<p>Your question:</p>

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

<p>Wikipedia should be taken with a grain of salt, but they define moons or moonlets within a ring system <a href="https://en.wikipedia.org/wiki/Moons_of_Saturn#Ring_moonlets" rel="nofollow noreferrer">as creating a gap or partial gap</a>, sometimes described as looking like a propeller.</p>

<blockquote>
  <p>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]</p>
</blockquote>

<p>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.</p>

<p>Some more details and images <a href="https://en.wikipedia.org/wiki/S/2009_S_1" rel="nofollow noreferrer">here</a> and <a href="https://en.wikipedia.org/wiki/Moonlet" rel="nofollow noreferrer">here</a></p>

CC BY-SA 4.0
2019-11-24T22:03:50.613Z
2019-11-24T22:03:50.613Z
userLTK
<p>A diameter of 0.5 kilometer will get you classified as a <a href="https://en.wikipedia.org/wiki/Moons_of_Saturn#Ring_moonlets" rel="nofollow noreferrer">moonlet</a>.</p>

<p>See <a href="https://en.wikipedia.org/wiki/Aegaeon_(moon)" rel="nofollow noreferrer">Aegaeon</a>. 
Apparently at 1.4 km × 0.5 km × 0.4 km, it is on the edge of the definition.</p>

CC BY-SA 4.0
2019-11-24T16:48:47.410Z
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?
usernumber
CC BY-SA 4.0
2019-11-24T17:36:26.677Z
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.
Wayfaring Stranger
CC BY-SA 4.0
2019-11-26T07:21:02.200Z
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.
usernumber
CC BY-SA 4.0
2019-11-24T16:12:40.887Z
2019-11-24T16:12:40.887Z
Wayfaring Stranger
<p>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 <a href="https://en.wikipedia.org/wiki/Shepherd_moon" rel="nofollow noreferrer">satellites in between the rings</a>. What distinguishes a moon from any other chunk in orbit around a planet? </p>

<p>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?</p>

CC BY-SA 4.0
2019-11-25T01:21:42.637Z
See also: https://astronomy.stackexchange.com/questions/27016/what-exactly-is-a-moon and https://astronomy.stackexchange.com/questions/27051/would-an-analogue-of-the-definition-for-planets-also-work-for-moons
Allure
<natural-satellites><planetary-ring><definition><moonlet>
What is the difference between a moon and a random chunk in the rings
usernumber
# What is the difference between a moon and a random chunk in the rings

(Asked by: username_0 on 2019-11-24)

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](https://en.wikipedia.org/wiki/Shepherd_moon). 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?

**Tags:** natural-satellites, planetary-ring, definition, moonlet

> **Comments:**
> **username_1**: See also: https://astronomy.stackexchange.com/questions/27016/what-exactly-is-a-moon and https://astronomy.stackexchange.com/questions/27051/would-an-analogue-of-the-definition-for-planets-also-work-for-moons  

---

## 2 answers

### Answer by username_2 on 2019-11-24. Score: 3 | Accepted answer

Your question:

> 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](https://en.wikipedia.org/wiki/Moons_of_Saturn#Ring_moonlets), 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](https://en.wikipedia.org/wiki/S/2009_S_1) and [here](https://en.wikipedia.org/wiki/Moonlet)


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### Answer by username_3 on 2019-11-24. Score: 2

A diameter of 0.5 kilometer will get you classified as a [moonlet](https://en.wikipedia.org/wiki/Moons_of_Saturn#Ring_moonlets).

See [Aegaeon](https://en.wikipedia.org/wiki/Aegaeon_(moon)).
Apparently at 1.4 km × 0.5 km × 0.4 km, it is on the edge of the definition.

> **Comments:**
> **username_0**: 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?  
> **username_3**: 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.  
> **username_0**: 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.  

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