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Since Titan and Ganymede fall into the same category as Deimos and Phobos or the components of planetary rings, the category of moons, it's like if all asteroids were considered planets. Tiny irregular satellites are called 'moonlets' but still fall into the category of moons.

Is there any distinction between the natural satellites and should there be one? From my point of view, only equilibrium-shaped spherical moons should be considered moons. While natural satellites are all satellites, the spherical ones should be moons. Otherwise it would be extremely weird that Deimos and Phobos fall into the same category as our Moon or the Galilean Moons. For example, Pluto should be considered having five natural satellites, and one of them (Charon) is a moon.

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    $\begingroup$ "Should there be one?" is a rather subjective question. There still are people upset about the fact that Pluto got its own category of celestial bodies. $\endgroup$ – usernumber Jan 30 '20 at 13:45
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    $\begingroup$ Pluto and Eris are planets and the IAU's definition is silly and wrong, but that's another topic. $\endgroup$ – user30007 Jan 30 '20 at 14:30
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    $\begingroup$ I don't see how this can be answered in a reasonable way. Every answer would start "in my opinion". There's no good reason why potato shaped satellites should or should not be called "moons", any more than there is a reason for feline shaped animals to be called "cats" $\endgroup$ – James K Jan 30 '20 at 17:23
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    $\begingroup$ @JamesK What's wrong with opinions? There are indeed good reasons to consider or not consider irregular satellites moons, or all asteroids planets. $\endgroup$ – user30007 Jan 30 '20 at 19:36
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    $\begingroup$ @user30007 re "What's wrong with opinions?" you might start by reading How to Ask, and follow up by reading our guidelines for great subjective questions and SE's blog post about how real questions have answers. One of the standard closure options is for questions that invite primarily opinion-based answers. Hope this helps :-) $\endgroup$ – Chappo Hasn't Forgotten Monica Jan 31 '20 at 3:23
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The need to distinguish between these potato-shaped and spherical moons hasn't arisen. As such, there aren't two different words to designate these two types of moons. And as long as the community of astronomers who study natural satellites doesn't feel the need for there to be an extra category, there likely won't be one created preemptively.

Perhaps, one day, we will discover that potato-shape and spherical satellites form in very different ways, of have very different properties (not just esthetic). Then the case might be made to sort them into two different categories.

On a side-note, defining arbitrary categories is never trivial. How spherical is spherical? The boundary has to be set somewhere, but there always will be edge cases where an object could reasonably fall in either category.

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  • $\begingroup$ A better definition perhaps would be 'differentiated' rather than 'spherical'. Phobos is a bit spherical too, but it's an asteroid and partially hollow. I'm talking about planetary-mass differentiated moons. These are our Earth's moon of course, the Galilean Moons, the seven spherical moons of Saturn, the five spherical moons of Uranus, Neptune's moon Triton, Pluto's moon Charon and possibly Eris' moon Dysnomia (of which we don't know whether it's spherical or not). $\endgroup$ – user30007 Jan 30 '20 at 14:27
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Apparently not! From Space.com's Here's Why Saturn's Inner Moons Are Shaped Like Ravioli and Potatoes

The odd shapes of Saturn's inner moons, which resemble objects ranging from ravioli to potatoes, may be due to mergers of tiny moonlets, a new study finds.

The new finding may help to explain how moons in general may form, researchers said in a new paper describing the work.

The paper in 21-May-2018 Nature Astronomy is The peculiar shapes of Saturn’s small inner moons as evidence of mergers of similar-sized moonlets (arXiv, slideshare) so these are definitely considered to be non-spherical moons made from mergers of "moonlets".

enter image description here

More (cropped) images of Pan from the NASA News item Cassini Reveals Strange Shape of Saturn's Moon Pan

enter image description here enter image description here

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  • $\begingroup$ The question is whether they should fall into the same category. I gonna adjust my question. $\endgroup$ – user30007 Jan 30 '20 at 14:28
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    $\begingroup$ @user30007 you have two answers now, it is strongly discouraged to change the question once people take the time to post answers. These are considered moons by astronomers, period. Also, a "should they" question solicits answers that are primarily opinion-based and therefore not appropriate for Stack Exchange. Instead of changing this question which would probably result in it being rolled back anyway, why not ask a follow-up question. You can link back here for background, and ask something like "Why...?" $\endgroup$ – uhoh Jan 30 '20 at 14:31
  • $\begingroup$ Too late, sorry. But I don't think it's the end of the world if I change "spherical" to "differentiated". $\endgroup$ – user30007 Jan 30 '20 at 14:33
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    $\begingroup$ @user30007 no it's not too late, I've rolled back. Seriously you should not change a question like that once answers have been posted. That's not how the site operates. The best way is to move forward and ask a new question about differentiated satellites and link back here. Next time you'll be more careful and write the question that you want answered the first time. $\endgroup$ – uhoh Jan 30 '20 at 14:38
  • $\begingroup$ I don't need to post it again for I've wrote that Phobos and Ganymede being in the same category is weird. One should conclude that I ain't speaking of tiny spherical moonlets but of planetary-mass moons. It is obvious. May I change the text of the question (not the question itself) to add "differentiated"? $\endgroup$ – user30007 Jan 30 '20 at 14:43

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