on them before we start redefining things.

Re:Why not set a defined width?

[Captain Nitpick]

Because some moons are larger than Pluto... would they be considered planets?

Ganymede (orbits Jupiter): 5262km
Titan (orbits Saturn): 5150km

In other words, simple definitions based on size are inadequate.

I think you missed the most important comparison to support your claim:

Mercury: 4880 km

Ganymede and Titan are both larger than Mercury. This is important because there's no argument about Mercury's standing as a "real" planet.

Re:silliness

[amRadioHed]

Because the traditional designation of what made up a planet was anything that we could find that orbits the sun. We didn't include comets because they looked different from planets and we didn't include asteroids because we couldn't resolve them. Now that we continue to find many large objects that are really little different from Pluto it has suddenly become important to have a real definition of which are planets and which are just big asteroids.

Also, as we find bodies orbiting other stars, the traditional designations for planets is obviously useless.

Bah

[DaLiNKz]

Why do they always need to complicate things. I thought size doesnt matter.

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

This is deeply troubling

[WankersRevenge]

As a representative one of the nine planets, I find this proposal deeply troubling, especially since there are not any other representatives from the other eight. Once a planet is classified as an "asteroid" or "floating piece of shit with gravity", it not only loses its prestige, but also, it cannot apply for federal grants, and hence, usually suffers a major economic blow. Laugh you may, but I've seen planets go from a heavenly body to a drunk spinning horizontal and finally distingrate into an asteroid belt in no time. We must support our planets because if we don't, then who will?

Seems pretty simple

[cpt kangarooski]

Is there a corresponding Sailor Senshi? If so, it's a planet. Ironically, this means that the Earth is not a planet, but the Moon is. Go figure.

A good "compromise"

[BTWR]

With the discovery over the past few decades of the Oort Cloud and Kupier Belt, it seems obvious that there are tens, if not hundreds/thousands of Pluto-sized objects out there. Obviously, we're not going to name all 10,000 of these rocks "planets." But then again, Pluto has a special place in history as the last "great" planet discovery on the level of Uranus and Neptune, so purists wouldn't want to ruin that by demoting it.

My solution? Define "Planet" as something bigger than Pluto, maybe with Mercury as the smallest, or whatever. But keep Pluto as a planet (as an exception ot the rule) for historical purposes. But, you may be thinking, "that's so stupid! Why give something a name if that name is now invalid?" The answer? We do it all the time. Here's an example...

Take a look at ANY diet softdrink/diet product with Nutrasweet. It warns you that this product contains "Phenylalanine" and should not be taken by "Pheylketonurics." Take a look at that word. It's called "Phenyl-keton-uria" (PKU) because years ago, people with this disease were diagnosed when "Phenylketones" were detected in their Urine. However, no one diagnoses PKU via a urine test anymore, they use another method. So should we change the name of the disease? Of course not. But due to historical significance, we keep it. Unlike the Indian/Native American designation, "Planet Pluto" should not offend anyone :-) Therefore, I say we define a planet however they want, but keep Pluto for historical significance. I knew med school would start paying off soon :-)

Re:A good "compromise"

[sysjkb]

it seems obvious that there are tens, if not hundreds/thousands of Pluto-sized objects out there

Let's rephrase that: there *might* be hundreds/thousands of Pluto-sized objects. But we certainly haven't found any yet!

• Pluto - 2300 km
• Quaoar - 1300km
• Varuna - 900km
• Ceres - 479km
• Chiron - ~175km

Note that Quaoar, the largest of the bunch, is half Pluto's size and barely larger than Pluto's moon, Charon.

As long as Pluto is substantially larger than any other known transneptunian object, it doesn't seem like we would need to worry about planetary definitions.

Yours truly,
Jeffrey Boulier

Lumpers and Splitters

[The Tyro]

This is just like taxonomy... some scientists like to lump similar creatures into one family or genus, while others like to split them up into different categories based on minor differences.

Looks like astronomers do it too.

Different discipline, same problem.

Re:Earth's moon

[gilroy]

Blockquoth the poster:

since they revolve around each other ... shouldn't we call earth/moon a two-planet system?

This is sort of handled(here):

It is also more accurate to say that the earth and moon together revolve about their common center of mass, rather than saying that the moon revolves about the earth. This common center of mass lies beneath the earth's surface, about 3,000 mi (4800 km) from the earth's center.

Since the COM is inside the Earth, I think it's fair to say that the Moon orbits the Earth (and not vice versa).

Absolutely moot...

[jtdubs]

This superficial naming convention makes absolutely no difference at all. It has no effect on anything.

It would be like if you changed the biological classification system so that bears were no longer Mammals. What difference does this make to the bears? None. What difference does this make in how we relate to bears? None.

It is simply an arbitrary naming convention. As are all naming conventions.

It reminds me of an old Zen saying that I am likely paraphrasing miserably:

"Before Zen, a mountain is a mountain. While one is practicing Zen, a mountain is no longer a mountain. After Zen, a mountain is once again a mountain."

Justin Dubs

Location Location Location

[infonography]

Just like in Real Estate, it's where it is not how big. The US States of Washington and Alaska have larger Island then Rhode Island. But Pluto is

Alone in it's orbit, it's moons orbits it

isn't too far out

WARNING: Pun ahead!

[Nathdot]

Why is there so much mention of radius and size and such. It's just so petty. I would think in this enlightened age we'd all know:

It's not the size that matters, it's how you orbit!

*Dodging tomatoes should be a sport*

Re:3 parameters

[B3ryllium]

Does jupiter have volcanic activity?

Use Star Trek's classification!

[Jugalator]

Just use the Star Trek planet classifications... Come on, it's time to make use of sci fi in astronomy for once. :-) Hmm, btw, I wonder what the heck the copyright at the top of the page is about? Courtesy JPL? Errr...

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?

Whoa, too many things to clarify

[helix400]

But doesn't it [Ceres] have a satellite? -- and -- What would we qualifty that as, because a satellite must orbit a planet.

It doesn't appear that Ceres has any satellites. But, there are 31 asteroids that do! That doesn't make them planets though...they're just small asteroids with really small moons.

Can anyone remind me what that sequence of numbers is called that vaguely predicts the distances of planets from the Sun?

Yep, its the Titius-Bode Law. Ceres does fit into this. But the reason we don't have a planet in between Mars and Jupiter is because "many astronomers think the asteroid belt is where a planet tried to form, but was pulled apart before it could solidify, caught between the strong opposing tugs of Jupiter and the sun's gravity." Quote taken from here.

Why does a planet _have_ to be a shpere...How perfect a sphere?

Well.... Ceres's shape is too distorted. Its shape is not spherical enough to be like regular planets. And, to get really technical, no planet is really a sphere. Due to rotation, all planets have a slightly distorted shape.

Re: How do you pronounce Quaoar?

[Ozymandias_KoK]

Badly?

Bah, use the traditional rule...

[gnovos]

If the crew of Enterprise would use the teleporter to reach the SURFACE of it, then it's a planet, if they are teleporting to a chamber inside it, it's most likely an asteroid or something.

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:silliness

[capologist]

scientific terms need an absolute definition

A rigorous definition may be necessary but not sufficient for the "usefulness" of a scientific term. In this case, it doesn't seem like the proposed definitions are useful.

If we determined (for example) that bodies above a certain mass had some other properties of interest, or that stellar systems with a certain number of bodies above a certain mass had some interesting properties, then it would be useful to define "planets" as being bodies of at least that mass. The class of such bodies would be a regular subject of analysis, and it's easier to say "planets" than to say "non-stellar gravitationally stellar-bound bodies of Werkeltroff-Schmeltergruber-Minayevich mass or greater."

See, in the ordinary course of developing a scientific lexicon, we discover scientifically useful concepts, and then define terms for those concepts in order to provide economy of expression.

In this case, however, it seems that we have a term that already exists in the popular lexicon, but no related scientific concept with a compelling need for the term.

So why bother? Why not just allow the term to continue its peaceful existence in the popular lexicon, without attaching an arbitrary definition to it? Are we trying to serve some purpose other than allowing slashdot geeks another way to point out where the popular press gets something wrong? If we can identify a purpose for a definition, I the definition will probably follow naturually from the identified purpose; if not, then the whole discussion is silly, at best.

More planets would be great for...

[Boss, Pointy Haired]

sysadmins who name their servers after planets.

Fine for your first 2 or 3 servers, but...

Lose Pluto?

[mikosullivan]

we could end up losing Pluto

Now if we could just lose the rest of Disney, our freedom might be safe.

-Miko

Re:3 parameters

[SEWilco]

Is the Earth 105% the size of the Sun?

Look down.
Look up at the Sun.
See? Earth is definitely much larger than the Sun.