Jupiter has 79 (known) moons, Saturn 82, Uranus 27 and Neptune 14 (numbers come from Wikipedia). These planets also all have rings. The rings are made up of chunks of rock and ice. There also are satellites in between the rings. What distinguishes a moon from any other chunk in orbit around a planet?

If a body is in orbit around a planet, how do you know if it's a moon or if it's part of the ring? Are there orbital characteristics that differ between the two? Is there a lower bound on size or mass for something to be a moon? Is it the density of surrounding bodies? Is there no formal (e.g from the IAU) definition?


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There also are satellites in between the rings. What distinguishes a moon from any other chunk in orbit around a planet?

Wikipedia should be taken with a grain of salt, but they define moons or moonlets within a ring system as creating a gap or partial gap, sometimes described as looking like a propeller.

In 2006, four tiny moonlets were found in Cassini images of the A Ring.[44] Before this discovery only two larger moons had been known within gaps in the A Ring: Pan and Daphnis. These are large enough to clear continuous gaps in the ring.[44] In contrast, a moonlet is only massive enough to clear two small—about 10 km across—partial gaps in the immediate vicinity of the moonlet itself creating a structure shaped like an airplane propeller.[45] The moonlets themselves are tiny, ranging from about 40 to 500 meters in diameter, and are too small to be seen directly.[9]

A moon, I would think, would need to clear out an entire gap within a ring. Clearing out a partial gap or "propeller" as noted above can be as small as 40 meters. Often the moonlets themselves aren't visible but the propellers they create in the ring system is visible.

Some more details and images here and here


A diameter of 0.5 kilometer will get you classified as a moonlet.

See Aegaeon. Apparently at 1.4 km × 0.5 km × 0.4 km, it is on the edge of the definition.

  • $\begingroup$ From what I understand in the first link, the difference between a moon and a moonlet is the amount of space it managed to clear in the rings. So there isn't a formal lower limit on the size of a moon. Aegaeon is just the smallest thing we decided to call a moon? $\endgroup$ – usernumber Nov 24 '19 at 16:48
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    $\begingroup$ As far as I know, the international Astronomical Union has not gotten around to a formal definition of moonlit. They're bigger than your usual ring particle, but we may well still find something under 0.5km, and call it a moonlet. $\endgroup$ – Wayfaring Stranger Nov 24 '19 at 17:36
  • $\begingroup$ I guess there hasn't been a problem like with the planets that forced the IAU to set a clear limit on what constitutes a moon. $\endgroup$ – usernumber Nov 26 '19 at 7:21

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