For moons, size does matter

By Phil Plait | January 20, 2012 7:00 am

One of these things is not like the others:

The Cassini spacecraft took this lovely image in December 2011, during a close pass of Saturn’s moon Dione. Ignoring Saturn’s rings slashing through the picture, we see, from left to right, the moons Dione, Prometheus, and Epimetheus. Which is the odd moon out?

Here’s a hint: Dione is 1100 km (700 miles) across, Prometheus 86 km (53 miles) along its longest axis, and Epimetheus 113 km (70 miles). Got it now?

Yeah, sure, Dione is far larger than the other two! But that’s not my point: Dione is round, while the other, smaller moons are lumpy and rather potato-shaped. Why?

Size matters. In this case, a bigger moon means more mass, and that means more gravity. In general, the force of gravity points toward the center of an object. As you add more mass to an object, gravity gets stronger. On a small moon, a big lump of rock like a mountain feels very little force downward, while on a more massive moon the force would be larger. If the moon has enough mass, and enough gravity, the force will be more than the internal strength of the rock itself, and the mountain crumbles.

So moons that are big and massive enough will tend to flatten their surface, or, more accurately, shape them into spheres. Dione is big enough to do that. Prometheus and Epimetheus are not. Dione is a big ball, the other two are spuds.

Note that gravity’s not the only thing that can make objects spherical. Water has surface tension, for example, caused by the electrostatic attraction between water molecules. In space, without gravity, drops of water are spherical. Random processes can generate round objects too: I bet if we could get a super-duper close look at Saturn’s rings, we’d see the trillions of chunks of ice that make up the rings are round too. But that’s from collisions; there are enough of those bits of ice that they smack into each other. Since they spin and tumble, over time any part of a chunk will have gotten hit by some other chunk, and that will tend to make them round.

So how big does an object have to be before it starts to become round via gravity? That’s complicated, and depends on its composition — a ball of ice the same size as a ball of iron will have far less gravity since it’s so much less dense, and will have lower mass. But for a ball of ice and rock — like Dione — that size is clearly no bigger than 1100 km across. And if you’re wondering how this might play into our concept of what a planet is, then you might want to read this. I’m way ahead of you!


Related posts:

Cassini gives Dione a close shave
The scale of Saturn
The scale of Saturn, redux
Cassini’s Pentaverate
A panoply of moons and rings

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (30)

  1. Messier Tidy Upper

    Neat photo. Cassini never fails to deliver in that respect! :-)

    I kinda tend to think of moons as gravitationally round & moonlets as smaller.

    As for the Great Planetary Definition debate, well, I’m just going to refer y’all to here :

    http://blogs.discovermagazine.com/badastronomy/2011/11/09/giant-sunspots-are-giant/comment-page-2/#comment-439108

    as near the starting point of a long discussion with Nigel Depledge & a few others and just say I stand by what I’ve said there.

    In summary I totally disagree with the IAU position which I think is utterly illogical and ridiculous and hope will soon be corrected.

    My preferred definition of planet would be that a planet is an object that :

    1) Is round or rounded by its own gravity thus not a comet or asteroid.

    2) Isn’t and has never been self-luminous by core nuclear fusion thus not a star or brown dwarf

    &

    3) Doesn’t directly orbit another planet thus not a moon.

    Yes, that would mean we have many more planets than before (which we’re getting anyhow via exoplanets btw.) which would then be broken down into categories such as gas giant (eg. Jupiter), rock dwarf (eg. Earth) and ice dwarf (eg. Pluto) and our solar system could be divided into the three main zones – the rocky, the gassy and the icy!

    I think a broader more inclusive definition – pretty much at the animal mineral vegetable level of astronomy where animal =star, mineral =planet and vegetable = smaller bodies – is best esp. given the surprises exoplanets have since delivered us!

    I’ll also direct folks to this :

    http://kencroswell.com/NinthRockFromTheSun.html

    great article by Ken Croswell who also offers an alternative definition for planet which makes much more sense than the IAU one – though I still like mine better! (As I guess is natural – & not that I’m claiming it as uniquely mine or original even.) ;-)

    Plus this more recent piece :

    http://www.abc.net.au/science/articles/2011/10/25/3346851.htm

    From the Aussie ABC online which makes some good points to such as where it observes :

    Kepler has found planets that orbit so close together they violate criterion number three. [The downright silly “orbital clearance” one which raises farmorequestions than it answers – ed.] Take the planetary system orbiting the star known as Kepler 11, for example. There are six planets, all heavier than Earth, five of which orbit closer to their parent star than Mercury — which orbits every 88 days — does to our Sun. Some of these planets are almost as close together as the Moon is to the Earth, but they’re too big to be moons. There’s also a star where two planets might travel on the same orbit. None of these planets could really be considered ‘gravitationally dominant’. Clearly we still have a problem here, and a satisfactory solution seems elusive.

    Indeed. Not to forget the possibilties of double planets, trojan planets and colliding planets as well. Oh & that alternative not-so-elusive potential solutions have been suggested which make much more sense to many people than the IAU’s (in my view) very poor definition.

  2. Gary Ansorge

    Planets,,,big, round things orbiting a star. As far as “clearing its local orbit” is concerned, given enough time, that would be the eventual fate of nearly every large object in orbit.

    Ah, Pluto, I knew thee well,,,when you were a planet,,,

    Gary 7

  3. HP

    Of course, the Classical definition of a planet was “a light in the sky that moves against the backdrop of the celestial sphere,” which works well and is perfectly consistent from a strictly anthropocentric POV, even if it includes the Sun and Moon.

    OTOH, the current definitions of “planet” being bandied about lump together gassy bodies and rocky bodies and icy bodies, and that makes no sense to me. If I were the guy defining non-stars, I’d create separate categories for gassy things, rocky things, and icy things, and then divide those into ones that orbit stars, ones that orbit other non-star things, and ones that orbit the galactic center of mass. Once you’ve done that, you can add the “sufficient mass to overcome material strength and clear orbits” class to your taxonomy.

    In other words, we need to approach non-stellar bodies using cladistics.

    No matter how hard the IAU tries to come up with a definition of “planet,” improved knowledge of extrasolar bodies is going to screw up their definition, and screw it up sooner rather than later. Let’s leave the word “planet” to the poets and the ancients.

  4. Sam

    Are you trying to tell us something here Phil?

  5. KDSmith

    Prometheus remarkably resembles the Jupiter II. Are we *sure* that’s a moon? ;)

  6. dominicjh

    So if a object that undergoes nuclear fusion due to gravitational pressure is in orbit around another such object, is it no longer a star?

    That seems to be the logical conclusion of extending the arguments about planets that exclude objects merely because of their location to other celestial objects.

  7. Color shot from the same flyby, showing Dione, Saturn, and the edge-on rings:

    http://www.flickr.com/photos/10795027@N08/6551753531/

  8. Scott P.

    So if you somehow were able to materialize an object the mass of Dione, but in the form of an oblate spheroid with a major axis, say, twice the length of its minor axes, how long before gravity compressed it into a sphere?

  9. CB

    @ HP:

    But they do distinguish between icy, gassy, and rocky objects — it’s why we have terms like “gas giant”, “ice giant”, “terrestrial planet” etc. Are you saying that the categorization of composition should come first, and then we apply the planet label (or not), as opposed to doing it in the opposite order? What difference would that make? And how does cladistics come into play since it’s about grouping things by ancestry? It’s not like all rocky bodies share a common ancestor separate from icy bodies.

    “Icy” or “Rocky” is one property of an object. “Big enough to be round” is another. They’re orthogonal (except obviously for making a difference how big something needs to be to be round), and trying to force it into a “tree of life”-style hierarchical taxonomy makes no sense to me.

    @ GP:
    Well, kinda the point is that Pluto isn’t large enough to do that. Jupiter’s gravity has vastly more effect on the vast majority of TNOs than Pluto’s does.

  10. HP

    CB: I’m saying we don’t apply the “planet” label at all. It’s a poetic term that’s been outstripped by reality. Consider the cladistics revolution that’s taken place in modern biological taxonomy. No modern biologist refers to “reptiles” outside the most colloquial circumstances. There’s basically no such thing as a reptile — it’s a “polyphyletic taxon.” Sure, you can say things that are generally true for lizards and snakes, but throw in turtles and crocodiles and tuatarans, and the whole category collapses under the weight of its contradictions.

    Likewise, “planet” is a useful concept if you are a human being standing on the surface of the Earth viewing the night sky with your own eyes and not doing the math. But it breaks down pretty quickly once you shift to a cosmological POV. Hence, Pluto. There will always be people talking about planets, there will always SF stories about planets around other suns, and this will make sense in a sloppy, anthropocentric way, but it’s useless to try to define “planet” in a scientifically literate way, in the same way that there’s no scientifically literate way to describe “reptiles.”

  11. HP

    In my previous comment #10, replace the phrase “scientifically literate” with the phrase “scientifically rigorous.” Sorry for the brain fart, and for not noticing it until the edit window had closed.

  12. As usual, a stunning image. I was just trying to explain this very thing yesterday to my history students (not quite sure how we got onto this subject). I think I may direct them here!

  13. CB

    @ HP:
    Fair enough. This sounds similar to the opinion of Neil Tyson, whom I agree with, that the important things are the properties of the object, not the name you give to collections of certain properties. Rather than grouping things, each object would have a bunch of tick boxes. Is it icy? Is it rocky? Is it round? Is it gravitationally bound to something else? Is it gravitationally dominant? And so on. Anyone interested in a certain subset of properties would just pick the objects that met the criterion.

    Of course people will still give names to subsets of these objects that share certain properties, and you can bet “planet” will be one of them. At the end of the day, I don’t really care what the actual words are. I care more about the properties. And it’s clear that “gravitational dominance” or “orbit clearing” is a property that the 8 things currently called “planets” have, and Pluto doesn’t (MTU’s deliberately erroneous representation of the property notwithstanding). Regardless of what you call having or not having that property.

    @ dominicjh:

    That would be a binary star system. The logical extension of this and the “clearing the orbit” criterion is that if you have a pair of round non-stellar objects that are bound to each other and of approximately the same size, then if that system dominates its orbit then they’re binary planets, otherwise they’re binary dwarf planets. The Pluto-Charon binary system does not dominate its orbit, ergo dwarf planets. The Earth-Moon system (if you chose to view this as a binary even though the barycenter is 1000 mi inside the earth) does dominate its orbit, ergo planets.

  14. As a lawyer, I think its odd to say “concepts are for scientists, definitions are for lawyers.” What, exactly, do you mean by that, and why do you think it’s true?

  15. @14 Jen Deland: Seems fairly straightforward to me. Laws deal with definitions. That’s why legalese is so… legales-ish. Everything has to be clearly defined.

    You can’t go before a judge and say “Ok, I know the law defines the crime of X as (exact thing that I did), but the concept of X is a bit more abstract, according to some social scientists.”

    Well, you could, but I doubt you’d get very far :D

  16. One of the arguements I often heard was that without this new definition of planet we might have dozens, or eventually hundreds, of planets. My thought was always “what’s wrong with that?”

    I like planets. The more the merrier.

  17. Dragonchild

    Judge me by my size, do you?

    @16 VinceRN – No serious problem; these oversized rocks flying through space don’t care what they’re called. The concern, though, is consensus. If we don’t at least agree on what’s a planet and what isn’t, then we have 7, 8, 9 or hundreds of planets. I know there are few absolutes in life, but it’s kind of hard to write a school textbook with that sort of ambiguity.

  18. Richard Woods

    To everyone attempting to apply the IAU definition of “planet” to exoplanets:

    The IAU 2006 definition of “planet” was _only_ for “our Solar System” (http://www.iau.org/public_press/news/detail/iau0603/), not for bodies in any other star system.

    For the 2003 IAU working group’s position statement on the working definition of “planet” for extrasolar systems, see: http://www.dtm.ciw.edu/boss/definition.html

  19. Al Viro

    @10 HP – _poly_phyletic? Paraphyletic – sure, but Reptilia in classical sense is Amniota – Mammalia – Aves. IOW, it doesn’t contain all descendents of its LCA, but it does contain all ancestors up to LCA.

    Unless you want to argue polyphyly of Amniota, Reptilia sensu Laurenti is not polyphyletic. And yes, there are tons of synapomorphies – shared with birds and mammals, for sure, but…

    As for applying cladistical methods to classification of, for the lack of better term, celestial bodies… You’d need to show that there’s a hierarchy of nested groups for that to work. In case of biology it is true – you can arrange species into a tree such that observable traits are distributed more or less along its subtrees. IOW, there is (more or less) a partial ordering on the traits – the things with jaws have backbones, the things with feathers have jaws, etc.

    For lumps of matter in space, though, it’s less than obvious. Moreover, we have no plausible mechanism that would lead to that. In case of biology we *do* – the common descent and inheritance of modified traits. For planets, OTOH…

  20. Messier Tidy Upper

    @13. CB :

    And it’s clear that “gravitational dominance” or “orbit clearing” is a property that the 8 things currently called “planets” have, and Pluto doesn’t

    Actually that’s NOT at all clear and merely asserting otherwise doesn’t make it so.

    Some planets have clear orbits due to their particular circumstances and orbital locations. whereas other planets – such as Pluto, Eris and Ceres – do not. Just as some planets have moons, weather or rings whilst others do not*. Put Earth where Pluto is and it couldn’t clear its orbit either. Put Pluto where Earth is and it could clear its orbit.

    (MTU’s deliberately erroneous representation of the property notwithstanding).

    You claim I’m deliberately erronous? Where?

    Please point out what you consider my error & why you think your opinion is more valid than mine.

    Note that I have supported my case with considerable logic and evidence whereas you have made an unsupported personal attack and committed the fallacy of attributing motives.

    ——

    * Pluto of course has at least moons and weather and possibly also rings as well. Mercury lacks moons, rings and weather. Earth has a Moon, weather but no ring system.

  21. Go get ‘em, MTU! :)

    Seriously though, I had the same questions…

  22. Gary B

    I’ll toss my hat into the ring, with another practical consideration that will (I hope) become more relevant in the future – an object that has a surface gravity greater than, say, that of the Moon. I can’t quite say ‘one can walk on it’ because of the big gas giants whose gravity is too large and the surface too indeterminate. But if/when we move out into the neighborhood and actually visit other worlds, no doubt looking for resources, the practical definition is likely to be how the mining crew has to operate. If you have to use tethers to stay attached while doing ‘normal work’, it’s a rock, not a planet.

  23. Nigel Depledge

    OK, I’ll try not to derail the entire discussion, and most of my arguments have been made in the thread to which MTU (#1) links, but there’s one point he makes here that I feel needs to be addressed. To whit:

    MTU (1) said:

    I think a broader more inclusive definition – pretty much at the animal mineral vegetable level of astronomy where animal =star, mineral =planet and vegetable = smaller bodies – is best esp. given the surprises exoplanets have since delivered us!

    It has been known for more than 100 years that animal / vegetable / mineral as categories are meaninglessly simplistic. Your analogy is all too true – your simplistic categorisation of objects renders your definition meaningless in anything but the most trivial way.

    For example: is coal (carbon-rich plant fossils) vegetable or mineral? Is limestone (the shells of long-dead sea creatures) animal or mineral? Is Euglena (a photosynthetic protist that is also motile and able to “eat”) animal or vegetable?

    Your comments about the IAU planet definition set me to thinking long and hard about it, and your arguments against the IAU’s definition have persuaded me that the IAU did the best job anyone could expect given our present state of knowledge.

  24. Nigel Depledge

    HP (3) said:

    No matter how hard the IAU tries to come up with a definition of “planet,” improved knowledge of extrasolar bodies is going to screw up their definition, and screw it up sooner rather than later.

    Not so. The IAU definition specifically excludes bodies orbiting other stars.

    Think about it – we know quite a lot about most of the large things orbiting Sol, but we know very little (and, yes, size, mass, orbital period and surface temperature are very little) about any exoplanet.

    Having said that, there’s nothing in the IAU definition that precludes it being rewritten at some future time when the state of our knowledge permits us to make a universal definition.

  25. Nigel Depledge

    Domincjh (6) said:

    So if a object that undergoes nuclear fusion due to gravitational pressure is in orbit around another such object, is it no longer a star?

    That seems to be the logical conclusion of extending the arguments about planets that exclude objects merely because of their location to other celestial objects.

    Ah, the “slippery slope” logical fallacy.

    To answer your question – no.

  26. Nigel Depledge

    HP (10) said:

    CB: I’m saying we don’t apply the “planet” label at all. It’s a poetic term that’s been outstripped by reality. Consider the cladistics revolution that’s taken place in modern biological taxonomy. No modern biologist refers to “reptiles” outside the most colloquial circumstances. There’s basically no such thing as a reptile — it’s a “polyphyletic taxon.” Sure, you can say things that are generally true for lizards and snakes, but throw in turtles and crocodiles and tuatarans, and the whole category collapses under the weight of its contradictions.

    Well, either that or the taxon “reptiles” has to include birds and mammals too!

    Likewise, “planet” is a useful concept if you are a human being standing on the surface of the Earth viewing the night sky with your own eyes and not doing the math. But it breaks down pretty quickly once you shift to a cosmological POV. Hence, Pluto. There will always be people talking about planets, there will always SF stories about planets around other suns, and this will make sense in a sloppy, anthropocentric way, but it’s useless to try to define “planet” in a scientifically literate way, in the same way that there’s no scientifically literate way to describe “reptiles.”

    Well, aside from the obvious fact that biological taxonomy males sense of the hierarchical nature of biological interrelatedness. Whereas planetary taxonomy can be entirely arbitrary. Jupiter and Saturn are similar objects, but they do not share ancestry. They are probably similar because similar processes occurred during their formation, but that’s not the same.

    There’s nothing wrong with professional astronomers coming up with a definition of the word “planet” for their use in technical discussion (and remember, all of you folks who object to the IAU’s definition, the definition was coined to facilitate technical discussion about objects that orbit our star, not to annoy everyone who learned about the “nine planets” in school).

  27. Nigel Depledge

    Vince RN (16) said:

    One of the arguements I often heard was that without this new definition of planet we might have dozens, or eventually hundreds, of planets. My thought was always “what’s wrong with that?”

    I like planets. The more the merrier.

    There’s nothing wrong with having several dozen planets, but:

    1) If the key defining characteristic of a “planet” is its size, then you end up with groups of similar objects (such as the KBOs and asteroids) in similar orbits of which some count as planets and some do not, based only on size.
    2) On the same basis, there is no specific size at which an object becomes a planet. If the size of an object becomes the key chracteristic (as opposed to a supporting characteristic), then you need to define how round something has to be to count. And the definition of “round” has to keep Saturn’s status as a planet without allowing every potato in the asteroid belt to become one too. Also, you have the strange property that, because ice is so much more ductile than rock, the size limit between “planet” and “not planet” would be at different sizes (in terms of both mass and diameter) for icy and rocky bodies.
    3) Because of the above two points, any definition that omits gravitational clearance is just as arbitrary and illogical as the current one*. But the current definition has the advantage of pragmatically recognising the obvious discontinuity that exists in our solar system.

    * I have seen some quite vehement diatribes against the IAU based on the alleged arbitrary and illogical nature of the “gravitational clearance” criterion, from people who have no objection to the “gravitational roundness” criterion, but that is hypocrisy of the highest order. Both criteria exist in a state of necessary pragmatism. To be logically consistent, one must object to both or neither.

  28. Nigel Depledge

    MTU (20) said:

    Actually that’s NOT at all clear and merely asserting otherwise doesn’t make it so.

    Except for the fact that the 8 planets dominate their orbital vicinity, and Pluto does not (hell, Pluto’s orbit is dominated by Neptune). This is self-evident from even a casual observation of the nature and context of solar system bodies.

    Some planets have clear orbits due to their particular circumstances and orbital locations. whereas other planets – such as Pluto, Eris and Ceres – do not.

    So what? Some planets dominate their orbits and some objects do not. To attempt to brush this criterion under the carpet is disingenuous at best. It is a clear difference between some solar system bodies and other solar system bodies.

    Just as some planets have moons, weather or rings whilst others do not*.

    Not really relevant, because these features are not exclusively a property of major bodies, whereas gravitational dominance is.

    Put Earth where Pluto is and it couldn’t clear its orbit either.

    So what? Earth isn’t out in the Kuiper Belt. For all we know, there could be a half-dozen Earth-size KBOs (well, they’d have to be pretty dark not to have been spotted, but you get the point). If they don’t dominate the region of their orbit, they don’t belong in the same category as those objects that do dominate their orbits.

    Put Pluto where Earth is and it could clear its orbit.

    Again, so what? All you are saying here is that it is easier to clear an orbit in the inner solar system than it is in the Kuiper Belt. Pluto isn’t in the inner solar system, it’s in the Kuiper Belt. And it does not dominate its orbital region.

    If anything, this strengthens the argument that a body’s context matters.

  29. Nigel Depledge

    Oh, dear, I seem to have killed the thread.

  30. Nigel Depledge

    MTU (20) said:

    Please point out what you consider my error & why you think your opinion is more valid than mine.

    This in relation to CB’s comment that MTU misrepresents the “gravitational clearance” criterion.

    I think CB may have overstepped the mark, but you certainly exaggerate any issue you think you have found with the criterion. You have certainly claimed it to be illogical without backing up that accusation with a rational argument.

    Your main claims about it, IIRC, are that (1) it defies comparison with reality since no orbit is truly “clear”; and that (2) if one could move the solar system bodies around like billiard balls, then one ends up with absurdities such as Earth not counting as a planet or any small rounded body counting as a planet if you plop it down into an empty bit of space; and that (3) we are likely to find examples of exoplanets that render the criterion more or less meaningless, such as double planets or whatever.

    As I have stated before, none of these objections really holds water.

    First, if you object on the basis of “how clear is clear?”, then you must also object to the gravitational roundness criterion on the basis of “how round is round”. It is true that every major planet has smaller bodies occupying the same orbital region, so they are not completely “clear”. However, the 8 major solar system bodies do gravitationally dominate everything else in their orbital region, not only in terms of dictating the movements of those bodies, but also in terms of outweighing them, in some cases by many orders of magnitude (compare Jupiter to its Trojan astroids, that you have in the past used as an objection to this criterion). By the same token, no planet is truly “round”, but we accept a bit of leeway in the definition of “round” for the sake of pragmatism. If you accept some leeway in the definition of “round”, why do you not do the same for the definition of “clear”?

    Second, we cannot move a planet from one orbit to another. Sure, if Pluto and Earth swapped places, then Earth would count as a dwarf planet and Pluto would be elevated to Planet status (well, until it evaporated, anyway), but this means nothing because we can’t make that happen. All this point illustrates is that the IAU considers the context in which a planet orbits to matter. To some extent, this is why a definition of “planet” was needed in the first place – because the discovery of some Pluto-sized KBOs showed us that Pluto was just the largest of a class of similar objects and not necessarily a unique object.

    Third, when we eventually start to learn some of the real details about exoplanets, this may well cause the IAU to revists the definition of “planet”, but for the time being we know too little about them so they are excluded from the IAU definition of “planet”.

    Finally, I will note (again!) that the IAU is not going to force anyone to use their definition unless they wish to publish papers in the primary astronomical literature. Astronomy is one of a very few areas of science in which amateurs may still make a significant contribution, but I don’t believe that any amateur astronomer will be publishing papers in the professional journals without the involvement of at least one professional astronomer.

    If you don’t like their definition, you don’t have to use it, but please stop whining about it and maligning the IAU over it.

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