Wherein I argue emotionally about the definition of "planet"

PS: See a million other blogs discussing this.

Plutophants in Disarray I'll say one thing: Not having a coherent scientific theory can lead to factionalization. Strains appeared today among the Plutophants, as many among their number were dismayed by the latest IAU attempt to keep Pluto a planet. The new transparently plutophantic scheme involved no less than four categories of planets: 1. Terrestrial 2. Gas Giant 3. Dwarf Non-Pluton (a special class for Ceres, formerly an asteroid) 4. Dwarf Pluton (for literally dozens of ice dwarfs) Or something like that. No one's really sure if you have to say "dwarf pluton," or whether all plutons are automatically dwarfs, or whatever. Appropriately, those of us following the Pluto issue have also spilt into four categories, the Pluto-haytas (who will win) and three groups of fractured plutophants. For those of you following at home, here's a quick rundown: 1. The No Iceball Left Behind Group The NILBies are the bunch who officially won today. They produced the bizarre scheme above, following the philosophy that planet-ness is primarily all about roundness. Asteroids, ice balls, whatever---if it's big enough for its own gravity to squish it into a sphere, but not so big it undergoes fusion (becoming a sun), and it's not a moon, it's a planet. Many Plutophants were happy at first, because this means that Pluto and UB313 are definitely planets. But soon the scales fell from their eyes, and they realized that the NILBie victory spells doom to Pluto in the long run. Why? Because it means scores of trans-Neptunian planets will appear almost instantly. Like a sweaty Joe McCarthy, the IAU brandished a lit of 43 known plutons, and admitted they have a secret list of dozens more. This is your solar system: This is your solar system on NILBie: Hmm. Of course, not all Plutophants want the nonsensical scribbles above, which brings me to: 2. The Culture Vultures The Vultures are a hardy band of simple folk who believe that the mnemonics of their childhood are in fact the laws of the universe. Oh, sure, they make claims about "planets" being a "cultural" term, because, you know, science changes, but culture has always been the same. The Vultures are sort of like those people who think that popular music was perfected while they were in high school, and wonder why all these new bands even exist. So Pluto will never change in their lifetime, anymore than Supertramp can ever be replaced. They're happy with the current mess, thinking that as the clearly insane NILBies inflate the term "planet" to meaninglessness, the rest of culture will fearfully retreat to the warm glowing warmth of the "nine historical planets." That's their new buzzword, "Nine historical planets." Because, like, history stops in 2006? The Vultures seem to forget that our culture: Once called the sun and moon "planets"; Once called whales "fish" (see the King James and Moby Dick); Once used the word "animal" to refer solely to non-human animals. In all of these cases, the scientific usage ultimately won. Maybe people sometimes say, "Delta doesn't allow animals onboard." But asked to define animal, they'll admit to the scientific definition after a moment's thought. And anyone who says that whales are fish, or the sun a planet, is pretty quickly shouted down. If scientists start saying "eight planets" a lot, so will the rest of us. We listen to scientists, at least when it comes to stuff like planets. That's part of our, you know, culture. More on that later. Because here's our final Plutophantic faction: 3. The UB313 Guy This is a faction of one: Mike Brown. He's one of the guys who discovered UB313, which will be made a planet under the new NILBie scheme. But he realizes all too well that his discovery will be made much less interesting, given that 50-something other planets will appear with the same stroke of the pen. (He coined the "No Iceball Left Behind" slur.) In the NY Times today, he freely admits that eight planets is the logical scientific number. But then he rhapsodizes a bit about the Culture Vulture argument, with one slight change . . . He thinks UB313 should be a planet, too. I'm biased, but I like to imagine this question through the eyes of the child I was in the 1970's . . . If I had heard back then about the discovery of something at the edge of the solar system, I wouldn't have waited for a body of astronomers to tell me what it was. I would have immediately cut out a little disk of white paper and taped it to the poster of planets on my bedroom wall. That night, I would have looked up, straining to see the latest addition to our solar system, hoping that I, too, might someday find a new planet. I hope the union . . . simply declares 2003 UB313 our 10th, full-fledged planet. Doing so might convince schoolchildren to put new paper disks on their walls, to look up to the sky and realize that exploration does continue, and that they can be part of it, too. Well, except they can't, Mike, because it's pretty much all iceballs from here on out.* But at least your iceball wouldn't be left behind. Which brings us to . . . 4. The Pluto-Haytas So while the Plutophants are crumbling into disarray, what does head Pluto-hayta Neil deGrasse Tyson have to say? "A Plutophile is well served by this definition," he said. "It is one of the few that allow you to utter Pluto and Jupiter in the same breath." Diss! Dissing aside, though, here's my new plan: Let's get rid of Pluto by whatever means necessary. It's a friggin' iceball, okay? Let's keep saying "eight classic planets" until everyone drops the "classic." But before we define the word "planet" for all times and all places, why not wait until we have observed ten or so other solar systems in their entirety? Out there in the rest of the galaxy, there may be all kinds of crazy stuff: Multiple accretion disks! Ice-worlds big enough to impress even me! Objects that aren't round, but are still totally planets! Captured rogue gas giants with wacky orbits! We just don't know yet. And we don't want to wind up like the ancients who thought the sun was a planet, until they figured out that we went around it. And we don't want to wind up with a bunch of lame iceballs lumped in with the cool stuff, just because of the pathetic Plutophants. This ain't about your bedroom walls, kiddies, it's about the universe. So let's get some more of the universe under our belts before making final judgements. So Pluto's not a planet. It's an iceball. Deal.

I agree totally. The best solution would be to demote Pluto from planethood to a Kuiper Belt object.

When I taught writing, I used to give an exercise requiring students to define (in their own words) various common nouns. The toughest one was 'planet.' One problem with the astronomers' definition that I don't think anyone has mentioned (but it came up in student essays) is that the definition needs to distinguish between actual planets and quasi-planetary objects in other solar systems. Do single-star planetary systems follow the Sol pattern of small inner planets, gas giants, and Kuiper Belt---type objects? When I was younger and paid more attention to astronomy, the speculation was that Kuiper Belt objects would continue to the next solar system and could be stepping stones to traveling to Barnard's Star or the Alpha Centauri system, i.e., some objects in between systems would be ambiguous as to which star they belonged to. Has that turned out to be the case? I have no idea but if so the region beyond Pluto should not be populated with planets, but some other class of object.

BTW, Isaac Asimov wrote a book about the Earth-Moon system called The Double Planet, so it's not at all a new proposal.

www.corneroak.com

The Bad Astronomer weighs in:
http://www.badastronomy.com/bablog/2006/08/15/congratulations-its-a-planet/

I agree totally. The best solution would be to demote Pluto from planethood to a Kuiper Belt object.

When I taught writing, I used to give an exercise requiring students to define (in their own words) various common nouns. The toughest one was 'planet.' One problem with the astronomers' definition that I don't think anyone has mentioned (but it came up in student essays) is that the definition needs to distinguish between actual planets and quasi-planetary objects in other solar systems. Do single-star planetary systems follow the Sol pattern of small inner planets, gas giants, and Kuiper Belt---type objects? When I was younger and paid more attention to astronomy, the speculation was that Kuiper Belt objects would continue to the next solar system and could be stepping stones to traveling to Barnard's Star or the Alpha Centauri system, i.e., some objects in between systems would be ambiguous as to which star they belonged to. Has that turned out to be the case? I have no idea but if so the region beyond Pluto should not be populated with planets, but some other class of object.

BTW, Isaac Asimov wrote a book about the Earth-Moon system called The Double Planet, so it's not at all a new proposal.

www.corneroak.com

Let's ask the geocentrists what THEY think.

Ghosty . . . ?

(snicker)

The proper way to handle this has nothing to do with rigid definitions. Instead, I propose the "Shotgun!" system of classification.

It works a lot like the system for deciding who gets to sit in the front passenger seat of the same name. When an astronomer spots a planet through his telescope, he screams out "I call planet!" if he wishes it to be classified as a planet, "I call asteroid!" for asteroids, and so forth for any object he wants it to be.

These classifications are binding forever until a human lands on the body in question. Whoever lands there gets to override the astronomer if he wishes.

In the event that the astronomer fails to make a call upon discovery, anyone who the astronomer informs may steal the call, and so forth as knowledge spreads. The same applies to the landing rule: if the first guy down doesn't call it, as soon as the second guy makes a footprint, he gets to call it.

I can see no downsides to this system.

# justawriter Says: August 16th, 2006 at 2:35 pm Actually, an honest species would say that this system had only four planets, Jupiter, Saturn, Uranus and Neptune, and there are a bunch of little turds floating around that are considered important by the fleas living on the third turd.

Dover, DE (FP) In a surprise move, lawers at Cheatem and Bilkem have filed a class action lawsuit on behalf of the Moon, Titan, Ganymede, Callisto, Io, Europa, Triton, Rhea, Iapetus, Dione, Tethys, Tritania, Oberon, Umbriel and Ariel at the International Court in The Hague. "Our clients are being denied their rightful billing as planets," said lead counsel Richard Visage, "and unfair billing is being distributed to smaller, insignificant objects in the outer rim." It is believed that the plaintiffs are seeking status and punitive damages from the IAU. When interviewed for comment, Mercury had this to say, "I don't know what they're on about. I've been a planet for a long time and it's not all it's cracked up to be. They named me after a naked man with wings on his feet... how do you live that down? Pass the sun screen please." Uranus added, "Feh. Listen to Mercury... like he has it bad in the name department." Ceres, traditionally quiet on the whole planet issue was more direct "4.5 billion years I floated out here, minding my own business. This lot, these 'moons' *HAD* to go float around Jupiter and Saturn or that icky Earth. Oh they were so cool back then. Now they're ticked because they can't be planets. They can kiss my asteroids. Patience paid off and I'm a planet now, so those bunch of moon rocks can go [expletive]. I made the big leagues and they're just jealous. Here's a moon for 'em" [photo redacted] Most of the plaintiffs were unreachable, but Io was adamant, "I got stuck here in the early days. I've been erupting crap for billions of years trying to blow this popsicle stand. Look, if that fat yellow blob in the middle didn't hog up all the mass when we were young, I might be orbiting a star in my own right. I'm bigger and sexier than Pluto and Quaor. People look at me all the time, nobody sees those other two. Now if you'll excuse me, I have to have an eruption." Lawyers for the IAU have not had time to review the court filing and had no comment at this time.

There's another major issue to deal with. People who study planets are often called "extraterrestrial geologists" or "exogeologists", or some such.

Well.... terrestrial geologists already have a use for the term "pluton" - it's an intrusive igneous body, meaning that it's a body of magma that never reached the surface of the Earth, and cooled off inside the Earth to form an igneous rock. The Rocky Mountains are composed of plutons. Stone Mountain in Georgia is a pluton. Heck, Devil's Tower (of Close Encounters of the Third Kind fame) is a pluton. There are many, many, many plutons within the Earth and exposed by erosion at its surface.

Can you imagine the confusion?

In zoology and botany, if an organism's name has been used once, it can't be used again. I think we have to do the same in this case.

Why would Pluto want to be a planet? As a planet, it's a tiny little, out-of-whack runt! As a Kuiper Belt Object, it's a rocking big heavyweight bruiser. Okay, not quite as big as UB313, but it's got more moons! So as a matter of common decency, we should realize that Pluto would rather rule in the icy reaches of the Kuiper Belt than be subject to mockery in the warm glowing warmth of the inner solar system. It's named after the god of the underworld, after all.

I agree totally. The best solution would be to demote Pluto from planethood to a Kuiper Belt object.

the whole point is that planet is not a scientifically useful category. it means something like "big local objects discovered before really good telescopes", and we group the names for the same reason we group the names of 1954 academy award winners. hence, planets should retain pluto, while more precise categories are used for scientific purposes. in the latter case, membership will vary according to well defined rules, and precisely none of the general public who date will know or care about those memberships

If we go down this route, soon people will be calling the Earth-Moon system a double-planet --- the earth-moon barycenter is a mere 1700 km below earth's surface, after all.

But... You mean the Moon isn't Earth's sister world? That it's not a double-planet system?

No, sorry, I've believed it is for... I forget how long, but a substantial part of my life anyway. It will take an awful lot to convince me otherwise.

What do we expect when a definition is so loaded yet unimportant that it is decided by committee instead of practice?

Pretty much what happened here:
- Inclusive definition.
- Some terms not currently welldefined.

I'm surprised they managed to find objective and welldefined criteria for planet and double planet both.

Okay, so the earth-moon system may eventually become a double planet instead, and caught or ejected planets will get or loose planet status. It is all contingent now. And so is the heliocentric ordering of the Neptune-Pluto-Charon bodies. It would have been easier if those suckers where nailed down! Who ordered them anyway?

And I had thought that the debates about phylocode and rank-free taxonomy were exhausting!

Heck, the great state of California is by itself 1,240 km long.

...and of course why does this discussion sound so similar to the discussions over "kinds" the creobots inevitably bring up?

The solution, of course, is to get the congresscritters to pass a "No Planet Left Behind" act, ensuring equal treatment of gas bags, dirt balls, and ice cubes.

brilliant!

and of course we will have to implement standardized "planet testing", in order to make sure all potential planetoids fulfill requirements.

Yes well I don't believe in planets,
they're just made up by a bunch of baby eating, godless and elitest scientists who want to impose a bunch of useless facts on our children.

If they get away with it, my children won't go to heaven, which if you believe scientists, is filled with gas giants, ice thingy's, dust and rocks , come on, you know it's not true, you can feel it.

AND Pluto isn't in the good book, so it just CAN'T be there.

AND Pluto isn't in the good book, so it just CAN'T be there.

I have a better solution.

Pluto should not be declared a planet, except on weekends.

On weekends, Pluto should be considered a planet.

The other planets must continue to chuck matter at Pluto at regular times so that Pluto would have a good chance to develop into a planet.

Planets, especially Neptune, must remain at least a gazillion kilometres from Pluto.

The more I think about this proposed definition, the more I like it.

What is really exciting about planets these days is that we keep finding them around other stars. This means we need a nice clear simple definition, that can be applied in many contexts. This is what the IAU is proposing to give us.

So what if Pluto is an ice-ball. Why should that rule it out? As for calling it a Kuiper belt object... so it is; but the Kuiper belt is a local solar system structure. Other stars have similar belts; but the structure of belts in other stars is going to vary a lot, and will depend on what other planets there are to push and pull on smaller planets. Kuiper belt is not a quality that can be used to distiguish planets and other bodies, because we can't apply it consistently for other systems.

The definition they have given is beautifully elegant. It's not a case of some arbitrary number chosen on permissible size, location, eccentricity or whatever. They don't give a number, but a quality. It's big enough for the surface to be defined by gravitational equilibrium. Now this is not perfect; it allows for a grey area as the shape is more and more constrained. But it is way better than just taking a number out of a hat; it is trying to identify a quality that transposes easily to other contexts.

And the asteroid Ceres is to be a planet! Great! I see this as an injustice rectified at last. I like it that we recognize this small world in the inner solar system. It's not devaluing the big guys... it's acknowledging one of the little guys, which even so all on its own accounts for about a third of the mass of the asteroid belt. I'd love to visit there one day -- and I hope with it will get increased recognition for that rather interesting part of our solar system.

Charon is a planet! How cool is that!? We have a binary planet in the solar system. It's been spoken of as such before this, but now this can become official, and we know what it means to say it is a binary planet. And if anyone else wants to find another binary planet, they know just what to look for.

It also opens the way for new discoveries. It's a good thing that we don't know how many planets there are in our solar system. It leaves open the way to search and find more worlds, and honour those who find them as discoverers of planets.

This is an excellent proposal.

Cheers -- Chris

I've got to agree with Nick on at least one point: Including Charon as a planet is bizarre, especially if Xena, Sedna, and Quaoar are excluded. I like the new IAU definition of planet, and am baffled as to how they can come up with such an elegant and useful definition of 'planet', and then catastrophically flub the application of it in the same stroke! Okay, okay - so the baricenter of the Pluto-Charon system is above the surface of Pluto - but is that really such a good definition of 'Double Planet'? Consider the gas giants - where are their 'surfaces'? Or objects with variable sizes or densities - if, when Plutos atmosphere freezes, the baricenter then falls below Pluto's new surface, is Charon somehow less of a planet?

I like the definition of planet:
Not a star.
Orbit a star.
Gravity strong enough to approximately sphericalize (is that a word?) itself.

But allow me to propose a new way to look at the definition of 'double planet' - the baricenter of the system should be close to the midpoint between the centers of mass of the bodies involved. What is 'close'? I'm flexible - but right now I'd be amenable to the middle one third of the distance between centers of mass. Gotta admit, I'm kinda thinkin' about the Roche limit & Roche sphere here....

So Terra-Luna? Not a double planet; the baricenter would have to about ~128000 km closer to Luna. Pluto-Charon? Again no, the baricenter needs to be ~5000 km closer to Charon.

And one more thought: we already know of dozens of extrasolar planets, soon we may know of hundreds, and one day I hope we will know of (and visit) millions. So, why the planet-o-phobia? Why is ten planets too many? Or nine? Or fifty? What is the rationale behind the 'our system shoud have a single digit numer of planets' chauvenism? I don't get it.

(*Shrug*) I am not an astronomer, nor do I play one on TV. Just my $0.02 worth.

I suppose if the larger satellites like Titan, Europa, Io, or Tritan for example, where orbiting the sun then they too would have been classed as planets ? I also remember Carl Sagen once saying that Jupiter was in fact a failed star. Obviously the line between planets and stars is also blured.

The young Earth creationist groups such as AIG still deny the existence of both the Kuiper belt and Oort cloud as sources for short and long period commets and, for some strange reason, persist with the "commets break up too quickly" claim. I can't understand why they haven't dropped this one since, in the light of recent discoveries, it has surely been shown to be nonsense.

Anyway, heres a site on planets you folks might enjoy:

http://www.nineplanets.org/hypo.html

For the definitive word, we must get the testimony of Michael Behe--will the new existence of plutons affect the scientificiness of astrology? We must also ask ourselves what the Designer had in mind when he created these ambiguous objects. Then we must answer our own question, the Designer is inscrutable and anyway might be an alien from the pluton Pluto.

I recall talk some years ago about Earth gaining a second moon, as the orbit of one of the asteroids brought it into control by Earth's gravity. I thought that asteroid was Ceres. Can anybody clue me in on that?

O.K. O.K. so just lets redefine everything so it fits everyones pre-conceived notions.... fine by me.

I'm not big headed enough to insist MY definition should be accepted by all...just one small..o.k. BIG request .......the next planet be named after me.

Planet k.e.

I was going to be magnanimous and allow it to be named planet Dembski....but since he is going to be buried in Westminster Abbey next to Charles Darwin with a cardboard cut out replica of the Nobel prize for something or other, I figure he has enough recognition already

Maybe we just need to establish two criteria for planets; as long as one is met, the circumsolar object in question is a planet.

1) The object is visible from Earth with the naked eye. Thus Mercury, Venus, Earth, Luna, Mars, Jupiter, and Saturn qualify as planets. But:

2) The object has a satellite of its own. Uranus, Neptune, and Pluto thus qualify as planets (and all of the above except Mercury & Venus doubly qualify), while Luna, Ceres, various iceballs, comets, and asteroids don't.

Yes, they're arbitrary criteria, and yes, Pluto wouldn't have qualified as a planet until Charon's discovery, but hey, it keeps the total well within the grasp of first-graders.

1) The object is visible from Earth with the naked eye. Thus Mercury, Venus, Earth, Luna, Mars, Jupiter, and Saturn qualify as planets.

Yes, they're arbitrary criteria, and yes, Pluto wouldn't have qualified as a planet until Charon's discovery, but hey, it keeps the total well within the grasp of first-graders.

I also remember Carl Sagen once saying that Jupiter was in fact a failed star.

Obviously the line between planets and stars is also blured.

I recall talk some years ago about Earth gaining a second moon, as the orbit of one of the asteroids brought it into control by Earth's gravity. I thought that asteroid was Ceres. Can anybody clue me in on that?

— mark

I personally favor dumping the "Planet" classification. Anything that puts Jupiter and Mercury in the same class seems surreal to me, a bit like the medieval taxonomic category "worms" containing snakes, eels, spiders and caterpillars.

Let's just demote the term 'planet' to a non-scientific, informal word and come up with a set of decent definitions of celestial objects. It's clear by now that there is no definition of planet that actually fits all the criteria for acceptability.

Pluto's rotation is opposite to most of the other planets in the solar system.

Pluto is smaller than seven of the solar system's moons (our moon, Io, Europa, Ganymede, Callisto, Titan, and Triton).

In addition, Pluto's "atmosphere" may be frozen for most of its long year except at perihelion, when it may emit its now-gaseous "atmosphere" into space, perhaps to interact with Charon.

Is this a Kuiper Belt object, or a comet that escaped the Oort Cloud?

Could we list that our solar system includes "planets" and "eccentric planets," or would that be too confusing?

I've got to agree with Nick on at least one point: Including Charon as a planet is bizarre, especially if Xena, Sedna, and Quaoar are excluded. I like the new IAU definition of planet, and am baffled as to how they can come up with such an elegant and useful definition of 'planet', and then catastrophically flub the application of it in the same stroke!

There are really only two broad categories of objects in the solar system: four gas giants, and zillions of tiny random bits of leftover debris. The debris varies somewhat in size and composition, and it just so happens that we live on a particularly large chunk of it, but the Earth isn't fundamentally different from, say, Ceres, or Xena, or what have you. Some bits of debris have moons, others don't. Some have atmospheres, to varying degrees, others don't. Some are round, to varying degrees, and others aren't. Composition might be metal, rock, and/or ice, with the percentages varying mostly by how close to the sun the object happened to end up. Add up the total mass of all this debris, and it's insignificant compared to the gas giants.

So if we really do need a rigorous definition of "planet", my vote is for four planets, just four, and everything else is either an asteroid or a moon. Doesn't get much easier to remember than that.

OK, I've thought about this more. The Right Solution is as follows.

There are main planets and dwarf planets.

1. The main planets (a) orbit in the ecliptic plane and (b) constitute the majority of mass in the region of their orbit. These planets are big and accreted directly out of the disc in something like their present positions.

We have 4 main rocky planets and 4 gas giants.

2. The dwarf planets orbit the sun, but are big enough to be at hydrostatic equilibrium and be roughly spherical, but don't meet the other criteria in #1.

(a) Ceres and other asteroids are "rock dwarfs", and are both dwarf planets and the largest representatives of the asteroids. There might be up to 4 of these.

(b) Pluto and other "plutons" are "ice dwarfs." Pluto has the honor of being their representative. Charon is not a planet, but a moon, because the barycenter is not really in-between the two (I like the middle 1/3 suggestion), but I guess you could call it a double-pluton if you want.

This ends up being quite close to the IAU proposal, but recognizes that the there really is something more significant about the main 8 planets.

IMHO...

atul - what about the sun? Wouldn't that make at least three categories of objects in the solar system? :)

Also, if bare simplicity is the goal, then how about just ONE broad category for everything but the sun, called "satellites." Besides, the gas giants are really just the same as the smaller objects, except that they happen to have especially thick atmospheres.

I've often thought that CA should be classified as a planet, in and of itself.

What is going to happen to the term 'planet' when we start seriously looking at other star systems? You can come up with all kinds of oddities:

1. A dwarf star with a couple of very small - but round - objects orbiting them. Are these planets?

2. How many stars? What if we find a relatively small body doing the n-body problem math in real time while whizzing among a triple-star system?

3. What about the diameter of the object has anything to do with it's planetary nature? We might find small, but very dense and massive objects smaller than Pluto. What if this small, dense object is orbited by large, less dense objects. Maybe the barycenter is inside the larger, lighter body? What then?

4. What about objects captured by a star's gravitational well which were not formed at the same time? ie. Objects in orbit that are not on the star's plane of rotation? Are these planets? Or not?

- The definition needs to refer to something intrinsic in the object itself, something observational, and tied to the nature of the universe. I can't see what - other than tradition - is wrong with the "orbital objects of sufficient mass to be spherical (or nearly so)".

Seriously, I like Nick's revised suggestion...but humorously, I love atul's. Can you imagine the reaction of those perpetually apoplectic wonkettes at the Discovery Institute getting a gander at the new science standards? Their "Privileged Planet," not even a planet! What, the privileged asteroid? "Stinkin' muh-terialists! A-thee-usts!"

I think there must be something wrong with me, because I couldn't care less what bodies are called planets. As far as I can see, it doesn't affect anything in science. Celestial navigation and gravitation concern themselves with the physical properties of bodies, not their classification in language.

The giveaway, as far as I can see, is that the justification that keeps getting trotted out is that "now first graders will have to memorize a different list". That's insane. I mean that it's insane to have first graders memorize lists of planets in the first place.

Is there any other country apart from the US where this pointless exercise is inflicted on children?

Well, our solar system certainly contains eccentrics...

Planets, schmanets. We should just tell kids "Look, there's lots of junk orbiting around the Sun. Some of it is bigger than others. We live on a rather smallish bit of it. But it's all just leftover junk."

What is all this "Luna" nonsense? The Earth's satellite is called the Moon. The Romans called it Luna, the Greeks called it Selenos. So what? Why do people think Latin names are better than English?

Whe I was at school, Jupiter had four moons. They had been discovered by Galileo. There were nine planets but my parents had only learned of eight when they were at school. Did it bother the post-1930 text book writers to add a ninth planet? Has it bothered text book writers to accept that Jupiter has more than sixty moons and still counting? Of course not.

It matters not what astronomers say or do, Pluto is a planet and has been for seventy-six years. Ordinary people know and accept that. New objects have been discovered and a definition of "planet" seems to be needed. Any definition has to include Pluto, people will not accept anything less. If this means that we must add another three, six, twelve or twenty-four planets then so be it. We accepted sixty new moons of Jupiter. I have no problem with more planets.

Including Charon as a planet might cause a few grumbles and it would probably be best not to do it immediately. Leave it for a few years, it isn't going away.

The solar system is much bigger than we thought it was only a few years ago. We have bigger and better telescopes seeing further and further into the outer reaches of the system. We are discovering new objects that should be called planets so why not do so? Be inclusive not exclusive.

Heh! IAU have worked seriously with this for two years after years of quibbling, and *now* people start to discuss.

Chris:
"What is really exciting about planets these days is that we keep finding them around other stars. This means we need a nice clear simple definition, that can be applied in many contexts. This is what the IAU is proposing to give us."

Sort of. We can probably in most cases assume sphericity instead of actually measuring it, et least while we only detect large bodies. Gravitational lensing may not give information if the body is gravitationally bound to a star or the period. The 200 year orbital period limit for plutons may be inappropriate elsewhere.

JB:
"Including Charon as a planet is bizarre, especially if Xena, Sedna, and Quaoar are excluded."

They aren't excluded: Xena is UB313, but the name isn't approved yet. The others will be included if their shape is spherical - they aren't approved yet.

mark:
"I recall talk some years ago about Earth gaining a second moon, as the orbit of one of the asteroids brought it into control by Earth's gravity. I thought that asteroid was Ceres."

Some objects are gravitationally influenced, but not Ceres. It is still between Mars and Jupiter.

"Many asteroids are now known to resonate with Jupiter or Mars, but only a few with Venus or Earth." ( http://www.nasm.si.edu/research/ceps/etp/asteroids/AST_where.html#where )

Kristine:
"Could we list that our solar system includes "planets" and "eccentric planets," or would that be too confusing?"

They have anticipated you by adding the official category "pluton" (Pluto, Charon, Xena).

"The IAU draft Resolution also defines a new category of planet for official use: "pluton".

Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they orbit beyond Neptune). Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically referred to as having a large orbital eccentricity).

All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets."

Why can't lumpers and splitters just get along?

On a more serious note, this idea that "people won't accept" a scheme that demotes Pluto, would seem to put the cart before the horse.

Pluto was believed to be unique and it was on that basis that it merited 'planet' status. Pluto is, in fact, representative of a class of objects. Those objects are substantially different in orbit, history and composition from the two other classes of 'planets.' If "people" got over geocentrism, I think they can eventually come to understand that Pluto was miscategorized in the past.

And "Luna" is a perfectly fine designation for "the Moon," especially since there are a couple of hundred other moons in the system, and they all have names.

It was the summer of 1966. I was a rising senior in high school, and obviously a very impressionable 15-year old, because I remember it rather well 40 years later.

At an NSF Secondary Science Training Program we (21 of us at the U of Aridzona SSTP) heard a lecture from Kuiper himself on why the moon should be considered a planet and Pluto should not be so considered. I guess it stuck.

And so I wonder: What's the big deal? Hasn't this been rather a long time in coming? Or is forty years not such a big deal in geologic or astronomical time?

The YEC's of course, still deny that the Kuiper belt is the reservoir for short period commets, despite all the recent dicoveries. They also claim the Oort cloud is a myth and persist in using the tired old argument: "commets break uup too quickly". I wonder how long it will be before they drop this one ?

Anyway for your amusement, here's an interesting site that I came across:

http://www.nineplanets.org/hypo.html

Why do people think Latin names are better than English?

— Tent O Field

Senda needs to be renamed for a Roman god/goddess, NOW. The media has already habitually taken to calling it Senda, and it was supposed to be given a new name. The rules state to use a Roman name.

A rule that was broken with Uranus, mind you, a Greek god with no Roman equivalent. Can we get around to renaming that, too? How about Juno?

As for "plutons"... What was wrong with "planetoid"?

Seems like any definition is going to have an upper and lower range that's ambiguous, unless some arbitrary limit is set. And with an arbitrary limit, objects near it might actually shift back and forth depending on conditions.

Big enough that gravity forces a more or less spherical shape: that seems a useful lower limit. (Noting that the composition of the object may affect exactly how much mass is required though, since sturdier materials could support a larger nonspherical shape than could a clump of gravel.)

Too small to support fusion: a useful upper bound.

Having a solid surface at all might also be a useful distinction in the middle - gas giants and rocky planets aren't really the same thing.

I'd be tempted to suggest ability to hold an atmosphere as a possible distinguishing feature, except that it would depend rather strongly on temperature (and perhaps on amount of solar wind in the area).

I'm not sure if orbital location really makes sense as a criteria for distinguishing "species". Consider three rocky objects of about the same size, mass, and composition: One orbiting a star, one orbiting a large planet, and one adrift in interstellar space. At least one of them is a planet by most definitions, but are the other two really different "species" than the definite planet, just because one is also a moon and one is a wanderer? (On a side note to that, didn't the word "planet" originally mean "wanderer", which would make the interstellar rock the planet and the other two not? LOL. )

Henry

A rule that was broken with Uranus, mind you, a Greek god with no Roman equivalent.

— Dexter

Senda needs to be renamed for a Roman god/goddess, NOW. The media has already habitually taken to calling it Senda, and it was supposed to be given a new name. The rules state to use a Roman name.

(A) Does the notion of planethood appear in any well-established astronomical laws? If not, then scientifically speaking who cares what we call things. If yes, then ...
(B) What is wrong with working out a way to have degrees of planethood? There's no intrinsic reason why a predicate has to be dicotomous.

(On a side note to that, didn't the word "planet" originally mean "wanderer", which would make the interstellar rock the planet and the other two not? LOL. )

The American Heritage® Dictionary of the English Language: Fourth Edition. 2000. planet NOUN: 1. A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. 2. One of the seven celestial bodies, Mercury, Venus, the moon, the sun, Mars, Jupiter, and Saturn, visible to the naked eye and thought by ancient astronomers to revolve in the heavens about a fixed Earth and among fixed stars. 3. One of the seven revolving astrological celestial bodies that in conjunction with the stars are believed to influence human affairs and personalities. ETYMOLOGY: Middle English, from Old French planete, from Late Latin plan ta, from Greek plan t s, variant of plan s, plan t-, from plan sthai, to wander. See pel -2 in Appendix I. Appendix I Indo-European Roots ENTRY: pel -2 DEFINITION: Flat; to spread. Oldest form *pel 2-; variant *ple 2-, colored to *pla 2-, contracted to *pl -. Derivatives include field, planet, plasma, plastic, and polka. 1. Suffixed form *pel( )-tu-. field, from Old English feld, open field, from Germanic *felthuz, flat land. 2. Suffixed form *pel( )-t-es- (by-form of *pel( )-tu-). a. feldspar, from Old High German feld, field; b. veldt, from Middle Dutch veld, velt, field. Both a and b from Germanic *feltha-, flat land. 3. Variant form *pl -. a. Suffixed form *pl -ru-. floor, from Old English fl r, floor, from Germanic *fl ruz, floor; b. suffixed form *pl -no-. llano, piano2, plain, planarian, plane1, plane2, plane3, planish, plano-, planula; esplanade, explain, pianoforte, from Latin pl nus, flat, level, even, plain, clear. 4. Suffixed zero-grade form *p -m -. palm1, palm2, palmary, from Latin palma (< *palama), palm of the hand. 5. Possibly extended variant form *plan-. a. planet; aplanatic, from Greek plan sthai, to wander (< "to spread out"); b. perhaps Germanic *flan-. flâneur, from French flâner, to walk the streets idly, from a source akin to Old Norse flana, to wander aimlessly. 6. Suffixed zero-grade form *pl -dh-. ---plasia, plasma, ---plast, plaster, plastic, plastid, ---plasty; dysplasia, metaplasm, toxoplasma, from Greek plassein (< *plath-yein), to mold, "spread out." 7. O-grade form *pol -. a. polynya, from Russian poly , open; b. Polack, polka, from Slavic polje, broad flat land, field. (Pokorny pel - 805.) See also extensions pl k-1 and plat-.

Peter,
Re "planet" = "wandering star" !

Of course, back then, "star" meant pretty much anything that stayed way up in the sky - sun, moon, planets (wandering stars), comets (hairy stars), actual stars, perhaps a galaxy if it was visible.

-----------

Keith,
Re "What is wrong with working out a way to have degrees of planethood?"

I'll second that - any definition that doesn't have somewhat arbitrary strict limits is going to have cases where it's ambiguous. Like a fuzzy set rather than a strict mathematical set.

Henry

I am facinated by the implications of the current discussion and the reaction of the creationists. It was actually antisipated by "Answers in Genesis" a few years ago; Kuiper Belt Objects: solution to short-period comets? Have recent 'Kuiper Belt' discoveries solved the evolutionary/long-age dilemma? by Robert Newton.

Who wants to take the next swing?

"Does the notion of planethood appear in any well-established astronomical laws? If not, then scientifically speaking who cares what we call things."

One reason to not include similar bodies which aren't gravitationally bound to stars is that they may be mainly remains of brown dwarfs. (But they could also be planetary ejects.) Plutons have a different history from closer planets.

But I too believe the definitions are historical and of less practical value, ie made by committee instead of consensus from practical science. OTOH, since naming bodies will remain cultural and historical, committees should have more valuable uses too. ;-)

I am fascinated by the implications of the current discussion and the reaction of the creationists. It was actually anticipated by "Answers in Genesis" a few years ago; Kuiper Belt Objects: solution to short-period comets? Have recent 'Kuiper Belt' discoveries solved the evolutionary/long-age dilemma? by Robert Newton. Who wants to take the next swing?

I just wonder how long they will persist with the "comets break up too quickly" claim.

"But the real Pluto cannot hold all the sinners of human history."

Maybe if you stacked them on top of each other?

I personally favor dumping the "Planet" classification. Anything that puts Jupiter and Mercury in the same class seems surreal to me

— Michael Suttkus

I personally favor dumping the "Planet" classification. Anything that puts Jupiter and Mercury in the same class seems surreal to me

— MaDeR

Same for yellow dwarfs (our sun) and hypergigants, eh?

From our solar system, it seems more reasonable to classify the terrestrial planets, gas giants and ice chunks separately.

Bad? Nothing really. Words mean only what we want them to mean. Planet used to mean anything that varied in position in the sky, including the sun and moon. If the ancients had been able to detect paralax, Alpha Centauri could have been a planet.

Tomorrow it may mean something else, or nothing at all.

I'd object to "Gas giant planet" simply because it's too long. It tells you nothing that "gas giant" doesn't communicate effectively. Yes, I know there are things that could be called gas giants that aren't (nobody refers to stars, despite fitting the literal meaning of the term, as gas giants). Terms shouldn't be overlong unless they're fun. (I, along with Calvin, vote we replace the term "Big Bang" with "Horrendous Space Kablooey!" Yes, use of exclamation mark will be required.)

It's all about creating an effective language that communicates terms effectively and efficiently, while still leaving room for poetry.

Darn, all the good stuff happens when I'm away! :( Anyway...

Anonymous_Coward said:

"Obviously, I don't think it's meant to be completely accurate. There are bigger objects (brown dwarfs) that are truly failed stars.

Still we don't know the exact history of Jupiter. But there's no evidence as of yet that Jupiter blew away its material before it could sustain fusion."

I don't think it's unfair to think of Jupiter as a failed star. Carl Sagan probably never thought that it did blow away any of its' own material. Just that during the early chaos of the developing solar system as all the planets (or whatever) were gaining mass, it was always possible that Jupiter could have gained more mass, achieving nuclear fusion and we could have ended up with a binary star-system. As it happened, the sun's influence stopped Jupiter from ever reaching that mass requirement (probably lucky for us). :)

Is a gas giant a failed star, or is a star a failed gas giant? ;)

Henry

Is a gas giant a failed star, or is a star a failed gas giant? ;)

The universe is a failed singularity. An IDer is a failed corpse. Scientists are failed idiots. X is a failed X'.

— Anonymous_Coward

And apparently it now seems official - Pluto is a failed planet.

http://news.bbc.co.uk/1/hi/world/5282440.stm

"I have a slight tear in my eye today, yes; but at the end of the day we have to describe the Solar System as it really is, not as we would like it to be," the Queen Mary University of London, UK, scientist told the BBC.

Leave the planets alone. Once you name something, you can't take it back. Are you going to say Rhode Island can no longer be a state because it's too small?

Also, what is going to happen to the saying that teaches our children the planets ... My Very Educated Mother, etc., etc., etc.

Leave It Alone

True. I'm happy to go along with whatever the astro dudes want. I got nothing against Pluto, but whether it's considered a planet or not, it don't matter to me. But while it may be that Pluto is no more special than a hundred other icy far away balls out there, there's no denying it's cultural impact.

"Uranus" is the Roman equivalent, actually, though Cicero calls him "Caelus;" it's "Ouranos" in Greek. No, the Romans didn't care about him much, but neither did the Greeks---he's just the personified sky.

In the diagram, Ceres, Charon, Pluto and 2003 UB313 are labeled as "dwarf planets".

From the link:

"The scientists agreed that for a celestial body to qualify as a planet:

it must be in orbit around the Sun

it must be large enough that it takes on a nearly round shape

it has cleared its orbit of other objects

Pluto was automatically disqualified because its highly elliptical orbit overlaps with that of Neptune. It will now join a new category of dwarf planets."

Why does it have to have its orbit cleared of other objects? As others have pointed out, what if we find other large planetary-like bodies with wierd orbits? The fact that they might get disqualified because their peculiar orbit happens to cross with something else (even if briefly) seems a bit strange.

Why does it have to have its orbit cleared of other objects? As others have pointed out, what if we find other large planetary-like bodies with wierd orbits? The fact that they might get disqualified because their peculiar orbit happens to cross with something else (even if briefly) seems a bit strange.

Yeah, too my miniscule pointy noggin, it would seem equally-arguable to claim that Neptune doesn't qualify either, because it hasn't yet cleared its orbit of Pluto and Charon.

Or does some uas-yet-unreported wrinkle in the definition handle that objection?

Distance from Neptune Pluto's orbit is often decribed as 'crossing' that of Neptune. In fact, Pluto's nodes (the points at which the orbit crosses the ecliptic) are both situated outside Neptune's orbit and are separated by a distance of 6.4 AU (that is, over six times the distance of the Earth from the Sun). Furthermore, due to the orbital resonance between them, Pluto executes 2 full cycles while Neptune makes 3; this means that when Neptune reaches the 'closest' point on the orbit, Pluto remains far behind and when Pluto in turn reaches that point, Neptune is far (over 50°) ahead. During the following orbit of Pluto, Neptune is half an orbit away. Consequently, Pluto never gets closer than 30 AU to Neptune at this point in its orbit. The actual closest approach between the Neptune and Pluto occurs at the opposite part of the orbit, some 30 years after Pluto's aphelion (its last aphelion was in 1866) when Neptune catches up with Pluto (i.e. Neptune and Pluto have similar longitudes). The minimum distance was 18.9 AU in June 1896. In other words, Pluto never approaches Neptune much closer than it approaches Saturn.

Dang, Coin, here I was all excited to click on your comment, because I was sure you were gonna straighten out my pinheaded confusion.

Now I'm even more confused.

But ta, anyway. I'm sure one of our savvy contributors or commenters will eventually 'splain it all to the likes of us.

Ok, Coin & steviepinhead, I'll 'splain it to you. The astronomers made the wrong decision and produced a quite ugly and useless definition of 'planet'. I don't think I could do much better, other than to list the eight planets of the solar system and proclaim that these eight, and only these eight, are planets. 'Cause I make the rules and I said so!

Satisfied now?

Oh, sod it. I have the solution:

BEER!

BEER!

Nah, the Stein Solution was the answer to the problem on some other recent thread.

Or maybe the Steiner Whatsis was the problem...

Or maybe [muzzily scratches head]...

Beer it is.

lol!

*clink*

Cheers!

(glug, glug)

:-P

Hauling myself inelegantly back over the rim of my beer stein, I have found this news article that does a better job than the earlier ones I had seen in explaining the "cleared out its neighborhood" requirement:
http://www.msnbc.msn.com/id/14489259/.

Apparently, it's not Pluto's failure to evict Neptune from Pluto's neighborhood that's problematic for the planet-formerly-known-as-ninth, but its failure to evict all those other plutons, er, dwarf ice planets, er, whatever-the-heck-they're-gonna-call-'em now spherical bodies from its orbit that has doomed Pluto to minor-planet ignominy.

The objection is raised in the article that all the "classic" planets still have asteroids travelling in their neighborhoods, but maybe the distinction is that Pluto hasn't evicted other spherical planettes from its 'hood.

So now if you are orbiting the sun and you've got the mass to collapse into a spheroid, and you've either captured all the other fellow-travelling spheroidal bodies in your neighborhood and turned them into your moons, and you've evicted all the other sperical bodies from your environs that you couldn't capture, ignoring non-sperical bodies like asteroids entirely, then you're a REAL planet...

As best, uh hic!, as I can tell.

Also, what is going to happen to the saying that teaches our children the planets ... My Very Educated Mother, etc., etc., etc.

Ahhh, now I remember ---- it was a way to remember the values of the different colored stripes on electrical capacitors . . . . .

Well, "gravitational dominance" would be better than "cleared his neighbourhood". Why?
1. Sounds nice. Like SF. Or something like that.
2. Clears his neighbourhood, too.
3. And no one will doubts that these bodies whom survive clearing ARE graviationally dominated by body in question. These bodies will be called "rings", "moons", "asteroids in L4 and L5", or simply "co-orbitals".

Todays definition only give ammo for proplutonists.

SciFi slogans that might be bent to apply to this "gravitational dominance" context:

Planet-wannabe to spheroidal objects it is evicting:
There can be only one!

Planet-wannabe to spheroidal objects it is converting to satellites:
Resistance is futile!

Unsuccessfully-evicted spheroidal object to planet-wannbe:
I'm ba-a-a-ack!

...there must be more. Help me out here!

Object being converted to moon or thrown out of orbit on facing the planet:

"The Force is strong with this one!"

Yeah, now we got it goin' on!

I'm trying to come up with one from the "Kung Fu" TV show. Or even from Samuel L's fisking of "Kung Fu" in Pulp Fiction...

it was a way to remember the values of the different colored stripes on electrical capacitors...

Re "...there must be more. Help me out here!"

I'm the chosen one, and you're dusted!

Incoming asteroid to planet-wannabe:

"Assimilate this!"

They will be classified according to standard Imperial er... standards!

"FEAR will keep the local systems in line!"

Or maybe: "This bickering is pointless!"

:-/

I just gotta get a few more of these, er, out of my system:

Planet-wannabe, to incoming impactor:
"Go ahead, make my day."

Planet-wannabe to competing spheroidals:
"I vant to be alone."

Planet-wannabe, musing over options for evicting debris:
"Shaken, not stirred."

Planet-wannabe, to spheroid in process of being evicted:
"Hasta la vista, baby!"

Pluto, sniffing:
"I coulda been a contender!"

Neptune, pompously, to Pluto:
"It's better to be looked over than overlooked."

One planet-wannabe to fellow planets, regarding a spheroid in need of eviction:
"Excuse me while I whip this out..."

Omniscient--but not necessarily intelligent--observer remarking upon spheroid-wannabes going through process of gravitational collapse:
"Round up the usual suspects."

Gravitationally-stressed satellite to massive planet:
"You're tearing me up!"

IAU, to Pluto:
"It's a hard world for little things."

Ah, back to the Empire again, eh? In that case, I leave it to the audience to place these final few lines (I promise!) in the "mouths" of the appropriate celestial bodies:

"I have a bad feeling about this."

"Try not. Do, or do not. There is no try."

"That's no moon!"

The backlash begins

The backlash is kind of dumb.
The issue of Neptune, IMO, is a red herring. The point about "clearing the orbit" is that, like Ceres, Pluto occupies a "belt" containing other similarly composed and shaped objects.

The same is not true of any of the eight "official planets. While their orbits may be cluttered up with post-accretion detritus, they are all by many orders of magnitude the largest spherical objects to be found.

Pluto isn't even the largest Kuiper belt object.

http://www.eonline.com/News/Items/0,1,19852,00.html

Although we think it's Dopey that Pluto has been downgraded to a dwarf planet, which has made some people Grumpy and others just Sleepy, we are not Bashful in saying we would be Happy if Disney's Pluto would join us as an eighth dwarf. We think this is just what the Doc ordered and is nothing to Sneeze at!

better make that resistors.

better make that resistors.

The object formerly informally known as Xena is now formally named Eris.

Henry

The object formerly informally known as Xena is now formally named Eris.

Henry

The object formerly informally known as Xena is now formally named Eris. Henry

PASADENA, Calif.--The International Astronomical Union (IAU) today announced that the dwarf planet known as Xena since its 2005 discovery has been named Eris, after the Greek goddess of discord. Eris's moon will be known as Dysnomia, the demon goddess of lawlessness and the daughter of Eris.