Daniel and Jorge Explain the UniverseDaniel and Jorge talk about how a planets gets its natural satellites and the stories that each one reveals.
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Thanks, Hey, hey, do you ever wish that we had more moons in our night sky?
Hmmm?
I think the universe as moon does enough times.
Actually, well, I like the moons, and I sometimes wish we had more going on in the night sky, like lots of little moons.
But then I wonder if we had the same line in Star Wars, you know, where he says that's no moon. That wouldn't work.
Maybe instead of the Death Star, they would have had the death constellation.
Or that song when the moon hits your eye like a big pizza pie. That wouldn't work with a lot of little moons. I guess it would work with lots of mini pizzas.
The personal pan pizza would have been invented earlier.
I am Horehem, a cartoonist and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvin and I've never really been a fan of the pan pizza, of.
The pan or the pizza, but just a combination of the two.
The pizza in the pan. Definitely a thin crust kind of guy over here.
Not a bread fan trying to curb your carbs.
I like tomato sauce, but I don't like the kiddie pool of Marinera that they call pizza in Chicago.
I feel like there's a thin line between a pan pizza and like a casserole or like a pot pie.
It's really just a Midwestern hot dish.
There you go, But anyways, welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio.
Where we love everything about the universe, the thick ready questions and the thin crunchy ones. We wonder about how everything out there in the universe works. We take a big curiosity motivated by it, of the universe and try to chew through all of it for you.
That's right, because the universe is a deep dish of amazing facts and incredible things happening in it, full of mysteries and wonderful conundrums for us to try to figure out.
And while physics has made incredible progress and understand the way the world works, we're still answering the kinds of questions we've been asking basically forever, just looking around us, seeing how the world is and wondering like, why is it this way? How did it get to be this way?
Why does deep dish pizza exist? Fundamental questions, that's what we ask here on the podcast, and why doesn't Daniel like them?
We know why deep Dish pizza exist for the same reason vanilla ice cream exists. Because you know, there's a whole spectrum of people out there, and everybody loves different things.
Are you saying the universe is kind of cheesy? I guess it will definitely give you a heart attack as well.
There's no topping that joke.
But yeah, we like to think about the universe and all of the perfecting things in it, like how did things come to be the way they are? Why are we here? And where are we in the universe? And what is around us?
One of the real, simple but enduring joys of being a curious person is just looking up at the night sky. Not just being amazed at it's beauty and odd at the depth of the view that you were looking at, but this shared feeling through time that humans one hundred years ago, a thousand years ago, twenty five thousand years ago probably looked up at almost the same night sky and wondered what was up there and why it looked the way it did, and whether it could have been different.
Yeah, you're looking at the exact same sky that our ancestors did, full of stars, comments, and even a moon. And they probably asked the same question that maybe a lot of you have out there, which is, why do we have a moon? And is it made out of cheese? Is it just a big, fat, giant, floating, deep dish of cheese.
It's a spherical pizza. In the end, I have my spherical pizza theory of the moon.
Do you do tell? Give us a slice of that knowledge there?
All right? He called my bluff I got nothing.
And it was a crusty joke. But yeah, sometimes we look out into the universe, into the night sky, and we wonder why are things there, and how did they come to be the way they are? What's going on? And it wasn't just our ancestors. I'd looked up at the Moon. It's like all of the planet that is looking up at the moon, right, it's feeling its facts and howling at it and rolling with the tides that go along with the moon. The moon has. It makes a big difference here on this planet.
Yeah, and these are not just questions asked by amateur astronomers, but planet heygologist at the Cunning Edge are still trying to figure out the details of how everything came to be in our night sky.
So to the other podcast, we'll be tackling the question how the planets get their moon or moons. Some planets have lots of moons, right.
Yeah, that's right. It turns out our planet is quite unusual in having approximately one moon.
So you do believe in the moon. The moon does exist, right.
I can see the Moon. I know it's there, but how.
You know it's round? It always looks the same.
Well, we've been to the Moon. Obviously we can see all around it, and you can tell that is round. Also, the curved edge between the bright side and the dark side of the moon tell us that it's got to be round.
Oh right, you get sort of like the shadow of it tells you it's round. M exactly is it perfectly round? Is the Moon perfectly round or is it kind of an ellipsoid like the Earth.
Nothing in the universe is perfectly round. There are no actual circles out there, maybe even not the event horizons of black holes due to quantum effects. So the Moon is definitely not perfectly round, and it's also pulled into something of a football shape thanks to the Earth's gravity. Tidal forces on the Moon by the Earth make it a little bit oblong.
Whoa, I guess it's not spinning in the way that the Earth is, so that you get this centropical force. But you're saying that the fact that the Earth is pulling on it kind of stretches it out.
Yeah, it is spinning. It's just spinning at exactly the right rate so that the same side of it is always facing the Earth. This is called a tidal locking. Eventually, the Earth pulls on the Moon to make it oblong, and then they get stuck in this stable equilibrium where that long bit is always pointing towards the Earth because gravity on that long bit is a little stronger, so it's sort of like a pendulum dangling towards the Earth.
Well, it is cool that we have a moon. I guess it to inspire all of these songs and all these stories and legends about.
It is nice to have a moon. It lights up otherwise very dark nights, and it's something very close by to look at. Right, So much of the rest of the night sky are just dots. They're so far away they look like pinpricks. But the moon has features on it. I remember as a kid looking up at it and studying those and like wondering what it would be like to walk across it and to be on it, or to look at the Earth from the moon. It's cool that it's both in the night sky and kind of close by.
Yeah, I guess it's kind of scary to think about it. Actually, it's just kind of swinging around the Earth. It's constantly falling around the Earth. That's what the moon is doing. And it's big, like you don't want to mess with the Moon either.
Yeah, the Moon is pretty massive, and not only is the Earth tugging on the Moon and forcing it to face us the same way, but the Moon is doing the same thing to Earth. Eventually, the Earth of the Moon will both be tidally locked to each other.
Mmmm.
Wait, what does that mean? That means that we will be going around the Moon.
It means that given enough time, the Moon will stay on the same side of the Earth. The Earth's spin and the Moon's spin will balance so that only one side of the Earth ever sees the Moon the same way only one side of the Moon ever sees the Earth.
Wait what for real? When is that going to happen.
It's not going to be for billions of years, but it has already had an impact. Like the rate that the Earth is spinning has slowed down to twenty four hours per spin over the last four billion years. Four billion years ago, it took about six hours for the Earth to spin, So the Moon is slowing down the spin of the Earth. Eventually, it'll take about forty seven of our current days to spin the Earth. That will match the Moon's orbital period, and the Earth and the Sun will be tidally locked to each other. But that wouldn't be for billions of years, and that would be assuming that the Sun doesn't gobble the Earth first.
Yeah, we would have bigger problems in the moon at that point.
But it would be kind of amazing if you could only see the moon from one half of the Earth. It would mean you could grow up your whole life and not see a moon, and then travel to another part of the Earth and see the moon for the first time. That would be incredible.
Whoa which SI gets to have the moon? Can they predict that? Or we even have the same We probably won't even have the same continents.
Right, Yeah, not in billions of years exactly.
I guess we have the moon to thank for having more time in our day. Then without the moon, things will be a lot more hectic.
Yeah, that's true. Those of you who like to work late at night would have much shorter nights to get stuff done.
I think the people during the day would also have a shorter time, right. But it is an interesting question how did we get this moon? Like how do planets get moons at all? And why do we only have one moon versus having lots of moons like other planets which have up to eighty four moons.
It is a really fun, fascinating question, and the answer tells us a lot about how solar systems form, whether our planet is weird, and maybe whether our solar system itself is kind of weird.
Well, you can get all that from the moon.
You can learn a lot just by asking questions.
Well, as you sure, we were wondering how many people out there, I thought about the question of where moons come from and how are they form? How do we get ours? So as usual Daniel went out there into the internet to ask people how do moons form?
Thanks very much to everybody who participates in this segment of the podcast. If you been listening for a while and thinking about participating, please let me encourage you. It's fun, it's easy, everybody enjoys it. Right to me. Two questions at Danielandjorge dot com.
So think about it for a second. How do you think planets get their moons? Here's what people had to say.
I think that most moons form from cloud discs around planets, and that Satan's rings are a picture of that process going on.
Moons form, how planets form, So there's small rocks and then they hit each other and create bigger ones. So when a planet is created, maybe when it gets created, some debris goes out and then creates like a miniature planet aka and Moon.
Maybe I suppose the Moon is just a small planet. It's kind of just a planet that gets tripped by another planet. Right, They're just rocks.
There's got to be like at least six ways moon forms. I don't know, like things crashing together. Apparently we might have stolen the Moon from Venus. Maybe probably a bunch of other ways they could form too.
I think Moon's form either just sort of alongside they're planet like Lucky, or from collisions like our own Moon came from an impact. Those are, I guess the only two I really know.
I would say that this is a collisions between among asceoids planets.
And then you have a planet that has been.
Hit by an asteroid and a small chunk of this planet would be placed in a sort of an orbit, and then you get the Moon.
I think it's when space dust is orbiting a planet and then it clumps together into Moon eventually, or if like an asteroid is caught in the gravitational pool of a planet and falls into its orbit.
I think moon's form from asteroids and other bits of space debris that get trapped in a planet's orbit.
I think that moon's form around planets the same way that planets form around stars. I think that basically there's a bunch of junk floating around the planet that aggregates into a moon or moons.
All right, a lot of interesting answers here. Somebody said at least six ways, that's all they said, But only at least six he or she had six ways in mind.
Mm reminds me that Paul Simon's song just get on the bus, gus don't need to discuss much. Yeah, there must be six ways to get a moon.
Sounds like clickbait. Six ways that we can get a moon. The six one will totally amaze you. Maybe we should be learning from this listener to title our podcast episodes. Although that you hear BuzzFeed is going down, it's going out of business or BuzzFeed news.
Do you think there's a lesson there for us podcasters? Yeah?
I think there are six amazing lessons. The sixth one will totally astound you.
I can't wait to hear.
But yeah, a lot of interesting theories here. From people. Some people think it happens from like a collision. Some people think we stole it from another planet. Is that true?
The truth is that there are lots of different ways to get moons, and we'll dig into several of them today. Hmmm.
The sixth one will amaze.
You if we get there.
If we get that right, know for here, we'll get there. Question is how long before we get there? Will we do it before the hour is up? So it's a close call. All right, Well, let's start with the basics, Daniel, how would you define a moon? Like what is a moon? And what's the difference between a moon and like a satellite or an asteroid or just a space jump.
Yeah, this is an interesting question. In astronomy. We have all these categories we've invented to describe the kinds of things we've seen out there, and then we find things that break those category and it turns out there are no real hard divisions and bright lines between stuff. It's just sort of like where humans like to draw a dotted line between things, and so it's kind of a mess what a moon is signs. Originally they called these things natural satellites, Like when you look at Jupiter and you see things going around it, you call those satellites of Jupiter. And for a long time, like before the space Age, the word moon just referred to the moon of the Earth, which is the name of our moon. But then we started launching artificial satellites, and so when Sputnik went up, people called it an artificial satellite. But that's sort of awkward and a mouthful. So people didn't like saying artificial satellite. They just start calling it satellite, and so that makes natural satellite kind of awkward.
To say, sort of like organic satellite exactly, or farm raised satellites.
But natural satellite is kind of a mouthful, and so now people, even scientists, sometimes say moons. Technically, we have artificial satellites, things we have launched in due space, and then we have natural satell lights, things that are in orbit anyway, naturally without the influence of humans.
But wait, wait, I think I've seen NASA call the moons of Jupiter the moons of Jupiter. They never say the satellites of Jupiter.
That's right, So technically we have artificial satellite and we have natural satellite, though typically we just say satellite for artificial satellite. And now we say moon for natural satellite, even in scientific publications and like official press releases. So now moon has come to mean natural satellite.
I see. But I guess if you put the in front of it, then it's our moon. Like the moon is our moon, but then other moons are just moons.
Yeah, exactly, the moon is our moon or Luna if you prefer. And moon with a lowercase M means any kind of natural satellite.
I guess you can have rocks, and you can have the.
Rock exactly, and we have lots of rocks in orbit, but I don't think we have the rock in orbit yet, though I haven't seen Fast and Furious twenty seven or whichever.
I'm assuing. The next one, the you know, speed up a ramp and somehow make it up to space and crash into a space station while he jumps and also launches a rocket launcher.
He's got to flex his muscles at some point. Another question is size, Like, is every object that's orbiting the Earth a moon? Every tiny little rock is at a moon of the Earth. Officially, there's no lower limit, right, Like, every natural object with an orbit around the planet, technically you could call it a moon.
Wait, what I mean. No, I mean at some point it's just the rock, right.
The moon is just a rock. Also makes the Moon different from other rocks orbiting the planet is just the size and there is no official lower limit to the size of a moon. You could have a moon of anti size and any limit you place is going to be totally arbitrary. Right.
I wonder if the definition also has to do with how stable its orbit is. Like, if I just throw a rock into space, it's going to be orbiting the Earth for a little bit. Does that mean it's a moon that's not really in orbit?
Right? I don't know how strong you are recent I haven't seen you in a while, but I don't think you could throw a rock and actually get it into orbit. It has to be in orbit.
Oh yeah, do you want to make that bet? I'll bet you a billion dollars because with a billion dollars I can throw the rock.
If you get a rock orbit in the Earth, I will campaign NASA to let you name it officially.
Well, if you get me on a rocket ship into space to throw a rock, we'll start the whole process.
But we can look at the typical sizes of these things, and like in our Solar system, there are a few hundred of these moons. There's six planets that have moons, for a total of two hundred and twenty six moons, and typically the planet to moon mass ratio is at least ten thousand to one, so moons typically have a much smaller mass than the planet they orbit.
Wait, there are six planets in our Solar system with moons. Who doesn't have a moon.
Neither Mercury nor Venus have moons. That's probably because they're too close to the Sun and so tidal disruption basically pulls those moons away.
Oh interesting, and I just figured it out. That's probably what the listener meant when they said at least six ways.
Oh nice. Probably every moon has a unique story, m they have their own origin story. Of course, our moon is a big exception to this ten thousand to one rule, right, because the mass of the Moon is about one eightieth of the mass of the Earth. It's like more than one percent of the mass of the Earth. So it's a big honk and moon it's very unusual.
So most of the moons in the Solar system, some of them are as big as our moon. But you're saying that the ratio compared to their planet. Most of them are small.
Most of them are small exactly. And then there's the case, for example, of Pluto. Pluto has a moon, which is Sharon, which is one eighth of its mass, and so like we call Pluto a dwarf planet, and we call Sharon a moon of Pluto. But you know, you could also argue that it's really like a dwarf planet binary system. Which one is the planet and which one is the moon. It all gets kind of fuzzy.
Whoa, it's like a double planet.
Man.
One way to distinguish the two scenarios, like having a double planet or a planet with a moon, is whether the center of mass of the system is within the surface of one of them. If you find the point that averages where all the mass is and the point around which the two objects really are orbiting, if that is underground one of the two objects, and you call the more massive one a planet and the other one a moon. Otherwise you call it a binary system. Again, that's still kind of arbitrary, right, We're just like giving these things names and drawing lines between them. Really, there's just a bunch of different rocks out there in the universe orbiting each other.
Except also, Pluto is not a planet, So can you have a moon around something that's not a planet? That can you can a moon or can a moon have a moon.
You can have a moon around a dwarf planet, though maybe you would want to call it a dwarf moon. I don't know. And it's possible in principle to have moons around moons. There are asteroids that have moons.
And you still call them moons, not master moons.
Some people want to call them moonlits or moon moons like the moon of a.
Moon or moony's or mini moons.
And there are even some moons like Rhea has its own ring system Saturn's moon Rhea.
Wait what some moons can have rings? Yeah, because they have so many mini moons.
If you're big enough that you dominate the gravitational environment nearby, then yeah, you can have your own rings.
I mean, I guess technically anything can have a satellite, right, Like I can take a baseball put in into space and then knock a you know, a speck of dust in orbit around it, right.
Yeah, exactly. That wouldn't be a planet or even a dwarf planet, but they would have an orbiting object. Is that a natural satellite? I'm not quite sure.
All right, what else do we know about moons?
If you try to dig into the ancient history of moons, it gets quite confusing to read about because until Copernicus, moons were actually called planets, like the Moon itself was referred to as a planet, like when you talked about astronomy in the fifteen hundreds, the Mars, the planet of Venus, the planet Luna. And it wasn't until Kepler, who was thinking about how these things orbit each other, who had a better understanding of these orbits, that we started calling the Moon a satellite of the Earth. And then the satellites of Jupiter are of course the moons of Jupiter. So there's a really fun interesting history to these words.
Wait, really, so like, for a moment in human history, we thought the Moon was another planet orbiting the Solar System. Technically it is orbiting the Sun.
Yeah, it is orbiting the Sun, and it is orbiting the Earth. Right, It's not that we thought the Moon was a planet like Mars. It's just that we categorized all these things the same way. All these words are just buckets, right, and they're just like, grab a bunch of the stuff that's out there and gather it all into a conceptual bucket. And where the lines between these buckets are is a little bit arbitrary. And so it used to be that we lumped the moon in with the planets. Now we have a separate category for things that orbit the planets.
Shoot for the moon. I guess when you're naming things right, Well, let's get a little deeper into where the moon came from. Where do moons in general come from, how do other planets get their moons, and what does it all mean about the history of the Solar system. But first, let's take a quick break.
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All right, we're getting a little looney here talking about the moon and how we got it and how do planets get their moons.
It is really fun in ancient question to wonder why that's is in our sky and why Jupiter has more moons than we do. And if history had been different, would we have had a bunch of moons? What would it be like to live in that scenario? Could we sail as well? If the tides were crazy and complicated from having like fifteen different little moons?
Hmmm? Interesting? Yeah, that would affect the tides, right, But that wouldn't affect navigation, would it.
It wouldn't affect navigation, but it would affect when it's easy to launch your ships or not, and so it might affect lots of industries and exploration.
M Also, if your were wolf would be pretty complicated, right, probably, just you know, just to plant ahead.
Maybe you changed into one kind of wolf for one moon and another kind of wolf for another moon, and like a Pomeranian for the little moon or a shit.
Sou for Yeah, there you go, different moons for different breeds.
Two moons up in the sky. Then you're a hybrid. Right, this sounds like a fun science fiction story. It sounds like a great Ya novel, so many spin offs.
All right, we talked about the word moon is kind of flexible and it's not quite super well defined, but basically it just kind of means like a big rock circling around a bigger rock. Right, that's right.
It's a big rock orbiting another big rock.
And as you said, six of the planets in our Solar system have them. Venus and Mercury don't because I guess they're too hot, you said. And if they had moons, then the Sun would have disrupted its orbit and probably made it crash into the planet, right or fly away.
Yeah, it's all about the tidal forces. It makes it basically impossible for Mercury or Venus to have moons and to keep them. It's because they're so close to the Sun. It's not their actual temperature, it's the gravitational tidal forces from the Sun.
All right, Well, what do we know about where moons come from and how they're formed?
So you can tell a lot about where a moon came from based on what it's made out of and how it's orbiting. If a moon is mostly in a circular orbit, and the circular orbit follows the tilt of the planet, so for example, it's orbiting around the equator in mostly a circle. Then it's very likely that that moon came from the same stuff that formed the planet. Remember how planets form in the beginning, it's a big cloud of gas, gas, and dust that forms the whole Solar System. Most of the gas is gobbled up by the star as it forms, and our Sun has ninety ninety percent of the mass of the Solar System. But if you get a little isolated pocket of heavy stuff that has its own gravity and can gather itself together, then you can get a planet. So a planet sort of forms the same way the Solar System does. It's the gravitational collapse of a big blob of.
Stuff, right, And initially it's just kind of like a big cloud of rocks and dust that's spinning or has kind of an overall spin.
Exactly. The reason you get moons around planets is the same reason you get planets around stars. Right. You don't just get all the mass of the Solar System collapsing into the Sun. You get these little pockets that have enough gravity to form themselves together. And then are moving at high enough speed that they can resist falling into the Sun. Now, around those planets, of course, you also have little clouds of gas and dust. Some of it collapses into the planet, most of it, but some of it pulls itself together and has enough velocity to avoid falling into the planet. And so if you have enough velocity and you can pull yourself to together, then you can form like a little planet's planet, a little miniature system around the planet, the same way the planet is going around the Sun.
Right, because I guess gravity. That's how gravity works, right, Like everything is attracted to everything else. It's not just like we're attracted to the Sun or just attracted to the planet Earth. It's like I'm attracted to my car and to this base bomb. The base will attracted to me. And so if we were out in space, we would both be falling towards the Earth, but then we would there would also be an attraction between us. And sometimes I think what you're saying is that if a clump of dirt and rocks when the planet is formed, it's kind of far enough out there, it will clump together before it comps with the Earth.
Yeah, that's exactly right, And there's sort of two steps there. One is have enough velocity, like are you spinning fast enough that you can basically get in orbit around the planet, And that's how you get like a protoplanetary disc. The planet forms and have some stuff out there that hasn't fallen into the planet. So initially it's like a disc and that can pull itself together using gravity into a ring. And then there's the question of whether that ring can pull itself together into a moon or not. Sometimes it stays a ring and sometimes it forms a moon, and that depends on how close you are to that planet. If you're really really close to that planet, closer than what we call the Roche limit, then tidle forces from the planet are too strong. If you try to form a moon, the tile forces will tear it apart. If you're outpast the Roche limit, then the tile forces are weak and you can gather together into a moon. So you have to have enough velocity to avoid falling into the planet, and then you have to be out past the Roche limit to have a ring get turned into a moon.
Because I think, as we've talked about before, gravity kind of depends on the distance between two things. Right, So if you're really close to the Earth, then like the difference between one side of the Moon and the other side of the Moon is they experience very different gravitational forces. But maybe if you're far out in it, beyond this limit, then you don't see this difference in pull from the Earth between one and the other, which kind of lets you clump together.
You'll still always feel that difference, right, and like the Moon does feel that difference. That's why we talked about earlier. The Moon is a football. It is being pulled by the Earth's tidal forces, but it's far enough away that those tidal forces are not strong enough to tear it apart. The roach limit for the Earth is around ten thousand kilometers. The Moon is like three hundred and eighty five thousand kilometers away, so it's well past the Roche limit. If the Moon was much much closer, if it was like less than ten thousand kilometers from the surface of the Earth, the Earth would tear it apart with those tidal forces into a massive ring system.
Instead, where some of the rocks in that ring are going at different speeds.
Right, it would be pretty cool to see the Moon get torn up into rocks. Not all of them would have the same velocity originally as the Moon. I'm sure it would be somewhat destructive and chaotic. Some of them would end up falling to the Earth, some of them would get lost, and some of them would end up in orbit.
Cool And I think that, as you said, also applies to planets, right like you kind of have to be a certain distance away from the Sun just to form a planet, too.
Exactly, if you're too close to the Sun, then the Sun's tidal forces, which are very powerful, will pull you apart. So the roche limit for the Sun is like seven hundred and fifty thousand kilometers. So if the Earth whereas that close to the Sun, not only will we be fried, of course, but the Sun would pull us apart with its tidal forces. As you said, the gravitational force depends on the distance, and so the Sun's gravit on the near side of the Earth would be so much more powerful than the Sun's gravity on the far side of the Earth. It's effectively pulling us apart, and the Earth is not strong enough to survive if it's closer than seven hundred and fifty thousand kilometers. Fortunately, we're like one hundred and fifty million kilometers from the Sun, so we're nowhere near the Roche limit. And this isn't like an exact, hard and fast number, it's like approximate. It depends on the mass of the object, and it depends on structural features of it, Like if you had a planet made out of diamond, it could get closer to the Sun than a planet made out of gravel.
But I guess generally speaking, it depends on the size of the thing in the middle, Like the Sun has a very big roach limit and the Earth as a smaller one.
Yeah, that's true. You can get closer to the Earth than you can get to the Sun. But if you see a moon around a planet and it's orbiting in a circular orbit and it mostly has the same angular momentum as the planet, then you suspect that probably it came from the same stuff that made the planet, and when that planet coalesced into stuff, not all of it got turned into the planet, some of it got left over and turned into a moon. So circular orbits with the same anglarmentum probably came from the same protoplanetary disk that formed the planet.
Okay, so then if a moon is orbiting a planet kind of in the same plane as the planet is spinning, then most likely they are like siblings, kind of like they were born at the same time from the same stuff.
Yeah, I suppose you could say so, though it's sort of the scenario where like one twin is really big and the other one is tiny.
Yeah, they're fraternal twins.
Exactly, and probably one's pretty grumpy about not getting as much of dinner.
All right, So that's one way maybe a moon can form, which is like it's borne along with the planet, and you can sort of tell which ones those are, Which of those do we have in our Solar system, Like, is our moon one of them?
Our moon is not one of those. Our moon is actually a very weird case. Most of the moons in the Solar System do not have nice circular orbits that are orbiting with the planets. In fact, most of them have elliptical orbits that have weird tilts, And that suggests a completely different history for how.
That moon formed, necessarily, because like I wonder if maybe the moon was formed as a sibling, but then it got not by something and then it got a skewed orbit.
Yeah, that's certainly possible, but we think that most of the ones with skewed orbits, that with tilted orbits that precess, for example, are probably captured objects, things that came nearby and were grabbed onto by the gravity of that planet. You're right, it's possible for moon to be formed with the planet and then get tilted through a collision. You can tell the difference by looking at what that moon is made out of. Is it made out of basically the same stuff that formed the planet, or is it made out of something totally weird and different. So that's sort of like the key piece of information for distinguishing.
Like a DNA test prove of your siblings are not all right. Well, then what does not having a circular orbit tell you about the moon.
It tells you that probably it was captured that the planet formed, and the Moon comes from somewhere else. It's like an asteroid or a dwarf planet or something that was floating around and just came too close to this massive object and its gravity took over. There's a vicinity of a massive object we call the hill sphere, which is the region sort of where its gravity dominates where everything else that's far away can basically be neglected, and if an object passes within the hillsphere, it's a candidate for getting captured.
But isn't that kind of weird? Or isn't it kind of unlikely that you'll just kind of catch a big rock out there and it'll have just the right speed and distance to fall into a stable orbit, Like, aren't stable orbits kind of hard to get into?
It is unlikely and it is weird. You're right. You have to match the radius and the velocity. Like the reason the Earth is in a stable orbit is because it has the right velocity for our radius. You have to be moving in a certain velocity at a given radius.
Like if we slowed down at all in our orbit, we would start to spiral into the Sun.
Right first orbit is actually quasi stable, so if we slowed down a little bit, gravitational forces would actually push us back towards our orbit. But that's a whole other topic.
Like if you slow down a lot, though, yes.
If we slow down a lot, we would fall in towards the Sun. If we sped up a bunch, we would be ejected from the Solar system. And so you have to have this match between these two quantities, and it's not trivial for that to happen. Not only do you have to have the right velocity and the right radius, but you also have to lose energy in order to fall into an orbit. Any object that falls into the Solar System by definition has enough energy to escape because it came from outside the Solar System. And so if it just passes through on like a hyperbolic trajectory, you know it has enough kinetic energy to climb out of the gravitational well of the Solar System because it came from outside the gravitational well. So in order to get captured, it not only does it have to come in at the right radius and the right velocity, got to lose a little bit of energy so that no longer has the energy to escape. That can happen if you like drag on the atmosphere the planet a little bit, which gives you a little bit of friction, or maybe another moon steals a little bit of your energy. So like, the more moons you have and the puffier your atmosphere, the more likely you are to be able to capture an object that comes near you.
It's kind of interesting to think of this idea the planets, like the Earth has a kind of a halo, kind of right, like a big sphere around it. It's basically like a giant net like it. Whatever falls into it, it's gonna get sucked in.
Yeah, exactly. And if you fall in too far and you slow don too far, then of course you're going to burn up as you enter the atmosphere. So it's a delicate operation. Physicists also think that sometimes a pair of objects that are orbiting each other can fall in the hill sphere and then one of them can get captured and the other one can get ejected. So all these sort of complicated things have to go just right in order for a planet to capture a moon.
And by capturing, that's a nice word for stealing, right right, you're like.
You're dancing, you're dancing with them. How of that?
Oh, I see see? Is that where that listener who mentioned earlier that maybe we got our moon by stealing it from Venus? Is that? Is there some truth to that?
We don't think that our moon was stolen from Venus, but we do think that lots of the moons of Jupiter and Saturn, for example, and these guys have like dozens of moons come from scattered objects in the early Solar system. Remember that very early on things were forming. There may even have been more planets than the ones we have now, and everything was quite chaotic. So now we have sort of an orderly solar system where everything is in place because it's been in place for so long, and things that were not in place have been lost or captured or fallen into the Sun. But in the early days there were a lot of things going in crazy orbits and crazy trajectories, and so Jupiter and Saturn sort of like hoovered up a bunch of them.
So like, if you look at the moons of Jupiter, for example, they're not all going to be like lined up in a plane like our solar system. They probably all have like crazy orbits around Jupiter.
Right, Yeah, that's exactly right, And that's why we think that most of these were captured. Also, in the cases that we've been able to try to study what these moons are made out of, they're all made out of totally different things, and so it doesn't look like they formed from the same sort of scoop of Solar system stuff that Jupiter did.
That's what you were saying, Like you can check the DNA basically the DNA of the moon and the planet to see if they came from the same stuff, and sometimes they don't, right.
Yeah, exactly. That's still tricky to do because we haven't landed on those moons and like really taken samples that we can study. But we can like do spectroscopy. We can see the light that bounces off of them, we can see what they emit these kind of things. We have done some flybys, and so we have ideas for what these moons are probably made out of.
And in the early Solar System, like you said, things were like there were probably like rock giant rocks flying all over the place, and so it wouldn't be that weird for some of them to fall into orbit around like a passing Jupiter or Center. So is that how we got our moon? Did we capture it or steal it from somebody?
So we think our moon is unusual because it doesn't fall into either of these categories. We don't think that the Moon was formed with the Earth. We also don't think that a wholly formed moon was captured by the Earth. Instead, we think it comes from a collision. We think that there was the proto Earth, this early planet, and then there was another Mars like planet that came by and smashed into the Earth, and there was this incredible collision where essentially these two planets merged, but they left a huge debris ring, and then that debris ring pulled together and made the Moon.
Does our moon have an orbit that's around our equator or is it tilted?
The Moon doesn't have a totally random orbit relative to the angular momentum of the Earth, because the two objects are essentially formed by the combined angular momentum of this collision. So you get this collision and basically you start from scratch. You have a new blob of stuff which is then going to coalesce again into a planet and a moon. And so there is a relationship of course between the Earth's spin and the Moon's orbit, and that's because they come from this combined blob from this colison.
It's pretty dramatic. I think you can look up videos of simulations of it online. It's like the Earth, the pro too Earth before Earth was just hanging out and then this giant rock just slams into it. It all sort of explodes together, but then gravity pulls it back together. Into Earth and the Moon exactly.
And the early Moon, we think, was much much closer to the Earth, something like a tenth of its current orbit, and then over time it spirals out and ends up becoming tightly locked to the Earth. One reason we think this is that we've been to the Moon and we've been able to land on it and study what it's made out of, and we see that it's made out of a lot of stuff that's very very similar to what the Earth is made out of, which suggests that they do have some sort of common origin.
And does the Earth also sort of have moon like materials in it that are not like the rest of the Earth.
Well, we see that the Earth and the Moon have a lot of very similar elements, which suggests they all came from the same stuff, But the Moon has fewer of like volatile elements. Things that vaporize at low temperatures were probably lost in this high energy event, and the Moon has small or gravity, and so it's not able to recapture these things the way the Earth did, so the Earth and the Moon don't have exactly the same kinds of stuff. The Moon also has a sort of surprisingly small iron core, which overall makes the Moon have lower density, and simulations confirm that this is what you would expect from this kind of collision, that the Moon was formed more out of the sort of external debris and the Earth sort of got a bigger sampling of the core stuff.
I guess the Moon is also smaller, so it doesn't have as much gravity compressing it, right, making it denser exactly.
And when they study like what's inside the Moon, they find samples that suggest the Moon was molten down to like a surprising depth. And you don't expect the small body like the Moon to have the gravitational pressure to like melt its inerts the way the Earth does. So they think that the Moon was probably molten because of this collision, not because of its like gravitational pressure.
Hmm.
Interesting. And I think they also found like the same cheese inside of the Moon as some of the cheese we have on Earth too, right, that's part of the.
Yeah, they found pan pizzas rejected on the surface of the Moon that nobody could.
Eat, that's right, the same mozzarella, cheese, the same cows, even space cows. All right, well, that's the origin of our moon. Let's get a little bit into what that means about the formation of all moons and the formation of our whole solar system. Why we're here and why are things the way they are. First, let's take another quick break.
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All Right, we are coming face to face with the lunacy here in the podcast talking about the origin of moons, and in particular our moon. You're saying, it's interesting because, like our moon is sort of a combination of how some of these other moons can form. Right, Like, we had a proto planet Earth, and when there was a visitor that was flying around came neros, it sort of got captured or collide with our Earth, and then it all became a big mess. And out of that mess, you know, the Earth and the Moon form sort of a siblings, but also not siblings, because there was it came from a different planet. The stuff.
There was a big fight early on, and we were left over cleaning up the mess. Yeah, it is really fascinating, and I love this sort of archaeology, this like detective story, figuring out what happened billions of years ago, reconstructing the story from the clues that are left behind, These really subtle hints. You know, why does the Moon seem to be made of the same stuff as the Earth, but it doesn't have sort of a close circular orbit the way you would expect. Why exactly is it so big? These stories are really fascinating because we missed so much of the Solar System history. You know, like billions of years happened with crazy fireworks in the sky and we weren't here to look at it. But we can still get hints about what happened.
Mmm.
Do you think that's an official job title out there, like space archaeologists.
Space murder mystery, you know, exactly.
Give me like Indiana solo combination of Janna Jones and hand solo.
And you know, we're still learning stuff about moons, like Mars has some funny moons, Phobos and Demos, and the smaller of those two moons, Demos, is really tiny. It's only like nine miles across, has a really weird short of blobby shape to it, and for a long time people thought it was probably a captured asteroid because of its orbit, but they recently send an orbiter very very close to it, super close approach by this spacecraft from the UA Emirates. Actually it's called Hope, and they were able to study what it's made out of and discover that it has sort of the same carbon and organics that Mars does, unlike the asteroids, sort of like the DNA test you were saying before, which means that Demos probably is a chunk of Mars that got blown off in some sort of prehistoric collision.
Isn't that also kind of the theory of how we got life on Earth like a possible way, and maybe Mars got hit by something. It threw a bunch of rocks out into space. Some of them became Demos, the moon of Mars, and maybe some of them came to Earth bringing like little bacteria.
Perhaps we're very sure that rocks from Mars have landed on Earth. We have found them and their geology matches Mars and doesn't match Earth. So there's no controversy about whether collisions from asteroids on planets can knock stuff off into outer space and have it land on other planets. Whether there's life in those rocks that then seeded life on Earth totally open question. There was this famous misdiscovery about twenty years ago when they found these weird little shapes inside a Martian rock on Earth, and they made this announcement that they were certain that there was life in them that these shapes could only be made by life, But then later analysis demonstrated that you could make those things without life. So there's no concrete proof that life has traveled between planets on an asteroid, but it's totally possible for it to happen. And Demos definitely is a chunk of Mars floating in space. Sort of like if you went out into space and you found like Manhattan floating out in space, you'd be like, WHOA, where'd this come from?
Yeah? Well people wonder about that.
Now, what's theory? How to Manhattan get so weird?
Where did New Yorkers come from their lunatics?
But they have the best pizza, right, so I don't want to have to go out of space to get my pizza.
You just insulted everyone in Chicago. You lost a big chunk of our listenership.
I love Chicago. I love Chicago Wins, and I love the Chicago Wins, love Chicago pizza. That's all fine with me, all right?
Well, what does all of this tells about the formation of the Solar System and like how we ended up where we are and how we are.
It's a story that we are still unraveling, right, we're still learning about our moon, We're still learning about our neighbors moons. We're still discovering moons. Right, Saturn recently past Jupiter. I think in the number of moons that it has, each one tells us a little bit of the story of the Solar system, and what we're looking for now are like more details for that story. As you said earlier, we're asking questions like can moons have moons? A lot of people think that it's impossible for the same reason that like Mercury and Venus don't have moons from the tidal forces of the Sun, that Jupiter's moons probably can't have their our own moons because of the tidal forces of Jupiter. But you know, Saturn's moon Rhea probably has rings which even though they're disrupted by the tidle forces, they can still be in orbit around Rhea. And there's like hundreds of minor planets deep out there in the Solar system that do have their own moons, like Pluto. But beyond that, we're also looking into other solar systems to try to understand whether the moons that we have are weird or typical. Right, Like, we only so far have this one solar system to study at this level of detail. Twenty years ago, we were able to see planets around other stars, which tells us a lot about whether the planets in our solar systems are weird. Now we're pushing those boundaries to try to look for moons in other solar systems. This thing we call exo moons to try to discover if the distribution of moons that we have is strange or pretty typical in the universe.
Well, that's wild. How are we seeing moons and other planets outside of our solar system? Can we see them or do we have to infer them from the gravity.
So there's a few ways that we discover planet. Then we can try to apply those ways to discover moons. One of the early ways we discovered planets around other stars was seeing their gravitational impact on the star, and that would wiggle the star and cause like a change in the frequency of the light, this Doppler shift, and so we could discover that there was something pulling on that star. We don't think that method will work for discovering moons because it's really hard to distinguish. It's the gravitational effect of a little moon around a planet from the planet itself. What we do think is possible is the transit method is eclipse method where a planet passes in front of a star and dips the light that comes from it, And if that happens in a regular way, we can tell that these little microeclipses come from a planet. Well, if those eclipses have their own little dips in them, because the moon going around the planet sometimes blocks this life from the star and sometimes doesn't, then you can discover moons around those planets. So like microeclipses within microeclipses.
Whoa exo eclipses exo exo moon eclipses.
Yeah, they're like eclipses squared. And we don't have any confirm exomoons yet though there are a couple of candidates from the Kepler telescopes that look promising, but people can't yet agree whether that actually is the discovery of a moon. And more recently we've developed this incredible technology to do direct imaging of exoplanets, like telescopes that actually take pictures of planets around other stars. Blows my mind. I never thought that would be possible. A lot of it involves like blocking out the light from the star itself, so you can see the ring around it. And sometimes that lets us see planets being formed. So we can see, for example, a star with a planetary disc around it. And in one case we have a direct image of a protoplanetary disc which seems to have a planet with its own disc around it. So like you can see the planet and it's got like a bunch of stuff around it, and maybe that stuff will form into a moon.
You can see the rings in it.
Yeah, exactly, you can see this disc that's going to form it either into rings or moons or something.
Yeah, it's pretty mind blowing because I wonder like if a lot of people know that you can actually see the moons of Jupiter kind of with your naked eye, or at least with a small telescope. You don't need like a super amazing telescope. You can just use something you can buy for your house and you can see the moons of Jupiter.
Yeah, it doesn't take a fancy telescope. It was one of the first things that Galleys saw when he pointed a telescope with the sky. It was one of the first people to ever do this, and he saw the moons of Jupiter. It's not hard. You can do it, and it's pretty exciting to see Jupiter expand from this tiny dot you can see with the naked eye to this whole orbital system with its own dynamics.
Hmm, pretty cool. All right. Well, one thing that's day announced recently is that we're sending more people to the Moon.
That's right, if Elon Musk ever gets that starship off the ground.
No, the new artem is missions right, It isn't one of the plants to send more people to the Moon.
Yeah, that's exciting. It'd be cool to go back to the Moon. With all of our advanced technology, we can take more detailed measurements. We can map its magnetic field, we can understand its geology and its history even better. Yeah.
I wonder why kind of pizza they would eat there though? It's a deep dish because but there's not much that much gravity, So can you have a deep dish pizza?
Probably every pizza would rise a lot more because there isn't great.
Oh my goodness, in space. Every pizza is a deep dish pizza. That's why you're not going to Earth Dannel.
Or maybe astronaut pizzas just freeze dried and gross no matter where it comes from.
I guess there's something one way to find out to take the podcast to space. All right, Well that kind of answers our question. How the Moon's form basically two main ways, right, They either capture something flying by in space, or they form together with the planet, or some combination of the tube where something comes from space crashes into you, creates a big mess, and then you both sort of form together or reform together.
And the incredible thing is that by looking at the Moon today we can mostly figure out how that happened, how this huge space rock ended up in orbit around the planet. Yeah.
And the cool thing is that you can see the Moon almost every night. Every night you step outside of your house, you can see this giant rock floating in space there for us to see, pretty shiny, pretty bright.
It's nice to have one big, fat moon. I agree.
All right, Well, we hope you enjoy with Thanks for joining us, see you next time.
Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last Sustainability to learn more.
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