# Why does Jupiter have so many moons?

The usual explanations one finds just say that Jupiter has a strong gravitational field, thereby being able to catch moons easier, and then they stop there. But this seems far from a satisfactory explanation. After all, an object which is not gravitationally bound to another object will never become gravitationally bound unless it interacts with other objects so it can shed some of its energy. Having a stronger gravitational field doesn't change this.

So then: Is there a more detailed explanation for why Jupiter has so many more moons than the other planets?

• Not that this is really relevant to the question but... doesn't Saturn have more moons than Jupiter? Mar 13 at 14:53
• @jacob1729 The numbers are in the same ballpark, true. I wouldn't mind a similar explanation for Saturn's moons, either. The two largest planets having the most moons might have similar reasons. Mar 13 at 15:00
• Not only are Jupiter and Saturn closer to the sun, so their moons are brighter in telescopes, we've also had a lot more closeup examination time with flybys and orbiters of Jupiter and Saturn than Uranus and Neptune, the latter two having only ever been visited by Voyager 2, making it far more difficult to hunt down the really small moons. Mar 13 at 15:14
• Would an argument work along the lines of: whatever process is at work to give planets moons, will be more effective if the planet excerpts it's gravitational control over a larger area...
– rfl
Mar 15 at 18:36
• Duplicate in astro.SE: astronomy.stackexchange.com/questions/321/… Mar 20 at 9:22

After all, an object which is not gravitationally bound to another object will never become gravitationally bound unless it interacts with other objects so it can shed some of its energy.

This is true, but you've forgotten about the Sun. Every interaction between a planetismal and Jupiter is a three-body interaction.

Above, a simulation of a low-mass planetismal moving in the effective potential in the rotating frame for a planet with mass $$10^{-3}$$ of its star's mass. The Lagrange points are marked with $$\color{orange}{\times}$$. The particle starts at $$%(0.83,0.47)$$ some random place I clicked; it moves ahead of the planet for two or three orbits, pausing at a couple of unstable stationary points in the rotating frame, then has a close interaction with the planet. In this case the close interaction doesn't lead to a capture, but you can see from the inset that the interaction is chaotic: it's extremely sensitive to the details of the closest approach. You can surely imagine a three-body interaction that ended in the particle being captured by the planet, even if I haven't hunted for one to show you.

You state

After all, an object which is not gravitationally bound to another object will never become gravitationally bound unless it interacts with other objects so it can shed some of its energy

italics mine

"never become" is not true if it is coming along with a correct angle and velocity for an elliptical orbit around the larger mass '

Jupiter, as well as other distant gas giants in the solar system such as Saturn and Neptune, has another ace up its sleeve. Not only does it have a very strong gravitational pull thanks to its mass, but it is also quite far away from the Sun. It’s about 5 times farther away from the Sun than Earth is, completing a full orbit every 11.86 years.

This great distance allows Jupiter to exert a larger area of influence or control as the Sun’s gravitational influence weakens the farther away you travel from it. With such a wide net cast, it’s no wonder that Jupiter has moons orbiting it as far away as 23.5 million miles, as is the case for Pasiphae and Sinope. Meanwhile, Venus and Mercury, the two closest planets to the sun in the solar system, have no moons at all, while Earth has a measly one to speak of and Mars has two tiny satellites.

I am copying from the comments below:

The above quotes say that it is not only its mass, but its location in the planetary system that gives it a large phase space at a first chance to catch something, its large volume dominance filtering off the inner orbits the majority of incoming masses.

• Anna, that is all correct, but doesn't address the OP's question given the extra thought that the gravitational potential is conservative, does it?
– rfl
Mar 15 at 18:35
• @rfl I believe it does.It is the volume of space spanned by the heavy planets, contrasted with the one close to the sun. Like a butterfly net it gets first chance from the volume the whole planetary system sweeps on its way orbiting the galaxy center. It is total energy that is conserved, and I do not see what "gravitational potentials are conserved means" Mar 15 at 18:59
• The point is just that Keplerian orbits are bound or unbound. "Catching" a moon is not as simple as making it pass through Jupiter's space of gravitational influence. That space scales with mass, sure. But the OP's question is why Jupiter's mass improves his chances of getting orbits from unbound to bound,
– rfl
Mar 15 at 20:19
• @rfl The quote adds that it is not only its mass, but its location in the planetary system that gives it a large phase space at a first chance to catch something, filtering it off the inner orbits. It answers the "more detailed explanation " in the question. Mar 16 at 5:12
• I don't think it addresses the valid point of the OP that is "an object which is not gravitationally bound to another object will never become gravitationally bound unless it interacts with other objects so it can shed some of its energy"
– rfl
Mar 16 at 11:36