WEBVTT - Which planet has the most moons? 0:00:07.800 --> 0:00:11.080 The Moon really doesn't get enough love. Of all of 0:00:11.119 --> 0:00:13.880 the features of the night sky. It's the only one 0:00:13.960 --> 0:00:17.760 with real texture visible to the naked eye. Everything else 0:00:17.960 --> 0:00:21.680 is either just a tiny distant point or an overwhelming 0:00:21.760 --> 0:00:24.799 ball of fire. There's nothing else out there that you 0:00:24.840 --> 0:00:28.960 can just sit and stare at and appreciate the feeling 0:00:29.120 --> 0:00:32.960 of looking through a gulf in space. We are here 0:00:33.000 --> 0:00:36.440 on a rock in space, and there's another whole rock 0:00:36.600 --> 0:00:40.560 right over there, so far away yet so weirdly big, 0:00:40.640 --> 0:00:44.040 that we can see it and see features on it. 0:00:44.040 --> 0:00:47.120 It challenges the mind, forcing you to come to griffs 0:00:47.159 --> 0:00:51.279 with our cosmic context of massive balls of rock spinning 0:00:51.320 --> 0:00:54.880 through a dark ocean. And there's so much more that 0:00:54.960 --> 0:00:57.760 moons do. Moons tell us about the history of the 0:00:57.800 --> 0:01:01.320 Solar System and help reveal what's going on inside their planets. 0:01:01.560 --> 0:01:04.480 They have volcanoes and weird colors and shoot jets of 0:01:04.520 --> 0:01:09.320 water into space. They might even harbor life. Seeing Jupiter's 0:01:09.360 --> 0:01:13.080 moons is what led Galileo to understand the structure of 0:01:13.080 --> 0:01:16.600 our Solar system. And the thing that our moon has 0:01:16.680 --> 0:01:21.160 misled us about is their number. Having one big, fat 0:01:21.200 --> 0:01:24.880 moon is not normal planets, and our Solar system have 0:01:25.200 --> 0:01:29.960 many moons, so many fabulous moons. So on this episode, 0:01:30.000 --> 0:01:33.520 we're going to give moons the love they deserve. Welcome 0:01:33.640 --> 0:01:37.840 to Daniel and Kelly's Extraordinary moon Malicious Universe. 0:01:51.080 --> 0:01:54.520 Hello. I'm Kelly Waitersmith. I study parasites and space, and 0:01:54.680 --> 0:01:57.200 I am not above jokes that involve mooning. 0:01:59.320 --> 0:02:03.000 Hiel, I'm a particle physicist, and I can't tell you 0:02:03.040 --> 0:02:05.440 how long it's been since I mooned somebody. 0:02:05.840 --> 0:02:10.360 Oh, yes, that was a joy to hear about mooning 0:02:10.440 --> 0:02:12.239 jokes when I was a kid. I don't remember actually 0:02:12.280 --> 0:02:14.040 mooning anyone in my teens. I was a bit of 0:02:14.120 --> 0:02:18.400 an insecure teen. But but ah, yes, I remember being 0:02:18.440 --> 0:02:19.040 mooned as. 0:02:18.919 --> 0:02:22.480 A teen and as an adult. Kelly, can I ask 0:02:22.560 --> 0:02:24.680 you how long has it been since you mooned anybody? 0:02:24.919 --> 0:02:29.040 I plead the fifth I married a man whose last 0:02:29.120 --> 0:02:31.720 name is Wiener, and so my sense of humor has 0:02:31.919 --> 0:02:33.160 become a bit more juvenile. 0:02:34.919 --> 0:02:37.040 Fill in the gaps yourself, everybody, that's. 0:02:36.960 --> 0:02:39.919 Right answer, Daniel. My question for you is, so today 0:02:39.919 --> 0:02:44.440 we are talking about moons. Have you ever heard of 0:02:44.480 --> 0:02:49.280 a convincing story of the moon influencing someone's behavior? Oh? 0:02:49.400 --> 0:02:51.920 I mean, I can imagine the moon influencing the tides, 0:02:51.960 --> 0:02:54.320 and the tides definitely influence people, m h. 0:02:54.880 --> 0:02:57.040 But you don't buy the like, you know, more people 0:02:57.080 --> 0:02:58.800 come into the er on a full moon. 0:02:59.360 --> 0:03:02.200 People do all all sorts of weird stuff, and I 0:03:02.240 --> 0:03:05.240 wouldn't be surprised if people acted weirder on a full 0:03:05.320 --> 0:03:08.120 moon and there were more visits to the er. So 0:03:08.240 --> 0:03:09.919 I'm gonna have to play the fifth on that one. 0:03:11.000 --> 0:03:13.800 Yeah, I mean, I don't think I buy that people 0:03:13.840 --> 0:03:17.760 are like more kooky on full moon nights unless they 0:03:17.760 --> 0:03:20.680 have been convinced that they ought to be more like 0:03:20.840 --> 0:03:23.000 risk taking on full moon nights and it's on their 0:03:23.080 --> 0:03:25.120 mind than I can imagine like they sort of have 0:03:25.160 --> 0:03:28.040 psyched themselves out. But I do like, if I get 0:03:28.520 --> 0:03:31.240 up early in the morning and it's brighter because of 0:03:31.240 --> 0:03:33.760 a full moon, I'll be more likely to like go 0:03:33.919 --> 0:03:37.440 jogging or something. And so I can imagine maybe people 0:03:38.280 --> 0:03:41.160 going out and doing more stuff outside during a full 0:03:41.160 --> 0:03:44.200 moon and maybe being more likely to like accidentally get 0:03:44.240 --> 0:03:46.360 hit by a car or something like that. And of 0:03:46.360 --> 0:03:49.720 course we were talking about how some organisms are influenced 0:03:49.720 --> 0:03:52.320 by the tide and so their's circadian rhythms are influenced 0:03:52.320 --> 0:03:54.800 by the tide, so their behaviors are influenced by the tide. 0:03:55.000 --> 0:03:57.440 But yeah, I don't think I buy that animals or 0:03:57.480 --> 0:04:01.320 people are more likely to be doing kooky crazy stuff 0:04:01.880 --> 0:04:04.560 because it's a full moon or anything like that. 0:04:04.720 --> 0:04:07.920 But I don't know. People are weird, and I think 0:04:07.920 --> 0:04:10.640 if you work in an er, you see the weirdest 0:04:10.640 --> 0:04:13.080 side of people. I remember Katrina worked at a hospital 0:04:13.160 --> 0:04:17.000 in Geneva, and near the entrance they had a huge 0:04:17.000 --> 0:04:19.960 display of the weirdest stuff they had pulled out of 0:04:20.000 --> 0:04:23.839 people's throats and other holes. Oh no, and boy was 0:04:23.839 --> 0:04:25.280 there some weird stuff there. 0:04:25.520 --> 0:04:29.680 Oh. I did once for a research project look for 0:04:29.800 --> 0:04:34.080 papers describing things that had been pulled from orifices. And 0:04:34.839 --> 0:04:39.279 we are an inventive species, and if it turns out 0:04:39.400 --> 0:04:42.599 that our inventive behaviors are tied to full moons, then 0:04:42.640 --> 0:04:45.400 we might be happy that we have only one moon 0:04:45.880 --> 0:04:47.440 and not many moons. 0:04:48.400 --> 0:04:51.360 And the moon figures so prominently in the sky and 0:04:51.440 --> 0:04:55.200 in our literature and in our imaginations that it's easy 0:04:55.200 --> 0:04:57.160 to imagine that aliens would look up to their sky 0:04:57.240 --> 0:04:59.559 and see the same thing. Or if you're a science 0:04:59.560 --> 0:05:03.000 fiction off third to imagine alien seeing a radically different sky, 0:05:03.600 --> 0:05:07.560 many moons, or something similarly weird. And there are lots 0:05:07.560 --> 0:05:10.560 of examples in science fiction of multiple suns or multiple 0:05:10.600 --> 0:05:14.960 moons or other variations on our experience, but you don't 0:05:14.960 --> 0:05:17.600 actually have to go that far visiting another solar system 0:05:17.920 --> 0:05:20.640 to see examples of multiple moons. 0:05:21.000 --> 0:05:23.159 And if I can just plug my friend's book for 0:05:23.160 --> 0:05:25.840 a second, Under Alien Skies by Phil Plait is an 0:05:25.839 --> 0:05:29.159 amazing book describing what might be like to visit alien 0:05:29.200 --> 0:05:32.240 planets and look at their skies. And Phil was a 0:05:32.240 --> 0:05:34.320 guest on our show early on when you and I 0:05:34.360 --> 0:05:35.920 started podcasting together. 0:05:36.120 --> 0:05:39.560 That's right, and everything he writes is insightful, well informed, 0:05:39.720 --> 0:05:41.599 and fun, so go check it out. 0:05:41.720 --> 0:05:46.719 Yes, all right, so you asked our extraordinaries which planet 0:05:46.880 --> 0:05:48.359 has the most moons? 0:05:48.960 --> 0:05:51.120 That's right, And if you would like to contribute for 0:05:51.160 --> 0:05:54.480 this segment of the show in future episodes, please don't 0:05:54.480 --> 0:05:56.520 be shy. We would love to add your voice to 0:05:56.560 --> 0:05:59.000 the chorus. In the meantime, think about it for yourself 0:05:59.040 --> 0:06:01.680 for a moment. Which planet in our solar system do 0:06:01.720 --> 0:06:05.560 you think has the most moons? Here's what the extraordinaries 0:06:05.839 --> 0:06:06.640 had to say. 0:06:06.880 --> 0:06:10.440 And believe it's Jupiter. That has the most moons. I 0:06:10.480 --> 0:06:14.239 think they just discovered a couple more, But technically really 0:06:14.279 --> 0:06:18.159 depends on what you define as moon, because rings are 0:06:18.200 --> 0:06:21.920 just particles orbiting the planet as well. In our Solar system, 0:06:22.000 --> 0:06:24.159 the planet with the most moons is probably Saturn. 0:06:24.400 --> 0:06:26.680 Outside of the Solar System, I have no clue. 0:06:26.839 --> 0:06:30.080 I would say Jupiter because of how big it is, 0:06:30.880 --> 0:06:33.440 but then it might have pulled some moons into it, 0:06:33.520 --> 0:06:35.720 So maybe Saturn has the most because it sits in 0:06:35.760 --> 0:06:36.480 a sweet spot. 0:06:36.920 --> 0:06:41.800 What do you think, glenn on? Going straight for Jupiter? Why? 0:06:42.000 --> 0:06:45.080 Because it has the most moons? All right? 0:06:45.720 --> 0:06:48.919 My initial thought would be Saturn or Jupiter, but I 0:06:48.920 --> 0:06:52.000 have been reading recent news reports about many new discoveries 0:06:52.000 --> 0:06:54.400 of moons around Jupiter, so I'm going to go with Jupiter. 0:06:54.600 --> 0:06:58.200 All right, So most votes for Jupiter. Kelly. If you 0:06:58.200 --> 0:07:00.000 hadn't read the outline, what would you have done? 0:07:00.000 --> 0:07:02.240 Cod Ah, I love that you think that I read 0:07:02.240 --> 0:07:04.040 the outline. 0:07:03.920 --> 0:07:06.560 Giving you the benefit of the doubt here just in case. 0:07:06.760 --> 0:07:08.640 No, I'm kidding. I did read the outline, so I 0:07:08.640 --> 0:07:11.840 would have guessed Saturn or Jupiter, or I would have 0:07:11.880 --> 0:07:15.640 guessed that because this is a dKu episode, maybe the 0:07:15.680 --> 0:07:20.720 answer is we don't know, because maybe maybe it's hard 0:07:20.720 --> 0:07:23.480 to count all the moons and we aren't sure if 0:07:23.520 --> 0:07:25.200 Saturn or Jupiter is the winner. 0:07:25.320 --> 0:07:30.360 Yet that's probably the right answer. Actually, Also it depends 0:07:30.400 --> 0:07:33.320 on what you mean by moon, the definition, which is 0:07:33.320 --> 0:07:36.520 always evolving as we discover more stuff out there in 0:07:36.560 --> 0:07:39.680 the Solar system, because though humans like to make tidy 0:07:39.760 --> 0:07:42.440 categories for the things that are orbiting the Sun, in 0:07:42.520 --> 0:07:45.520 reality the universe is chaotic and there's just like a 0:07:45.680 --> 0:07:49.720 huge smooth spectrum of stuff from tiny little dust grains 0:07:49.760 --> 0:07:53.320 all the way up to Jupiter and basically everything in between. 0:07:53.640 --> 0:07:55.520 And if you try to put everything in boxes and 0:07:55.520 --> 0:07:58.080 make artificial dotted lines to separate it, you'll find a 0:07:58.120 --> 0:08:01.200 bunch of stuff in fuzzy category and you'll argue about 0:08:01.200 --> 0:08:03.480 whether it's a moon or whether it's not a moon, 0:08:03.800 --> 0:08:05.520 and that will probably change the answer. 0:08:05.800 --> 0:08:08.200 Well, at least that's a bit more satisfying than like 0:08:08.280 --> 0:08:10.600 our discussion on the ort cloud, where you're like, actually, 0:08:10.640 --> 0:08:13.000 we're not even really sure there's an ork cloud, and 0:08:13.040 --> 0:08:14.520 so you know, I was a little worried you we 0:08:14.520 --> 0:08:16.520 were going to be like, actually, we're not even sure 0:08:16.560 --> 0:08:20.120 that there's moons that could be an optical illusion. But anyway, Okay, 0:08:20.120 --> 0:08:22.840 we're sure that there's moons. We're not quite sure where 0:08:22.840 --> 0:08:26.200 the cutoff should be, but we've arbitrarily said it somewhere. 0:08:26.400 --> 0:08:30.240 Yeah, exactly. So first let's clarify what we mean by 0:08:30.320 --> 0:08:33.360 a moon. And this has a fascinating history, even just 0:08:33.400 --> 0:08:37.959 the word is fairly recent and modern. Astronomically, the category 0:08:38.000 --> 0:08:41.880 officially is natural satellite, and we use the word moon 0:08:42.120 --> 0:08:46.880 sort of colloquially after the moon of Earth. Historically, people 0:08:46.920 --> 0:08:51.520 called our moon a planet, like until Copernicus in fifteen hundred, 0:08:51.760 --> 0:08:54.440 the term planet was basically used to describe like things 0:08:54.480 --> 0:08:57.280 that move in the sky, which we assume to move 0:08:57.360 --> 0:08:59.760 around the Earth, and the moon was just like another thing, 0:09:00.200 --> 0:09:02.440 like the other planets, and like the Sun that people 0:09:02.520 --> 0:09:07.240 assumed moved around the Earth. Then, of course Galileo saw 0:09:07.280 --> 0:09:10.880 the moons of Jupiter and thought, ooh, the planets themselves 0:09:10.960 --> 0:09:13.520 are like mini systems, and so you can have this 0:09:13.640 --> 0:09:17.560 hierarchical structure, and so it's not required for everything to 0:09:17.840 --> 0:09:21.280 orbit one thing or to orbit the Earth, and so 0:09:21.360 --> 0:09:24.000 that gives way to a more nuanced structure of the 0:09:24.000 --> 0:09:26.200 Solar System. And now you have to have different words 0:09:26.280 --> 0:09:29.600 to define these things that are not directly orbiting the sun, 0:09:29.720 --> 0:09:32.160 but are orbiting something that is orbiting the sun. 0:09:32.320 --> 0:09:35.319 Okay, now, hold on, So recently you asked me what 0:09:35.440 --> 0:09:38.360 killyfish means, and then you ask me what hymenopterin means. 0:09:39.040 --> 0:09:41.679 Oh no, are we about to get some linguistic revenge? 0:09:42.280 --> 0:09:44.880 Gosh? I hope so, because usually when I'm like, oh, 0:09:44.880 --> 0:09:48.440 I'm about to get Daniel, You're like, oh, because Daniel 0:09:48.480 --> 0:09:50.320 just knows. But I see you're about to hit your keyboard. 0:09:50.640 --> 0:09:53.360 Does the word moon mean something? Did they take that 0:09:53.400 --> 0:09:56.360 from something else? Yes? 0:09:57.160 --> 0:10:01.600 Yes, so you know, the Latin name for the moon 0:10:01.880 --> 0:10:05.080 is Luna. But the word moon itself actually comes from 0:10:05.080 --> 0:10:08.600 the Old English word, which comes from a Germanic word, 0:10:08.960 --> 0:10:12.480 which comes from a Proto Indo European word which might 0:10:12.520 --> 0:10:15.800 be related to the measurement of time. And so yes, 0:10:15.880 --> 0:10:19.800 the moon has this like ancient historical meaning connected with, 0:10:19.960 --> 0:10:22.440 you know, the passage of time. And that's super fun 0:10:22.480 --> 0:10:24.560 because we know that, like looking at patterns in the 0:10:24.600 --> 0:10:27.439 sky is how a lot of ancient peoples first developed 0:10:27.480 --> 0:10:30.960 like astronomy and mathematics and you know, trying to predict 0:10:31.000 --> 0:10:33.200 the future. So it's like a deep rabbit hole all 0:10:33.240 --> 0:10:34.960 the way to the origins of astronomy. 0:10:35.320 --> 0:10:38.440 Okay, I mean that's it's not quite as much fun 0:10:38.440 --> 0:10:40.640 as I was hoping, but like sort of fun. All right. 0:10:40.679 --> 0:10:42.640 So you're hoping it was related to butts, weren't you? 0:10:42.720 --> 0:10:46.079 I mean probably, like, yes, the act of dropping one's 0:10:46.120 --> 0:10:50.080 pants is what I was hoping for, but maybe this 0:10:50.160 --> 0:10:53.120 is I got to keep this kid friendly. Also, teeny 0:10:53.160 --> 0:10:56.000 pet peeve of mine. I feel like, whenever you're referring 0:10:56.040 --> 0:10:58.240 to the moon of Earth, you need to be careful 0:10:58.280 --> 0:11:01.280 to capitalize it because we're referring to it's a specific moon. 0:11:01.400 --> 0:11:04.720 Then the other moons can get a lower case because 0:11:04.720 --> 0:11:07.679 we're referring to moons in general exactly. 0:11:08.160 --> 0:11:09.880 And you know, for a long time, we refer to 0:11:09.960 --> 0:11:12.880 all of these things as just satellites, so instead of 0:11:12.880 --> 0:11:15.559 calling them moons, you would say the satellites of Jupiter. 0:11:16.200 --> 0:11:19.720 Then with Sputnak and the advent of artificial satellites, you know, 0:11:19.920 --> 0:11:22.960 half a century ago it became very awkward to constantly 0:11:22.960 --> 0:11:27.440 say artificial satellite, natural satellite, artificial satellite. And you know, 0:11:27.520 --> 0:11:30.800 English is constantly smoothing and shortening things, and so people 0:11:30.840 --> 0:11:34.880 just started referring to artificial satellites as satellites, and then 0:11:35.040 --> 0:11:38.720 instead of referring to natural satellites, they just called everything moons, 0:11:39.240 --> 0:11:44.000 sort of colloquially after our moon. So technically, astronomically we 0:11:44.080 --> 0:11:47.640 have natural satellites and artificial satellites, but more practically we 0:11:47.720 --> 0:11:51.320 have satellites, which mean artificial satellites which are not moons 0:11:51.480 --> 0:11:55.000 or natural satellites, and then natural satellites that we call moons. 0:11:55.280 --> 0:11:57.559 And I think the word sputnik is just the Russian 0:11:57.559 --> 0:11:59.040 word for satellite. 0:11:59.200 --> 0:12:04.079 Yeah. And then aside from the linguistic fuzziness, there is 0:12:04.200 --> 0:12:07.560 also this question of like, well, what makes something a moon? 0:12:07.640 --> 0:12:09.360 Like it's pretty clear if you're looking at the Earth 0:12:09.360 --> 0:12:11.600 and the Moon, the Earth is bigger, and so you 0:12:11.640 --> 0:12:14.120 would say that the moon is a moon and the 0:12:14.200 --> 0:12:17.240 Earth is a satellite of the Sun. But physically speaking, 0:12:17.559 --> 0:12:19.920 the Earth and the Moon are orbiting each other, right, 0:12:20.000 --> 0:12:22.360 and there's a center of mass of the Earth Moon system, 0:12:22.640 --> 0:12:25.280 and that is what's orbiting the Sun. And so it's 0:12:25.280 --> 0:12:27.800 a little bit arbitrary to say, Okay, this one's a 0:12:27.800 --> 0:12:30.760 moon and this one's a planet. You can imagine, for example, 0:12:30.800 --> 0:12:32.760 a scenario where you have two objects of the same 0:12:32.800 --> 0:12:37.000 mass orbiting each other, like a binary dwarf planet system, 0:12:37.559 --> 0:12:40.320 which is the moon, which is a planet, right, you 0:12:40.400 --> 0:12:43.920 need some way of categorizing it, and so in our 0:12:44.000 --> 0:12:47.360 Solar system, typically if the center of mass of the 0:12:47.440 --> 0:12:51.000 system is within the surface of one of the objects, 0:12:51.800 --> 0:12:54.680 so that's like, you know, where is the average bit, 0:12:55.080 --> 0:12:57.760 The average bit is under the surface, like between the 0:12:57.800 --> 0:12:59.559 Earth and the Moon. The center of mass of the 0:12:59.600 --> 0:13:03.320 Earth Moon system is within the volume of the Earth. 0:13:03.480 --> 0:13:06.800 Then you say that's the planet and anything else outside 0:13:06.840 --> 0:13:09.839 of that is a moon. It's a little bit arbitrary, 0:13:10.080 --> 0:13:14.360 but it's helpful to settling debates among astronomers, which you know, 0:13:15.000 --> 0:13:16.640 you have to have a reason for those guys to 0:13:16.880 --> 0:13:20.360 stop drinking and go to sleep. And there's the other 0:13:20.440 --> 0:13:23.040 side of it, which is like, well, what's the smallest 0:13:23.120 --> 0:13:27.600 possible moon? Right? Like is every dust grain that's orbiting 0:13:27.640 --> 0:13:31.280 the Earth a natural satellite if it formed naturally? Like, 0:13:31.400 --> 0:13:34.679 is there a lowest cutoff? Is every proton that's in 0:13:34.840 --> 0:13:39.000 orbit around the Earth a satellite? Technically according to the 0:13:39.040 --> 0:13:43.320 definition of natural satellite, yes, those are natural satellites. You 0:13:43.360 --> 0:13:46.360 don't count them as moons of Earth. And later we'll 0:13:46.360 --> 0:13:48.720 talk about how the Earth actually does have a weird 0:13:48.880 --> 0:13:51.960 second body that's kind of orbiting it but not really 0:13:51.960 --> 0:13:54.680 that you could argue is kind of like a second 0:13:54.720 --> 0:13:58.439 moon of the Earth. Anyway, the point is it depends 0:13:58.840 --> 0:14:02.360 on the definition. And so, yeah, Kelly, really nobody knows. 0:14:02.679 --> 0:14:06.280 Yes, yes, I knew, I was right. That's great, But 0:14:07.480 --> 0:14:09.720 it does seem like, you know, you talk to ninety 0:14:09.800 --> 0:14:12.440 nine point nine nine percent of the people on the planet, 0:14:12.480 --> 0:14:16.199 they'd say that Earth has one moon, and is that 0:14:16.360 --> 0:14:18.240 just because that's what we were told as kids and 0:14:18.240 --> 0:14:20.320 it's stuck. But if you talk to an astronomer, they'd 0:14:20.320 --> 0:14:24.360 be like, maybe we have a billion, or like, is 0:14:24.400 --> 0:14:27.920 there essentially a definition that we all agree to for 0:14:28.040 --> 0:14:29.080 convenience sake? 0:14:29.600 --> 0:14:32.920 There is really no lower limit. But we call these 0:14:32.960 --> 0:14:36.120 things moons because they're interesting. They reveal the structure of 0:14:36.160 --> 0:14:40.320 the Solar System. They affect planets because of their tidal forces, 0:14:40.760 --> 0:14:43.800 and tiny little dust grains don't do that as much, 0:14:43.840 --> 0:14:46.080 and so they really are sort of a different kind 0:14:46.120 --> 0:14:48.720 of thing. Even if it is an arbitrary dotted line 0:14:48.720 --> 0:14:51.360 that we draw, and there really isn't a place to 0:14:51.360 --> 0:14:54.040 put it. It doesn't make sense to put an individual 0:14:54.040 --> 0:14:56.680 proton in the same category as the moon of Earth, 0:14:57.040 --> 0:15:00.480 just because they both orbit the Earth, and so is 0:15:00.520 --> 0:15:03.800 a qualitative difference, even if there isn't a crisp line 0:15:03.840 --> 0:15:05.720 that we can draw. And so, no, I don't think 0:15:05.720 --> 0:15:08.240 any astronomer is going to be like, actually, there are 0:15:08.240 --> 0:15:12.000 many moons of Earth, but you know, probably maybe there 0:15:12.040 --> 0:15:13.080 is one out there somewhere. 0:15:13.480 --> 0:15:15.480 Is that what you think astronomers sound like? Because I 0:15:15.480 --> 0:15:17.600 think you got to watch it. There might be some 0:15:17.640 --> 0:15:19.880 astronomers that listen to this show and their feelings might 0:15:19.920 --> 0:15:20.520 get hurt. 0:15:20.840 --> 0:15:24.480 No, I think most astronomers sound like they're really fit. 0:15:24.840 --> 0:15:27.360 They're very suave, but just the annoying ones sound like that. 0:15:27.520 --> 0:15:29.800 Got it? Okay, So you said we have like this 0:15:29.960 --> 0:15:37.000 arbitrary definition. Does this arbitrary definition have some features or 0:15:37.080 --> 0:15:39.720 is it just like, is it always a gut feeling 0:15:39.960 --> 0:15:44.120 or do we have some criteria even if it was 0:15:44.240 --> 0:15:45.880 happened upon somewhat arbitrarily. 0:15:46.440 --> 0:15:51.400 So, typically planets have something like ten thousand times the 0:15:51.440 --> 0:15:53.840 mass of their natural satellite. That's what we tend to 0:15:53.840 --> 0:15:56.320 see in the Solar System. But you know there are 0:15:56.360 --> 0:15:59.200 already exceptions to that, Like the moon is one eightieth 0:15:59.240 --> 0:16:02.520 of the Earth, Pluto has a moon that's one eighth 0:16:03.000 --> 0:16:05.880 of its mass, right, and so like you can make 0:16:05.880 --> 0:16:09.160 these broad categories. Almost all the moons in our Solar 0:16:09.200 --> 0:16:13.160 system have a mass that's less than ten thousands of 0:16:13.200 --> 0:16:16.640 the planetary mass. But there are already major exceptions. 0:16:17.600 --> 0:16:19.760 Well, let's take a break. When we come back, we'll 0:16:19.760 --> 0:16:22.320 talk about why we have moons. And if there are 0:16:22.360 --> 0:16:24.960 any kids hanging out with you while you're listening to 0:16:25.000 --> 0:16:28.280 this episode, kids, don't get any ideas. No mooning your 0:16:28.320 --> 0:16:31.400 parents during the commercial break. When we get back, why 0:16:31.440 --> 0:16:54.880 do we have moons? And we're back, So, Daniel, we 0:16:54.920 --> 0:16:57.880 talked about how moons are defined, but why do we 0:16:57.920 --> 0:17:00.320 have moons in the first place, and why did the Earth, 0:17:00.480 --> 0:17:04.600 clearly the greatest planet in the Solar System, only get one. 0:17:05.160 --> 0:17:07.719 Yeah, this is the most interesting thing about moons, in 0:17:07.720 --> 0:17:10.879 my opinion, is that they reveal something about the history 0:17:10.920 --> 0:17:13.440 of the Solar System. If you read about the Solar 0:17:13.440 --> 0:17:15.080 System and look out at the sky, you get the 0:17:15.080 --> 0:17:18.760 impression that it's this sort of calm parade. There's like 0:17:18.920 --> 0:17:22.040 slow moving truds through space that's been going this way 0:17:22.080 --> 0:17:23.520 for a long time and going to be going this 0:17:23.560 --> 0:17:26.919 way forever. But that's only because we're used to living 0:17:27.160 --> 0:17:31.520 at a sort of human timescale seconds, minutes, even centuries, 0:17:32.080 --> 0:17:34.760 and in those timescales, yeah, not much really happens in 0:17:34.800 --> 0:17:37.679 the Solar System. But the Solar System is very old. 0:17:37.760 --> 0:17:40.600 It's four and a half billion years old, and its 0:17:40.720 --> 0:17:45.400 history is filled with chaos. All sorts of crazy cosmic 0:17:45.440 --> 0:17:49.280 cataclysms have occurred in our Solar System, from planets moving 0:17:49.359 --> 0:17:52.040 in and out and switching orbits and losing planets. And 0:17:52.200 --> 0:17:54.960 moons are a great way to understand this history because 0:17:55.000 --> 0:17:58.040 they are in effect records of this history. And if 0:17:58.080 --> 0:18:00.480 you understand it why a planet has a moon, you 0:18:00.480 --> 0:18:04.159 can understand something about what happened to that planet. So 0:18:04.200 --> 0:18:08.040 there's basically three different ways that planets can get moons, 0:18:08.400 --> 0:18:10.479 from the least interesting all the way up to the 0:18:10.520 --> 0:18:11.600 most exciting. 0:18:12.000 --> 0:18:14.840 Well, I bet we are all super excited for you 0:18:14.920 --> 0:18:17.119 to start with the least interesting. Explanation? 0:18:18.040 --> 0:18:20.320 Was that a great lead in, right, lead into that 0:18:22.000 --> 0:18:26.840 ready to get bored here, I'm in, Well, you don't 0:18:26.880 --> 0:18:29.320 want to drop people with the most exciting one, and 0:18:29.359 --> 0:18:31.000 then you know they're sort of spent and like, I 0:18:31.000 --> 0:18:32.320 don't want to listen to the rest of this. You 0:18:32.400 --> 0:18:33.600 got to build up to it, don't you. 0:18:33.680 --> 0:18:35.200 But you don't even have to tell them there's a 0:18:35.320 --> 0:18:36.480 least interesting option. 0:18:37.680 --> 0:18:40.680 Great, all right, Well, the least interesting is already very 0:18:40.720 --> 0:18:45.400 exciting because it tells you about the formation of the planet. 0:18:45.480 --> 0:18:48.720 So sometimes moons form at the same time as the 0:18:48.720 --> 0:18:51.080 planet in the same way that, like the structure of 0:18:51.119 --> 0:18:54.160 our Solar System, didn't just make a sun, but also 0:18:54.200 --> 0:18:57.200 made a bunch of planets. As those planets are forming, 0:18:57.400 --> 0:18:59.439 they don't just make a planet, they also make their 0:18:59.480 --> 0:19:00.720 own little. 0:19:00.359 --> 0:19:04.159 Orbiting guys and gals space gals. 0:19:04.200 --> 0:19:07.080 I like that. So why does the Solar System have 0:19:07.160 --> 0:19:09.960 this kind of structure? Well, remember that the Solar System 0:19:10.080 --> 0:19:13.679 formed from a central blob that collapse gravitationally. You have 0:19:13.760 --> 0:19:17.560 like a seed somewhere where gravity started this runaway effect. 0:19:17.880 --> 0:19:19.760 It got denser, so it had more gravity, so it 0:19:19.760 --> 0:19:21.399 pulled on stuff, so it got denser, so it had 0:19:21.400 --> 0:19:23.760 more gravity, et cetera, et cetera. But you never just 0:19:23.800 --> 0:19:26.800 have exactly one seed. Sometimes you have like another nearby, 0:19:26.960 --> 0:19:30.479 smaller seed that resists being pulled in because it has 0:19:30.600 --> 0:19:33.399 enough relative velocity, So it goes into orbit and it 0:19:33.400 --> 0:19:37.320 starts gathering its own stuff. So that's how planets form. Right. 0:19:37.320 --> 0:19:40.080 You have this central blog which absorbs most of the mass, 0:19:40.320 --> 0:19:43.520 but then you have these other seeds nearby which sometimes 0:19:43.560 --> 0:19:46.119 form stars and you get like binary star systems. But 0:19:46.200 --> 0:19:49.000 sometimes they're smaller and they'll form planets in the same 0:19:49.040 --> 0:19:52.080 way as that planet is forming. It makes a proto 0:19:52.160 --> 0:19:55.720 planetary disc. Right. It's a big swirling mass of stuff, 0:19:56.000 --> 0:19:58.719 most of which will collapse into the planet, but some 0:19:58.800 --> 0:20:01.240 of it is going fast enough and has its own 0:20:01.320 --> 0:20:05.000 little gravitational seed, so can form an object which comes 0:20:05.040 --> 0:20:07.640 into orbit around the planet rather than falling in. 0:20:08.160 --> 0:20:10.440 Ah okay, and if it were to slow down, it 0:20:10.440 --> 0:20:11.960 would fall in, but it's not. 0:20:12.119 --> 0:20:14.879 Slowing down exactly. If it were to slow down, then 0:20:14.920 --> 0:20:17.400 it would fall in. And you know, in that initial 0:20:17.440 --> 0:20:19.359 planetary disc there is a lot of friction, and so 0:20:19.400 --> 0:20:21.080 a lot of stuff does fall in. But if you 0:20:21.160 --> 0:20:24.399 survive that and you form, then there's much less friction 0:20:24.480 --> 0:20:26.880 because things have cleared out and you can mostly orbit 0:20:26.920 --> 0:20:29.880 in a stable way. But it depends also on your 0:20:29.960 --> 0:20:33.040 distance from the planet. So for example, if you're too 0:20:33.200 --> 0:20:35.160 close to the planet, you're going to be feeling its 0:20:35.200 --> 0:20:39.040 tidal forces. The planet is a huge gravitational object. And 0:20:39.119 --> 0:20:42.200 remember that gravity depends on distance. The closer you are 0:20:42.280 --> 0:20:45.240 to something, the stronger the gravitational force. The further you 0:20:45.240 --> 0:20:48.159 are from something, the weaker the gravitational force. Now, if 0:20:48.200 --> 0:20:50.640 you are just a point object, that doesn't really matter 0:20:50.680 --> 0:20:52.760 because your front and back are the same thing. But 0:20:52.800 --> 0:20:55.280 if you're big enough that your front and back are 0:20:55.320 --> 0:20:58.280 substantially different, and your front is closer to the planet 0:20:58.280 --> 0:21:00.480 and your back is further from the planet, then a 0:21:00.560 --> 0:21:02.719 planet is going to be pulling on your front harder 0:21:02.760 --> 0:21:06.000 than it's pulling on your back. These are tidal forces. Essentially, 0:21:06.000 --> 0:21:08.400 it's pulling your front and back apart from each other. 0:21:08.680 --> 0:21:10.840 It tends to pull you into like a football shape. 0:21:11.119 --> 0:21:13.959 If you're strong enough internally you're made of diamond, you 0:21:14.000 --> 0:21:16.240 can resist that. But if you're not, you're going to 0:21:16.280 --> 0:21:19.440 get shredded by the tidal forces. This is a mini 0:21:19.560 --> 0:21:23.080