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Hello, and welcome to season two of Exolore, the show that helps you imagine other worlds with facts and science. I am your host, Dr. Moiya McTier. And wow, it is so nice to be here with you again on the proverbial Exolore stage chatting about science and worlds and the things that go into both of those. The astute listener probably would have noticed that there's a bit of an addition to my introduction. Two little extra letters. I am officially a doctor now! It's really cool. I defended my PhD dissertation a little over a week ago, you know, at the time of this recording, which is not the same day as when the episode will be released, because, you know, that's how podcasts work. But I recently defended my PhD dissertation. So I wanted to take this first episode back as an opportunity to tell you a little bit about the process of a dissertation defense, you know, what is that, like? I want to tell you about the research that I've been doing over the past five years. And I want to tell you how I think that connects to worldbuilding with a couple of examples from different fictional worlds. So that's what this episode is about. That's what you can expect for the next 50 or so minutes. For those of you who don't know, necessarily about academia, or how PhD dissertations work, or you know, I'm sure it's different in different disciplines to this is how mine went down. I gave an hour long public talk, there were 40 minutes plus about 15 or 20 minutes for questions. And then I went into a little Zoom Room with a small group of faculty, my Defense Committee, I actually got to choose them, which was nice. And I think probably not that common. But I went into a little room with my committee, and they asked me questions about my research and how it fits into the larger context of astronomy as a field for two hours and like 20 minutes. It was a long time, but I was fully prepared when I went in to answer their questions for three whole hours. So I was actually relieved when they cut it short for about 30 minutes. And then after the questions were done, they sent me into, a Zoom breakout room for about five to ten minutes while they deliberated on, I guess whether or not they thought I should be a doctor. Luckily, it was short. You know, if they take a long time to decide that type of thing, that's probably a red flag. It's also a red flag if your committee lets you defend without thinking you're going to pass. So you know, all of that aside, they brought me out of the Zoom breakout room after about five or ten minutes, and they said, "Congratulations, Dr. McTier!" So I knew I had passed. And it was a really proud and exciting day, lots of happy tears afterwards, lots of nervous and anxious tears before the defense. But after it was just a lovely day. So that's how the defense worked for me, that public talk, that hour long talk that I told you about that I gave at the beginning, that is available on YouTube, it was public in almost the truest sense of the word. And now I should actually tell you that there is a slide show that goes along with this episode of Exolore, and you can find that slideshow by going to "exolorepod.com". That slideshow has links and stuff in it. If you want to follow along over the course of the episode, you can do that. If you just want to check out the slides later and follow the links with some resources and some information, you can do that too. I just wanted to make sure you know that at the beginning so that we don't get to the end of the episode and I'm like, "oh by the way, there are slides". So if you want to follow along with the slides go to "exolorepod.com" right now and find the slideshow. Okay, cool.
So I am a doctor now. I'm officially a Doctor of Astronomy from Columbia University. It feels really good to say and I want to use this episode to talk to you about my research. Last season, I introduced or I started every episode saying, "Hi, I'm your host Moiya McTier. And I study planets outside of our solar system. Those are called exoplanets". And I'm really sorry to say that that was kind of a lie. I do study exoplanets, but that is not the most complete description of what I study. So in this episode, I want to walk you through my research, tell you what I've been doing for the last five years of my life. And I also want to tell you about how I think it relates to worldbuilding, because it's not a coincidence that I am an astrophysicist who studies these exoplanets and habitability who [is] also the host of this worldbuilding podcast. It's not just random that I do both of those things, because in my mind, they are very strongly tied together. I got into fictional worldbuilding as a process, I got into facts based fictional worldbuilding because of my astronomy work. So I want to make that tie really explicit for you today. So I'm just going to go through the slideshow here. It starts with, "I'm a doctor now," which is very exciting. You can watch my public thesis defense up on YouTube. A lot of people have been asking me what's next? And I know that that's because they are kind and generous people who are interested in what's going on in my life. And I appreciate that, thank you for your interest. But also it elicits this anxious, nervous, panicky response in me every single time because I don't really totally know what I'm going to be doing next. I decided early on in grad school, like in my second or third year that I didn't want to stay in academia, that I wanted to leave to do science communication full time. But the thing about science communication is that it can take so many different forms. You can be a science communicator who writes for so many different types of organizations, you can be a science communicator who speaks, one who works with children, one who works in museums, and of course, all of these different descriptions that I'm using, they aren't mutually exclusive, and they definitely aren't, you know, collectively exhaustive, if you're a fan of the MECE framework. So it's not MECE, but science communication can mean a lot of different things. So I know that I want to do science communication full time, [but] the immediate next step for me is to finish writing my nonfiction popular science book about the Milky Way galaxy, and how it has evolved over time. But that book is being written from the Milky Way's perspective as if it's writing its own autobiography. I expect that to come out sometime in 2022, because I am supposed to send it to my editor this July. So hopefully, we will -- I will, unless you want to help... But hopefully I'll get that done by July. And after that, I don't really know. I'll keep doing this. I will keep creating content for the growing Exolore family. I'll keep doing talks about science and worldbuilding. I don't know what I'm gonna do, and you are now all witness to my complete and total shrug-a-tude. [I'm] giving a real big shrug right now. So that's what's happening next. I may not know what's coming in the future. But I do know very deeply and intimately what's been going on for the last five years in my life.
For the last five years, I have been doing research that some astronomers would say touches on the search for the "Galactic habitable zone". So breaking that down "galactic habitable zones" are the place in the galaxy, where habitable planets are most likely to form. This is a relatively new field of study. The first paper that explicitly used the phrase, "galactic habitable zone", the first peer reviewed scientific paper was published in 2001 by a team led by Guillermo Gonzalez, I think he actually may have been a grad student when he wrote it, or at least he was pretty early in his career. So that's what the "galactic habitable zone" is. It's kind of a zoomed out version of the "circumstellar habitable zone". Or maybe you've heard this referred to as the "Goldilocks zone", the distance from a star, where the temperature is just right to have liquid water, if you're any closer to the star than the water would evaporate away, because it's too hot. And if you're any further away from the star, then the water would freeze because it's too cold. So the circumstellar habitable zone really depends on basically how hot the star is and how far away the planet is from it. Galactic habitable zones aren't quite so simple. There's a little bit more nuance in there. And we're looking at different factors, right? In the last 20 years since the first galactic habitable zone paper was published, people have studied GHZs in a bunch of different ways. What do you need to know right now just for the sake of understanding the rest of my research is that the location of the Galactic habitable zone depends on a couple different things. It depends on metallicity and metallicity is the abundance of elements heavier than helium. So astronomers consider all of the elements heavier than helium -- beryllium, lithium, carbon, iron, nitrogen, all of those -- we call them metals, which you know, is not what other people and other scientists use as that definition, but it's what we use. It's the jargon that astronomers use. So it's what I'm going to use today. So the GHZ's location depends on metallicity. You want to make sure you have enough metals to produce these rocky planets like Earth that are made of astronomers metals. And you also need to have metals to build people, right, like we're carbon based life forms. So we need carbon around. For the GHZ, you also want to be pretty far from intense sources of dangerous radiation. These are things like Gamma-ray bursts. Although astronomers don't totally understand what causes Gamma-ray bursts or what they are, so it's hard to avoid those. The other big source of dangerous radiation is a supernova. A supernovae can give off as much energy in just a month, the same amount of energy as our sun can produce in its entire 10 billion year lifespan. So these supernovae are really energetic, they can be very dangerous, we want to be far away from those for life. The other things that the Galactic habitable zones location depends on are like the age of the star. In past research, astronomers have figured that stars between 4 and 8 billion years old are probably in the sweet spot for life. The GHZ depends on the stellar number density, how many stars are around, because those can also give off radiation and do funky things that we're going to get to in the stellar encounters part of this episode. So you know, keep that in the back of your mind. The Galactic habitable zone depends on the path of the stars orbit and a bunch of other factors. So when astronomers are trying to define this galactic habitable zone, it's mostly a case of us figuring out, "okay, what would be bad for life? And where in the galaxy, can we avoid those things"? What we've discovered is that the Galactic habitable zone is an annulus, which is like a ring. It's just a doughnut. It's a flat doughnut, but really big. And it's a ring that goes from 7-9 kiloparsecs from the galactic center. I know that kiloparsec is probably not a unit that most people are familiar with, but it's just a unit of length. And for some context here, our sun, our solar system, is about 8 kiloparsecs from the galactic center. So this galactic habitable zone ring, pretty much is centered on us. And you know, maybe that should start ringing some alarm bells like, "Oh, that's mighty coincidental, we're so lucky". But that's why it's really important for me to say here that the Galactic habitable zone is not the only place in the Milky Way or in any galaxy, where life can form. Life theoretically can form anywhere in the galaxy, especially if it's non earth-like life it can totally be found outside of this galactic habitable zone that we have defined. But I think that the Galactic habitable zone is interesting to study because it narrows down our search for extraterrestrial life or aliens.
We've been unintentionally broadcasting our position out into space for the last 50 years, at least, you know, with high frequency radio signals and TV signals being sent out at the speed of light into the interstellar medium. We are letting people know that we're here, if they want to listen. And we have done, I say "we" [but] I mean the SETI Institute, "the Search for ExtraTerrestrial Intelligence" and other organizations that are interested in finding aliens. They have intentionally sent out some signals. But that's kind of difficult because you have to think about whether or not you want to direct those signals. Where would you want to direct them? We have to think about the fact that light does travel at a finite speed. So even if we send something to a civilization 100 light years away, we're not going to get an answer back for at least 200 years if they respond immediately. And I know I don't respond to messages immediately -- I'm sorry if I owe you a message. But I think that means it's pretty safe to say that we are not expecting contact with aliens anytime soon. And the universe is really big. The Galaxy is really big. There are 100 billion stars in the Milky Way. And we think that on average, there are like two planets for every star. So that's a lot to go searching through. If we can define the Galactic habitable zone, it means that we don't have to look in as many places to find aliens. If we don't find aliens there fine, we can widen the search criteria or we can give up whatever. But my point here is that the Galactic habitable zone is not the only place where life can form. I spent way longer on that point than I originally planned on, but I think it's important because I don't want people walking away from this episode or from my research thinking, "oh Moiya found where the aliens are". That is not the case. I just wanted to make that very clear.
So the point of this episode is to take you through my research, explain a little bit what I did, I'm not going to go into too much detail. If you want more detail, then you can go and watch the public defense talk that I gave on YouTube. And if you want even more detail than that, then I guess you can reach out to me and ask for my thesis, but it's you know, like a technical piece of writing. [I'm] happy to share that with anyone but you know, know what you're getting into for that. So here is my research. I did four different research projects over the course of my five year grad school career. I started in my first year with a project called "Exotopography". Introducing exotopography. Exotopography is a word that I made up, it means searching for topographical features, like mountains and volcanoes and trenches. But on exoplanets, which, if you listen to the first season of Exolore, you know, are planets outside of our solar system. So in that first project, I was really trying to understand what makes planets habitable from the inside. This project is about galactic habitability, but to do that, I first had to understand the planetary scale of habitability. There are characteristics of individual planets that would make them more or less suitable for life. Things like: how reflective is the surface? What is the planet itself made of? What is its atmosphere made of? And this exotopography project got at some of those internal, intrinsic characteristics of the planet. In my second year, I moved on to a project about the chemistry of moving groups of stars. And this project was really to help me understand, how do stars and the elements that they're made of get mixed throughout the galaxy, because stars are moving around the galaxy all the time. Our own sun is moving at about 230 kilometers per second around the galaxy, which to do the conversion is a little bit more than 500,000 miles per hour. So our sun is zipping around the galaxy, and they carry with them all of the elements that make them up. They carry gas and dust with them as they move, [which are also] orbiting the galaxy. And so I wanted to understand how that worked. And later in this episode, I'll tell you what that means, or what that could mean for worldbuilding and habitability. In my third year of grad school, I started thinking about the motions of stars, I started wondering, can fast stars around the Sun hold on to their planets as well as slow stars. And so I'll talk about that. And then in my fourth year, I did a project to understand how common close stellar flybys are in the Milky Way bulge. And then in my fifth year, I actually went back to working on the chemistry of moving groups project. So I backtracked a little bit. But those are the four projects that I worked on in my PhD. And now I'm going to go into each of them in a little bit more detail, and talk about some cool worldbuilding consequences.
So the first project I did, like I said, was searching for exotopography. What I really did was develop a method that astronomers can use to figure out whether or not exoplanets have these topographical features. This project was based on simulated [and] observed data. And if you want to learn more about that you can watch the public defense talk. But this method, exotopography, relied on a way of finding exoplanets called transit photometry. It is the most common discovery method for confirmed exoplanets that the astronomy community has found. Transit photometry traditionally works by staring at a patch of sky and with a telescope, or the instruments on the telescope, measuring how much light you get from that patch of sky over time. If a planet passes in front of the star from our point of view, which is rare, [you would] have to get lucky to have the right orientation of the planet compared to its star so that it does line up with our point of view. When that happens, the planet will block some of the stars light. And because we're measuring how much light we get from this star over time, we see a dip in what we call a light curve. We can study that dip to learn things about the planet that made it. Essentially, we are studying the shadows that these planets cast on their stars. Traditionally, transit photometry assumes that the planet passing in front of the star is totally spherical. And when that happens, you get a totally flat bottom, at the bottom of this transit light curve. But the thing that we all know from living on a planet and from seeing shows and video games and movies, where they depict other planets with mountains and volcanoes, and these really cool topographical features, the thing that we know is that real planets have bumps and those bumps should affect the planets light curve. So if a planet has mountains and features, let's imagine a planet like Earth with you know the Himalayas on it. If it's rotating in front of its star as it transits, then the silhouette of the planet is changing. And that means it's blocking out different amounts of light coming from the star that creates a jagged light curve bottom. The thing that I did was look at elevation data from different rocky bodies in our solar system like Mercury, Venus, Earth, Mars, and the Moon; and I simulated their light curves to see how jagged the bottom of their light curves would be based on the topography that they had. And so I came up with a relationship that astronomers might one day be able to use to look at light curves and back out how bumpy the planets that made them are. And that is cool in its own right, but that's not like the coolest thing about this project, at least in my opinion. I think that the coolest thing about exotopography is what it implies for the planet itself, [there's] the presence of mountains and volcanoes and other topographical features, they imply certain internal characteristics for the world. Things like volcanism and tectonic plate movement, you actually can get volcanoes without tectonic plate movement. But there are different types of volcanoes like the ones that we see in Hawaii, where you get just magma flowing up through the same part in the crust because the crust isn't moving. So you know, that is possible. But it's really helpful to know whether or not a planet has volcanism and tectonic plate movement because those two characteristics, those two features, they're pretty helpful for habitability.
If a planet has internal volcanism that is a source of heat, so that planet can be further away from its star and still be maybe warm enough to host life. That means that it's extending the circumstellar habitable zone, that's a call back to earlier in the episode in case, you were paying attention. volcanism, you know, on our own planet was responsible for creating the atmosphere that we have, or at least an older version of the atmosphere that we have. And so there are interactions between the inside and the outside of a planet in that way. Mountains, do really cool things to weather and climate patterns, you know, if you have a big mountain range, a chain of mountains, close to an ocean, for example, that can create this cool system where moisture coming in from the ocean gets stopped by the mountains, and you end up with a rainy side of the mountain and then a dry side of the mountain. That's not the only thing that can happen, but what I'm trying to get at here is that mountains do influence weather and climate in really interesting ways that can be used in worldbuilding. So that's what I'm gonna talk about now. If you have been listening to me talk about worldbuilding and my process for building a world from scratch. Or if you happen to have taken my worldbuilding class, either with Atlas Obscura, or with Silver beach, [then] you will know that I separate the process of worldbuilding into six different steps. The first three of those steps are figuring out basically what is the main characteristic of your world. Like, how is your world different from our own? The latter three steps of that process are figuring out what the environment of your world is, what the biology is, and what the culture is. So we've already established that mountains obviously influence the environment, mountains are literally part of the environment. I think that's like the strongest way you can possibly influence something. But what about biology, this might be a little bit less intuitive for some people. There have been a few studies that suggest or claim that mountains and other topographical features are important for habitability. One way that that works is that mountains can help promote biodiversity. They create these different environments or terrains, that's why I mentioned the example of rainy side and a dry side. Having those different terrains means that the animals in those respective locations will adapt to those terrains. So mountains can force scenarios that lead to genetic variation and adaptation. In fact, there are some examples of humans adapting to high altitude. There have been examples of humans adapting to high altitude in Tibet, in Ethiopia and in the Andes. And they adapt in different ways. Being at such high altitudes where the oxygen level is low can change the way that humans breathe. [It] can change the way that oxygen spreads throughout human bodies. And that happens on relatively short evolutionary timescales. You know, in the last 12,000 years-ish, these humans bodies have actually changed, like, this is a micro version of human evolution right here, which is very cool, and can happen because of mountains. Mountains also affect culture in a couple different ways. So they can influence settlement patterns. Maybe people want to settle near a chain of mountains, because it might protect them from potential invading nations. Maybe they want to be near a mountain because it provides a source of water. The snow at the top of the mountain can melt down [and] create rivers that maybe irrigate farmland for these people. Maybe people want to be very far away from mountains, or maybe people want to be close to a sacred mountain, which is another thing, right? These mountains can become sacred, religious spots. There are a lot of sacred mountains around Earth. One that I am pretty familiar with is Mauna Kea in Hawai'i. I actually wrote my undergraduate senior thesis on the fact that astronomers are trying to build a really big telescope called the 30 Meter Telescope on top of this sacred mountain called Mauna Kea. And there were Hawaiians who were protecting this mountain. They called themselves protectors, [and] were demonstrating on the mountain so that this telescope wouldn't be built. There was a lot of back and forth. The Supreme Court of Hawai'i got involved. It definitely caused some tension in the astronomy community, some astronomers saying, "hey, maybe we shouldn't build this telescope on this mountain", and other astronomers saying, "oh, but the science, we have to!"
So yeah, mountains can be very culturally important. And these are just a handful of examples that I came up with off the top of my head. There are countless interesting ways that you can incorporate mountains and volcanoes and other features. I haven't even talked about deep trenches. There are countless interesting ways that you can incorporate those into your world building and into your stories. There are a couple examples of mountains or geology, geography in general being used in fiction. The obvious example that immediately comes to mind is "The Broken Earth" trilogy by N.K. Jemisin. I don't want to spoil that too much for anyone, but I will say that the motion of things in the earth are very important for the plot of that trilogy. And if you have other examples of books or movies or TV shows or video games, where maybe tectonic plate movement, or volcanism or mountains are important to the worldbuilding, please let me know tag @exolorepod on Twitter or Instagram probably Twitter because that's where the words are, but tag @exolorepod and let me know about your examples.
Hi, there, sorry to interrupt this monologue with just more of my same old voice, but I have a message for you. If you like my worldbuilding work, if you are enjoying Exolore, and you want to support it, a great way to do that is by joining my Patreon. Patrons do get some perks like early access to episodes and you get to see my research notes, which I guess are post hoc research notes for different episodes, and I'm adding benefits over time as I learn more about what Exolore is, and what it can be. But your monthly recurring support on Patreon would help me do things like pay my editor and eventually get to pay my guests and just keep the lights on here at Exolore. So if you are inclined and able, I would love for you to go to patreon.com/exolorepod, and see whatever you can give.
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That was one project. Now I'll move on to the next one. The next project that I did was about the chemistry of moving groups, moving groups are groups of stars. Astronomers are not that great at naming things. But moving groups are groups of stars that are moving at similar speeds and in similar directions to each other. In other words, these are stars that are clustered in velocity space. And one of the big challenges that I had in this project, especially when explaining it to other people was to get them to understand the difference between velocity space and position space because the interesting or maybe annoying, depending on your point of view thing about these these moving groups is that these stars aren't necessarily anywhere near each other physically, they can be on opposite sides of the galaxy. I studied this in a pretty localized volume close to the sun that they can be on opposite sides of this sphere, this big, spherical region that I defined and still be in the same moving group. To study these moving groups, I used a machine learning code to identify the groups. And then I compared the chemistry and orbits of stars in these groups to try and figure out how the groups formed, and also where they formed - this is a big mystery. In astronomy, we actually don't know much about the evolutionary history of the Milky Way, because you know, we weren't here to see it. And it's actually pretty difficult to observe features in our Milky Way because we're in it, right, like we can't send a telescope or a camera or anything outside of the Milky Way to turn around and then take a picture of us. So it's hard for us to understand what's happening in the core of the Milky Way. It's all covered in in dust and stars. And it's hard to see through that. So I was doing this project comparing the chemistry and orbits of moving groups to figure out how and where they formed. Did I do that? Kinda, it kind of worked. I don't know, the paper for this project isn't done yet. It took a bit of thinking to figure out how this project relates to worldbuilding, but I did it. So chemistry is related to metallicity. And remember I told you in the beginning of the episode or released earlier in the episode, that metallicity is the abundance of elements that are heavier than helium. So we've been studying metallicity for a while to see if we can notice any trends, [and] what we've found is that stars in the center of the galaxy or closer to the center of the galaxy have higher metallicity than stars closer to the edge of the galaxy. So we call this a descending or decreasing gradient, a metallicity gradient. What it means is that the closer you are to the edge of the galaxy, the fewer metals you have, the lower your metallicity is. We call those stars "metal poor', it means that they don't have a lot of elements that are heavier than helium. So that's one trend we noticed that as you move out from the center of the galaxy to the edge, you get fewer and fewer metals. The other trend we noticed that we've known about is that older stars tend to have lower metallicities than younger stars. And that's because stars are responsible for creating these heavier elements inside their cores. This is the fusion process at work. And stars can fuse hydrogen into helium, helium into other things, I actually don't know what the whole scale is, but it works its way up to iron. And some of those stars are massive enough to explode, which is very exciting, as supernovae or as just regular old novae. And they send out those elements into the space between stars. And then new stars form from the gas clouds that were receeded with those heavy elements. So over time, the stars that are born have more and more metals in them from the previous generations of stars, it's actually like, really beautiful and like a bit macabre. I love thinking about stellar generations and how the death of one generation leads to more metal enrichment for the next. It's cool stuff. So metallicity, and how it relates to worldbuilding, it mostly directly affects the environment. So stars that have more metals tend to have more planets around them, especially they tend to have more gas giants. So if you want to build a world where there are a lot of neighboring planets in one solar system, and if you want to definitely have a lot of gas giants, then you would want the host star in your in your stellar system to be metal rich, you would want it to have a lot of elements heavier than helium. If it is very metal rich, then because of that trend that I just told you about, that likely means that the star is relatively young. But relatively young on galactic timescales can still mean it's billions of years old. Our own sun is about 4 1/2 billion years old. And I would say it's in like a medium generation of stars. So that might help you calibrate your ages, if you care about this at all. The other way that metallicity directly effects environment is that it would influence or it would determine what your planet is made of. So it would determine the composition of your planet. And that will tell you what type of natural resources are available on your planet. One example of this in fiction is naquadah, in the Stargate franchise, I'm pretty sure that of all of the episodes I've recorded so far for season two, I've referenced Stargate and every single one of them. So you have that to look forward to good luck. But naquadah is a mineral. I don't think it's an element, but it's a mineral, which is like a combination of elements happening together. So naquadah is a mineral that can store and produce energy really well. It's also fairly explosive. So they use it in a lot of different technologies in the Stargate franchise, and there are some worlds where naquahad is pretty abundant. And there are some worlds like Earth where there's no naquadah at all. And I really love that they included that in the Stargate worldbuilding because it's so easy to just assume that every single world has the same minerals - has the same elements on it, but that's not necessarily the case. And sometimes it's really fun to imagine a world that has elements or resources that we just don't have here on Earth. So I encourage you to do that in your worldbuilding.
I'm going to move on to the next project. But I'm going to lump a couple of projects in together. So the third project that I did was looking at stellar velocities and how that impacted planet occurrence. What I did for this project was compare the speeds of stars with planets, to the speeds of stars without planets. I was doing this to see whether or not fast stars could actually hold on to their planets, I imagined that maybe they might have some trouble doing that. But what I found (after I corrected for a selection effect on the part of the telescope that collected this data) was that there is no difference in the stellar speeds. So fast stars are just as likely to have planets as slow stars. But that's in the solar neighborhood in the region of the galaxy around the sun. And I told you before that the sun is zipping by about 230 kilometers per second. And that's a typical speed for this part of the galaxy. But it's actually pretty slow compared to other parts of the galaxy. So I turned my attention to the Milky Way bulge in my first year of grad school, I took a class about stars and galaxies and there was a professor who would talk about the Milky Way bulge and I sat in the front row of that class and had the audacity not only to fall asleep in almost every class, but also to laugh every time this man said bulge because apparently, I'm a 12 year old boy at heart. But I turned my attention to the Milky Way bulge to see how often stars there would have close stellar flybys. And I'm not talking about collisions, because space is so big. It's so big, y'all. Stars are so far apart that even when galaxies collide, it's very rare for stars to touch each other. One of the first problems that a fledgling astronomy student does is calculate the number of stellar collisions that we can expect when Andromeda and the Milky Way galaxy collide in about 5 billion years. And it's like a handful of stars. And each of these galaxies has, you know, hundreds of billions of stars. So it's very rare. So I'm not talking about stellar collisions. But stellar flybys, which it turns out, are much more common. I found that by simulating the orbits of stars in the Milky Way bulge and counting pretty much how many encounters each star has. And what I found was that about 80%, 8-0, 80% of stars have at least one encounter every billion years within 1,000 AU. So AU stands for "astronomical unit", and it's the distance between Earth and the Sun. It's the average distance between Earth and the Sun, actually, because you know, in case there's anyone out there who's like, "oh, but the Earth orbit isn't totally circular. So sometimes it's closer to the sun bla, bla bla", well, to you, it's the average distance. 1,000 AU technically is like within the solar system, the furthest planet Neptune, I want to say is about 50 AU, I might be wrong on that. So don't cite me look it up. It's pretty easy to Google, "the semi major axis of Neptune", if you want to be fancy, but I think that's about 50 AU. But outside of Neptune, outside of Pluto, outside of the Kuiper Belt, there is this field of asteroids called the "Oort cloud", and that goes out to like 5000 AU. So if a star came within the 1000 AU of our sun, it wouldn't necessarily touch the planets, but it would pass within the Oort cloud, which could be very bad. Closer encounters like encounters within 100 AU or within 10 AU, those are rarer.
Those are less common, but not unheard of what I found in my research was that about one in 5,000, stars should have an encounter within 10 AU every billion years, and there are about 10 billion stars in the Bulge. So 10 billion over 5,000 is math that I don't want to do when I'm on a microphone, but you can do it. And that will tell you how many 10 AU encounters there will be. These encounters can be pretty dangerous for planets around stars, they can rip planets away from their host stars, which would be I think so so traumatic, you know, in working on this Milky Way book and personifying the Milky Way, I've gotten into the habit of personifying other astronomical objects as well. And I'm just thinking about the therapy that a planet would need, if it got ripped away from its host star in one of these dangerous encounters. That's years' worth of therapy, that's millions of years worth of therapy for a planet right there. That bill is astronomical, pun intended, I'm sorry, I'll keep talking about science. So these planets can get ripped away from their host stars, or their orbits can become destabilized so that maybe nothing happens immediately after the encounter, but like, a million years later, maybe this planet spins into its host star, or it gets flung out of its system. And if these encounters happen early enough, then it can disrupt the planet formation process altogether. So encounters are potentially very dangerous based on a lot of different factors. Like you have to consider, how massive are these stars, how fast are they going, where are the planets in their respective orbits? Like, if all the planets are on one side of the star and the other star flies by on the other side - that's unlikely, but you know, think about the edge cases.
The way that stellar motion interacts with worldbuilding is through environment, biology and culture. So it hits on all three. For environment, the dense stellar environments that you have in the bulge can provide a lot of dangerous radiation. And if you remember, earlier in the episode, I said that radiation is one of the factors that you consider in the Galactic habitable zone. I didn't say why. It's because we don't necessarily know what the exact consequences of like a supernova explosion would be to human life, but based on some studies of exposure to radiation that we've done here on Earth, radiation can affect mutation rates that can lead to cancer, it can affect the way that plants photosynthesize, really strong radiation from the supernovae can actually alter the makeup of the atmosphere, it could totally destroy Earth's ozone layer, if a supernova went off within 8 parsecs. Let's say [that] if a supernova goes off within a few 100 light years, like we're probably screwed, I did not say that in my defense talk, because I couldn't say, "we're screwed'. But like, we would be screwed. These unstable orbits that planets might be on after a stellar flyby, if one happens, those unstable orbits can yield unpredictable weather patterns. And those patterns just get less predictable over time, as the planet either swirls into its star, or it gets flung out. Or maybe it interacts with another planet in the system, especially if that star was very rich in metals. And so it's much more likely that there are a lot of planets in the system altogether. See, it connects, I promise. For biology, there's more potential for what I called "Interstellar pollination", and what I mean there is that because these stars are coming closer to each other, these stars are also moving around more, they're moving faster than stars in the solar neighborhood in our part of the galaxy. I expected that that means that you know, if life happens on one of these planets, it might be a lot easier for like a rock with a little microbe on it to get flung out of a system and crash onto another planet and another system. This is related to a concept called panspermia, which is the idea that life didn't necessarily originate on Earth, but that it originated elsewhere, and hitched a ride on like a comet or an asteroid or something, and then that managed to find its way to earth. And that's why there's life on our planet. So I think that that might be something that you could play with more in a dense stellar environment like the Milky Way bulge, or these things called globular clusters, which are dense clusters of stars in the Milky Way Halo. If you want to learn more about the Milky Way Halo, I guess, you can Google it, or again, you can watch the public defense that I did on YouTube. For culture, this is related to the interstellar pollination and panspermia idea. But for culture, I imagine there's more potential for Interstellar civilizations here in this dense stellar environment. Maybe it's easier for civilizations to travel around this region, and interact with other stellar systems, just because they're moving faster, maybe they do some cool science where they figure out how they can use the gravity wells from stars to slingshot their way and travel faster through the galaxy. I just feel like in these dense stellar environments, there's more opportunity to have these interesting, multi stellar systems civilizations or interactions between different civilizations that you know are forced closer together in this denser environment. I think that that would be really cool. I do have an episode in the works, a worldbuilding episode where I imagine a world like this, a world in a denser environment, and I don't want to give too much away. So I'm not gonna say anything else about that, but be on the lookout for it.
That's all of my research. That's five years worth of intense grad student research, and then a little bit about how it connects to worldbuilding. This really is the research that got me thinking about habitability and worldbuilding in the first place. So it's really nice to share it with you and move beyond that lie that I was telling all of last season. And now hopefully you understand more about my research and how I use that in the worldbuilding that we do here on Exolore. If you've made it this far, you're probably a nerd, and as I've said before, nerds are my favorite people. So thank you. That's all I have about my research. I want to tell you just briefly what you can expect from season two of Exolore. It's going to be a lot of worldbuilding, and interview[ing] people who have created worlds. I've already interviewed some people that you recommended when I asked for suggestions on Twitter, so thank you for that. In this season, I want to get more in depth with worldbuilding. When I put out the survey last season to ask for your feedback on season one, a lot of you said that you wanted longer episodes and I can't do longer episodes because I'm a human. Also that's very expensive. But what I can do is draw out the worldbuilding process. So in this season two of Exolore, one thing that I'm going to experiment with is doing three part worldbuilding trilogies where I spend one episode building out the environment, the next building out the biology, and the third building out the culture. So that's one thing that I'm going to try this season, I want to do more how-tos, more instructional episodes. Many of them will probably be interviews to help you develop more of the skills that I think are important if you want to do in-depth worldbuilding work, and I want to incorporate more fantasy. I feel like I've totally misrepresented myself in season one, because I read almost exclusively fantasy. And yet, if you only listen to season one, you would probably think that I was much more into science-fiction than I actually am. So I want to do more fantasy worlds in season two. Just because they're fantasy does not mean that they're not going to be based on facts. That's my whole thing. Facts will be in everything that I do. So yeah, that's what you can expect for season two.
Thanks for joining me for this first episode, and I hope to see you back for the next one. Exolore's cover art is by Steven J. Reisig. The transcript is by Iesir Moss and the music is from purple-planet.com. Exolore is a member of Multitude, an independent podcasting collective and production studio, I highly recommend checking out the other Multitude shows, all you have to do is type Multitude into the search bar of your favorite podcast app. If you want to support my worldbuilding work, there are a couple ways you can do that. The first is to rate and review the show on Apple podcasts. It's free, you don't need an Apple device, and it really does help the show grow. Second, you can support me on Patreon. Your monthly support would make it possible for me to continue working on this passion project of mine. So if you're able, please head on over to patreon.com/exolorepod. If you liked this episode, be sure to share it with your friends, and subscribe to the show. That way you can catch me next time on another world.