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 griffs with our cosmic context of massive balls of rock spinning through a dark ocean. 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, and 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 Malicious Universe.
Hello. I'm Kelly Waitersmith. I study parasites and space, and I am not above jokes that involve mooning.
Hiel, I'm a particle physicist, and I can't tell you how long it's been since I mooned somebody.
Oh, yes, that was a joy 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 but ah, yes, I remember being mooned as.
A teen and as an adult. Kelly, can I ask you how long has it been since you mooned anybody?
I plead the fifth I married a man whose last name is Wiener, and so my sense of humor has become a bit more juvenile.
Fill in the gaps yourself, everybody, that's.
Right answer, 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? Oh?
I mean, I can imagine the moon influencing the tides, and the tides definitely influence people, m h.
But you don't buy the like, you know, more people come into the er on a full moon.
People do all 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 play the fifth on that one.
Yeah, I mean, I don't think I buy that 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 than 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. 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's circadian rhythms are influenced by the tide, so their behaviors are influenced by the tide.
But yeah, I don't think I buy that animals or people are more likely to be doing kooky crazy stuff because it's a full moon or anything like that.
But 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. Oh no, and boy was there some weird stuff there.
Oh. I did once for a research project look for papers describing things that had been pulled from orifices. And we are an inventive species, and 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.
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 to their sky and see the same thing. Or if you're a science fiction off third to imagine alien 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.
And if I can just plug my friend's book for a second, Under Alien Skies by Phil Plait is an amazing book describing what 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.
That's right, and everything he writes is insightful, well informed, and fun, so go check it out.
Yes, all right, so you asked our extraordinaries which planet has the most moons?
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.
And 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. In our Solar system, the planet with the most moons is probably Saturn.
Outside of the Solar System, I have no clue.
I would say Jupiter because of how big it is, but then it might have pulled some moons into it, So maybe Saturn has the most because it sits in a sweet spot.
What do you think, glenn on? Going straight for Jupiter? Why? Because it has the most moons? All right?
My initial thought would be Saturn or Jupiter, but I have been reading recent news reports about many new discoveries of moons around Jupiter, 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 done?
Cod Ah, I love that you think that I read the outline.
Giving you the benefit of the doubt here just in case.
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 maybe it's hard to count all the moons and we aren't sure if Saturn or Jupiter is the winner.
Yet that's probably the right answer. Actually, Also it depends on what you mean by moon, the 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 category and you'll argue about whether it's a moon or whether it's not a moon, and that will probably change the answer.
Well, at least that's a bit more satisfying than like our discussion on the ort cloud, where you're like, actually, we're not even really sure there's an ork cloud, and so you know, I was a little worried you we were going to 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 somewhere.
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 fifteen hundred, the term planet was basically used to describe like things that move in the sky, which we assume 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 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.
Okay, now, hold on, So recently you asked me what killyfish means, and then you ask me what hymenopterin means.
Oh no, are we about to get some linguistic revenge?
Gosh? I hope so, because usually when I'm like, oh, I'm about to get Daniel, You're like, oh, 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? Yes?
Yes, 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 you know, trying to predict the future. So it's like a deep rabbit hole all the way to the origins of astronomy.
Okay, I mean that's it's not quite as much fun as I was hoping, but like sort of fun. All right.
So you're hoping it was related to butts, weren't you?
I mean probably, like, yes, the act of dropping one's pants is what I was hoping for, but maybe this is I got to 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 it's a specific moon. Then the other moons can get a lower case because we're referring to moons in general exactly.
And you know, for a long time, we refer to all of these things as just satellites, so instead of calling them moons, you would say the satellites of Jupiter. Then with Sputnak 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, they just 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 which are not moons or natural satellites, and then natural satellites that we call moons.
And I think the word sputnik is just the Russian word for satellite.
Yeah. And then aside from the linguistic fuzziness, there is also this question of 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, 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 that'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. It's a little bit arbitrary, but it's helpful to settling debates among astronomers, which you know, 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? 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.
Yes, yes, I knew, I was right. That's great, But it does seem like, you know, you talk to ninety nine point nine nine percent 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?
There is really no lower limit. But we call these things moons because they're interesting. 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 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.
Is that what you think astronomers sound like? Because I think you got to watch it. There might be some astronomers that listen to this show and their feelings might get hurt.
No, I think most astronomers sound like they're really fit. They're very suave, but just the annoying ones sound like that.
Got it? Okay, 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.
So, typically planets have something like ten thousand 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 one eightieth of the Earth, Pluto has a moon that's one eighth 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 ten thousands of the planetary mass. But there are already major exceptions.
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? 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 the Earth, clearly the greatest planet in the Solar System, only get one.
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. There's like slow moving truds through space that's been going this way for a long time and going to be going this
way forever. But that's only because we're used to living at a sort of human timescale seconds, minutes, even centuries, and in those timescales, 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, 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 it 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.
Well, I bet we are all super excited for you to start with the least interesting. Explanation?
Was that a great lead in, right, lead into that ready to get bored here, I'm in, 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 got to build up to it, don't you.
But you don't even have to tell them there's a least interesting option.
Great, 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 are forming, they don't just make a planet, they also make their own little.
Orbiting guys 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 collapse gravitationally. 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 blog 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. But sometimes they're smaller and they'll form planets in the same
way as that planet is forming. It makes a proto planetary disc. 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.
Ah okay, and if it were to slow down, it would fall in, but it's not.
Slowing down exactly. If it were to slow down, then it would fall in. And you know, in that initial planetary disc 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, 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 spagetification when you approach a black hole, and so a planet has a thing called a roche limit. Anything closer to the planet than the roach 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. 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 that 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, ok 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 roach 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 roach 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 pre formed object that then comes inside the roach limit, the planet will tear it apart. So you have rings are 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 roach limit. Yes, but rings within the roach limit are probably never were moonduns because there's just too strong a tidal force for them to pull themselves together.
So this sounds like an unlikely path to moonhood.
Yeah, exactly, you need to be outside the roach limit in order to be a moon. And so, for example, in the Earth moon system, the roach limit is like ten thousand kilometers. Anything closer to the Earth than ten thousand kilometers you're going to get pulled apart. The Moon is safely like three hundred and eighty thousand kilometers away, so you're safe. And the Sun also has a roach limit. Planets that approach closer than like seven hundred and fifty
thousand kilometers to the Sun would get shredded. We're safely one hundred and fifty million kilometers from the Sun, and these roach limits are a little bit fuzzy because an object doesn't have a fixed roach limit depends on what you're bringing near it. Like if you bring a moon made of jello versus the 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 roach limit and where the moon is is so far there. Oh wait, or is the Moon getting farther away from us?
The Moon is getting further away by the best centimeter a year, and it 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 eventually the Earth and the Moon are going to get tidily locked, which will stop this process.
Okay, good few. 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 moon's 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 and the same axis as a planet is spinning, probably it formed with the planet.
WHOA, 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 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 that's 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, in 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 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, because 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 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.
And was hill a guy or gal was Hill an astronomer, presumably not one with the voice like you were making before.
Yes, it 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 outs. 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 at a 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.
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.
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. That'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. 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 hillsphere. So it's really just like 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. And 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 snacks 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 with 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 protoplanet that hid 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 it creates two new objects.
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.
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 critics 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, like maybe junk would get thrown off Mars and would hit up 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're 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 impactor and basically shredded, then some significant fraction of it is definitely going to hit Earth.
Yeah, given that you are willing to sacrifice all of humanity just to talk to aliens for thirty 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 word, Well, how valuable that is? Like, we have this incredible, complicated plan to 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.
One 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 fastening into a lot of deep signs 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?
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.
Why.
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 and 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 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 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, 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 should 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.
Okay, Well, Daniel had hinted earlier that there is one other candidate for a moon for Earth, and after the break he's going to tell you about it.
And we're going to hear Kelly try to pronounce it.
Oh, 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 being.
And uh, how was that pronounced, Kelly? Oh?
I was trying to avoid it, Kriefney. 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 the etymology of it is, or why anybody would choose this unpronounceable word. C r u I t h n E.
I wait, you're not even going to try And you had me do it.
First about to you interrupted me as I was about to give it a try. I was gonna say I would call it krithne 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 Croithney 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. Yeah, okay, all right, 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. 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 pindo bean or something. It's just that it has a funny orbit. So its
orbit is kind of being shaped. So it's got this elliptical orbit and 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, 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, the grange 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, 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.
Danielle, I have very strong opinions that this is not a moon. 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?
I mean, it's a fuzzy thing, but I would have to say no, right they 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
thirty times farther than our current moon. So officially astronomers call this a quasi satellite, which is like such a fudge.
Word, wishy washy. It doesn't influence our songs or mythology. I'm 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 and every part of people's lives. I don't think we're pro space bean being promoted to Moon.
I call beino 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 the Mars. And they also have awesome names Phobos and demos. These things mean fear and dread.
Who means that 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 name the moon's 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 twenty two 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 fifty 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.
But you know, you could Phobos is pretty small. Yeah, you could push Phobos to a different orbit or something.
Yeah, 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 twenty two kilometers, so it's it's not enormous.
I mean, there is so many problems with living on Mars. I don't know that this is the one that's the show stopper.
No, just added to your list. And Demos is even smaller. It's thirteen kilometers and slowered orbits further out every thirty hours. And there was a recent visit to Demos, 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 Demos and looked at it carefully and learned something interesting. People had long thought that Demos was a captured asteroid, and
Phobos almost certainly is. But it turns out that Demos 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. And the solution is that probably Demos is an object that was tossed up from some
other major collision. 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 Demos is like a rejected piece of Mars.
Ah.
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.
I 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 hmmm.
And these are moons. We've known about it for quite a while. They were seen in the late eighteen hundreds by astronomer aesof Hall, So not as long as we've known about Jupiter's moons. But like you know, one hundred and fifty 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, Some 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, do give me four moons? Nope, five moons, five moons.
We have Pluto right now. Pluto, of course, famous for being an astronomical fuzzy territory. Is it a planet, is it a dwarf planet? Whatever. Also, it has this weird moon, Sharon. So Sharon 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 Sharon system
its 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 one 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 Sharon as tidally locked binary dwarf planets.
WHOA, And so because you're not referring to Sharon as a moon, does that mean that the center of mass is somewhere between Pluto and Sharon.
Okay, yeah, so technically it is a moon, but like it violates that definition as well. Okay, yeah, so really fascinating system. Plus there's four more small moons. Hubble discovered these just like twenty 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 fat 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 win. Even Hubble is challenged to see tiny little moons that are not glowing right. You can only see them when photons leave the Sun happy to bounce off of these dark objects and come back to Earth. And so if that doesn't happen, you don't see it. 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.
Well, I like that idea, Melo 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 wind. 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 sixteen moons. Neptune has sixteen moons, all of which are named after water gods, which is super awesome. Maybe 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 seventeen days after Neptune
itself was discovered. It's 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 log book. You can go back and reconstruct it.
And it wasn't until two hundred years later that people figured out where Neptune was and saw it because of its pull on Urinus. A really whole fascinating scientific history story. I love when there's a discovery made, 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. 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, Trenton 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 Trident 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're 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.
WHOA okay.
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 did not it went the wrong direction. It'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 fifteen moons of Neptune are re
accretions of the rubble disc from Triton's capture. So Triton comes in destroys all the moons, and then eventually they gathered 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, because they're affected by this collision, and they seem to be all sort of mixed up, a various mishmashs of the moons and they're sort of loosely held and so they haven't like formed together for a long time.
Amazing. And then is that all of Neptune's moons.
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 in the list is Urinus. Urinus has twenty nine 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 are found like last year,
so twenty twenty five with James web'space telescope. 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 Web 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 twenty the mass of our moon, and this we've known about for
like more than two hundred years. Are you gonna make a Titania urinus joke? Kelly has lost at people.
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're 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 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 could produce volcanism of its own? You know, yes, exactly, And so the history here is also funny because it was discovered in seventeen eighty seven, just after the planet itself was discovered. But the guy who discovered Titania also
claimed the discovery four more moons which don't exist. What so they were like spurious moons which later people are 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 war fits geer?
Moving on, outside of the major moons are ten 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.
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 were 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 one hundred moons. Wow, right,
seven counted? Ninety seven 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.
Could we all count as moons of the Sun should the Sun win?
Are you 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. 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.
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 five hundred kilometers above the surface. It's the most geologically active object in the Solar System.
Wow.
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, and 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, But Europa. Is Europa really hot?
Eurrope 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 ten kilometers of ice and
below that maybe like one hundred kilometers of subsurface ocean. Wow, 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 we're 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, and there's a mission being sent Europa Clipper to go and sample these things, like look for microbes or whatever, tiny alien octopi or something so exciting.
Yeah, 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. Wow. So if you're like impressed by our moon, like Ganymede is bigger than our moon by a factor of.
Two, Well, don't rub it in. Our moon's doing a good job.
Again. I mean, 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.
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 all right, So Jupiter's got one hundred moons? Are there any other moons that we need to talk about before we move on to our winter?
So the third largest moon in the Solar system is also a moon of Jupiter. It's Callisto. Calisto is super interesting because its surface, unlike some of the other ones, is very very old, so lots of craters on the surface of Calisto. 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?
Calisto sounds beautiful, but in a different way. Daniel, all right, So Daniel, drum roll please.
The drama has been removed by the process of elimination. But the winner is Saturn, and not by a little bit Saturn has two hundred and seventy four moons right Jupiter at ninety seven, Saturn has two hundred and seventy four. 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. You've known about it for hundreds of years. Discovered by
Huygens in sixteen fifty five. Another one of my favorite moons in the Solar system is ensuladis this thing like Europa amidst 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 Yupitus, 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. It's unbelievable. So Saturn has all of these moons, most of them are very far from Saturn, like two hundred and fifty 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 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 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.
Amazing. If you could visit any moon, Daniel, would it be Yapitus? 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. 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.
That would be absolutely epic.
Yeah, and not included in today's list are the three hundred and thirty four minor planets in our Solar system that have their own moons.
Wow.
These things you might call moon moons or moonlitz aw.
Moonlits is cute, but moon moons is funner.
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 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 star. But people are working on this, and direct imaging of planetary discs might help us discover exo moons.
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.
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 the 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.
Well, stay tuned. If we find moon moons or exo moons, 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, everything that went down in their cosmic.
Chaos, all of the chaos from the kind of boring stuff Daniel will still tell us about anyway, so the much more exciting stuff, all of which we'll explain in an exciting way. Here a Daniel and kill.
All exciting because some of it's more boring anyway. Thanks for tuning in, everyone, and for ficking it out for this countdown for which planet has the most moons.
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