This week, we're focusing on the physical environment of a planet orbiting an M dwarf star. We talk plate tectonics, atmospheric loss, compact multi-planet systems, and how uranium makes things HOT!
HOSTED by Dr. Moiya McTier (@GoAstroMo), astrophysicist and folklorist
Dr. Wendy Bohon is an earthquake geologist, science communicator, and AAAS If/Then Ambassador. You can follow Wendy on twitter at @DrWendyRocks and learn about all of her amazing work on her website, drwendybohon.com!
Will Waalkes is a PhD candidate in astronomy at UC Boulder who specializes in characterizing planets around M dwarf stars. You can check out his work on his website will-waalkes.github.io
- Listen to HORSE wherever you get your podcasts, or at horsehoops.com
Hello there friends. Welcome to Exolore, the show that helps you imagine other worlds with facts and science. I am your host, Dr. Moiya McTier. I'm an astrophysicist who studied pretty much everything in space from planetary orbits to the radiation leftover from the Big Bang to star formation and black holes and galaxy evolution. But I am especially interested in the motion of stars and how that affects the habitability of exoplanets, which are planets outside of our solar system. I am also a folklorist who specializes in building and analyzing fictional worlds. And this podcast is my way of sharing those worlds and that knowledge with you. So let's get started.
This episode is the first in a three part series and every episode in the series will focus on building out the same world. The first episode, this one will focus on building out the environment. The second episode will focus on biology and the third will focus on culture. And the world characteristic for this series was inspired by my first ever exoplanet research project that I did the summer before my senior year of college. And that summer, I was studying a system called Kepler 186, I was really interested in learning the eccentricity of the fifth planet in the solar system, Kepler 186 F. If you want to know more about eccentricity, which is a measure of how elliptical or not round or circular a planet's orbit is, then I would recommend going back and listening to season one, Episode Four, "The World of Dancing Seals", where we imagine[d] what life might be like on a planet that has a very eccentric orbit. But in today's episode and the two others in the series, we're focusing on a different element of that planetary system, we're focusing on its star, because the cool thing about that system was that all of the planets orbited what's called an M dwarf star. And so in this three part series, we're going to be imagining a world that orbits an M dwarf star. And we're going to get into the details of how that really will affect the environment, the biology and the culture. And so without further ado, here are my guests for this environment episode. Thanks, friends for joining me, it's very exciting to have you here. The first thing I'll ask you to do is introduce yourself and Wendy, your left most on my screen. And I typically read from left to right. So do you want to tell me and the listeners who you are and what you do?
Absolutely. My name is Wendy Bohon. And I'm an earthquake geologist at the incorporated research institutions for seismology. I also do science communication. So I work a lot on trying to make sure that we are communicating well about earthquakes and other rapid onset geologic hazards to all of the different stakeholders. So politicians and the public and educators, everybody needs to know more about earthquakes.
That's awesome. How do you communicate that differently to those different groups?
Well, there's a lot of different kind of recommended practices, depending on your audience. But clear, consistent messaging is really important. And also empathetic messaging, the things we study actually affect people's lives. And they can cause a lot of fear and anxiety. And so we love our science. We think it's really interesting and fascinating, but we recognize that it really does impact people's lives and mental health and well-being. So that's really important to keep at the forefront when we're talking about any of these hazards.
It's a really great message. I think it'd be nice if more scientists approach[ed] communication and their work with more empathy.
Yeah, absolutely. Will, what about you?
Well, I am your friend.
I've known Will, for what is it now? 5-6 years.
[It's] going on six year[s]. But aside from that, I'm a fourth year PhD candidate at the University of Colorado Boulder, where I work on detecting and characterizing small exoplanets that orbit small stars. So typically, that means M dwarf stars, these sort of very low mass, typically active nearby stars that are very numerous in the galaxy. Without getting [too into it], I try to find these smaller, closer to terrestrial sized planets surrounding stars and ideally study their atmospheres. I'm working on different projects right now about that.
That's gonna be real relevant. I noticed that you were very intentional about using quotes around "small planets" and "small stars." Who are you talking to that 's saying M dwarfs aren't small?
Okay, so it's not so much that there's disagreement about M dwarfs being small. What a small planet is and what a small star is just means different things to different scientists. So specifically, I'm interested in planets with long periods and in the era of exoplanets that we've discovered, a long period is anything greater than 5 days, so long period planets that are less than 4 Earth radii. This is the sub-Neptune, super Earth, and Earth-like exoplanet regime.
So that means there's lots of range for the size of our planet today. We're not necessarily limiting ourselves to earth-like.
Right. It doesn't have to be one Earth radius to be a terrestrial planet with an atmosphere.
Awesome. One thing I always ask my guests is what fictional worlds they've been inhabiting lately. So Will, movies, TV shows, books -- what do you got?
Mostly, I've been in World of Warcraft. I've been I've been playing lots of World of Warcraft lately. I mean, that's where my sense of community comes from right now. It's an online game. So it's not like different in a quarantine, you know, so, it's actually been a really nice environment to hang out in. And I have friends, I have a community. I've just been spending lots of time in there. I started reading Assata: An Autobiography by Assata Shakur. Jesus, I'm two chapters in and I would already die for her. I mean, you know, I would have before. It's a very emotional [and] really difficult read. And it was a very interesting place to put my energy while I was kind of processing this shooting that happened.
Wow, that's a lot.
Sorry. Well, you asked what I was reading.
No, you don't have to apologize. I admire your ability to choose to take on that sort of emotional toll when we're already dealing with everything else going on. Wendy, what about you? Are you also inhabiting some emotionally fraught fictional worlds?
I am too emotionally fraught, to seek that out in my escapist realities. So I've been reading, Uprooted [by Naomi Novak].
Have you read that book? It's so interesting. And I don't really know where it's supposed to be. It's an earth-like planet with trees and whatnot, but they're malevolent and it's very much wizards and escapists in the Nth degree, and then I've been playing Lego Indiana Jones with my six year old twins. So they're having a great time.
So are you like acting out the the Indiana Jones movies? Have your kids seen Indiana Jones?
They have. I mean, we have some questionable parenting during the pandemic.
I'm not here to judge anyone. Also, I definitely watched those when I was little.
You know, it's fine. We cover their eyes, you know, during the face melting scenes and all those kinds of things. And it's a little bit more violent than, say, SpongeBob, which is our normal go-to, but you know, it's fun. And it's Indiana Jones. It's sort of like Star Wars. Like there's guns and they're fighting, but it's kind of a classic. So we're playing the Lego version of that on Xbox and having a good time.
Okay, this is going to show how out of touch I am. I didn't even know you could play Lego on the Xbox. Is it still building?
Basically, you're in a Lego world that's made out of Legos. And so you're interacting just like any other video game, but it's more intended for kids. It's kind of like Minecraft where everything's square, and it's scenes from the movie. So they have hints on what they're supposed to do based on what they remember from the movies.
Yes, they're very entertaining.
I have a huge weak spot in in video gaming, because I have just never been into it. I didn't have any consoles growing up. And now so much good worldbuilding happens in video games, and I just I want to learn more about it. And today I learned about Lego video games. So thanks.
If it makes you feel better, a lot of bad worldbuilding happens in video games, too.
Oh, bad worldbuilding happens everywhere. Yeah, but hopefully not bad worldbuilding here today.
No, we're gonna be great. We're gonna build a much better world than the one we have inherited.
Oh, absolutely. I think the bar is set pretty damn low. Let's clear it with flying colors. In today's fictional world, we're doing something a little bit different from the way we did things in Season One. Instead of building out an entire world, in one episode - in one hour long sitting, we're going to split it up into three different sections: environment, biology, and culture. And in today's episode, we're really going to focus on building out the environment of this world, the physical setting. So today, we're going to be building out a world that orbits an M dwarf star, which is why Will is here, and one really cool thing about M dwarfs -- actually a couple really cool things about M dwarfs because I can't limit myself to just one: they are the longest lived and most abundant stars in definitely our galaxy, probably the universe. We think that those things are constant throughout the universe. They can live for like a trillion years.
Yeah, some of them longer than that.
That's not me exaggerating. I think when people mean "really big number" they say "like a trillion." I literally mean 1 trillion, like 10 to the 12[th power].
Not a single M dwarf that was ever born since the beginning of the universe has aged off of its main sequence.
Yeah, we've never seen an M dwarf die.
Yeah, I'm sorry. You're just bringing up now, but I want to add something about why this is such an important characteristic of building worlds, which is that if you have a planet around a star that more or less the star, I mean, we'll talk about sort of like stellar activity and stuff, I'm sure, but in the sense of the number of stars on the main sequence for a really, really long time, means that you can have planets around them that have evolved in really interesting ways, and that have potentially more opportunities to create and also to lose atmospheres compared with planets and much shorter lived systems.
Yeah, absolutely. And I am so excited to get deeper into the details of what might happen to our planet as it's evolving, both outside with the atmospheres and inside with all of the potential tectonic plates. And Wendy is here to help us decide, you know, what's going on in there. But I do want to say just a couple more things about m dwarfs to really contextualize this. Wendy, I don't know how familiar you are [with M dwarfs].
Not very. So this is great.
Awesome, so here's some extra info. M dwarfs are smaller than our sun, our sun is actually a pretty average star. M dwarfs have masses between 10 and about 60%, the mass of our sun. They have temperatures that are much lower than our sun's temperature between about 2,000-4,000 Kelvin. That's 3,500 to about 7,000 degrees Fahrenheit, which is still very hot, that's still something that would like burn you to a crisp, immediately. Another cool thing about M dwarfs is that they're really magnetically active, they have lots of flares, they give off high amounts of X ray and ultraviolet radiation. And the astronomy community is actually kind of split on whether or not planets around these stars would be habitable. There are lots of papers debating this, so many on both sides. And so today, we're just going to start with the assumption that life is going to happen on this world. And our job here in this episode is to set up the environment for that life to exist on, which we'll get to in the next episode. Will, anything you want to add about M dwarfs?
Oh, geez. Yeah, I mean, they're really cool in every sense of the word. And, there's a lot of them. This isn't a stellar type that you hear about very much, because we can't see any M dwarfs with the naked eye. So when you're looking at the night sky, you know, none of those dots are endorsed, because even the closest endorse are just so dim, compared to every other type of star.
Hold up, so you're telling me that all of the stars that I see none of them are this type of star? And yet this type of star is the most plentiful star in the universe?
And we can't see them?
Not with our naked eyes.
That is blowing my mind.
Right? Isn't it so cool?
So many stars.
And so when I'm trying to find or characterize planets that orbit these stars, even M dwarf stars that are fairly close to us, in our stellar neighborhood are too far away. So they're too dim for us to be able to study.
Yeah, and I want to clarify just a couple of things here. We're talking about these stars being cool and dim, but they're dim for a couple of different reasons. They're dim inherently, like in an absolute sense. If an M dwarf were sitting right next to our sun, it would literally be dimmer, because an object's brightness and its temperature are linked in that way, these stars are cooler, and therefore they give off less light. But they also can appear dimmer because they're far away. Right? So there are two reasons for astronomical objects to to appear dim.
So what I was trying to say is, if an M dwarf is the same distance as another star, a planet around that M dwarf might be impossible to study its atmosphere, whereas a planet with atmosphere around a brighter star at the same distance, maybe we could do that because of how much brighter it is.
Any questions about M dwarfs, Wendy, before we get into building out this world and thinking about the planet around it?
I'm trying to think about reasons that planets would be habitable, and I'm assuming if it's more dim, our planet would have to be closer to the star than say, maybe Earth is from our Sun.
Yes, absolutely. These M dwarf systems have smaller or closer in habitable zones, then brighter and hotter stars like our sun.
That's good to know. Okay.
And you figured it out. That's the best part of this. So I really want to focus first on these flares. A lot of people who say M dwarf planets aren't habitable say that because of the magnetic activity of these stars. So, first, what do these flares do? How can they actually affect the planet?
Yeah, so the sun has sunspots and the solar activity cycle, and there are occasionally solar flares, you know, it's been 115 years since the last solar flare powerful enough to disrupt our energy grid, but it does happen, you know, stars do stuff. M dwarf stars, like you said, are magnetically active, they are much more active, especially when they're young, and so M dwarfs, don't maintain that same level of activity their whole life. But there is a significant period of time when most types of M dwarfs woud be so active that they likely will have stripped the atmospheres of terrestrial planets around them. So stars can do two things with these flares. They're emitting high energy radiation, and they're emitting particles and ions, what we call "plasma" into interplanetary space. And if all the solar activity interacts with the planet's atmosphere, it has a tendency to excite and destroy molecules in that atmosphere so that those molecules and atoms escape to space. In particular, water, [which] in any kind of volatile state, if you have a water ocean, you might have vapor in your atmosphere, ultraviolet photons interact with that water, they break it up into OH and H and the hydrogen escapes.
That's so rude. I like my hydrogen. I like my hydrogen here on my planet.
It turns out that all life we know of needs water.
That's kind of important. Yeah.
Just as an aside, if you have a larger mass planets outside of the context of habitability, you know, Jupiter is not a habitable planet.
At least not for humans.
Oh, fair, okay, but by all of our current definitions of habitability. But if you have mass, you can hold on to more stuff, especially if you're very massive, you can hold on to single protons, you know, these hydrogen atoms that have been dissociated or something. So this is why larger planets, if you're already past a certain mass limit, somewhere in the vicinity of being a little bit less massive than Neptune. So past the mass limit, you actually can hold on to your atmosphere, even with a star that's very active.
That's why you can have these massive gas giants really close to their stars, these hot Jupiters, as they're called, that don't lose their atmosphere because they have such a strong gravitational field due to lots of mass. They have lots of junk in the trunk, if you will, and magnetic fields. Yes, they have these massive things that can help them hold on to their atmospheres, even if they're much closer to their stars.
Yeah, we call it a B field, and the B stands for badonkadonk.
Did you just make that up? I've never heard that before that.
No, you're the one that brought junk in the trunk into it.
Okay, so these flares, they can strip away atmospheres when you eventually get to the point of a civilization that may require electricity. And depend on some sort of electricity grid, these flares can interfere with that. But that's for a future episode in this series. Let's talk about the planet itself. Wendy, you very smartly pointed out that it's going to have to be closer to its star. And Will, you said that if it's bigger, it might have an easier time holding on to its atmosphere. So my question for both of you is how big do we want to make this planet?
Not too small because if it's too small, it's not going to be tectonically active. And we want it to be tectonically active, because that's going to help us to maintain life that will allow volatiles to leave the planetary system and create atmosphere, create liquid water, as long as our you know, Labrador retriever puppy of a sun isn't shooting all of these flares out, which are then causing the volatiles to escape all of the hydrogens running away. So, you know, geologic activity requires heat, either from primordial heat from the formation of the planet, or heat from the decay of radioactive materials inside of the planet. And so if you have a larger planet, then it's going to take longer for that heat to dissipate. And so you have kind of a longer lifespan for tectonic activity. I don't know how big is too big, but we don't want it to be too small, because then it would turn into like the moon, or perhaps even Mars, where it just has one kind of outer shell instead of having, you know, the heat escaping and things moving around the way that we do on earth. And even like you see on Venus.
Yeah, that's a really great point. And we especially want to be able to hold on to heat for a long time, because it's a system that's going to last for a really long time up to a trillion years.
And thanks to how spherical objects work, even just making it a little bit larger in size means that it can hold on to its feet a lot longer. I would say lower limit Earth radius, smaller than that tectonics become an issue like Wendy is saying, but there's an upper limit for this based on observational evidence of exoplanet statistics, which is that we probably won't have a terrestrial planet with an atmosphere the way that we're imagining unless that planet is less than 1.6 Earth radii.
I love that paper.
The title of the paper is literally like "Most Planets bigger than 1.6 Earth radii aren't rocky." It's such a straightforward title. And I love it.
So I bought a sticker from Erin May who runs the sticker shop and she has the Fulton gap plot sticker. So I'm waiting until I get a new laptop. I'm putting the slot on it.
Do you want to explain what the Fulton gap is?
Sure. So it's a histogram of how many planets we see of a certain size and essentially what it's saying is around 1.6 Earth radii we see this huge drop in the number of plants we see. And then we see another peak come up after like 1.8 - 2 Earth radii. So an interpretation of this is that once you've reached a certain size, well, then you have enough mass to actually hold on to a much larger atmosphere. And you're no longer this small terrestrial planet trying to hold on to this, you know, paper, thin layer, you've passed some threshold where now you can hold on to significantly more atmosphere.
Okay, well, then, let's zero in on something that's like one and a half times the size of Earth. So it's not too small to limit tectonic activity. It's not too big. That way we can make sure it's still rocky. What do we want it to be made up? Because right now we're talking about size. But that's slightly different from mass, right? If we make this planet out of really dense materials, it can be much more massive. Wendy, if you had to build your like, ideal planet from scratch? What would you make it out of?
That's a good question. And I don't know that I've ever really thought about it coming from quite that way. When I think about planetary composition, I think about the way the planet evolves. And so you start off with kind of a particular primordial oozy magma, that's all basaltic. You know, it's like kind of heavy and dense. And then as you continue through this plate tectonics cycle, you start to differentiate out the minerals, it's almost like a milkshake, you know, the foamy part rises to the top, and then your other stuff sinks to the bottom. And so the foamy stuff, the earth minerals that are silica-rich, the quartz, those sorts of things will come up to the top. And so you get this differentiation of the heavy stuff sinking to the bottom and forming the core, and then the lighter materials rising up and forming the lithosphere. And you continue that differentiation throughout the history of the planet. So what we should start with is not something I've thought a lot about.
Okay, cool. Do you want to start with something that's very similar to Earth? And then we can take away or add as we see fit?
Yeah, that sounds good. What do you think Will?
Well, we've talked about how old M dwarfs can be, but we also need to star that's not too old because if it formed in the galaxy, before there were sufficient supernovae, then there aren't going to be any heavy elements to make our planets out of. So you know, we also want to make sure that our star is is young enough.
I see that Lineweaver 2004 paper coming up. The Lineweaver paper is a paper that's very relevant to my dissertation research on galactic habitability, which talks about how you need to have a sufficient amount of metals, [and] for astronomers, anything heavier than helium is a metal. And I know that drives a lot of people nuts. But that's our vernacular. And so this paper talks about how you need a lot of metals to make a habitable planet and metals increase in abundance over time, because stars are forming these metals in their cores and supernovae are forming these heavier metals and then pushing them out into space. So yeah, that's a really good point. We don't want a very old star. We want a young M dwarf that has lots of metals in it -
Okay, but we can't have it too young, because then it's sending out too many flares, which will destroy our atmosphere. So we have a sweet spot in dwarf age.
Exactly. We want it to be pretty juvenile, like 6-8 billion years old.
Basically, it's got its driver's license.
I love that. So our M dwarf has a driver's license, it's about 7 billion years old. So it's older than our sun, which means potentially any civilizations on it could be more advanced. That's an interesting thing that we might pick up later in this world series. Wendy, I'm really interested in what you said about the radioactive material helping to provide heat in this planet. What types of material? What's actually making the heat?
Well, there's lots of different radioactive materials inside of the earth, including things that most people have heard of, like uranium, which decays into thorium. So you have these decay reactions. And when they decay, they produce heat. So we do need radioactive elements to be inside of the earth, or our planet. I said the earth because I always say the earth, I am also a planetary scientist. It's just that the earth is my planet, so we definitely have to have radioactive materials to allow that additional source of heat, uranium for sure. Let's go with uranium. We don't even need the other ones. Let's just say uranium to make it easy.
All right, just uranium Do you want a bunch of uranium in this planet?
That's an interesting thing, right? If we have a lot of heat, and we're going to be diffusing that heat out. And so that means you would have really active tectonics, you would have really active volcanism, so that would definitely impact our societies for habitability, where they would be able to stay on the planet.
Well, I mean, that tectonic activity can decrease over time like it did on the earth. So I think more in the direction of what you're describing, like having more uranium or outgassing, all the stuff, that's a good source of a secondary atmosphere for a planet.
Yeah, so okay, we've got a bigger planet than Earth, which means it's going to hold heat better. And if we have more radioactive material inside the earth, that's going to be creating additional heat, we would have a more active plate tectonic or more active tectonic regime, which means, in particular, increased volcanism, but also increased hazards from things like earthquakes and the types of gases that we don't want, like sulfur, sulfuric acid, and, and that sort of thing, which can be problematic for crops and society and all those things for life.
For human life.
Yeah. So one thing I want to give you the freedom to do here is not limit yourself to what would be okay for human life. Basically, whatever type of planet we come up with here, the biologist in the next episode is going to have to come up with a lifeform that can survive there.
Well, I'm glad were first.
So you have all the freedom here.
What are all those extremophile environments, right, like in the hot springs in Yellowstone, and all of that, like, wow, that'd be fun.
Well, okay. I mean, we solve our questionable gases in the atmosphere problem by just having it be a water civilization.
Okay, haven't done one of those in a while, it'd be nice to revisit that.
I mean, I guess it's not our job to decide what civilization is. But I'm just thinking ahead.
I know, we're moving on to a different episode and far outside of our area of expertise here, but like, I want them to live symbiotically with those bacteria that live in super hot environments [where they] form this whole ecosystem together.
Noted, I will let the people in the next episode know.
So if there's increased tectonic activity on this world, like if you're a person, not necessarily a human person, but a person standing, or crawling, or whatever existing on the surface of this planet, how do you notice or engage with increased tectonic activity? Like how does it affect your life?
Well, the speed of plate tectonics could be dependent on the amount of heat that's being released. So I wonder if there were plates if they're moving around much faster, which means that your mountain building processes would be faster, you would have more volcanic activity, so you'd have all kinds of different types of volcanoes around they'd be more plentiful. And in more places, the crust would turn over faster,[because] in some places, the crust is created, and in other places, it has to go somewhere. So it would be destroyed faster, but you would also maybe have increased erosion, if you have a different atmosphere, things could be working more quickly to get rid of the rocks. So it could be a changing environment, but I think the things you would notice the most would be the results of the tectonics and day to day life, which would be more volcanic eruptions, more earthquakes and those sorts of things.
Yeah, I love the idea of a world that's more constantly in flux than ours.
Yeah, this reminds me of N.K. Jemisin's, "The Broken Earth" Trilogies, where in this universe - not getting into the kind of more supernatural elements of it, they live in this place where there's a lot more sort of geological peril, and they sort of just get used to whole cities of people dying every once in a while. So again, this is back to the civilization part, but I think they could get used to it.
Yeah, find ways to adapt.
So I have a question then that has to do maybe with how close our world is to the star. Would it be rotating? Or would it be like the moon always facing one direction? I don't know what that's called.
Yes. This is an amazing question to bring up for planets around M dwarf stars. So the phrase I think you're looking for is "tidally locked".
Yes, that's the one!
Yeah, the moon is tidally locked to the earth, which means the same side of the moon is always facing us. So there is a far side of the moon, but not a dark side of the moon. Just making that very clear. There is no dark side of the moon, all sides of the moon get illuminated by the sun at some point in the month. One of the other big contentions for astronomers thinking about habitability around M dwarfs is that because the planets have to be closer to their stars, they might be more at risk of being tidally locked, we could decide whether or not that's the case, especially since this planet has a lot of internal heating mechanisms, because of the buttload of uranium that we've decided it has. It doesn't have to be as close to its star to be a good temperature for life.
I agree. And I also think it's way more interesting if the world is tidally locked.
I am doing another episode about a tidally locked world.
Alright, fine. Okay, so yes, so given the scenario, where we have a lot more internal heating, and you know, the radioactive decay may be playing a much larger role than it does for other terrestrial planets. We could be far enough away not to be tidally locked, so that's not unreasonable.
Or we could be an intermediate distance and the planet is becoming tidally locked. Can you imagine living on a planet that is in the process of becoming tidally locked so that the days are slowly becoming longer and longer as it reaches that like equilibrium state with its star?
What is the timescale there? So if these organisms live for a 1,000 years, would it be something that would happen over the course of their lifetime?
They would definitely notice things starting to change over their lifetime. But I think depending on basically how squishy the planet is, and how close it is, to its star, it can happen like over a billion years, I think is typically the timescale that people cite for tidally locked planets.
Yeah, to give it even more of a coarse estimate. I think that by the time a secondary atmosphere and a civilization evolve on this planet, it will have become tidally locked, that timescale would be much shorter than the development of civilization. I mean, as if I know.
But we get to control everything here.
So I do know.
Yeah, you do know.
Neil Gaiman calls worldbuilding, "the joy of getting to play God." And I think that that's accurate.
Okay, let's go with the intermediate then. I like the idea that everything on our planet is in flux.
The ever-changing world. I like this.
The biologists that you get are gonna be equal parts horrified and interested?
How does that change the evolution of the beings and the things that live on the planet? If things are constantly in flux, and you're constantly changing your external stressors, that's got to be really interesting for them to consider.
And I mean, think about human perception. There's a certain time scale at which we experience reality. And that's different than the timescale that other things experience reality. I mean, it's weird.
On that note, let's take a little break. And then when we come back, I'd love to talk more about tectonic plates under oceans because I'm curious about those.
One thing you might not know about me is that I played basketball for eight years starting in third grade. So I know a lot about the sport of basketball, but not much about the world that surrounds the game, which is why I've been listening to Horse lately. Horse is another show from the Multitude Collective and it's a podcast about ridiculous stories, internet drama, and some of the biggest and baddest personalities out there today in the world of basketball. The hosts Adam Mamawala, and Mike Schubert are hilarious. And they just want the world to know how unbelievable the history and culture of basketball are. They're here to fight gatekeeping and prove that it's entertaining for everyone to follow even if you've never seen a basketball game before. New Episodes release every other Monday. And to find them, all you have to do is search "Horse" in your podcast app or check out horsehoops.com. And you should check it out. That's Horse, the podcast because basketball is more than what happens on the court.
And now for a couple of quick housekeeping notes. First is that I've decided to take the entire month of August off. Some of you may know that I just defended my PhD dissertation back in April. And I went right from that into working on the Milky Way autobiography that I'm writing for Grand Central Publishing, all of that without any time off. So I am taking a much needed vacation in August, which means I'm skipping the next episode on August 12. And the next episode that you hear in this M dwarf series is going to be on August 26. But that takes me to my next point, which is I know that I'm going to miss you all while I'm away this month. So I am leaving you with an exciting prompt at the end of this episode. And if you respond to that prompt, you'll be entered into a raffle where the prize is a 30 minute one on one worldbuilding session with me. So to hear the details of this raffle and prompt, be sure to listen to the very end of the episode. Or if you'd prefer to see those details written out instead of spoken, you can go to "Exolorepod.com/raffle" that's Exolorepod.com/RAFFLE. And you can hear the details. But let's get back to the episode and be sure to listen to the very end because you don't want to miss this exciting raffle. All right, are we ready to start up again?
Well, I have a question for Wendy about tectonics. So Wendy, maybe this is a thing that we know well, or maybe it's a thing that we really don't know and it's very theoretical. But you know, if we took the earth and we scale this up to the planet that we're talking about, you know, it's a little bit larger, a little bit more massive, but still treshold plan. I mean, would there be more plates, you're kind of mentioning like a faster subduction of plates and things like that, like with the surface of this world be more fractured or less?
It depends. I mean, the surface of our world has been more fractured and less fractured at different times, because plates are in continual states of flux. So right off the coast of Oregon and Washington, you know, there's a huge subduction zone there. It's actually a tiny little remnant of a plate called the Juan de Fuca plate. That's the only piece that's left of a giant plate called the Farallon plate which we can still image underneath North America. So the whole thing has gone down underneath North America except for this tiny little draggy edge, and so soon that entire plate's gonna be gone. It's like a tectonic plate graveyard in the mantle, you can still see them in some places, and they're just gone, plates get bigger in some places, and then they're getting smaller and disappearing and other places. And so we are always changing how fractured we are in how many plates there are, and how fast they're moving. how this relates to our planet. You know, it depends on how thick your lithosphere is, which has something to do with the chemical composition of the world. So if you have a thicker lithosphere, that may change how the plates interact and how they move. If you have a thinner lithosphere. That's also going to change how they interact and how they move.
What's a lithosphere?
When you look at how you can differentiate the inside of rocky planets, you can do it chemically or mechanically. And so it's based on what its composition is or how it behaves. The lithosphere is the part of the crust that's brittle, so it breaks during earthquakes, whereas the Venus sphere is more like the mantle in terms of how it works. Even though its composition is different than the mantle. It's still like Silly Putty, so it doesn't behave brutally. It behaves ductily.
Yeah, Moiya and I were just sitting here with our mouths open for most of that description.
That's pretty cool.
That's such cool stuff. You said that you can see the plates even after they've been subducted.
That X-ray machine must be gigantic.
Science magic. It's called seismic tomography. So seismologists are very creative scientists in the same way that astrophysicists are very creative scientists, because you're having to study something that you can't really see, that you can't touch, that you can't interact with. And so you have to come up with creative ways to image those things. And so just like you use a telescope to look up into the sky, we use things like earthquakes as an earth scope to stare down into the ground. Seismic waves move differently through different types of rocks, and the temperature of the rocks changes the speed of the seismic waves. It's called "seismic tomography". So looking under the ground, seeing how the seismic waves are behaving under the ground can show us places where there abnormalities or things we wouldn't expect underground. And that's how we were able to image the plates in different places. Basically, they're cold blobs inside the mantle.
You saying that the seismic waves speed depends on temperature makes me think that this planet that we're building has a much hotter interior? Do you think it'd be easier for seismologists to study the interior of this world?
It would be easier because there would be more passive source seismology, right? If you have more earthquakes, then you have more opportunities to see how the waves travel. I don't know if the temperature would matter so much as the frequency of opportunities to look inside the ground.
That's so cool. It's terrible for them, potentially. Who knows? Maybe they thrive on earthquakes. We'll figure it out in the next episode.
Well, and that's what we're doing on Mars, right? There's a seismometer on Mars from the insight mission that is looking at seismology on Mars, but also at like the rate of meteorite impacts, because that also puts energy into the ground, which sends energy waves through the planetary interior, which has allowed us to look inside of Mars. So we've discovered that the core is larger than we thought it was.
Do you mean it's measuring these things as meteorites strike the surface? It's like sensing the vibrations?
Theoretically. I don't think any have actually struck the surface since the seismometer was working. But there have been more than 400 Mars quakes that have been measured so far.
Is that more than the number of earthquakes in the same amount of time?
Oh, Lord, no. We have like 14 earthquakes a day just in California every day.
There's like 100,000 earthquakes recorded in Japan every year. There are hundreds of thouands of earthquakes happening around the world all the time. And as we have more and better seismometers, we're able to detect smaller and smaller events. So you know, we can basically increase the resolution of what we're looking at. And so in terms of seismicity, Mars is in between the Earth and the Moon.
[This is] some really cool stuff.
Wow. Right. So our planet is going to have more seismic activity, because it's more active.
I want to think about some other potential consequences. And this is like a brainstorming session at this point. What do we think might be some consequences of being around an M dwarf star that we haven't talked about? So one thing that comes to mind for me is maybe there would be more aurora, especially if this planet has a magnetic field? And it seems like it might because it's big, and there's gonna be lots of stuff happening inside to create a strong magnetic field.
I think there would be more Aurora. I don't know so much about them because they fall outside of the lithosphere. But, you know, we do you use seismometers to study the Aurora. So I know that they interact, you know, with the ground surface, and the magnetic field can actually couple with the ground surface. And we can measure that using magnetotellurics and things. I don't know so much about it. But it seems to me that if we have all of the flares, and we have a strong magnetic field that we should have Aurora and other really amazing things, but we wouldn't have a moon, would we? Because we'd be too close to the star.
That's a good question.
But in this context, we've already decided we're at an intermediate distance where the planet isn't necessarily even tidally locked, like, I guess I can't definitively say, you know, or even competently say that we would have them. But I don't see why not?
Could we have two moons?
We can have as many moons as you want.
How do our moons get affected if we're becoming tidally locked? Does that affect the orbiting bodies of us?
It can. Yeah, I mean, the planet definitely dominates the gravitational interaction between moons and the planet. But the interaction between the planet and the star definitely has some effect.
I'm sure that there is a distance at which when you become close enough to the star, then the moon-planet interaction gets disrupted, I think we could probably assume for our case, that there's a moon in a stable orbit. Yeah. So if an M dwarf has a bunch of UV flux, that's not necessarily great for life, but obviously, the sun has some UV flux, because those ultraviolet photons get funneled into our magnetic field, and they excite stuff in the atmosphere and stuff glows, and it's pretty, but that's depending on the atmosphere of this planet, you might not be able to see an Aurora if it was happening. Because the stuff in the atmosphere that the ultraviolet photons excite, has to glow in visible wavelengths, you know. There could be aurora, and if there are, depending on the composition of the atmosphere, it may or may not even show up to the eyes, then again, we're not talking about humans on this planet.
Wendy, you said that the aurora can interact with the ground somehow?
It's not my research, [but] we have a whole array of seismic stations in Alaska. And when the aurora are strong, it disrupts the seismic signal, you actually have to take out the aurora signal from the seismometers in order to be able to see the smaller earthquakes.
Whoa. So I'm imagining [that] there [could] potentially be some creatures who feel the aurora, especially if they can't see it through the atmosphere?
I mean, what is the process by which is disrupting the seismometer?
I don't remember. I mean, I wrote a paper about the paper, but it was like six months ago, and I can't even remember what I had for breakfast. So it's asking a lot.
Yeah, this is a facts based worldbuilding show. But that doesn't mean that everything has to strictly follow facts, like, I'm definitely going to be pushing for lifeforms who can feel the aurora in the ground. Any other consequences you can think of, of being around an M dwarf star, thinking as expansively as you want?
I'm interested in the wavelengths of light that are coming in, and what that will look like and how that will impact things that can be grown, or, you know what I'm saying?
Well, there is lots to say about that. And if I tried to say even half of it, it would all be wrong. So an M dwarf is radiating in different wavelengths than stars like the sun. The sun emits the majority of its photons coming out of the green. Alright, so the sun is peeking its emission in the green, we don't see green, because there's lots of other wavelengths neighboring green on either side that our eyes, combine all those wavelengths and say, "Oh, it's like white." And so with an M dwarf, you have shifted the spectrum of the star to the red, like quite red. I mean, they look red to our eyes, we could see them because it's color, its actual visible color is representative of its temperature, right. And we kind of talked about this in the beginning. So the photons that are leaving the star and coming into this planet's atmosphere, are going to be skewed toward the redder wavelengths. And that doesn't mean that they're going to be photons that aren't admitted by our sun, it just means that the relative amount of red photons to blue photons is gonna be much higher for an M dwarf compared to the sun. I mean, the sky is gonna look different, the sunsets will look different -
To human eyes.
Yes, sorry to human eyes.
Assuming they were plants, they would not be green.
Yeah. So there is some interesting stuff in how plants have evolved to look the way they do. They have chlorophyll in them, which helps them photosynthesize. But the green color that a lot of plants have is because they've done this really cool balancing act of figuring out, "do I want the most energetic photons from the sun? Or do I want the most abundant photons from the sun?" And basically, they've decided to absorb all of the abundant photons, the reds and the blues and reject or reflect the most energetic photons, the green ones that are coming from our sun, but as Will said, on this planet, because it's orbiting an M dwarf, all of those wavelengths get shifted down, they get shifted towards the red. And so maybe plants on this planet would reject the red photons and absorb everything else. So maybe they would look red if humans went and looked at them, but maybe not.
Maybe they would absorb all the photons and they would maybe look dark, [and] be purple or black.
Because they don't have the energy budget to be reflecting that many photons from a cool star.
That's true. Yeah, there are just fewer photons. So maybe they can't do the same balancing act as plants here on Earth, so they'd have to absorb everything.
I have another light related question, what would the star look like to us in the sky? Would it be really big? Would it be small and dim?
So the sun is a disk. You know, I mean, you can see it, it's the size of the moon, right? If we take that, and we say, "now it's an M dwarf" and let's say that we're at the same distance from the M dwarf, then that star is going to look proportionally smaller compared to the sun. I mean, it would be you know, less than half the radius of the sun. But if you just took that and you shrunk it, then it's way too cool at the Earth's orbit. I mean, we're freezing to death, it would be a teeny tiny little red dot. Catastrophe! Life on Earth ends suddenly, because our stars decided to become an M dwarf, right. But if you move that orbit in such as the M dwarf is the same size as the sun, then it's going to be cooler. If two stars are the same size disk in the sky, whichever one is dimmer or redder, that's going to be giving you less radiation. So if we have to move close enough that we're on a temporary M dwarf orbit, then that star is going to be much bigger in the sky.
I'm trying to imagine that sunrise, you know, watching this huge disc come up over the horizon. That would be awesome.
That would be beautiful.
That would be so incredible.
Especially with the two moons that our world has. Yeah, I think it would look larger, but not as bright probably.
Yeah, I mean, without doing like back of the envelope calculations, I would guess that the angular size of the M dwarf star in the sky is going to be between 2 - 8 times the size of the sun. If we're on a temperate orbit.
Let's go with 8.
Yeah, let's go with 8. That really would dominate, like almost the entire horizon.
Our sky would be full of moons and suns. It would be a full sky.
Yeah, definitely lots of room there to play with how it would influence folklore, because so many gods and mythical figures are based on celestial objects. And on this world, it really would seem like they're there, watching over you.
Okay, I'm just really on this roll now with M dwarfs and planets. So if we're talking about the spectrum, these photons coming off these M dwarfs that are different from the sun, depending on the atmospheric composition, you know, that's really the next deciding factor, like what things would look like, because you wouldn't have a blue sky, right, we see a blue sky because the blue photons are getting scattered. So if your star doesn't have those blue photons or just not nearly as many of them, the sky's not going to be blue.
I'm imagining like a gray, hazy sky. I mean, we've said that there's lots of heavy volatiles in the atmosphere, right, coming from all of the volcanic activity.
And probably particulate matter. Because, you know, as you erupt, you put particulate matter into the atmosphere.
We didn't really pin down an atmospheric composition. But assuming it is different, someone would have to know what wavelength photons get scattered by, you know, a silicon atmosphere or whatever. And the other thing I just want to bring up about, because this is an M dwarf, what might be different about the system. M dwarfs can have very compact multi terrestrial planet systems. And you can have multiple terrestrial planets that are also habitable. So maybe we have a system where you have other planets in the sky that are close enough on some orbital positions. You know, there's just like, "Oh, this planet is there this month".
Wendy's reaction to that was priceless.
I didn't know that was the thing. I didn't think about what that would mean. I want there to be other planets that will see such a full sky will have moons and a sun and and a planet but only part of the time.
I mean, a gas cloud condenses, it collapses into a star. And if you have a big chunk of gas becomes a big heavy star. If you're in the kind of dregs of the cloud, that's just forming these small little clumps of stars. Those stars have just like a ton of just leftover material that didn't get turned into big stars. M dwarfs tend not to form large Jupiter sized planets. Not that they can't or don't ever, but they just tend to form very few of those and way more terrestrial planets.
Shout out to the TRAPPIST-1 system, which is probably the most famous M dwarf system that we've discovered. There are seven nearly kind of Earth-sized planets orbiting around this M dwarf star TRAPPIST-1. And this entire system of a star plus seven planets, if you brought it into our solar system and plopped it down on top, the whole system could fit inside the orbit of Mars.
Yeah, it's very compact. I mean, just the scale of an M dwarf planetary system is smaller than the scale of a solar type star system.
Y'all have ruined my night. I'm just gonna be sitting here reading about all of this stuff for legitimately the rest of the night, so RIP to my plans.
I mean, I would heavily encourage you to look at some of the TRAPPIST-1 art. There is beautiful art out there of people drawing their imagination of what the skyline might look like if you could see these other nearby planets appear over your horizon like a moon. It's gorgeous.
That could be this system.
It could be. Any other consequences of M dwarf systems before we wrap up?
Multiplicity, you can have other circumbinary or circumtrinary. Yeah, there's possibilities. Lots of M dwarfs are in binary-plus systems.
I like to say the only reason our sun is special is because it's alone. A lot of stars exist in these multiple star systems.
Well, that's sad.
Yeah, but it's probably good for us.
We have a sad, lonely star.
A sad, middle aged, lonely, average star. Yea.,
We should get the sun a cat.
Oh, my God. Yes. I've had a lot of fun today. I hope you have to.
I'm so excited to go and read all about this now.
I've absolutely loved watching your reactions to you know, "how old can M dwarfs get?" "How compact are these systems?" It's been such a joy to see the wonder on your face, Wendy. When the listeners want to learn more about you and your work, if they want to stay up to date on what you're doing? How could they do that?
You can find me on my website, which is "drwendybohon.com", or on Twitter, YouTube Instagram, @DrWendyRocks.
Oh, you got it on all three. Good for you.