Daniel and Kelly’s Extraordinary Universe - How big is the smallest habitable planet?

Episode Date: September 14, 2023

Daniel and Kelly talk about what it would take to live away from Earth, and whether a small planet could host humanity.See omnystudio.com/listener for privacy information.

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have resources available for you at loveyourmind today.org. Hey, Kelly, do you prefer to have like a huge, spacious house with lots of rooms or a cozy little house? So I like to have spacious land, but the house can be like whatever size. We've got like a nice size house. It's not huge or anything, but we've got a lot of land and we love that space. What about you? I agree.

I think big houses can be too big. We moved here from a little Chicago apartment into a huge Southern California house where like the bedroom was as big as our old apartment. It just feels like too much space. How can a house be too big? What do you mean? That sounds great. It's like too big to have a conversation across. You know, we want the kids to come down for dinner.

We end up like texting them. It's ridiculous. All right. Well, so even in my tiny house, I text Zach from like the other room. But we don't have to worry about that with our kids because they're like younger and super energetic. So they're more like quantum particles. They're like simultaneously in every part of the house at once. That's a great analogy because kids also have their wave functions collapse eventually when they're exhausted.

You know, usually it's my wave function that collapses first. And I could just hopefully get them in the like air. area of their bedroom beforehand, but lots of collapsing, yes. Check out our upcoming book, Quantum Parenting by Daniel and Kelly. I'm not sure anyone should be following my lead, or at least paying for my advice, but hopefully I'm doing something right. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine. And I'm Kelly Weiner-Smith. I'm a parasitologist and I'm adjunct with race university.

And as much as I used to like to travel all around the world, these days I really like being a homebody. I like having a comfortable place to sit on my couch and read great novels and look at pictures of my travels around the world. How would you, Kelly? Are you traveling a lot these days? Are you sticking close to home? to home. You know, COVID squashed a bunch of my travels and they haven't picked up too much again. I like getting out of the house, but I also just really like being at home with my family and my bugs. And as much as I like looking up at the stars and thinking about space travel, I'm not really very excited about going into space myself. I'd rather stay here where the atmosphere is nice and warm and there's plenty of fresh water while other people break new ground. Amen. And welcome to the

podcast, Daniel and Jorge, explain the universe in which we mostly cast our minds, not our bodies out into the universe to try to understand what's out there. How does it all work? And is it possible to sustain life anywhere else in this vast and glittering cosmos other than our cozy couches? My friend Jorge and usual co-host can be here today, but I'm very happy to be talking to our regular guest host, Kelly. Kelly, thank you very much for connecting to us from your comfortable couch. Well, thanks. I'm actually in my comfortable barn, but I'm excited to be connecting on such an interesting topic. And today we are thinking about couches and barns, but not here on Earth, where everybody's comfortable. We're thinking about out in space. It feels like the standard trope

in science fiction is to think about the future of humanity as out in the stars. And of course, lots of people talk about how it's vital that humans get off this planet and establish colonies or outpost in other places in the solar system and maybe even around the galaxy to avoid being extinguished by one single rock from space. Do you think that's a good argument? I'm going to derail us real early. I think that depends on whether you mean good for humanity or like good for the galaxy. It might be best if we're knocked off the table early on before we do any more damage.

Fair enough. I hear that argument from ecologists a lot. I mean, there might be fascinating bits of life here and they're all over the solar system or all over the galaxy. And we don't exactly have the greatest history about treating local ecosystems with respect. So from the point of view of like nascent life in other places of the galaxy, it might be best if we just stay where we are. It might be, especially if they're bacteria, since I'm not sure we would have a ton of respect for bacteria on another planet. But as a booster for humanity, of course, I want my kids. to survive and their kids to survive and also your kids and you know other people's kids too

and I want us to all have cozy places to live and fun places to run around then yeah I think it makes absolute sense for us to think about places to live other than earth because our time here is limited even if we do take good care of it yeah you know humans make a lot of mistakes but I still remain pro-human so so yes I like this plan B argument way to take a really controversial opinion in there, Kelly. I know people who aren't super pro-human, but I think my species has some good features. I think so, too. I've got a soft spot for humanity over here. And Earth does seem like a very wonderful and comfortable place to live, but it's important that we think about other places

that we might be able to survive. Places where humanity might be able to get a foothold and even flourish. Places, of course, other than Earth. Even if a meteor doesn't hit the planet, And even if we don't boil the oceans with climate change, eventually the sun will get brighter and larger and will boil our oceans for us and may even envelop the Earth in its outer layers. So if we're thinking really, really long term, it's important that we find another place for humans to run around. Well, so just before we scare everybody, because I think a theme is going to be that technology is nowhere near prepared for us to inhabit another planet. We've got like a couple billion years before the sun consumes. the earth or whatever, right? Calculations vary, but we have at least a cool billion years. So, yeah, this is not something we need to plan for next week. You don't need to cancel your travel

plans. Your kids can finish high school, all that kind of stuff. But it is something that we need to be thinking about. You know, humans also have a record for kicking the can down the road. So it's important to be aware of these things well before we actually need to take any action. Well, and frankly, it's fun. What a fun project to start on. So yeah, let's get started for the fun of it. That's right. And something that's very exciting in the last couple of decades is that we've been able to discover planets around other stars. Until about 20 years ago, we only really knew about planets in our solar system. Out of all the stars out there in the cosmos, we'd never seen a planet orbiting another star. But now we have the technology to discover those planets and to think about which ones might actually be homes for humanity.

A common way to thinking about this is identifying the habitable zone, a regenerative. around those stars where there's enough heat, enough solar radiation to keep people warm and toasty on the surface. Fun aside here, the people who think about how many generations you would need to get to one of these habitable planets outside of our, you know, neighbors, you know, farther out past Mars or whatever, they call their field arc geology because it's like your arc to get there. Or maybe Zach is the one who came up with that name. Anyway, it makes me laugh.

Wait, so you're not sure if that's a scientific term or just a joke invented by your husband whose job is to troll science? This is my life. Everything blurs. I don't know, but I'm always having fun. Well, a lot of the planets that we have found around other stars are pretty big because big planets are easier to find. A lot of these planets are close to their sun or very, very massive. And so it makes some of us wonder, like, is it possible to live on a really big planet? or are there other small planets out there that we might live on?

Is it the case that the smaller the planets we can live on, the more options we would have? Or is it more complicated? Because science! We'll get into that at the end of the episode, whether there are more small or large planets available. But today on the podcast, we're going to be digging deep into one extreme of possible habitats for humanity. So on today's episode, we'll be answering the question. What's the smallest habitable planet size? Ooh, you know what I'm wondering?

Are you thinking about The Little Prince? No. I have no idea what you're talking about. That's not what I'm thinking about at all. Wait, you've never read the book, The Little Prince? No. This is a famous story about a little prince who lives on a tiny little planet. He can like see the curvature.

He can like run around the whole planet in about two minutes. Whoa. That sounds lonely. Wow. Way to just like pan-famous literature. Crime and punishment, sounds depressing. I mean, that's Russian literature, right? You don't need to take the class.

It's just, that's depressing. You know, I think you should put out a book with one-word reviews of famous literature. Well, you know, if you're interested in brief reviews, you can check out, Zach's A Bridge Beyond the Point of Usefulness series, where he did that for Shakespeare, and it was hilarious. All right, but today we are talking about a tiny little place. places to live. The Earth is small compared to many planets like Jupiter and many planets we've discovered around other stars, but it's really also very, very big. And there's lots of much smaller chunks of rock, even in our solar system. And you might wonder whether it's possible

to have life on those little bits of carbon. And so as usual, I was wondering what people had thought about this question. If people imagine living on tiny little rocks or people thought that Earth was basically the smallest thing that could keep an atmosphere and maintain habitable conditions. So I went out there and asked folks who listen to the podcast their thoughts on this question. I really value hearing everybody's opinions before I prepare the podcast helps me align the episode with what people already know. If you would like to share your thoughts for an upcoming episode, please don't be shy.

We want to hear your voice. Write to me to questions at danielanhorpe.com. So think about it for a moment before you hear these answers. what do you think is the smallest habitable planet size? Here's what some listeners had to say. I think that the smallest planet for humans to inhabit would be one that would support our kind of life without a need of lots of artificial equipment like astronaut suits.

So it would need to be big enough to hold its atmosphere only through gravity, its own gravity. This question gives me real the little print spot. If we're talking habitable in terms of life as we know it, then I believe we have to have an atmosphere, which means it has to have the mass to retain one. Like Mars is about half our mass, but it has lost that atmosphere due to solar wind or sheer mass issues. So maybe there's a way to have a smaller mass where something lives deep in the rock or whatnot,

but I believe you would need some sort of liquid water, liquid methane or whatnot, which would require again an atmosphere. So my thing would be the requirement of an atmosphere would be a minimum. So those were some great answers. And I think that they point to an important question that you need to clarify before we move on. Does smallest habitable planets, how much technology and equipment are we allowed to use? Do we make it habitable by like making pressurized domes where we provide the atmosphere? Or are you talking about habitable with like no help from technology. You could plop a naked person down and they would survive. Yeah, you're right. And we could spend like 45 minutes even just discussing like the definition of habitable. And as we'll discover,

the literature on this question is beginning more and more sophisticated as people add more conditions to the definition of habitable. You know, people when they were first thinking about habitable planet, they were just like, is there roughly the sun's amount of sunlight? And then as time goes on and on, they add conditions like, you know, is the gravity okay? And is there liquid water on the surface and can you breathe and you know personally when I think about a habitable planet I'm imagining my kids running around and touching grass you know I can't really imagine raising kids and living in like a glass bubble on the surface of a desolate asteroid though technically that does make something habitable right you put down a habitat on something in principle you can inhabit it

but I'm imagining sort of a warmer cozier future for humanity one where you can and grow crops and go outside without a breathing apparatus. What do you think? That is the future I would want to inhabit if I was not living on the Earth, but I wouldn't want to leave all the amazing tiny little synipid gall wasps that we have on Earth, but maybe that makes me weird. But certainly I've talked to a lot of people who would be totally fine living in pressurized habitats and going outside only after like pre-breathing oxygen and putting on a space suit.

So I think there's a lot of variability and what people think they would be happy dealing with. Although I'd argue that maybe those people after like three or four years of living like that might not be so enamored of the idea anymore. But maybe I'm wrong. And maybe I just have very high standards. I do like a lot of space. I suspect you're right. And I notice in science fiction where people do live in like glass bubbles, there are all these like soft focus memories of walking through fields of wheat on earth, you know, or dipping their toes in water. Or in that recent show, Silo, where everybody lives underground in this surviving.

tube. They're all like nostalgic about walking along the beach. And, you know, I get it. That sounds pretty terrible. But, you know, if we are thinking about just technically habitable, like could we put a colony there and have people survive? Like, for example, Elon Musk, I think is imagining on Mars. He might have like longer term plans to terraform Mars by nuking the polar ice caps and releasing more atmosphere. Such a good idea. But he's always thinking shorter term, right? Can we get people there in the next 10 years or so. He's definitely not planning to have people be able to walk around. And of course, we're lucky enough to know somebody who's something of an expert on whether it's possible to build habitats on Mars or other solar system bodies. That's you, Kelly. That's right. You and Jackson's

wrote a book about this called literally a city on Mars. So tell me, do you think it's possible with current or near future technology to build something so that humans could make essentially, you know, any rock out there habitable? No. Is that the whole book? No. It's a short pamphlet, a bridged beyond usefulness. No, actually, the book is over 400 pages long, so that's a much more nuanced argument. Is it just like no on 400 different pages? No, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no. No, no, no, no. No, what are the major obstacles? Well, so, it depends on who you talk to. Everybody has their favorite obstacle. And actually, it was very interesting going through the space medicine literature, because you'd find multiple different

titles that were of the form, blank is the biggest barrier to living on space, but everybody would have different blanks. And one of them would be like, you know, the way microgravity messes with vision and another would be space radiation. And so there's like a lot that we don't know. But in terms of technology, in my mind, the thing that we really don't know is how to create closed loop ecosystems that don't break and are like reliable on a planet like Mars. So, you know, for example, So you want to be able to grow food on Mars and you want to be able to like use those plants to sort of clean your atmosphere. But right now, you know, we don't have ecological systems figured out to the right level of detail to know exactly how many plants you need, for example. There was this simulation that happened in China that had the amazing acronym Lunar Palace.

Like they are clearly rivaling NASA's acronym game. Watch out, guys. Wait, Lunar Palace is the name of it and also every letter stands for something? So lunar, no, this was a simulation for the moon, but palace, it all stood for something. So just to be clear, you're not just talking about having an outpost where we're sending supplies from Earth. You're talking about like a self-sustaining colony, is that right? Well, so, you know, if you're talking about Mars, for example, habitable kind of requires self-sustaining because the orbital window only opens every two years. And a lot of food is not shelf-stable for that long.

And like, so you sort of need to be able to pack as little as possible because it's still pretty expensive to ship stuff to Mars. And you need to have redundancy and you need to have, you know, you need to have all of this stuff working in a way that won't break. And at the moment, you know, resupplies to the ISS happen pretty often when things break. But that's not going to happen on Mars. So you need to make sure that you've got this all figured out before you leave. Otherwise, you all dead. So like a big fancier version of the ISS would not qualify as a real habitat, right? if we like outfitted an asteroid with a bigger version of the ISS and space panels,

but still required a lot of resupply from Earth, that feels like just an extension of Earth, right? Because then if Earth goes out, if Earth gets hit by a meteor, then obviously that's going to die out. Well, and I don't think that's a problem initially. I think relying on resupply from Earth for a while is fine. And you're certainly going to need that as you're trying to get towards self-sustaining. But when you're out as far as Mars, you already need to have a lot of this stuff figured out because you are going to be on your own for years. And at the moment, we don't have that kind of stuff figured out. And so, like, you know, that Lunar Palace sim that I was talking about,

they actually had a crew of three guys, and they didn't have enough oxygen for the three guys, so they had to swap out two of the guys for smaller women. And then they had enough oxygen for everybody. And, like, that's where we are right now. Like, if you're on Mars and you're like, shoot, we should have brought two shorter people. Like, you're just out of luck.

You can't just be like breathe less, guys. You can make people shorter, I guess, you know, sorry, you got to lose some height there, buddy. We got to take off one of your arms, pal. Too much aerobic respiration happening. So anyway, that's where we are right now. Like, we don't know how to keep these systems stable, even on Earth, for periods of time of less than two years.

And so, and there's not a lot of money going into that because it's not, like, exciting science. And so I don't think we can do this now. If we start investing in it heavily in the near future, I think we could. And some people are doing that science, but not on the scale. I think we need to be, like, creating settlements on Mars. for example. How much of that is the context and how much of that is just like we don't know how to get things started? Say, for example, we found a copy of Earth, you know, a rocky planet with our atmosphere and our temperature and flowing water and all that kind of stuff. But barren of life, completely sterile. Could we get agriculture, et cetera, started? Or is it just really hard to boot up a whole ecosystem? I mean, it's hard to boot up a whole ecosystem. So for example, in Biosphere 2, they tried to boot up a whole ecosystem.

And in some cases, it went like hilariously wrong. I'm sure it wasn't hilarious for them. But so like these scorpions that they didn't mean to bring along, they did bring along. And it turns out that these scorpions are like deadly. And so they had to figure out what to do with the scorpion. Wait, doesn't that problem solve itself? I mean, scorpions are eaten in some parts of the world, right? Can't you turn that problem into your afternoon snack?

Sure. But like, I would rather that my snacks didn't have the chance to kill me, you know? like, if I had to choose between mealworms and scorpions that could kill me, I'd go with the mealworms. And those are probably going to be your early choices for protein on space, something like that. I mean, so, like, for example, agriculture on the moon would be hard because there is hardly any carbon there. And so in our theoretical or hypothetical situation, are you saying that all of the stuff

that plants need to grow is already on Earth, but just like microbes haven't shown up yet? Yeah, exactly. I'm wondering in like the best case scenario, a planet with all the resources we, we need, but without life having gotten started. Could we like bring enough of our ecosystem with us to actually make that livable? I would bet we could. I bet that there would be some things that get sort of out of balance and there could be some difficult moments where, you know, like an insect that we brought to help us out sort of does something we weren't expecting and then we have to figure out how to control it. Like I imagine there'd be some tense moments,

but but that sounds like a much easier project to me than trying to like garden and the perchlorate-laden soil that you find on Mars, yeah, that would be better. But that doesn't happen in our solar system, I don't think. All right. So it sounds like building a technological habitat in an otherwise completely unfriendly to life situation is currently far outside of our technical abilities. Though, you know, maybe in 100 or 500 years or 1,000 years, we would be able to build habitable bubbles that could go anywhere and making this question of habitability is sort of

irrelevant. Until then, though, we do need to find places. where we can get a foothold, where we could bring our ecosystem and boot it up and provide, you know, fields for playing baseball and soccer for our great, great grandchildren so they can have some kind of life. Okay, so let's take a break and then we'll move on to talking about what kind of characteristics the small habitable planet would need to have in order for us to be able to dip our toes in the ocean or run our hands through fields of wheat. I'm Simone Boyce, host of the Brightside podcast, and on this week's episode, I'm talking to Olympian, World Cup champion, and podcast host, Ashlyn Harris. My worth is not wrapped up in how many things I've won, because what I came to realize is I valued winning so much that once it was over, I got the blues, and I was like, this is it.

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Okay, that's, okay, yours might be worse. We can never get married. Listen to this episode with Whitney Cummings and check out new episodes of bananas every Tuesday on the exactly right network listen to bananas on the iHeart radio app apple podcasts or wherever you get your podcasts okay we're back so right daniel smallest habitable planet tell me more about what kind of characteristics it would need to have so i'm usually thinking about things like you know, can you grow plants, but as a non-biologist, I bet you're going to tell me that's something other than biology matters, which is hard to imagine, but give me a shot.

No, I think that can you grow plants is essentially the requirement number one, because that brings together a lot of the other issues. I think the basic requirements for building a habitat are like liquid water on the surface, meaning you have water and it's not presently being boiled off the surface into the atmosphere or freezing into giant ice sheets, right? You want that water to last for like a long time, things like billions of years so that either life can evolve or so that we can like detect it and travel to it and have it like still be there. We don't want to find some planet with water on it. And then when we get there, it's a frozen wasteland. But does it need to be flowing? So like on Mars, it was flowing at some point. And near the equator, you definitely might

underground have water that isn't frozen. Is flowing important or just like water that is accessible? I think if we're making a wish list, then definitely flowing water is a lot easier. And if you need to burn energy and use technology just to get frozen water out of the ground and available to your crops, then it's going to make everything much, much more difficult. So I think it might be possible with places on frozen water, but I think that sounds a lot less comfortable. So let's put flowing water on our wish list. And from the perspective of psychology, we all love flowing water. We do exactly. Nobody wants to dip their toes in a frozen lake, right? That's right. That's right. So we need flowing water on the surface. We also need an atmosphere.

We need this planet to have an atmosphere, meaning there's an envelope of gas that surrounds it, and that envelope is sticking around. It's not like boiling off into space or getting zapped off by solar radiation, you know, that it's going to be there for us to breathe. And it needs to be a certain composition, right? Like if it's all carbon dioxide, it doesn't matter if it's being held to the surface. Exactly. All carbon dioxide will poison us and also lead to a runaway greenhouse effect. So we need a certain mixture of atmosphere to be there.

And all of this in the end comes down to gravity on the planet. Like in order to have liquid water, you need temperature. In order to have that temperature, you need an atmosphere because the atmosphere is what keeps the solar radiation on the surface. You know, I see this on Twitter from politicians who don't. understand any better, they're confusion about like why space is cold because you're going towards the sun. Like if you lift off the surface of the earth, why does it get colder and colder if you're getting closer and closer to the sun, right? Is this confusion I see sometimes on social media? And the answer is that space is very, very cold. It just happens to be warm here because we're under

a blanket. So imagine that you're like camping and you're under a thick blanket and there's a fire there. But if you crawl out from under your blanket, you're going to get very, very cold before you get close to the fire. But you got to be careful about that blanket because Venus got too many blankets. They overheated. Exactly. So you need that atmosphere and you have to have that atmosphere be the right situation, not too thick, not too thin. And to keep that atmosphere on your planet, what you need is gravity, right? You need gravity to hold that gas to the planet because in the end, what else is keeping it there? Atmospheres don't hang out just because they think the planet is cool. They don't like hop from planet to planet looking for the best place to hang out.

Ours would have left us a long time ago. It's gravity that keeps our atmosphere here and it's gravity that determines its composition. What's going to boil off into space? What isn't going to boil off into space? And gravity is also directly important, right? You can't live in a place with too high gravity. If we land on a planet that has like 10G of gravity, 10 times our normal surface gravity, it's not going to be a lot of fun. You can't like walk through that fields of wheat.

and when you're grasped through it, right? No, smooosh. And so in the end, it all comes down to gravity. If you have enough gravity, then you can have a blanket of an atmosphere, and you can keep enough solar radiation, and then you can keep your water liquid. And so the gravity of the planet really is crucial, which is why I think this question is interesting,

the smallest habitable planet. Because often when people think about the habitable zones, the Goldilocks region around a star, they imagine that it's defined by the star. You find the star, you know, the bird, brightness of the star, that you can calculate the region in which that star gives off about as much solar radiation as our star. And so when you talk about the habitable zone of a star, you're kind of ignoring the planetary characteristics. But it turns out the planetary

characteristics are super important, right? If you're on a tiny little rock in the habitable zone, it's not going to have an atmosphere. It's not going to be habitable. And so because science is never straightforward, is there an interaction between like the size to get the gravity right and how close you are. So like if you were too close, do you need to have more gravity because the solar wind is trying to kick your atmosphere away? And so like size varies depending on how close you are.

Or is this one of those amazing instances in science where it's not that complicated? Unfortunately, your pessimistic instincts, I think are correct here. For a given mass and volume of planet, it's sort of a little window where it's well suited. If you look at the Earth, for example, The Earth, if it was like 5% closer to the sun, it would not have liquid oceans.

The sun would boil away our oceans and we'd get a runaway greenhouse effect, sort of like Venus. And if we were a couple percent further from the sun, then our oceans would freeze and we would be globally glaciated. So the size and mass of your planet really does determine where around that star you can survive. Dikes. Okay. So it's complicated. Is it even more complicated? There is one additional crazy complication, which is that the mass of a planet and its volume are not a simple connection. It's not like if you want a bigger planet, you just got to like add more stuff to it. Like you want to make Earth bigger, you couldn't just like add a layer of rock to the Earth and then you'd have like a larger planet because adding more stuff means more gravity and more gravity means it gets more compressed. So in order to make the Earth bigger, you need to add a lot more stuff because then the Earth gets more and more dense.

So this is complicated relationship between the mass and the density of a planet that determines in the end the surface gravity, which remember is the crucial thing to determining whether you can hold an atmosphere and keep liquid water. Okay. So the answer to this episode, what is the smallest habitable planet is going to be? It's complicated. And I'm totally down for that answer as a scientist. Okay, so it depends then probably also like what, you know, if you have mostly metals versus if you have other stuff that probably determines the size that can hold an atmosphere too, right? It does exactly. It depends like on how much the atoms squeeze together. What is like the structural strength?

Because if you're building a planet out of like diamond, for example, it wouldn't take as much diamond to make a large planet because diamond is super duper strong. And if you're building that planet out of like oatmeal or something much more compressible or helium, which is very compressible, right, then it takes a lot more. And it's really fascinating if you look at the connection between the mass of planets and their density, a very weird relationship. Like, Earth turns out to be the densest planet in the solar system because it's close to the largest rocky planet size. As you keep adding stuff to rocky planets, they get denser faster than they get. get larger. And so it's hard to grow a rocky planet a lot bigger than Earth. This is like this maximum size, probably around 10,000 kilometer in radius, which is only like an Earth and a half. So we're like up there sort of close-ish to the limit of the largest possible rocky planet you

could ever build. The other planets like Jupiter, they're much more massive, but they're much less dense than the Earth because they're made of really compressible stuff. You know, I was prepared to be impressed by all of that. But now that you've mentioned that you could have a planet made out of compressed oatmeal. I'm totally depressed because I'd rather live there. But okay, all right, that's all interesting. And so what you want to do in the end is have the right surface gravity. And so you need enough gravity to hold onto your atmosphere, but there's also a maximum amount of gravity, right? Like if you have too much gravity for people, then they just can't even walk around to live on the planet. And the studies I read suggest that people could live

comfortably up to like one and a half G, like one and a half times Earth's gravity would be comfortable for people to live on. How about in the other direction? My sense is that we don't know what the lowest gravity humans could remain healthy in is. Is that your sense as well? I think biologically, you're right. We know something about the highest gravity people could tolerate, but we don't know anything about this lower end, right? People wonder if we could live in basically zero G for a lifetime. And I know in your book, you guys talk about, like, whether you can reproduce and whether babies can develop and all this fun stuff in space. But I think here we should just focus on, like, could you build a planet that has lower

than Earth, gravity, and still keep water on the surface and an atmosphere and high enough temperatures and all that kind of stuff? We'll put those rails on the conversation. Okay, so we're going to focus on how do you maintain an atmosphere and what gravity do you need before humans are squished or their will to live is squished. And after we get back from the break, let's talk about the history of how we've thought about that and the what we know so far about the math behind this question.

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All right, Daniel, so I have been reading books from decades and decades ago where people talk about going out and inhabiting space. When did we start thinking about this question mathematically, as opposed to just sort of like chatting about it over beer or tea? It's really interesting to dig into that history and think about what people were thinking about. decades and decades ago. I love that. The earliest study that I found is a book from 1964, written by a guy Dole at Rand Corporation, thinking about habitable planets for man. So this is, you know, very early space age. We hadn't even landed on the moon. He's already like thinking about these calculations. And this book is really fun because he brings together a lot of really early science from all sorts of different areas, you know, atmosphere chemistry

and planetary geology to think about how. how to put a planet together. And so it's very forward thinking for its time. It's also sort of simplistic from the point of view of modern studies, which have gotten progressively more and more sophisticated as time goes on. But you know, you've got to start from somewhere. And this is, so 64, that would be like before the Venera probes that the Soviet Union sent out landed on Venus and like immediately got squished and boiled.

So is this like back when we still thought like Venus could potentially be habitable? Is that sort of playing into the way he thinks about things? Yeah, exactly. his question was like, how does this work? What do our calculations suggest? And he started from a pretty simple model. He was like, let's just imagine a big blob of rock and let's calculate the gravity on the surface and then try to estimate whether it can hold onto an atmosphere. Then think about the solar radiation, how much energy would be absorbed by that atmosphere and could you have liquid water on the surface. So he makes this specific list of requirements in his book about what it

would take to be habitable. And he requires basically what we've been talking about, which is reasonable amount of gravity, a reasonable temperature, and liquid water on the surface. Okay, cool. And what does he decide? What is the smallest habitable planet? His model suggests that a planet a little bit smaller than Earth, something with like half of Earth's gravity, only like 20% of Earth's mass and like 60% of Earth's radius would be sufficient to maintain those conditions. That you would have an atmosphere on the surface and you would have enough atmosphere to keep the solar radiation so that water could be liquid on the surface. So this is something pretty small. Yeah. Okay. That's cool. So that probably opens up

a lot of possibilities when we get out past our solar system. How have his calculations held up over time? So his calculations were a great first step, of course, in this field. But as always, when you make a first step, you make approximations. And so later studies have dug into some of those approximations and those assumptions and try to make them more realistic. For example, he assumes a very simple atmosphere. He assumes it's optically very thin, has a very fixed amount of reflection, and that, like, what's happening on the surface in terms of clouds and water vapor being formed isn't changing the reflection at all.

So he, for example, ignores, you know, greenhouse feedback effects and all sorts of stuff like that, which really do change the answer. Yeah, I wouldn't want to move to another planet based on these calculations. I remain impressed, but I'm not going to sell my house. So then in the 90s, this was picked back up again and there's a paper by casting at all that really do consider the presence of liquid water and this greenhouse effect and allow the possibility of a runaway greenhouse effect in their calculations. And they rule out some of the possibilities that Dole allowed. But more recently, there was a really fascinating study by a group at MIT and Harvard that trying to make these calculations much more realistic. And does that mean that they needed supercomputers and all kinds of fancy stuff?

How did they make it more realistic? You don't really need fancy computers. You just need to take this model that you're building in your mind and make it more realistic. Add more whizzes and bangs to it. This is a really interesting and sort of philosophically important step in science. When you're trying to answer a question, you have to decide like which details are important and which details are irrelevant.

And in the end, when we're doing science, we're never answering questions about like the actual universe out there, some specific planet. We're always building a little toy model in our mind, one where we have mathematical equations that we know how to wrangle with and answering questions about that and then wondering, you know, how well does that apply to the real universe out there? And so the history of this field is like making those models more and more sophisticated, bringing them what we think is closer and closer to reality. And so, for example, the recent study by MIT and Harvard included an important effect, which is that as the planet gets lower and lower mass, the atmosphere

grows in size. Like, you can have the same amount. of atmosphere, but it can get more fluffy. And as the atmosphere gets hotter, it can also get large because a hot atmosphere means those little particles are moving around faster, which means that they can get like further from the planet. And this has an important effect on the temperature of the planet because like a larger atmosphere both captures more solar radiation and also emits more solar radiation. So they have this more nuanced model of how planetary atmospheres work, how they grow and shrink, how they emit and absorb radiation. And this lets them do more realistic modeling of what these planets would actually be like.

So I wonder how much research on the greenhouse effect on Earth has helped with being able to do models like this. Or maybe even the other way around, maybe studying the greenhouse effect on Venus helps with our greenhouse effect models on Earth. Or maybe they're both helping each other. Yeah, exactly. It's really fascinating. What they found is that Earth mass planets can also get too hot.

You can get this runaway greenhouse. effect if you're too close to the sun, even if you're Earth mass, right? But there's this interesting wrinkle, which is that smaller planets can actually avoid this runaway greenhouse effect because their atmospheres can get bigger and they can emit more. So this helps balance the absorption. As the temperature on the planet goes up, the sort of the gap between how much you're emitting and how much you're absorbing also goes up. And so it helps like release some of that energy into space. That's really interesting. Could we blow part of Venus off and make it habitable? Yeah, that's exactly the point. So if Venus was smaller, it might not have this

greenhouse effect, because it would allow its atmosphere to be a little fluffier because the gravity would be weaker, and that would help it radiate more energy out into space. Okay, so I'm definitely not recommending that we do this, but maybe Elon Musk, you could imagine him saying something like, what if we blow off a part of Venus to make it smaller, and then maybe it would become habitable in enough time, although you'd have to make sure that none of the parts of Venus you blew off hit Earth. That would be embarrassing. Exactly. And then you also have to worry about like disrupting the atmosphere with

your planet exploding nuclear weapons. I think there's a lot of details to nail down there before you pitch that to investors. Yeah, but none of those details are fun. And in this paper, they do this fun calculation where they think about how long water can exist on the surface of this planet.

Because as the planet gets really, really small, it's not as good at holding onto its water. the water then becomes vapor and then increases the greenhouse effect. So they do this calculation where they determine like how long water will be liquid on the surface for these planets and they do it at a whole range of sizes and distances from the star and local surface temperature. It's really fascinating. So if they were to find a planet and another solar system and they ran this model on it

to give you a yes, no, this is going to be habitable, would you sell your house and trust these calculations and go move out there? Or do you think we need a couple more decades of adding additional variables to these models? I'm pretty sure the authors of this paper would not bank on these results. No, I do not think so. I think this is part of the process, you know, and it's informing our understanding, and it's giving us a sense for what's out there. And it's also helping to inform what we're looking for. It's fascinating what they find in this paper is that you can live on a pretty small planet. If your planet is like 40% water by mass,

they do a calculation and they suggest that a local gravity of just one and a half meters per second squared, which is like 15, 20% of Earth gravity, would be enough to keep liquid water on the surface and the atmosphere and everything else we need. Whoa, that's pretty tiny. That's very cool. It's pretty small.

I mean, it's like 50% of the size of Mercury. It's about twice the size of our moon. So that's a lot smaller. than the earlier calculations. These planets benefit from having the like fluffy atmosphere effect, which helps prevent the runaway greenhouse effect. Interesting. So did the authors note that there's like anything that they know they should have included

in the model, but it was just getting too complicated? Or are they like, this is all the stuff that we think is important so far? No, they definitely understand that this is just one step in the process of building more and more sophisticated models of alien planet atmospheres. You know, for example, they ignore clouds, right? They're only thinking about a few atmospheric species, and they're not including the effect of, like, having clouds in the atmosphere. They're also, for example, not thinking about tidal forces.

Tidal forces are the effect of having different gravitational force from the sun on one side of the planet and the other. So, Mercury, for example, is very close to the sun. It's as much stronger gravity on one side of Mercury than on the other side of Mercury, which squeezes and compresses mercury and gives internal heating the same way like jupiter has tidal heating of all of its moons and so this is something they completely ignore in their calculations and they're very upfront about that and in their paper they're like here's you know future work and so there's definitely going to be more studies of this kind of thing especially as we discover

alien planets and learn about their atmospheric signatures by looking through telescopes and seeing what kind of light they emit and looking at that spectra we're going to be discovering planets that probably disagree with all these calculations are going to make us go back and figure out how to modify them. And have we identified any planets yet that meet the criteria, for example, that are laid out in this particular paper that might be habitable? So one disadvantage of our methods for finding exoplanets is that they're very good at finding really big planets and very good at finding planets close to their star because those are the ones that like block the light from the star the most or make that star wiggle the most. And those are

are two best techniques for finding exoplanets. So what that means is that a lot of the exoplanets we have found are really big and really close to their star, which is like not what we're looking for at all. Nobody wants to move to like a hot Jupiter, even though that sounds like a fun dance club. It does. I'd go there. But there's a lot of work in understanding what are the exoplanets that are out there that we're not as good at finding.

We had astronomers on the show recently talking about like how to extrapolate. If you know that you have a bias in your sort of sampling method, then you can try to unfold that bias and say, well, what's out there that I'm not seeing? Like, if you walk through the woods and write down all the animals you see, that's not necessarily all the animals that are in your woods, right? Because you imagine there's some that you can't see or some that run away from you. And so if you know the scale of that effect, you can like invert it to try to estimate, like, what's out there that you're not seeing.

So we do the same thing with planets and try to estimate, like, what is the true population of planets, not just the ones that we can see. And is it additionally complicated by the fact that they're far away and the light takes a long time to get to us and by the time we're analyzing it, maybe the planet is no longer habitable? It is, in fact, complicated by that. And the fact that we don't really understand how solar systems form, right? If we had a great model for a solar system formation, then we could make a very confident prediction for what the distribution of planets are out there. But it's very chaotic, very complicated. You and I've talked about that even the history of our own solar system is not one that we understand very.

well. We think Jupiter moved in and then got pulled out again and Saturn followed it and there may have been another planet that was lost. It's very chaotic and sensitive to initial conditions, which makes it very hard to model sort of ab initio what these solar systems should look like. But we have found some small planets. There's a planet called Kepler 37B, which is like the smallest exoplanet ever found. Its radius is like 35% of the Earth's radius. So it's like a little bit larger than the moon. Though, based on its orbit around its sun, they estimate its surface temperature is like 700 Kelvin,

which is like 800 degrees Fahrenheit. So there'll be no dipping your toes in the water around on Kepler 37B. There'll be no toes on Kepler 37B. Yeah, that's right. Yeah, exactly. But, you know, it is fascinating to think that there might be smaller planets than we ever imagined possible

that are habitable. Then when we look out into those solar systems and imagine where we might live, we don't just have to look for other Earths, bigger Earths or slightly smaller Earths, that there might be a whole range of planets out there. And, you know, the experience of living on those planets could be very different. Imagine living on a planet where, like, the curvature is much more visible, right? You're, like, out on the ocean and you could, like, see the curvature of your planet. I think Phil Plate recently wrote a book that's all about, like, imagining what it would be like if you were living on another planet and sort of look. out at like their atmosphere that has different color sunsets and different curvatures. Yeah, that sounds super awesome.

Imagine if you lived on a small planet that had a moon and that moon was tidily locked and you were tidily locked, that would mean you could only see the moon from like one side of the planet. These effects are much more likely on a small planet than a larger planet with small moons. And so the whole experience of living on those planets could really be very different. You are maybe sort of kind of making me think that it might be fun to leave my house for another planet. That does all sound pretty awesome. Now, do these planets have different wasps that haven't been described yet? Because then I'm in. You mean, like, are there weird alien parasites that

might suck the brains out of your children? Is that what you're asking? No, no, not at all. I'm hoping that they're sucking the brains out of each other and I can take notes, but that they're not well adapted to me. I would like to be at the top of the food chain there as well. Well, maybe you'll enjoy spicy wasps, you know, on your salad. We all have to make compromises when we're camping on exoplanets. So so far we found about 5,000 planets, mostly hot Jupiter's, but a smattering of smaller ones. And we hope that as our ability to find these planets improves, we might find smaller and smaller rocky planets, maybe even ones that have everything we need to travel there and boot up our ecosystem on its surface. Let's hope that we bring small enough people so they don't

use up all the oxygen. Yes, it would be nice if we could have interchangeable people. But yes, Small people is a good decision. And thanks to all the listeners for coming along with us on this journey to imagine small habitable planets we might one day send our great, great, great, great, great, great, great kids to run along in. And thanks, Kelly, for joining us on today's episode. Thanks for having me. As always, it was a ton of fun. All right, tune in next time.

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