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Recently some new moons of Jupiter were discovered. However the moons are pretty damn small indeed - only a few kilometers wide. That makes me wonder if they should even be called moons in the first place. The Wikipedia article on natural satellites says there's no clear lower limit (or upper limit for that matter). But if we accept that, then Jupiter already has countless numbers of moons in its ring system, and even the Earth has multiple moons since the International Space Station is a "moon".

What exactly is a moon? Is it really a vague notion of "something that goes around a planet"? If so, why hasn't the IAU clarified what exactly a moon is? They've already done so for planets; it seems natural to do so for moons as well.

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Unlike "planet" the IAU hasn't attempted to precisely define "moon". General usage requires that a "moon" is a natural satellite of a planet (or dwarf planet, asteroid, or perhaps even of another moon?) and it is big enough for us to have seen it as an independent body.

This contrasts with the "moonlets" that have been detected in Saturn's rings by their graviational effects on the even smaller ring particles. We know that a body exists there, but it is too small to be directly imaged.

As such the question "How many moons does Jupiter have" is probably unanswerable, since it depends on the sensitivity of your telescope and how close you are to the planet.

The smallest moons are about 10km across, but moonlets in the rings exist that are only a few hundred metres across. So in current usage, 10k might be roughly where people stop saying "moon" and start saying "moonlet".

Language exists to serve us, the definition of planet was only made to solve a particular naming problem. No such problem has arisen for moons. So there is no authoritative answer.

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    $\begingroup$ "Language exists to serve us" is a very good point which can hardly be repeated too much. (Though whether the IAU's naming proclivities have helped or not is debatable.) $\endgroup$ – Mark Olson Jul 20 '18 at 12:42
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    $\begingroup$ What is the "particular naming problem" the definition of planet was made to solve? I thought it was because of the discovery of a planet larger than Pluto that led to the IAU redefining planets. $\endgroup$ – Allure Jul 23 '18 at 3:33
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    $\begingroup$ Previously there had been no definition of "planet". The IAU didn't want Eris and other kuiper belt objects to be called a planets, so chose to define "planet" in such a way as to exclude them. $\endgroup$ – James K Jul 23 '18 at 6:26
  • $\begingroup$ "The IAU didn't want Eris and other kuiper belt objects to be called a planets". Sure. But why? What scientific end is served? $\endgroup$ – Mark Olson Jul 24 '18 at 15:14
  • $\begingroup$ Just as they didn't want Ceres, Vesta, Juno and so on to be planets. But naming isn't really about science. It is about language (or less politely, its about stamp collecting) $\endgroup$ – James K Jul 24 '18 at 16:26
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According to NASA - What is a Moon?:

Planets and asteroids orbit the Sun. Moons—also known as natural satellites—orbit planets and asteroids. Moons come in many shapes, sizes and types. Most are airless, but a few have atmospheres and even hidden oceans. There are hundreds of moons in our solar system—even a few asteroids have small companion moons.

The same question on Universe Today - What is a Moon? says:

A moon is defined to be a celestial body that makes an orbit around a planet, including the eight major planets, dwarf planets, and minor planets. A moon may also be referred to as a natural satellite, although to differentiate it from other astronomical bodies orbiting another body, e.g. a planet orbiting a star, the term moon is used exclusively to make a reference to a planet’s natural satellite.

Many other sites have similar explanations. By any of the definitions, though, the International Space Station is not a moon, as a moon is defined as a natural satellite orbiting a planet, dwarf planet or minor planet.

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    $\begingroup$ The particles in Saturn's rings meet all the criteria you listed above. They're generally not considered moons but part of a ring system. I think your answer is generally correct, but it misses the point of the question, on whether there is a specific and clear cut definition where a moon ends and debris in orbit begins. $\endgroup$ – userLTK Jul 20 '18 at 8:20
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    $\begingroup$ A fleck of paint from the Apollo 10 mission... We get down to absurdity. $\endgroup$ – Wayfaring Stranger Jul 27 '18 at 16:43
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I wont attempt to propose a complete definition, but only suggest a few criteria, that could be meaningful when defining a moon. As I see it, a formal definition, should either serve a very specific purpose (none given in question) or clarify a term already in general use. as a clarification of an everyday term, the formal definition should strive to be consistent with how, the word is already in common use, and not break drastically with existing understandings of the word. I would propose, that moons are first and foremostnatural satelites orbiting around planets, dwarf-planets and exo-planets (a satelite orbiting smaller objects, such as an asteroid, would probably not meet most people's idea of a real moon. As for size, and clearing it's orbit, I would suggest, that the satelite, should be large enough and distinct enough from other objects in its orbital neighbourhood, to be clearly visible, as a distinct object, with the naked eye of a hypothetical human being, standing on the surface of the orbited (parent) object(at least, during part of its orbit).

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"Moon" is a very broad term used for any natural satellite of a planet or asteroid. They can be as small and irregular as Deimos and Phobos (satellites of Mars) or the components of planetary rings or as big and planet-like as our Moon or the Galilean moons or Saturn's moon Titan.

That is not alright because it is like if we called every celestial body which orbits directly the Sun a planet including the tiniest asteroids. You're right that one must establish a definition of moon. Only ellipsoidal satellites should be called moons. According to this definition, Earth of course has one moon, Jupiter has four moons, Saturn seven, Uranus five, Neptune one, Pluto one and Eris possibly one (we don't know whether Dysnomia is spherical). Irregular satellites are asteroids that orbit a planet rather than a star. Asteroid satellites. Let's say a red dwarf orbits a blue main-sequence star, this doesn't make the red dwarf a moon or planet, it is still a star in a double star system.

Dr Alan Stern is even proposing to call the spherical moons planets / satellite planets. However that goes too far in my opinion, it would make our Moon a planet. It is too late to consider Earth-Moon a double planet system.

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  • $\begingroup$ Two key problems with your proposed metric: (1) Define "spherical", or "ellipsoidal". There is no such thing as a perfectly spherical or ellipsoidal moon, so if perfection is your metric, there are no moons. (2) As perfection is out, a numerical metric with a boundary is needed. What's your metric, and what's the boundary? Is there a huge gap between moons and non-moons, as there is with planets and non-planets? The metric proposed by Dr. Alan Stern, for example, has a six order of magnitude gap between planets and non-planets. Pluto is not a planet, by any sane metric. $\endgroup$ – David Hammen Feb 16 '20 at 3:01
  • $\begingroup$ I don't know perfectly well what you mean. No one, including Dr. Stern, knows perfectly well what you mean because no one has spelled out what deviations from hydrostatic equilibrium are acceptable. An interesting well-known fact: Not a single one of the three most intensively studied terrestrial bodies (the Earth, the Moon, and Mars) is in hydrostatic equilibrium. The Earth is still recovering from the last ice age, and the Moon and Mars appear to be frozen a bit out of hydrostatic equilibrium. Regarding Saturn, all but two of its larger moons are known not to be in hydrostatic equilibrium. $\endgroup$ – David Hammen Feb 16 '20 at 12:14
  • $\begingroup$ To make matters worse, there's a concept called the "potato radius." Objects smaller than the potato radius tend to be markedly lumpy while larger objects tend to be rounded (if one squints hard enough). A number of issues arise with this concept. One is that the potato radius varies with composition. Another is that there is no clear-cut boundary and that any boundary has known exceptions. Some objects significantly larger than the potato radius are visibly "not round", and some objects significantly smaller than the potato radius appear to be quite round. $\endgroup$ – David Hammen Feb 16 '20 at 12:30
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    $\begingroup$ There is a very good, unambiguous metric that distinguishes planets from non-planets. That you happen not to like it is irrelevant. There is no good metric for what distinguishes a moon from a moonlet. Any such metric would inevitably be flawed. $\endgroup$ – David Hammen Feb 16 '20 at 13:51
  • $\begingroup$ You might be on to something with that metric. From what I have read, no one has proposed such a metric. This proposal of yours flies in the face of your answer as this is quite different from vague concepts of roundness or hydrostatic equilibrium. All three extant metrics regarding "clearing the neighborhood" are explicitly or implicitly tailored toward a central body of about one solar mass. Adjusting those metrics to accommodate small or large stars is quite possible. Adjusting them to accommodate planets so as to distinguish moons from moonlets is a very different matter. But have at it! $\endgroup$ – David Hammen Feb 16 '20 at 15:05

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