Which planet has the most moons?

26:06The closer you are to something, the stronger the gravitational force. The further you are from something, the weaker the gravitational force. Now, if you are just a point object, that doesn't really matter because your front and back are the same thing. But if you're big enough that your front and back are substantially different and your front is closer to the planet and your back is further from the planet, then a planet is going to be pulling on your front harder than it's pulling on your back. These are tidal forces. Essentially, it's pulling your front and back apart from each other. It tends to pull you into like a football shape.

26:35If you're strong enough internally, you're made of diamond, you can resist that. But if you're not, you're going to get shredded by the tidal forces. This is a mini version of like spaghettification when you approach a black hole. And so a planet has a thing called a Roche limit. Anything closer to the planet than the Roche limit is feeling tidal forces that are too strong to survive. Those are going to get shredded into like a ring. Anything outside the Roche limit can hold itself together and stay as a moon. All right. So you've got your central blob. It's formed into a planet. And then I was imagining everything around it is a ring.

27:10And so I guess when we're discussing this Roche limit thing, at that point, everything that was in the ring has already been sort of pulled into the football shape. And it got really hard when it got pulled into the football shape. And instead of getting shredded, then it forms the football moon. Yeah, you're right. Everything starts as a ring. But if you have a ring and it's far enough out, then gravity in that ring is going to pull the ring together into a moon. because things in the same orbit are going to attract each other along that orbit and they'll pull itself together. Now, if you're far enough away beyond the Roche limit, the planet is not going to interfere with that and you're going to gather yourself together into a moon.

27:49If you are within the Roche limit, then the planet is going to interfere with that. And as soon as you get any big objects, the planet's going to tear them apart or even prevent them from forming in the first place. Or as we'll talk about later, if you have a preformed object that then comes inside the Roche limit, the planet will tear it apart. So yeah, rings that formed with the planet probably never were moons, if that's really what you're asking. Rings that formed with the planet never were moons, but could become moons. They could become moons if they're outside the Roche limit, yes. But rings within the Roche limit probably never were moons because there's just too strong a tidal force for them to pull themselves together.

28:29So this sounds like an unlikely path to moonhood. Yeah, exactly. You need to be outside the Roche limit in order to be a moon. And so, for example, in the Earth-Moon system, the Roche limit is like 10 ,000 kilometers. Anything closer to the Earth than 10 ,000 kilometers, you're going to get pulled apart. The moon is safely like 380 ,000 kilometers away, so you're safe. And the sun also has a Roche limit. Planets that approach closer than like 750 ,000 kilometers to the sun would get shredded. We are safely 150 million kilometers from the sun. And these Roche limits are a little bit fuzzy because an object doesn't have a fixed Roche limit.

29:07It depends on what you're bringing near it. Like if you bring a moon made of jello versus a moon made of diamond, they're going to get pulled apart at different places, if that makes sense. And the moon each year is getting pulled a little bit closer to Earth. But it sounds like the difference between the Roche limit and where the moon is is so far. Oh, wait. Or is the moon getting farther away from us? The moon is getting further away by about a centimeter a year. And that has to do with the angular momentum, yeah. But it is a tidal force effect also. Okay. Are we going to lose the moon then?

29:40Eventually, the Earth and the moon are going to get tidally locked, which will stop this process. Okay, good. Phew. Okay, so it sounds like this might not be how we got our moon. Well, this is not how we got our moon. And you can tell which moons formed with the planet because they tend to have a very nice circular orbit and to be orbiting the planet around its equator, essentially on the same axis that the planet is spinning, because it all formed from the same blob of stuff. Just like most of the planets in the solar system are orbiting the sun around the same axis that the sun is rotating, because it all came from the same original blob of stuff.

30:18So if you find a moon around a planet and it's got a mostly circular orbit and it's orbiting around the same axis as the planet is spinning, probably it formed with the planet. Whew, we got through the boring stuff. All right. Actually, I thought that was really cool. But okay, so let's move on. Relatively boring. Relatively boring. All right. Super exciting, just not the most exciting thing we'll talk about today. All right, all right. So let's move on to something way more epic. What's the next method? The next method is to capture something. Imagine an object is floating through the solar system and it comes near a planet.

30:54Planets have a lot of gravity. If it comes in at the right velocity, in the right distance, in the right angle, it can basically enter orbit around a planet. The way like a spaceship can approach a planet and go into orbit around it. Or you can launch a satellite into orbit. If a visiting rock has the right trajectory, it can be captured by a planet. And you can tell moons that have this history because they're not made of the same stuff as the planet and because they tend to have like weird orbits. They're highly elliptical or they're tilted relative to the axis of rotation of the planet. How many of these do we think we have in our solar system?

31:31Because that seems like, depending on where they came from, a really cool opportunity for science. It is a very cool opportunity for science. And we have lots and lots of these examples in the solar system. And we'll talk about a few of them in a minute when we do the countdown for the ranking of the most moons. But it happens quite a bit. And astronomers like to think about the region of a planet near a planet where its gravity dominates. Because imagine you're some rock and basically you're orbiting the sun and then you come near the Earth. How close to the Earth do you have to get before the Earth's gravity dominates over the sun's gravity?

32:05That's called the hill sphere. and if a rock enters the hill sphere of a planet, then it has a chance to become captured. And was Hill a guy or a gal? Was Hill an astronomer? Presumably not one with the voice like you were making before.

32:24Yes, Hill was an astronomer and someone who thought about the early formation of the solar system because remember, back in the early days, things weren't as well cleared out. There were lots of rocks floating around. Jupiter and Saturn moved into the inner solar system and then back out. And that threw lots of asteroids everywhere. So there were lots of scattered objects. Some of them just get lost, fly outside the solar system, eventually maybe get captured by another solar system. Some of them will get captured by other planets. But each one is like a time capsule of crazy events that happened in our solar system.

32:58It sounds like if a giant object comes in, that would cause a lot of chaos in the process. Like, does it cause – I mean, I guess if it's small enough relative to a planet that it can get captured, it's probably not going to cause a lot of chaos for the planet that captures it. So maybe it's not going to cause that much chaos. How chaotic is this, Daniel? Well, the bigger the thing, the more chaos it causes, right, because the more gravitational pull it has. But the solar system already has a lot of chaos in it because every system that has more than two bodies in it is chaotic. You know, it's called the three-body problem.

33:31You can't have very many stable configurations of three objects orbiting each other. And the only way to do that is to have configurations where like two of them are really, really far from the other one. So even just having like Earth, Sun, and Jupiter already is a little bit chaotic. Jupiter is pulling on the Earth. It's one of the reasons why our orbits are changing. and we have these Malkovich cycles where we get further and closer from the sun. So there's already a little bit of chaos in the solar system. So something coming in from the outside is definitely going to inject more chaos. And most likely chaos causes us to lose things.

34:04Being captured is rare because you have to come in at like the right angle and you can't be close enough that you enter the atmosphere and then drag and then hit the planet and not so far that you're going to miss the hill sphere. So it's really just like a quite a narrow window for things to get captured. So the things in the solar system that are captured are a tiny fraction of all the chaos that has flown through our solar system. So most of the stuff that does get captured and become a moon, do we know where it tends to come from? We don't because we haven't studied a lot of these enough. We haven't like landed on them or done spectrometry on them in great detail.

34:40And as you'll see, the ones we have studied, we already have a lot of questions about. Well, I also thought that that was pretty awesome. But now we're going to get to the most awesome explanation, which is collisions. And I agree. That is probably the most awesome out of all the explanations. So, all right. Collisions. Hit me with it, Daniel. So imagine you have a planet and it's doing its thing. And then, like, another planet comes and smacks into it. And the two planets are vaporized, essentially. And they have to, like, coalesce again into molten blobs. And that process doesn't always give two objects of the same size.

35:18This is the leading theory for the formation of Earth and its moon, that there was a proto-Earth without a huge moon, and it was smacked into by like a Mars-sized planet, which essentially vaporized the surface of the Earth, and a huge chunk of stuff was then thrown out into orbit, gathered into a ring as it cooled, and then formed into a moon. and there's lots of evidence of this from the fact that like the moon is made out of basically the same stuff as the earth and because of the way the moon formed and cooled you can tell that had fairly recent activity on it also recent studies of the mantle of the earth has shown like actual deposition from this original proto planet that hit it it's like there's bits of that still leaving an imprint on the inner part of the earth it's really fascinating and so essentially that Like, I don't know if that really adds a moon or if it just sort of like deletes a planet and then creates two new objects.

36:15All right. So I'm adding a qualifier to my earlier statement. This is the most awesome way to make a moon as long as it never happens again in my lifetime to my planets. Yeah, exactly. But it's awesome that it happened long ago. It would have been very dramatic to watch, right? I wouldn't mind seeing this happen to another planet. You would learn a lot about this solar system as long as, of course, nobody was living on it. No critters were harmed in this cosmic collision. Okay, so let's assume that there are no microbes on Mars. If something like this happened to Mars and we were watching it, would there be any implications for us?

36:48Like maybe junk would get thrown off Mars and would hit us? Yeah, would anything happen to us if this happened to Mars? Oh, for sure. We would be hit by debris from it. We're already hit by debris from Mars. When things hit the surface of Mars, stuff is thrown out into space and some of it lands on the Earth. We have found rocks on the surface of the Earth that we know come from Mars because they are geologically incompatible with Earth's history and perfectly compatible with Mars. So that already happens that like Mars's garbage lands on Earth. So if Mars gets like massively slammed by a huge impact or basically shredded, then some significant fraction of it is definitely going to hit Earth.

37:27Yeah. Given that you are willing to sacrifice all of humanity just to talk to aliens for 30 seconds, would you risk Earth getting hit by large chunks of Mars just so you could see how this would all play out? For sure. Plus, we would get samples of Mars here on Earth. You're the worst. You know how valuable that is? Like, we have this incredible complicated plan to, like, dig up stuff on the surface of Mars and send it back to Earth for study. So valuable scientifically, but it's going to bring back, like, you know, a small amount of Mars. If you could, like, deliver huge chunks of Mars dropped into the Pacific or something, yeah, let's do it.

38:03One of us studies life and one of us studies non-life, and it is very clear which one does which right now. All right. And while the history of Earth's moon and this collision is really fascinating, a lot of deep science we could dig into there. Today's episode is not just a history of Earth's moon, but we wanted to do a ranking of which planets have the most moons. All right. And so we know, unfortunately, that the answer is not Earth. That's right. And in shortstanding tradition established on this episode, we're going to start with the least exciting examples first. That's right. Okay. And so, well, should we start closest to the sun?

38:46Yes. And that coincidentally starts us at the end of the list, the planets with the least moons, and that would be Mercury and Venus. Both of these planets have zero moons. Why? Why? The reason is that they are so close to the sun that the sun's tidal forces will perturb any orbits. It's really hard to have a three-body system if those three are close to each other. The only way to have a three-body stable system is if two of them are far enough away. So for example, the sun, the earth, and the moon is a three-body system. Why is that stable? Because the moon and the earth are fairly close to each other and both are fairly far away from the sun.

39:24If you bring that two-body system close enough to the sun, then it becomes a three-body system and the whole thing is chaotic. So basically, the sun is going to be pulling on those moons in a way that makes none of those orbits stable. So it sounds like you've just said that the Earth is the closest planet that could possibly have a moon in our solar system. Yeah. How much closer could the Earth be and still have a moon? Are we essentially at the limit? Yeah, great question. It is possible for Mercury or Venus to have a moon. It's just that their hill sphere is really, really small. So for example, for Mercury to have a moon, you'd have to have a moon that orbits like pretty close above the surface, which would be like a super high velocity.

40:07And then Mercury would have to have like no atmosphere at all because you wouldn't want any drag and no like mountains for the moon to smash into. So in principle, it is possible for Mercury to have a moon. And Venus is like a very thick atmosphere. So this would be tricky, but they don't have a moon and it'd be very hard for them to get one. You could bring the Earth closer to the sun and still have a moon. We're quite a cozy distance from the sun. I don't know exactly the number of how close it could be. OK, well, Daniel had hinted earlier that there is one other candidate for a moon for Earth.

40:40And after the break, he's going to tell you about it. And we're going to hear Kelly try to pronounce it. Oh, no.

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43:36We're back, and there is one other object in the sky that is a potential candidate for another moon for Earth. It is colloquially referred to as the space bean. And how is it pronounced, Kelly? Oh, I was trying to avoid it. Kruthny. I probably got it right on the first try, right? How would you say it, Daniel? Are you looking up the pronunciation? That's cheating. That's cheating. I have no idea how to pronounce this. It is such a weird word. I don't even know what the etymology of it is or why anybody would choose this unpronounceable word. C-R-U-I-T-H-N-E. Wait, you're not even going to try and you had me do it first?

44:23I was about to. This is your episode. You interrupted me as I was about to give it a try. I was going to say, I would call it chrythne also. But it's probably something weirder. You know, anytime you have these weird combinations of vowels in English, you're like, oh, actually, it's kreuthne or something. So I have no idea. Let's call it the space bean. That's great. And I'll note the three times you and I said it, we did say it differently each time. Oh, did we? We did. Yeah. Okay. All right. It's evolving through time. That's right. Tell me about the space bean. So obviously the Earth has the moon, which is the major moon and the real moon we consider.

44:59But there's also this funny object called the space bean. This is a rock like five kilometers in diameter. And it's not shaped like a bean. You might think, oh, does it look like a kidney bean or a pinto bean or something? It's just that it has a funny orbit. So its orbit is kind of bean shaped. So it's got this elliptical orbit. In principle, it's orbiting the sun and not the earth. But it's close enough to the earth that the earth is also tugging on it. And so it's in resonance with the earth. Like it's a complicated three body system that has found this stability to it. And so if you look at its orbit relative to the earth, it moves in this sort of bean shaped pattern, again, relative to the earth.

45:41It's not really orbiting the earth. I mean, sometimes the earth is on one side of the sun and the space bean is on the other side of the sun. So it's more like how Jupiter has asteroids that follow it and precede it in its orbit. because those are stable points, Lagrange points in the Jupiter-Sun system. Those are the Trojans, right? Yes, exactly. The Trojans and the Greeks. You got to keep them separated. That's right. I'll stop. Exactly. And in the same way, there are like some stable patterns in the Earth-Sun system and this object has fallen into one of them. Daniel, I have very strong opinions that this is not a moon.

46:26this just does not feel like a moon at all given what you've just told me what do you think yes or no thumbs up thumbs down nowhere in between i mean it's a fuzzy thing but i would have to say no right it probably also it's not really captured but it is gravitationally influenced by the earth it's not just in orbit around the sun on its own but the thing that pushes me against calling it a moon is it never really gets very close to the earth the closest it ever gets to the earth is like seven and a half million miles away, which is like 30 times farther than our current moon. So officially, astronomers call this a quasi-satellite, which is like such a fudge word.

47:06Wishy-washy. Exactly. It doesn't influence our songs or mythology. I'm not emotionally feeling any connection to the space bean. I say no. Even in our family, which are big lovers of beans and promoters of beans, in every part of people's lives. I don't think we're pro-space bean being promoted to moon. I call Beano on the space bean. All right. So Earth has one moon and the space bean. So let's take a step up and talk about Mars. Mars has two moons. And both of these moons are fascinating because both of them are very likely captured objects, not formed with Mars. And they also have awesome names, Phobos and Deimos.

47:50These things mean fear and dread. Oh, who named them? They were having a bad day when they named them, I bet. Dark, right? So dark. Oh, but Mars is the god of war, right? So of course you'd name the moons something dark. Yeah, exactly. And as we'll hear later on, the name of the moons does keep in the theme of the name of the planet, which is kind of cool, honestly. So Phobos is the closer one. It's larger. It's like 22 kilometers in size. It orbits Mars three times every Earth day and is slowly losing its orbit because of the atmospheric drag from being so close. So in 50 million years, just after Elon Musk finishes terraforming, Phobos is going to hit Mars.

48:34So that's going to be a cataclysm. Yeah. I know he's really worried about, like, the sun expanding and eventually destroying the Earth in a billion years. But, like, this is much sooner. Mm-hmm. But, you know, you could. Phobos is pretty small. Yeah. you could push Phobos to a different orbit or something. Oh, yeah, probably. You could strap a bunch of starships on it and push it to – it's not that big, right? It's 22 kilometers, so it's not enormous. I mean, there's so many problems with living on Mars. I don't know that this is the one that's the showstopper. No, just add it to your list. That's right.

49:07And Deimos is even smaller. It's 13 kilometers and slower. It orbits further out every 30 hours. And there was a recent visit to Deimos. We didn't land on it, but it came to a close approach. This is actually a satellite launched by the Emirates and their orbiter called Hope. So this Hope orbiter visited Deimos and looked at it carefully and learned something interesting. People had long thought that Deimos was a captured asteroid, and Phobos almost certainly is. But it turns out that Deimos actually has a composition more similar to Mars than we expected. So it probably isn't a captured asteroid.

49:43And yet it's in this weird orbit. so it doesn't seem like it formed with the planet. And the solution is that probably Deimos is an object that was tossed up from some other major collision. Wow. Right? So like something hit Mars, and as we talked about, this like blows up chunks of Mars out into space, some of which land on Earth, but some of which could be in just the right angle to end up in orbit. So Deimos is like a rejected piece of Mars. Oh, but that's not how I think of the moon. I don't think of it as a rejected piece of the Earth. Earth. You watch your language, Daniel. The moon might be listening.

50:19I mean, its name is Dread, okay? I don't think it needs to be dressed up in happy language. All right, all right. It's probably the goth equivalent of a moon, yeah. Mm-hmm. And these are moons we've known about for quite a while. They were seen in the late 1800s by astronomer Asaph Hall. So not as long as we've known about Jupiter's moons, but like, you know, 150 years or so. These are not recent discoveries. Wait, we knew about Jupiter's moons before we knew about Mars's moons? Yeah, Galileo discovered the first moons of Jupiter. Wow. Yeah, quite a long time ago. But those moons are huge, right?

50:54Some of those are big enough that they would be planets if they weren't in orbit around Jupiter. Holy cow. All right, so we've got Earth has one moon and a bean. Mars has two moons. Does anybody have three moons? No. No. All right. Give me four moons. Nope. Five moons? Five moons, we have Pluto, right? Now, Pluto, of course, famous for being in an astronomical fuzzy territory. Is it a planet? Is it a dwarf planet? Whatever. Also, it has this weird moon, Charon. So Charon is super weird because it's almost as big as Pluto. It's half the size of Pluto, right? And so it really challenges the whole concept of a moon.

51:38It's more natural to think of the Pluto-Sharon system as binary dwarf planets. Basically, the reason that Pluto was demoted from a planet to dwarf planet is that there's many things like Pluto out there. And so if you include Pluto, then you've got to include like hundreds and hundreds of these things. And people didn't want hundreds and hundreds of planets. Why not? I don't know, because they wanted to think planets are special because we're on one and they wanted to be a protected category. This whole thing is like so arbitrary and cultural and ridiculous. But anyway, if you demote it to a dwarf planet, then you really should think about Pluto and Charon as tidally locked binary dwarf planets.

52:16Whoa. And so because you're not referring to Charon as a moon, does that mean that the center of mass is somewhere between Pluto and Charon? Okay. Yeah. So technically it is a moon, but it violates that definition as well. Okay. Yeah. So really fascinating system. Plus, there's four more small moons. Hubble discovered these just like 20 years ago, that there's four more little bits orbiting the Pluto-Sharon system. So it's a really complex little system. They're orbiting each other, and then orbiting the two of them are four smaller moons further out. So it's crazy. It's crazy that we know any of this, in my opinion.

52:55It's happening so far away, and that's amazing. Go us. It's amazing. And there's probably more small moons around Pluto. We just haven't seen them. right? Because Pluto is super far away. And even Hubble is challenged to see tiny little moons that are not glowing, right? You can only see them when photons leave the sun, happen to bounce off of these dark objects and come back to earth. And so if that doesn't happen, you don't see it. If it's too small, you don't see it. And so there's almost certainly more moons of Pluto yet to be discovered. You could find them and name them after your dog. Oh, I like that idea.

53:31Milo and biscuit moons. But I did read the outline. So I know that even if we missed a handful of moons around Pluto, that would not give Pluto the win. So I'm guessing Pluto's not going to win. So who is next in our list? So next up in the ranking, we got to go all the way up to 16 moons. Neptune has 16 moons, all of which are named after water gods, which is super awesome. Because of course, Neptune, Poseidon, god of the sea. And Neptune has a really dramatic moon with probably a very dramatic history. So its biggest moon is called Triton. And it was actually discovered only 17 days after Neptune itself was discovered.

54:12That's exciting. Super fun. Exactly. And the whole history of the discovery of Neptune is really fascinating because you can see Neptune in Galileo's logbook. He was observing Jupiter and he was looking at the moons. And it just so happens that if you're looking in that direction in the sky at that time of year, that you would see Neptune. And he saw it there, but he thought it was a star. And so it's there in his logbook. You can go back and reconstruct it. And it wasn't until 200 years later that people figured out where Neptune was and saw it because of its pull on Uranus, a really whole fascinating scientific history story.

54:47I love when there's a discovery made, and then you can go back and find, oh, this data actually already existed. We could have made that discovery earlier if people had recognized it. Those are fun moments. Yes, amazing. Although not fun if you're alive to realize that you missed that moment. But what it says is that there's probably data in a logbook right now that is enough to support some crazy discovery and we won't realize it until somebody else figures it out. Anyway, Triton is incredible because it orbits retrograde to Neptune's rotation. So most moons orbit the same direction the planet spins because either formed with the moon or they're captured that way.

55:27But Triton orbits the opposite direction that Neptune spins, which is crazy. And probably this is because it's captured. It's some huge object that Neptune captured. I always thought that retrograde was just a word you heard in astrology horoscopes. No, it just means that you are orbiting the opposite of the, okay. Yeah, exactly. Like when planets go into retrograde, it's because relative to the Earth, they're moving the opposite direction. Wow. Okay. Yeah. And so probably what happened here is that Neptune had a tidy little set of moons, and then Triton came in and destroyed all of them. Because it came in and it went the wrong direction.

56:07It's like driving a semi the wrong way on a freeway, right? Not cool. Yeah. And so it looks like all the other moons, the other 15 moons of Neptune, are reaccretions of the rubble disk from Triton's capture. So Triton comes in, destroys all the moons, and then eventually they gather themselves together into these pathetic little objects, you know, just remnants of this collision with Triton. So how could you know the difference between moons that were there before and moons that recollected after Triton destroyed everything? So they're not in tidy circular orbits around Neptune. So they probably are not there historically since the beginning, right?

56:45Because they're affected by this collision. And they seem to be all sort of mixed up, a various mishmash of the moons, and they're sort of loosely held. And so they haven't formed together for a long time. Amazing. And then is that all of Neptune's moons? Those are the ones we found. And these distant planets probably have many smaller moons orbiting far out that we just haven't been able to see. But next up on the list is Uranus. Uranus has 29 moons. And these are so many that it's useful to categorize them. You've got the inner moons. These are like really small objects just above the Roche limit, just able to hold themselves together.

57:25And many of these were found like last year, so 2025, with James Webb Space Telescope. They are so small and dark that they are not visible in the optical. You can only see them in the infrared. And so that's why James Webb can see them. Move a little bit further out, and you've got five major moons. Some of these are big enough to have things like volcanism and internal magma and flow on the inside of them. The largest one is Titania, which is one-twentieth of the mass of our moon. And this we've known about for like more than 200 years. Are you going to make a Titania Uranus joke? Kelly is lost at people.

58:11I have been trying to not make any comments because I feel like there's nothing good that I can say. But, you know, all the other moons were named after things similar. And so, you know, I'm wondering, you know, why, you know, why is it Titania and not, you know, other butt related things. And so I'm just, I'm not going to, okay, but I'm going to focus on the science and ask you, Daniel, is this the first moon we've talked about that could have volcanism? It is. Yes, exactly. What an adult I am right now. For those of us just after the holidays who've eaten a big meal and then, you know, produced something titanic of our own and wondered, I wonder if that has its own gravity and could produce volcanism of its own.

58:57You know, yes, exactly. And so the history here is also funny because it was discovered in 1787, just after the planet itself was discovered. But the guy who discovered Titania also claimed the discovery of four more moons, which don't exist. So they were like spurious moons, which later people were like, yeah, I don't see those. I don't know what you were looking at. And so the history here is a little bit checkered. Did he name the other moons like rectum, orifice? Yes. Keister. Moving on. Okay. Outside of the major moons, there are 10 more irregular moons, some of which just discovered in the last couple of years.

59:39So, you know, this is an area where we are actively learning new things about these planets. Amazing. And what makes an irregular moon? Is it the shape? Mm-hmm. These things are not, like, big enough to have pulled themselves into spheres. And so they're, like, weird blobs. Some of them also with weird orbits, probably captured objects. Cool. All right. So then stepping up to Jupiter, this was most people's candidate for having the most moons just because Jupiter is the mostest of most of the stuff, right? It's got most of the non-sun mass in the solar system. And indeed, it has almost 100 moons.

1:00:13Whoa. 97 moons counted for Jupiter so far. The largest four of which were discovered hundreds of years ago by Galileo. Whoa. Way to go, Galileo. Yeah, exactly. Wait, could we all count as moons of the sun? Should the sun win? Are you in moons of the sun? Well, I think moons of the sun we call planets, right? Yeah, yeah. Okay, I'm sorry. I'll put down the banana peels. Let's, all right. The last four discovered by Galileo. Way to go, Galileo. Let's talk about Io. I like Io. Yeah, these moons are huge and likely formed with Jupiter. They're in nice orbits around Jupiter. Io is bigger than our moon.

1:00:52A lot of people think the moon is the biggest moon in the solar system. Not true. Not even the second biggest moon in the solar system. Io is not even the biggest moon in the solar system, but it is larger than our moon. What moon is the biggest, Daniel? We'll get there. Oh, okay. All right. All right. But Io, of course, super awesome because it has hundreds of active volcanoes on it. And some of these things shoot plumes out like 500 kilometers above the surface. It's the most geologically active object in the solar system. Yeah, the reason it's got so much going on is that it's pretty close to Jupiter.

1:01:27And so these tidal forces from Jupiter are not strong enough to pull it apart, but they do squeeze it into a football. And then Io also is orbiting. So which part of Io is getting squeezed into a football shape is changing. So sort of from Io's point of view, if you're just looking at it, it's a football, but like different parts of it are getting footballed as it orbits Jupiter and as it spins. So that creates a lot of internal friction. It's just like Jupiter is reaching out with huge cosmic hands and squeezing this thing. It's like kneading dough, right? And so this is tidal heating. So just this gravitational interaction between Io and Jupiter is enough to heat up the inside of Io.

1:02:07And that's where you get all these flows inside of it and this cracking of the surface and all these volcanoes. So is Io really hot in some spots then? Inside of it is very toasty. Yes, absolutely. All right. Right. But Europa, is Europa really hot? Europa is fascinating because it's icy on the surface, right? So it's got like a crust of ice. But we've done studies of it and we've seen that Jupiter's magnetic field creates a current inside Europa. What? Which means probably there's salt water, which is capable of conducting electricity inside. So probably you have like 10 kilometers of ice and below that, maybe like 100 kilometers of subsurface ocean.

1:02:51And we think that that's water because as you get closer to the center of Europa, like with many of these moons, tidal heating makes things warm. And so not only is it directly heated because of the friction, but also they're probably like geothermal vents or hydrothermal vents on a moon where heat from the core from this tidal heating is then leaking up, equivalent to like a volcano. So you could have enormous quantities of liquid water under this frozen surface on Europa. I would love to know if there's life there. I know, right? So much possibility there for life. And the cool thing is that sometimes it shoots stuff out into space because you get cracks and geysers.

1:03:30And there's a mission being sent Europa Clipper to go and sample these things and look for microbes or whatever, tiny alien octopi or something. So exciting. Okay, all right. Let's get to the largest moon now. So the largest moon in the solar system is Ganymede, also Jupiter's moon. This thing is two times the mass of our moon. Whoa. So if you're impressed by our moon, Ganymede is bigger than our moon by a factor of two. Well, don't rub it in. Our moon's doing a good job. Ganymede is big enough to have an atmosphere, probably has a lot of oxygen in it. Also very likely to have a subsurface ocean.

1:04:09We think it has a metal core. This thing has its own magnetic field. It's a monster. But I'm guessing you wouldn't want to live there because proximity to Jupiter would make it uninhabitable. Is that right? Yes. Jupiter puts out a lot of radiation. It's not a star like the sun, and it's not even a brown dwarf. There's no fusion happening inside of it, but there's still a lot of radiation being pumped off of Jupiter. So not a safe place to live without a lot of shielding. But often the place you'll find settlements in very well-written hard sci-fi novels, for example. Like The Expanse. Exactly.

1:04:41All right, so Jupiter's got 100 moons. Are there any other moons that we need to talk about before we move on to our winner? So the third largest moon in the solar system is also a moon of Jupiter. It's Callisto. Callisto is super interesting because its surface, unlike some of the other ones, is very, very old. So lots of craters on the surface of Callisto. Europa, in contrast, has like a really smooth surface, very young surface. It's constantly being reformed. So you get a crater, it gets deleted. Callisto is showing all of its scars, which means it's a great way to understand the history of the solar system.

1:05:13Like, when was there a lot of impacts? When wasn't there? Callisto sounds beautiful, but in a different way, Daniel. All right, so Daniel, drumroll, please. The drama's been removed by the process of elimination, but the winner is Saturn. And not by a little bit. Saturn has 274 moons. What? Right? Jupiter at 97, Saturn is 274. It's crazy. And these moons are amazing also. The biggest one is Titan. This thing is more massive than the planet Mercury. Okay. It's not a small moon. It's huge. We've known about it for hundreds of years. It was discovered by Huygens in 1655. Another one of my favorite moons in the solar system is Enceladus.

1:06:03This thing, like Europa, emits jets of ice, probably because it has a subsurface ocean. It's another great candidate for where life could form. But maybe my favorite moon in the solar system is this moon of Saturn called Iupetus, which is the craziest name for a moon, but it's shaped like a walnut. It has this enormous ridge all the way around its equator, like this vast set of mountains. And then the top half of it is black and the bottom half of it is white. It's like a black and white cookie. Yes. This thing is crazy. Exactly. Somebody took a huge space walnut and dipped half of it in frosting.

1:06:42It's unbelievable. I'm in. So Saturn has all of these moons. Most of them are very far from Saturn. Like 250 plus of these moons are distant from Saturn, orbiting with high inclinations, almost certainly captured objects of Saturn. Saturn is in a great place to capture all of these objects, many of which were like scattered by Jupiter. So it's sort of like the garbage collector of the solar system. Okay. Yes. You were able to tell us that Mercury and Venus are too close to the sun. That's why they probably don't have any. And Saturn probably has the most because it's just in a good position to pick up junk.

1:07:17Exactly. It's got a lot of gravity and it's far away from the sun and it's nearby Jupiter, which creates lots of stuff tossing off of it. Exactly. So over the years, it's picked up a lot of moons. Amazing. If you could visit any moon, Daniel, would it be Yappetus? Is that how you said it? Well, there's another moon of Saturn called Rhea, which they think might have rings, right? Because moons can have rings. Usually that's unusual because the tidal effects of the planet will disrupt it. And this is why, for example, moons tend to not have moons, but it's easier to have rings than moons because they're basically already torn into shreds.

1:07:55So that would be pretty awesome to be on a moon of Saturn, see the rings of Saturn and the rings of Rhea around it. That would be pretty awesome. That would be absolutely epic, yeah. And not included in today's list are the 334 minor planets in our solar system that have their own moons. Wow. These things you might call moon moons or moonlets. Aw, moonlets is cute, but moon moons is funner. Yeah, exactly. I mean, moon moons technically reserved for moons that have their own moons. We haven't found any of those yet. It depends on whether you call these minor planets moons. And we're all looking forward to the day when we can discover exomoons, moons around planets in other solar systems.

1:08:39It's particularly tricky because the techniques that we have for finding exoplanets are good at finding planets like really close to the sun that are really big. And it's hard for those planets to have moons. where they're so close to their star. But people are working on this, and direct imaging of planetary disks might help us discover exomoons. I am hoping that this happens in my lifetime. I don't know. There's been a lot of, I mean, just hearing you talk today, there's been a lot of cool stuff in our solar system that's been discovered in our lifetimes. I don't know. Maybe we'll start discovering even more cool stuff in other solar systems in our lifetimes.

1:09:13I don't think it's impossible. Yeah, and I should say there are already candidates for exomoons, because in some of these eclipse methods where like the planet passes in front of the star, you can see deviations from those that might be consistent with a moon going around that. But these are just candidates and unconfirmed. And of course, there's lots of controversy. So I think we might be on the verge of discovering exomoons. Well, stay tuned. If we find moon moons or exomoons, we will let you know. And one day when we visit another system, studying their moons will help us understand the history of that solar system, the cataclysms, the captures, the collisions.

1:09:50everything that went down in their cosmic chaos. All of the chaos from the kind of boring stuff that Daniel will still tell us about anyway to the much more exciting stuff, all of which we'll explain in an exciting way here on Daniel and Killers. It's all exciting, just some of it's more boring anyway. Thanks for tuning in, everyone, and for sticking it out for this countdown for which planet has the most moons.

1:10:19Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you. We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at danielandkelly.org. Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe. Don't be shy. Write to us. This July 4th, come celebrate at America's Block Party, hosted by America 250.

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