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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, 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.

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  • $\begingroup$ Possible duplicate of astronomy.stackexchange.com/questions/13419/… $\endgroup$
    – Barry Jenakuns
    May 26, 2020 at 10:08
  • $\begingroup$ 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. $\endgroup$
    – uhoh
    May 26, 2020 at 10:53
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    $\begingroup$ @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. $\endgroup$
    – Barry Jenakuns
    May 26, 2020 at 11:50
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    $\begingroup$ 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. $\endgroup$
    – AtmosphericPrisonEscape
    May 26, 2020 at 12:21
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    $\begingroup$ @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. $\endgroup$
    – David Hammen
    May 26, 2020 at 21:17

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Gravitational mass is not the be-all and end-all. Two other metrics are the Hill sphere and gravitational sphere of influence. 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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