On August 24, the International Astronomical Union is going to vote on a proposal (here is the official resolution) to define the term “planet” such that Pluto stays in, and three bodies get added. This would require the re-writing of textbooks and make millions of first-graders learn 12 planets instead of nine. The planet status of Pluto has long provoked heated and fairly pointless and silly debate, much of it by people who are only vaguely familiar with astronomy but feel strongly about the definition of planet, a tradition which I fully intend to continue here.
At first I thought that the IAU proposal was to include Pluto, Xena (UB313), Sedna, and Quaoar as planets, perhaps getting the “ice dwarf” category. This was obviously the right thing to do, since rewriting textbooks is a good thing, and I think 21st-century first graders can handle it, and those various ice dwarves were probably tossed out of the inner solar system by other planets during the formation of the early solar system and so probably formed in a similar fashion originally. This also made for a nice symmetrical classification: 4 inner rocky planets, 4 outer gas giants, and 4 ice dwarf planets even further out. Everyone can remember that, even after we add more ice dwarf planets as we are likely to do.
But then I learned that the candidates for official planethood were not the above, but instead Pluto, Pluto’s moon Charon, Sedna, and the asteroid Ceres. Pluto and Sedna I can deal with, but Charon clearly belongs with the other two moons of Pluto. Pluto is 9 times more massive than Charon, we can’t let it schlepp itself up to planet status just because it happens to be just big enough to move the barycenter outside the surface of Pluto. 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.
And Ceres – I should say up front I’ve got nothing against Ceres, she’s a spunky little planetoid. And clearly we need to send a probe to get some decent pictures as soon as possible, because the Hubble shots are frustratingly fuzzy. And sure, she’s vaguely spherical. But c’mon, let’s get real. She’s less than 1000 km across. Heck, the great state of California is by itself 1,240 km long. If you get up early and take I-5 you can drive the whole thing by 9 pm. I know some people think California seems like it is its own planet already, but if we let Ceres in, we’ll have to let in Vesta, Pallas, and Hygiea, and while this would make another nice group of four, 16 is way too many for the first graders to learn. And Hygiea is only 300 x 500 km. I mean, Oregon is 420 x 580 km, and if we start calling Hygiea a planet pretty soon Oregon will want to be treated like California, or at least a moon of California. Clearly, it’s a slippery slope, and that way lies chaos.
It looks like I’m taking on consensus of the astronomers over 2 years of debates, so maybe I’m off my rocker. Are they right? Have at it in the comments.
(Note: any similarities between this post and a Stephen Colbert report are purely accidental.)





PS: See a million other blogs discussing this.
Best post yet. Not that I necessarily agree, but it’s hilarious.
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/[…]ts-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.
Here’s a good one:
More news:
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.
Best argument for the ice-dwarf category yet: http://scottwesterfeld.com/blog/?p=161#comments
Lindsey Eck wrote:
Well, if you got a giant pool cue and knocked Pluto towards the sun it would most probably give you a comet’s tail as it warmed up. So, it might be a comet on the inner most edge of the Oort cloud.
However, I would counter your argument with the “death threat from a third-grader argument.” When the Hayden Planetarium eliminated Pluto from its planet display the astronomer in charge got death threats from third-graders. When third-graders send letters to astronomers demanding that Pluto remain a planet – it should remain a planet.
What about the children?
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!
I’ve often thought that CA should be classified as a planet, in and of itself.
…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.
Some translations of the good book uses “Hades”. So technically, Pluto is in the good book. But the real Pluto cannot hold all the sinners of human history.
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.
As does Vesta. But not Ceres. Still want to go this route?
Roy
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…
better make that resistors.
http://www.elexp.com/t_resist.htm
Black = 0 Brown = 1 Rred = 2 Orange = 3 Yellow = 4 Green = 5 Blue = 6 Violet = 7 Gives = 8 White = 9
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[…]9852,00.html
fnxtr‘s link is pretty cute stuff–the Seven Dwarfs have got fellow Disney-toon Pluto’s back:
Well, that’s why I got a “C” in physics.
;)
Cause resistors were futile?
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
This is true.
“Xena” was better IMO…besides, there can’t be many more real mythological Greek and Latin figures left. Pretty soon they will have to start using TV characters. I am waiting for the Trek system where they name the extrasolar planets Kirk, Spock, McCoy, etc. I half-recall that this has already happened in fiction somewhere.
Another article on Eris and its moon Dysnomia:
The Dwarf Planet Formerly Known as Xena Has Officially Been Named Eris, IAU Announces
Henry
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