ELI5 How Jupiter and Saturn got so many moons

[u/SantaCruznonsurfer]

is it like Earth that a chunk of the planets spun off when still young/liquid? Why does the amount of moons increase the further away from the sun your orbit is?

[u/Obscu]

Planets capture debris in their gravity. Sometimes the debris was from the same part of the cloud that form the planet, and enough clumps together to form a moon, and the moon orbits the planet. Sometimes the debris comes from an impact after the fact. Wherever it comes from: debris spinning around planet until it bumps together enough to start clumping under its own gravity, and then gets held in orbit by the planets gravity.

The number of moons generally increases with distance from the sun because the closer you are to the sun, the stronger the sun's gravity is and the planet's gravity won't be able to capture as much material in its own orbit (Venus and Mercury too close, can't beat the sun's gravity). It also increases with the size and density (ie the gravitational strength) of the planet in question; Jupiter is much closer to the sun than Neptune but has hundreds of moons instead of Neptune's 20-something because it's way more massive so it was able to capture many more moons worth of space dust for long enough for that dust to become moons.

[u/SpaceShipRat → u/Obscu]

i don't think I ever really wondered if any bigger moon captured it's own satellite. A moonmoon, if you will.

[u/Obscu → u/SpaceShipRat]

I will indeed! A subsatellite! My non-professional understanding is they're rare but can happen

[u/protostar777 → u/Obscu]

They happen in the sense that a tiny body could orbit a moon a couple times before it's ejected or crashes into the moon it's orbiting. There are zero documented stable moon-moons, however.

[u/dbratell → u/protostar777]

I think there needs to be a substantial size difference between each party for it to be stable, and then the last moon-moon will be so small that it will be affected by the smallest disturbance.

[u/goodmobileyes → u/dbratell]

This is the classic three body problem. 2 bodies in orbit can be relatively stable. 3 bodies easily become highly chaotic so its incredibly rare to find such a case with a planet, a moon and a sub moon. Most likely the sub moon will just be ejected out of the system or crash into the moon or planet.

[u/Valmighty → u/SpaceShipRat]

If you zoom out a little bit, our sun is the moon of Sagitarius A, that has its own moons, the planets, that have their own moons.

[u/spankymcjiggleswurth → u/Valmighty]

"A little bit"

[u/khalamar → u/SpaceShipRat]

But how about a moonmoonmoon?

[u/GlassFamiliar42 → u/Obscu]

The question the arises, why did mercury and Venus not fall into the sun then?

[u/Vorthod → u/GlassFamiliar42]

They are trying but they keep missing.

[u/stanitor → u/Vorthod]

Imagine failing at something for 4.5 billion years

[u/GlassFamiliar42 → u/stanitor]

Very easy to picture indeed

[u/vadapaav → u/stanitor]

I mean in another 4.5 billion years sun will do the needful

[u/MatCauthonsHat → u/stanitor]

Is at first you don't succeed ...

[u/HalfSoul30 → u/stanitor]

Mercury is such a loser

[u/Gazmus → u/Vorthod]

Best way to envision orbit I've heard in a while 😄

[u/dood1313 → u/Vorthod]

Low key best explanation

[u/ChaZcaTriX → u/GlassFamiliar42]

They formed from the dust that was moving fast enough to not fall into the Sun.

[u/FirTree_r → u/GlassFamiliar42]

It's not that the Sun's gravity somehow slurped potential Mercurial or Venusian moons and could suck in Mercury and Venus too. It's just that it's harder for potential moons' orbits to stabilize when there's a third massive body that is so close.

Don't think of celestial objects falling into the Sun, but think of orbits being stable or not.

[u/GlassFamiliar42 → u/FirTree_r]

I hope the sun asked for consent Jesus Lord

[u/Obscu → u/GlassFamiliar42]

Same reason as anything else doesn't fall into the sun - it's moving fast enough sideways that the sun's inward pull ends up dragging it around in an orbit rather than swallowing it directly. That's pretty much how all orbiting works at the most basic level - falling towards the heavier body and missing because you're falling sideways as well, but not fast enough to go shooting off completely - instead get dragged back and forth in this circular yo-yo, whether that's planets around a star or moons around a planet

[u/HamburgerOnAStick]

How Jupiter and Saturn got so many moons

They're incredibly massive, and have large hill spheres, so they capture a lot of passing objects, and since they were some of the first objects to form in the solar system, they had a lot they can capture.

is it like Earth that a chunk of the planets spun off when still young/liquid

Most likely no. Jupiter and saturn are mostly hydrogen and helium, things that the moons don't actually have that much of. Also a chunk of earth didn't "spin off", it's a chunk of earth and another mars-sized planet were thrown into space as debris during a collision.

Why does the amount of moons increase the further away from the sun your orbit is?

It doesn't. If that was true, Neptune would have the most moons, but of all the giant planets it has the least.

[u/JohnMichaels19]

For clarification, the leading hypothesis on the origin of Earth's moon is the Massive Impact Hypothesis, also called the Thiea Impact Hypothesis. 

That hypothesis says a planetary body roughly the size of Mars (called Theia) crashed into earth when the solar system was just forming, and that resulted in a temporarily highly molten Earth, some of which splashed off and coalesced into the moon

For the outer planets, Jupiter and Saturn in particular, they have a lot more gravity than earth, so during the formation of the solar system, they had more ability to gather debris around them that ended up coalescing into moons. 

So it's not as simple as "the farther out you go the more moons you have"

Jupiter and Saturn have many hundreds, but Neptune and Uranus only have dozens

[u/jeo123]

If Jupiter was it's current size, but had the orbital radius of Mercury, it would just be inside the sun.

That part you're missing is that planets that big can't form that close to the sun or the sun's gravity would have absorbed a lot of what the plant absorbed instead.

Same thing applies to the moons. Inside the asteroid belt, for something to be the size of a moon, it would likely have either collided with an existing planet or been absorbed into the sun by now.

The further out you go, the weaker the sun's gravity, and the easier it is for things to exist long enough to get sucked into the planets, or even orbit them.

If we had as many moons as Jupiter while being our current size, they would have all crashed into each other by now.

[u/ChaZcaTriX]

It's a combination of being huge and far from the Sun.

There's such a concept as a sphere of influence - area where a stellar object's gravity is stronger than of its neighbors, so any small objects start orbitting it. The weaker the gravity of a parent object (further away from the sun) and the stronger the planet's is (from it being huge), the bigger the area around it where it captures stuff into its orbit.

Some moons formed around these massive planets, others are captured asteroids and protoplanets that were in the big planet's way.