<p>Likely there's two different explanations, which can even work in conjunction:</p>
<p>a) An explanation along the same as goes for the size sorting of the planets in the solar system. For those regular moons which formed along with Neptune or in orbit of the protoplanet we see that the further outward the moon forms in the accretion disk around proto-Neptune, the more material is present due to the greater circumference of the orbit. Thus it can grow larger in size.</p>
<p>b) Neptune exerts tides onto the moons. Typically no moons can exist within the <a href="https://en.wikipedia.org/wiki/Roche_limit" rel="nofollow noreferrer">Roche limit</a>. How close a moon can exist depends on the rigidity of the body, though. Big moons (mass <span class="math-container">$M_m$</span>) cannot exist as far inward (distance <span class="math-container">$d$</span> to Neptune) as a small moon (Neptune's radius <span class="math-container">$R_N$</span> and mass <span class="math-container">$M_N$</span>):
<span class="math-container">$$ M_m = \left(\frac{R_N}{d}\right)^3\cdot2M_N$$</span> or easier expressed as ratio of the moon's radius <span class="math-container">$r_{moon}$</span> and the moon's distance to Neptune <span class="math-container">$d$</span> as well as its density <span class="math-container">$ \varrho_{moon}$</span>: <span class="math-container">$$ \left(\frac{r_{moon}}{d}\right)^3\cdot\varrho_{moon} = constant $$</span>
Thus the further out, the larger the moon can be without falling apart due to tidal forces. Wikipedia has a page on <a href="https://en.wikipedia.org/wiki/Moons_of_Neptune" rel="nofollow noreferrer">Neptune's moons</a> and references which indicates that the inner moons are somewhere at the tidal limit for bodies which are of somewhat granular nature as is expected for at least the smaller moons or maybe even all with the exception of Triton.</p>

CC BY-SA 4.0
2020-11-11T14:02:29.847Z
$\varrho_{moon}$ being the density of the moon I suppose?
Jean-Marie Prival
CC BY-SA 4.0
2020-11-11T14:14:59.863Z
yes, $ \varrho $ is as usal the density
planetmaker
CC BY-SA 4.0
2020-11-23T14:08:53.237Z
Can this explanation be valid for other planets or is it for just for Neptune? Note that moons of other planets also experience tides. Neptune moons follow a pattern. I kind of plotted d-M graph for Neptune moons and gave a [cusp graph](https://en.wikipedia.org/wiki/Cusp_(singularity)) (ignoring some exceptions - Hippocamp and Neso). Uranus moons follow a different pattern (increasing for the first 4 innermost moons and then alternating with low and high masses) but no pattern for Jupiter or Saturn's moons.
Nilay Ghosh
CC BY-SA 4.0
2020-11-11T13:11:57.513Z
2020-11-11T14:16:25.237Z
2020-11-11T14:16:25.237Z
planetmaker
<p>The five moons closest to Neptune are sorted by increasing size. Naiad &lt; Thalassa &lt; Despina &lt; Galatea &lt; Larissa. The trend continues with Proteus and Triton, only interrupted by Hippocamp, which is smaller than the closer moons.</p>
<p>Is this ordering due to pure chance? Is there a reason why moons closer to Neptune should be smaller? Is it possibly an observational bias?</p>

CC BY-SA 4.0
2020-11-25T05:12:30.303Z
I have drawn the d-M graph for Neptune moons ([one excluding Triton](https://imgur.com/hoJB6jp) and [one including Triton](https://imgur.com/TFAOwdq) (Triton was well out of scale, hence I drew one without it)) in Excel sheet. It kinda looks like a [cusp graph](https://imgur.com/dib1qet) . So obviously, there is a kind of a pattern   for Neptune moons.
Nilay Ghosh
<natural-satellites><neptune>
Why are the moons of Neptune sorted by diameter?
usernumber
# Why are the moons of Neptune sorted by diameter?

(Asked by: username_0 on 2020-11-11)

The five moons closest to Neptune are sorted by increasing size. Naiad < Thalassa < Despina < Galatea < Larissa. The trend continues with Proteus and Triton, only interrupted by Hippocamp, which is smaller than the closer moons.

Is this ordering due to pure chance? Is there a reason why moons closer to Neptune should be smaller? Is it possibly an observational bias?

**Tags:** natural-satellites, neptune
> **Comments:**
> **username_1**: I have drawn the d-M graph for Neptune moons ([one excluding Triton](https://imgur.com/hoJB6jp) and [one including Triton](https://imgur.com/TFAOwdq) (Triton was well out of scale, hence I drew one without it)) in Excel sheet. It kinda looks like a [cusp graph](https://imgur.com/dib1qet) . So obviously, there is a kind of a pattern   for Neptune moons.  

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## 1 answers

### Answer by username_2 on 2020-11-11. Score: 2 | Accepted answer

Likely there's two different explanations, which can even work in conjunction:

a) An explanation along the same as goes for the size sorting of the planets in the solar system. For those regular moons which formed along with Neptune or in orbit of the protoplanet we see that the further outward the moon forms in the accretion disk around proto-Neptune, the more material is present due to the greater circumference of the orbit. Thus it can grow larger in size.

b) Neptune exerts tides onto the moons. Typically no moons can exist within the [Roche limit](https://en.wikipedia.org/wiki/Roche_limit). How close a moon can exist depends on the rigidity of the body, though. Big moons (mass $M\_m$) cannot exist as far inward (distance $d$ to Neptune) as a small moon (Neptune's radius $R\_N$ and mass $M\_N$):
$$ M\_m = \left(\frac{R\_N}{d}\right)^3\cdot2M\_N$$ or easier expressed as ratio of the moon's radius $r\_{moon}$ and the moon's distance to Neptune $d$ as well as its density $ \varrho\_{moon}$: $$ \left(\frac{r\_{moon}}{d}\right)^3\cdot\varrho\_{moon} = constant $$
Thus the further out, the larger the moon can be without falling apart due to tidal forces. Wikipedia has a page on [Neptune's moons](https://en.wikipedia.org/wiki/Moons_of_Neptune) and references which indicates that the inner moons are somewhere at the tidal limit for bodies which are of somewhat granular nature as is expected for at least the smaller moons or maybe even all with the exception of Triton.

> **Comments:**
> **username_3**: $\varrho_{moon}$ being the density of the moon I suppose?  
> **username_2**: yes, $ \varrho $ is as usal the density  
> **username_1**: Can this explanation be valid for other planets or is it for just for Neptune? Note that moons of other planets also experience tides. Neptune moons follow a pattern. I kind of plotted d-M graph for Neptune moons and gave a [cusp graph](https://en.wikipedia.org/wiki/Cusp_(singularity)) (ignoring some exceptions - Hippocamp and Neso). Uranus moons follow a different pattern (increasing for the first 4 innermost moons and then alternating with low and high masses) but no pattern for Jupiter or Saturn's moons.  

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