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

Transcript

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

• 0:08
  Speaker 1: Hey, Kelly, do you prefer to have like a huge,
• 0:11
  Speaker 1: spacious house with lots of rooms or a cozy little house?
• 0:15
  Speaker 2: Oh? So I like to have spacious land, but the
• 0:18
  Speaker 2: house can be like whatever size we've got, like a
• 0:21
  Speaker 2: nice size house. It's not huge or anything, but we've
• 0:24
  Speaker 2: got a lot of land and we love that space.
• 0:26
  Speaker 2: What about you?
• 0:27
  Speaker 1: I agree, I think big houses can be too big.
• 0:30
  Speaker 1: We moved here from a little Chicago apartment into a
• 0:34
  Speaker 1: huge southern California house where like the bedroom was as
• 0:37
  Speaker 1: big as our old apartment. It just feels like too
• 0:39
  Speaker 1: much space.
• 0:39
  Speaker 2: How can a house be too big? What do you mean?
• 0:42
  Speaker 2: That sounds great?
• 0:43
  Speaker 1: It's like too big to have a conversation across you know,
• 0:46
  Speaker 1: we want the kids to come down for dinner, we
• 0:48
  Speaker 1: end up like texting them. It's ridiculous, all right.
• 0:50
  Speaker 2: Well, so even in my tiny house, I text Zach
• 0:54
  Speaker 2: from like the other room. But we don't have to
• 0:56
  Speaker 2: worry about that with our kids because they're like younger
• 0:59
  Speaker 2: and super andredic so they're more like quantum particles. They're
• 1:03
  Speaker 2: like simultaneously in every part of the house at once.
• 1:07
  Speaker 1: That's a great analogy because kids also have their wave
• 1:10
  Speaker 1: functions collapse eventually when they're exhausted.
• 1:14
  Speaker 2: You know, Usually it's my wave function that collapses first,
• 1:18
  Speaker 2: and I can just hopefully get them in the like
• 1:20
  Speaker 2: area of their bedroom beforehand. But lots of collapsing, Yes.
• 1:25
  Speaker 1: Check out our upcoming book, Quantum Parenting by Daniel and.
• 1:28
  Speaker 2: Count I'm not sure anyone should be following my lead
• 1:33
  Speaker 2: or at least paying for my advice, but but hopefully
• 1:35
  Speaker 2: I'm doing something right.
• 1:52
  Speaker 1: Hi. I'm Daniel. I'm a particle physicist and a professor
• 1:55
  Speaker 1: at UC Irvine.
• 1:56
  Speaker 2: And I'm Kelly Wienersmith. I'm a parasitologist adjunct with Race University.
• 2:02
  Speaker 1: And as much as I used to like to travel
• 2:04
  Speaker 1: all around the world, these days, I really like being
• 2:07
  Speaker 1: a homebody. I like having a comfortable place to sit
• 2:10
  Speaker 1: on my couch and read great novels and look at
• 2:13
  Speaker 1: pictures of my travels around the world. How about you, Kelly,
• 2:17
  Speaker 1: are you traveling a lot these days? Are you sticking
• 2:19
  Speaker 1: close to home?
• 2:20
  Speaker 2: You know, COVID squashed a bunch of my travels and
• 2:23
  Speaker 2: they haven't picked up too much. Again. I like getting
• 2:25
  Speaker 2: out of the house, but I also just really like
• 2:27
  Speaker 2: being at home with my family and my bugs.
• 2:30
  Speaker 1: And As much as I like looking up at the
• 2:31
  Speaker 1: stars and thinking about space travel, I'm not really very
• 2:34
  Speaker 1: excited about going into space myself. I'd rather stay here
• 2:38
  Speaker 1: where the atmosphere is nice and warm and there's plenty
• 2:41
  Speaker 1: of fresh water while other people break new ground. Amen,
• 2:46
  Speaker 1: and welcome to the podcast. Daniel and Jorge explain the
• 2:49
  Speaker 1: universe in which we mostly cast our minds, not our bodies,
• 2:53
  Speaker 1: out into the universe to try to understand what's out there,
• 2:57
  Speaker 1: how does it all work, and is it possible to
• 2:59
  Speaker 1: sustain in life anywhere else in this vast and glittering
• 3:03
  Speaker 1: cosmos other than our cozy couches. My friend Jorge and
• 3:07
  Speaker 1: usual co host can't be here today, but I'm very
• 3:09
  Speaker 1: happy to be talking to our regular guest host, Kelly. Kelly,
• 3:13
  Speaker 1: thank you very much for connecting to us from your
• 3:15
  Speaker 1: comfortable couch.
• 3:16
  Speaker 2: Well, thanks, I'm actually in my comfortable barn, but I'm
• 3:19
  Speaker 2: excited to be connecting on such an interesting topic.
• 3:22
  Speaker 1: And today we are thinking about couches and barns, but
• 3:25
  Speaker 1: not here on Earth where everybody's comfortable. We're thinking about
• 3:29
  Speaker 1: out in space. It feels like the standard trope in
• 3:32
  Speaker 1: science fiction is to think about the future of humanity
• 3:35
  Speaker 1: as out in the stars, And of course lots of
• 3:38
  Speaker 1: people talk about how it's vital that humans get off
• 3:41
  Speaker 1: this planet and establish colonies or outposts in other places
• 3:45
  Speaker 1: in the Solar System and maybe even around the galaxy
• 3:48
  Speaker 1: to avoid being extinguished by one single rock from space.
• 3:53
  Speaker 2: Do you think that's a good argument I'm going to
• 3:55
  Speaker 2: derail us real early.
• 3:57
  Speaker 1: I think that depends on whether you mean good for
• 3:59
  Speaker 1: humanity or like good for the galaxy. It might be
• 4:02
  Speaker 1: best if we're knocked out at the table early on
• 4:05
  Speaker 1: before we do any more damage.
• 4:07
  Speaker 2: M fair enough. I hear that argument from ecologists a lot.
• 4:11
  Speaker 1: I mean, there might be fascinating bits of life here,
• 4:14
  Speaker 1: and they're all over the Solar System or all over
• 4:16
  Speaker 1: the galaxy, and we don't exactly have the greatest history
• 4:19
  Speaker 1: about treating local ecosystems with respect. So from the point
• 4:23
  Speaker 1: of view of like nascent life in other places of
• 4:26
  Speaker 1: the galaxy, it might be best if we just stay
• 4:28
  Speaker 1: where we are.
• 4:29
  Speaker 2: It might be especially if they're bacteria, since I'm not
• 4:31
  Speaker 2: sure we would have a ton of respect for bacteria
• 4:35
  Speaker 2: on another planet.
• 4:36
  Speaker 1: But as a booster for humanity, of course, I want
• 4:39
  Speaker 1: my kids to survive and their kids to survive, and
• 4:42
  Speaker 1: also your kids, and you know other people's kids too,
• 4:45
  Speaker 1: And I want us to all have cozy places to
• 4:47
  Speaker 1: live and fun places to run around. Then yeah, I
• 4:50
  Speaker 1: think it makes absolute sense for us to think about
• 4:52
  Speaker 1: places to live other than Earth, because our time here
• 4:56
  Speaker 1: is limited, even if we do take good care of it.
• 4:58
  Speaker 2: Yeah, you know, humans make a lot of misis, but
• 5:00
  Speaker 2: I still remain pro human. So yes, I like this
• 5:04
  Speaker 2: plant b argument.
• 5:06
  Speaker 1: Way to take a really controversial opinion there.
• 5:12
  Speaker 2: I know people who aren't super pro human, but I
• 5:15
  Speaker 2: think my species has some good features.
• 5:19
  Speaker 1: I think so too. I've got a soft spot for
• 5:20
  Speaker 1: humanity over here, and Earth does seem like a very
• 5:23
  Speaker 1: wonderful and comfortable place to live. But it's important that
• 5:26
  Speaker 1: we think about other places that we might be able
• 5:29
  Speaker 1: to survive, places where humanity might be able to get
• 5:32
  Speaker 1: a foothold and even flourish. Places of course, other than Earth.
• 5:36
  Speaker 1: Even if a meteor doesn't hit the planet, and even
• 5:39
  Speaker 1: if we don't boil the oceans with climate change, eventually
• 5:42
  Speaker 1: the Sun will get brighter and larger and will boil
• 5:45
  Speaker 1: our oceans for us and may even envelop the Earth
• 5:48
  Speaker 1: in its outer layers. So if we're thinking really really
• 5:51
  Speaker 1: long term. It's important that we find another place for
• 5:54
  Speaker 1: humans to run around.
• 5:56
  Speaker 2: Well, so, just before we scare everybody, because I think
• 5:59
  Speaker 2: a theme is going to be that technology is nowhere
• 6:02
  Speaker 2: near or prepared for us to inhabit another planet. We've
• 6:05
  Speaker 2: got like a couple billion years before the Sun consumes
• 6:08
  Speaker 2: the Earth or whatever.
• 6:10
  Speaker 1: Right, calculations vary, but we have at least a cool
• 6:12
  Speaker 1: billion years. So yeah, this is not something we need
• 6:15
  Speaker 1: to plan for next week. You don't need to cancel
• 6:17
  Speaker 1: your travel plans, your kids can finish high school, all
• 6:20
  Speaker 1: that kind of stuff, but it is something that we
• 6:22
  Speaker 1: need to be thinking about. You know, humans also have
• 6:25
  Speaker 1: a record for kicking the can down the road, so
• 6:28
  Speaker 1: it's important to be aware of these things well before
• 6:30
  Speaker 1: we actually need to take any action.
• 6:32
  Speaker 2: Well and frankly, it's fun, what a fun project to
• 6:35
  Speaker 2: start on. So yeah, let's let's get started for the
• 6:37
  Speaker 2: fun of it.
• 6:38
  Speaker 1: That's right. And something that's very exciting in the last
• 6:40
  Speaker 1: couple of decades is that we've been able to discover
• 6:43
  Speaker 1: planets around other stars. Until about twenty years ago, we
• 6:47
  Speaker 1: only really knew about planets in our Solar system. Out
• 6:51
  Speaker 1: of all the stars out there in the cosmos. We'd
• 6:53
  Speaker 1: never seen a planet orbiting another star. But now we
• 6:56
  Speaker 1: have the technology to discover those planets and to think
• 6:59
  Speaker 1: about which ones might actually be homes for humanity. A
• 7:03
  Speaker 1: common way of thinking about this is identifying the habitable zone,
• 7:07
  Speaker 1: a region around those stars where there's enough heat enough
• 7:10
  Speaker 1: solar radiation to keep people warm and toasty on the surface.
• 7:15
  Speaker 2: Fun aside here the people who think about how many
• 7:18
  Speaker 2: generations you would need to get to one of these
• 7:21
  Speaker 2: habitable planets outside of our you know, neighbors, you know,
• 7:25
  Speaker 2: farther out past Mars or whatever they call their field
• 7:28
  Speaker 2: arc eology, because it's like your arc to get there
• 7:33
  Speaker 2: you're or maybe Zach is the one who came up
• 7:36
  Speaker 2: with that name. Anyway, it makes me laugh.
• 7:38
  Speaker 1: So you're not sure if that's a scientific term or
• 7:40
  Speaker 1: just a joke invented by your husband, whose job is
• 7:43
  Speaker 1: control science.
• 7:44
  Speaker 2: This is my life. Everything blurs. I don't know, but
• 7:49
  Speaker 2: I'm always having fun.
• 7:50
  Speaker 1: Well. A lot of the planets that we have found
• 7:52
  Speaker 1: about in other stars are pretty big, because big planets
• 7:56
  Speaker 1: are easier to find. A lot of these planets are
• 7:59
  Speaker 1: close to their Sun or very very massive, and so
• 8:02
  Speaker 1: it makes some of us wonder like, is it possible
• 8:04
  Speaker 1: to live on a really big planet or are there
• 8:07
  Speaker 1: other small planets out there that we might live on.
• 8:10
  Speaker 2: Is it the case that the smaller the planets we
• 8:12
  Speaker 2: can live on, the more options we would have, or
• 8:14
  Speaker 2: is it more complicated because science.
• 8:16
  Speaker 1: We'll get into that at the end of the episode,
• 8:18
  Speaker 1: whether there are more small or large planets available. But
• 8:22
  Speaker 1: today on the podcast, we're going to be digging deep
• 8:24
  Speaker 1: into one extreme of possible habitats for humanity. So on
• 8:29
  Speaker 1: today's episode, we'll be answering the question what's the smallest
• 8:38
  Speaker 1: habitable planet size?
• 8:40
  Speaker 2: Ooh, you know what I'm wondering.
• 8:42
  Speaker 1: Are you thinking about The Little Prince?
• 8:44
  Speaker 2: No, I have no idea what you're talking about. That's
• 8:46
  Speaker 2: not what I'm thinking about at all.
• 8:47
  Speaker 1: Well, you've never read the book The Little Prince.
• 8:49
  Speaker 3: No.
• 8:50
  Speaker 1: This is a famous story about a little prince who
• 8:52
  Speaker 1: lives on a tiny little planet. He can like see
• 8:55
  Speaker 1: the curvae, he can like run around the whole planet
• 8:57
  Speaker 1: in about two minutes.
• 8:58
  Speaker 2: WHOA, that sounds lonely.
• 9:05
  Speaker 1: Wow, waited just like pan famous literature Crime and Punishment
• 9:09
  Speaker 1: sounds depressing.
• 9:13
  Speaker 2: I mean that's Russian literature, right you don't need to
• 9:15
  Speaker 2: take the class. It's just that's depressing.
• 9:18
  Speaker 1: You know. I think you should put out a book
• 9:19
  Speaker 1: with one word reviews of famous literature.
• 9:22
  Speaker 2: Well, you know, if you're interested in brief reviews, you
• 9:24
  Speaker 2: can check out zax a Bridge Beyond the Point of
• 9:26
  Speaker 2: Usefulness series where he did that for Shakespeare and it
• 9:29
  Speaker 2: was hilarious.
• 9:32
  Speaker 1: All right, But today we are talking about tiny little
• 9:35
  Speaker 1: places to live. The Earth is small compared to many
• 9:37
  Speaker 1: planets like Jupiter and many planets we've discovered around other stars.
• 9:42
  Speaker 1: But it's really also very, very big, and there's lots
• 9:45
  Speaker 1: of much smaller chunks of rock, even in our Solar system,
• 9:49
  Speaker 1: and you might wonder whether it's possible to have life
• 9:51
  Speaker 1: on those little bits of carbon. And so, as usual,
• 9:55
  Speaker 1: I was wondering what people had thought about this question,
• 9:58
  Speaker 1: if people imagine living on tiny little rocks, or if
• 10:00
  Speaker 1: people thought that Earth was basically the smallest thing that
• 10:03
  Speaker 1: could keep an atmosphere and maintain habitable conditions. So I
• 10:07
  Speaker 1: went out there and asked folks who listen to the
• 10:10
  Speaker 1: podcast their thoughts on this question. I really value hearing
• 10:13
  Speaker 1: everybody's opinions before I prepare the podcast helps me align
• 10:17
  Speaker 1: the episode with what people already know. If you would
• 10:19
  Speaker 1: like to share your thoughts for an upcoming episode, please
• 10:22
  Speaker 1: don't be shy. We want to hear your voice right
• 10:25
  Speaker 1: to me. Two questions at Danielandjorge dot com. So think
• 10:28
  Speaker 1: about it for a moment before you hear these answers.
• 10:30
  Speaker 1: What do you think is the smallest habitable planet size?
• 10:34
  Speaker 1: Here's what some listeners had to say.
• 10:36
  Speaker 3: I think that the smallest planet for humans to inhabit
• 10:39
  Speaker 3: would be one that would support our kind of life
• 10:44
  Speaker 3: without a need of lots of artificial equipment like astronaut suits.
• 10:52
  Speaker 3: So it would need to be big enough to hold
• 10:54
  Speaker 3: its atmosphere only through gravity.
• 10:57
  Speaker 1: It's on gravity.
• 10:58
  Speaker 4: This question gives me real the little Prince vibes. If
• 11:01
  Speaker 4: we're talking habitable in terms of life as we know it,
• 11:04
  Speaker 4: then I believe we have to have an atmosphere, which
• 11:07
  Speaker 4: means it has to have the mass to retain one
• 11:09
  Speaker 4: like Mars is about half our mask, but it has
• 11:12
  Speaker 4: lost that atmosphere due to solar wind or sheer mass issues.
• 11:17
  Speaker 4: So maybe there's a way to have a smaller mass
• 11:21
  Speaker 4: where it lives something lives deep in the rock or whatnot.
• 11:24
  Speaker 4: But I believe you would need some sort of liquid water,
• 11:27
  Speaker 4: liquid methane or whatnot, which would require again an atmosphere.
• 11:30
  Speaker 4: So my thing would be the requirement of an atmosphere
• 11:33
  Speaker 4: would be a minimum.
• 11:34
  Speaker 2: So those were some great answers, and I think that
• 11:37
  Speaker 2: they point to an important question that you need to
• 11:41
  Speaker 2: clarify before we move on. Does smallest habitable planets? How
• 11:46
  Speaker 2: much technology and equipment are we allowed to use? Do
• 11:48
  Speaker 2: we make it habitable by like making pressurized domes where
• 11:53
  Speaker 2: we provide the atmosphere, or are you talking about habitable
• 11:56
  Speaker 2: with like no help from technology. You could PLoP a
• 11:59
  Speaker 2: naked person down and they would survive.
• 12:02
  Speaker 1: Yeah, you're right, And we could spend like forty five
• 12:04
  Speaker 1: minutes even just discussing like the definition of habitable. And
• 12:07
  Speaker 1: as we'll discover, the literature on this question is begetting
• 12:10
  Speaker 1: more and more sophisticated as people add more conditions to
• 12:13
  Speaker 1: the definition of habitable. You know, people when they were
• 12:17
  Speaker 1: first thinking about habitable planet, they were just like, is
• 12:19
  Speaker 1: there roughly the sun's amount of sunlight? And then as
• 12:22
  Speaker 1: time goes on and on, they add conditions like, you know,
• 12:24
  Speaker 1: is the gravity okay? And is there liquid water on
• 12:27
  Speaker 1: the surface, and can you breathe. And you know, personally,
• 12:30
  Speaker 1: when I think about a habitable planet, I'm imagining my
• 12:33
  Speaker 1: kids running around and touching grass. You know. I can't
• 12:37
  Speaker 1: really imagine raising kids and living in like a glass
• 12:41
  Speaker 1: bubble on the surface of a desolate asteroid. Though technically
• 12:45
  Speaker 1: that does make something habitable, right You put down a
• 12:48
  Speaker 1: habitat on something, and principle you can inhabit it. But
• 12:53
  Speaker 1: I'm imagining sort of a warmer, cozier future for humanity,
• 12:57
  Speaker 1: one where you can like grow crops and go outside
• 13:00
  Speaker 1: that a breathing apparatus. What do you think that is.
• 13:02
  Speaker 2: The future I would want to inhabit if I was
• 13:04
  Speaker 2: not living on the Earth, But I wouldn't want to
• 13:07
  Speaker 2: leave all the amazing tiny little signipet gall wasps that
• 13:10
  Speaker 2: we have on Earth. But maybe that makes me weird,
• 13:13
  Speaker 2: But certainly I've talked to a lot of people who
• 13:15
  Speaker 2: would be totally fine living in pressurized habitats and going
• 13:18
  Speaker 2: outside only after like breathing oxygen and putting on a spacesuit.
• 13:22
  Speaker 2: So I think there's a lot of variability and what
• 13:24
  Speaker 2: people think they would be happy dealing with, although I'd
• 13:28
  Speaker 2: argue that maybe those people after like three or four
• 13:30
  Speaker 2: years of living like that might not be so enamored
• 13:33
  Speaker 2: of the idea anymore. But maybe I'm wrong, and maybe
• 13:36
  Speaker 2: I just have very high standards. I do like a
• 13:38
  Speaker 2: lot of space.
• 13:38
  Speaker 1: I suspect you're right. And I notice in science fiction
• 13:42
  Speaker 1: where people do live in like glass bubbles, they are
• 13:44
  Speaker 1: all these like soft focus memories of walking through fields
• 13:49
  Speaker 1: of wheat on Earth, you know, or dipping their toes
• 13:52
  Speaker 1: in water, or in that recent show Silo where everybody
• 13:55
  Speaker 1: lives underground in this survival tube, they're all like nostalgia
• 14:00
  Speaker 1: about walking along the beach, And you know, I get
• 14:02
  Speaker 1: it that sounds pretty terrible. But you know, if we
• 14:05
  Speaker 1: are thinking about just technically habitable, like could we put
• 14:09
  Speaker 1: a colony there and have people survive, Like for example,
• 14:12
  Speaker 1: Elon Musk, I think is imagining on Mars. He might
• 14:16
  Speaker 1: have like longer term plans to terraform Mars by nuking
• 14:19
  Speaker 1: the polar ice caps and releasing more atmosphere.
• 14:21
  Speaker 2: Such a good idea, Yeah, but he's.
• 14:23
  Speaker 1: Always thinking shorter term, right, can't we get people there
• 14:26
  Speaker 1: in the next ten years or so. He's definitely not
• 14:28
  Speaker 1: planning to have people be able to walk around, And
• 14:30
  Speaker 1: of course we're lucky enough to know somebody who's something
• 14:33
  Speaker 1: of an expert on whether it's possible to build habitats
• 14:36
  Speaker 1: on Mars or other Solar System bodies. That's you, Kelly,
• 14:40
  Speaker 1: that's right you and zactually has wrote a book about
• 14:42
  Speaker 1: this called Literally a City on Mars. So tell me,
• 14:46
  Speaker 1: do you think it's possible with current or near future
• 14:49
  Speaker 1: technology to build something so that humans could make essentially,
• 14:52
  Speaker 1: you know, any rock out there habitable? No? Is that
• 14:56
  Speaker 1: the whole book? No, it's a short pamphlet, a bridge
• 14:59
  Speaker 1: to beyond usefulness.
• 15:01
  Speaker 2: Now, actually the book is over four hundred pages long,
• 15:04
  Speaker 2: so that's a much more nuanced argument.
• 15:06
  Speaker 1: Is it, just like no on four hundred different pages? No, no, no, no.
• 15:09
  Speaker 2: No no no no no no no no no no
• 15:11
  Speaker 2: no no no no no no.
• 15:13
  Speaker 1: What are the major obstacles?
• 15:15
  Speaker 2: Well, so it depends on who you talk to. Everybody
• 15:17
  Speaker 2: has their favorite obstacle. And actually it was very interesting
• 15:20
  Speaker 2: going through the space medicine literature because you'd find multiple
• 15:24
  Speaker 2: different titles that were of the form blank is the
• 15:27
  Speaker 2: biggest barrier to living on space. But everybody would have
• 15:30
  Speaker 2: different blanks, and one of them would be like, you know,
• 15:33
  Speaker 2: the way microgravity messes with vision, and another would be
• 15:36
  Speaker 2: space radiation. And so there's like a lot that we
• 15:39
  Speaker 2: don't know. But in terms of technology, in my mind,
• 15:43
  Speaker 2: the thing that we really don't know is how to
• 15:45
  Speaker 2: create closed loop ecosystems that don't break and are like
• 15:50
  Speaker 2: reliable on a planet like Mars. So you know, for example,
• 15:54
  Speaker 2: so you want to be able to grow food on Mars,
• 15:57
  Speaker 2: and you want to be able to like use those
• 15:59
  Speaker 2: plans to sort of clean your atmosphere. But right now,
• 16:02
  Speaker 2: you know, we don't have ecological systems figured out to
• 16:05
  Speaker 2: the right level of detail to know exactly how many
• 16:08
  Speaker 2: plants you need. For example, there was this simulation that
• 16:10
  Speaker 2: happened in China that had the amazing acronym Lunar Palace.
• 16:14
  Speaker 2: Like they are clearly rivaling NASA's acronym game Watch out, guys.
• 16:18
  Speaker 1: Wait, Lunar Palace is the name of it. And also
• 16:21
  Speaker 1: every letter stands for something.
• 16:22
  Speaker 2: So Lunar No, this was a simulation for the Moon,
• 16:26
  Speaker 2: but palace it all stood for something.
• 16:28
  Speaker 1: So just to be clear, you're not just talking about
• 16:30
  Speaker 1: having an outpost where we're sending supplies from Earth. You're
• 16:33
  Speaker 1: talking about like a self sustaining colony. Is that right?
• 16:36
  Speaker 2: Well, so you know, if you're talking about Mars, for example,
• 16:39
  Speaker 2: habitable kind of requires self sustaining because the orbital window
• 16:43
  Speaker 2: only opens every two years and a lot of food
• 16:46
  Speaker 2: is not shelf stable for that long, and like so
• 16:49
  Speaker 2: you sort of need to be able to pack as
• 16:51
  Speaker 2: little as possible because it's still pretty expensive to ship
• 16:54
  Speaker 2: stuff to Mars, and you need to have redundancy, and
• 16:57
  Speaker 2: you need to have you know, you need to have
• 16:59
  Speaker 2: all of this stuff working in a way that won't break.
• 17:02
  Speaker 2: And at the moment, you know, resupplies to the ISS
• 17:04
  Speaker 2: happened pretty often when things break, but that's not going
• 17:07
  Speaker 2: to happen on Mars. So you need to make sure
• 17:09
  Speaker 2: that you've got this all figured out before you leave. Otherwise,
• 17:11
  Speaker 2: y'all did so like.
• 17:12
  Speaker 1: A big fancier version in the ISS would not qualify
• 17:15
  Speaker 1: as a real habitat right, We like outfitted an asteroid
• 17:19
  Speaker 1: with a bigger version of the ISS and space panels,
• 17:21
  Speaker 1: but still required a lot of resupply from Earth. That
• 17:24
  Speaker 1: feels like just an extension of Earth, right, because then
• 17:27
  Speaker 1: if Earth goes out, if Earth gets hit by a meteor,
• 17:29
  Speaker 1: then obviously that's going to die out.
• 17:31
  Speaker 2: Well, and I don't think that's a problem. Initially, I
• 17:33
  Speaker 2: think relying on resupply from Earth for a while is fine,
• 17:36
  Speaker 2: and you're certainly going to need that as you're trying
• 17:38
  Speaker 2: to get towards self sustaining. But when you're out as
• 17:41
  Speaker 2: far as Mars, you already need to have a lot
• 17:43
  Speaker 2: of this stuff figured out because you are going to
• 17:45
  Speaker 2: be on your own for years. And at the moment,
• 17:48
  Speaker 2: we don't have that kind of stuff figured out. And so,
• 17:50
  Speaker 2: like you know, that Lunar Palace sim that I was
• 17:52
  Speaker 2: talking about, they actually had they had a crew of
• 17:55
  Speaker 2: three guys, and they didn't have enough oxygen for the
• 17:58
  Speaker 2: three guys, so they had to swap out of the
• 18:00
  Speaker 2: guys for smaller women, and then they had enough oxygen
• 18:03
  Speaker 2: for everybody. And like that's where we are right now. Like,
• 18:07
  Speaker 2: if you're on Mars and you're like, shoot, we should
• 18:09
  Speaker 2: have brought two shorter people, like you're just out of luck.
• 18:12
  Speaker 2: You can't just be like breathe less guys.
• 18:14
  Speaker 1: You can make people shorter. I guess you know. Sorry,
• 18:16
  Speaker 1: you gotta lose some height there, buddy.
• 18:18
  Speaker 2: We got to take off one of your arms. Pal
• 18:20
  Speaker 2: too much aerobic respiration happening. So anyway, that's where we
• 18:23
  Speaker 2: are right now, Like, we don't know how to keep
• 18:25
  Speaker 2: these systems stable, even on Earth for periods of time
• 18:29
  Speaker 2: of less than two years and so, and there's not
• 18:31
  Speaker 2: a lot of money going into that because it's not
• 18:33
  Speaker 2: like exciting science, and so I don't think we can
• 18:36
  Speaker 2: do this now if we start investing in it heavily
• 18:39
  Speaker 2: and then your future I think we could, And some
• 18:41
  Speaker 2: people are doing that science, but not on the scale.
• 18:43
  Speaker 2: I think we need to be like creating settlements on Mars,
• 18:46
  Speaker 2: for example.
• 18:47
  Speaker 1: How much of that is the context and how much
• 18:49
  Speaker 1: of that is just like we don't know how to
• 18:50
  Speaker 1: get things started. Say, for example, we found a copy
• 18:54
  Speaker 1: of Earth, you know, a rocky planet with our atmosphere
• 18:58
  Speaker 1: and our temperature and flowing water and all that kind
• 19:00
  Speaker 1: of stuff, but barren of life, completely sterile. Could we
• 19:04
  Speaker 1: get agriculture, etc. Started? Or is it just really hard
• 19:08
  Speaker 1: to boot up a whole ecosystem?
• 19:10
  Speaker 2: I mean, it's hard to boot up a whole ecosystem.
• 19:12
  Speaker 2: So for example, in Biosphere Too, they tried to boot
• 19:15
  Speaker 2: up a whole ecosystem and in some cases it went
• 19:18
  Speaker 2: like hilariously wrong. I'm sure it wasn't hilarious for them,
• 19:22
  Speaker 2: but so like these scorpions that they didn't mean to
• 19:24
  Speaker 2: bring along, they did bring along, and it turns out
• 19:27
  Speaker 2: that these scorpions are like deadly, and so they had
• 19:31
  Speaker 2: to figure out what to do with the scorpion.
• 19:33
  Speaker 1: Wait, doesn't that problem solve itself? I mean, scorpions are
• 19:35
  Speaker 1: eaten in some parts of the world, right, can't you
• 19:38
  Speaker 1: turn that problem into your afternoon snack?
• 19:40
  Speaker 2: Sure, but like I would rather that my snacks didn't
• 19:44
  Speaker 2: have the chance to kill me. You know, like if
• 19:46
  Speaker 2: I had to choose between meal worms and scorpions that
• 19:49
  Speaker 2: could kill me, I'd go with the meal worms. And
• 19:51
  Speaker 2: those are probably gonna be your early choices for protein
• 19:53
  Speaker 2: on space something like that. I mean, so like, for example,
• 19:56
  Speaker 2: agriculture on the Moon would be hard because there's hardly
• 19:58
  Speaker 2: any carbon there. And so in our theoretical or hypothetical situation,
• 20:03
  Speaker 2: are you saying that all of the stuff that plants
• 20:06
  Speaker 2: need to grow is already on Earth, but just like
• 20:09
  Speaker 2: microbes haven't shown up yet.
• 20:10
  Speaker 1: Yeah, exactly, I'm wondering, in like the best case scenario,
• 20:13
  Speaker 1: a planet with all the resources we need, but without
• 20:16
  Speaker 1: life having gotten started, could we like bring enough of
• 20:20
  Speaker 1: our ecosystem with us to actually make that livable.
• 20:23
  Speaker 2: I would bet we could. I bet that there would
• 20:27
  Speaker 2: be some things that get sort of out of balance,
• 20:29
  Speaker 2: and there could be some difficult moments where, you know,
• 20:32
  Speaker 2: like an insect that we brought to help us out
• 20:34
  Speaker 2: sort of does something we weren't expecting, and then we
• 20:36
  Speaker 2: have to figure out how to control it. Like I
• 20:38
  Speaker 2: imagine there'd be some tense moments. But that sounds like
• 20:41
  Speaker 2: a much easier project to me than trying to like
• 20:45
  Speaker 2: garden in the perchlorate laden soil that you find on Mars. Yeah,
• 20:49
  Speaker 2: that would be better, but that doesn't happen in our
• 20:51
  Speaker 2: solar system, I.
• 20:52
  Speaker 1: Don't think, all right, So it sounds like building a
• 20:54
  Speaker 1: technological habitat in an otherwise completely unfriendly to life situation
• 21:00
  Speaker 1: is currently far outside of our technical abilities. Though you know,
• 21:04
  Speaker 1: maybe in one hundred or five hundred years or a
• 21:06
  Speaker 1: thousand years, we would be able to build habitable bubbles
• 21:09
  Speaker 1: that could go anywhere, and making this question of habitability
• 21:12
  Speaker 1: sort of irrelevant. Until then, though, we do need to
• 21:15
  Speaker 1: find places where we can get a foothold where we
• 21:18
  Speaker 1: could bring our ecosystem and boot it up and provide
• 21:22
  Speaker 1: you know, fields for playing baseball and soccer for our
• 21:25
  Speaker 1: great great grandchildren, so that can have some kind of life.
• 21:29
  Speaker 2: Okay, so let's take a break and then we'll move
• 21:31
  Speaker 2: on to talking about what kind of characteristics this small
• 21:35
  Speaker 2: habitable planet would need to have in order for us
• 21:38
  Speaker 2: to be able to dip our toes in the ocean
• 21:40
  Speaker 2: or run our hands through fields of wheat. Okay, we're back,
• 21:57
  Speaker 2: soll right, Daniel, smallest habitable planet, tell me more about
• 22:01
  Speaker 2: what kind of characteristics it would need to have. So
• 22:04
  Speaker 2: I'm usually thinking about things like, you know, can you
• 22:07
  Speaker 2: grow plants? But as a non biologist, I bet you're
• 22:10
  Speaker 2: going to tell me that's something other than biology matters,
• 22:13
  Speaker 2: which is hard to imagine, but give me a shot.
• 22:16
  Speaker 1: No, I think that can you grow plants is essentially
• 22:20
  Speaker 1: the requirement number one, because that brings together a lot
• 22:22
  Speaker 1: of the other issues. I think the basic requirements for
• 22:25
  Speaker 1: building a habitat are like liquid water on the surface,
• 22:29
  Speaker 1: meaning you have water and it's not presently being boiled
• 22:32
  Speaker 1: off the surface into the atmosphere or freezing into giant
• 22:36
  Speaker 1: ice sheets. Right. You want that water to last for
• 22:40
  Speaker 1: like a long time, things like billions of years, so
• 22:45
  Speaker 1: that either life can evolve or so that we can
• 22:47
  Speaker 1: like detect it and travel to it and have it
• 22:49
  Speaker 1: like still be there. We don't want to find some
• 22:52
  Speaker 1: planet with water on it and then when we get
• 22:54
  Speaker 1: there it's a frozen wasteland.
• 22:55
  Speaker 2: But does it need to be flowing? So like on
• 22:57
  Speaker 2: Mars it was flowing at some point and near the equator,
• 23:01
  Speaker 2: you definitely might under ground have water that isn't frozen.
• 23:06
  Speaker 2: Is flowing important or just like water that is accessible.
• 23:09
  Speaker 1: I think if we're making a wish list, then definitely
• 23:11
  Speaker 1: flowing water is a lot easier. I mean, if you
• 23:14
  Speaker 1: need to burn energy and use technology just to get
• 23:17
  Speaker 1: frozen water out of the ground and available to your crops,
• 23:21
  Speaker 1: then it's going to make everything much much more difficult.
• 23:24
  Speaker 1: So I think it might be possible with places on
• 23:25
  Speaker 1: frozen water, but I think that sounds a lot less comfortable.
• 23:28
  Speaker 1: So let's put flowing water on our wish list.
• 23:30
  Speaker 2: And from the perspective of psychology, we all love flowing water.
• 23:34
  Speaker 1: We do exactly. Nobody wants to dip their toes in
• 23:37
  Speaker 1: a frozen.
• 23:38
  Speaker 2: Lake, right, That's right, that's right.
• 23:41
  Speaker 1: So we need flowing water on the surface. We also
• 23:43
  Speaker 1: need an atmosphere. We need this planet to have an atmosphere,
• 23:47
  Speaker 1: meaning there's an envelope of gas that surrounds it, and
• 23:50
  Speaker 1: that envelope is sticking around. It's not like boiling off
• 23:53
  Speaker 1: into space or getting zapped off by solar radiation. You
• 23:57
  Speaker 1: know that it's going to be there for us to breathe.
• 23:59
  Speaker 2: It needs to be a certain composition, right, Like if
• 24:02
  Speaker 2: it's all carbon dioxide, it doesn't matter if it's being
• 24:04
  Speaker 2: held to the surface exactly.
• 24:06
  Speaker 1: All of carbon dioxide will poison us and also lead
• 24:08
  Speaker 1: to a runaway greenhouse effect. So we need a certain
• 24:12
  Speaker 1: mixture of atmosphere to be there. And all this in
• 24:15
  Speaker 1: the end comes down to gravity on the planet. Like
• 24:18
  Speaker 1: in order to have liquid water, you need temperature. In
• 24:20
  Speaker 1: order to have that temperature, you need an atmosphere because
• 24:23
  Speaker 1: the atmosphere is what keeps the solar radiation on the surface.
• 24:27
  Speaker 1: You know. I see this on Twitter from politicians who
• 24:29
  Speaker 1: don't understand any better. They're confusion about like why space
• 24:33
  Speaker 1: is cold because you're going towards the Sun. Like if
• 24:36
  Speaker 1: you lift off the surface of the Earth, why does
• 24:38
  Speaker 1: it get colder and colder if you're getting closer and
• 24:40
  Speaker 1: closer to the Sun. Right? Is this confusuency sometimes on
• 24:44
  Speaker 1: social media. And the answer is that space is very
• 24:46
  Speaker 1: very cold. It just happens to be warm here because
• 24:49
  Speaker 1: we're under a blanket. So imagine that you're like camping
• 24:52
  Speaker 1: and you're under a thick blanket and there is a
• 24:54
  Speaker 1: fire there. But if you crawl a lot from under
• 24:56
  Speaker 1: your blanket, you're going to get very very cold before
• 24:58
  Speaker 1: you get close to the fire.
• 25:00
  Speaker 2: But you got to be careful about that planket. Because
• 25:02
  Speaker 2: Venus got too many blankets, they overheated exactly.
• 25:06
  Speaker 1: So you need that atmosphere and you have to have
• 25:08
  Speaker 1: that atmosphere be the right situation, not too thick, not
• 25:11
  Speaker 1: too thin. And to keep that atmosphere on your planet,
• 25:14
  Speaker 1: what you need is gravity, right, You need gravity to
• 25:18
  Speaker 1: hold that gas to the planet because in the end,
• 25:20
  Speaker 1: what else is keeping it there. Atmospheres don't hang out
• 25:23
  Speaker 1: just because they think the planet is cool. They don't
• 25:25
  Speaker 1: like hop from planet a planet looking for the best
• 25:28
  Speaker 1: place to hang out.
• 25:29
  Speaker 2: Ours would have left us a long time ago.
• 25:32
  Speaker 1: It's gravity that keeps our atmosphere here, and it's gravity
• 25:35
  Speaker 1: that determines its composition. What's going to boil off into space?
• 25:38
  Speaker 1: What isn't going to boil off into space? And gravity
• 25:41
  Speaker 1: is also directly important, Right, you can't live in a
• 25:43
  Speaker 1: place with too high gravity. If we land on a
• 25:46
  Speaker 1: planet that has like ten g of gravity, ten times
• 25:49
  Speaker 1: our normal surface gravity, it's not going to be a
• 25:51
  Speaker 1: lot of fun. You can't like walk through that field
• 25:53
  Speaker 1: of wheat and when you're grassed through it, right.
• 25:56
  Speaker 2: No smoosh.
• 25:57
  Speaker 1: And so in the end, it all comes down to gravity.
• 26:00
  Speaker 1: If you have enough gravity, then you can have a
• 26:02
  Speaker 1: blanket of an atmosphere, and you can keep enough solar radiation,
• 26:05
  Speaker 1: and then you can keep your water liquid. And so
• 26:08
  Speaker 1: the gravity of the planet really is crucial, which is
• 26:11
  Speaker 1: why I think this question is interesting the smallest habitable planet,
• 26:14
  Speaker 1: because often when people think about the habitable zones the
• 26:18
  Speaker 1: goldilocks region around a star, they imagine that it's defined
• 26:21
  Speaker 1: by the star. Right, you find the star, You know
• 26:24
  Speaker 1: the brightness of the star. Now you can calculate the
• 26:26
  Speaker 1: region in which that star gives off about as much
• 26:29
  Speaker 1: solar radiation as our star. And so when you talk
• 26:33
  Speaker 1: about the habitable zone of a star, you're kind of
• 26:35
  Speaker 1: ignoring the planetary characteristics. But it turns out the planetary
• 26:39
  Speaker 1: characteristics are super important. Right if you're on a tiny,
• 26:42
  Speaker 1: little rock inhabitable zone. It's not going to have an atmosphere,
• 26:46
  Speaker 1: it's not going to be habitable.
• 26:47
  Speaker 2: And so, because science is never straightforward, is there an
• 26:51
  Speaker 2: interaction between like the size to get the gravity right
• 26:55
  Speaker 2: and how close you are, So like, if you are
• 26:58
  Speaker 2: too close, you need to have more gravity because the
• 27:02
  Speaker 2: solar wind is trying to kick your atmosphere away, and
• 27:05
  Speaker 2: so like size varies depending on how close you are,
• 27:08
  Speaker 2: or is this one of those amazing instances in science
• 27:11
  Speaker 2: where it's not that complicated.
• 27:12
  Speaker 1: Unfortunately, your pessimistic instincts I think are correct here. For
• 27:16
  Speaker 1: a given mass and volume of planet, it's sort of
• 27:19
  Speaker 1: a little window where it's well suited. If you look
• 27:22
  Speaker 1: at the Earth, for example, the Earth, if it was
• 27:25
  Speaker 1: like five percent closer to the Sun, it would not
• 27:28
  Speaker 1: have liquid oceans. The Sun would boil away our oceans
• 27:31
  Speaker 1: and we'd get a runaway green house effect sort of
• 27:34
  Speaker 1: like Venus. And if we were a couple percent further
• 27:36
  Speaker 1: from the Sun, then our oceans would freeze and we
• 27:39
  Speaker 1: would be globally glaciated. So the size and mass of
• 27:42
  Speaker 1: your planet really does determine where around that star you
• 27:46
  Speaker 1: can survive dikes.
• 27:48
  Speaker 2: Okay, so it's complicated. Is it even more complicated?
• 27:53
  Speaker 1: There is one additional crazy complication, which is that the
• 27:57
  Speaker 1: mass of a planet and its volume are not a
• 28:00
  Speaker 1: simple connection. It's not like if you want a bigger planet,
• 28:04
  Speaker 1: you just got to like add more stuff to it.
• 28:06
  Speaker 1: Like if you want to make Earth bigger, you couldn't
• 28:08
  Speaker 1: just like add a layer of rock to the Earth
• 28:11
  Speaker 1: and then you'd have like a larger planet, because adding
• 28:14
  Speaker 1: more stuff means more gravity, and more gravity means it
• 28:18
  Speaker 1: gets more compressed. So in order to make the Earth bigger,
• 28:21
  Speaker 1: you need to add a lot more stuff because then
• 28:24
  Speaker 1: the Earth gets more and more dense. So this is
• 28:27
  Speaker 1: complicated relationship between the mass and the density of a
• 28:30
  Speaker 1: planet that determines in the end the surface gravity, which
• 28:33
  Speaker 1: remember is the crucial thing to determining whether you can
• 28:36
  Speaker 1: hold an atmosphere and keep liquid water.
• 28:38
  Speaker 2: Okay, so the answer to this episode, what is the
• 28:42
  Speaker 2: smallest habitable planet is going to be It's complicated, and
• 28:47
  Speaker 2: I'm totally down for that answer as a scientist. Okay,
• 28:50
  Speaker 2: So it depends then probably also like what you know,
• 28:53
  Speaker 2: if you have mostly metals versus if you have other stuff,
• 28:57
  Speaker 2: that probably determines the size that can hold an atmosphe.
• 29:00
  Speaker 1: Right, it does exactly. It depends on how much the
• 29:03
  Speaker 1: atoms squeeze together. What is like the structural strength. Because
• 29:07
  Speaker 1: if you're building a planet out of like diamond, for example,
• 29:10
  Speaker 1: it wouldn't take as much diamond to make a large
• 29:12
  Speaker 1: planet because diamond is super duper strong. And if you're
• 29:16
  Speaker 1: building that planet out of like oatmeal or something much
• 29:19
  Speaker 1: more compressible, or helium, which is very compressible, right, then
• 29:23
  Speaker 1: it takes a lot more. And it's really fascinating if
• 29:26
  Speaker 1: you look at the connection between the mass of planets
• 29:29
  Speaker 1: and their density, a very weird relationship. Like Earth turns
• 29:33
  Speaker 1: out to be the densest planet in the Solar System
• 29:36
  Speaker 1: because it's close to the largest rocky planet size. As
• 29:41
  Speaker 1: you keep adding stuff to rocky planets, they get denser
• 29:44
  Speaker 1: faster than they get larger, and so it's hard to
• 29:47
  Speaker 1: grow a rocky planet a lot bigger than Earth. It's
• 29:50
  Speaker 1: like this maximum size probably around ten thousand kilometer in radius,
• 29:54
  Speaker 1: which is only like an Earth and a half. So
• 29:57
  Speaker 1: we're like up there, sort of closest to the limit
• 29:59
  Speaker 1: of the life largest possible rocky planet you could ever build.
• 30:03
  Speaker 1: The other planets like Jupiter, they're much more massive, but
• 30:06
  Speaker 1: they're much less dense than the Earth because they're made
• 30:09
  Speaker 1: of really compressible stuff.
• 30:11
  Speaker 2: You know, I was prepared to be impressed by all
• 30:13
  Speaker 2: of that, but now that you've mentioned that you could
• 30:15
  Speaker 2: have a planet made out of compressed oatmeal, I'm totally
• 30:18
  Speaker 2: depressed because I'd rather live there. But okay, all right,
• 30:21
  Speaker 2: that's all interesting.
• 30:22
  Speaker 1: And so what you want to do in the end
• 30:24
  Speaker 1: is have the right surface gravity, and so you need
• 30:27
  Speaker 1: enough gravity to hold onto your atmosphere. But there's also
• 30:30
  Speaker 1: a maximum amount of gravity, right, Like, if you have
• 30:32
  Speaker 1: too much gravity for people, then they just can't even
• 30:35
  Speaker 1: walk around and live on the planet. And studies I
• 30:37
  Speaker 1: read suggest that people could live comfortably up to like
• 30:40
  Speaker 1: one and a half G, like one and a half
• 30:43
  Speaker 1: times Earth's gravity would be comfortable for people to live on.
• 30:46
  Speaker 2: How about in the other direction, My sense is that
• 30:48
  Speaker 2: we don't know what the lowest gravity humans could remain
• 30:52
  Speaker 2: healthy in. Is Is that your sense as well?
• 30:55
  Speaker 1: I think biologically you're right, we know something about the
• 30:58
  Speaker 1: highest gravity people could tolerate, but we don't know anything
• 31:01
  Speaker 1: about this lower end. People wonder if we could live
• 31:04
  Speaker 1: in basically zero G for a lifetime and I know
• 31:07
  Speaker 1: in your book you guys talk about like whether you
• 31:09
  Speaker 1: can reproduce and whether babies can develop and all this
• 31:11
  Speaker 1: fun stuff in space. But I think here we should
• 31:14
  Speaker 1: just focus on, like could you build a planet that
• 31:17
  Speaker 1: has lower than Earth gravity and still keep water on
• 31:21
  Speaker 1: the surface and an atmosphere and high enough temperatures and
• 31:25
  Speaker 1: all that kind of stuff.
• 31:26
  Speaker 2: We'll put those rails on the conversation. Okay, So we're
• 31:29
  Speaker 2: gonna focus on how do you maintain an atmosphere and
• 31:33
  Speaker 2: what gravity do you need before humans are squished or
• 31:37
  Speaker 2: their will to live is squished, And after we get
• 31:41
  Speaker 2: back from the break, let's talk about the history of
• 31:43
  Speaker 2: how we've thought about that and the what we know
• 31:46
  Speaker 2: so far about the math behind this question. All right, Daniels, So,
• 32:01
  Speaker 2: I have been reading books from decades and decades ago
• 32:05
  Speaker 2: where people talk about going out and inhabiting space. When
• 32:08
  Speaker 2: did we start thinking about this question mathematically as opposed
• 32:11
  Speaker 2: to just sort of like chatting about it over beer
• 32:13
  Speaker 2: or tea.
• 32:14
  Speaker 1: It's really interesting to dig into that history and think
• 32:16
  Speaker 1: about what people were thinking about decades and decades ago.
• 32:19
  Speaker 1: I love that the earliest study that I found is
• 32:22
  Speaker 1: a book from nineteen sixty four written by a guy
• 32:26
  Speaker 1: Dole at Rand Corporation thinking about habitable planets for man.
• 32:31
  Speaker 1: So this is, you know, very early space age. We
• 32:33
  Speaker 1: hadn't even landed on the Moon. He's already like thinking
• 32:37
  Speaker 1: about these calculations. And this book is really fun because
• 32:40
  Speaker 1: he brings together a lot of really early science from
• 32:42
  Speaker 1: all sorts of different areas, you know, atmosphere, chemistry, and
• 32:46
  Speaker 1: planetary geology to think about how to put a planet together.
• 32:50
  Speaker 1: And so it's very forward thinking for its time. It's
• 32:53
  Speaker 1: also sort of simplistic from the point of view of
• 32:55
  Speaker 1: modern studies, which have gone progressively more and more sophisticated
• 32:58
  Speaker 1: as time goes on. But you know, you got to
• 33:00
  Speaker 1: start from somewhere.
• 33:01
  Speaker 2: And this is so sixty four. That would be like
• 33:03
  Speaker 2: before the Venera probes that the Soviet Union sent out
• 33:07
  Speaker 2: landed on Venus and like immediately got squished and boiled.
• 33:10
  Speaker 2: So is this like back when we still thought like
• 33:13
  Speaker 2: Venus could potentially be habitable. Is that sort of playing
• 33:16
  Speaker 2: into the way he thinks about things?
• 33:17
  Speaker 1: Yeah, exactly. I think his question was like, how does
• 33:20
  Speaker 1: this work? What do our calculations suggest? And he started
• 33:23
  Speaker 1: from a pretty simple model. He was like, let's just
• 33:25
  Speaker 1: imagine a big blob of rock, and let's calculate the
• 33:28
  Speaker 1: gravity on the surface and then try to estimate whether
• 33:31
  Speaker 1: it can hold onto an atmosphere. Then think about the
• 33:34
  Speaker 1: solar radiation, how much energy would be absorbed by that atmosphere,
• 33:37
  Speaker 1: and could you have liquid water on the surface. So
• 33:40
  Speaker 1: he makes this specific list of requirements in his book
• 33:43
  Speaker 1: about what it would take to be habitable, and he
• 33:45
  Speaker 1: requires basically what we've been talking about, which is reasonable
• 33:47
  Speaker 1: amount of gravity, a reasonable temperature, and liquid water on
• 33:51
  Speaker 1: the surface.
• 33:52
  Speaker 2: Okay, cool. And what does he decide what is the
• 33:53
  Speaker 2: smallest tabitable planet.
• 33:55
  Speaker 1: His model suggests that a planet a little bit smaller
• 33:58
  Speaker 1: than Earth, something with like half of Earth's gravity, only
• 34:02
  Speaker 1: like twenty percent of Earth's mass and like sixty percent
• 34:06
  Speaker 1: of Earth's radius would be sufficient to maintain those conditions
• 34:10
  Speaker 1: that you would have an atmosphere on the surface, and
• 34:13
  Speaker 1: you would have enough atmosphere to keep the solar radiation
• 34:17
  Speaker 1: so that water could be liquid on the surface. So
• 34:20
  Speaker 1: this is something pretty small.
• 34:21
  Speaker 2: Yeah, okay, that's cool. So that probably opens up a
• 34:24
  Speaker 2: lot of possibilities when we get out past our solar system.
• 34:28
  Speaker 2: How have his calculations held up over time.
• 34:31
  Speaker 1: So his calculations were a great first step of course
• 34:34
  Speaker 1: in this field. But as always when you make a
• 34:35
  Speaker 1: first step, you make approximations, and so later studies have
• 34:39
  Speaker 1: dug into some of those approximations and those assumptions and
• 34:42
  Speaker 1: try to make them more realistic. For example, he assumes
• 34:44
  Speaker 1: a very simple atmosphere. He assumes it's optically very thin,
• 34:48
  Speaker 1: has a very fixed amount of reflection, and that like
• 34:51
  Speaker 1: what's happening on the surface in terms of clouds and
• 34:54
  Speaker 1: water vapor being formed isn't changing the reflection at all.
• 34:58
  Speaker 1: So he for example, ignores you know, greenhouse feedback effects
• 35:02
  Speaker 1: and all sorts of stuff like that, which really do
• 35:04
  Speaker 1: change the answer.
• 35:05
  Speaker 2: Yeah, I wouldn't want to move to another planet based
• 35:08
  Speaker 2: on these calculations. I remain impressed, but I'm not going
• 35:13
  Speaker 2: to sell my house.
• 35:14
  Speaker 1: So then in the nineties this was picked back up again,
• 35:16
  Speaker 1: and there's a paper by Casting at All that really
• 35:19
  Speaker 1: do consider the presence of liquid water and this greenhouse
• 35:22
  Speaker 1: effect and allow the possibility of a runaway greenhouse effect
• 35:25
  Speaker 1: in their calculations, and they rule out some of the
• 35:28
  Speaker 1: possibilities that Dole allowed. But more recently there was a
• 35:31
  Speaker 1: really fascinating study by a group at MIT and Harvard
• 35:35
  Speaker 1: that trying to make these calculations much more realistic.
• 35:38
  Speaker 2: And does that mean that they needed supercomputers and all
• 35:40
  Speaker 2: kinds of fancy stuff. How did they make it more realistic?
• 35:43
  Speaker 1: You don't really need fancy computers. You just need to
• 35:46
  Speaker 1: take this model that you're building in your mind and
• 35:48
  Speaker 1: make it more realistic, add more wizzes and bangs to it.
• 35:52
  Speaker 1: This is a really interesting and sort of philosophically important
• 35:55
  Speaker 1: step in science. When you're trying to answer a question,
• 35:57
  Speaker 1: you have to decide like which details are important and
• 36:00
  Speaker 1: which details are irrelevant. And in the end, when we're
• 36:02
  Speaker 1: doing science, we're never answering questions about like the actual
• 36:05
  Speaker 1: universe out there, some specific planet. We're always building a
• 36:09
  Speaker 1: little POI model in our mind, one where we have
• 36:11
  Speaker 1: mathematical equations that we know how to wrangle with and
• 36:15
  Speaker 1: answering questions about that and then wondering how well does
• 36:18
  Speaker 1: that apply to the real universe out there. And so
• 36:21
  Speaker 1: the history of this field is like making those models
• 36:24
  Speaker 1: more and more sophisticated, bringing them what we think is
• 36:27
  Speaker 1: closer and closer to reality. And so, for example, the
• 36:30
  Speaker 1: recent study by MIT and Harvard include an important effect,
• 36:34
  Speaker 1: which is that as the planet gets lower and lower mass,
• 36:38
  Speaker 1: the atmosphere grows in size, Like you can have the
• 36:42
  Speaker 1: same amount of atmosphere, but it can get more fluffy.
• 36:44
  Speaker 1: And as the atmosphere gets hotter, it can also get
• 36:47
  Speaker 1: large because a hot atmosphere means those little particles are
• 36:51
  Speaker 1: moving around faster, which means that they can get further
• 36:54
  Speaker 1: from the planet. And this that's important effect on the
• 36:56
  Speaker 1: temperature of the planet because like a larger atmosphere more
• 37:00
  Speaker 1: solar radiation and also emits more solar radiation. So they
• 37:04
  Speaker 1: have this more nuanced model of how planetary atmospheres work,
• 37:09
  Speaker 1: how they grow and shrink, how they emit and absorb radiation,
• 37:13
  Speaker 1: and this lets them do more realistic modeling of what
• 37:15
  Speaker 1: these planets would actually be like.
• 37:17
  Speaker 2: Oh so, I wonder how much research on the greenhouse
• 37:19
  Speaker 2: effect on Earth has helped with being able to do
• 37:22
  Speaker 2: models like this, or maybe even the other way around.
• 37:24
  Speaker 2: Maybe studying the greenhouse effect on Venus helps with our
• 37:27
  Speaker 2: greenhouse effect models on Earth, or maybe they're both helping
• 37:29
  Speaker 2: each other.
• 37:30
  Speaker 1: Yeah, exactly. It's really fascinating. What they found is that
• 37:34
  Speaker 1: Earth mass planets can also get too hot. You can
• 37:37
  Speaker 1: get this runaway greenhouse effect if you're too close to
• 37:40
  Speaker 1: the Sun, even if you're Earth mass, right. But there's
• 37:43
  Speaker 1: this interesting wrinkle which is that smaller planets can actually
• 37:46
  Speaker 1: avoid this runaway greenhouse effect because their atmospheres can get
• 37:50
  Speaker 1: bigger and they can emit more. So this helps balance
• 37:53
  Speaker 1: the absorption. As the temperature on the planet goes up,
• 37:56
  Speaker 1: the sort of the gap between how much you're emitting
• 37:58
  Speaker 1: and how much you're absorbing also goes up, and so
• 38:01
  Speaker 1: it helps like release some of that energy into space.
• 38:04
  Speaker 2: That's really interesting. Could we blow part of Venus off
• 38:08
  Speaker 2: and make it habitable?
• 38:09
  Speaker 1: Yeah, that's exactly the point. So if Venus was smaller,
• 38:12
  Speaker 1: it might not have this greenhouse effect because it would
• 38:15
  Speaker 1: allow its atmosphere to be a little fluffier because the
• 38:17
  Speaker 1: gravity would be weaker, and that would help it radiate
• 38:20
  Speaker 1: more energy out into space.
• 38:22
  Speaker 2: Well, okay, so I'm definitely not recommending that we do this,
• 38:25
  Speaker 2: but maybe Elon Musk you could imagine him saying something like,
• 38:29
  Speaker 2: what if we blow off a part of Venus to
• 38:32
  Speaker 2: make it smaller, and then maybe it would become habitable
• 38:35
  Speaker 2: at enough time, although you'd have to make sure that
• 38:36
  Speaker 2: none of the parts of Venus you blew off it Earth.
• 38:38
  Speaker 2: That would be embarrassing exactly.
• 38:41
  Speaker 1: And then you also have to worry about like disrupting
• 38:43
  Speaker 1: the atmosphere with your planet exploding nuclear weapons. I think
• 38:47
  Speaker 1: there's a lot of details to nail down there before
• 38:49
  Speaker 1: you pitch that to investors.
• 38:51
  Speaker 2: Yeah, but none of those details are fun.
• 38:56
  Speaker 1: And in this paper they do this fun calculation where
• 38:59
  Speaker 1: they think about how long water can exist on the
• 39:01
  Speaker 1: surface of this planet. Because as the planet gets really
• 39:04
  Speaker 1: really small, it's not as good at holding onto its water.
• 39:07
  Speaker 1: The water then becomes vapor and then increases the greenhouse effect.
• 39:11
  Speaker 1: So they do this calculation where they determine like how
• 39:13
  Speaker 1: long water will be liquid on the surface for these planets,
• 39:16
  Speaker 1: and they do it at a whole range of sizes
• 39:19
  Speaker 1: and distances from the star and local surface temperature. It's
• 39:23
  Speaker 1: really fascinating.
• 39:24
  Speaker 2: So if they were to find a planet in another
• 39:27
  Speaker 2: solar system and they ran this model on it to
• 39:30
  Speaker 2: give you a yes, no, this is going to be habitable,
• 39:33
  Speaker 2: would you sell your house and trust these calculations and
• 39:38
  Speaker 2: go move out there, or do you think we need
• 39:41
  Speaker 2: a couple more decades of adding additional variables to these models.
• 39:44
  Speaker 1: I'm pretty sure the authors of this paper would not
• 39:47
  Speaker 1: bank on these results. Ok No, I mean I think so.
• 39:50
  Speaker 1: I think this is part of the process, you know,
• 39:52
  Speaker 1: and it's informing our understanding, and it's giving us a
• 39:54
  Speaker 1: sense for what's out there, and it's also helping to
• 39:57
  Speaker 1: inform what we're looking for deciding. What they find in
• 40:01
  Speaker 1: this paper is that you can live on a pretty
• 40:04
  Speaker 1: small planet if your planet is like forty percent water
• 40:07
  Speaker 1: by mass. They do a calculation and they suggest that
• 40:09
  Speaker 1: a local gravity of just one and a half meters
• 40:12
  Speaker 1: per second squared, which is like, you know, fifteen twenty
• 40:16
  Speaker 1: percent of Earth gravity, would be enough to keep liquid
• 40:19
  Speaker 1: water on the surface and the atmosphere and everything else
• 40:22
  Speaker 1: we need.
• 40:22
  Speaker 2: Wow, that's pretty tiny. That's very cool.
• 40:25
  Speaker 1: It's pretty small. I mean, it's like fifty percent of
• 40:28
  Speaker 1: the size of mercury. It's about twice the size of
• 40:31
  Speaker 1: our moon. So that's a lot smaller than the earlier calculations.
• 40:35
  Speaker 1: These planets benefit from having the like fluffy atmosphere effect,
• 40:38
  Speaker 1: which helps prevent the runaway greenhouse effect.
• 40:41
  Speaker 2: Interesting, So did the authors note that there's like anything
• 40:45
  Speaker 2: that they know they should have included in the model,
• 40:47
  Speaker 2: but it was just getting too complicated, or are they like,
• 40:50
  Speaker 2: this is all the stuff that we think is important
• 40:52
  Speaker 2: so far.
• 40:53
  Speaker 1: No, they definitely understand that this is just one step
• 40:55
  Speaker 1: in the process of building more and more sophisticated models
• 40:59
  Speaker 1: of alien planet atmospheres. You know, for example, they ignore clouds, right,
• 41:04
  Speaker 1: They're only thinking about a few atmospheric species and they're
• 41:08
  Speaker 1: not including the effect of like having clouds in the atmosphere. Also,
• 41:11
  Speaker 1: for example, not thinking about tidal forces. Tile forces are
• 41:15
  Speaker 1: the effect of having a different gravitational force from the
• 41:19
  Speaker 1: Sun on one side of the planet and the other.
• 41:22
  Speaker 1: So mercury, for example, is very close to the Sun,
• 41:24
  Speaker 1: it is much stronger gravity on one side of mercury
• 41:27
  Speaker 1: than on the other side of mercury, which squeezes and
• 41:30
  Speaker 1: compresses mercury and gets internal heating the same way like
• 41:33
  Speaker 1: Jupiter has tidal heating of all of its moons. And
• 41:36
  Speaker 1: so this is something they completely ignore in their calculations,
• 41:40
  Speaker 1: and they're very upfront about that, and in their paper
• 41:42
  Speaker 1: they're like, here's you know, future work. And so there's
• 41:45
  Speaker 1: definitely going to be more studies of this kind of thing,
• 41:47
  Speaker 1: especially as we discover alien planets and learn about their
• 41:51
  Speaker 1: atmospheric signatures by looking through telescopes and seeing what kind
• 41:55
  Speaker 1: of light they emit and looking at that spectra, we're
• 41:57
  Speaker 1: going to be discovering planets that probably disagree with all
• 42:01
  Speaker 1: these calculations that are going to make us go back
• 42:03
  Speaker 1: and figure out how to modify them.
• 42:04
  Speaker 2: And have we identified any planets yet that meet the criteria,
• 42:08
  Speaker 2: for example, that are laid out in this particular paper
• 42:11
  Speaker 2: that might be habitable.
• 42:12
  Speaker 1: So one disadvantage of our methods for finding exoplanets is
• 42:16
  Speaker 1: that they're very good at finding really big planets and
• 42:19
  Speaker 1: very good at finding planets close to their star, because
• 42:23
  Speaker 1: those are the ones that like block the light from
• 42:25
  Speaker 1: the star of the most or make that star wiggle
• 42:27
  Speaker 1: the most, and those are two best techniques for finding exoplanets.
• 42:31
  Speaker 1: So what that means is that a lot of the
• 42:32
  Speaker 1: exoplanets we have found are really big and really close
• 42:36
  Speaker 1: to their star, which is like, not what we're looking
• 42:38
  Speaker 1: for at all. Nobody wants to move to like a
• 42:40
  Speaker 1: hot jupiter, even though that sounds like a fun dance club.
• 42:44
  Speaker 2: It does, I'd go there.
• 42:46
  Speaker 1: But there's a lot of work in understanding what are
• 42:49
  Speaker 1: the exoplanets that are out there that we're not as
• 42:51
  Speaker 1: good at finding. We had astronomers on the show recently
• 42:55
  Speaker 1: talking about like how to extrapolate. If you know that
• 42:57
  Speaker 1: you have a bias in your sort of sampling method,
• 43:00
  Speaker 1: then you can try to unfold that bias and say, well,
• 43:02
  Speaker 1: what's out there that I'm not seeing? Like if you
• 43:04
  Speaker 1: walk through the woods and write down all the animals
• 43:06
  Speaker 1: you see, that's not necessarily all the animals that are
• 43:08
  Speaker 1: in your woods, right, because you imagine there's something that
• 43:11
  Speaker 1: you can't see or something that run away from you.
• 43:13
  Speaker 1: And so if you know that the scale of that effect,
• 43:15
  Speaker 1: you can like invert it to try to estimate, like
• 43:18
  Speaker 1: what's out there that you're not seeing. So we do
• 43:20
  Speaker 1: the same thing with planets and try to estimate, like,
• 43:22
  Speaker 1: what is the true population of planets, not just the
• 43:25
  Speaker 1: ones that we can.
• 43:27
  Speaker 2: See, And is it additionally complicated by the fact that
• 43:30
  Speaker 2: they're far away and the light takes a long time
• 43:32
  Speaker 2: to get to us, and by the time we're analyzing it,
• 43:35
  Speaker 2: maybe the planet is no longer habitable.
• 43:37
  Speaker 1: It is in fact complicated by that and the fact
• 43:39
  Speaker 1: that we don't really understand how solar systems form. Right,
• 43:42
  Speaker 1: if we had a great model for solar system formation,
• 43:46
  Speaker 1: then we could make a very confident prediction for what
• 43:48
  Speaker 1: the distribution of planets are out there. But it's very chaotic,
• 43:51
  Speaker 1: very complicated. You and I have talked about it. Even
• 43:54
  Speaker 1: the history of our own Solar system is not one
• 43:56
  Speaker 1: that we understand very well. We think Jupiter moved in
• 43:59
  Speaker 1: and then got pulled out again, and Saturn followed it,
• 44:01
  Speaker 1: and there may have been another planet that was lost.
• 44:03
  Speaker 1: It's very chaotic and sensitive to initial conditions, which makes
• 44:07
  Speaker 1: it very hard to model, sort of ab initio, what
• 44:10
  Speaker 1: these solar systems should look like. But we have found
• 44:12
  Speaker 1: some small planets. There's a planet called Kepler thirty seven B,
• 44:16
  Speaker 1: which is like the smallest exoplanet ever found. Its radius
• 44:20
  Speaker 1: is like thirty five percent of the Earth's radius, so
• 44:23
  Speaker 1: it's like a little bit larger than the Moon. Though
• 44:26
  Speaker 1: based on its orbit around its Sun, they estimate its
• 44:29
  Speaker 1: service temperature is like seven hundred kelvin, which is like
• 44:32
  Speaker 1: eight hundred degrees fahrenheit, So there'll be no dipping your
• 44:36
  Speaker 1: toes in the water around on Kepler thirty seven B.
• 44:39
  Speaker 2: There no toes on Kepler thirty seven B's.
• 44:43
  Speaker 1: Right, yeah, exactly, But you know, it is fascinating to
• 44:48
  Speaker 1: think that there might be smaller planets than we ever
• 44:51
  Speaker 1: imagined possible that are habitable. Then when we look out
• 44:54
  Speaker 1: into those solar systems and imagine where we might live,
• 44:57
  Speaker 1: we don't just have to look for other earth bigger earths,
• 45:01
  Speaker 1: or slightly smaller earth. That there might be a whole
• 45:03
  Speaker 1: range of planets out there, and you know, the experience
• 45:06
  Speaker 1: of living on those planets could be very different. Imagine
• 45:09
  Speaker 1: living on a planet where like the curvature is much
• 45:12
  Speaker 1: more visible. Right, You're like out on the ocean and
• 45:14
  Speaker 1: you could like see the curvature of your planet.
• 45:17
  Speaker 2: I think Phil Plait recently wrote a book that's all
• 45:19
  Speaker 2: about like imagining what it would be like if you
• 45:21
  Speaker 2: were living on another planet and sort of looked out
• 45:23
  Speaker 2: at like their atmosphere that has different color sunsets and
• 45:26
  Speaker 2: different curvatures.
• 45:27
  Speaker 1: Yeah, that sounds super awesome. Imagine if you lived on
• 45:30
  Speaker 1: a small planet that had a moon, and that moon
• 45:32
  Speaker 1: was tidly locked, and you were tidally locked tho would
• 45:35
  Speaker 1: mean you could only see the moon from like one
• 45:37
  Speaker 1: side of the planet. These effects are much more likely
• 45:40
  Speaker 1: on a small planet than a larger planet with small moons,
• 45:44
  Speaker 1: and so the whole experience of living on those planets
• 45:46
  Speaker 1: could really be very different.
• 45:47
  Speaker 2: You are maybe sort of kind of making me think
• 45:51
  Speaker 2: that it might be fun to leave my house for
• 45:53
  Speaker 2: another planet. That does all sound pretty awesome. Now, now,
• 45:57
  Speaker 2: do these planets have different wasps that haven't been described yet?
• 46:01
  Speaker 1: Because then I'm in you mean, like, are there weird
• 46:03
  Speaker 1: alien parasites that might suck the brains out of your children?
• 46:05
  Speaker 1: Is that you're asking?
• 46:06
  Speaker 2: No, No, not at all. I'm hoping that they're sucking
• 46:10
  Speaker 2: the brains out of each other and I can take notes,
• 46:12
  Speaker 2: but that they're not well adapted to me. I would
• 46:15
  Speaker 2: like to be at the top of the food chain
• 46:17
  Speaker 2: there as well.
• 46:17
  Speaker 1: Well, maybe you'll enjoy spicy wasps, you know, on your salad.
• 46:20
  Speaker 1: We all have to make compromises when we're camping on exoplanets.
• 46:24
  Speaker 1: So so far we found about five thousand planets, mostly
• 46:27
  Speaker 1: hot Jupiters, but a smattering of smaller ones. And we
• 46:30
  Speaker 1: hope that as our ability to find these planets improves,
• 46:33
  Speaker 1: we might find smaller and smaller, rocky planets, maybe even
• 46:37
  Speaker 1: ones that have everything we need to travel there and
• 46:40
  Speaker 1: boot up our ecosystem on its surface. Let's hope that
• 46:44
  Speaker 1: we bring small enough people so they don't use up
• 46:46
  Speaker 1: all the oxygen.
• 46:49
  Speaker 2: Yes, it would be nice if we could have interchangeable people,
• 46:51
  Speaker 2: but yes, small people is a good decision.
• 46:54
  Speaker 1: And thanks all the listeners for coming along with us
• 46:56
  Speaker 1: on this journey to imagine small habitable planets might one
• 47:00
  Speaker 1: day send our great great great great great grandkids to
• 47:04
  Speaker 1: run along in And thanks Kelly for joining us on
• 47:06
  Speaker 1: today's episode.
• 47:07
  Speaker 2: Thanks for having me. As always, it was a ton
• 47:09
  Speaker 2: of fun.
• 47:09
  Speaker 1: All right, tune in next time. Thanks for listening, and
• 47:20
  Speaker 1: remember that Daniel and Jorge Explain the Universe is a
• 47:22
  Speaker 1: production of iHeartRadio. For more podcasts from iHeartRadio, visit the
• 47:27
  Speaker 1: iHeartRadio app, Apple Podcasts, or wherever you listen to your
• 47:31
  Speaker 1: favorite shows.