Speaker 1: The Moon really doesn't get enough love. Of all of
Speaker 1: the features of the night sky. It's the only one
Speaker 1: with real texture visible to the naked eye. Everything else
Speaker 1: is either just a tiny distant point or an overwhelming
Speaker 1: ball of fire. There's nothing else out there that you
Speaker 1: can just sit and stare at and appreciate the feeling
Speaker 1: of looking through a gulf in space. We are here
Speaker 1: on a rock in space, and there's another whole rock
Speaker 1: right over there, so far away yet so weirdly big,
Speaker 1: that we can see it and see features on it.
Speaker 1: It challenges the mind, forcing you to come to griffs
Speaker 1: with our cosmic context of massive balls of rock spinning
Speaker 1: through a dark ocean. And there's so much more that
Speaker 1: moons do. Moons tell us about the history of the
Speaker 1: Solar System and help reveal what's going on inside their planets.
Speaker 1: They have volcanoes and weird colors and shoot jets of
Speaker 1: water into space. They might even harbor life. Seeing Jupiter's
Speaker 1: moons is what led Galileo to understand the structure of
Speaker 1: our Solar system. And the thing that our moon has
Speaker 1: misled us about is their number. Having one big, fat
Speaker 1: moon is not normal planets, and our Solar system have
Speaker 1: many moons, so many fabulous moons. So on this episode,
Speaker 1: we're going to give moons the love they deserve. Welcome
Speaker 1: to Daniel and Kelly's Extraordinary moon Malicious Universe.
Speaker 2: Hello. I'm Kelly Waitersmith. I study parasites and space, and
Speaker 2: I am not above jokes that involve mooning.
Speaker 1: Hiel, I'm a particle physicist, and I can't tell you
Speaker 1: how long it's been since I mooned somebody.
Speaker 2: Oh, yes, that was a joy to hear about mooning
Speaker 2: jokes when I was a kid. I don't remember actually
Speaker 2: mooning anyone in my teens. I was a bit of
Speaker 2: an insecure teen. But but ah, yes, I remember being
Speaker 2: mooned as.
Speaker 1: A teen and as an adult. Kelly, can I ask
Speaker 1: you how long has it been since you mooned anybody?
Speaker 2: I plead the fifth I married a man whose last
Speaker 2: name is Wiener, and so my sense of humor has
Speaker 2: become a bit more juvenile.
Speaker 1: Fill in the gaps yourself, everybody, that's.
Speaker 2: Right answer, Daniel. My question for you is, so today
Speaker 2: we are talking about moons. Have you ever heard of
Speaker 2: a convincing story of the moon influencing someone's behavior? Oh?
Speaker 1: I mean, I can imagine the moon influencing the tides,
Speaker 1: and the tides definitely influence people, m h.
Speaker 2: But you don't buy the like, you know, more people
Speaker 2: come into the er on a full moon.
Speaker 1: People do all all sorts of weird stuff, and I
Speaker 1: wouldn't be surprised if people acted weirder on a full
Speaker 1: moon and there were more visits to the er. So
Speaker 1: I'm gonna have to play the fifth on that one.
Speaker 2: Yeah, I mean, I don't think I buy that people
Speaker 2: are like more kooky on full moon nights unless they
Speaker 2: have been convinced that they ought to be more like
Speaker 2: risk taking on full moon nights and it's on their
Speaker 2: mind than I can imagine like they sort of have
Speaker 2: psyched themselves out. But I do like, if I get
Speaker 2: up early in the morning and it's brighter because of
Speaker 2: a full moon, I'll be more likely to like go
Speaker 2: jogging or something. And so I can imagine maybe people
Speaker 2: going out and doing more stuff outside during a full
Speaker 2: moon and maybe being more likely to like accidentally get
Speaker 2: hit by a car or something like that. And of
Speaker 2: course we were talking about how some organisms are influenced
Speaker 2: by the tide and so their's circadian rhythms are influenced
Speaker 2: by the tide, so their behaviors are influenced by the tide.
Speaker 2: But yeah, I don't think I buy that animals or
Speaker 2: people are more likely to be doing kooky crazy stuff
Speaker 2: because it's a full moon or anything like that.
Speaker 1: But I don't know. People are weird, and I think
Speaker 1: if you work in an er, you see the weirdest
Speaker 1: side of people. I remember Katrina worked at a hospital
Speaker 1: in Geneva, and near the entrance they had a huge
Speaker 1: display of the weirdest stuff they had pulled out of
Speaker 1: people's throats and other holes. Oh no, and boy was
Speaker 1: there some weird stuff there.
Speaker 2: Oh. I did once for a research project look for
Speaker 2: papers describing things that had been pulled from orifices. And
Speaker 2: we are an inventive species, and if it turns out
Speaker 2: that our inventive behaviors are tied to full moons, then
Speaker 2: we might be happy that we have only one moon
Speaker 2: and not many moons.
Speaker 1: And the moon figures so prominently in the sky and
Speaker 1: in our literature and in our imaginations that it's easy
Speaker 1: to imagine that aliens would look up to their sky
Speaker 1: and see the same thing. Or if you're a science
Speaker 1: fiction off third to imagine alien seeing a radically different sky,
Speaker 1: many moons, or something similarly weird. And there are lots
Speaker 1: of examples in science fiction of multiple suns or multiple
Speaker 1: moons or other variations on our experience, but you don't
Speaker 1: actually have to go that far visiting another solar system
Speaker 1: to see examples of multiple moons.
Speaker 2: And if I can just plug my friend's book for
Speaker 2: a second, Under Alien Skies by Phil Plait is an
Speaker 2: amazing book describing what might be like to visit alien
Speaker 2: planets and look at their skies. And Phil was a
Speaker 2: guest on our show early on when you and I
Speaker 2: started podcasting together.
Speaker 1: That's right, and everything he writes is insightful, well informed,
Speaker 1: and fun, so go check it out.
Speaker 2: Yes, all right, so you asked our extraordinaries which planet
Speaker 2: has the most moons?
Speaker 1: That's right, And if you would like to contribute for
Speaker 1: this segment of the show in future episodes, please don't
Speaker 1: be shy. We would love to add your voice to
Speaker 1: the chorus. In the meantime, think about it for yourself
Speaker 1: for a moment. Which planet in our solar system do
Speaker 1: you think has the most moons? Here's what the extraordinaries
Speaker 1: had to say.
Speaker 3: And believe it's Jupiter. That has the most moons. I
Speaker 3: think they just discovered a couple more, But technically really
Speaker 3: depends on what you define as moon, because rings are
Speaker 3: just particles orbiting the planet as well. In our Solar system,
Speaker 3: the planet with the most moons is probably Saturn.
Speaker 1: Outside of the Solar System, I have no clue.
Speaker 3: I would say Jupiter because of how big it is,
Speaker 3: but then it might have pulled some moons into it,
Speaker 3: So maybe Saturn has the most because it sits in
Speaker 3: a sweet spot.
Speaker 1: What do you think, glenn on? Going straight for Jupiter? Why?
Speaker 1: Because it has the most moons? All right?
Speaker 3: My initial thought would be Saturn or Jupiter, but I
Speaker 3: have been reading recent news reports about many new discoveries
Speaker 3: of moons around Jupiter, so I'm going to go with Jupiter.
Speaker 1: All right, So most votes for Jupiter. Kelly. If you
Speaker 1: hadn't read the outline, what would you have done?
Speaker 2: Cod Ah, I love that you think that I read
Speaker 2: the outline.
Speaker 1: Giving you the benefit of the doubt here just in case.
Speaker 2: No, I'm kidding. I did read the outline, so I
Speaker 2: would have guessed Saturn or Jupiter, or I would have
Speaker 2: guessed that because this is a dKu episode, maybe the
Speaker 2: answer is we don't know, because maybe maybe it's hard
Speaker 2: to count all the moons and we aren't sure if
Speaker 2: Saturn or Jupiter is the winner.
Speaker 1: Yet that's probably the right answer. Actually, Also it depends
Speaker 1: on what you mean by moon, the definition, which is
Speaker 1: always evolving as we discover more stuff out there in
Speaker 1: the Solar system, because though humans like to make tidy
Speaker 1: categories for the things that are orbiting the Sun, in
Speaker 1: reality the universe is chaotic and there's just like a
Speaker 1: huge smooth spectrum of stuff from tiny little dust grains
Speaker 1: all the way up to Jupiter and basically everything in between.
Speaker 1: And if you try to put everything in boxes and
Speaker 1: make artificial dotted lines to separate it, you'll find a
Speaker 1: bunch of stuff in fuzzy category and you'll argue about
Speaker 1: whether it's a moon or whether it's not a moon,
Speaker 1: and that will probably change the answer.
Speaker 2: Well, at least that's a bit more satisfying than like
Speaker 2: our discussion on the ort cloud, where you're like, actually,
Speaker 2: we're not even really sure there's an ork cloud, and
Speaker 2: so you know, I was a little worried you we
Speaker 2: were going to be like, actually, we're not even sure
Speaker 2: that there's moons that could be an optical illusion. But anyway, Okay,
Speaker 2: we're sure that there's moons. We're not quite sure where
Speaker 2: the cutoff should be, but we've arbitrarily said it somewhere.
Speaker 1: Yeah, exactly. So first let's clarify what we mean by
Speaker 1: a moon. And this has a fascinating history, even just
Speaker 1: the word is fairly recent and modern. Astronomically, the category
Speaker 1: officially is natural satellite, and we use the word moon
Speaker 1: sort of colloquially after the moon of Earth. Historically, people
Speaker 1: called our moon a planet, like until Copernicus in fifteen hundred,
Speaker 1: the term planet was basically used to describe like things
Speaker 1: that move in the sky, which we assume to move
Speaker 1: around the Earth, and the moon was just like another thing,
Speaker 1: like the other planets, and like the Sun that people
Speaker 1: assumed moved around the Earth. Then, of course Galileo saw
Speaker 1: the moons of Jupiter and thought, ooh, the planets themselves
Speaker 1: are like mini systems, and so you can have this
Speaker 1: hierarchical structure, and so it's not required for everything to
Speaker 1: orbit one thing or to orbit the Earth, and so
Speaker 1: that gives way to a more nuanced structure of the
Speaker 1: Solar System. And now you have to have different words
Speaker 1: to define these things that are not directly orbiting the sun,
Speaker 1: but are orbiting something that is orbiting the sun.
Speaker 2: Okay, now, hold on, So recently you asked me what
Speaker 2: killyfish means, and then you ask me what hymenopterin means.
Speaker 1: Oh no, are we about to get some linguistic revenge?
Speaker 2: Gosh? I hope so, because usually when I'm like, oh,
Speaker 2: I'm about to get Daniel, You're like, oh, because Daniel
Speaker 2: just knows. But I see you're about to hit your keyboard.
Speaker 2: Does the word moon mean something? Did they take that
Speaker 2: from something else? Yes?
Speaker 1: Yes, so you know, the Latin name for the moon
Speaker 1: is Luna. But the word moon itself actually comes from
Speaker 1: the Old English word, which comes from a Germanic word,
Speaker 1: which comes from a Proto Indo European word which might
Speaker 1: be related to the measurement of time. And so yes,
Speaker 1: the moon has this like ancient historical meaning connected with,
Speaker 1: you know, the passage of time. And that's super fun
Speaker 1: because we know that, like looking at patterns in the
Speaker 1: sky is how a lot of ancient peoples first developed
Speaker 1: like astronomy and mathematics and you know, trying to predict
Speaker 1: the future. So it's like a deep rabbit hole all
Speaker 1: the way to the origins of astronomy.
Speaker 2: Okay, I mean that's it's not quite as much fun
Speaker 2: as I was hoping, but like sort of fun. All right.
Speaker 1: So you're hoping it was related to butts, weren't you?
Speaker 2: I mean probably, like, yes, the act of dropping one's
Speaker 2: pants is what I was hoping for, but maybe this
Speaker 2: is I got to keep this kid friendly. Also, teeny
Speaker 2: pet peeve of mine. I feel like, whenever you're referring
Speaker 2: to the moon of Earth, you need to be careful
Speaker 2: to capitalize it because we're referring to it's a specific moon.
Speaker 2: Then the other moons can get a lower case because
Speaker 2: we're referring to moons in general exactly.
Speaker 1: And you know, for a long time, we refer to
Speaker 1: all of these things as just satellites, so instead of
Speaker 1: calling them moons, you would say the satellites of Jupiter.
Speaker 1: Then with Sputnak and the advent of artificial satellites, you know,
Speaker 1: half a century ago it became very awkward to constantly
Speaker 1: say artificial satellite, natural satellite, artificial satellite. And you know,
Speaker 1: English is constantly smoothing and shortening things, and so people
Speaker 1: just started referring to artificial satellites as satellites, and then
Speaker 1: instead of referring to natural satellites, they just called everything moons,
Speaker 1: sort of colloquially after our moon. So technically, astronomically we
Speaker 1: have natural satellites and artificial satellites, but more practically we
Speaker 1: have satellites, which mean artificial satellites which are not moons
Speaker 1: or natural satellites, and then natural satellites that we call moons.
Speaker 2: And I think the word sputnik is just the Russian
Speaker 2: word for satellite.
Speaker 1: Yeah. And then aside from the linguistic fuzziness, there is
Speaker 1: also this question of like, well, what makes something a moon?
Speaker 1: Like it's pretty clear if you're looking at the Earth
Speaker 1: and the Moon, the Earth is bigger, and so you
Speaker 1: would say that the moon is a moon and the
Speaker 1: Earth is a satellite of the Sun. But physically speaking,
Speaker 1: the Earth and the Moon are orbiting each other, right,
Speaker 1: and there's a center of mass of the Earth Moon system,
Speaker 1: and that is what's orbiting the Sun. And so it's
Speaker 1: a little bit arbitrary to say, Okay, this one's a
Speaker 1: moon and this one's a planet. You can imagine, for example,
Speaker 1: a scenario where you have two objects of the same
Speaker 1: mass orbiting each other, like a binary dwarf planet system,
Speaker 1: which is the moon, which is a planet, right, you
Speaker 1: need some way of categorizing it, and so in our
Speaker 1: Solar system, typically if the center of mass of the
Speaker 1: system is within the surface of one of the objects,
Speaker 1: so that's like, you know, where is the average bit,
Speaker 1: The average bit is under the surface, like between the
Speaker 1: Earth and the Moon. The center of mass of the
Speaker 1: Earth Moon system is within the volume of the Earth.
Speaker 1: Then you say that's the planet and anything else outside
Speaker 1: of that is a moon. It's a little bit arbitrary,
Speaker 1: but it's helpful to settling debates among astronomers, which you know,
Speaker 1: you have to have a reason for those guys to
Speaker 1: stop drinking and go to sleep. And there's the other
Speaker 1: side of it, which is like, well, what's the smallest
Speaker 1: possible moon? Right? Like is every dust grain that's orbiting
Speaker 1: the Earth a natural satellite if it formed naturally? Like,
Speaker 1: is there a lowest cutoff? Is every proton that's in
Speaker 1: orbit around the Earth a satellite? Technically according to the
Speaker 1: definition of natural satellite, yes, those are natural satellites. You
Speaker 1: don't count them as moons of Earth. And later we'll
Speaker 1: talk about how the Earth actually does have a weird
Speaker 1: second body that's kind of orbiting it but not really
Speaker 1: that you could argue is kind of like a second
Speaker 1: moon of the Earth. Anyway, the point is it depends
Speaker 1: on the definition. And so, yeah, Kelly, really nobody knows.
Speaker 2: Yes, yes, I knew, I was right. That's great, But
Speaker 2: it does seem like, you know, you talk to ninety
Speaker 2: nine point nine nine percent of the people on the planet,
Speaker 2: they'd say that Earth has one moon, and is that
Speaker 2: just because that's what we were told as kids and
Speaker 2: it's stuck. But if you talk to an astronomer, they'd
Speaker 2: be like, maybe we have a billion, or like, is
Speaker 2: there essentially a definition that we all agree to for
Speaker 2: convenience sake?
Speaker 1: There is really no lower limit. But we call these
Speaker 1: things moons because they're interesting. They reveal the structure of
Speaker 1: the Solar System. They affect planets because of their tidal forces,
Speaker 1: and tiny little dust grains don't do that as much,
Speaker 1: and so they really are sort of a different kind
Speaker 1: of thing. Even if it is an arbitrary dotted line
Speaker 1: that we draw, and there really isn't a place to
Speaker 1: put it. It doesn't make sense to put an individual
Speaker 1: proton in the same category as the moon of Earth,
Speaker 1: just because they both orbit the Earth, and so is
Speaker 1: a qualitative difference, even if there isn't a crisp line
Speaker 1: that we can draw. And so, no, I don't think
Speaker 1: any astronomer is going to be like, actually, there are
Speaker 1: many moons of Earth, but you know, probably maybe there
Speaker 1: is one out there somewhere.
Speaker 2: Is that what you think astronomers sound like? Because I
Speaker 2: think you got to watch it. There might be some
Speaker 2: astronomers that listen to this show and their feelings might
Speaker 2: get hurt.
Speaker 1: No, I think most astronomers sound like they're really fit.
Speaker 1: They're very suave, but just the annoying ones sound like that.
Speaker 2: Got it? Okay, So you said we have like this
Speaker 2: arbitrary definition. Does this arbitrary definition have some features or
Speaker 2: is it just like, is it always a gut feeling
Speaker 2: or do we have some criteria even if it was
Speaker 2: happened upon somewhat arbitrarily.
Speaker 1: So, typically planets have something like ten thousand times the
Speaker 1: mass of their natural satellite. That's what we tend to
Speaker 1: see in the Solar System. But you know there are
Speaker 1: already exceptions to that, Like the moon is one eightieth
Speaker 1: of the Earth, Pluto has a moon that's one eighth
Speaker 1: of its mass, right, and so like you can make
Speaker 1: these broad categories. Almost all the moons in our Solar
Speaker 1: system have a mass that's less than ten thousands of
Speaker 1: the planetary mass. But there are already major exceptions.
Speaker 2: Well, let's take a break. When we come back, we'll
Speaker 2: talk about why we have moons. And if there are
Speaker 2: any kids hanging out with you while you're listening to
Speaker 2: this episode, kids, don't get any ideas. No mooning your
Speaker 2: parents during the commercial break. When we get back, why
Speaker 2: do we have moons? And we're back, So, Daniel, we
Speaker 2: talked about how moons are defined, but why do we
Speaker 2: have moons in the first place, and why did the Earth,
Speaker 2: clearly the greatest planet in the Solar System, only get one.
Speaker 1: Yeah, this is the most interesting thing about moons, in
Speaker 1: my opinion, is that they reveal something about the history
Speaker 1: of the Solar System. If you read about the Solar
Speaker 1: System and look out at the sky, you get the
Speaker 1: impression that it's this sort of calm parade. There's like
Speaker 1: slow moving truds through space that's been going this way
Speaker 1: for a long time and going to be going this
Speaker 1: way forever. But that's only because we're used to living
Speaker 1: at a sort of human timescale seconds, minutes, even centuries,
Speaker 1: and in those timescales, yeah, not much really happens in
Speaker 1: the Solar System. But the Solar System is very old.
Speaker 1: It's four and a half billion years old, and its
Speaker 1: history is filled with chaos. All sorts of crazy cosmic
Speaker 1: cataclysms have occurred in our Solar System, from planets moving
Speaker 1: in and out and switching orbits and losing planets. And
Speaker 1: moons are a great way to understand this history because
Speaker 1: they are in effect records of this history. And if
Speaker 1: you understand it why a planet has a moon, you
Speaker 1: can understand something about what happened to that planet. So
Speaker 1: there's basically three different ways that planets can get moons,
Speaker 1: from the least interesting all the way up to the
Speaker 1: most exciting.
Speaker 2: Well, I bet we are all super excited for you
Speaker 2: to start with the least interesting. Explanation?
Speaker 1: Was that a great lead in, right, lead into that
Speaker 1: ready to get bored here, I'm in, Well, you don't
Speaker 1: want to drop people with the most exciting one, and
Speaker 1: then you know they're sort of spent and like, I
Speaker 1: don't want to listen to the rest of this. You
Speaker 1: got to build up to it, don't you.
Speaker 2: But you don't even have to tell them there's a
Speaker 2: least interesting option.
Speaker 1: Great, all right, Well, the least interesting is already very
Speaker 1: exciting because it tells you about the formation of the planet.
Speaker 1: So sometimes moons form at the same time as the
Speaker 1: planet in the same way that, like the structure of
Speaker 1: our Solar System, didn't just make a sun, but also
Speaker 1: made a bunch of planets. As those planets are forming,
Speaker 1: they don't just make a planet, they also make their
Speaker 1: own little.
Speaker 2: Orbiting guys and gals space gals.
Speaker 1: I like that. So why does the Solar System have
Speaker 1: this kind of structure? Well, remember that the Solar System
Speaker 1: formed from a central blob that collapse gravitationally. You have
Speaker 1: like a seed somewhere where gravity started this runaway effect.
Speaker 1: It got denser, so it had more gravity, so it
Speaker 1: pulled on stuff, so it got denser, so it had
Speaker 1: more gravity, et cetera, et cetera. But you never just
Speaker 1: have exactly one seed. Sometimes you have like another nearby,
Speaker 1: smaller seed that resists being pulled in because it has
Speaker 1: enough relative velocity, So it goes into orbit and it
Speaker 1: starts gathering its own stuff. So that's how planets form. Right.
Speaker 1: You have this central blog which absorbs most of the mass,
Speaker 1: but then you have these other seeds nearby which sometimes
Speaker 1: form stars and you get like binary star systems. But
Speaker 1: sometimes they're smaller and they'll form planets in the same
Speaker 1: way as that planet is forming. It makes a proto
Speaker 1: planetary disc. Right. It's a big swirling mass of stuff,
Speaker 1: most of which will collapse into the planet, but some
Speaker 1: of it is going fast enough and has its own
Speaker 1: little gravitational seed, so can form an object which comes
Speaker 1: into orbit around the planet rather than falling in.
Speaker 2: Ah okay, and if it were to slow down, it
Speaker 2: would fall in, but it's not.
Speaker 1: Slowing down exactly. If it were to slow down, then
Speaker 1: it would fall in. And you know, in that initial
Speaker 1: planetary disc there is a lot of friction, and so
Speaker 1: a lot of stuff does fall in. But if you
Speaker 1: survive that and you form, then there's much less friction
Speaker 1: because things have cleared out and you can mostly orbit
Speaker 1: in a stable way. But it depends also on your
Speaker 1: distance from the planet. So for example, if you're too
Speaker 1: close to the planet, you're going to be feeling its
Speaker 1: tidal forces. The planet is a huge gravitational object. And
Speaker 1: remember that gravity depends on distance. The closer you are
Speaker 1: to something, the stronger the gravitational force. The further you
Speaker 1: are from something, the weaker the gravitational force. Now, if
Speaker 1: you are just a point object, that doesn't really matter
Speaker 1: because your front and back are the same thing. But
Speaker 1: if you're big enough that your front and back are
Speaker 1: substantially different, and your front is closer to the planet
Speaker 1: and your back is further from the planet, then a
Speaker 1: planet is going to be pulling on your front harder
Speaker 1: than it's pulling on your back. These are tidal forces. Essentially,
Speaker 1: it's pulling your front and back apart from each other.
Speaker 1: It tends to pull you into like a football shape.
Speaker 1: If you're strong enough internally you're made of diamond, you
Speaker 1: can resist that. But if you're not, you're going to
Speaker 1: get shredded by the tidal forces. This is a mini
Speaker 1: version of like spagetification when you approach a black hole,
Speaker 1: and so a planet has a thing called a roche limit.
Speaker 1: Anything closer to the planet than the roach limit is
Speaker 1: feeling tidal forces that are too strong to survive. Those
Speaker 1: are going to get shredded into like a ring. Anything
Speaker 1: outside the roche limit can hold itself together and stay
Speaker 1: as a moon.
Speaker 2: All right, So you've got your central blob. It's formed
Speaker 2: into a planet. And then I was imagining everything around
Speaker 2: it is a ring. And so I guess when we're
Speaker 2: discussing this roche limit thing, at that point, everything that
Speaker 2: was in the ring has already been sort of pulled
Speaker 2: into the football shape and it got really hard when
Speaker 2: it got pulled into the football shape, and instead of
Speaker 2: getting shredded that it forms the football moon.
Speaker 1: Yeah, you're right. Everything starts as a ring. But if
Speaker 1: you have a ring and it's far enough out, then
Speaker 1: gravity in that ring is going to pull the ring
Speaker 1: together into a moon, ok right, because things in the
Speaker 1: same orbit are going to attract each other along that
Speaker 1: orbit and they'll pull itself together. Now, if you're far
Speaker 1: enough away beyond the roach limit, the planet is not
Speaker 1: going to interfere with that, and you're going to gather
Speaker 1: yourself together into a moon. If you are within the
Speaker 1: roach limit, then the planet is going to interfere with that.
Speaker 1: And as soon as you get any big objects, the
Speaker 1: planet's going to tear them apart or even prevent them
Speaker 1: from forming in the first place, or as we'll talk
Speaker 1: about later. If you have a pre formed object that
Speaker 1: then comes inside the roach limit, the planet will tear
Speaker 1: it apart. So you have rings are formed with the
Speaker 1: planet probably never were moons, if that's really what you're asking.
Speaker 2: Rings that formed with the planet never were moons, but
Speaker 2: could become moons.
Speaker 1: They could become moons if they're outside the roach limit. Yes,
Speaker 1: but rings within the roach limit are probably never were
Speaker 1: moonduns because there's just too strong a tidal force for
Speaker 1: them to pull themselves together.
Speaker 2: So this sounds like an unlikely path to moonhood.
Speaker 1: Yeah, exactly, you need to be outside the roach limit
Speaker 1: in order to be a moon. And so, for example,
Speaker 1: in the Earth moon system, the roach limit is like
Speaker 1: ten thousand kilometers. Anything closer to the Earth than ten
Speaker 1: thousand kilometers you're going to get pulled apart. The Moon
Speaker 1: is safely like three hundred and eighty thousand kilometers away,
Speaker 1: so you're safe. And the Sun also has a roach limit.
Speaker 1: Planets that approach closer than like seven hundred and fifty
Speaker 1: thousand kilometers to the Sun would get shredded. We're safely
Speaker 1: one hundred and fifty million kilometers from the Sun, and
Speaker 1: these roach limits are a little bit fuzzy because an
Speaker 1: object doesn't have a fixed roach limit depends on what
Speaker 1: you're bringing near it. Like if you bring a moon
Speaker 1: made of jello versus the moon made of diamond, they're
Speaker 1: going to get pulled apart at different places, if that
Speaker 1: makes sense.
Speaker 2: And the Moon each year is getting pulled a little
Speaker 2: bit closer to Earth. But it sounds like the difference
Speaker 2: between the roach limit and where the moon is is
Speaker 2: so far there. Oh wait, or is the Moon getting
Speaker 2: farther away from us?
Speaker 1: The Moon is getting further away by the best centimeter
Speaker 1: a year, and it has to do with the angular momentum. Yeah,
Speaker 1: but it is a tidal force effect.
Speaker 2: Also, Okay, are we going to lose the moon?
Speaker 1: Then eventually the Earth and the Moon are going to
Speaker 1: get tidily locked, which will stop this process.
Speaker 2: Okay, good few. Okay, So it sounds like this might
Speaker 2: not be how we got our moon.
Speaker 1: Well, this is not how we got our moon. And
Speaker 1: you can tell which moon's formed with the planet because
Speaker 1: they tend to have a very nice circular orbit and
Speaker 1: to be orbiting the planet around its equator essentially on
Speaker 1: the same axis that the planet is spinning. Because it
Speaker 1: all formed from the same blob of stuff, just like
Speaker 1: most of the planets in the Solar System are orbiting
Speaker 1: the Sun around the same axis that the Sun is rotating,
Speaker 1: because it all came from the same original blob of stuff.
Speaker 1: So if you find a moon around a planet and
Speaker 1: it's got a mostly circular orbit and it's orbiting around
Speaker 1: and the same axis as a planet is spinning, probably
Speaker 1: it formed with the planet.
Speaker 2: WHOA, we got through the boring stuff. All right, Actually
Speaker 2: I thought that was really cool. But okay, so let's
Speaker 2: move on.
Speaker 1: Relatively boring, relatively super exciting, just not the most exciting
Speaker 1: thing we'll talk about today, all.
Speaker 2: Right, all right, so let's move on to something way
Speaker 2: more epic. What's the next method?
Speaker 1: The next method is to capture something. Imagine an object
Speaker 1: that's floating through the Solar System and it comes near
Speaker 1: a planet. Planets have a lot of gravity. If it
Speaker 1: comes in at the right velocity, in the right distance
Speaker 1: and the right angle, it can basically enter orbit around
Speaker 1: a planet the way like a spaceship can approach a
Speaker 1: planet and go into orbit around it, or you can
Speaker 1: launch a satellite into orbit. If visiting rock has the
Speaker 1: right trajectory, it can be captured by a planet. And
Speaker 1: you can tell moons that have this history because they're
Speaker 1: not made of the same stuff as the planet, and
Speaker 1: because they tend to have like weird orbits, they're highly
Speaker 1: elliptical or they're tilted relative to the axis of rotation
Speaker 1: of the planet.
Speaker 2: How many of these do we think we have in
Speaker 2: our Solar system, because that seems like, depending on where
Speaker 2: they came from, a really cool opportunity for science.
Speaker 1: It is a very cool opportunity for science. And we
Speaker 1: have lots and lots of these examples in the Solar System,
Speaker 1: and we'll talk about a few of them in a
Speaker 1: minute when we do the countdown for the ranking of
Speaker 1: the most moons. But it happens quite a bit. And
Speaker 1: astronomers like to think about the region of a planet
Speaker 1: near a planet where its gravity dominates. Because imagine you're
Speaker 1: some rock and basically you're orbiting the Sun and you
Speaker 1: come near the Earth. How close to the Earth you
Speaker 1: have to get before the Earth's gravity dominates over the
Speaker 1: Sun's gravity. That's called the hill sphere. And if a
Speaker 1: rock enters the hill sphere of a planet, then it
Speaker 1: has a chance to become captured.
Speaker 2: And was hill a guy or gal was Hill an astronomer,
Speaker 2: presumably not one with the voice like you were making before.
Speaker 1: Yes, it was an astronomer and someone who thought about
Speaker 1: the early formation of the Solar System, because remember, back
Speaker 1: in the early days, things weren't as well cleared outs.
Speaker 1: There were lots of rocks floating around Jupiter and Saturn
Speaker 1: moved into the inner Solar System and then back out,
Speaker 1: and that threw lots of asteroids everywhere, So there were
Speaker 1: lots of scattered objects. Some of them just get lost
Speaker 1: at a fly outside the Solar System. Eventually maybe get
Speaker 1: captured by another Solar system. Some of them will get
Speaker 1: captured by other planets. But each one is like a
Speaker 1: time capsule of crazy events that happened in our Solar system.
Speaker 2: It sounds like if a giant object comes in that
Speaker 2: would cause a lot of chaos in the process, Like
Speaker 2: does it cause? I mean, I guess if it's small
Speaker 2: enough relative to a planet that it can get captured,
Speaker 2: it's probably not going to cause a lot of chaos
Speaker 2: for the planet that captures it. So maybe it's not
Speaker 2: going to cause that much chaos. How chaotic is this, Daniel.
Speaker 1: Well, the bigger the thing, the more chaos it causes, right,
Speaker 1: because the more gravitational pull it has. But the Solar
Speaker 1: System already has a lot of chaos in it because
Speaker 1: every system that has more than two bodies in it
Speaker 1: is chaotic. You know, it's called the three body problem.
Speaker 1: You can't have very many stable configurations of three objects
Speaker 1: orbiting each other, and the only way to do that
Speaker 1: is to have configurations where like two of them are
Speaker 1: really really far from the other one. So even just
Speaker 1: having like Earth, Sun and Jupiter already is a little
Speaker 1: bit chaotic. Jupiter is pulling on the Earth. That's one
Speaker 1: of the reasons why our orbits are changing, and we
Speaker 1: have these Malkovich cycles where we get further and closer
Speaker 1: from the Sun. So there's already a little bit of
Speaker 1: chaos in the Solar System. So something coming in from
Speaker 1: the outside is definitely going to inject more chaos, and
Speaker 1: most likely chaos causes us to lose things. Being captured
Speaker 1: is rare because you have to come in at like
Speaker 1: the right angle, and you can't be close enough that
Speaker 1: you enter the atmosphere and then drag and then hit
Speaker 1: the planet, and not so far that you're going to
Speaker 1: miss the hillsphere. So it's really just like quite a
Speaker 1: narrow window for things to get captured. So the things
Speaker 1: in the Solar system that are captured are a tiny
Speaker 1: fraction of all the chaos that has flown through our
Speaker 1: solar system.
Speaker 2: So most of the stuff that does get captured and
Speaker 2: become a moon, do we know where it tends to
Speaker 2: come from?
Speaker 1: We don't because we haven't studied a lot of these enough.
Speaker 1: We haven't like landed on them or done spectrometry on
Speaker 1: them in great detail. And you'll see the ones we
Speaker 1: have studied we already have a lot of questions about.
Speaker 2: Well. I also thought that that was pretty awesome, But
Speaker 2: now we're going to get to the most awesome explanation,
Speaker 2: which is collisions, and I agree that is probably the
Speaker 2: most awesome out of all the explanations. So all right, collisions,
Speaker 2: hit me with it, Daniel.
Speaker 1: So imagine you have a planet and it's doing its thing,
Speaker 1: and then like another planet comes and snacks into it,
Speaker 1: and the two planets are vaporized essentially, and they have
Speaker 1: to like coalesce again into molten blobs, and that process
Speaker 1: doesn't always give two objects of the same size. This
Speaker 1: is the leading theory for the formation of Earth. And
Speaker 1: its moon. That there was a proto Earth with a
Speaker 1: huge moon and it was smacked into by like a
Speaker 1: Mars sized planet, which essentially vaporized the surface of the Earth,
Speaker 1: and a huge chunk of stuff was then thrown out
Speaker 1: into orbit, gathered into a ring as it cooled, and
Speaker 1: then formed into a moon. And there's lots of evidence
Speaker 1: of this from the fact that, like the Moon is
Speaker 1: made out of basically the same stuff as the Earth,
Speaker 1: and because of the way the Moon formed and cooled,
Speaker 1: you can tell that had fairly recent activity on it. Also,
Speaker 1: recent studies of the mantle of the Earth has shown
Speaker 1: like actual deposition from this original protoplanet that hid it.
Speaker 1: It's like there's bits of that still leaving an imprint
Speaker 1: on the inner part of the Earth. It's really fascinating,
Speaker 1: and so essentially that like I don't know if that
Speaker 1: really adds a moon or if it just sort of
Speaker 1: like deletes a planet and it creates two new objects.
Speaker 2: All right, So I'm adding a qualifier to my earlier statement,
Speaker 2: this is the most awesome way to make a moon,
Speaker 2: as long as it never happens again in my lifetime
Speaker 2: to my planets.
Speaker 1: Yeah, exactly.
Speaker 2: But it's awesome that it happened long ago.
Speaker 1: It would have been very dramatic to watch, right. I
Speaker 1: wouldn't mind seeing this happen to another planet. You would
Speaker 1: learn a lot about this Solar System as long as,
Speaker 1: of course, nobody was living on it. No critics were
Speaker 1: harmed in this cosmic collision.
Speaker 2: Okay, So let's assume that there are no microbes on Mars.
Speaker 2: If something like this happened to Mars and we were
Speaker 2: watching it, would there be any implications for us, like
Speaker 2: maybe junk would get thrown off Mars and would hit
Speaker 2: up Yeah, would anything happen to us if this happened
Speaker 2: to Mars.
Speaker 1: Oh, for sure, we would be hit by debris from it.
Speaker 1: We're already hit by debris from Mars. When things hit
Speaker 1: the surface of Mars, stuff is thrown out into space
Speaker 1: and some of it lands on the Earth. We have
Speaker 1: found rocks on the surface of the Earth that we
Speaker 1: know come from Mars because they're geologically incompatible with Earth's
Speaker 1: history and perfectly compatible with Mars. So that already happens
Speaker 1: that like Mars's garbage lands on Earth. So if Mars
Speaker 1: gets like massively slammed by a huge impactor and basically shredded,
Speaker 1: then some significant fraction of it is definitely going to
Speaker 1: hit Earth.
Speaker 2: Yeah, given that you are willing to sacrifice all of
Speaker 2: humanity just to talk to aliens for thirty seconds, would
Speaker 2: you risk Earth getting hit by large chunks of Mars
Speaker 2: just so you could see how this would all play out?
Speaker 1: For sure? Plus we would get samples of Mars here
Speaker 1: on Earth. You're the word, Well, how valuable that is? Like,
Speaker 1: we have this incredible, complicated plan to dig up stuff
Speaker 1: on the surface of Mars and send it back to
Speaker 1: Earth for study. So valuable scientifically, but it's going to
Speaker 1: bring back, like, you know, a small amount of Mars.
Speaker 1: If you could like deliver huge chunks of Mars dropped
Speaker 1: into the Pacific or something, Yeah, let's do it.
Speaker 2: One of us studies life and one of us studies
Speaker 2: non life, and it is very clear which one does
Speaker 2: which right now?
Speaker 1: All right, And while the history of Earth's moon and
Speaker 1: this collision is really fastening into a lot of deep
Speaker 1: signs we could dig into there. Today's episode is not
Speaker 1: just a history of Earth's moon, but we wanted to
Speaker 1: do a ranking of which planets have the most.
Speaker 2: Moons, all right, And so we know unfortunately that the
Speaker 2: answer is not Earth.
Speaker 1: That's right, and in shortstanding tradition established on this episode,
Speaker 1: we're going to start with the least exciting examples first.
Speaker 2: That's right, okay, and so well, should we start closest
Speaker 2: to the Sun?
Speaker 1: Yes, and that coincidentally starts us at the end of
Speaker 1: the list the planets with the least moons, and that
Speaker 1: would be Mercury and Venus. Both of these planets have
Speaker 1: zero moons.
Speaker 2: Why.
Speaker 1: The reason is that they are so close to the
Speaker 1: Sun that the Sun's tidal forces will perturb any orbits.
Speaker 1: Like it's really hard to have a three body system
Speaker 1: if those three are close to each other. The only
Speaker 1: way to have a three body stable system is if
Speaker 1: two of them are far enough away. So, for example,
Speaker 1: the Sun and the Earth and the Moon is a
Speaker 1: three body system. Why is that stable Because the Moon
Speaker 1: and the Earth are fairly close to each other and
Speaker 1: both fairly far away from the Sun. If you bring
Speaker 1: that two body system close enough to the Sun, then
Speaker 1: it becomes a three body system and the whole thing
Speaker 1: is chaotic. So basically, the Sun is going to be
Speaker 1: pulling on those moons in a way that makes none
Speaker 1: of those orbits stable.
Speaker 2: So it sounds like you've just said that the Earth
Speaker 2: is the closest planet that could possibly have a moon
Speaker 2: in our solar system. Yeah, how much closer could the
Speaker 2: Earth be and still have a moon? Are we essentially
Speaker 2: at the limit?
Speaker 1: Yeah, great question. It is possible for Mercury or Venus
Speaker 1: to have a moon. It's just that their hill sphere
Speaker 1: is really really small. So for example, for Mercury to
Speaker 1: have a moon, you'd have to have a moon that
Speaker 1: orbits like pretty close above the surface, which would be
Speaker 1: like a super high velocity, And then Mercury would have
Speaker 1: to have like no atmosphere at all because you wouldn't
Speaker 1: want any drag and no like mountains for the moon
Speaker 1: to smash into. So in principle, it is possible for
Speaker 1: Mercury to have a moon, and Venus is like a
Speaker 1: very thick atmosphere, so this should be tricky. But they
Speaker 1: don't have a moon, and it'd be very hard for
Speaker 1: them to get one. You could bring the Earth closer
Speaker 1: to the Sun and still have a moon. We're quite
Speaker 1: a cozy distance from the Sun. I don't know exactly
Speaker 1: the number of how close it could be.
Speaker 2: Okay, Well, Daniel had hinted earlier that there is one
Speaker 2: other candidate for a moon for Earth, and after the
Speaker 2: break he's going to tell you about it.
Speaker 1: And we're going to hear Kelly try to pronounce it.
Speaker 2: Oh, we're back, and there is one other object in
Speaker 2: the sky that is a potential candidate for another moon
Speaker 2: for Earth. It is colloquially referred to as the space being.
Speaker 1: And uh, how was that pronounced, Kelly? Oh?
Speaker 2: I was trying to avoid it, Kriefney. I probably got
Speaker 2: it right on the first try. Right, how would you
Speaker 2: say it? Daniel? Are you looking up the pronunciation? That's cheating,
Speaker 2: that's cheating.
Speaker 1: I have no idea how to pronounce this. It is
Speaker 1: such a weird word. I don't even know the etymology
Speaker 1: of it is, or why anybody would choose this unpronounceable word.
Speaker 1: C r u I t h n E.
Speaker 2: I wait, you're not even going to try And you
Speaker 2: had me do it.
Speaker 1: First about to you interrupted me as I was about
Speaker 1: to give it a try. I was gonna say I
Speaker 1: would call it krithne also, but it's probably something weirder,
Speaker 1: you know, anytime you have these weird combinations of vowels
Speaker 1: in English, you're like, oh, actually it's Croithney or something.
Speaker 1: So I have no idea. Let's call it the space bean.
Speaker 2: That's great, And I'll note the three times you and
Speaker 2: I said it. We did say it differently each time. Yeah, okay,
Speaker 2: all right, through time. That's right. Tell me about the
Speaker 2: space bean.
Speaker 1: So obviously, the Earth has the Moon, which is the
Speaker 1: major moon and the real moon we consider. But there's
Speaker 1: also this funny object called the space bean. This is
Speaker 1: a rock like five kilometers in diameter, and it's not
Speaker 1: shaped like a bean. You might think, oh, does it
Speaker 1: look like a kidney bean or a pindo bean or something.
Speaker 1: It's just that it has a funny orbit. So its
Speaker 1: orbit is kind of being shaped. So it's got this
Speaker 1: elliptical orbit and principle it's orbiting the Sun and not
Speaker 1: the Earth, but it's close enough to the Earth that
Speaker 1: the Earth is also tugging on it, and so it's
Speaker 1: in resonance with the Earth like it's a complicated three
Speaker 1: body system that has found this stability to it. And
Speaker 1: so if you look at its orbit relative to the Earth,
Speaker 1: it moves in this sort of bean shaped pattern. Again
Speaker 1: relative to the Earth, it's not really orbiting the Earth.
Speaker 1: I mean sometimes the Earth is on one side of
Speaker 1: the Sun and the space bean is on the other
Speaker 1: side of the Sun. So it's more like how Jupiter
Speaker 1: has asteroids that follow it and precede it in its orbit,
Speaker 1: because those are stable points, the grange points in the
Speaker 1: Jupiter Sun system.
Speaker 2: Those are the Trojans, right.
Speaker 1: Yes, exactly, the Trojans and the Greeks. You got to
Speaker 1: keep them separated, that's right, exactly. And in the same way,
Speaker 1: there are like some stable patterns in the Earth Sun system,
Speaker 1: and this object has fallen into one of them.
Speaker 2: Danielle, I have very strong opinions that this is not
Speaker 2: a moon. This just does not feel like a moon
Speaker 2: at all. Given what you've just told me. What do
Speaker 2: you think, yes or no? Thumbs up? Thumbs down?
Speaker 1: I mean, it's a fuzzy thing, but I would have
Speaker 1: to say no, right they probably also it's not really captured,
Speaker 1: but it is gravitationally influenced by the Earth. It's not
Speaker 1: just in orbit around the Sun on its own. But
Speaker 1: the thing that pushes me against calling it a moon
Speaker 1: is it never really gets very close to the Earth.
Speaker 1: The closest it ever gets to the Earth is like
Speaker 1: seven and a half million miles away, which is like
Speaker 1: thirty times farther than our current moon. So officially astronomers
Speaker 1: call this a quasi satellite, which is like such a fudge.
Speaker 2: Word, wishy washy. It doesn't influence our songs or mythology.
Speaker 2: I'm I'm not emotionally feeling any connection to the space bean.
Speaker 1: I say no, even in our family, which are big
Speaker 1: lovers of beans and promoters of beans and every part
Speaker 1: of people's lives. I don't think we're pro space bean
Speaker 1: being promoted to Moon.
Speaker 2: I call beino on the space bean.
Speaker 1: All right. So Earth has one moon and the space bean.
Speaker 1: So let's take a step up and talk about Mars.
Speaker 1: Mars has two moons, and both of these moons are
Speaker 1: fascinating because both of them are very likely captured objects
Speaker 1: not formed with the Mars. And they also have awesome
Speaker 1: names Phobos and demos. These things mean fear and dread.
Speaker 2: Who means that they were having a bad day when
Speaker 2: they named them.
Speaker 1: I bet dark, right? So dark?
Speaker 2: Oh, but Mars is the god of war, right, so
Speaker 2: of course you name the moon's something dark.
Speaker 1: Yeah, exactly, And as we'll hear later on, the name
Speaker 1: of the moons does keep in the theme of the
Speaker 1: name of the planet, which is kind of cool, honestly.
Speaker 1: So Phobos is the closer one. It's larger, It's like
Speaker 1: twenty two kilometers in size. It orbits Mars three times
Speaker 1: every Earth day and is slowly losing its orbit because
Speaker 1: of the atmospheric drag from being so close. So in
Speaker 1: fifty million years, just after elon Musk finishes terraforming, Phobos
Speaker 1: is gonna hit Mars. So that's gonna be a cataclysm. Yeah.
Speaker 1: I know he's really worried about like the Sun expanding
Speaker 1: and eventually destroying the Earth in a billion years, but
Speaker 1: like this is much sooner.
Speaker 2: But you know, you could Phobos is pretty small. Yeah,
Speaker 2: you could push Phobos to a different orbit or something.
Speaker 1: Yeah, yeah, probably you could. You could strap a bunch
Speaker 1: of starships on it and push it to It's not
Speaker 1: that big, right, It's twenty two kilometers, so it's it's
Speaker 1: not enormous.
Speaker 2: I mean, there is so many problems with living on Mars.
Speaker 2: I don't know that this is the one that's the
Speaker 2: show stopper.
Speaker 1: No, just added to your list. And Demos is even smaller.
Speaker 1: It's thirteen kilometers and slowered orbits further out every thirty hours.
Speaker 1: And there was a recent visit to Demos, Like we
Speaker 1: didn't land on it, but it came to a close approach.
Speaker 1: This is actually a satellite launched by the Emirates and
Speaker 1: their orbiter called Hope. So this Hope orbiter visited Demos
Speaker 1: and looked at it carefully and learned something interesting. People
Speaker 1: had long thought that Demos was a captured asteroid, and
Speaker 1: Phobos almost certainly is. But it turns out that Demos
Speaker 1: actually has a composition more similar to Mars than we expected,
Speaker 1: so it probably isn't a captured asteroid, and yet it's
Speaker 1: in this weird orbit, so it doesn't seem like it
Speaker 1: formed with the planet. And the solution is that probably
Speaker 1: Demos is an object that was tossed up from some
Speaker 1: other major collision. Right, So, like something hit Mars, and
Speaker 1: as we talked about this, like blows up chunks of
Speaker 1: Mars out into space, some of which land on Earth,
Speaker 1: but some of which could be in just the right
Speaker 1: angle to end up in orbit. So Demos is like
Speaker 1: a rejected piece of Mars.
Speaker 3: Ah.
Speaker 2: But that's not how I think of the Moon. I
Speaker 2: don't think of it as a rejected piece of the Earth.
Speaker 2: You watch your language Daniel, the moon might be listening.
Speaker 1: I mean, its name is dread Okay, I don't think
Speaker 1: it needs to be dressed up in happy language, all right.
Speaker 2: All right, it's probably the goth equivalent of a moon. Yeah,
Speaker 2: mm hmmm.
Speaker 1: And these are moons. We've known about it for quite
Speaker 1: a while. They were seen in the late eighteen hundreds
Speaker 1: by astronomer aesof Hall, So not as long as we've
Speaker 1: known about Jupiter's moons. But like you know, one hundred
Speaker 1: and fifty years or so, these are not recent discoveries.
Speaker 2: Wait, we knew about Jupiter's moons before we knew about
Speaker 2: Mars's moons.
Speaker 1: Yeah, Galileo discovered the first moons of Jupiter. Wow. Yeah,
Speaker 1: quite a long time ago. But those moons are huge, right,
Speaker 1: Some of those are big enough that they would be
Speaker 1: planets if they weren't in orbit around Jupiter.
Speaker 2: Holy cow. All right, So we've got Earth has one
Speaker 2: moon and a bean, Mars has two moons. Does anybody
Speaker 2: have three moons?
Speaker 1: No?
Speaker 2: No, all right, do give me four moons? Nope, five moons,
Speaker 2: five moons.
Speaker 1: We have Pluto right now. Pluto, of course, famous for
Speaker 1: being an astronomical fuzzy territory. Is it a planet, is
Speaker 1: it a dwarf planet? Whatever. Also, it has this weird moon, Sharon.
Speaker 1: So Sharon is super weird because it's almost as big
Speaker 1: as Pluto. It's half the size of Pluto, right, and
Speaker 1: so it really challenges the whole concept of a moon.
Speaker 1: It's more natural to think of the Pluto Sharon system
Speaker 1: its binary dwarf planets. Basically, the reason that Pluto was
Speaker 1: demoted from a planet to dwarf planet is that there's
Speaker 1: many things like Pluto out there, and so if you
Speaker 1: include Pluto, then you've got to include like hundreds and
Speaker 1: hundreds of these things. And people didn't one hundreds and
Speaker 1: hundreds of planets. Why not, I don't know, because they
Speaker 1: wanted to think planets are special because we're on one,
Speaker 1: and they wanted to be a protected category. This whole
Speaker 1: thing is like so arbitrary and cultural and ridiculous. But anyway,
Speaker 1: if you demote it to a dwarf planet, then you
Speaker 1: really should think about Pluto and Sharon as tidally locked
Speaker 1: binary dwarf planets.
Speaker 2: WHOA, And so because you're not referring to Sharon as
Speaker 2: a moon, does that mean that the center of mass
Speaker 2: is somewhere between Pluto and Sharon.
Speaker 1: Okay, yeah, so technically it is a moon, but like
Speaker 1: it violates that definition as well. Okay, yeah, so really
Speaker 1: fascinating system. Plus there's four more small moons. Hubble discovered
Speaker 1: these just like twenty years ago, that there's four more
Speaker 1: little bits orbiting the Pluto Sharon system. So it's a
Speaker 1: really complex little system. They're orbiting each other and then
Speaker 1: orbiting the two of them are four smaller moons further out.
Speaker 1: So it's crazy.
Speaker 2: It's crazy that we know any of this in my
Speaker 2: fat so far away and that's amazing.
Speaker 1: Go us, it's amazing. And there's probably more small moons
Speaker 1: around Pluto. We just haven't seen them, right, because Pluto
Speaker 1: is super far win. Even Hubble is challenged to see
Speaker 1: tiny little moons that are not glowing right. You can
Speaker 1: only see them when photons leave the Sun happy to
Speaker 1: bounce off of these dark objects and come back to Earth.
Speaker 1: And so if that doesn't happen, you don't see it.
Speaker 1: It's too small, you don't see it. And so there's
Speaker 1: almost certainly more moons of Pluto yet to be discovered.
Speaker 1: You could find them and name them after your dog.
Speaker 2: Well, I like that idea, Melo and Biscuit moons. But
Speaker 2: I did read the outline, so I know that even
Speaker 2: if we missed a handful of moons around Pluto, that
Speaker 2: would not give Pluto the wind. So I'm guessing Pluto's
Speaker 2: not going to win. So who is next in our list?
Speaker 1: So next up in the ranking we got to go
Speaker 1: all the way up to sixteen moons. Neptune has sixteen moons,
Speaker 1: all of which are named after water gods, which is
Speaker 1: super awesome. Maybe of course Neptune Poseidon, god of the sea,
Speaker 1: and Neptune has a really dramatic moon with probably a
Speaker 1: very dramatic history. So its biggest moon is called Triton,
Speaker 1: and it was actually discovered only seventeen days after Neptune
Speaker 1: itself was discovered. It's super fun, exactly, and the whole
Speaker 1: history of the discovery of Neptune is really fascinating because
Speaker 1: you can see Neptune in Galileo's logbook, like he was
Speaker 1: observing Jupiter and he was looking at the moons, and
Speaker 1: it just so happens that if you're looking in that
Speaker 1: direction in the sky at that time of year, that
Speaker 1: you would see Neptune. And he saw it there, but
Speaker 1: he thought it was a star, and so it's there
Speaker 1: in his log book. You can go back and reconstruct it.
Speaker 1: And it wasn't until two hundred years later that people
Speaker 1: figured out where Neptune was and saw it because of
Speaker 1: its pull on Urinus. A really whole fascinating scientific history story.
Speaker 1: I love when there's a discovery made, then you can
Speaker 1: go back and find oh, this data actually already existed.
Speaker 1: We could have made that discovery earlier if people had
Speaker 1: recognized it.
Speaker 2: Those are fun moments, yes, amazing, although not fun if
Speaker 2: you're alive to realize that you missed. You missed that moment.
Speaker 1: But what it says is that there's probably data in
Speaker 1: a logbook right now that is enough to support some
Speaker 1: crazy discovery, and we won't realize it until somebody else
Speaker 1: figures it out. Anyway, Trenton is incredible because it orbits
Speaker 1: retrograde to Neptune's rotation. So most moons orbit the same
Speaker 1: direction the planet spins, because either they're formed with the
Speaker 1: moon or they're captured that way. But Trident orbits the
Speaker 1: opposite direction that Neptune spins, which is crazy, and probably
Speaker 1: this is because it's captured. It's some huge object that
Speaker 1: Neptune captured.
Speaker 2: I always thought that retrograde was just a word you
Speaker 2: heard in astrology horoscopes. No, it just means that you're
Speaker 2: orbiting the opposite of the Okay.
Speaker 1: Yeah, exactly like when planets go into retrograde, it's because
Speaker 1: relative to the Earth, they're moving the opposite direction.
Speaker 2: WHOA okay.
Speaker 1: And so probably what happened here is that Neptune had
Speaker 1: a tidy little set of moons and then Triton came
Speaker 1: in and destroyed all of them because it came in
Speaker 1: and it did not it went the wrong direction. It's
Speaker 1: like driving a semi the wrong way on a freeway, right,
Speaker 1: not cool. Yeah, And so it looks like all the
Speaker 1: other moons, the other fifteen moons of Neptune are re
Speaker 1: accretions of the rubble disc from Triton's capture. So Triton
Speaker 1: comes in destroys all the moons, and then eventually they
Speaker 1: gathered themselves together into these pathetic little objects, you know,
Speaker 1: just remnants of this collision with Triton.
Speaker 2: So how could you know the difference between moons that
Speaker 2: were there before and moons that recollected after Triton destroyed everything?
Speaker 1: So they're not in tidy circular orbits around Neptune. So
Speaker 1: they probably are not there historically since the beginning, right,
Speaker 1: because they're affected by this collision, and they seem to
Speaker 1: be all sort of mixed up, a various mishmashs of
Speaker 1: the moons and they're sort of loosely held and so
Speaker 1: they haven't like formed together for a long time.
Speaker 2: Amazing. And then is that all of Neptune's moons.
Speaker 1: Those are the ones we found, and you know, these
Speaker 1: distant planets probably have many smaller moons orbiting far out
Speaker 1: that we just haven't been able to see. But next
Speaker 1: up in the list is Urinus. Urinus has twenty nine moons,
Speaker 1: and these are so many that it's useful to categorize them.
Speaker 1: You've got the inner moons. These are like really small
Speaker 1: objects just above the roche limit, just able to hold
Speaker 1: themselves together. And many of these are found like last year,
Speaker 1: so twenty twenty five with James web'space telescope. They are
Speaker 1: so small and dark that they are not visible in
Speaker 1: the optical and you can only see them in the infrared,
Speaker 1: and so that's why James Web can see them. Move
Speaker 1: a little bit further out. And you've got five major moons,
Speaker 1: some of these are big enough to have things like
Speaker 1: volcanism and internal magma and flow on the inside of them.
Speaker 1: The largest one is Titania, which is one twenty the
Speaker 1: mass of our moon, and this we've known about for
Speaker 1: like more than two hundred years. Are you gonna make
Speaker 1: a Titania urinus joke? Kelly has lost at people.
Speaker 4: I have been trying to not make any comments because
Speaker 4: I feel like there's nothing good that I can say.
Speaker 4: But you know, all the other moons were named after
Speaker 4: things similar, and so you know, I'm wondering, You're why,
Speaker 4: you know, why is it Titania and not you know,
Speaker 4: other butt related things, and so I'm just I'm not gonna, okay,
Speaker 4: but I'm gonna focus on the science and ask you, Daniel,
Speaker 4: is this the first moon we've talked about that could
Speaker 4: have volcanism?
Speaker 1: It is, yes, exactly.
Speaker 2: What an adult I am right now.
Speaker 1: For those of us just after the holidays who've eaten
Speaker 1: a big meal and then, you know, produced something titanic
Speaker 1: of our own and wondered, I wonder if that has
Speaker 1: its own gravity could produce volcanism of its own? You know, yes, exactly,
Speaker 1: And so the history here is also funny because it
Speaker 1: was discovered in seventeen eighty seven, just after the planet
Speaker 1: itself was discovered. But the guy who discovered Titania also
Speaker 1: claimed the discovery four more moons which don't exist. What
Speaker 1: so they were like spurious moons which later people are like, yeah,
Speaker 1: I don't see those. I don't know what you were
Speaker 1: looking at. And so the history here is a little
Speaker 1: bit checkered.
Speaker 2: Did he name the other moons like rectum war fits geer?
Speaker 1: Moving on, outside of the major moons are ten more
Speaker 1: irregular moons, some of which just discovered in the last
Speaker 1: couple of years. So you know, this is an area
Speaker 1: where we are actively learning new things about these planets.
Speaker 2: Amazing, and what makes an irregular moon? Is it the shape?
Speaker 1: Mm hmm. These things are not like big enough to
Speaker 1: have pulled themselves into spheres, and so they're like weird blobs.
Speaker 1: Some of them were also with weird orbits, probably captured objects. Cool,
Speaker 1: all right. So then stepping up to Jupiter, this was
Speaker 1: most people's candidate for having the most moons, just because
Speaker 1: Jupiter is the mostest of most of the stuff, right,
Speaker 1: It's got most of the non sun mass in the
Speaker 1: Solar System, and indeed it has almost one hundred moons. Wow, right,
Speaker 1: seven counted? Ninety seven moons counted for Jupiter so far,
Speaker 1: the largest four of which were discovered hundreds of years
Speaker 1: ago by Galileo.
Speaker 2: WHOA, way to go, Galileo.
Speaker 1: Yeah, exactly.
Speaker 2: Could we all count as moons of the Sun should
Speaker 2: the Sun win?
Speaker 1: Are you moons of the Sun? Well? I think moons
Speaker 1: of the Sun we.
Speaker 2: Call planets, right, yeah, yeah, okay, I'm sorry, I'll put
Speaker 2: down the banana peels. Let's all right, the last four
Speaker 2: discovered by Galileo. Way to go, Galileo. Let's talk about Io.
Speaker 2: I like Io.
Speaker 1: Yeah, these moons are huge and likely formed with Jupiter.
Speaker 1: They're in nice orbits around Jupiter. Io is bigger than
Speaker 1: our moon. A lot of people think the moon is
Speaker 1: the biggest moon in the Solar system. Not true, not
Speaker 1: even the second biggest moon in the Solar system. Io
Speaker 1: is not even the biggest moon in the Solar system.
Speaker 1: But it is larger than our moon.
Speaker 2: What moon is the biggest? Daniel We'll get there.
Speaker 1: Oh okay, all right, all right, But Io, of course
Speaker 1: super awesome because it has hundreds of active volcanoes on it,
Speaker 1: and some of these things shoot plumes out like five
Speaker 1: hundred kilometers above the surface. It's the most geologically active
Speaker 1: object in the Solar System.
Speaker 2: Wow.
Speaker 1: Yeah. The reason it's got so much going on is
Speaker 1: that it's pretty close to Jupiter, and so these tidal
Speaker 1: forces from Jupiter are not strong enough to pull it apart,
Speaker 1: but they do squeeze it into a football. And then
Speaker 1: Io also is orbiting, so which part of Io is
Speaker 1: getting squeezed into a football shape is changing, so sort
Speaker 1: of from Io's point of view, if you're just looking
Speaker 1: at it, it's a football, but like different parts of
Speaker 1: it are getting footballed as it orbits Jupiter and as
Speaker 1: it spins, so that creates a lot of internal friction.
Speaker 1: It's just like Jupiter is reaching out with huge cosmic
Speaker 1: hands and squeezing this thing. It's like kneading dough, right,
Speaker 1: And so this is tidal heating. So just this gravitational
Speaker 1: interaction between Io and Jupiter is enough to heat up
Speaker 1: the inside of Io, and that's where you get all
Speaker 1: these flows inside of it and this cracking of the
Speaker 1: surface and all these volcanoes.
Speaker 2: So is Io really hot in some spots.
Speaker 1: Then inside of it is very toasty, yes.
Speaker 2: Absolutely all right, But Europa. Is Europa really hot?
Speaker 1: Eurrope is fascinating because it's icy on the surface, right,
Speaker 1: so it's got like a crust of ice. But we've
Speaker 1: done studies of it and we've seen that Jupiter's magnetic
Speaker 1: field creates a current inside Europa. What which means probably
Speaker 1: there's salt water which is capable of conducting electricity inside.
Speaker 1: So probably you have like ten kilometers of ice and
Speaker 1: below that maybe like one hundred kilometers of subsurface ocean. Wow,
Speaker 1: And we think that that's water because as you get
Speaker 1: closer to the center of Europa, like with many of
Speaker 1: these moons, tidal heating makes things warm, and so not
Speaker 1: only is it directly heated because of the friction, but
Speaker 1: also they're probably like geothermal vents or hydrothermal vents on
Speaker 1: a moon, where we're heat from the core from this
Speaker 1: tidal heating is then leaking up equivalent to like a volcano.
Speaker 1: So you could have enormous quantities of liquid water under
Speaker 1: this frozen surface on Europa.
Speaker 2: I would love to know if there's life there.
Speaker 1: I know, right, so much possibility there for life. And
Speaker 1: the cool thing is that sometimes it shoots stuff out
Speaker 1: into space because you get cracks and geysers, and there's
Speaker 1: a mission being sent Europa Clipper to go and sample
Speaker 1: these things, like look for microbes or whatever, tiny alien
Speaker 1: octopi or something so exciting.
Speaker 2: Yeah, okay, all right, let's get to the largest moon now.
Speaker 1: So the largest moon in the Solar System is Ganymede,
Speaker 1: also Jupiter's moon. This thing is two times the mass
Speaker 1: of our moon. Wow. So if you're like impressed by
Speaker 1: our moon, like Ganymede is bigger than our moon by
Speaker 1: a factor of.
Speaker 2: Two, Well, don't rub it in. Our moon's doing a
Speaker 2: good job.
Speaker 1: Again. I mean, is big enough to have an atmosphere,
Speaker 1: probably has a lot of oxygen in it, also very
Speaker 1: likely to have a subsurface ocean. We think it has
Speaker 1: a metal core. This thing has its own magnetic field.
Speaker 1: It's a monster.
Speaker 2: But I'm guessing you wouldn't want to live there because
Speaker 2: proximity to Jupiter would make it uninhabitable. Is that right?
Speaker 1: Yes, Jupiter puts out a lot of radiation. It's not
Speaker 1: a star like the Sun, and it's not even a
Speaker 1: brown dwarf. There's no fusion happening inside of it, but
Speaker 1: there's still a lot of radiation being pumped off of Jupiter,
Speaker 1: so not a safe place to live without a lot
Speaker 1: of shielding, but often the place you'll find settlements in
Speaker 1: very well written hard sci fi novels, for example, like.
Speaker 2: The Expanse exactly all right, So Jupiter's got one hundred moons?
Speaker 2: Are there any other moons that we need to talk
Speaker 2: about before we move on to our winter?
Speaker 1: So the third largest moon in the Solar system is
Speaker 1: also a moon of Jupiter. It's Callisto. Calisto is super
Speaker 1: interesting because its surface, unlike some of the other ones,
Speaker 1: is very very old, so lots of craters on the
Speaker 1: surface of Calisto. Europa, in contrast, has like a really
Speaker 1: smooth surface, very young surface. It's constantly being reformed, so
Speaker 1: you get a crater, it gets deleted. Callisto is showing
Speaker 1: all of its scars, which means it's a great way
Speaker 1: to understand the history of the Solar system, like when
Speaker 1: was there a lot of impacts? When wasn't there?
Speaker 2: Calisto sounds beautiful, but in a different way. Daniel, all right,
Speaker 2: So Daniel, drum roll please.
Speaker 1: The drama has been removed by the process of elimination.
Speaker 1: But the winner is Saturn, and not by a little
Speaker 1: bit Saturn has two hundred and seventy four moons right
Speaker 1: Jupiter at ninety seven, Saturn has two hundred and seventy four.
Speaker 1: It's crazy and these moons are amazing. Also, the biggest
Speaker 1: one is Titan. This thing is more massive than the
Speaker 1: planet Mercury. Okay, it's not a small moon. It's huge.
Speaker 1: You've known about it for hundreds of years. Discovered by
Speaker 1: Huygens in sixteen fifty five. Another one of my favorite
Speaker 1: moons in the Solar system is ensuladis this thing like
Speaker 1: Europa amidst jets of ice, probably because it has a
Speaker 1: subsurface ocean. It's another great candidate for where life could form.
Speaker 1: But maybe my favorite moon in the Solar system is
Speaker 1: this moon of Saturn called Yupitus, which is the craziest
Speaker 1: name for a moon. But it's shaped like a walnut.
Speaker 1: It has this enormous ridge all the way around its equator,
Speaker 1: like this vast set of mountains, and then the top
Speaker 1: half of it is black and the bottom half of
Speaker 1: it is white. It's like a black and white cookie. Yes,
Speaker 1: this thing is crazy, exactly. Somebody took a huge space
Speaker 1: walnut and dipped half of it in frosting. It's unbelievable.
Speaker 1: So Saturn has all of these moons, most of them
Speaker 1: are very far from Saturn, like two hundred and fifty
Speaker 1: plus of these moons are distant from Saturn, orbiting with
Speaker 1: high inclinations, almost certainly captured objects of Saturn. Saturn is
Speaker 1: in a great place to capture all of these objects,
Speaker 1: many of which are were like scattered by Jupiter, so
Speaker 1: sort of like the garbage collector of the Solar system.
Speaker 2: Okay, yes, you were able to tell us that Mercury
Speaker 2: and Venus are too close to the Sun. That's why
Speaker 2: they probably don't have any And Saturn probably has the
Speaker 2: most because it's just in a good position to.
Speaker 1: Pick up junk exactly. It's got a lot of gravity,
Speaker 1: and it's far away from the Sun, and it's nearby Jupiter,
Speaker 1: which creates lots of stuff tossing off of it exactly.
Speaker 1: So over the years it's picked up a lot of moons.
Speaker 2: Amazing. If you could visit any moon, Daniel, would it
Speaker 2: be Yapitus? Is that how you said it?
Speaker 1: Well, there's another moon of Saturn called Rhea, which they
Speaker 1: think might have rings, right, because moons can have rings
Speaker 1: usually that's unusual because the tidal effects of the planet
Speaker 1: will disrupt it. And this is why, for example, moons
Speaker 1: tend to not have moons, but it's easier to have
Speaker 1: rings than moons because they're basically already torn into shreds.
Speaker 1: So that would be pretty awesome to be on a
Speaker 1: moon of Saturn, see the rings of Saturn and the
Speaker 1: rings of Rhea around it. That would be pretty awesome.
Speaker 2: That would be absolutely epic.
Speaker 1: Yeah, and not included in today's list are the three
Speaker 1: hundred and thirty four minor planets in our Solar system
Speaker 1: that have their own moons.
Speaker 2: Wow.
Speaker 1: These things you might call moon moons or moonlitz aw.
Speaker 2: Moonlits is cute, but moon moons is funner.
Speaker 1: Yeah, exactly. Moon moons technically reserved for moons that have
Speaker 1: their own moons. We haven't found any of those yet,
Speaker 1: but it depends on you whether you call these minor
Speaker 1: planets moons. And we're all looking forward to the day
Speaker 1: when we can discover exo moons moons around planets in
Speaker 1: other Solar systems. It's particularly tricky because the techniques that
Speaker 1: we have for finding exoplanets are good at finding planets
Speaker 1: like really close to the Sun that are really big
Speaker 1: and it's hard for those planets to have moons they're
Speaker 1: so close to their star. But people are working on this,
Speaker 1: and direct imaging of planetary discs might help us discover
Speaker 1: exo moons.
Speaker 2: I am hoping that this happens in my lifetime. I
Speaker 2: don't know. There's been a lot of I mean, just
Speaker 2: hearing you talk today, there's been a lot of cool
Speaker 2: stuff in our solar system that's been discovered in our lifetimes.
Speaker 2: I don't know. Maybe we'll start discovering even more cool
Speaker 2: stuff in other solar systems in our lifetimes. I don't
Speaker 2: think it's impossible.
Speaker 1: Yeah, and I should say there are already candidates for
Speaker 1: exo moons because in some of these eclipse methods, where
Speaker 1: like a planet passes in front of the star, you
Speaker 1: can see deviations from those that might be consistent with
Speaker 1: the moon going around that. But these are just candidates
Speaker 1: and unconfirmed, and of course there's lots of controversy. So
Speaker 1: I think we might be on the verge of discovering
Speaker 1: exo moons.
Speaker 2: Well, stay tuned. If we find moon moons or exo moons,
Speaker 2: we will let you know.
Speaker 1: And one day when we visit another system studying their
Speaker 1: moons will help us understand the history of that solar system,
Speaker 1: the cataclysms, the captures, the collisions, everything that went down
Speaker 1: in their cosmic.
Speaker 2: Chaos, all of the chaos from the kind of boring
Speaker 2: stuff Daniel will still tell us about anyway, so the
Speaker 2: much more exciting stuff, all of which we'll explain in
Speaker 2: an exciting way. Here a Daniel and kill.
Speaker 1: All exciting because some of it's more boring anyway. Thanks
Speaker 1: for tuning in, everyone, and for ficking it out for
Speaker 1: this countdown for which planet has the most moons.
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