<p>Gravitational mass is not the be-all and end-all. Two other metrics are the <a href="https://en.wikipedia.org/wiki/Hill_sphere" rel="nofollow noreferrer">Hill sphere</a> and <a href="https://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)" rel="nofollow noreferrer">gravitational sphere of influence</a>. These are widely used metrics that describe the volume over which a planetary body dominates gravitationally over than of the Sun. Saturn has a larger Hill sphere and a larger gravitational sphere of influence than does Jupiter.</p>

<p>This suggests that Saturn might have many more moons than does Jupiter. Saturn's much greater distance from the Sun compared to Jupiter's suggests that discovering a new Saturnian moon is over three times harder than is discovering a new comparably sized Jovian moon. This might be why the ratio is as close as it is.</p>

CC BY-SA 4.0
2020-05-27T22:05:42.533Z
2020-05-27T22:05:42.533Z
David Hammen
<p>Given that Jupiter is roughly 3x as massive compared to Saturn and is located right next to the asteroid belt, it feels that Jupiter should easily have a larger amount of captured moons. However currently, due to a <a href="https://www.nationalgeographic.com/science/2019/10/discovery-20-new-moons-gives-saturn-solar-system-record/" rel="nofollow noreferrer">recent survey of Saturn discovering 20 new moons</a>, Saturn has 57 observed captured moons compared to Jupiters 71. </p>

<p>Also summing the mass of the 6 largest captured moons for each planet gives:<br>
- Jupiter: 7.844*10^18kg<br>
- Saturn: 8.372*10^18kg</p>

<p>Why isn't there a larger gap between the number and mass of Jupiter and Saturns captured moons?</p>

<p>Note:
While Jupiter has a very large amount of Trojan asteroids, the main focus of the question is moons directly in orbit around the main planet.</p>

CC BY-SA 4.0
2020-05-26T10:08:00.117Z
Possible duplicate of https://astronomy.stackexchange.com/questions/13419/why-does-the-planet-saturn-have-numerous-62-moons-compared-to-the-rest-of-the
Barry Jenakuns
CC BY-SA 4.0
2020-05-26T10:53:14.060Z
It's unusual to see you leave a "possible duplicate" message under your own question. In Stack Exchange what makes it a duplicate is if answers there also answer your question. If you feel they sufficiently answer your question, then you can click "close" and start the process yourself. If you feel you need an answer that differs or goes beyond what's there, then you can mention what that might be *in your question* so that this doesn't get closed.
uhoh
CC BY-SA 4.0
2020-05-26T11:50:05.590Z
@uhoh I don't think it's a duplicate, but it felt like there was a lot enough in common between the two questions to argue closure. In this case, I wanted to get the ball rolling. However I've adjusted the focus of the question to specify Jupiter to the point where I think this is it's own question.
Barry Jenakuns
CC BY-SA 4.0
2020-05-26T12:21:50.843Z
The number of moons is not expected to be a fundamental physical parameter. More the total mass in the moons. And for the all moons-to-planet mass ratios, all giant planets have similar values, indicating a similar formation mechanism for the moons.
AtmosphericPrisonEscape
CC BY-SA 4.0
2020-05-26T13:22:27.270Z
What precisely do you mean by "the main focus of the question is moons directly in orbit around the main planet" ?  All the moons are "directly in orbit".
StephenG - Help Ukraine
CC BY-SA 4.0
2020-05-26T21:17:20.227Z
@StephenG - The OP is rejecting objects captured at Jupiter's triangular libration points (L4 and L5), which are called pseudo moons by some, as moons.
David Hammen
CC BY-SA 4.0
2020-05-26T23:46:33.947Z
There are studies, eg.: [Satellite capture mechanism in a sun-planet-binary four-body system](https://arxiv.org/abs/1804.10415) and references therein. If we knew the probabilities of capture/ejection, we could estimate the number.
Keith McClary
<jupiter><natural-satellites><saturn>
Why isn't there a large gap in the number of captured moons Jupiter and Saturn have?
Barry Jenakuns
# Why isn't there a large gap in the number of captured moons Jupiter and Saturn have?

(Asked by: username_0 on 2020-05-26)

Given that Jupiter is roughly 3x as massive compared to Saturn and is located right next to the asteroid belt, it feels that Jupiter should easily have a larger amount of captured moons. However currently, due to a [recent survey of Saturn discovering 20 new moons](https://www.nationalgeographic.com/science/2019/10/discovery-20-new-moons-gives-saturn-solar-system-record/), Saturn has 57 observed captured moons compared to Jupiters 71.

Also summing the mass of the 6 largest captured moons for each planet gives:  
- Jupiter: 7.844\*10^18kg  
- Saturn: 8.372\*10^18kg

Why isn't there a larger gap between the number and mass of Jupiter and Saturns captured moons?

Note:
While Jupiter has a very large amount of Trojan asteroids, the main focus of the question is moons directly in orbit around the main planet.

**Tags:** jupiter, natural-satellites, saturn
> **Comments:**
> **username_0**: Possible duplicate of https://astronomy.stackexchange.com/questions/13419/why-does-the-planet-saturn-have-numerous-62-moons-compared-to-the-rest-of-the  
> **username_1**: It's unusual to see you leave a "possible duplicate" message under your own question. In Stack Exchange what makes it a duplicate is if answers there also answer your question. If you feel they sufficiently answer your question, then you can click "close" and start the process yourself. If you feel you need an answer that differs or goes beyond what's there, then you can mention what that might be *in your question* so that this doesn't get closed.  
> **username_0**: @uhoh I don't think it's a duplicate, but it felt like there was a lot enough in common between the two questions to argue closure. In this case, I wanted to get the ball rolling. However I've adjusted the focus of the question to specify Jupiter to the point where I think this is it's own question.  
> **username_2**: The number of moons is not expected to be a fundamental physical parameter. More the total mass in the moons. And for the all moons-to-planet mass ratios, all giant planets have similar values, indicating a similar formation mechanism for the moons.  
> **username_3**: What precisely do you mean by "the main focus of the question is moons directly in orbit around the main planet" ?  All the moons are "directly in orbit".  
> **username_4**: @StephenG - The OP is rejecting objects captured at Jupiter's triangular libration points (L4 and L5), which are called pseudo moons by some, as moons.  
> **username_5**: There are studies, eg.: [Satellite capture mechanism in a sun-planet-binary four-body system](https://arxiv.org/abs/1804.10415) and references therein. If we knew the probabilities of capture/ejection, we could estimate the number.  

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## 1 answers

### Answer by username_4 on 2020-05-27. Score: 2 | Accepted answer

Gravitational mass is not the be-all and end-all. Two other metrics are the [Hill sphere](https://en.wikipedia.org/wiki/Hill_sphere) and [gravitational sphere of influence](https://en.wikipedia.org/wiki/Sphere_of_influence_(astrodynamics)). These are widely used metrics that describe the volume over which a planetary body dominates gravitationally over than of the Sun. Saturn has a larger Hill sphere and a larger gravitational sphere of influence than does Jupiter.

This suggests that Saturn might have many more moons than does Jupiter. Saturn's much greater distance from the Sun compared to Jupiter's suggests that discovering a new Saturnian moon is over three times harder than is discovering a new comparably sized Jovian moon. This might be why the ratio is as close as it is.


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