The IAU has voted on a series of resolutions on what a planet is and what a planet isn’t, and the verdict is…
Pluto is not a planet.
At least, not a major one.
This is a big turnaround from the initial resolution, which would have given our solar system at least 12 planets, and potentially many, many more. Here is the first resolution that passed:
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way:(1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
Ignoring for the moment, once again, that it’s silly to try to scientifically define a class of objects that are really only defined culturally, these definitions are still unsatisfying to me. A planet-sized object between stars is not a planet? How round is round? How do you define its "neighborhood"? These are still the same objections I made before in my earlier post about this.
But I suppose what people want to know is how Pluto fits in this. Pluto is round, and orbits the Sun, but has not cleared out its local neighborhood. Smaller objects that orbit the Sun in nearly the same orbit will get absorbed by or ejected by the larger object. As planets form, their gravity either pulls in smaller bits of junk, causing them to impact, making the planet grow, or it slingshots the smaller object away, putting it in a very different orbit. That’s why big objects in the solar system tend not to have anything else near them (except moons).
Pluto fails this. As I understand it (the news is still sketchy from the IAU meeting) there are other objects in similar orbits as Pluto, and therefore Pluto has not cleared out its neighborhood. I’m not sure if Charon, Pluto’s moon, is included in that list of uncleared objects. Now, this is a little confusing: lots of planets have moons, so just having a moon doesn’t mean a planet has not cleared its area (since the moon is bound gravitationally by the planet). But Charon orbits Pluto far enough out that the center-of-mass of the system is outside Pluto’s surface (again, see see my earlier post about this). Ironically, with the original resolution, this made both Pluto and Charon a planet. Now, under the new rules, this may mean neither is.
So: according to the new rules, passed by the IAU, Pluto is no longer a planet. I guess Neil Tyson will have to go on Colbert again.
The IAU made this pretty official with another resolution:
RESOLUTION 6A
The IAU further resolves:Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
This sits better with me, actually, than calling Pluto a planet, but a lot of people aren’t gonna like it.
Incidentally, there were two resolutions voted down:
Insert the word “classical” before the word “planet” in Resolution 5A, Section (1)
so that if it had passed we’d call the 8 major planets "classical". The other resolution would have been added to 6A about the dwarf planets:
This category is to be called “plutonian objects.”
Since this last bit was voted down (narrowly, 187 to 183!), the IAU will decide what to call this class of objects at the next meeting, in Rio in 2009. Rio, hmmmm… maybe I’d better go to that one.
Let me once again reiterate that trying to define what a planet is is very, very silly. The very fact that all this is so bizarrely confusing is good evidence of this.
Want another reason this is silly? If the reason Pluto isn’t a planet is because of Charon, then we’re in trouble: as I pointed out in my other post, in a billion years or so the Moon will be far enough away that the Earth-Moon center-of-mass will be outside the Earth. So at that time, if I understand this correctly (and I may not), Earth will no longer be a planet. I need to find out more about all this, but as I said, details about why exactly Pluto isn’t a planet anymore are still a little sketchy. I’ll post more when I find out.
And here’s another point. Pluto crosses Neptune’s orbit. Due to the delicate dance of gravity between the two, they never actually get near each; Pluto is always on the opposite side of the Sun from Neptune when it crosses the bigger planet’s orbit. So, if Pluto’s orbit actually overlaps Neptune’s, doesn’t that mean Neptune hasn’t cleared out its neighborhood? I think you might argue that. So why don’t we have 7 planets?
I’m really torn over this. Scientifically, this whole debate is a tempest in a teapot. It’s ridiculous, and serves no purpose. How is scientific knowledge furthered in any way by debating and resolving this?
On the other hand, it’s gotten a lot of interest by the public, and it’s been positive interest so far. People are talking about what it means to be a planet, and given the abysmal level of science education in the US, it’s great that folks are actually talking about astronomy. Maybe it’ll lead to some of them looking into it more, and that’s a good thing.
And now, finally, just maybe, we can actually get back to studying these objects instead of arguing about what to call them. There’s much to learn about them, real stuff, interesting stuff. The planets — however many you may think there are — are waiting. Let’s get going.
August 24th, 2006 at 8:37 am
great shock that is. Now the solar system has 8 planets.
August 24th, 2006 at 8:41 am
That which we call a rose, by any other name…
August 24th, 2006 at 8:43 am
But without a decent definition how are we supposed to know what anyone’s talking about when they tell us that a new planet has been discovered in some other solar system?
August 24th, 2006 at 8:44 am
In addition to the several good questions BA has about the defiinition of “planet”, what do we call the thingies that we’ve discovered orbiting other stars? The IAU’s definition says a planet “(a) is in orbit around the Sun”. So there are no other planets in the universe. So much for ET.
I had the same thought about Neptune too: I’m not sure what other objects there are that Pluto is supposed not to have cleared from its orbit; one would hope that this does not include satellites, or we’re down to Mercury and Venus. But is Pluto supposed to have cleared Neptune from its orbit??? Talk about tail wagging dog. If those two are, despite the difference in inclination, in the same orbit, then Neptune is not a planet.
August 24th, 2006 at 8:47 am
I prefered the 12+ planet proposal far better than this. Just like we have multiple stars, I have no problem with multiple planets. The original proposal also has a better structure, starts general and than creates sub-categories. This new definition is a confused muddle. The idea of clearing a zone is far less scientific than any thing in the original proposal. I think IAU made a major mistake here. I rather have thousands of planet with a easy to remember definition than eight with an unexplainable one.
August 24th, 2006 at 8:48 am
I dunno. Seems that Neptune has cleared its orbit of everything that it possibly could, leaving the only objects that, due to orbital resonance, just happen to occupy an orbit that avoids any interaction. If not for Neptune, the Kuiper belt might extend closer to the Sun, with hundreds of bodies like Pluto/Charon. As it stands, the latter are just the lucky ones which by their very existence imply just how effective has cleared its orbital neighborhood.
August 24th, 2006 at 8:53 am
… and even if they change the definition to allow extra-solar planets, the definition is still faulty, since it would appear to make stars in orbit about other, larger stars into planets. Nothing there about being too small to ignite fusion.
We don’t have that issue as long as we are dealing with the solar system, but hey, we found out that we were not the center of the universe some time ago.
August 24th, 2006 at 8:58 am
hehehe. now not only our solar system has 8 plannets, the entire universe has only 8 planets. and all orbit our sun.
So what are those things orbiting other stars they have found? Whatever they are, they are not planets.
August 24th, 2006 at 9:01 am
I like this outcome much better. I have always thought of Pluto being anything other than an official planet; its composition being a Kuiper Belt Object that just happens to be large. If the 12 planet proposal was approved, I can see out Solar System growing even larger as larger KBO’s are discovered - and the thought of labelling a large asteroid a planet seems silly to me.
Why not have different classes of planets: planets, dwarf planets, irregular planets, gasseous planets… Stars have multiple classifications so this is not that far fetched. The benefit is that our Solar System will remain with the (now) 8 planets in the classical sense with the addition of multiple irregular planets and dwarf planets (or plutons if you will - which is a silly name as well in my opinion).
Do I see and H-R like diagram for planets in the near future?
August 24th, 2006 at 9:07 am
The BA leaves me a little confused. He says that a planet classification system is a cultural definition and silly. So is the solar classification system he often refers to also cultural and silly? His writings would seem to suggest he endorses classifying stars. Why is classifying planets different?
August 24th, 2006 at 9:09 am
Good points BA - and what about the Trojans or the many near-Earth asteroids?
It would have been far more sensible to label the 9 planets that we have (had) the 9 “classical” planets and to call large Kuiper belt objects large Kuiper belt objects with Pluto having a dual designation.
It seems that planets are like pornography - hard to define but you know it when you see it.
August 24th, 2006 at 9:09 am
Well between having a 100 planet solar system or an 8 planet configuration, I’ll pick the latter.
Secondly, neighbourhood is not necessarily an unscientific term. We can define it to be a small region of space around an object, but I can see the word’s shortcomings.
August 24th, 2006 at 9:10 am
I don’t see why this definition would exclude the extrasolar planets. The whole controversy was about where do we draw the line between the large and the small planets in our system. All ESPs found so far are large, so it’s a moot point. Later in the future we might define them more carefully, but presently there is no need for that as we don’t know of any extrasolar asteroids.
Pluto and Neptune don’t really have crossing orbits, they are in a mean-motion resonance and never meet as they go around the Sun. The same is true for Jupiter and its Trojans. Moons, by definition, are bound to planets gravitationally, so they also can’t be kicked out of the system by close encounters (which must be hyperbolic). The eight planets have truly cleared out their environment of almost everything else, and the present small bodies they are encountering (NEAs, comets, Centaurs) are just passing through and were not original inhabitants of those regions.
For those who are really interested in what “clearing out” exactly means, I recommend Steven Soter’s paper on astro-ph (it’s free, but quite long): http://arxiv.org/abs/astro-ph/0608359
August 24th, 2006 at 9:14 am
In the BA blog today (Breaking News– Pluto not a planet!), the BA asserts that “as I pointed out in my other post, in a billion years or so the Moon will be far enough away that the Earth-Moon center-of-mass will be outside the Earth.”
Because the tidal-slowing process is limited, that will not happen.
August 24th, 2006 at 9:16 am
If we allowed Pluto to have planetary status then it would be a short step to polygamy and people marrying dogs.
August 24th, 2006 at 9:22 am
I would have been happy with two types of planet–major planets and minor planets–with Pluto becoming a minor planet. But I guess that “planet” and “dwarf planet” is close enough. Fine with me.
I suspect that if we even come across planet-sized objects that do not orbit a star, we will simply create a new category of planet–either rogue planet or, more likely, wandering planet. No big deal.
The irony is that it’s pretty easy to see why Pluto doesn’t belong in the list of “classical planets”, but it’s proving darned difficult to come up with a rigorous specification that explains why Pluto is not a classical planet.
August 24th, 2006 at 9:24 am
Matija,
How come you don’t see why the definition excludes the extrasolar objetcs? It says right there - orbit the SUN.
Who cares how large the ESP is, as long as it does not orbit the SUN it falls out of the definition of the planet. ESPs under this definition are not planets!
August 24th, 2006 at 9:25 am
“People are talking about what it means to be a planet” - well, I’ve never been one as far as I can remember, although I have a brain the size of one…
Seriously though, it is a good thing as it is getting people who wouldn’t normally talk about astronomy doing just that - and maybe doing a bit of thinking too.
August 24th, 2006 at 9:34 am
[…] Update: The Bad Astronomer has more. […]
August 24th, 2006 at 9:34 am
Maybe that thing of “cleared the neighbourhood around its orbit” means “to be more massive than all other bodies in the orbit together”. Earth would still qualify as a planet when the Moon finally leaves it. I remember that Neptune is way bigger than Pluto, and still qualifies as a planet…
Nice work of IAU guys: their clarifying definition confuses things even more.
August 24th, 2006 at 9:38 am
Kit,
The IAU definition starts with:
“The IAU therefore resolves that planets and other bodies in our Solar System be defined into three distinct categories in the following way…”
I think this makes clear that the definition says nothing directly about ESPs. However, by analogy, one would expect the same distinction bewteen major and dwarf planets to be made in other systems, once there is a need.
August 24th, 2006 at 9:39 am
I applaud the new definition of a planet. Let’s remember that the term was coined by our ancestors in reference to star-like objects that appear to wander relative to the so-called fixed stars. Invisible objects due to small size or great distance were understandably not included. In particular, the term did not apply to unseen bodies circling fixed stars. Earth was added to the set following an historic debate regarding heliocentricity. The later discovered Uranus and Neptune unquestionably had the size and orbital characteristics of planets.
The characteristics and methods of formation of other bodies in the solar system seem to mandate classes of their own. Regarding bodies circling fixed stars, the term extra-solar planets may be sufficient for that set, although a new term could be devised.
It would have been simplest if the IAU had defined the set of planets as Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune. Apparently they feel that defining the set by its properties provides justification. I suspect that very soon the simple naming of the members of the set will be deemed sufficient, especially for fourth grade textbooks.
August 24th, 2006 at 9:45 am
“That which we call a rose, by any other name…”
Would still have thorns!
August 24th, 2006 at 9:47 am
I can’t see how it can be claimed that Saturn has cleared the neighborhood around it’s orbit. It may be in the process of doing so, but it hasn’t finished the job just yet. Come to think of it, the other gas planets have rings that demonstrate that they are not quite done clearing their orbits either.
Eventually we will have 8 planets it seems, but for now there might only be 4 or 5.
August 24th, 2006 at 9:51 am
Has anybody read to the bottom of the AP reports on the decision? My favorite part of the press coverage is this gem from the latest AP article:
“It was unclear how Pluto’s demotion might affect the mission of NASA’s New Horizons spacecraft, which earlier this year began a 9 1/2-year journey to the oddball object….”
It’s a bit late now to cancel the mission, does the AP think New Horizons will get out to Pluto’s orbit and find just empty space (maybe a “Pardon our dust” sign with the IAU logo)?
Lorne
August 24th, 2006 at 9:52 am
Very good discussion in this preprint
http://xxx.lanl.gov/abs/astro-ph/0608417
August 24th, 2006 at 9:54 am
> A planet-sized object between stars is not a planet?
Science-fiction writers have speculated about “rogue planets” - planets that don’t orbit a star, but is there any evidence that such a thing can exist? We don’t know everything about planet formation, but maybe all planets are around stars. Also, the only thing that makes the moon not a planet is it’s location, so I don’t see what’s wrong with applying the same criteria to interstellar bodies.
>Pluto is a dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.
I’m happy with “trans-Neptunian object” but “dwarf planet” sticks in my craw a little. It makes it sound like it’s a planet that’s small, but I don’t think it should qualify as even a “dwarf” planet. A SmartCar is a dwarf car, but a Bonneville Triumph with a sidecar is a motorcycle, not a car, even though it’s nearly as big as the SmartCar. I think Pluto should just be considered “the largest of the TNOs”, because there’s a smooth continuum of sizes among TNOs. Still, I’m glad it’s not a full planet any more.
August 24th, 2006 at 10:00 am
I like this decision. Since Pluto and Charon are essentially Kuiper Belt objects, with 100s more bodies just like them, it appears to me that we have either 8 planets, or we have 1,000+. This makes sense to me. In the end, it maintains the cultural importance of the inner planets. And we’ll always remember Pluto.
August 24th, 2006 at 10:09 am
I think the better definition is the center-of-mass, ok in a billion year or so, like you said the Earth will no longer a planet with this one, but i think we are going to destroy Earth sooner than that.
(first comment i made :P)
August 24th, 2006 at 10:17 am
I agree with many above comments: the way the decision was formed was pretty ugly, but the end results seems pretty, well, right. I have to call this a victory for the IAU.
August 24th, 2006 at 10:17 am
Ashley: Yes. Google “planemo”. Or check this article: http://www.spaceref.com/news/viewpr.html?pid=20537
So anyway… Pluto is a “dwarf planet” now. (And presumably so is Ceres, maybe one or two other Asteroids, and then perhaps a number of other Kuiper Belt objects.) I’m not sure about the “cleared its neighborhood” business. How big is the neighborhood? Aren’t there over 1,500 asteroids which share Jupiter’s orbit but just happen to be far enough from the planet and orbitting in sync with it so as to remain unmolested by it? I think they are called “trojans”?
August 24th, 2006 at 10:23 am
Ah well, at least the fingercounting crowd can still count the planets on their fingers or some ridiculous mnemonic.
Hukt awn fahniks wirks 4 IAU.
I truly appreciate the sick irony of a heliocentric definition for planets, the Christians must be drooling over the possibility of restoring Earth’s place as “special” in the universe.
August 24th, 2006 at 10:28 am
The problem is not that they’re trying to define what a planet is. They already have a very good idea what a planet is! The problem is that they’re trying to find a set of criteria that cover all the objects they call planets. And then suddenly they realize that they all have a lot in common with objects they don’t want to call planets. Must be rather frustrating to keep coming up with rules that just don’t fit like they want them to!
August 24th, 2006 at 10:30 am
[…] The blogosphere is all a buzz about this new turn of events, with the Bad Astromoner objecting to the defintion again, Frinkers crusing about it (as always) and inkycircus discussing the pains of demotion. […]
August 24th, 2006 at 10:30 am
If you note the definition states that the dwarf planets are not a subclass of planets but a new class. It doesm’t make sense. Earth and Jupiter are in the same class of objects. Pluto and the Earth are not. All the TNOs and Pluto could fit inside the Earth but then all the other planets could fit inside Jupiter. The only criterion is the “cleaning of the orbit”.
August 24th, 2006 at 10:31 am
Ashley Zinyk: Yes, there exist “free floating objects” of jovian mass, but right now, they form just like stars, by a collapsing cloud. Indeed, some weeks ago a double object was discovered, one just above the brown dwarf limit and one well below (14 and 7 jovian masses).
August 24th, 2006 at 10:33 am
YES!! YES!!! MY LIFE LONG DREAM HAS BEEN FULFILLED!!! PLUTO IS NOT A PLANET!! YAY!! THIS IS A CAUSE FOR CELEBRATION!!! W00T W00T!!!! YES YES YES!!!!!!!!!! 8 PLANTES YES!
Hey what about Xena? D= Xena should be a planet!
August 24th, 2006 at 10:37 am
What I think is silly is that this was never defined in the first place.
August 24th, 2006 at 10:44 am
You know, I think that I’m more confused now than ever about our system.
And Fender, “Xena” is UB313 and it is not a planet. It’s now a dwarf planet like Pluto and Ceres
August 24th, 2006 at 10:44 am
[…] Stop the presses, the official decision has been made, and Pluto is no longer a planet (so neither are Charon, Ceres, and UB313). […]
August 24th, 2006 at 10:55 am
Demote Pluto…
The debate over Pluto rages: What is a planet? It is a question that calls us to take sides: the rationalists who insist that it can no longer be considered a planet, and the sentimentalists who will hear none of……
August 24th, 2006 at 11:13 am
BA wrote: “…given the abysmal level of science education in the US, it’s great that folks are actually talking about astronomy.”
But what are the abysmally ignorant talking about? The inability of astronomers to solve a non-poblem, it seems - if the Daily Show take on this is any example.
So what does this say about a group of the best astronomers in the world failing to arrive at even a partially successful solution which seems could be resolved with the existing science covering bodies which inhabit solar systems? It says to me that they behave just like a group of high school teachers when put on a committee to decide a like problem: the number of solutions (and those who defend them) are directly proportional to the number of teachers on the committee, notwithstanding the nature of the problem or the expertise of the committee participants, and the solution doesn’t satisfy anyone. (And, I’ve seen teenagers get to the heart of the matter and make clearer decisions than such faculty-peopled committees.) Perhaps we can’t ever get clear answers to problems which are (a) not crucial, and (b) which don’t have clearly defined data with which to address them. This jury wasn’t deciding a life or death decision, and the planets/orbiting bodies/objects/etc. will still be there when we do get enough data to finally classify them (should we decide we need to).
August 24th, 2006 at 11:15 am
“Let me once again reiterate that trying to define what a planet is is very, very silly.”
How can it be that the definition of a basic term in your field is “silly”? Can you imagine any other field of science where the definition of a basic term would be an exercise in silliness?
Imagine chemists saying that attempting to define the term “element” is silly. Would anyone take them - or their field of study - seriously?
August 24th, 2006 at 11:17 am
“And Fender, “Xena†is UB313 and it is not a planet. It’s now a dwarf planet like Pluto and Ceres”
I know what Xena is, I’m not retarded. I just don’t want to waste 3 seconds of my life typing UB313 when insted I could use that time to utter my last important words with my last breath.
August 24th, 2006 at 11:18 am
If anyone is interested, over at the Jodcast we have edited down the very lengthy IAU session that took place this afternoon to a more reasonable 20 minutes or so (the MP3 is around 10MB). Many of the questions raised in the comments here were also raised by astronomers in the session (we took out some parts just to keep the whole thing brief) such as the status of Neptune being a planet and why Charon is not a dwarf planet. The BA is not going to be happy though, as people kept calling Charon “Sharon”.
August 24th, 2006 at 11:34 am
I find one thing kind of funny, anytime someone mistakenly calls a meteorite a meteor or vise versa, Phil is quick to point out their mistake. Yet he states that coming up with rules that define what is a planet and what isn’t is “sillyâ€.
August 24th, 2006 at 12:38 pm
So far NASA has said this will not affect the NH mission, I just hope this is not used as an excuse to scrub a mission underway so nobody will be there to hear thge NH signal.
I read one report that most of the astronomers had gone home and the vote was taken by a small number. Any word on that?
Is there some avenue of appeal?
August 24th, 2006 at 12:38 pm
I like the new definition.
Now maybe I’m wrong, but I thought that the objects that pluto failed to clear from its neighborhood was the rest of the Kupier belt Objects, not Neptune.
Also, concerning the recession of Earth’s moon, unless I’m missing something, the moon does not neccessarily gain planethood, nor do we lose ours, as the moon could still be considered our satellite.
Finally, I don’t think that they meant that ALL debris was cleared from the neighborhood, just a considerable chunk. So while their are still Near Eart Astroids, the population density is not near that of the asteriod belt. Also, Trojans, Moons, and rings, while they are debris, are under the influence of the planet policing the neighborhood (as opposed to the asteroids and KBO, which have no obital “policing” planet).
August 24th, 2006 at 12:40 pm
Wow, it’s amazing how many people ignore the “in our Solar System” bit and get all het up over the “orbiting the Sun” bit. Perhaps poor reading comprehension is the reason for the sad state of American education?
[b]Ed Minchau[/b]: When was the last time you heard a debate about what an “element” is? There is a simple definition, and there are things that clearly do and don’t meet it. Whereas “planet” is clearly not so simple a definition. It’s a lot foggier, and everything that fits it also technically fits the definition “satellite,” so “planet” is, indeed, a human and essentially worthless construct.
Still, it makes elementary school education easier, I suppose.
August 24th, 2006 at 12:40 pm
Sticks,
Even if true, those that went home probably did not feel that the vote was important, or they didn’t care. Afterall, we all knew a week ago that a vote on the definition was supposed to be coming today.
August 24th, 2006 at 12:45 pm
Gillianren,
From a technical standpoint, the definition is quite worthless, but setting a good definition is important for the purposes of public outreach. Now we just have to show the public that this is how science works: our way of seeing the universe changes based on observations and does not mere stand still just do to traditions.
August 24th, 2006 at 12:54 pm
Pluto’s characterization has bugged me since I was an early teen, today I can finally rejoice at its demotion. I drink to you, Pluto, the planet that never was.
August 24th, 2006 at 12:56 pm
This definition is far more amenable to extra-solar planets than the one first floated by the sub-committee, with its arbitrary 200-year orbital period dividing line. I think it’s a reasonable definition, but there is the issue of what it means to “clear the neighborhood around its orbit”. Pluto’s problem on this front is not Charon or its other moons, but the several other objects (previously dubbed “Plutinos”) that are in the same 3:2 orbital resonance with Neptune that Pluto is in. Or, maybe its problem is that its orbit crosses the orbit of Neptune (although it is stable due to that 3:2 resonance). Or maybe its because its orbit crosses the orbits of other Kuiper Belt Objects. The asteroids are on crossing orbits, but they are also on planet-crossing orbits, so some additional distinction is needed about just how clear the “neighborhood” has to be.
August 24th, 2006 at 12:59 pm
From now on, I’m going to call Earth a “large, wet asteroid.”
August 24th, 2006 at 1:04 pm
That is because what is fundamental in this situation is physics. Physically, what we call a meteor and what we call a meteorite are completely different things, completely separate phenomena. With the term ‘planet’, however, you make the definition however you like. As far as physics goes, Pluto is absolutely no different today than it was a month ago, and it is therefore very very silly for a group of scientists to try to change via nomenclature what has not changed via the laws of physics. Historically, the term “planet” has much more to do with Earthbound observers than it has to do with physics, solar system formation theory, or hydrostatics. Earth and Jupiter are physically different places, yet we use the same name - planet - for each. We acknowledge the physical difference when we describe the separate searches for extrasolar “Earths” and extrasolar “Jupiters.” The nomenclature is a cultural and traditional decision, not a description of the physics at play in the body in question.
August 24th, 2006 at 1:04 pm
So, why is UB313 not a planet? Because it hasn’t cleared out the Kuiper belt?
August 24th, 2006 at 1:24 pm
[…] The IAU did what I originally thought they would do and demoted Pluto from a “planet” to a “dwarf planet”? I agree with the decision. The reasoning is pretty simple. If we want the word “planet” to be reserved for a small, unique group of large objects orbiting a star, then there is no easy way to have Pluto be a “planet” the way Jupiter and the Earth are planets. Pluto is one of the largest (but not the largest) members of a vast swarm of objects orbiting the Sun in what is called the Kuiper Belt. The new definition excludes objects that are not the dominant objects in their orbital zones. This has some physical basis for it, as planets originally grow by gravitationally gobbling up (or kicking away) everything in their gravitational zone of influence. If you can’t manage to unite your neighborhood, then the best you can be is a dwarf planet. But the resolution is pretty vaguely worded about just how “clear” the “neighborhood of the orbit” has to be. Jupiter, the king of the planets, shares its orbit with thousands of Trojan asteroids that are in a special 1:1 resonance with Jupiter. So what rules out Pluto: the dozen or more “Plutino” Kuiper Belt Objects that share its 3:2 resonance with Neptune; the fact that its orbit crosses that of Neptune; other KBOs whose orbits cross that of Pluto? Maybe it’s like porn and you know it when you see it. […]
August 24th, 2006 at 1:29 pm
I think of it as the first steps in refining the idea of “Planet Classification”. It is at its earliest and broadest start. As our understanding of the planets and other objects that orbit our Sun advances, so to will the definitions. Yes it has gaps but so does our understanding of our solar system.
August 24th, 2006 at 1:42 pm
Wait, what about Cruithne? It shares an orbit with Earth, so Earth has not cleared out its neighborhood! OMG, Earth is not a planet!!!
August 24th, 2006 at 1:46 pm
DJ: Yes.
I’m pretty pleased with this, but then again I was pleased with last week’s definition, too. What would make me really, really happy is if we formalised different classes of planets for rocky planets and gas giants. So I would have four classes of object where the IAU has three: gas planets, rocky planets, dwarf planets, and SSBOs. The first two categories fall under the term “classical planets”, and the first three categories fall under the broader umbrella of “planets”. But “planets” plus “dwarf planets” with “SSBOs” hanging around the edges is ok with me.
August 24th, 2006 at 1:48 pm
“our way of seeing the universe changes”
Fundamental Zen error #1: the name is not the thing.
Fundamental Zen error #2: The name change is not the result of observations, but of “interpretations” of the observations.
Habits from millenia gone-by still clinging … bad hygiene.
August 24th, 2006 at 1:49 pm
“if Pluto’s orbit actually overlaps Neptune’s, doesn’t that mean Neptune hasn’t cleared out its neighborhood? I think you might argue that. So why don’t we have 7 planets?”
No. This is obviously intended to mean bodies that have the same orbit. Pluto’s orbit is different to Neptune’s, so it is not in Neptune’s neighbourhood, no more than Halley’s comet is in Earth’s neighbourhood, despite the fact that their orbits cross.
Personally, I’m quite pleased that this non-planet is no longer considered a planet, for whatever reason. It’s not that contrived, either, although I agree that it’s nonsense to try to give a failsafe definition of a planet.
August 24th, 2006 at 2:09 pm
So we are saying that 187 elitist professional astronomers voted for this and 183 were against?
Out of how many members?
And the next meeting is when?
In 2009?
Ok, here is what I think.
Let’s strip the funding from IAU member Astronomers totally and
see how they like that!
Here’s what Johns Hopkins astronomers are saying:
http://groups.google.com/group/sci.space.news?lnk=oa&hl=en
Touche.
August 24th, 2006 at 2:21 pm
Never bothered me if Pluto was a “Planet” or a “Dwarf Planet.”
BTW, if Pluto were the same size as Mars it would still not be a planet, right? If not, what is the maximum diameter of a “dwarf planet”?
August 24th, 2006 at 2:42 pm
Why not call Pluto a moon of Neptune, since it is gravitationnaly bound to it? (resonance orbit and all…)
August 24th, 2006 at 3:00 pm
@Chip:
If Pluto were the size of Mars, I’d expect it would have done a better job at clearing its orbit (greater mass, stronger gravitational tugs, etc.). Big planets keep the spaceways tidy.
I haven’t done the calculations on this, but my first blog-commentor’s guess is that by our current understanding of How Things Work, such a problem isn’t likely to arise.
August 24th, 2006 at 3:05 pm
And what about this “clearing” business?
Sounds to me like this definition says that the Earth is not
a planet because it hasn’t cleared it’s orbit!
What in heck do we call these NEO’s? Space Dust?
What about comets such as Halley’s that regularly
pass through our orbit?
And what about meteors?
Clearly, the Earth has not cleared it’s orbit yet either!
If anyone asks me I will ask them which definition?
August 24th, 2006 at 3:06 pm
honestly. sol has one planet, three minor planets, and a lot of debris.
August 24th, 2006 at 3:07 pm
As long as the name xena did not stick is the only thing i care about and i feel sorry for anyone who has to teach an intro astromony course or is tour guide at a telescope in about year from hearing the same question over and over ” is pluto a planet what are your thoughts”
August 24th, 2006 at 3:18 pm
[…] Bad Astronomy shares my issues with this new definition, but I disagree that it is a silly argument. Maybe it is my innate drive to name and classify everything, but I do think it is a worthwhile discussion. I have the same feelings about lakes and ponds, streams and rivers. IMHO we should set an official definition of these as well. […]
August 24th, 2006 at 3:20 pm
Whoa B.A, chill bud. You and many others here have stated this is unscientific and your right. Why get your knickers in a twist over it? All the IAU has done is to define what is and what isn’t a planet for the ordinary layperson. And let’s face it, there are a heck of a lot more of them then there are scientist types.
So which aspect of you is balking Pluto not being a planet? The scientist or the regular everyday joe with a wife and 2.1 kids? (i’m counting the dogs too
)
Until you scientists get together and get a scientific definition of what a planet is, including ours, then the rest of us will just have to deal with the fact that Pluto has been demoted to “dwarf” status. No big deal.
August 24th, 2006 at 3:28 pm
The planet definition should be named only to its size and mass.
Maybe adding the category of ice, rocky, and gas planets depending of the main sustance they are from.
Planets should not be tied to its orbit. One day we may find a solar system with a planet the size of jupiter with an orbit like the comets. But it would be called a comet or a planet???.
Other posibility that we may find a rock of the size of Jupiter but not orbiting a star but only static on a gas cloud. Still this would be called a rock or a planet. In our minds there are constraints from our learning past, althought they are great we should not be tied to that. We can learn a new definition. On the past a man could only walk or swin, now we can fly to outer space. Free from the clasical view and evolve to a more scientific model. Keep it scientific not semantical.
Again sorry for my English.
Thank you for your time.
August 24th, 2006 at 3:34 pm
What about the asteroid belt. Wouldn’t that be in the neighbourhood of both Mars and Jupiter? Would they be therefore declassified as pplanets, becuase they haven’t cleaned up that part of their neighbourhood?
I think we now need a decent definition of neighbourhood to stop these sorts of silly questions about what is or is not part of a planet’s neighbourhood.
August 24th, 2006 at 3:34 pm
As I stated before this definition is far worse the previous proposal that was floated. In stead of defining planet and then creating sub-categories, it clutter the definition itself with pointless things like this clearing its neighborhood.
The previous proposal is simply: A planet is a round object that orbits the sun, and may co-orbit another planet.
This new proposal that now has being made official is: planet is a round object orbiting the sun that cleared its neighborhood.
The amount of NEOS is an indication that earth has not cleared its neighborhood. I would venture to wager that the number of NEOS-like objects within certain fixed volume arounod pluto is less than around earth. Certainly not a fair comparison but within the vaguery of the definition. I cannot think any way clearing the neighborhood can be made precise the way co-orbit and round can be made precise.
August 24th, 2006 at 3:42 pm
Dagger,
They haven’t defined it at all they have only muddied the whole discussion.
They didn’t even talk about a planet having an atmosphere or setting a definite limit in size.
They left it to the very last.
Thank God I don’t let these folks do my taxes or make any life or death decisions for me!
August 24th, 2006 at 3:47 pm
Using the word “neighborhood” to to describe astronomical distances is just [i]weird[/i]. Could we not have some formulas, or numbers, or something?
August 24th, 2006 at 4:02 pm
P. Edward Murray,
No argument with the tax assessment, but as to muddled, we’ll have to agree to disagree. I don’t believe it’s muddled at all. They really didn’t get into proper definitions and certainly didn’t apply any sort of science to it at all. I prefer to think of it as the precursor to when the scientists do develop a valid system of defining a planet. But we’ll have to be careful with that. Until now, we’ve only been good a defining what we can see. With 190 or so extrasolar planets now in the catalogue, it’s a daunting task to find a robust enough system to be able to catagorize them all. Scientifically.
August 24th, 2006 at 4:29 pm
I found the following comment on the BBC “Have your say” web page. I hope Mr. Slawson doesn’t mind me reprinting it here. He’s summed it all up succinctly thus:
“If Pluto is a planet then I’m the Ravenous Bug Blatter Beast of Traal. It is quite clearly a lump of irrelevant rubble left over after the Solar System coalesced out of the primordial gloop. It doesnt have a propper orbit and worst of all I cant find it in my telescope. I’m happy to have it described as a Pluton along with all the other garbage in the Oort cloud. Propper planets have tunes written for them by Gustav Holst and thats the end of it.”
jeremy slawson, plymouth, United Kingdom
August 24th, 2006 at 5:03 pm
HERE HERE to Phil’s suggestion to study the planets. I think, however, that the category of “planet” can still be retained, and the idea retain its coherency, without getting in the way of science.
Why not just give the name of “planet” to any body that is sufficiently round but has not achieved nuclear fusion? Then we’ll have billions of stars, billions of planets, and billions of smaller objects (everything from irregularly shaped asteroids to dust particles), and we can start sub-classifying planets the way we do with stars. We can have terrestrial-type, comet-type, gas giant, etc.
Leave the question of orbit out. Phil just demonstrated how it makes things too complicated — and I’m beginning to think myself that defining a body according to its relationship to another body is complicating things too much. We could indicate whether something is a satellite of something else, or not, and leave it at that. Best to study the planets according to their properties: size, atmosphere, chemistry (biology?).
That would make scientists happy and it would still give the public something like the old definition: we can still say that we have 8 large planets (not including satellites) in the inner reaches of the solar system (our “neighborhood”), half of them terrestrial and half of them gas giants.
August 24th, 2006 at 5:24 pm
I frankly think that the IAU’s decision was HORRIBLE — not because of any scientific reasons, but because it was a Public Relations disaster. Let me explain….
As we know the public’s interest in Astronomy and science in general is at best iffy and short term. In general, only an important first or discovery tends to attract any meaningful attention from the average person. That is why the idea of having new “Planets” discovered would be such a boon to public relations. All people are drawn to the new and unknown if they realize its significance. What better possibility as far as Astronomy is concerned other than finding a new Planet in our own solar system?
Don’t bother to complain that science shouldn’t bow to public opinion — the two are hardly mutually exclusive! It is all about the presentation. If, for instance, the IAU had listed the first eight planets as “classical” it would have allowed for the discovery of more “planets” while keeping the more scientific definition that there is difference between “Classical” and “Dwarf” planets. It is a compromise with great PR potential — there would be many “planets” to be found, just of different types. To the general public the discovery/reclassification would represent something new, an addition to their understanding of the universe that made it more expansive and interesting.
By comparison, by not qualifying the classic Eight planets as just a subset of the larger term “planet” what you get — and the reaction from the public and media already reflects this — is the widespread understanding that a “dwarf planet” really isn’t a “planet” at all, but something LESS interesting. By not only taking away part of the public understanding of the “universe”, but by effectively stating that no more planets will ever be found (in our solar system, which is the only planets most people are likely to care about) you turn this whole question into a serious NEGATIVE.
It’s the sort of thing that will just leave a bad taste in people’s mouths — the IAU “took away” Pluto, not to mention other potential candidates. In short, psychologically speaking for the average person the universe just became a less mysterious, less relevant place. Sure, those of us interested in Astronomy know better, but that is far less likely to be the case for the average person.
Worse, the IAU may very well have added fuel to the anti-intellectual bias that is such an evident thing in our society (especially in the U.S.). It will only strengthen the perception (partially because of all the arguing that has already gone on over this subject) that scientists are just remote ivory tower intellectuals who obsess over trivia that has little or no importance to “real life”. Given how this decision was made by just a tiny percentage of all Astronomers it will also encourage the other common negative impression that scientists are merely a bunch of self-serving elitists making decisions for everyone else.
That is why this decision — and the handling of the matter in general — is turning into such a negative. It makes the scientific community seem petty and elitist. Yet, it didn’t have to be this way. With some common sense and savvy salesmanship this could have been a great PR triumph, yet without harming the basic science at all.
August 24th, 2006 at 6:12 pm
Alan,
I think you’ve just hit the proverbial nail on the head.
Yes, it is a PR disaster and yes it flows right into this Anti-Intellectual bent
that we seem to be stuck in here in the States.
To me, it “seems” that they were hell bent on determining that Pluto was not a planet. I might be wrong but it smells of an “Elitist” mentality as in
Non-Professional Astronomers don’t or shouldn’t make major discoveries.
And that’s the attitude that we don’t need in the sciences today, rather we need a welcoming attitude to those who want to contribute and those who just want to learn. Because if we don’t foster that attitude who will?
Science & Technology after all are the engines of change and the engines that create. If we, in the United States don’t DO others in other nations WILL and leave us behind and then our standard of living will be left behind.
I think the IAU has outlived it’s usefulness.
All of us Astronomers, Pro’s and Ams need to create a new organization.
An organization that is vibrant and full of life.
August 24th, 2006 at 6:19 pm
I do prefer this resolution with 8 classical planets, and the rest as “dwarf planets”.
One must wonder if they can really come up with a satifying answer. I suppose they won’t.
Perhaps the IAU should have set up a classification system. You have major planets like the classical eight, which themselves are either terrestrial or gas giants, and then you have dwarf planets, with Pluto as the prototype. And add that a dwarf planet is one that fits neither into the gas giant or terrestrial planet catagories.
Then we could drop the ’roundness’ and ‘clearing the neighborhood’ issues.
Oh yes, perhaps this would also be a start; define a planet as a celestial object that is not a star. And then you have the classification mentioned above.
Maybe the IAU will think of that for their next meeting. Think they’ll take suggestions?
August 24th, 2006 at 6:20 pm
“Neighborhood” is actually a well-defined mathematical term, but it requires a parameter. The neighborhood of size epsilon around a point P is the set of all points whose distance from P is less than epsilon.
This can be pretty easily extended to: the neighborhood of size epsilon around a set of points S (such as an orbit) is the union of the neighborhoods of size epsilon around each point of S.
So the orbit of Pluto or any other body has not just one well-defined neighborhood, but infinitely many of them (of different sizes). Do they mean the 1-km neighborhood, or the 1,000-km neighborhood, or the 1,000,000-km neighborhood, or the 1,000,000,000-km neighborhood?
Furthermore, unless the neighborhood size is set *really* small, Pluto and Charon are in Neptune’s neighborhood and thus Neptune is a dwarf planet. The absurdity of a gas giant being classified as a “dwarf” planet is patent.
And what does “cleared” mean with respect to double planets (double dwarf planets?), moons, rings, Trojans and miscellaneous small objects and dust? What if some body of substantial size *enters* the neighborhood of a planet? Does it lose planet status until it gets rid of the intruder?
Any definition is going to be arbitrary, but this one is *vague*, too. Ick.
P.S. Charon is still a dwarf planet, right? The not-a-satellite criterion reduces to the center-of-mass criterion from the previous definition?
August 24th, 2006 at 6:25 pm
I say math is fundamental to all science. Lewis Carroll demonstated a basic mathmatical principle when he wrote, ‘”When I use a word,”Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean — neither more nor less.”‘ I think math trumps the IAU.
Spread the word, Pluto is a planet whether the IAU likes it or not. In fact I now know there are 12 planets, Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Neptune, Uranus, Pluto, Charon, and 2003 UB313.
August 24th, 2006 at 6:35 pm
But think of all the ping-pong balls that will be saved.
August 24th, 2006 at 6:41 pm
Huh. Well, THIS is some surprising news. I suppose there were enough 11th-hour objections for the initial resolution to get voted down. While I am somewhat glad that the initial terms weren’t set in stone, I am a bit uneasy about the ones that just were. I can definitely see some controversy over this for a long time to come.
Now, here’s a thought I had as how one would go about making a definition for a “planet” if they absolutely had to: I was looking through my college astro textbook last night (from ~2003, so not so old), and I noticed that, while the outer “planets” (Jupiter, Saturn, Uranus, Neptune) were different from the inner ones in several ways, a) they all follow the general “rule” of what major planets should be like based on distance from the parent star; and b) they all have differentiated (i.e. layered) structures. I suspect that a) may have something remotely to do with the “clearing the neighborhood” rule in one way or another, but b) really got me thinking. From what I understand, differentiated structures are the exception rather than the rule in terms of non-solar objects, and differentiation suggests processes such as tectonic activity, etc. that don’t happen on just any or every object around. Objects such as asteroids, KBOs, etc. are for the most part NOT differentiated the way that the “classic” 8 planets are. AFAIK, Pluto is not. Other large KBOs are not. Most asteroids are not, although there has been some research to suggest that Ceres has a differentiated structure. So, could this be a possible criterion by which objects could be called “planets” or not? Incidentally, I’d actually have less of a problem with Ceres being called a “planet”, since it lives out amongst the failed planets of ages long past…
August 24th, 2006 at 6:55 pm
To me Pluto is still a planet. Xena is also a planet to me. Maybe there hasn’t been a scientific definition of planet until now, but it has been popularly accepted that Pluto is a planet. According to that, Xena, which is larger than Pluto would also be a planet.
I find it quite ironic that it has been the questions of the public, in large part, that has lead to this: a public that has always accepted Pluto as a planet, and certainly would have accepted Pluto also.
Also, some astronomers believe we may eventually find objects as large as Mars in the Kuiper Belt. The IAU will have a difficult time justifying those objects’ not being planets. After all, they would be larger than one accepted planet (Mercury) and at least as large as another (Mars). Yet, according to the IAU definition, they more than likely could not be planets, since they would not be the dominant body in their neighborhood.
I’d really be interested on your thoughts of how that scenario would be handled. Would such an object be a planet in your opinion? What would the IAU do?
August 24th, 2006 at 6:59 pm
The debate rages and is fascinating for folks like me, who are learning immense amounts from it. I thought you, BA, and the rest of the commenters would be amused by a link my friend send me: http://www.brianlarter.com/wp-content/uploads/2006/08/pluto-astroid.jpg
August 24th, 2006 at 7:10 pm
Its too bad there that no one at this conference decided to develop a classification system like the one that is used in Star Trek. Granted, we are talking about sci-fi, but what is fantasy now can be fact later on. It just needs a thoughtful approach. Something like:
Location: Hot Zone, Eco Zone, Cold Zone… based on our current understanding about water being a prerequisite for life.
Size: Asteroidal or Spheroidal… depending on whether gravity can shape it into a round body.
Geological Activity: Is the body geologically active or inactive? Can it evolve from being active to inactive like the planet Mars is?
Atmosphere: Is it transitory or largely permanent? What is it composed of?
So and so on… and eventually a pattern will emerge that could satisfy almost everybody. I mean come on, it was reasoning like this that gave us our stellar classification system and its familiar refrain ‘Oh Be A Fine Girl, Kiss Me’. Right now, its too simplistic, vague, and more about showmanship. Is Pluto a planet? Its such a non-issue. Its a Class C Planet.
August 24th, 2006 at 7:13 pm
Ok, these new rules still do not fit. they are no better than the ones before.
Ok here are a few contridictions:
1. the largest asteroid we have found (I forget the name.. is it ceres?) is round, and HAS cleared its neighborhood. Just because it crosses the orbits of other planets, still means that it has “cleared its neighborhood”
2. If the “cleared the neighborhood” rule were followed, then Jupiter IS NOT A PLANET because there are at least 2 groups of asteroids that currently share jupiter’s orbit.
3. if the above said rule was followed, then that would also mean that plants with satilites do not qualify as planets as sattilites are objects in their “neighborhood” Then there would only be 2 “planets” which comprise of Venus and Mecury.
4. If a sattilite, say titan, was somehow ejected, and assumes a stationary orbit around the sun between saturn and uranus. Would titan be reclassified as a planet if this happens?? Probably not.
5. What about sedna, quoar, and the other elipical orbiting “planetoids” that were found?? Since we do not know of where the objects in the Oort cloud are or how many are near the orbits of Sedna, Quoar and the other recently discovered planetoids, then it would be inappreopriate to classify them at all as they can possibly fit those rules.
6. Then would mercury be classified as a dwarf planet? (not following these rules, but looking at the size and mass of mercury, it would be “appropriate” to classify mercury as a dwarf planet.
August 24th, 2006 at 7:15 pm
too many of the posters here need to take a deep breath and chill out. Hydrostaticicity is probably the best limiter of planetdom. As for neighborhood, man, it can be narrowed down later. Pluto/Charon, Ceres, and Xena pretty obviously haven’t cleared their neighborhood, and the 8 by definition have. Therefore, their epsilon is somewhere between the two.
I also don’t understand why BA thinks this is silly, starting a definition of the most important (to laypeople) astronomical object.
I think planet-likes that don’t orbit stars shouldn’t qualify, and should get their own name, because there’s a qualitative difference - namely, the orbit of a star. Nyah.
When they start to work on an extrasolar definition of planet, then we can add ‘not big enough to start fusion’.
August 24th, 2006 at 7:34 pm
[…] We’ll see if that classification sticks. Probably not. This whole thing is pretty futile. For a good discussion of that vist the blog of the unbiquitous Bad Astronomer here. […]
August 24th, 2006 at 7:36 pm
It’s One Flew Over The Kuiper’s Nest for sure. I wants me Pluto, and anybody denying me my Pluto can kiss my ice. All them other planets are on aSteroids and that’s why they became so biggie sized.
If Wendy’s (can’t say that) can finally sell a vanilla Frosty (it’s for a pun, yes I can) which one might enjoy Charon with their friends, then Pluto can be a planet because it’s still a shake even if it can melt. If in doubt, ask William Orbit, I’m sure he has a Neptune to cover it. Discrimination against ellipses such as this shall not stand!
World up ; talk to the hand!
Begin Hyperbola: Pluto Sez: Foci your momma, and your daddy and the IAU rode end upon end on. Uranus ain’t the only thing stinking sideways about this decision, and if you think otherwise I have a parabolas I can sell you on ebay. End Hyperbola.
What’s next, if you have an atmosphere or not? I loathe that. So, now we have a case of atmosphere and loathing, and in the least, beer and loafing. I’m Ceres. Stop laughing. I need a Sednative now. Somebody help open the bottle here, who Titan’ed this lid? I never Xena one so tight.
Punography aside, the original definition was better, with the Round and In Orbit and Party of 12. Made Sense. This makes only Specialist Sense. And thus, no sense. So I’m further incensed. Pooey. Somebody tell New Horizons it’s flying to one that no longer exists.
August 24th, 2006 at 7:41 pm
The universe is so huge with uncountable stars and planets in it, and somewhere in the depths there is an unimportant planet named earth where unimportant people discussing/deciding whether pluto is a planet or not. I mean, who cares? If people on earth call it a planet of their system or not … Pluto is still what it was before.
August 24th, 2006 at 7:42 pm
Check this out:
For those of us who think Pluto is a Planet..
there will be a rematch!
http://www.msnbc.msn.com/id/14489259/
August 24th, 2006 at 7:43 pm
Actually I’m beginning to like this definition of a planet a bit better. I think one reason why people aren’t taking to it, because the science behind it hasn’t been explained too well.
The “cleared neighborhood” criteria is really a backdoor statement on formation and probably more important than equilibrium. If you compare an object’s mass and period (mass squared divided by period) and compare the traditional 9 planets to the new KBOs and asteroids, there’s a big “gap” bewtween the planets and non-planets. Pluto and UB313 fall dramatically on the wrong side of the gap in the non-planet category.
August 24th, 2006 at 7:52 pm
Earth & Jupiter don’t meet the criteria set out in clause (c). From: http://msnbc.msn.com/id/14489259/
—————————————————————
“I’m embarrassed for astronomy. Less than 5 percent of the world’s astronomers voted,” said Alan Stern, leader of NASA’s New Horizons mission to Pluto and a scientist at the Southwest Research Institute.
“This definition stinks, for technical reasons,” Stern told Space.com. He expects the astronomy community to overturn the decision. Other astronomers criticized the definition as ambiguous….Stern, in charge of the robotic probe on its way to Pluto, said the language of the resolution is flawed. It requires that a planet “has cleared the neighborhood around its orbit.” But Earth, Mars, Jupiter and Neptune all have asteroids as neighbors.
“It’s patently clear that Earth’s zone is not cleared,” Stern told Space.com. “Jupiter has 50,000 Trojan asteroids,” which orbit in lockstep with the planet.
—————————————————————-
Also, Saturn doesn’t fit into the “nearly round” criteria of clause (b). It’s an oblate spheroid, that’s flat at it’s poles.
August 24th, 2006 at 7:58 pm
Yeah Kevin 424 people vote out of 10,000 some odd professional astronomers worldwide.
This decision won’t stand.
August 24th, 2006 at 8:03 pm
When Ceres was found they called it a planet for a while then they found a lot of other objects were found in same orbit. The same with pluto and other obects that are in kuiper belt or a trans neptune objects, in about a hundred years nobody will think of pluto as a planet.
August 24th, 2006 at 8:21 pm
So is this new resolution truly a “Cosmic Demotion,” or in the words of Frank Zappa, “Kosmik Debris”? I got sarcastic with some friends and some rather pointed remarks about Pluto’s “demotion” were made. For example:
Does this mean that Pluto will have to take a cut in pay? What about “his” benefits, such as medical, dental, vision and retirement? What about Pluto’s mortgage, alimony and child support payments?
“Failed to clear the neighborhood,” does this mean that Pluto was too lazy to mow the yard or participate in a neighborhood watch program? This demotion seems a little harsh for some long grass and blackberry bushes.
Pluto is NOT a “Dwarf” Planet, “he” is merely “size challenged.”
It seems that Pluto is being discriminated against - who’s going to call the IAASCP? [The Interplanetary Association for the Advancement of Size Challenged Planets].
“I am NOT a Trans-Neptunian “object,” I am a PLANET BEING!”
This is your brain on drugs, this is a bunch of supposedly “professional” astronomers trying to act like linguists. Any questions?
Why, oh, WHY wasn’t Phil Plait of Bad Astronomy allowed to sit in and tell it like it is?
August 24th, 2006 at 8:40 pm
Not the first time the definition of “planet” has changed. Anybody read any Shakespeare lately? In the Bard’s day, the sun and moon were also considered “planets,” the operative definition (from astrology) being any object that appeared to move across the sky (instead of appearing “fixed” in the sky, like the stars). Of course, this definition came from the today much laughed-at “discipline” (word loosely used) of astrology (and, indeed, modern astrologers still refer to sun and moon as “planets”).
Before you baste me in lamb for mentioning astrology in this forum, consider that, as kooky as it seems in comparison to the modern science of astronomy, astrology was the stargazer’s primary outlet back in those daze, from which astronomy later developed as the paradigm shift occurred (from religion/superstition to science). Certain lauded historical astronomers (for instance, Galileo) were actually astrologers.
Also, regarding the moon thing, I remember hearing Michio Kaku say on the radio a while back that in a couple zillion years or so, we would actually loose the moon, which would effectively end life on Earth as we know it, for, without the moon’s gravity, the Earth would begin to tumble, causing our weather to go haywire. If this is actually the case, how come having a proportionately sized moon isn’t included in the Drake equation (because Drake didn’t think of it?)? Just wondering…
August 24th, 2006 at 8:46 pm
From the 5th grade point of view (my son’s, to be precise), it’s much better to demote Pluto and have only 8 planets than to promote dozens, if not hundreds, of somewhat big objects to “planetary” status. He told me he didn’t think he could enjoy astronomy anymore if he had to remember the names (and orbits) of a couple dozen (not to mention a couple hundred) “planets.”
August 24th, 2006 at 9:03 pm
[…] Bad Astronomy Blog » Breaking News– Pluto not a planet! […]
August 24th, 2006 at 9:15 pm
The time it takes to change books in this country, we’ll be lucky if our great-great-great grand kids find out about this change.
August 24th, 2006 at 9:49 pm
Apparently Pluto itself is somewhat ticked off; read the tirade here:
http://hippobean.blogspot.com/2006/08/pluto-says.html
August 24th, 2006 at 9:51 pm
Until reading through some of the comments here, most of my thinking on this debate has been about what the news headlines say the issue is - Pluto’s status as a planet. But I realized after reading some of these comments that despite the headlines, the demotion of Pluto, and all the interesting technical points raised in criticism of the new definition, the debate was never about anything besides one particular, near-term problem.
It was about Sedna, 2003 UB313 and other big trans-Neptunian iceballs to be discovered in the next few years, and the new defintion, however unsaisfactory from the general point of view, does exactly what it was designed to do: it disposes of 2003 UB313 in particular as a dwarf planet and not as a “classic” planet.
Pluto’s demotion was collateral damage of this short term-fix. The other issues don’t really matter, as none of them raise near-term definitional and discovery-credit issues. To cite one example, whatever “clearing the neighborhood around an orbit” means, since the IAU certainly did not intend to raise questions about any of the “inner eight” planets, we can assume as a practical matter that it includes whatever those eight are doing. And whatever those things are, 2003 UB313 and its to-be-discovered siblings don’t have what it takes to do them. Over the next few years, the orbital clearing criterion will be dissected to shavings, but by the time it becomes wholly unsatisfactory, 2003 UB313’s status as something other than a “classic” planet will have become set. (We saw what happened when the original proposal included reviving Ceres as a planet. Thwack!)
Once the dust settles, “planet” will be reasserted as a common-sense notion as extrasolar planet discoverers continue to discover what they discover and continue to call them extrasolar planets, and as the recent-historical idea of terrestrial planets and gas giant planets continues to be used whether or not, for instance, Earth can be said to have cleared its orbit with that pesky lump of debris out there at 239,000 miles. And I’ll bet the IAU revises the definition of “planet” before the Earth-Moon barycenter emerges from the earth’s crust to endanger Earth’s planetary status.
Bottom line: 2003 UB313 and like trans-Neptunians (including Pluto) didn’t make the cut as planets; everything else will sort itself out in time.
August 24th, 2006 at 10:33 pm
The moment I heard the news I knew there’d be a nice discussion going on here.
My thoughts: on the bright side, Pluto and Ceres are unremarkable as planets, but awesome as smaller objects.
August 24th, 2006 at 11:34 pm
Well I for one am pretty pleased with this decision. Not only have I never really be all the happy about calling Pluto a “planet” due to it’s orbit being off the solar system’s natural plane, but its proximity to the Kuiper Belt seems to make it more belonging to that formation that that of the inner planets’.
But the other reason why I’m content with this call is it may take the name of 2003UB313’s fate out of the IAU’s hands. If it’s no longer a planet but now considered one of possibly millions of similar objects yet to be discovered like asteroids and comets, then it would fall to is discoverer to name it, as it should have been all along. Dr. Michael Brown decided to name his find “Xena” and it’s moon (or whatever it’s going to be cataloged as now) “Gabriele”. And though a tad geeky, I feel that should stand.
August 25th, 2006 at 12:16 am
Pluto still a high-flyer for astrologers. Gee thanks, Reuters. *rolls eyes*
August 25th, 2006 at 12:42 am
Now I know what a planet is…
The astronomical community chose to downgrade Pluto to a dwarf planet instead of going with their original draft proposal. Clyde Tombaugh is rolling in his grave and millions of school children worldwide will now recite an incomplete sentence trying to…
August 25th, 2006 at 3:01 am
Shock Horror! Never mind in the great scheme of things it matters not.
August 25th, 2006 at 4:28 am
Dead Boy — while I am a fan of the tradition wherein discoverers of such objects get to name them, I am very very opposed to “Xena” and “Gabrielle,” and how it makes the astronomical community look to the general public. As in, we’re all guys who think Xena is hot and the idea of Xena and Gabrielle together is so exciting as to be worthy of this honor.
August 25th, 2006 at 5:53 am
Annie,
Xena & Gabrielle are just nicknames and not the Official names, the IAU will have to meet or the committee on naming objects, at a later date.
August 25th, 2006 at 6:03 am
I think IAU are covering themselves with ridiculous with this Pluto downgrading.
Did they feel the need to change the definition of moons? In fact, planets have two types of moons: the ones which formed together with the planet (I think about Earth’s moon or the largest Jupiter moons), and the ones which were captured later by the planet’s gravity (I think about smaller moons of Jupiter, maybe Phobos and Deimos). No one in the IAU never thought about changing the definition for these object, afaik. Maybe because the claim “I discovered a new moon of Saturn” is not so strong as “I discovered a new solar system’s planet” ?
August 25th, 2006 at 6:08 am
We need a plan of action!
First, we have to recall New Horizon’s since it is a terrible expenditure of money to investigate a dwarf planet.
Second, we should file suit as this ruling clearly discriminates against dwarves.
Third, I am buying stock in any company that publishes science textbooks.
Fourth, we should be prepared to meet some really upset Plutoniums in a few years.
August 25th, 2006 at 6:16 am
don’t we have a certain probe out there with a certain disc on it saying our solar system has 9 planets to recall before any aliens find it and laugh at us for not being able to count properly?
August 25th, 2006 at 6:35 am
What if the solar system was a neighborhood association, and Pluto was a slacker resident?
August 25th, 2006 at 6:49 am
P. Edward Murray — I know that, as my post indicates (and I was referring to a preceding post which stated that Dr. Brown should be able to name UB313 whatever he chooses), but the point still stands as far as the mainstream press and public are concerned.
August 25th, 2006 at 6:50 am
They still need to rename it back to Yuggoth.
August 25th, 2006 at 7:06 am
Well, even if Pluto can’t make up its mind if it wants ‘in’ on the planet thing or not at least it’s keeping us all entertained! It also seems Pluto has two more moons, Nyx and Hydra! Here’s the link to the NASA article about the moons. Pluto has been very busy for a small whatever it is!
http://nssdc.gsfc.nasa.gov/planetary/text/pluto_pr_20060626.txt
August 25th, 2006 at 7:07 am
Uh, looks like “cleared his neighbourhood” was nof fortunate choosing of words. I propose “gravitationally dominant”. Why?
1. Sounds cool. Like SF. Or something.
2. Clears his neighbourhood, too.
3… and anything that survive clearing (or joins later) is UNDOUBTEDLY gravitationally dominated by body in question.
This is why Pluto is not a planet, but Neptun is. In addition to gigantic difference in mass, Pluto is controlled gravitationally (by resonance). Same for solar system moons, asteroids in L4 and L5, and many other deribs.
Only one problem: what the hell is neighbourhood? Maybe epsilon in mathematical definition of neighbourhood should be proportional to mass of body in question, creating virtual torus (in center lies orbit).
August 25th, 2006 at 7:12 am
Just wild thoughts… Mercury actually did “clean his neighborhoods” or maybe was the Sun the responsible in cleaning its (of Mercury) orbit from other objects?
August 25th, 2006 at 7:52 am
[…] Pluto is no longer a planet because it hasn’t cleared Neptune from its neighbourhood. Neighbourhood is another problem word, but it’s the logic that’s the real problem. If Pluto hasn’t cleared Neptune then no can Neptune have cleared Pluto - it can’t be a planet either. I worked this out as I read Phil Plait’s entry on the topic and was feeling pretty smug about this line of reasoning until I saw further down the page that he’d noticed it too. It is an obvious hole. But where does this leave Earth? […]
August 25th, 2006 at 8:43 am
[…] Bad Astronomy Blog » Breaking News– Pluto not a planet! - I’m actually glad that this resolved this way, but Phil makes a good point that the new definition isn’t terribly satistying. I think, however, that the proposal that would have added 3 more planets to our Solar System would have been even worse. It would’ve opened up the flood gates for far too many objects that had little else in common than that they were round. As for Pluto, from what I understand, it should really be considered with the Kuiper Belt objects since it’s more similar to them than to any of the planets. It’s certainly a fascinating question, and if it gets people talking about classifying objects in a way that make sense, then I’m all for it.  There’s also a recent Point of Inquiry podcast with Dr. Neil deGrasse Tyson in which he talks about how the Hayden Planetarium (of which he is the director) organizes their presentation of the Solar System, among other things. Check it out! […]
August 25th, 2006 at 9:30 am
[…] Was ist passiert? Die Internationale Astronomie Union (IAU) hat in Prag getagt und gestern (24. August) eine Definition der Kategorie “Planet” verabschiedet. Nach dieser Definition, die man bei Badastronomy im englischen Wortlaut nachlesen kann, ist Pluto kein Planet, sondern ein Zwergplanet. […]
August 25th, 2006 at 9:46 am
I think the new definition is stupid. I don’t really care whether Pluto is a planet or not, but I find the definitions vague and unscientific.
Look linguistically, we have already been saying that anything that orbits a star is a planet. We even had a word for those asteroids and comets and things: “Minor Planets”. Yes, that means that Ceres already was a Planet, it just wasn’t a Major Planet.
Now the IAU has generated a new term, “Dwarf Planet”. This is to set Pluto and Ceres and the others off from “real planets”. But they’re already set off by composition and orbit and such, just like Earth is set off from Jupiter by being terrestrial and inhabited, and Mercury is set off from Earth by being so toasty hot.
Culturally a planet is something that primarily orbits a star and is not just a small rock. I like the definitions offered for “planemo” and fusors and such.
Something that fuses = star
Something that does not fuse but is in hydrostatic equilibrium = planemo
A planemo orbiting a star = planet
A planemo orbiting a planet = moon
A body not in hydrostatic equilibrium = small solar system body (need better term, or abbreviation - SSSB? SS debris?)
The clearing it’s neighborhood is poorly defined and a general mess.
I don’t think we should worry about keeping “planet” somehow special. We discovered that the Sun is just a run of the mill star, but that doesn’t mean we don’t think it’s special. We have 9 classical planets (8 big ones and Pluto for cultural reasons), we have 8 major planets. But Ceres and 2003 UB313 and Sedna and Quoaor all count as planets. There are probably others out there in the Kuiper Belt. Wheee!
That still leaves the question of “double planets” to be solved. What is the best means to determine it? Ratio of diameters? Location of barycenter? Primary gravitational influence?
August 25th, 2006 at 9:59 am
Pl