How Many Dwarfs Are There?

(Image courtesy Wikipedia)

Most people know by now that Pluto has been downgraded. Astronomers have decided that, conceptually, we should reserve the word "planet" for the small number of dominant bodies in the solar system. Pluto doesn't come close to making the cut. But it didn't just get shoved into the corner as "insignificant object," it got to be part of a brand new class of objects never before defined, the "dwarf planets."

Now, before you complain that, clearly, by virtue of the power of the English language, a "dwarf planet" must certainly be a planet first, a dwarf second, I would just like to mention two things. First all adjective noun combination in the English language are not noun first, adjective second. A matchbox car is, in fact, not a real car. It's OK if a dwarf planet is not a real planet. Second, though, I will acknowledge that the language is unfortunate and misleading. I preferred the term "planetoid" myself, rather than the (intentionally?) misleading "dwarf planet."

Still, forgetting the vagaries of language, we are left with dwarf planets which are not planets. How many are out there besides Pluto? And what is a dwarf planet? The International Astronomical Union (the group responsible for all astronomical nomenclature) has officially declared there to be five dwarf planet (in order of mass: Eris, Pluto, Makemake, Haumea, Ceres), and we are likely in for a dry spell on new dwarf planets. The preliminary searches of the sky are all but complete, and (as far as I know) no one has any new objects the size of Haumea hiding in their back pockets. We'll probably be at five official dwarf planets for a while.

Now is a good time, then, to remind ourselves what a dwarf planet really is.

When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined. The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best description I can come up with is that a dwarf planet is something that looks like a planet, but is not a planet. The official definition is that dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction.

Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object which is geologically dead and one which might have interesting processes worthy of study.

[Haumea is, of course, not round, but that is only because it is spinning so fast. If you stopped it spinning it would become a sphere. That still counts.]

So how many dwarf planets are there? Five, of course. The IAU says so.

But let's ask the more scientifically interesting question: how many (non-planet) objects in the solar system are large enough to be round due to their own gravitational pull?

Still five, right?

Well, no. Here is where the IAU and reality part ways.

There are many more objects that precisely fit the definition of dwarf planet but that the IAU chosen not to recognize. But if the category of dwarf planet is important, then it is the reality that is important, not the official list. So let's examine reality.

So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. Vesta, the next largest, is close, but has a large crater blasted out of its side that makes it distinctly oblong. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round.

The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger. We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet. We might be a bit off in one direction or another, but 400 km seems like a good estimate.

How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects, but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of now, about 50 objects this size or larger are known in the Kuiper belt (including, of course, Eris, Pluto, Makemake, and Haumea). Our best estimate is that a complete survey of the Kuiper belt would double this number, so there are roughly 100 dwarf planets in the Kuiper belt, of which 50 are currently known.

The new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth.

Does it matter how many dwarf planets we say there are?

I think the answer is "yes." If you believe that there are only 4 dwarf planets in the Kuiper belt then you place an oversized importance on those 4 objects and you get an exceedingly warped picture of what the outer solar is like. The important thing about the Kuiper belt is that beyond Neptune there are many many many objects with hundreds being large enough to be round. The four "IAU Dwarf Planets" in the outer solar system are all fascinating objects -- hey! I discovered 3 of them, I must think there are at least a little interesting -- but it would be a gross exaggeration to think of them as the only objects, or even the only important objects, in the fascinating region of space beyond Neptune.

I love dwarf planets. All hundred of them or so.

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Mark Dow

Are those satellites dwarf planet moons? Dwarf moons? Moonoids? Moonenites?

Is Dysnomia misnamed?
ill lich

So. . . these planets are inhabited by nothing but dwarfs? We have found extraterrestrial life!!
DoubleTee

“That’s no moon…”
Blinde Schildpad

Can I just say dat not calling these things planettes is a horrible, horrible, HORRIBLE mistake?

Thank you. That itch needed scratching for nigh on five years…
lava

the sliver of earth is good, but another helpful scale reference would have been our moon.
Crispy Critter

They still have yet to find Yuggoth, though.
Blinde Schildpad

Can I just say that not calling these things planettes has been a horrible, horrible, HORRIBLE mistake?

Thank you. That itch needed scratching for nigh on five years…
- nutbastard
  
  I’m sure there are many short haired women in Berkeley who would take umbrage with that proposal…
Phikus

How Many Planets?
Walt Guyll

I unashamedly take the opportunity to pass along a bit of doggerel composed a couple of years ago:

In a series of gaseous emissions the I. A. U. shat a plutoid.
While not carin’ to bash the decision the air is something to avoid.
Robert

I’m reminded of the explosion in taxonomy (as opposed to taxidermy, in which explosions also play a part, albeit different). One wonders if the ancients threw a similar hissy fit when the rat became something other than “big mouse”.
- Ito Kagehisa
  
  A Linnaean taxonomist, a DNA phylogenist, and a cladist walk into a bar.
  
  The Linnaen says “Ow!”
  
  The Wet Chemistry boy says “Ouch!”
  
  The cladist says “YOU’RE BOTH WRONG!!!”
Phobophilic

I’m not fully aware of the reasons for defining planets the way we do, and correct me if I’m wrong, but isn’t Pluto closer in size to the Earth than the Earth is to Jupiter or Saturn. Wouldn’t it be more principled to define the planets by less arbitrary criteria. For instance:
1) Spherical shape due to its own gravity – but no fusion
2) Orbiting a star, not a planet
Antinous / Moderator

I see that Dwarf Planet is Today’s featured article at Wikipedia. Coincidence?
muteboy

I prefer the nomenclature used by Harry Dean Stanton’s character in ‘Alien’.
“We shouldn’t have landed on this damn ball, I know that”
MikeBrown

Hey Alan –
[and you guys should all go read his book: The Case for Pluto; he and I agree on reality, disagree on classification. But, more importantly, we both agree it's a book you should read]. You’re actually wrong here. The IAU only imposed a magnitude limit on their weird word “Plutoid” (rhymes with hemorrhoid, I guess).

But, seriously, if we are to consider reality, rather than IAU arbitrariness, the interesting question, I think, is “how many objects are there out there in hydrostatic equilibrium” not “how many objects is the IAU interested in classifying as Plutoids based on some arbitrary magnitude limit”!

I’m pretty sure you and I would agree on that one!

@littlebrother: cool, huh? I suspect it will turn out to be true that Eris is a few km smaller in diameter than Pluto (I could still be wrong about this, though; the size of Pluto remains a bit uncertaint). With Eris significantly more massive, but the same size or smaller, they turn out to be very different objects, making them much much more scientifically interesting than we thought they were before!
- Alan Boyle
  
  Thanks, Mike … It seems to me that when you’re talking about the trans-Neptunian region (another one of those weird terms), “plutoid” equals “dwarf planet” … and that’s why there are only four trans-Neptunian dwarf planets, er, plutoids, at this time. That situation is likely to persist for a while. Here’s more about the plutoids from Wikipedia:
  
  http://en.wikipedia.org/wiki/Plutoid
  
  You’re right that this is overly legalistic. The only reason to bring it up is to explain why the list of “official” dwarf planets is likely to stay at five for a while longer. You’re also right that there could be scores or hundreds of objects out there that are (relatively) small and round. I bow down to your expertise on that, and I think that’ll be a fascinating frontier to explore in future years.
  
  And you’re right again when you say that everyone should read both our books. ;-)
MikeBrown

plus, I really love “star fish” and “sea lion” as good analogies for the fact that poorly constructed terminology does not determine reality. I apologize now for the fact that I am going to appropriate these analogies forever.
chris23

Please be more sensitive. They prefer the term “spherically challenged”.
littlebrother

Three teams of astronomers watched through
telescopes as the icy Eris passed in front of a
distant star over the weekend. The length of the
occultation â€” as the event is called â€” showed that
Eris is likely less than 1,454 miles (2,340
kilometers) wide, the magazine Sky & Telescope
reported.

As reported elsewhere….
http://www.space.com/scienceastronomy/eris-smaller-than-pluto-101109.html

Eris is smaller than Pluto, not larger as this image suggests.

More evidence, to me, that we should say we have sixteen planets, or nine.
- Anonymous
  
  The eight major bodies of the solar system at least have things uniting them: they each make up most of the mass in their region of the solar system, they all orbit within the same plane, they are all more massive than any of the moons, and so on. Other than tradition, what would you gain by drawing a line right below Pluto?
  - Alan Boyle
    
    Oops … A couple of moons (Titan and Ganymede) are bigger than Mercury, and Mercury’s orbit is inclined 7 degrees out of plane with respect to Earth … so I guess Mercury is next on the hit list. “First they came for Pluto, and I didn’t speak up because I wasn’t a dwarf planet… and then they came for Mercury…” ;-)
    
    With respect to Matchbox cars: I’d love to have some Matchbox planets, but please include some dwarf planets in the set. I would categorize dwarf planets as a different type of planet, but I’m sure folks will eventually get used to the idea that there are four giant things, four Earth-scale things and a whole bunch of smaller things that may or may not be round. Pluto and Ceres will be the most interesting of those small round things because those will be the only things of their kind that we’ll ever see close up.
    
    With respect to dwarf planets: Mike hasn’t mentioned the absolute magnitude rule that the IAU is using to determine whether something that’s found should be named as a dwarf planet. If the shape of an object can’t be determined by telescopes, and it comes time to name it, it would go through the dwarf-planet process if its absolute magnitude is +1 or brighter. The reason why all the other objects Mike has referred to aren’t considered dwarf planets is because they’re not that bright. However, when we get more powerful telescopes … or when the rules change again … we could well have more dwarf planets officially recognized by the IAU.
Anonymous

Weywot where? when? who?
Anonymous

Would it be worth looking outside of the plane of the solar system for more objects? In that region of the solar system orbit perturbing interactions are more likely.
Art Carnage

Where’s Sneezy?
entropy

um, matchbox cars are actually real cars…
- Jonathan Badger
  
  Precisely. While I understand Mike’s point, namely that these objects can be interesting things to study whether or not they are planets, a name like “dwarf planet” *is* saying that they are planets, just small like a dwarf in the same way that a “matchbox car” is a car that is small like a matchbox. If they aren’t planets, don’t call them by a name that includes “planet” as the the noun!
  - Anonymous
    
    How about “seahorse”? But if you want a better precedent here, may I point out that people have talked about “minor planets” left out from the official count since the 19th century.
    - Jonathan Badger
      
      Well, you can argue that “seahorse” is a compound noun, which is a different issue than an adjective + noun (“seahorse” != “sea horse”). But still, I know there has been a movement by several aquariums to get the public used to “seastar” as a replacement name for those star-shaped marine echinoderms which are not at all fish…
      - Anonymous
        
        Sure, but they’re are closer to fish than they are to stars. At any rate, I stand by my minor planet example. If names like that and starfish didn’t bother people who understood what they were for the last two centuries, why are we afraid of them confusing people now?
      - Anonymous
        
        Sea lion, then.
Drabula

Trans-Neptunian Objects will be the name of my next band.
Anonymous

A matchbox car is a real car. Just like a yellow car is a real car, or a styrofoam car is a real car, or a polar bear is a real bear. The descriptor is used to imply characteristic qualities of the object.

The “matchbox” descriptor implies the “toy” descriptor which additionally implies that the car carries similar distinguishable but not functional characteristics of other cars. As the purpose of “toy” is to imply that. Toy things are real things, just with different functional characteristics (usually reduced). In the case of “matchbox car” the “car” term is not metaphorical as perhaps is the case of “bear” in “water bear,” which is not intended to imply that water bears are real bears.

You would face this same identity problem if you went with “planetoids” instead, as the same sets of arguments can be made. By using the word “planet” as the noun, most people will imply that you mean planet. Similarly, a “kitchenette” is a real kitchen, in this case (opposite to the previous), it is functionally similar to other kitchens, while -ette implies that it is distinguishably different from other kitchens based on scale (and perhaps sexuality, though context is important here). I’m not entirely sure what -oid implies, but I’d guess it’s similar to -ette.

While definitions are useful, and both you and the IAU argue them, people communicate through the implications that terms provide, not necessarily strict definitions.

It seems to be generally held among laymen that the term “dwarf planet,” regardless of official definition, implies an object that is “planet-like” in function, if not scale — as that is also the implication of “dwarf person,” a real person, even if distinguishable through scale.

If it can be concluded by experts such as yourself that TNO’s and objects like them in other systems are functionally planet-like, yet primarily distinguishable through scale, then either dwarf planet or planetoid are still fitting terms.
blatant fool

How many dwarfs are there?

Seven, of course.
Dewi Morgan

I still don’t get what’s wrong with the old definition of “planet” – something round through its own gravity, and orbiting a star.

Then you can have sub-classifications like “gas giant” and “dwarf planet” that are applied to types of those planets.

Why *must* the term “planet” be so exclusive? What was the problem the IAU was trying to fix with their hamhanded, publicity-seeking attempt to redefine the word?

Then we have this:
planet (gas giant, minor planet, mesoplanet, dwarf planet, goldilocks planet, etc)

Why was that system deemed messy, evil, or bad enough to go with something random, arbitrary and contentious?
- Anonymous
  
  Something round and orbiting the star, like Ceres? That hasn’t been the definition of a planet for a long time. Ceres has been excluded for a century because it is part of a debris belt; now Eris and Pluto are the same. That’s what clearing the orbit is supposed to represent.
billstewart

It’s a planet with dwarves on it. What’s misleading about that?
Axx

THERE…ARE…FOUR….DWARFS…!
Anonymous

Why do all the others get cool names and that one is stuck with 2007 OR 10?
Anonymous

Pshh.. a matchbox car is a hell of a lot more car than matchbox.
Alex

What about Rupert?
deckard68

I think I used to drive a Toyota Sedna.
Clayton

A matchbox car isnâ€™t aa car, huh? What is it, then? A toy? A toy what?

Youâ€™ve opened an argument about identity you cannot win, Mike. This isnâ€™t merely about language.
- Anonymous
  
  Is a toy horse actually a horse, and a virtual woman actually a woman? If a contest said that I won a car and they gave me a matchbox car, I’d still consider it false advertising.
Marja

It seems odd to use different minimum sizes in the inner [900 km] and outer [400 km] solar systems.
Ugly Canuck

And upon which of these could we find the Little Prince?

http://home.pacific.net.hk/~rebylee/text/prince/contents.html
Kevin Kenny

Hmm, the sensible divisions are: Jupiter + other stuff; the four gas giants + other stuff; butt then things get fuzzy. The IAU definition of “has enough gravity to have cleared its orbit” is enough to distinguish Mercury, Venus, Earth, Mars from the others. But if you’re trying to sort by size, the rocky and icy bodies look pretty much like a continuum; there are no natural breakpoints.

But these Kuiper belt objects are fascinating beasts; thanks for finding them!

And, yes, strange things happen when you concatenate nouns; consider a cat-house housecat.
Anonymous

I don’t think the searches are “all but complete” unless you’re only talking about the plane of the ecliptic exclusively.

For example, how about 2004 XR190, aka “Buffy”? The IAU hasn’t yet declared it a dwarf planet yet but it looks to be only a matter of time. It’s certainly large enough, but its orbit is tilted way off the plane of the ecliptic, where we generally don’t look for solar system objects.

I’m certain we’ll have plenty more surprises along this vein.
yepmatt

Even Dwarf Planets Started Small
JIMWICh

One word: Murgatroyds

Most people have forgotten this term, suggested long ago for trans-Neptunian objects by part-time astronomer, Snagglepuss. He repeatedly advocated using that term for all significant bodies in the heavens beyond the orbits of our major planets.
adamnvillani

I’ll use an analogy I got from David Letterman — CARPET — neither a car nor a pet!

Anyway, size-wize, I think the clear categorical distinction is clearly hydrostatic equilibrium vs. not. The part of the definition that I never understood was the one about “clearing its orbit.” Is there a clear definition for that? Otherwise it seems like one too many earth-crossing asteroids and suddenly the earth is no longer a planet! Or if there was some kind of limit on how many moons Jupiter or Saturn could have before we stopped calling them planets. What if Mars had a counter-Mars at one of its Lagrangian points?

It seems tangential to our conceptual definition of what is and isn’t a planet. I mean, it just kind of happens to be that the small, icy round bodies in our solar system are the ones way out past Neptune, outside of the ecliptic, failing to “clear their orbits,” however that’s defined, etc.

But now imagine a solar system similar to ours except that instead of just weird, small, icy bodies out at its limits, not clearing their orbits, we had a rocky body the size of Earth, or even another gas giant. It seems arbitrary to call these bodies not planets. I just don’t see how clearing the orbit fits into whether something is or isn’t a planet.
adamnvillani

(I do realize that if you had a larger body out on the edge like that, it would be more likely to have cleared its orbit of any pesky neighbors, or at least captured them as moons. But just imagine a solar system just like ours, except younger. Does Earth not achieve planet status until it clears the last remaining interloper from its orbit?)
DoubleTee

Classification will become much more complicated when artificially-constructed planetoids are made.

I’m visualizing a hollow geodesic sphere that looks like the Death Star.

The bonus is that above a [REDACTED] metre diameter, the scale is impossible to discern in person.
- Pantograph
  
  An object called Makemake simply has to be the work of Boingboing reading makers.
  - IronEdithKidd
    
    It can’t really be pronounced “make-make”, can it?
    - rikchik
      
      No, it’s “mah-keh-mah-keh”. Hawai`ian doesn’t have the silent e.
      - Anonymous
        
        It’s actually Rapa Nui Maori.
  - The Mudshark
    
    http://www.mikebrownsplanets.com/2008/07/whats-in-name-part-2.html
Anonymous

The key word (at least in your article here) seems to be “dominant”, is there a definition of “dominant” in the context of that term put forth by The International Astronomical Union?

And does it matter if every body in some future star system is no more massive than pluto? That is: “This star has no planets, it only has eight dwarf planets”? or is it some statistic independently applied for each star?
Joseph Hertzlinger

Can we name the next seven dwarf planets to be discovered Doc, Grumpy, Happy, Sleepy, Bashful, Sneezy, and Dopey? That would fit in with a name like Pluto (followed by Mickey and Goofy, of course).
fxq

OK, without referring to notes, name them out loud.

Go!
Anonymous

I thought there were 7 dwarfs?
Chris Williams

“The four “IAU Dwarf Planets” in the outer solar system are all fascinating objects — hey! I discovered 3 of them, I must think there are at least a little interesting — ”

From this statement I can conclude that the man who made it is considerably cooler than nearly everyone else.

The question is, who is the IAU? Who gets to decide policy? Is it a peak body of national scientific societies like the Royal Society, or what?
dross1260

“Does it matter how many dwarf planets we say there are?”
Seven would be optimal.