How many moons does the Earth have?

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Speaker 1: Hey, Kelly, do you ever wish that our night sky was I don't know, a little bit more dramatic, more dramatic. We've got a huge moon, planets, stars, and all of the Milky Way. You've got pretty high standards for the night sky. What did you have in mind? I don't know. Sometimes I think it's boring to have just that one huge moon. It seems like it might be more exciting to have lots of little moons up there. Okay, all right, that's true. It's kind of like having an only child. Yeah, and not just an only child, like a single mega child. You want drama. What's more dramatic than the Earth's mega child hovering above you in the sky. Yeah, I guess I wanted drama. But now I'm worried that the idea of a mega child is going to give me nightmares. Yeah, me too. I like my two little ones. Hi. I'm Daniel. I'm a particle physicist, and I'm very happy to have too normal sized children instead of one mega child. And I'm Kelly. I'm a parasitologist, and I also have two normal sized children, and that works well for me. You having considered the concept of like combining them together into one child unification of the children. You know, Zach likes to joke that they stack when he stacks them. They don't seem very happy about it, and so you know, I haven't thought too hard about trying to combine them. I don't think they'd be big fans of that. I'm pretty sure my kids wouldn't be into that either. Well, Welcome to the podcast Daniel and Jorge explain the universe and which we ask the hard questions of the universe. We talked about the crazy stuff that's out there deep in the cosmos, and we talked about the tiny little particles that are wriggling around inside my children, your children, and everybody's children. We try to explore the entire universe and explain all of it to you. Normally we have my friend and co host Jorge him here on the podcast, but he's unavailable today, so we have our wonderful, hilarious guest host, Kelly Weener Smith. Hello, I'm excited to be back. Thanks very much for joining us and for talking to us about crazy stuff in the universe. So on the podcast, we sometimes like to dig into stuff that's really complicated, like how does quantum field theory work, and what is the real charge of the electron? Or what is it like to go inside a black hole? These are deep and complicated questions about the universe that we try to take apart and explain to you. But sometimes we also like to dive into simpler seeming in questions, questions that might seem like they have an obvious answer but then have some interesting little wrinkles. It's good to have diversity, yeah, because questions are at the heart of science, and they're not just the questions that scientists ask when they're doing their research, but we're also interested in the questions that everybody asks. And sometimes the best questions are the questions that children ask, the most obvious questions which sometimes have a tricky answer. And I remember looking up at the night sky with my kids, who are now twelve and fourteen, but earlier asked very simple, basic, fun questions about the universe, like why do we only have one moon? Or do we only have one moon? Kelly, what kind of questions do your kids ask when they look at the night sky? I was just trying to think of a good example of Like, my daughter is seven, and she's always asking me questions that I don't know the answer to some of them super fundamental, which makes me think, how is it that I'm walking around not knowing the answer to all of these fundamental things, but yet I still managed to survive. But no good example is coming to mind right now. But we've got one of those apps on our phone where you can sort of point it at whatever in the night sky and it tells you what it is. And we spend a lot of time sitting on our car at night looking up at stuff, and it's a lot of fun. She asked lots of questions, but nothing's coming to mind right now that I did not know the answer to. What's your favorite question your kids have asked you? One of my favorite questions my kids have asked me are questions like where are we in the universe. You know, they're just getting used to the idea of like our houses in the neighborhood, and the neighborhood is in the city, and that city is in a state somewhere we have like a location and address, and so they wanted to know, like, where are we in the universe? What's our like universe address? And of course, you know, the hardcore physics answer is that every place in the universe is the same, and physics doesn't care about location and translationally and variant and stuff. But that answer is not very satisfying for a four year old. Yeah, that's why it's a bummer to have a physicist for a dad. But instead I try to describe to them, you know, what the galaxy looks like, and how the galaxy fits in with other galaxies, and what we know about sort of our cosmic neighborhood. And the thing I love about their questions is that they are both easy and hard. They're like easy to ask but sometimes hard to answer. And they're the obvious questions, as you said, that you don't always think about when you're walking around the world. But that's why I think that children really are sometimes the most natural scientists, because they don't just accept things as they are. They ask questions about the and say why is it this way or is it that way? After all? Totally, And what I love about Ada is that she'll tell me that I'm wrong. I wish I had that confidence. So you know, not only do they ask good questions, but they still have the confidence of children to be like, no, I don't think so. And so today in the podcast, we'll be talking about a question which seems to have an obvious answer. So on today's program will be answering the question how many moons does the Earth have? So, Kelly, tell me what you thought when you saw that we would be talking about this today. So you sent me a list of topics we could talk about, And when I read this one, I thought, well, the answer seems obvious. The answer is one. And if the answer seems obvious, then either Daniel needs to be fired because he's not asking good questions anymore, or there's something fundamental that I'm missing, and it's probably the second. And so I suggested that we talked about this one because I figured this was a question that would teach me something new. Awesome. This is a fun question to dig into. We get to hear about like what exactly is a moon, what counts as a moon, even a little bit about the etymology of the word moon and when it came into use, and what else is out there orbiting around the Earth and could you call it a moon? So, as usual, before I dug into this, I wanted to know what everybody else out there thought. Sometimes I used to walk around the campus if you see Irvine and ask students there these questions to see what the general level of knowledge was or confusion. These days, of course, because the pandemic, I'm sending emails out into the Internet asking listeners to speculate baselessly without the chance to do any research on these difficult physics questions. Did you actually stop students on the campus and ask them how many moons does they are? Have? No, I haven't because this question is too recent, But I wonder what they would say. I've noticed that our listeners tend to give better informed answers than the random you see Irvine student. Nothing against you see Irvine. Of course, our listeners may just be like a subset of the universe that's more interested in understanding the universe. But I hope one day to get back to campus and getting to ask the students those questions. It was fun to sort of see their faces when I asked them these questions. I bet you have a reputation as being one of the like super weird professors. Well, you know, I don't dress very fancy, so maybe they just think I'm some random homeless person asking them hard physical questions. Every college has homeless person or physics professor, it can be hard to tell the difference, physics professor, you can't tell the difference between Yeah, anyway, Thank you to everybody who volunteered to answer these questions. If you'd like to participate for a future episode of the podcast, please write to me two questions at Daniel and Jorge dot com. Here's what some of our listeners had to say. The obvious answer that pops into my head immediately would be one. So I'm pretty sure that answer is wrong. Well, we have just our celebrity moon, the moon, and I don't know, maybe we might have caught up some small objects there, probably a Deslo car elon musk scent, who knows. I don't know exactly. I'm one sure that the Earth has two moons, the classic moon that we see in the sky and Keith Moon. I would say the Earth has one moon. I don't know if you would consider satellites or a space station a moon because the orbit the Earth, but I would probably just consider one moon. Well, I guess this depends on your definition of moon the natural moons. It has the one large natural moon, and I believe they've discovered a couple of asteroids that have been captured in loose enough for bits that they're probably temporary. If you're going to call any artificial uh satellite a moon, which they've been called artificial moons, then that number goes into the millions, depending on how small a particle you want to count. There's nuts and bolts and pieces of rocket and larger things that satellites, fragments of satellites that have collided with each other, so uh, a whole lot of them, all right, Kelly, So what do you think about our listeners responses here? I thought there were some very insightful responses, you know. One of them was essentially asking questions about maybe how do we define moons. I thought somebody had a clever thought that the Tesla car might now be sort of orbiting, although I think of the Tesla car was shot Mars word instead of moonword. But it was clear that people sort of thought through their answers, and yeah, they had some clever answers. What do you think? Yeah, I was impressed as well. In the end, of course, it's going to come down to the definition. I hope that we end up with the definition of moon that doesn't allow Elon Musk to have created his own personal moon. That would seems sort of unsatisfying. That would be too much power for one person. The dude's already a billionaire and owns three like really valuable companies like it, doesn't need to create astronomical objects on top of all of it. Although he wants to create a Mars colony, which is maybe even more powerful than creating a moon. So I guess we'll have to see what the future holds. Do you think that's because he wants to be dictator of Mars? I like joking about that with Zack. Yes, he claims he wants a representative not a direct democracy, So we'll see what happens when the time comes. Well, if he gets to pick who goes to Mars, and they're probably going to elect him president anyway. But let's get back to the topic. We're not talking about Elon Musk's plan to colonize Mars. We're talking about moons and how many moons the Earth has and what exactly is a moon? So let's dig into that first and talk about what qualifies as a moon. So, without doing any research, Kelly, what would you use to define a moon? So I would have guessed that a moon is a big thing that isn't bigger than the planet that it orbits. So a big thing that orbits around a planet, and it has to be big, but it can't be bigger than the thing that it's orbiting around. That's how I would have defined it. But but probably that's wrong. And then your definition would it have to be natural or could be manned? Me that? Could we build an artificial moon? Oh h no, No, I've always thought of it as being natural. Yeah, what about you? Well, I really wasn't sure because it seems like in lots of directions there are arbitrary distinctions you have to make. And that's the problem often in astronomy is that the categories between things get blurred, and you always find something right on the edge between the different categories, and in the end, you know, we're just we're looking at the universe and we're trying to categorize it to understand is that we can talk about it together. Doesn't mean that those categories actually reflect something deep and true about the universe. There's sometimes just the words we've used to describe stuff, and so as we see more stuff out there in the universe, we're gonna find things that break those categories. But the definition that I found for moon is that it has to be a quote natural body, right, So it needs to be a natural body that's orbiting a planet. And to me already this is problem that like a natural body means not like a human made satellite. So we built a death Star for example, Officially that wouldn't qualify as a moon. But what if we just like you know, pulled out a huge chunk of Earth and threw it out into space and then it went into orbit around the Earth. Is that a natural body? It's like made of rock, right, It's essentially the same process that created our moon as we understand it. But I think according to this definition, it wouldn't be a moon because you know, it would have been like as the result of a human process. It really does seem like if you just took a chunk of earth and you put it up there, it should count. But I suppose satellites are just chunks of Earth that we've we've done things too, that we send up into space, and so maybe it shouldn't count. Yeah, exactly, That's where I was going. What is a satellite other than like an ultra refined piece of Earth that you've somehow, you know, manipulated into a complex technological object. But in the end, it's a piece of the Earth that we've launched into space. And you know, for those of you who don't know the history of our moon, we think that our moon is basically a big chunk of Earth that was vaulted out into space when another proto planet hit Earth when it was very very young, and this collision led to this enormous blab of stuff getting tossed out there, which eventually coalesced into the Moon. And when we studied the surface of the Moon, we found elements of it which look exactly the same as elements of the surface of the Earth, which suggests that they really do have sort of a common origin. So this definition of a moon being a natural body not a human made object is a little tricky, and I don't think the definition there is very clear. Can I ask a sort of tangential question, of course, So, well, Zach and I were reading about the Moon. We came across this fact that the Moon is sort of carbon poor, like maybe there's some carbon up there, but you're gonna have trouble growing plants and stuff because there's not a lot of carbon. If the Moon is just a chunk of Earth that got expelled, why do we have so much carbon and they have so little. Is that the result of natal processes that happened after the Moon got cut off partly, Yeah, Mostly the Earth and the Moon are made up of very similar stuff. It's mostly oxygen and silicon and calcium and iron and that kind of stuff. That's because we think that the Moon and the Earth are made up of the remnants of a huge collision between the proto Earth and a Mars sized planet, like fifty million years after the Solar system formed, and the remnants of that huge collision coalesced into the Earth and into the Moon. But you're right that there is very little carbon on the Moon, but there is carbon on the Earth's surface, and we're not sure why. One theory is that carbon tends to form very volatile compounds, things that would end up in an atmosphere if the Moon could keep one. So the Earth hangs onto some of these things, some of this carbon, because it's big enough to keep hold of its atmosphere it has more gravity, But the Moon can't. Anything like an atmosphere on the Moon just boils away. Interesting okay, thanks. So the definition of a moon is that needs to be a natural a body in orbit around a planet, and you said that it needs to be big. Officially, technically there's no minimum size to a moon. A moon could be like huge, like our moon, or you know, according to astronomers, it's okay to have a moon that's like a kilometer in size. I find that very unsatisfying. Like to me, a moon is something that you should be able to, like put a settlement on one day, and you can't really do that with a one kilometer sized quote unquote moon. I don't know. I don't want to buy that, but apparently it's part of the definition. It would be like the little prints right walking around this tiny little planet. But I think that comes from our experience, right, just because we have this one big moon, we tend to think that's what you gotta have in a moon. But as well talk about later, so many planets in the Solar System don't have a huge moon, to have a lot of smaller moons, And so part of the joy of astronomy and and of exploring the universe is discovering that our corner of the universe is weird, is unusual, is not representative, and we've got to sort of like stretch our mind then get used to a different way of thinking about what a moon is. Yeah, it is really hard to wrap your mind around just how different it is. And yeah, I'm very earth centric. I guess I would like moons to all sort of fits in a nice little mold that matches the moon that I have, and you touch it on really all the important issues. I think that was really tremendous. The other thing you mentioned is that it shouldn't be bigger than the planet, right, And in the definition of a moon, officially, the upper limit on the size of a moon is pretty vague, Like what happens when the moon approaches the size of the planet. Then you might ask, like, well, which one is the planet and which one is the moon? Right? Or if they're exactly equal size, could you consider them to be like a binary planet system? So where does our moon fall relative? And they'll like Earth to Moon ratio side, Like, is our moon big for a moon? Is it the biggest relative to the size of the planet it orbits is? So is our moon weird relative to the other moons? Our moon is very weird. Our moon is basically that megachild. The diameter of the Moon is more than a quarter of the diameter of the Earth, and that's the biggest moon to planet ratio in the Solar System by like a factor of five. There's no other planet out there that has such a big moon relative to the planet. Like there are moons of Jupiter that are roughly the same size as our moon, but of course Jupiter is huge, and so relative to the size of Jupiter, those moons are very very small, So we have a really weird big moon. You know, it's kind of amazing that we've managed to figure out anything, which is sort of a phrase that, if I'm remembering correctly, I've said in almost every single one of these episodes we've done together. But it seems like, you know, our Solar System is so weird, and our planet and the moon is so weird. It gives us such a weird frame of reference. But I know, it's exciting that we can still figure stuff out. Yeah, and it's a real journey to peel away these preconceptions and understand that things we thought were fundamental, things we thought we're obvious or universal, are really just unusual. You know. It's just like when you go traveling and you discover like, oh, not everybody drinks coffee for breakfast. You know, it turns out there are other ways to live, and that's the joy of exploring the universe, you know, discovering the tea is also acceptable at breakfast times. No, no, no, no, no, no, that's just wrong. One fact that I really liked while I was reading through here was that those like one kilometer size moons are sometimes called moonlits. And I thought that was super cute. So I asked my seven year old today. I was like, how many moons does the Earth have? So she, being a very clever kid, was like, well, I thought it was one, but you're asking me this, so what does that mean? And and so I told her like, well, you know you can have these little things called moonlits. And she said, well, what does that mean? And so I tried to explain that let's means little things and she was like, well, when else does it mean that? And I had a like, uh, you know, like a moment of not being able to figure it out, but I came up with piglets and outlets, and I was very proud of myself. So anyway, that was a fun moment with the seven year old today. Yeah, I love those little endings lits and like, it's just like the diminutive in Spanish, you know, eto or eno or these kind of things in romance languages. So I'm glad that we have those in English. Also, you could really stick them on anything, you know, like I'm not having dessert, I'm having dessert lits. It makes everything cuter, it really does. So where are we with the moons? Moons have to be a natural body orbiting a planet. No minimum size. The oppera limit is kind of vague. One thing I read suggested that you could effectively describe an upper limit on the moon by requiring that the center of mass of the planet and moon together should be inside the diameter of the planet. Right, So the center of mass of two objects is sort of like the weighted average of all of their stuff. If you've got more stuff in the planet than in the moon, it's going to be closer to the planet mathematically and physically. It's actually the location that the two things are orbiting. Right. We say that the Moon, for example, orbits the Earth, but actually the two things are orbiting each other, or more precisely, they're both orbiting the center of mass of the two things. And if you have two things that are equal mass, then their center of masses right between them. If one of them is infinitely massive, then the center of mass is at the center of that very massive object. In a more realistic scenario, where one thing is very massive and one thing is much smaller, you can have the center of mass of the system of the two things be close to the center of the larger object. And so here the definition is like, is the center of mass of the two objects underneath the ground within the radius of the larger object. That's sort of a cool definition. That is a cool So I have a couple of questions to see if I understand. So one, does it depend on how far apart the things are or is it like you just when you make this comparison, you you know, sort of mentally put these things right next to each other and then compare the center of mass. It definitely depends on how far apart they are, right because for example, if they're equally massive, you'd be right between them, and that definitely depends on the distance between the two objects. Yes, and so that would mean you have two planets as opposed to a moon and a planet in that definition. And so so you have two things that are like one is just a little bit smaller than the other, and one is big er, but the smaller one is made out of something much more massive or dense or I'm not quite sure what word I'm looking for here. Could the smaller thing end up being the planet that gets orbited by a bigger thing because it has more like gravitational pull? Absolutely? Yeah. For example, if you had something orbiting a black hole, right, a super duper dense object, then the center of mass of the system is definitely going to be within the event horizon, even if though the black hole could be smaller. Right, black holes are very very compact. They don't have to be very very large, but they are supermassive. So that's awesome. You're really like pushing the boundaries of this definition. But I don't know. I guess a black hole could have a planet orbiting it. I'm not sure if you would call that a planet or a moon, right, if you're orbiting a black hole, Yeah, it's trippy, I guess, I, as a not physicist, find it sort of unsatisfying that if the moon is too far away, it might or if something that feels sort of moon like is too far away, it wouldn't count as a moon because the center of masses is not within the bigger body or something. It feels like distance shouldn't matter for the definition, But that's a biologist take on it. Well, it definitely has to be in orbit. And I think that one problem with this whole categorization is that this is like a natural hierarchy. I mean, you have the Sun and the stuff orbiting around the Sun, and then you call those planets, but that's sort of arbitrary because they're orbiting around the Sun. And then there's things orbiting around the planets, right, and you call those moons. In the end, it's just sort of a natural hierarchy of stuff orbiting stuff that's orbiting other stuff, and our categorization of it is you're really going to try to draw lines there where they really aren't lines. Yeah, but that's like a fundamental problem with science. You know, in biology, we categorize things, and constantly nature does not care how we categorize it. She moves away outside of our categories. Yeah, I guess it's tough just finding a definition that is useful I guess in biology there was a huge revolution a couple hundred years ago, right because you guys had been categorizing stuff based on how it looked and what it did, and then discovered the genetic relation ships between everything, and that there was actually a real, true organization to all these animals that didn't necessarily respect how they looked or how we observe them well. And then it turns out that, you know, how do you define species? That's actually a pretty difficult question. The tree of life doesn't just branch, it sort of comes, the branches come back together and you get horizontal gene transfer. And how do you categorize species of bacteria when they're swapping jeans all the time, and it gets complicated fast. Well, in physics, it's interesting they think about how these words were used for a long time. People use the word satellite to also refer to moons, and this is before we had things in space like GPS and telecommunications, satellites and you know, laser weapons and all that kind of stuff. Now, when you say satellite, you think of something people build, maybe electronics and launched into space. But before spot Nick, before ninety seven, astronomers used the word satellite to talk about anything orbiting another object, and so they didn't really use the word moon scientifically. They referred to Luna, for example, as Earth satellite. Then when they launched sput Nick, they realized, whole on a second, what are we going to call that? And so they started to use the word artificial satellite. But that was a little bit too cumbersome, and so now we just say satellite when we mean artificial satellite. And then moon, which was a word mostly used like in science fiction and a sort of popular culture, became a more official scientific word. I think that's sort of cool that the scientists adopted a word from sort of common usage. That's totally fascinating. And I've been reading about the space race and I didn't know this history of the word satellite. And when I saw that they named spot Nick an artificial satellite, I was like, well, that's sort of clunk. Why not just call it satellite? But it makes sense now. A lot of the fathers of rocket science or whatever. We're really into sci fi. I think almost all of the fathers of rocket science claimed that they were inspired by Jules Verns. What is it called to the moon? I wonder if the fact that they were all super into sci fi sort of helped get the word moon into the scientific literature. Yeah it makes me one or what else? Like will in the future be part of science. You know, things that we say but you wouldn't hear a scientists say, you know, like a shooting star is our word for a meteor, but you never see that in the science paper. But maybe someday in the future, you know, the language will transmogrify itself into official scientific physics speaker could be. All right, So we have our definition of a moon. It has to be a natural body, right, not something engineered by humanity or elon Musk. Has to orbit a planet. But there's really no minimum size to it, though there seems to be some sort of sense of an upper limit that you shouldn't have your moon being larger than your planet. All right, So, now that we have our definition, or at least a definition we're gonna work with for the rest of this episode, let's take a break before we move back to figuring out exactly how many moons Earth has. So we have a definition. Now, how many moons do we have, Daniel, So let's count them first. Of course, we have the number one candidate. The classic moon, the one you're familiar with, the one we all grew up loving. Right, this is the moon we call the moon, which is sort of a silly name, right, calling the moon the moon. It reminds me actually, when I was a kid, we had a cat and nobody could agree on what to call the cat, so we just called it the cat, and that just became its name. And for a while I felt sad for like, oh, poor cat doesn't get a name, But he didn't care. It got fed, it got pet it doesn't need a name. I think it's good that you became more creative that you got I can't tell if calling it the moon is very egocentric, like that is our moon, so it is the moon, or if it just represents a lack of creativity. Hard to say. Well, as we were saying earlier, it's really weird, Like it's huge. It's more than a quarter of the diameter of the Earth in the Solar System. The next largest moon relative to his planet is one of the moons of Neptune, which is almost six percent of the diameter of Neptune. So that's a pretty big gap. Like mostly moons are small compared to their planet. The moon is really a very weird and Or and I did a whole episode about how the Earth got its moon and the various theories and how people are testing those, and so if you're interested in the details of the history of the moon, then good check out that episode. But it's interesting that, you know, in the sort of the history of astronomy I was reading about this, people didn't really think about the Moon as a moon until they discovered Jupiter's moons. Like people were looking up in the sky and they saw, of course stars and they saw the moon, but they just thought of it as like another celestial object. They were wrong. And then you know, when Galileo and other folks saw the moons of Jupiter and they're like, hey, look at that, there are things orbiting Jupiter. That's when it clicked in the place and they're like, wait a second, what Jupiter's got over there is the same thing that we've got over here. And that's when they really eyes that this was a whole new kind of thing in space. That's so interesting. That's why it's so important to you know, to learn more and to step outside your comfort zone because there's so many things that become obvious when you see how other things are done, and the moon is actually considered a planet for a while, though the definition of a planet has also changed. Back then, a planet was just like another astronomical object. People didn't really understand the relationship between the planets and the stars and all this kind of stuff. So if you go back to very early astronomy, what these words mean is totally different from what they mean today. So it's pretty tricky to read, like you know, early astronomical texts. That sounds super confusing. So it makes me wonder. So if you can be changing what the definition of a planet is, can you have things that are not planets that have moons? You can definitely have things that are not planets that have stuff orbiting them, and we will talk about that later in the episode. There are things in the Solar System orbiting other things that are not planets, for sure. Sorry I got excited. It is very cool. And for example, you know, just as a teaser, Pluto no longer considered a planet, but it does have things orbiting it, right, Pluto has something which you would call a moon, Sharon, which is half the diameter of Pluto. So that's pretty fuzzy, like which one is a planet, which one is the moon. Pluto is only twice as big as its moon. So if Sharon or Charon, however you say, it ends up being called the moon, that makes our moon a little bit less exciting. Yeah, Or if Pluto gets repromoted back into it being a planet, then Sharon would be the largest moon relative to a planet in the Solar System. But because Pluto's no longer technically a planet, Sharon loses that distinction. Sharon shure, Sharon, Alright, so everybody knows about the moon. That's not what people want to hear about. Are there other things orbiting the Earth that could also be considered moons and so at? In fact, there are, but they're sort of on the border line of whether you would really technically call them a moon. One of my favorites, they say, the object which I won't even try to pronounce. You're gonna have to look it up, but it goes by the name of the space being, and it's this object like five kilometers in diameter that's orbiting the Sun in the vicinity of the Earth. But it seems like we've got to try to pronounce it, right, So I'm gonna try crew with me, Crew with crew with me. That sounds about right. If I had no idea how to pronounce it, I would guess that was correct. And I have no idea how to pronounce it, so I guess that's correct. I don't even know what the like, you know, origin of that, what language that comes from. It looks Irish to me, and I have I have no reason. I don't actually know anything about the Irish language. So mean, maybe I should stop. Maybe it's Gaelic for space being. Somebody out there who speaks Gaelic let us know how to say space being in Gaelic. So this is a fun object because it's in orbit technically around the Sun, but it's ring resonance with the Earth. This is something maybe people haven't realized that, like things that are orbiting the Sun are also interacting with each other. Right, our orbit around the Sun is mostly a big lips and it's dominated by the Sun. The shape of that lips is mostly controlled by the gravitational power of the Sun. But there are other things also orbiting with us, and they tug on us, they pull on us, and they can change our orbit. And sometimes you get two objects and what you call a resonance, which means that they are very regularly tugging on each other in a systematic way that changes both of their orbits and makes it stable, so that, for example, they will like meet two or four times while going around the Sun, depending on their relative speed, and nudge each other in just the right way. So, for example, Saturday and Jupiter are both really really powerful and they're in an orbital residence with each other. So this is an object that's moving around the Sun, but it's in resonance with the Earth, so it's strongly influenced by the Earth's gravity. But does it does it orbit in the same shape the Moon duns? So how is an orbit and a resonance different? So this thing moves around the Sun, you know, but so does the Moon. Right if you tracked the moon. The Moon goes around the Sun. The Earth goes around the Sun one but the Moon is gravitationally mostly bound to the Earth. This thing is primarily mostly bound to the Sun, but because of the influence of the Earth, if you look at its orbit from the Earth, it looks like it makes a bean shaped motion around the Earth. Like if you put the Earth, if you're sending of your reference frame and you plot where the space being is and you like take data points from day to day and imagine where it is. It makes a bean shape all the way around the Earth. So does that mean it doesn't quite fit the definition? Because like do orbits need to be elliptical and not not being shaped? It means that there's not really a clear distinction. We have a three body system. We have the Sun, we have the Earth, and we have the space being, and they're all tugging on each other. And so to ask the question like what is the space being orbiting? Is it orbiting the Earth? Is it orbiting just the Sun, doesn't have a clear precise answer unless you define an arbitrary distinction. But there is no natural distinction between these two scenarios. Is like a smooth continuum of being like the Moon where you're very clearly disorbiting the Earth and being like Mercury, where you're very clearly disorbiting the Sun. And then these quasi satellites of Earth that are very clearly affected by Earth and sort of moving around Earth as they move around the Sun. And that's what the space bean is. It's a quasi satellite of the Earth. All right, So two things. One, every time you say space being I'm going to continue to chuckle. I don't think I'm gonna stop doing that, because it's just funny and cute and too you know, I feel like it would be really nice in science classes if like, how hard it is to do these definitions and how so few things fit into these definitions was sort of made more clear, and maybe they do that in school today. It constantly blows my mind how much I think that we have the world bend nicely. But everything breaks the bins. Everything breaks the bins because the world is continuous, right, and it's huge, and so there's always going to be an example of something in between. These crisp distinctions are are our artificial way to try to impose order on the universe. And back when we hadn't seen so many things, those gaps might have seen meaningful. But as we keep looking at the universe, we find stuff that feels in those gaps, and you know, the same must be true in biology, right, that's what the duck bill platypus is. Right, It's like, hold on a second, what do you mean mammals? You know, right, right, it's nature's way of being, like, you cannot put me in a box. Yeah, so that's the space being. Also, I'm saying the space being because it is fun to say, and because I'm trying to avoid having to pronounce the actual name of the thing. Yeah. No, sure, that makes sense, It's okay. So here's what I want to know. So we're not taught that the space being is a moon in school, or at least I wasn't. So if you were to walk into a random astronomer's office at some university and ask them does the space being count as a moon? Are they gonna like roll their eyes and be like not this again. Or are they going to be like, oh, that's a really interesting question, Like how does the astronomy community do this question? That's a great question. I bet that half of them would roll our eyes and half of them would be like, you know, I was on the committee that decided what a quasi satellite is, and I have a very strong opinion about it, because you know, they get hotheaded about this stuff, right, And you will now sit for three hours while I explain the difference to you and you regret asking immediately. Yeah, And sometimes I wonder, like what does it matter? You know, people argue about whether Pluto is a planet or a dwarf planet, like, what are the consequences? Why does anybody really care? We're just making these names up. It doesn't change what Pluto is. It's not like Pluto shrinks if we call it a dwarf planet or grows in importance if we call it a real planet. These are just our arbitrary categorizations of the universe. So the affect how we think and how we talk, but really it's it's inconsequential. You know. I can see why it would matter to you if you are an astronomer, because maybe it affects the way you model things based on how these definitions fall out. But I imagine to the lay public it's like, what are we paying these people for. That's a good point. That's a good point. And so we have this thing that said the moons to the quasi satellite, and that's just one example. You know, there are lots of these things that are sort of loosely bound to the Earth orbiting the Sun that can be fairly small, like this thing is only five kilometers in diameter. As you get smaller and smaller, there are many many more of these things. Some of them are more loosely bound to the Earth. But you know, if you're gonna call this thing a moon, then you've got to call all those other little things moons also, So if you're gonna open the door that wide, then Earth is going to have a lot of little moons, and high school astronomy becomes totally untenable. And unfortunately, it doesn't change what it looks like in the sky, right, You can't look up in the sky and see this thing. The closest it ever comes to Earth is like seven and a half million miles away, which is thirty times further away from the Earth than our moon is. Plus, of course it's super tiny, so you have almost no change of spotting this thing. It was difficult for astronomers to even see it in the first place. It was only discovered in the last twenty years. Is it safe to assume that it's center of masses within the Earth or the center of mass for the two bodies falls within Earth? Absolutely? Yeah, because the Earth totally dominates this thing when it comes to mass. This thing is only five kilometers in diameter, and it's just a big chunk of rock, and so the Earth dwarfs this thing. Okay, even though it's super far away, it still works. Okay, that's our like best candidates, is there like a second best candidate. There's something else which I think is super fun, which astronomers and science communicators called a temporary moon for a while. And this is a weird object which at the end of last year sort of swooped in from deep space and orbiting the Earth a couple of times, and people are like, oh, what is this thing? It was like only forty ft long, and it has this strange cylindrical shape, and they spotted it on his telescope and they're like, wait a second, what is this thing? And it looks like a piece of a rocket. What it's space debris. It's space debris. And so it turns out this is a long lost piece of an ill faded NASA mission. There was this mission in nineteen sixty six called Surveyor two. Is a robotic spacecraft that was going to go land on the Moon and it was going to go explore the Moon, etcetera. But mid course there was a failure in the sort of ballistics and the trajection and they lost contact with the craft and one of the thrusters failed to ignite, and so basically they lost this thing. Part of it crashed on the Moon, and part of it just like tumbled out into deep space sort of lost, they thought forever. And so this thing has been like out there in space for you know what, seventy years, almost bouncing around, swooshing around, and somehow it ended up coming back to Earth. That's crazy, first of all, I think it's more like fifty years though, right, But like, it's amazing that we have that much resolution that we could find it again. It's really incredible. But you know, we are monitoring stuff that comes close to Earth, and we're looking for stuff that's sort of like tumbling towards the Earth. This is a pretty small object though that it's also a weird shape. Right. If you're looking out in the space and you see this forty ft long cylindrical object, my first guess is gonna be like aliens, right, And so how disappointed to look at it and then see, like you say, written down the side, it's just our old trash. But it's fascinating. People have done a study to figure out like what happened to it, and it turns out that when it tumbled out into space. It was very gently pushed further into space by the pressure of sunlight. We talked about this on the podcast, that you could build a spaceship that sails on light. It's called a solar sail. Just have a huge sheet of material and the photons from the sun when they hit it are going to transfer their momentum to that, just like wind on Earth transfers its momentum to a sail on a sailboat. And so that's like by design. But here, of course, it's just a rocket tumbling in space, was very gently, over decades and decades pushed deeper out into space. Are you familiar with Project Westward? No? Tell me, Oh my gosh. So the idea here was so this was like during the Cold War, and the US was worried that they would lose their ability to communicate with other nations if our undersea cables got cut by the Soviets, and so in that case we would have to rely on sending radio waves up and we would have to hope that they would bounce off the ionosphere. But that's sort of a not super reliable way of sending radio communications, and so I think it was m i T came up with the solution where they were like, well, we could be sure that the ionosphere would have stuff that would bounce the radio waves back if we put up over four hundred million copper needles. That's a great idea, right, And so the argument was that you would put these needles up and it would give us some information about how well this was gonna work. But in about two to three years, the combination of gravity and the sun pushing these needles back into the atmosphere would clear everything out, clear everything out. Does that mean these copper needles come raining back down to the service to impale people. Well, they do start raining back down towards Earth, but they're like literally only a couple of maybe centimeters long, like maybe two centimeters or something. These aren't huge, and that's why they needed four million of them. That sounds like approximately the size is of a bullet. You're talking about space bullets raining down on the Earth. But that would definitely burn up before it hits Earth, right, right, Yes, of course I think so. Yeah, so our atmosphere is like a bulletproof vest. Yes, thank you atmosphere. But so the first one that they shot up there, it didn't deploy the right way, so a bunch of these needles clumped instead of spreading nicely, and so some of these clumps fifty years later are still tracked as space debris because the clumps didn't de orbit the way that they thought they would. And then they did a second shot, and the second shot dispersed and they were able to confirm, yes, the radio waves bounced back more when we have supplemented the ionosphere with all of these needles. But then we had to go to the United Nations and promise we were going to stop messing up the space environment and that we were going to consult people from here on out. And one of the articles of the Outer Space Treaty actually requires people to do consultations. And that was motivated by Project Westward, because what the heck is the US doing? So anyway, tangent related to the Sun pushing things back into orbits or sorry, back into the atmosphere. Yeah, and we have a whole fun podcast episode about space junk and how dangerous it is and that's all smashing into itself. And if we don't get this stuff under control, all of space might be useless to us. We might be trapped down here on Earth because we'd be surrounded by our own garbage. So check out that episode if you're interested. But this is one particularly interesting object, and it came in November twenty and made a few large loops around the Earth. And so that's why they called this sort of like a temporary moon, because even though it was lost in deep space, somehow it ended up close enough for the Earth to grab it again and loop around. But it wasn't a stable orbit, which is really fascinating. It was like stable enough to loop around a few times and then get tossed back out into space. So in March continued his journey and is now back into deep space somewhere. WHOA. So, so was this like a sycom angle for getting people interested in this, because it seems like because it was artificial, it shouldn't count as a moon at all, not a temporary moon. It shouldn't be a moon. Or do different people use different definitions than the one that we settled upon at the beginning of the episode, Yeah, exactly. I think this was just a clickbait moon, not really by any definition. If you're going to require that moons be natural, may not be manufactured by humanity. Then this is definitely not a mood. But it's cool that we capture something from deep space and it orbited us a little while before going back out into the dark deep emptiness. Yeah, no, that's super cool. All right, So let's take our second break before we start to talk a bit more about moons on planets other than our own. Okay, so we've talked a bit. How about how Earth has one for sure moon and a couple maybe moons, quasi moons, quasi moons. Right, tell me a bit more about what else is typical in the Solar System. How many moons do the other planets have. It's really interesting because the Earth, as we were saying, is really weird, Like having a really big moon is weird. Also, having just one moon is weird. So the eight planets in the Solar System, of course, the Earth has one moon. Two of the planets have no moons, Mercury and Venus, and the other five planets have two hundred and four moons in between them. So that's really fascinating and you might wonder like, well, why do Mercury and Venus have no moons? And the reason is that they're so close to the Sun. The Sun is this like really terrible overwhelming gravitational force and it tugs on things. We talked to the program before about tidal forces. The closer you are to the Sun, the more it tugs on you. So if you're an object, it's not just like a point. If you have like an extent to you, and the Sun is going to tug harder on the bit that's closer and not as hard on the bid that's further away, and that's an effective force on you. So the Sun is, for example, squeezing mercury and venus all the time. It's pulling on them, and that makes it very hard for them to have any in a stable orbit around them. Basically, if they have a moon, the Sun is going to steal it when it swoops around the planet on the close side. So here I'm finding that I want to ask you if that means in other Solar systems, the close planets don't have moons. But I'm jumping if I do that, aren't I We don't know the answer to that question yet. We'll talk about that in a minute, but yeah, it's a fascinating question. We know that in our Solar system, the innermost planets Mercury and Venus have no moons, and you know, that might be part of the answer of why the Earth only has one moon and it's a really big one. It might be that anything else the Sun is basically going to steal, and that we're only capable of having like either really big moons or quasi moons that were sort of sharing with the Sun. That might be the partial answer to that question. Interesting, So we have the Sun to thank for the space being. Yeah, the Sun is toasting our space being for us. But then further out in the Solar system, there are lots and lots of moons. You know, the other five planets with moons have two hundred and four of these things combined, you know, in Jupiter and sat And have like dozens of them. And you know, Saturn has this crazy ring system, and inside those rings are rocks that are big enough to be considered moons. Sometimes they're called you know, moonlits, sometimes are called minor moons. But there are all sorts of things, and so the deeper you go out into our Solar system, the more you find these objects that have moons and lots and lots of moons. To do any of the moons have moons. I wish they did have moons. And what would you call that anyway? Would that be like a moon moon or squared? That's a great question. And so far we haven't found any moons that have moons. And the reason is similar to why Mercury doesn't have moons. If you're a moon, then you're near a big planet, and that means that there are tidal forces, and so it's pretty hard to hold onto something and not have it gets slurred up and just become another moon of the planet you're orbiting. So if you're Io and you're orbiting Jupiter, right, Jupiter is an enormous gravitational pull. The reason that Io is like hot inside, it's because of Jupiter's tidal forces, and so anything orbiting Io is just going to get pulled away. It's very very hard to have a stable orbit. Although there's this really crazy moon of Saturn. This moon is called Rhea, and recently they've spotted some stuff in orbit around Rhea, maybe sort of like these very small particles. It almost looks like this moon of Saturn has its own ring system. What I know, isn't that cool? I never thought about like a moon with rings around. It is pretty cool. That's awesome. So so how if it can't have moons, how does it manage to maintain its rings? Well, the reason you sometimes have rings instead of moons are because of tidal forces. Basically, rings are moons that have been torn apart by tidal forces. And so it might be that there was an object there and then it got sort of shredded by tidal forces, and what we're seeing is something transient that these rings might not last for very long. You know, by very long we mean on astrophysical time scale. So they might be there a million years or so. But if you leave the Solar system and come back and five million, you might find that they're not there anymore. Okay, so let's move out of our solar system. What does this look like in other solar system? So, for example, do the mercury and venus equivalents and other solar systems lack moons like we see in our solar system. It's such a good question and one we just don't know the answer to yet because we have never seen a moon in another solar system, Like every moon that any human has ever seen has only ever been in our solar system. Isn't that like crazy and slightly frustrating. It's both of those things, And it reveals to me, like just how often I assume that science has figured out so many things that are much harder than I apparently understand. So why do we know that all these other solar systems have planets but we don't know that they have moons? Well, moons are much smaller, and they're just harder to see them planets. You know, we now sort of take it for granted that we can see planets around other stars, But that's pretty recent. It was like that we started to be able to really do this and spot planets and other soldiers. Until then, we had never seen a planet around a star that wasn't our star. We didn't know until then, if, for example, our star was the only one that had planets, we could have been like the weirdest star in the universe. Now, of course, we know, because we've looked and we've seen that lots and lots and lots of stars have lots and lots of planets. We're also learning, of course, that our Solar system is weird in all sorts of interesting ways. But the problem is that the techniques we used to see those planets are better when the planets are large and when they're close to the Sun, and that makes it pretty hard to see moons, which tend to be small and further from the star. We haven't found any instances of a moon that's like huge or one of those instances where it's hard to determine which one is the planet and which one is the moon because they're both pretty big, and so there's no big things that break this rule. We haven't confirmed any moons and other solar systems are you're looking for the duck built platypus of moons out there somewhere No so far zero. There have been some candidates. People have some really clever ideas for how you might see moons around planets in other solar systems. Like one way that we see the planet is that the planet passes in front of the star makes like a little eclipse, it like dims the star a little bit. And if it does that several times, you can see a pattern, so you can get a sense for like, oh, this is really something that's passing in front of the star. It must be a planet. Now, if there's a moon around that planet and it's orbiting that planet. As it passes around the planet, it might add to that eclipse a little bit and then pass behind the planet and dip again. And so if you have really refined measurements, you might see these like wiggles on the wiggles that tell you that there's something going around that planet. And there are some cases where people look at the data and they're like, yeah, I think those are wiggles and wiggles, but other people analyze the same data and they're like, no, there's no evidence for that. So we're really at the edge of being able to detect XO moons. We haven't seen any yet. Well that's exciting, Like, hopefully in our lifetime we figure out how to do that. Well, yeah, hopefully in our lifetime. I mean, we're on the edge of learning so much about these other solars to times, like figuring out what's any atmosphere of those planets, seeing what moons there are. There was also a lot of excitement. I don't know if you remember about this one particular star called Tabby Star, which dimmed really weirdly all of a sudden, and people thought like, wait a second, what's blocking this star? Is this evidence of like an alien megastructure. Is somebody out there building a dison spear around this star, which was really fun and you know inspired, I'm sure a lot of science fiction novels. But it turns out that it was just like a big cloud of dust and debris. But it might be that, like, you know, there was a planet out there and it had a moon, and that moon got like tidally disrupted and torn up and ended up being this big cloud of stuff that partially blocked the star. So that might have been like, you know, evidence of the destruction of an excellent moon. WHOA, that's pretty cool. That's pretty cool. Yeah, But I'm looking forward to scientists figuring that out and so we can get a sense for like how unusual is it to have just like one big moon, How unusual is it to have inner planets with no moons? How weird is it to have huge gas giants with like a dozen moons. We ask these basic questions because we just don't know the answers, and really the only way to figure it out is to go out there and explore the universe and to like actually look with our eyeballs and our telescopes and get the answers. It's a cool time to be alive. It is a cool time to be alive. So, Kelly, what are you going to go back and tell your seven year old about how many moons the Earth has? Well, I think she's going to be super excited. She We've been talking to her about Apollo and the space race, and I think she'll be super excited to hear that one of those you know, boosters came back around again and is still floating in deep space. So I'll tell her about that, and I definitely think she's going to love that there's something called a space being or something we referred to as a space being because we can't pronounce its real name. I think she's going to get a total kick out of that information. What about you, what are you gonna tell your kids? Yeah, my kids also will tell them that there's weird stuff floating out there, and that in the end, it's all just sort of arbitrary. What we call a moon and what we call a quasi satellite is really just an arbitrary distinction, but that their questions are valuable questions, and that these really simple, basic questions about the universe are sometimes the most interesting, the hardest to answer and reveal something about you know, what we've thought about our place in the universe, what we think is natural, what we think is normal, what we think is typical, and it probably is really unusual. And that's somewhere out there there's an alien kid looking up at the sky and asking the alien mom, like, why do we have five moons? You know, it's just every planet to have five moons. I love that idea that we're all asking the same questions, but just very much separated. All right, So thank you very much everybody for going on this curiosity journey with us and listening to us talk about the number of moons around the Earth. If you have stuff you'd like to hear us talk about, please send them to us two questions at Daniel and Jorge dot com. Thanks again, Kelly for joining us today and thanks to all of you for tuning in. Thanks for having me. It was a lot of fun. Thanks for listening, and remember that Daniel and Jorge explained. The Universe is a production of I Heart Radio or more podcast from my Heart Radio visit the I Heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.