Which planet has the most moons?

Daniel and Kelly revel in the amazing multiplicity of moons in our solar system and what they reveal about our cosmic history. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

  This is an iHeart Podcast. Guaranteed Human.

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• 1:12Daniel Whiteson?

  The moon really doesn't get enough love. Of all of the features of the night sky, it's the only one with real texture, visible to the naked eye. Everything else is either just a tiny distant point or an overwhelming ball of fire. There's nothing else out there that you can just sit and stare at and appreciate the feeling of looking through a gulf in space. We are here on a rock in space, and there's another whole rock right over there, so far away, yet so weirdly big that we can see it and see features on it. It challenges the mind, forcing you to come to grips with our cosmic context of massive balls of rock spinning through a dark ocean.

• 1:57Daniel Whiteson?

  And there's so much more that moons do. Moons tell us about the history of the solar system and help reveal what's going on inside their planets. They have volcanoes and weird colors and shoot jets of water into space. They might even harbor life. Seeing Jupiter's moons is what led Galileo to understand the structure of our solar system. And the thing that our moon has misled us about is their number. Having one big fat moon is not normal. Planets in our solar system have many moons, so many fabulous moons. So on this episode, we're going to give moons the love they deserve. Welcome to Daniel and Kelly's extraordinary Moon-licious Universe.

• 2:55Kelly Weinersmith?

  Hello, I'm Kelly Weinersmith. I study parasites and space, and I am not above jokes that involve mooning.

• 3:03Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist, and I can't tell you how long it's been since I mooned somebody.

• 3:09Kelly Weinersmith?

  Oh, yes, that was a joy to, uh, to hear about mooning jokes when I was a kid. I don't remember actually mooning anyone in my teens. I was a bit of an insecure teen, but, uh, but, uh, yes, I remember being mooned as a teen.

• 3:24Daniel Whiteson?

  And as an adult, Kelly, can I ask you, how long has it been since you mooned anybody?

• 3:28Kelly Weinersmith?

  I plead the fifth. I married a man whose last name is Weiner, and so my sense of humor has become a bit more juvenile.

• 3:39Daniel Whiteson?

  Fill in the gaps yourself, everybody.

• 3:40Kelly Weinersmith?

  That's right. All right, so Daniel, my question for you is, so today we are talking about moons. Have you ever heard of a convincing story of the moon influencing someone's behavior?

• 3:53Daniel Whiteson?

  Oh, I mean, I can imagine the moon influencing the tides, and the tides definitely influence people.

• 3:58Kelly Weinersmith?

  Mm hmm. But you don't buy the like, you know, more people come into the ER on a full moon.

• 4:03Daniel Whiteson?

  People do all sorts of weird stuff, and I wouldn't be surprised if people acted weirder on a full moon and there were more visits to the ER. So I'm gonna have to plead the fifth on that one.

• 4:15Kelly Weinersmith?

  Yeah, I don't, I mean, I don't think I buy that, uh, people are like more kooky on full moon nights. Unless they have been convinced that they ought to be more like risk-taking on full moon nights, and it's on their mind, then I can imagine. Like they sort of have psyched themselves out. But I do like if I get up early in the morning and it's brighter because of a full moon, I'll be more likely to like go jogging or something. Uh, and so I can imagine maybe people going out and doing more stuff outside during a full moon and maybe being more likely to like accidentally get hit by a car or something like that. And of course, we were talking about how some organisms are influenced by the tide. And so their circadian rhythms are influenced by the tide. So their behaviors are influenced by the tide.

• 4:59Kelly Weinersmith?

  But yeah, I don't think I buy that animals or people are like more likely to be doing kooky, crazy stuff, uh, because, because it's a full moon or anything like that. But, uh,

• 5:09Daniel Whiteson?

  I don't know. People are weird. And I think if you work in an ER, you see the weirdest side of people. I remember Katrina worked at a hospital in Geneva and near the entrance, they had a huge display of the weirdest stuff they had pulled out of people's throats and other holes.

• 5:26Kelly Weinersmith?

  Oh, no.

• 5:26Daniel Whiteson?

  And boy, was there some weird stuff there.

• 5:29Kelly Weinersmith?

  Oh, I, I did once for a research project, uh, look for papers describing things that had been pulled from orifices. And we are an inventive species. And, uh, if it turns out that our inventive behaviors are tied to full moons, then we might be happy that we have only one moon and not many moons.

• 5:52Daniel Whiteson?

  And the moon figures so prominently in the sky and in our literature and in our imaginations, that it's easy to imagine that aliens would look up at their sky and see the same thing. Or if you're a science fiction author, to imagine aliens seeing a radically different sky, many moons or something similarly weird. And there are lots of examples in science fiction of multiple suns or multiple moons or other variations on our experience. But you don't actually have to go that far, visiting another solar system to see examples of multiple moons.

• 6:25Kelly Weinersmith?

  And if I can just plug my friend's book for a second, Under Alien Skies by Phil Plate is an amazing book describing what it might be like to visit alien planets and look at their skies. And Phil was a guest on our show early on when you and I Started podcasting together.

• 6:40Daniel Whiteson?

  That's right. And everything he writes is insightful, well informed and fun. So go check it out.

• 6:45Kelly Weinersmith?

  Yes. All right. So you asked our extraordinaries, which planet has the most moons?

• 6:52Daniel Whiteson?

  That's right. And if you would like to contribute for this segment of the show in future episodes, please don't be shy. We would love to add your voice to the chorus. In the meantime, think about it for yourself for a moment. Which planet in our solar system do you think has the most moons? Here's what the extraordinaries had to say.

• 7:11Speaker 04

  I believe it's Jupiter that has the most moons. I think they just discovered a couple more, but technically really depends on what you define as moon because rings are just particles orbiting the planet as well.

• 7:24Speaker 05

  In our solar system, the planet with the most moons is probably Saturn. Outside of the solar system, I have no clue.

• 7:30Speaker 06

  I would say Jupiter because of how big it is, but then it might've pulled some moons into it. So maybe Saturn has the most because it sits in a sweet spot. What do you think, Glenn?

• 7:42Speaker 07

  Uh, I'm going straight for Jupiter.

• 7:45Speaker 06

  Why?

• 7:46Speaker 07

  Because it has the most moons.

• 7:48Speaker 06

  All right. My initial thought would be Saturn or Jupiter, but I've been reading recent news reports about many new discoveries of moons around Jupiter.

• 7:57Daniel Whiteson?

  So I'm going to go with Jupiter. All right. So most votes for Jupiter. Kelly, if you hadn't read the outline, what would you have voted?

• 8:04Kelly Weinersmith?

  Oh, I love that you think that I read the outline.

• 8:08Daniel Whiteson?

  Giving you the benefit of the doubt here, just in case.

• 8:10Kelly Weinersmith?

  No, I'm kidding. I did read the outline. So I would have guessed Saturn or Jupiter, or I would have guessed that because this is a DKU episode, maybe the answer is, we don't know. Because maybe we, maybe it's hard to count all the moons and we aren't sure if Saturn or Jupiter is the winner yet.

• 8:30Daniel Whiteson?

  That's probably the right answer, actually. Also, it depends on what you mean by moon, a definition which is always evolving as we discover more stuff out there in the solar system. Because though humans like to make tidy categories for the things that are orbiting the sun, in reality, the universe is chaotic and there's just like a huge, smooth spectrum of stuff, from tiny little dust grains all the way up to Jupiter and basically everything in between. And if you try to put everything in boxes and make artificial dotted lines to separate it, you'll find a bunch of stuff in fuzzy categories and you'll argue about whether it's a moon or whether it's not a moon, and that will probably change the answer.

• 9:09Kelly Weinersmith?

  Well, at least that's a bit more satisfying than like our discussion on the Oort Cloud, where you're like, actually, we're not even really sure there's an Oort Cloud. And so I was a little worried you were gonna be like, actually, we're not even sure that there's moons, that could be an optical illusion. But anyway, okay, we're sure that there's moons, we're not quite sure where the cutoff should be, but we've arbitrarily said it's somewhere.

• 9:30Daniel Whiteson?

  Yeah, exactly. So first, let's clarify what we mean by a moon. And this has a fascinating history, even just the word is fairly recent and modern. Astronomically, the category officially is natural satellite. And we use the word moon sort of colloquially after the moon of Earth. Historically, people called our moon a planet, like until Copernicus in 1500, the term planet was basically used to describe like things that move in the sky, which we assumed to move around the Earth. And the moon was just like another thing, like the other planets and like the sun that people assumed moved around the Earth. Then, of course, Galileo saw the moons of Jupiter and thought, ooh, the planets themselves

• 10:15Daniel Whiteson?

  are like mini systems. And so you can have this hierarchical structure. And so it's not required for everything to orbit one thing or to orbit the Earth. And so that gives way to a more nuanced structure of the solar system. And now you have to have different words to define these things that are not directly orbiting the sun, but are orbiting something that is orbiting the sun.

• 10:36Kelly Weinersmith?

  Okay, now, hold on. So recently, you asked me what killifish means, and then you asked me what hymenopterin means.

• 10:43Daniel Whiteson?

  Oh, no. Are we about to get some linguistic revenge?

• 10:46Kelly Weinersmith?

  Gosh, I hope so. Because usually when I'm like, oh, I'm about to get Daniel, you're like, oh, blah, blah, blah, blah, blah, blah, because Daniel just knows. But I see you're about to hit your keyboard. Does the word moon mean something? Did they take that from something else?

• 11:00Kelly Weinersmith?

  Yes. Yes.

• 11:04Daniel Whiteson?

  So, you know, the Latin name for the moon is Luna, but the word moon itself actually comes from the Old English word, which comes from a Germanic word, which comes from a Proto Indo-European word, which might be related to the measurement of time. And so, yes, the moon has this like ancient historical meaning connected with, you know, the passage of time. And that's super fun because we know that like looking at patterns in the sky is how a lot of ancient peoples first developed like astronomy and mathematics and, and, you know, trying to predict the future. So it's like a deep rabbit hole all the way to the origins of astronomy.

• 11:39Kelly Weinersmith?

  Okay. I mean, that's, it's not quite as much fun as I was hoping, but like sort of fun. All right. So.

• 11:45Daniel Whiteson?

  You were hoping it was related to butts, weren't you?

• 11:46Kelly Weinersmith?

  I mean, probably like, yes, the, the act of dropping one's pants is what I was hoping for. But, uh, maybe this is, I gotta keep this kid friendly. Also, teeny pet peeve of mine, I feel like whenever you're referring to the moon of Earth, you need to be careful to capitalize it because we're referring to a specific moon. And then the other moons can get a lowercase because we're referring to moons in general.

• 12:11Daniel Whiteson?

  Exactly. And, you know, for a long time, we referred to all of these things as just satellites. So instead of calling them moons, you would say the satellites of Jupiter. Then with Sputnik and the advent of artificial satellites, you know, half a century ago, it became very awkward to constantly say, artificial satellite, natural satellite, artificial satellite. And, you know, English is constantly smoothing and shortening things. And so people just started referring to artificial satellites as satellites. And then instead of referring to natural satellites, it is called everything moons, sort of colloquially after our moon. So technically, astronomically, we have natural satellites and artificial satellites. But more practically, we have satellites, which mean artificial satellites,

• 12:54Daniel Whiteson?

  which are not moons or natural satellites, and then natural satellites that we call moons.

• 12:59Kelly Weinersmith?

  And I think the word Sputnik is just the Russian word for satellite.

• 13:03Daniel Whiteson?

  Yeah. And then aside from the linguistic fuzziness, there is also this question. Like, well, what makes something a moon? Like, it's pretty clear if you're looking at the Earth and the moon, the Earth is bigger. And so you would say that the moon is a moon and the Earth is a satellite of the sun. But physically speaking, the Earth and the moon are orbiting each other, right? And there's a center of mass of the Earth-moon system, and that is what's orbiting the sun. And so it's a little bit arbitrary to say, okay, this one's a moon and this one's a planet. You can imagine, for example, a scenario where you have two objects of the same mass orbiting each other, like a binary dwarf planet system, which is the moon, which is a planet, right? You need some way of categorizing it.

• 13:46Daniel Whiteson?

  And so in our solar system, typically, if the center of mass of the system is within the surface of one of the objects. So it's like, you know, where is the average bit? The average bit is under the surface, like between the Earth and the moon. The center of mass of the Earth-moon system is within the volume of the Earth. Then you say, that's the planet. And anything else outside of that is a moon. So a little bit arbitrary, but it's helpful to settling debates among astronomers, which you have to have a reason for those guys to stop drinking and go to sleep. And there's the other side of it, which is like, well, what's the smallest possible moon, right?

• 14:29Daniel Whiteson?

  Like, is every dust grain that's orbiting the Earth a natural satellite? If it formed naturally, like, is there a lowest cutoff? Is every proton that's in orbit around the Earth a satellite? Technically, according to the definition of natural satellite, yes, those are natural satellites. You don't count them as moons of Earth. And later, we'll talk about how the Earth actually does have a weird second body that's kind of orbiting it, but not really that you could argue is kind of like a second moon of the Earth. Anyway, the point is, it depends on the definition. And so, yeah, Kelly, really, nobody knows.

• 15:06Kelly Weinersmith?

  Yes, yes, I knew I was right. That's great. But it does seem like, you know, you you talk to 99.99999% of the people on the planet, they'd say that Earth has one moon. And is that just because that's what we were told as kids and it's stuck? But if you talk to an astronomer, they'd be like, maybe we have a billion. Or like, is there essentially a definition that we all agree to for convenience sake?

• 15:33Daniel Whiteson?

  There is really no lower limit, but we call these things moons because they're interesting. And they reveal the structure of the solar system. They affect planets because of their tidal forces. And tiny little dust grains don't do that as much. And so they really are sort of a different kind of thing, even if it is an arbitrary dotted line that we draw, and there really isn't a place to put it. It doesn't make sense to put an individual proton in the same category as the moon of Earth, just because they both orbit the Earth. And so there is a qualitative difference, even if there isn't a crisp line that we can draw. And so, no, I don't think any astronomer is going to be like, actually, there are many moons of Earth. But, you know, probably, maybe there is one out there somewhere.

• 16:17Kelly Weinersmith?

  Is that what you think astronomers sound like? Because I think you you got to watch it. There might be some astronomers that listen to this show and their feelings might get hurt.

• 16:24Daniel Whiteson?

  No, I think most astronomers sound like they're really fit and very suave, but just the annoying ones sound like that.

• 16:31Kelly Weinersmith?

  Got it. Okay. So so you said we have like this arbitrary definition. Does this arbitrary definition have some features? Or is it just like, is it always a gut feeling? Or do we have some criteria, even if it was happened upon somewhat arbitrarily?

• 16:50Daniel Whiteson?

  So typically planets have something like 10,000 times the mass of their natural satellite. That's what we tend to see in the solar system. But, you know, there are already exceptions to that, like the moon is 1/80 of the Earth. Pluto has a moon that's 1/8 of its mass. Right? And so like, you can make these broad categories. Almost all the moons in our solar system have a mass that's less than 1/10,000 of the planetary mass, but they're already made. Major exceptions.

• 17:20Kelly Weinersmith?

  Hmm. Well, let's take a break. When we come back, we'll talk about why we have moons. And if there are any kids hanging out with you while you're listening to this episode, kids don't get any ideas. No mooning your parents during the commercial break. When we get back, why do we have moons?

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• 19:46Kelly Weinersmith?

  And we're back. So, Daniel, we talked about how moons are defined, but why do we have moons in the first place? And why did Earth, clearly the greatest planet in the solar system, only get one?

• 19:58Daniel Whiteson?

  Yeah, this is the most interesting thing about moons, in my opinion, is that they reveal something about the history of the solar system. If you read about the solar system and look out at the sky, you get the impression that it's this sort of calm parade, this like slow-moving trudge through space that's been going this way for a long time and gonna be going this way forever. But that's only because we're used to living at a sort of human time scale, seconds, minutes, even centuries. And in those time scales, yeah, not much really happens in the solar system. But the solar system is very old. It's four and a half billion years old, and its history is filled with chaos. All sorts of crazy cosmic cataclysms have occurred in our solar system,

• 20:42Daniel Whiteson?

  from planets moving in and out and switching orbits and losing planets. And moons are a great way to understand this history because they are, in effect, records of this history. And if you understand why a planet has a moon, you can understand something about what happened to that planet. So there's basically three different ways that planets can get moons, from the least interesting all the way up to the most exciting.

• 21:05Kelly Weinersmith?

  Well, I bet we are all super excited for you to start with the least interesting explanation.

• 21:11Daniel Whiteson?

  Wasn't that a great lead-in, right?

• 21:13Kelly Weinersmith?

  What a great way to lead into that.

• 21:15Daniel Whiteson?

  Ready to get bored? Here we go.

• 21:17Kelly Weinersmith?

  I'm in.

• 21:20Daniel Whiteson?

  Well, you don't want to drop people with the most exciting one, and then, you know, they're sort of spent and like, I don't want to listen to the rest of this. You gotta build up to it, don't you?

• 21:27Kelly Weinersmith?

  But you don't even have to tell them that there's a least interesting option. Just get them. They're all great.

• 21:32Daniel Whiteson?

  All right. Well, the least interesting is already very exciting because it tells you about the formation of the planet. So sometimes moons form at the same time as the planet. In the same way that like the structure of our solar system didn't just make a sun, but also made a bunch of planets, as those planets Planets are forming. They don't just make a planet. They also make their own little orbiting guys.

• 21:55Kelly Weinersmith?

  And gals.

• 21:56Daniel Whiteson?

  And gals, space gals. I like that. So why does the solar system have this kind of structure? Well, remember that the solar system formed from a central blob that collapsed gravitational. You have like a seed somewhere where gravity started this runaway effect. It got denser, so it had more gravity, so it pulled on stuff, so it got denser, so it had more gravity, et cetera, et cetera. But you never just have exactly one seed. Sometimes you have like another nearby smaller seed that resists being pulled in because it has enough relative velocity. So it goes into orbit and it starts gathering its own stuff. So that's how planets form, right? You have this central blob which absorbs most of the mass. But then you have these other seeds nearby which sometimes form stars and you get like binary star systems.

• 22:39Daniel Whiteson?

  But sometimes they're smaller and they'll form planets. In the same way, as that planet is forming, it makes a protoplanetary disk, right? It's a big swirling mass of stuff, most of which will collapse into the planet. But some of it is going fast enough and has its own little gravitational seed. So can form an object which comes into orbit around the planet rather than falling in.

• 23:01Kelly Weinersmith?

  Ah, okay. And if it were to slow down, it would fall in, but it's not slowing down.

• 23:06Daniel Whiteson?

  Exactly. If it were to slow down, then it would fall in. And, you know, in that initial planetary disk, there is a lot of friction. And so a lot of stuff does fall in. But if you survive that and you form, then there's much less friction because things have cleared out and you can mostly orbit in a stable way. But it depends also on your distance from the planet. So for example, if you're too close to the planet, you're going to be feeling its tidal forces. The planet is a huge gravitational object. And remember that gravity depends on distance. The 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

• 23:50Daniel Whiteson?

  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. If 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.

• 24:33Kelly Weinersmith?

  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. And 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.

• 24:57Daniel Whiteson?

  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.

• 25:05Kelly Weinersmith?

  Okay.

• 25:05Daniel Whiteson?

  Right. 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. If 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.

• 25:41Kelly Weinersmith?

  Rings that formed with the planet never were moons, but could become moons.

• 25:47Daniel Whiteson?

  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.

• 25:58Kelly Weinersmith?

  So this sounds like an unlikely path to moonhood.

• 26:02Daniel Whiteson?

  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. It 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.

• 26:46Kelly Weinersmith?

  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 that... Oh, wait, or is the moon getting farther away from us?

• 26:59Daniel Whiteson?

  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.

• 27:07Kelly Weinersmith?

  Okay, are we gonna lose the moon then?

• 27:09Daniel Whiteson?

  Eventually, the Earth and the moon are going to get tidally locked, which will stop this process.

• 27:13Kelly Weinersmith?

  Okay, good. Phew. Okay, so it sounds like this might not be how we got our moon.

• 27:20Daniel Whiteson?

  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. So 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.

• 27:58Kelly Weinersmith?

  Whew, we got through the boring stuff. All right. Actually, I thought that was really cool. But okay, so let's move on.

• 28:05Daniel Whiteson?

  Relatively boring.

• 28:07Kelly Weinersmith?

  Relatively boring. All right.

• 28:08Daniel Whiteson?

  Super exciting, just not the most exciting thing we'll talk about today.

• 28:12Kelly Weinersmith?

  All right, all right. So let's move on to something way more epic. What's the next method?

• 28:16Daniel Whiteson?

  The next method is to capture something. Imagine an object is floating through the solar system and it comes near a planet. Planets have a lot of gravity. If it comes in at the right velocity and the right distance and 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.

• 28:57Kelly Weinersmith?

  How many of these do we think we have in our solar system? Because that seems like, depending on where they came from, a really cool opportunity for science.

• 29:04Daniel Whiteson?

  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 you have to get before the earth's gravity dominates over the sun's gravity? That's called the Hill sphere. And if a rock enters the Hill sphere of a planet, then it has a chance to become captured.

• 29:42Kelly Weinersmith?

  And was Hill a guy or a gal? Was Hill an astronomer? Presumably not one with the voice like you were making before.

• 29:53Daniel Whiteson?

  Yes, Hill was an astronomer and somebody 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.

• 30:27Kelly Weinersmith?

  It 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?

• 30:47Daniel Whiteson?

  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. You 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.

• 31:31Daniel Whiteson?

  And most likely, chaos causes us to lose things. Being 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.

• 31:55Kelly Weinersmith?

  So most of the stuff that does get captured and become a moon, do we know where it tends to come from?

• 32:02Daniel Whiteson?

  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. And as you'll see, the ones we have studied, we already have a lot of questions about.

• 32:13Kelly Weinersmith?

  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.

• 32:28Daniel Whiteson?

  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. This 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.

• 33:10Daniel Whiteson?

  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 it had fairly recent activity on it. Also, recent studies of the mantle of the Earth have shown like actual deposition from this original protoplanet 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.

• 33:44Kelly Weinersmith?

  All 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.

• 33:53Daniel Whiteson?

  Yeah, exactly.

• 33:54Kelly Weinersmith?

  But it's awesome that it happened long ago.

• 33:57Daniel Whiteson?

  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 large system, as long as, of course, nobody was living on it, no critters were harmed in this cosmic collision.

• 34:08Kelly Weinersmith?

  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? Like maybe junk would get thrown off Mars and would hit us? Yeah, would anything happen to us if this happened to Mars?

• 34:23Daniel Whiteson?

  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' garbage lands on Earth. So if Mars gets like massively slammed by a huge impactor and basically shredded, then some significant fraction of it is definitely gonna hit Earth, yeah.

• 34:57Kelly Weinersmith?

  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?

• 35:09Daniel Whiteson?

  For sure. Plus, we could get samples of Mars here on Earth.

• 35:12Kelly Weinersmith?

  You're the worst.

• 35:13Daniel Whiteson?

  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 gonna 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.

• 35:32Kelly Weinersmith?

  One of us studies life, and one of us studies non-life, and it is very clear which one does which right now.

• 35:39Daniel Whiteson?

  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.

• 35:54Kelly Weinersmith?

  All right, and so, we know, unfortunately, that the answer is not Earth.

• 36:02Daniel Whiteson?

  That's right. And in short-standing tradition established on this episode, we're gonna start with the least exciting examples first.

• 36:09Kelly Weinersmith?

  That's right. Okay, and so, well, should we start closest to the Sun?

• 36:15Daniel Whiteson?

  Yes, 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.

• 36:26Kelly Weinersmith?

  Why?

• 36:27Daniel Whiteson?

  The reason is that they are so close to the Sun that the Sun's tidal forces will perturb any orbits. Like, 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. If 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 gonna be pulling on those moons in a way that makes none of those orbits stable.

• 37:07Kelly Weinersmith?

  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.

• 37:12Daniel Whiteson?

  Yeah.

• 37:13Kelly Weinersmith?

  How much closer could the Earth be and still have a moon? Are we essentially at the limit?

• 37:18Daniel Whiteson?

  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. And 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.

• 38:02Kelly Weinersmith?

  Okay, well, Daniel had hinted earlier that there is one other candidate for a moon for Earth. And after the break, he's gonna tell you about it.

• 38:11Daniel Whiteson?

  And we're gonna hear Kelly try to pronounce it.

• 38:14Kelly Weinersmith?

  Oh, no.

• 38:23Speaker 01

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• 38:53Daniel Whiteson?

  Hey, I'm Nile Rodgers, producer, songwriter, guitarist, and co-founder of the group Chic. A great song has a way of defining a moment, a moment of joy, heartbreak, or anything in between. This fall, I'll be driving across the country, dropping in on some of the coolest, most fascinating artists working today. We'll go deep into their lives and careers and hear what it was like before the moment that made them. From hitmakers like Miguel... That last year, finishing that record was the hardest experience of my life. Regardless of how everyone else buys into this, I believe in this and I'm gonna push.

• 39:39Daniel Whiteson?

  To rising stars like Alex Warren.

• 39:42Speaker 06

  You know, a song is a story and the verse is setting the scene. And for me, at least, it's allowing me to be personal and vulnerable with the audience, whether it's about my wife and I'm being vulnerable about her, or it's about, you know, my parents and I'm being vulnerable to them. And it's almost like a cinematic piece in a way where each song tells that beautiful story.

• 40:00Daniel Whiteson?

  It's gonna be one hell of a trip. Experience Before the Moment, a Lincoln original series on the iHeart Radio app, YouTube, or wherever you get your podcasts.

• 40:23Kelly Weinersmith?

  We'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 uh...

• 40:37Daniel Whiteson?

  How is it pronounced, Kelly?

• 40:38Kelly Weinersmith?

  Oh, I was trying to avoid it. Cruithne. I probably got it right on the first try, right? How would you say it, Daniel?

• 40:49Kelly Weinersmith?

  Are you looking up the pronunciation? That's cheating. That's cheating.

• 40:55Daniel Whiteson?

  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.

• 41:08Kelly Weinersmith?

  Wait, you're not even gonna try and you had me do it first?

• 41:10Daniel Whiteson?

  I was about to.

• 41:11Kelly Weinersmith?

  This is your episode.

• 41:13Daniel Whiteson?

  You interrupted me as I was about to give it a try. I was gonna say, I would call it Cruithne 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 Cruithne or something. So I have no idea. Let's call it the space bean.

• 41:30Kelly Weinersmith?

  That's great. And I'll note the three times you and I said it, we did say it differently each time.

• 41:35Daniel Whiteson?

  Oh, did we?

• 41:36Kelly Weinersmith?

  We did, yeah, okay. All right, so tell me about...

• 41:38Daniel Whiteson?

  It's evolving through time.

• 41:39Kelly Weinersmith?

  That's right. Tell me about the space bean.

• 41:41Daniel Whiteson?

  So obviously, the Earth has the moon, which is the major moon and the real moon we consider. But 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

• 42:22Daniel Whiteson?

  its orbit relative to the Earth, it moves in this sort of bean-shaped pattern, again, relative to the Earth. It'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.

• 42:50Kelly Weinersmith?

  Those are the Trojans, right?

• 42:51Daniel Whiteson?

  Yes, exactly. The Trojans and the Greeks. You gotta keep them separated.

• 42:55Kelly Weinersmith?

  That's... Hey, I'll stop.

• 42:58Daniel Whiteson?

  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.

• 43:08Kelly Weinersmith?

  Daniel, I have very strong opinions that this is not a moon.

• 43:11Daniel Whiteson?

  [laughing]

• 43:13Kelly Weinersmith?

  This 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?

• 43:19Daniel Whiteson?

  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.

• 43:53Kelly Weinersmith?

  Wishy washy. I mean, I feel like it doesn't influence our songs or mythology. I'm not emotionally feeling any connection to the space bean. I say no.

• 44:04Daniel Whiteson?

  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.

• 44:13Kelly Weinersmith?

  I call Bino on the space bean.

• 44:18Daniel Whiteson?

  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 Demos. These things mean fear and dread.

• 44:40Kelly Weinersmith?

  Oh, who named them? They were having a bad day when they named them, I bet.

• 44:45Daniel Whiteson?

  Dark, right? So dark.

• 44:47Kelly Weinersmith?

  Oh, but Mars is the god of war, right? So of course you'd name the moons something dark.

• 44:52Daniel Whiteson?

  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 gonna hit Mars. So that's gonna 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.

• 45:30Kelly Weinersmith?

  Mm hmm. But you know, you could, Phobos is pretty small.

• 45:34Daniel Whiteson?

  Yeah.

• 45:34Kelly Weinersmith?

  You could push Phobos to a different orbit or something.

• 45:38Daniel Whiteson?

  Oh, yeah, probably. You could, 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, it's not enormous.

• 45:45Kelly Weinersmith?

  I mean, there's so many problems with living on Mars. I don't know that this is the one that's the showstopper.

• 45:52Daniel Whiteson?

  No, just add it to your list.

• 45:54Kelly Weinersmith?

  That's right.

• 45:54Daniel Whiteson?

  And 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. Like, 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. And yet it's in this weird orbit, so it doesn't seem like it formed with the planet.

• 46:35Daniel Whiteson?

  And the solution is that probably Deimos is an object that was tossed up from some other major collision.

• 46:42Kelly Weinersmith?

  Wow.

• 46:42Daniel Whiteson?

  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.

• 46:57Kelly Weinersmith?

  Aw, but that's not how I think of the moon. I don't think of it as a rejected piece of the Earth. You watch your language, Daniel. The moon might be listening.

• 47:06Daniel Whiteson?

  I mean, its name is Dread, okay? I don't think it needs to be dressed up in happy language.

• 47:11Kelly Weinersmith?

  All right, all right. It's probably the Goth equivalent of a moon, yeah.

• 47:14Daniel Whiteson?

  Mm hmm. And these are moons we've known about for quite a while. They were seen in the late 1800s by astronomer A. S. 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.

• 47:30Kelly Weinersmith?

  Wait, we knew about Jupiter's moons before we knew about Mars's moons?

• 47:34Daniel Whiteson?

  Yeah, Galileo discovered the first moons of Jupiter.

• 47:36Kelly Weinersmith?

  Wow.

• 47:37Daniel Whiteson?

  Yeah, quite a long time ago. But those moons are huge, right? Some of those are big enough that they would be planets if they weren't in orbit around Jupiter.

• 47:45Kelly Weinersmith?

  Holy cow. All right, so we've got, uh, Earth has one moon and a bean. Mars has two moons. Does anybody have three moons?

• 47:56Daniel Whiteson?

  No.

• 47:57Kelly Weinersmith?

  No. All right. Do we, give me four moons.

• 48:00Daniel Whiteson?

  Nope.

• 48:02Kelly Weinersmith?

  Five moons?

• 48:03Daniel Whiteson?

  Five moons, we have Pluto, right? Now Pluto, of course, famous for being in 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. It's more natural to think of the Pluto Charon 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

• 48:48Daniel Whiteson?

  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 tightly locked, binary dwarf planets.

• 49:03Kelly Weinersmith?

  Whoa. 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.

• 49:13Daniel Whiteson?

  Yeah, so technically it is a moon, but like it violates that definition as well.

• 49:18Kelly Weinersmith?

  Okay.

• 49:18Daniel Whiteson?

  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 Charon 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.

• 49:39Kelly Weinersmith?

  It's crazy that we know any of this, in my opinion. It's happening so far away and that's amazing. Go us.

• 49:46Daniel Whiteson?

  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.

• 50:17Kelly Weinersmith?

  Oh, I like that idea. Milo 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 gonna win. So who is next in our list?

• 50:32Daniel Whiteson?

  So next up in the ranking, we gotta 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.

• 50:59Kelly Weinersmith?

  That's exciting.

• 50:59Daniel Whiteson?

  Super fun. Exactly. And the whole history of the discovery of Neptune is really fascinating, because you can see Neptune in Galileo's logbook. Like, 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. I 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.

• 51:43Kelly Weinersmith?

  Yes, amazing. Although not fun if you're alive to realize that you missed, you missed that moment.

• 51:49Daniel Whiteson?

  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 they're formed with the moon, or they're captured that way. But 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.

• 52:25Kelly Weinersmith?

  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.

• 52:34Daniel Whiteson?

  Yeah, exactly. Like when planets go into retrograde, it's because relative to the earth, they're moving in the opposite direction.

• 52:41Kelly Weinersmith?

  Wow, okay.

• 52:42Daniel Whiteson?

  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, it did not, it went the wrong direction. It's like driving a semi the wrong way on a freeway, right?

• 52:57Kelly Weinersmith?

  Not cool.

• 52:58Daniel Whiteson?

  Yeah. And so it's, looks like all the other moons, the other 15 moons of Neptune, are re-accretions 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.

• 53:17Kelly Weinersmith?

  So how could you know the difference between moons that were there before? And moons that recollected after Triton destroyed everything?

• 53:25Daniel Whiteson?

  So they're not in tidy circular orbits around Neptune. So they probably are not there historically since the beginning, right? Because they're affected by this collision. And they seem to be all sort of mixed up, of various mishmash of the moons. And they're sort of loosely held. And so they haven't like formed together for a long time.

• 53:43Kelly Weinersmith?

  Amazing. And then is that all of Neptune's moons?

• 53:47Daniel Whiteson?

  Those are the ones we found. And you know, 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. And many of these were found like last year, so 2025, with James Webb Space Telescope.

• 54:19Kelly Weinersmith?

  Cool.

• 54:19Daniel Whiteson?

  They are so small and dark that they are not visible in the optical. And 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 1/20th of the mass of our moon. And this we've known about for like more than 200 years.

• 54:50Daniel Whiteson?

  Are you gonna make a Titania Uranus joke?

• 54:55Daniel Whiteson?

  Kelly has lost it, people.

• 54:59Kelly Weinersmith?

  I 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 gonna, okay, but I'm gonna focus on the science and ask you, Daniel, is this the first moon we've talked about that could have vulcanism?

• 55:29Daniel Whiteson?

  It is. Yes, exactly.

• 55:30Kelly Weinersmith?

  What an adult I am right now.

• 55:32Daniel Whiteson?

  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 vulcanism of its own, you 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 or more moons, which don't exist.

• 55:59Kelly Weinersmith?

  What?

• 56:00Daniel Whiteson?

  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.

• 56:08Kelly Weinersmith?

  Did he name the other moons like Rectum?

• 56:17Kelly Weinersmith?

  Keister?

• 56:18Daniel Whiteson?

  Moving on. Outside of the major moons, there are 10 more irregular moons, some of which just discovered in the last couple of years. So, you know, this is an area where we are actively learning new things about these planets.

• 56:31Kelly Weinersmith?

  Amazing. And what makes an irregular moon? Is it the shape?

• 56:33Daniel Whiteson?

  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.

• 56:43Kelly Weinersmith?

  Cool.

• 56:44Daniel Whiteson?

  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.

• 57:00Kelly Weinersmith?

  Whoa.

• 57:01Daniel Whiteson?

  97 counted, 97 moons counted for Jupiter so far, the largest four of which were discovered hundreds of years ago by Galileo.

• 57:10Kelly Weinersmith?

  Whoa, way to go, Galileo.

• 57:12Daniel Whiteson?

  Yeah, exactly.

• 57:12Kelly Weinersmith?

  Wait, could we all count as moons of the sun? Should the sun win?

• 57:17Daniel Whiteson?

  Are you in moons of the sun? Well, I think moons of the sun we call planets, right?

• 57:23Kelly Weinersmith?

  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.

• 57:32Daniel Whiteson?

  Yeah, these moons are huge and likely formed with Jupiter. They're in nice orbits around Jupiter. Io is bigger than our moon. A 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.

• 57:50Kelly Weinersmith?

  What moon is the biggest, Daniel?

• 57:52Daniel Whiteson?

  We'll get there.

• 57:53Kelly Weinersmith?

  Oh, okay. All right, all right.

• 57:55Daniel Whiteson?

  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.

• 58:10Kelly Weinersmith?

  Whoa.

• 58:10Daniel Whiteson?

  Yeah, the reason it's got so much going on is that it's pretty close to Jupiter. And 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.

• 58:54Daniel Whiteson?

  And that's where you get all these flows inside of it and this cracking of the surface and all these volcanoes.

• 58:59Kelly Weinersmith?

  So is Io really hot in some spots then?

• 59:02Daniel Whiteson?

  Inside of it is very toasty. Yes, absolutely.

• 59:06Kelly Weinersmith?

  All right, but Europa. Is Europa really hot?

• 59:10Daniel Whiteson?

  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.

• 59:23Kelly Weinersmith?

  What?

• 59:24Daniel Whiteson?

  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.

• 59:38Kelly Weinersmith?

  Wow.

• 59:38Daniel Whiteson?

  And 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, 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.

• 1:00:05Kelly Weinersmith?

  I would love to know if there's life there.

• 1:00:07Daniel Whiteson?

  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 and there's a mission being sent Europa Clipper to go and sample these things and like look for microbes or whatever, tiny alien octopi or something. So exciting.

• 1:00:27Kelly Weinersmith?

  Okay. All right. Let's get to the largest moon now.

• 1:00:30Daniel Whiteson?

  So the largest moon in the solar system is Ganymede, also Jupiter's moon. This thing is two times the mass of our moon.

• 1:00:36Kelly Weinersmith?

  Whoa.

• 1:00:37Daniel Whiteson?

  So if you're like impressed by our moon, like Ganymede is bigger than our moon by a factor of two.

• 1:00:43Kelly Weinersmith?

  Well, don't rub it in. Our moon's doing a good job.

• 1:00:49Daniel Whiteson?

  Ganymede is big enough to have an atmosphere, probably has a lot of oxygen in it. Also very likely to have a subsurface ocean. We think it has a metal core. This thing has its own magnetic field. It's a monster.

• 1:01:01Kelly Weinersmith?

  But I'm guessing you wouldn't want to live there because proximity to Jupiter would make it uninhabitable. Is that right?

• 1:01:07Daniel Whiteson?

  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. Live without a lot of shielding. But often a place you'll find settlements in very well-written hard sci-fi novels, for example.

• 1:01:26Kelly Weinersmith?

  Like the expanse.

• 1:01:27Daniel Whiteson?

  Exactly.

• 1:01:28Kelly Weinersmith?

  All 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?

• 1:01:34Daniel Whiteson?

  So the third largest moon in the solar system is also a moon of Jupiter. It's Callisto. Callisto's 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. Like, when was there a lot of impacts? When wasn't there?

• 1:02:03Kelly Weinersmith?

  Callisto sounds beautiful, but in a different way, Daniel.

• 1:02:09Kelly Weinersmith?

  All right. So Daniel, drum roll, please.

• 1:02:14Daniel Whiteson?

  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.

• 1:02:23Kelly Weinersmith?

  What?

• 1:02:24Daniel Whiteson?

  Right? Jupiter had 97. Saturn has 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. This 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 Yippidis, which is the craziest name for a moon.

• 1:03:07Daniel Whiteson?

  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. It's unbelievable.

• 1:03:30Kelly Weinersmith?

  I'm in.

• 1:03:31Daniel Whiteson?

  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 are, were like scattered by Jupiter. So it's sort of like the garbage collector of the solar system.

• 1:03:53Kelly Weinersmith?

  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.

• 1:04:04Daniel Whiteson?

  Exactly. 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.

• 1:04:15Kelly Weinersmith?

  Amazing. If you could visit any moon, Daniel, would it be Yapidus? Is that how you said it?

• 1:04:22Daniel Whiteson?

  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. So 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.

• 1:04:50Kelly Weinersmith?

  That would be absolutely epic. Yeah.

• 1:04:52Daniel Whiteson?

  And not included in today's list are the 334 minor planets in our solar system that have their own moons. These things you might call moon moons or moonlets.

• 1:05:03Kelly Weinersmith?

  Aww. Moonlets is cute, but moon moons is funner.

• 1:05:07Daniel Whiteson?

  Yeah, exactly. Moon moons technically reserved for moons that have their own moons. We haven't found any of those yet, but it depends on, you know, whether you call these minor planets moons. And we're all looking forward to the day when we can discover exo moons, moons around planets in other solar systems. It'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. They're so close to their But people are working on this and direct imaging of planetary discs might help us discover exo-moons.

• 1:05:46Kelly Weinersmith?

  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. I don't think it's impossible.

• 1:06:01Daniel Whiteson?

  Yeah, and I should say there are already candidates for exo-moons because in some of these eclipse methods where like a 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 exo-moons.

• 1:06:21Kelly Weinersmith?

  Well, stay tuned. If we find moon moons or exo-moons, we will let you know.

• 1:06:26Daniel Whiteson?

  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, everything that went down in their cosmic chaos.

• 1:06:40Kelly Weinersmith?

  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 Kelly's.

• 1:06:50Daniel Whiteson?

  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.

• 1:07:06Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 1:07:12Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 1:07:18Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 1:07:24Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 1:07:31Kelly Weinersmith?

  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.

• 1:07:40Daniel Whiteson?

  Don't be shy. Write to us.

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