Daniel and Kelly’s Extraordinary Universe - Are Earths rare in the Universe?

Episode Date: October 15, 2024

Daniel and Kelly discuss the Rare Earth Hypothesis, that argues that Earth-like life is vanishingly rare.See omnystudio.com/listener for privacy information.

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And asking basic questions about it for a long time. What are those points? of light? Why do some of them move? Are there patterns that we can understand? And though it's taken us thousands of years, we've actually made a lot of progress in figuring it out. Unlike our ancient ancestors, we know something about what we're looking at. We can appreciate that the points of light are not nearby, but are mind-bogglingly distant, that we're looking out over a vast, black ocean of space, seeing the few and incredibly distant islands of light in the cosmos.

It's really one of the most spectacular views you'll ever see. And from that view, we have extrapolated quite incredibly into an understanding of the bigger picture of the universe. We now know a lot about how vast the universe is, how galaxies form, how they cluster, how the universe expands. But in all of those thousands of years, we've made almost zero progress on one of the most basic questions, one of the most pressing. As we've been able to expand our mental picture of the universe from just the neighborhood

of Earth to the 90-some billion light years of the observable universe, this unanswered question has just become more pressing. That question is, of course, are we alone? As we stare up into the night sky and do our best to unravel its workings, to put pen to paper and capture the mechanisms of the universe in our little mathematical stories, we wonder if we are the only ones doing that. Is there someone else up there around a distant planet looking back at us, wondering if we too are here looking back at them? Well, until the aliens arrive or send us an email, Instagram, DM, or a LinkedIn connection request, do we have any hope of figuring

this out? We have managed to learn incredible things about the nature of the wider universe without ever leaving the immediate neighborhood of our planet, just from gathering the photons that happen to splash down here. Can we do the same to understand the prevalence of alien life, what it might be like, and whether it thinks and wonders and eats and screams when it steps on a Lego at 3 a.m? We are curious humans, and so we are never satisfied just throwing our hands in the air and giving up because it's hard. We want to extract every last bit of knowledge from the rare data that we have. So on today's episode, we're going to explore one particular theory that tries to answer the age-old question.

Is life rare in the universe? Hi, I'm Kelly Weiner-Smith. I'm an adjunct faculty at Rice University, and welcome to Daniel and Kelly's Extraordinary Universe. We're happy to have you here. And I'm here with my co-host, Daniel. Hey, everyone, I'm Daniel. I'm a particle physicist and a professor at UC Irvine.

And I'm so happy to be here today with you, Kelly, on our extraordinary podcast about the extraordinary universe. Yeah, super exciting. I have an extraordinary question for you. So I wrote this book about space, which means I get to talk to the space community. The question that I get most often that I don't know how to answer, I'm going to ask you what you think, is what about? All that UFO stuff that we've been hearing about lately, like the Senate hearing and stuff,

what is your thought on the UFO situation? My thought is that there are two totally different categories of work. One is like real scientific questions about where alien life might be and how we might find it and like hard hitting scientific projects to look for biosignatures around other stars. And then there's UFOology, which is a very fascinating community of folks. who desperately want to believe that aliens are already here and the government is covering it up somehow and the fact that we don't have evidence for it

is somehow evidence of that cover up. And there are some folks in there who are doing their best to try to sort through the evidence in a methodical way. And I think there are also a lot of gristers in there who are playing on people who really want to believe. And I think you'll find a lot of scientists are eager to dig into the alien life stuff and eager to avoid the conspiracy theories of UFOlogy.

I think our takes pretty much agreed. If anybody wants to check my Goodreads account, there's this great book, and I'm forgetting the title and the name, which is very helpful. But there was a woman who essentially engaged with the UFO community, and it was sort of like an anthropological look at that community, and it was a very fun book to read. But my take on most of these things that are coming through the Senate hearing is that it's probably technology from some other country that's like in our airspace and we happen to run upon it at their wrong time and they didn't mean for us to see it. or it's a U.S. technology that we're working on and some pilots saw it when they weren't supposed to. I think there's almost always some explanation that doesn't require aliens. And it would also surprise me if aliens like came and were accidentally seen and then left. Like that's a long journey to just like go home.

Yeah. But anyway, I think we're on the same page. No, I totally agree. And I want to emphasize that like I want to believe if there was really any sort of shred of hard evidence that we could dig into about aliens, I would be the first one to trumpet it. I'm definitely a mark for these folks. And I want to believe it, but it just doesn't really hold up. But I know that what we've seen is a tiny fraction of what the government has, which is something that the UFO community talks about a lot.

I actually once had a congressman reach out to me, say, Daniel, I have questions about what I've been shown. Can we talk? And so I gave him a call, of course. And I tried to explain to him what we were looking at and what we knew and what physics said and what we didn't know about the universe. And then I asked him. I said, hey, have you seen all the classified footage from the DoD?

He said, yes, I have. And I said, what's in it? And he says, nothing interesting. It's all drones and balloons and it's less interesting than the stuff you have seen, which was like exciting to get a glimpse behind the scenes, but also deflating. Because I was really hoping he was going to say, oh, it's amazing stuff. You won't believe it when they finally release it. So unfortunately, but I do want to believe I'm waiting for.

the day that the aliens come and I will be first in line to talk to them about the secrets of the universe. I'm excited too and I think if our species persists long enough, it's just a matter of time before we encounter something. But this is a good transition into today's topic. So the title of today's show is, is life rare in the universe? So what is the probability that there's life out there that we could encounter? We reached out to the listeners and we told them that one approach to thinking about life in the universe is the rare earth hypothesis. We wanted to what they had heard about this, and here are their responses. I think the rare Earth hypothesis is the idea that Earth, and specifically its conditions

for life, are really rare in the universe, and that life being able to form is rare because a planet able to support life is rare. I don't know, but I think it's that Earth is the only planet that has life that we know of. I think the rare Earth hypothesis has to do with how common it is that a planet just like ours would form with all the crazy factors that went into our history, or it has to do with the composition of elements that we can find in our crust, mantle, and core.

Either way, rare earth. Planets that can foster life orbit in the Goldilocks zone allowing for liquid water. They have a molten core, iron core, creating a magnetic field, protecting it from high energy, harmful radiation, and many other characteristics required to sustain life. So I think the rare earth hypothesis,

hypothesizes it's rare but possible to have planets similar to Earth throughout the universe because I have no idea I will use context clues and say maybe it has something to do with minerals the rare earth hypothesis is the way our Earth born does compared to other rocky planets and other solo systems is not a common occurrence like it's rare like we don't see them ever every day. All right, Daniel, what do you think? Yeah, I think the listeners are right on here.

The rare hypothesis is one of those ideas in science that is rare because it's well-named. You know, you can actually understand what it means from the title. It's the hypothesis that Earth is rare, that life on Earth is rare, that our existence is unusual in the universe, which of course would be a devastating outcome for those of us who hope that the universe is filled with all sorts of intelligent alien life we can one day talk to. but it's a possibility we have to face, right? It's one of the potential outcomes to this question, are we alone in the universe?

It might be yes. It might be that we're the only thinking creatures out there trying to understand how the universe works. Yeah, maybe, or maybe they've just seen human history and they're like, they're just not ready for us yet. So every conversation about aliens has to start with the Drake equation, which I think is both interesting and useful. I've reviewed a lot of proposals for students

are into science communication and almost everyone who's in physics wants to do a video about the Drake equation. And so I'm like, I'm kind of over it. It's like mitosis for biologists. Like everybody does the video on mitosis. But all right. So Daniel, the bar is high. Make the Drake equation interesting to me again. Like it was the first time I heard it. All right. All right. Well, this episode is not just about the Drake equation, but I think that Drake equation is a very useful way to organize our thinking. Like, look, this is a big. problem, right? Is there life elsewhere in the universe? And in science, when we tackle a really big problem, what are our strategies is like, hey, break it into pieces. You know, every time I have a big research project, I don't know how I'm going to get from here to there. You got to break it into steps and then take on each of those steps. And the Drake equation is just that is to say, look, if there is going to be life elsewhere in the universe, then a lot of things have to line up. You need to have enough stars. You need to have enough planets around those stars. You need to have life on those planets. That life. has to develop intelligence that intelligence has to be technological and so it breaks it up into

all these sub-questions like well we can think about how many stars there are and we can think about the physics of whether there are planets around them and we can argue about whether there is life on planets or what the probability of those is so it's a really useful way to try to think about the problem it also shows you how hard this problem is because you know how do you know what fraction of planets have life on them remember that all the information we have comes either from Earth or from the few measly photons that have crawled their way through the universe to get to Earth. And so it's a really difficult task.

But that doesn't mean we shouldn't take it on, right? Every really big question in science got started at some point when people were like, hmm, this seems really hard. What's the first thing we can do to tackle it? So to me, the Drake equation is just like, hey, let's take a step towards answering this really hard but really important problem. Yeah, I think that's fair. figuring out the like parameters that need to be measured and trying to get, you know, a handle on the errors in our data, those are all good things to do.

When was the Drake equation proposed? Because I think we have actually like made pretty good progress on getting sort of estimates for some of these things since the Drake equation was proposed, right? Yeah, the Drake equation is pretty old by now. It's 1961 by Frank Drake, famous astronomer, of course. And so that's, you know, 60 years ago. And we definitely know a lot more about some of these numbers than we did when it was first proposed, which is really fascinating because in the early days, they had to do a lot more sort of guessing. And it's interesting to read, like, what Carl Sagan thought about these things. He was a big believer in this principle of mediocrity.

I mean, I know his famous quote is like, extraordinary claims require extraordinary evidence. But he also hit hard on this principle of mediocrity, which I think is fascinating. And it's a very simple argument. It says, like, look, if you only have one example of something, probably. It's a typical example. You know, imagine, for example, you have a bag full of marbles. You draw one. It's an orange one.

What are the chances it's the only orange marble in a bag of millions of marbles? Pretty small. Probably it's the most likely kind of marble, right? Probably it's mostly orange marbles. So that's a pretty basic argument. And Carl Sagan uses it to suggest, hey, look, there's life here on Earth. There's planets around this star. You know, we develop technological civilization.

So probably all of these numbers in the Drake equation are not zero or very, very, very close to zero. They're probably around half, right? And so that's like a really powerful, not that convincing, but useful first argument to say like, hey, look, life in the universe shouldn't be rare. And it's sort of astonishing how far you can get from just like, hey, we're going to write down all the numbers. You have to multiply together to calculate the number of alien species. We're going to make this very vague argument. And then we're going to basically conclude that life is everywhere in the universe. So I don't remember.

Does the Drake equation give you an estimated number of intelligent civilizations out there or like a probability of finding it? And are there recent estimates that we could point to? Yeah. So the Drake equation calculates the number of civilizations in the galaxy with which communication might be possible, right? So it has the rate of star formation in the galaxy, the fraction of those that have planets, the number of planets that have life, the first.

fraction of those that actually develop life, the fraction of those that have intelligent life, a fraction of those that have civilization, and then the time that they survive, like the length of their survival. Because if aliens are civilized for only five seconds before they nuke themselves to smithereens, then we're not likely to hear from them. We might get the shock waves from their nuclear holocaust, but that's not exactly communication. Yeah, no, I mean, that would be a lesson that would be important to learn, though. Yeah. But you're right that more recently people have started to wonder about this because we've learned some things about these numbers like we know that there are billions and billions more than a hundred billion stars in the

Milky Way so like we're starting from a really big number and more recently we've learned thanks to exoplanets that there are a lot of planets out there there's like many planets per star and a significant fraction of stars out there have earthlike planets in a habitable zone you know so that's not a tiny number so now we're talking like tens of billions of billion of rocky planets out there in the Milky Way. And even a few decades after the Drake equation, people were wondering, like, hmm, there are all these stars out there in the galaxy. There are all these planets out there. Life can't be that rare to form.

So where are all the aliens, right? And so that's the famous Fermi paradox. People are wondering, like, Sagan's principle of mediocrity in the Drake equation all suggest that the universe should be teeming with life, that the galaxy should be overflowing with life. And also, crucially, that there's time. for it to have reached us. Like the galaxy is about a hundred thousand light years across and it's more than 10 billion years old. So there's time for like a civilization to flourish, to form, to travel the stars, to even explore the galaxy. I once sat down and calculated like how long would it take for a civilization to send a probe to every single rocky planet in the galaxy? And the answer is not that long. If you can build, self-replicating probes that land on a planet, do a little bit of mining, make more of themselves, then the number of probes grows exponentially. And so it takes like much less than a million years to explore the entire galaxy. And the galaxy's billions of years old. Surely somebody

would have thought of this idea. I mean, if I've had this idea and other people have had it, certainly some clever alien has done it. And yet we haven't been visited as far as we know by self-replicating alien robots. But timing matters too. So, you know, if those self-replicating alien robots showed up in the age of the dinosaurs, then they would have missed us. The Drake equation definitely accommodates those windows. And you're right that if self-replicating any robots had shown up in the age of the dinosaurs, we wouldn't have been here to greet them. But, you know, they might have left a mark and maybe we'll discover traces of them on the

moon or something. And they probably would have replicated themselves and occupied near my solar systems. I don't know that they would have been really subtle. But, hey, maybe that's a possibility. It's certainly one of the answers to this question of the Fermi paradox. of where is everybody, right? Why haven't we been yet visited by aliens if it seems like the universe should be filled with them?

It's not like a claim that there have to be aliens out there. It's more like a response to Sagan's argument. It says like, okay, you make that argument. It suggests that there should be aliens everywhere, but there aren't. So what's wrong, right? Which piece have we misunderstood? Sure. And the co-host with anxiety and insecurity is thinking maybe we're the stinky kids.

And that's why we haven't been visited. but let's assume that it's not about us. It's not that they just dislike us. So one rebuttal here is that actually we've overestimated the probability that there's intelligent life out there that could visit us. And this is the rare earth hypothesis. And we will dive into the rare earth hypothesis after this break. Hello, puzzlers. Let's start with a quick puzzle.

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And then, as we try the whole thing out for real, wait, what? Oh, that's the run right. I'm looking at this thing. Listen to no such thing on the Iheart radio app, Apple Podcasts, or wherever you get your podcasts. All right, Daniel. We're not the stinky kids in the universe.

I'm sure of it. Wait, first I want to know why that makes you feel anxious. Like, if we are rejected by aliens, you're going to take that personally? Like, it matters to you what aliens think? I would be a little bit bummed out if there was like a network of alien civilizations that were all sort of sharing secrets and information about science together, but they just decided that the humans were not quite ready for it. Maybe we're too violent at this point and we couldn't handle what they had to teach us.

that would kind of bum me out. I think my psychiatrist calls that catastrophizing. It would bum me out if there were aliens and they decided we're not worth talking to. What about you? Well, it would bum me out, but not in a personal way. I wouldn't feel like, wow, all the cool kids don't think I'm cool. I just feel like it would be a tragedy if they're out there and we don't get to meet them. We're going to learn about how the universe works and how life could be so different or so similar to ours.

It just seems like a tragic missed opportunity. So it's not really important to me that aliens think we're all. awesome or that we smell good, but I do want to meet them. So anything that stands in the way of that is definitely a big bummer. Yeah, fair enough. All right. So the rare earth hypothesis posits that we should expect to see life like ours much less in the universe. So give me some background here. Whose idea is this? Yeah. So the rare earth hypothesis came out around 2000. It's a famous book by the astronomer Donald Brownlee and the geologist Peter Ward. And they try to take a careful look at some of these numbers, like how often is there a planet in a habitable zone that

could develop life like ours? What are the conditions that are really necessary and how common are those conditions? And again, there's a lot of guesswork here, but there's always going to be a lot of guesswork. You can't just criticize any work in this area by saying it's not definitive. You know, the first steps are never definitive. But their studies lead them to conclude the opposite of Sagan. Sagan says, you know, hey, we should be typical because we're one of many. But they say essentially, maybe we're the only one of many because we're unusual. We're weird and we're different and we're the only place in the universe that life could arise.

So their argument is that we're not typical. There are not many other cases, you know, that we are the one orange marble from the bag. And that's why we're the ones asking the question. We're one and there's not many. Yeah, that's right. Yeah. Okay. Got it.

Got it. What factors did we look into then to be the ones who ended up evolving as the intelligent species? Yeah. So I like their book a lot because it does dig into the details of like what it takes for life to form. And they start big. So let's think about the galaxy. Like where in a typical galaxy could you have life? We talked about 100 billion stars, but could you really have intelligent life around all of those stars? And it turns out the answer is no. There's a lot of the galaxy that's basically a dead zone for life like ours. So for example, if you're near the center of the galaxy, there's a lot more. radiation. Like we talk about often how out in space, it's not just a vacuum where there's nothing. There's a lot of radiation out there in space. Astronauts get dosed with radiation. On the International Space Station, they have basically a panic room where they can go if there's a solar flare. Space is filled with flying particles that will kill you and we're

buffered from that because of our atmosphere. But also, our solar system is not in the most intensely radiation-filled part of the galaxy. As you get closer and closer to the center, the black hole and all the other stars are pumping out a lot of radiation that could really damage you. This is a different kind of radiation. So maybe this argument isn't a good one. But so like there's fungus that are growing in like the abandoned Chernobyl nuclear reactors. And so there are apparently species that can handle much higher levels of radiation. They do it better than humans do.

Maybe Merlin Sheldrake or some other fungus enthusiasts would say that they communicate intelligently with one another. but I know that's not what we're quite talking about. But could there be species that are intelligent and are just way more radiation resistant than we are? Of course there might be, right? And you put your finger on the fundamental weakness of this entire argument, which is that they're thinking about life like ours. Sure, if you took humans and you teleported them to some planet around a star close to the center of the galaxy, they wouldn't last very long.

They'd all get cancer and it wouldn't be very fun because life like ours didn't evolve in those conditions. But it's much harder to say, could life that's not like ours arise under different conditions? And that's really the challenge of thinking about alien life is thinking out of the box of Earth-like life. So really this book is asking, what's the probability of Earth-like life arising, not on Earth? Which, of course, is going to be a small number. And so that's the fundamental criticism of this book. You really put your finger on it. But it does highlight how different the conditions are in different parts of the galaxy.

and therefore how different life might be. If you are evolving close to the center of the galaxy and near the black hole, you're going to somehow have to have a different biochemistry because you're getting pummeled by radiation all the time. And maybe you figured out a way to take advantage of that. Maybe radiation is like lunch for you, right? I wonder if these factors would also influence, like, the order that visits would have to happen.

Like humans couldn't go towards a high radiation area to, like, try to meet those aliens because we'd get cancer and die. But organisms that are from a high radiation part of the galaxy could come visit us if they could handle less radiation, unless they're eating radiation, then they might be very hungry. But we could feed them at Chernobyl or something. Yeah, exactly. I guess you don't have to physically go somewhere to communicate with someone. Yeah, that's right. I think the Drake equation is mostly thinking about communication.

But of course, I want to meet the aliens. And I think if we found a civilization near the center of the galaxy that was eating radiation, we just have to develop good technology to present. protect ourselves. We could probably figure out how to get there. But it's fascinating to think about which parts of the galaxy life could evolve in and how it would be different. So there's the center of the galaxy filled with radiation. That's a dead zone for lifelike hours. It's also less likely to develop life like hours at the edge of the galaxy, like far out from the center. It's also not a great place to have life like hours because far out there, the stars have less metal. So the solar systems have less like hard, rocky stuff in order to build

life. So does that mean you're more likely to get gas giants that you can't land on out there? Yeah, exactly. And remember where stuff comes from in the universe. Very abbreviated history of the universe is you have like protons and electrons and they form hydrogen. So most of the universe is hydrogen with a little bit of helium from fusion during the Big Bang. But then the universe is just like clouds of hydrogen for a long, long time. And that hasn't really changed. You know, we have stars that have formed and have made helium and carbon and nitrogen and oxygen and all the way up to iron. And then we have supernova that make really heavy stuff. But the fraction of the universe that is anything but hydrogen helium is still pretty small. So this is the stuff that like

makes life interesting. It makes complicated biochemistry possible. And like, you know, information, storage and DNA, all that stuff needs more complex molecules. Those rely on those heavier atoms. And those are still pretty rare. And they tend to be near the center of the galaxy. That's That's where the mass is. The heavy stuff falls there. And so out in the sort of backwaters of the galaxy, there are fewer metals. And so, yeah, you have stars there that are more hydrogen, more helium, less metallic, which changes how they operate.

And then you have fewer rocky planets and more gas giants. And again, that's bad for life like ours. But if you're some sort of weird alien that's like a plasma cyclone in a star, you don't really care. Or if you're a weird alien that evolved in the clouds of a gas giant, you also don't necessarily care. But for lifelike ours, it would be harder to evolve far away from the center of the galaxy. In our atmosphere, there's bacteria that are, you know, floating around up in the sky and the clouds and stuff like that.

You couldn't even get stuff like that at the far reaches of the galaxy because bacteria have more than hydrogen and helium in them. Yeah, exactly. And it's not like there are no metals out there. It's just rarer. And so that like suppresses the chances for all those kind of stuff happening. So that makes the galaxy have like a window of habitability, you know, habitable zone. You can't get too close to the center or you get fried.

You can't get too far out or there aren't enough of the building blocks. And then in the habitable zone, you're not even safe. Like there are places in the habitable zone that are more dangerous for life like ours than other places. And how is this danger measured? Is this radiation? This is the chances that a nearby star are going to come by, disturb the solar system, and cause an extinction event due to like a comet hitting a planet. So the density of stars nearby really affects how likely you are to have a comet

get nudged from the outer solar system and fall into the inner solar system and impact on your planet and fry everybody. If you have a lot of nearby stars, then their little gravitational tugs are going to interfere with your solar system and make it very chaotic. If you're pretty far away from everybody else, then you're safer. But we've been hit by comets and have massive extinction events, but still some stuff eeked through and humans popped out. I feel like chaos just kind of pushes diversity back a little bit.

Yeah. And then it rebounds, whoever makes it through. That doesn't seem like a showstopper so much as a show slower downer. Yeah, and it might also be crucial, right? Maybe evolution gets stuck in some local minimum and it needs a reboot. It needs to get like kicked into higher gear, right? We certainly know that a massive impactor played a big role in the history of life on Earth. And we wouldn't be here if it has.

hadn't hit 65 million years ago. So I agree with you, this is not an open and shut case. But again, it does show you that elements of the galaxy are different. And that growing up in different parts of the galaxy, the neighborhoods really are different. They're more crowded neighborhoods. And then there are quiet and peaceful neighborhoods. And, you know, where we are in the galaxy changes because the most dangerous parts in the habitable zone are in the galactic arms.

Like, you know, the galaxy is a big flat disk, but it spins and has these arms. you see these when you look at pictures of galaxies. And it's not like the sun is either in the arms or not in the arms. Those arms pass through the galaxy. So sometimes during the galactic cycle, we are in an arm. An arm passes through us. It's more of a density wave than a structure. It's sort of like traffic, right?

Sometimes you can be on the road. There's no traffic, and then a traffic wave will pass through you. So there are epics when you're inside one of these arms, and there are more stars around you and more comets are hitting your planet for good or for ill. But where you are in the galaxy really influences the experience of life on that planet. But what if instead that spurs innovation? Like what if you're a civilization that's sort of on the edge of being able to, you know, do space travel? And you're like, all right, we really got to figure it out.

Our neighborhoods are going to be colliding. It's time to go. I can imagine that spurring innovation if the timing works out. Yeah, I think that really highlights the weakness of these arguments that you can almost always make the opposite argument. Like, this seems plausible. So does the opposite argument. I guess the answer is we really just don't know. And I think that's the takeaway message.

It's like, we really don't know anything about any of these questions. And what we really need is more data. We have n equals one. We really need n equals two. We need another example of life out there in the universe to tell us. Like, oh, wow, it's very similar ours. Or wow, it's dramatically different. But right now we just have n equals one and we're extrapolating blindly.

You know, I think maybe we should have called the show Daniel and Kelly's not really explainable yet universe. But that's all right. We're enjoying ourselves. All right. So what's the next thing that they want us to think about? The next thing is to think about where you are in the solar system, right? So say you have a star where you are convinced Earth-like life could evolve, then do you

have a planet that's in the right region? And so people talk a lot about this habitable zone. You have to be close enough to the star for water to melt, but not so close that water is going to get turned into vapor and everything on the planet is going to get fried. And again, this is assuming that you have water-based life, and so you need liquid water on the surface. And like, we certainly have water-based life here on Earth, but it's also not that hard to imagine life on a planet where there isn't liquid water on the surface. I disagree as a biologist. It's hard for me to imagine life without water.

But I think that's just because this is the planet that I'm on and this is what I know. But questions like this, I waffle between thinking. well maybe it could be totally different than what we know and then thinking okay but we have all these different conditions on earth and we're not seeing like life evolving independently on our deserts like if it was that easy we'd expect n equals two on earth for the origin of life coming up in the desert and so we don't know yeah and that's actually a question I have is like how do we know that life didn't start multiple times on earth that there aren't remnants of that somewhere or history of that somewhere but let me give you a counter example what about life forming

on subsurface oceans. Say you have, for example, the moon of a gas giant, and it's frozen solid on the surface because it's cold and far from the sun, but there's enough pressure and enough tidal forces that internally there's heat,

and then you have a subsurface ocean. Why can't you have life forming there? So it's still water, but it's not liquid water on the surface. And technically, that's outside of these guys' habitable zone. I would not be too surprised if we did find life there.

It would be really fun to have someone on the show who studies extremophiles and see what their take is on this stuff. But yeah, I guess I wouldn't be surprised to find life on Mars, like in the caves, because it used to be a wet planet and maybe bacteria might still be living under the surface or something that looks like bacteria to us or on these subsurface oceans. Yeah, I think the extremophiles are a really important lesson. For those of you who don't know, these are examples of life that live in extreme conditions. And my wife is a microbiologist, and she basically tells me it's impossible to ever make anything

sterile because life is everywhere and it always will be. And she tells me these examples of like probes that we send up into space. We try to make them sterile because we don't want to like land on Mars and infect it with earth microbes and then later rediscover those microbes and think we're finding Mars life. But every time we do, we discover that there are some microbes that are going to be happy living in whatever conditions we put this probe in. You know, they develop all these poison to kill microbes and then some micro evolves to eat that poison. And it's like, yum, thanks. And so it's basically impossible to kill off life on Earth, which tells you that life is pretty hardy and it might survive or even evolve in crazy conditions.

The other thing that's really going to change your experience on the planet is not just are you in the habitable zone, but what kind of star do you have? Like, do you have a star that's very stable that's always pumping out the same amount of energy all the time so that the habitable zone is constant? You're always in it. Or you could also have a star that's pretty variable. Some stars out there get much brighter and then much dimmer and then much brighter and then much dimmer again. What would it be like for life to evolve under those conditions?

You know, like a sephid, the famous variable stars that get brighter and dimmer in very regular cycles. We use those to learn about the distance ladder in the universe, how far away things are. I could imagine that spurring competition, for plants at least, you know, the trees that get the tallest are the ones that are going to get the sunlight during those dim light periods. I don't necessarily see that that would have to kill life on the planet so much as spur competition so that you're the one who succeeds during the less good periods. Yeah, exactly. It might just create different conditions and it might just create a different kind of life. It's fascinating to think about the other component here is how many stars there are in your solar system.

We used to think of the galaxy is just a bunch of individual stars. And maybe under exotic conditions, you would have two stars in a solar system. a fun topic for science fiction novels, but recently we've learned that actually it's not uncommon to have two stars in a system, that binary stars are very common. And this is because of how stars form. Stars don't form from like one gas cloud that collapses into a star. Typically, you have a huge star forming region that simultaneously is collapsing into a bunch of stars. So you have like baby star neighbors and they're not isolated gravitationally. They come together and they form a binary or even a trinary system. And so something like 50% of stars out there have a partner.

Oh, wow. That probably would have been very surprising to Drake. So the Drake's equation came out before we even landed on the moon. Yeah. And to think about how mind blowing it would be to be able to get an update on all of this stuff that we know. Okay. So this is common. Why would it be harder to evolve in a neighborhood with two sons? It's just twice as much to photosynthesize. Yeah, it also makes for less regular cycles, though, right? You don't have the kind of cycles we have here on Earth with seasons and days that are so regular. You have a more chaotic system, right? You could be like swapped between the two stars or the stars themselves could be fairly chaotic.

So you might just not have as regular a system. I don't know that that means that life can't evolve. It might just have to be more hardy and more adaptable or just different from the kind of life that we have. Again, I think this is evidence that like these rare earth guys are not really. thinking that far out of the box of like what life could be like. They're really just asking how common are our specific conditions in the galaxy? And that could be very unusual, but that doesn't mean that there are no aliens. Yeah, I can imagine the dynamics you just discussed, just driving life underground, for example, where things like temperature extremes would be less pronounced and things

would just kind of be more stable in general. And basically means there's no California over there, right? No place with like constant good weather all year long. Well, that's good because that's so boring to have constant good weather all year long. No, I want to meet the alien Californians. They're going to be the nicest ones. They're going to be the chillest, you know. I don't want to meet the grumpy aliens. They'll have to dig out of the snow and, like, survive the serious heat.

You know, in Virginia, we have seasons and I love it. And, you know, not everybody in the California cities. You know, some of them are a little snooty. There, I said it. Oh, really? Oh, my gosh. We're only snooty because we know we're only snooty. in the best place in the universe and we're not shy to say it i think virginia is more beautiful i

said it all right right in if you have an opinion about that and whether aliens are more likely to sit down in california or virginia when they come virginia so much better come to virginia aliens all right let's take a break and then we'll get back on track sure how to make it. Maybe you felt stuck in a job, a place, or even a relationship. I'm Emily Tish Sussman, and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Whitmer, Jody Sweeten. Monica Patton. Elaine Welteroff. I'm Jessica Voss. And that's when I was like, I got to go. I don't know how, but that kicked off the pivot of how to make the transition. Learn how to get comfortable pivoting

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Wait, what? Oh, that's the run right. I'm looking at this thing. Listen to no such thing on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. So Daniel, what was? is the next criteria that Brownlee and Ward wanted us to think about? They wanted us to think about not just where you are around the star, but what other planets

there are and whether that's an important part of life evolving. They argue that the reason life evolved on Earth is not just because the Earth is an habitable zone where water can be liquid on the surface, and therefore you can have a California, which we all know is super important, but that the other planets in our solar system have played a big role. For example, having a big cousin like Jupiter out in the outer solar system might be shielding us from comets. So again, they're like really trying to create a situation where you don't get hit by comets a lot. I know you're a big fan of life evolving under multiple commentary impacts. I mean, life is hard, man. You've got to be able to survive. Is that what you teach your kids?

You're like, hey kids, asteroids could hit any day. Check the weather. As you know from our prior interactions, I try to not make my kids lives filled with existential dread. But I do to try to prepare them for difficult situations. That's a difficult line to walk there, Kelly. I'm not sure you're doing it. Oh, God. Thanks for critiquing my parenting. This got intense.

Yeah, exactly. I'm going to have to talk to your kids and let them know that you're actually pro-astroid impact. I'm not going to let you talk to my kids. All right. Fair enough. I wouldn't either.

Anyway, the idea is that Jupiter is out there and when comments do fall towards the inner solar system, maybe it's acting like a celestial vacuum cleaner. It's like blocking for us. You know, it's making sure that these things are not hitting the Earth. It's making the inner solar system like a little walled garden where life is free to evolve while it's still vulnerable. All right. Well, so we've already talked about how I don't necessarily think asteroids are a showstopper,

but how common is it for other systems to have a Jupiter to act as the celestial vacuum cleaner? So we actually really don't know the answer to that question. And it's one where the answer has been changing a lot in the last few years. When we first developed the ability to see planets around other stars, it was a huge moment, right? Because we'd only ever seen planets around our star. So we didn't know how common is it to have planets at all? And is our arrangement common or totally unusual? But the first planets we could see were planets that were really big and planets that were really close to their star.

And that's because we were seeing the effects of those planets tugging on their stars, like the gravity of the planet. wiggling the star a little bit. And later on, we were seeing planets pass in front of their stars, making like little mini planet eclipses. Both of those are really good for seeing planets that are big and close to their star and really bad at seeing anything else. So of course, first planets we discover

are hot Jupiters, big planets close to their star. So then everybody thought, oh my gosh, wow, maybe the universe is filled with hot Jupiters. And then folks were like, well, or maybe that's all we can see, and there's a bunch of invisible planets out there there that we don't know about. But it made people think really carefully about the history of our own solar system

because they were like, well, if hot jupiter's are really common, why don't we have one? Why is our jupiter so cold? It's so far out in the solar system. And that inspired a lot of really interesting theory about like the origins of the arrangements of our planets. And one fascinating theory is that the typical thing is for a gas giant to form in the outer solar system. It has to be far out there so we can like grab enough gas because the sun tends

to grab all the gas in the inner solar system, but then they typically fall into the inner solar system because of chaos and dynamical friction and whatever, they migrate inwards. And so that's what we're seeing out there in other solar systems. Jupiter's that formed far out and then work their way in. And eventually those planets are going to plummet into their stars, right? They're not really stable. But that's not what we have here. And so the theory is that our Jupiter formed and our Saturn formed and some now lost of the stars. gas giant in our solar system formed, and they migrated in, as you typically expect. But then some chaotic interaction happened, and Jupiter and Saturn got kicked to the outer solar

system, and the now lost gas giant ejected from the solar system. So we have a missing planet that got like rejected from the solar system is now out there floating alone in space. Wow. Would we expect to be able to find it, or could it have gotten pretty far away? It could have gotten pretty far away. And these days, we know that there are lots and lots. of these planets. They're called rogue planets that have been ejected from their solar system.

So, like, our pretty picture of planets having nice circular orbits around their stars is kind of an ancient idea. And now we know that, like, solar system dynamics are pretty chaotic. It's not that unusual to lose a planet. In fact, it's kind of amazing if you don't, if you can hang onto a planet for billions of years. And so anyway, that's how we ended up with this Jupiter and the outer solar system sort of protecting us. That might be unusual. It could be that typically the big giants just end up falling into their stars. And that's what we're seeing out there in other solar systems. But this is a question where we're learning new stuff every day because we've only seen a few thousand exoplanets. We're rapidly seeing more and more and

more and more. And so the picture is changing constantly, which is my favorite thing in science when we're like our eyeballs are opening up and we're learning more and more about the universe. Yeah. No, it's exciting. Also, I think someone should start a band, hot Jupiter and the rogue planets. And there are also people who make the opposite argument. They say that actually having a Jupiter in your solar system is not good for reducing impacts. That actually having a Jupiter could cause impacts because it makes these gravitational instabilities. It like tugs on stuff. And it might actually pull stuff towards the Earth.

There was a case in our solar system in 1770. There was a near miss when a comet came really close to the Earth. It was called Lexell's Comet. And you can reconstruct its path. and they're pretty sure that it was Jupiter's fault, that like it was not going to come anywhere near the Earth and then Jupiter tugged on it and basically aimed it right at the Earth.

It was a really close call. And so there's examples there were like, yeah, Jupiter can save you, but Jupiter could also, you know, send something bad your way. So how would you figure out if on balance it was better to have a Jupiter? I guess it would depend on so many different factors

that with what we know, you couldn't come down on this argument one way or another probably. Yeah. It's a really chaotic thing, right? If you start with the solar system, you don't really know whether having a Jupiter would be better or worse. You can't sit down and calculate it. All you can do is run a bunch of simulations. And those simulations depend on the initial conditions. Like, what is the distribution of gas and where are the metals and where the rock and the ice? Very sensitive to all those details. And we don't know how most solar systems start. And so people run a lot of simulations, but everybody starts from different places and gets different answers. And so it's a big mystery right now. We just really don't know the answer to this. question. So in the rare earth hypothesis, they suggest our Jupiter is rare and it was really important, but we actually don't know how rare it is and we don't know if it's important or not. So it's really much more of an open question. All right. So what's the next level that we need to be considering? The next thing is about the size of our planet. Like rocky planets have to be the right size they argue in order to have life on them. And you know, we have some examples of that.

Like if you look at Mars, it's basically in the habitable zone, but it's kind of kind of small. It's a lot smaller than Earth, and it's so small that it doesn't have much of an atmosphere. It's like 1% of the atmosphere of Earth because it doesn't have the gravity to hold onto it the way the moon doesn't, for example. So you can't have Earth-like life evolving on Mars the way it is now. And if you have too much stuff, you're over-dense, then you can have a really dense atmosphere the way like Venus does. You know, their atmosphere is like something you couldn't even breathe. It's like soup, right? It's also like sulfuric acid. But it's very different. from the conditions on Earth.

And so they argue that like having a rocky planet with just the right mass to have an atmosphere like ours so we can all enjoy breezy sunny days in California. It's pretty unusual. You know, the breezy sunny days in Virginia are nice also. There's maybe a little more humidity. But the deal with needing an atmosphere is that what? It protects you from radiation. It moderates temperature swings.

Are those the main reasons why you need an atmosphere? You need an atmosphere, absolutely, for those reasons. and if you think oxygen is important for all the biochemistry of Earth, then you need to be able to hang on to the oxygen when it's produced. You know, oxygen is not something that we had in our atmosphere early on. It took a long, long time for oxygen to be produced by early life so that more complex life that used oxygen could be very efficient, could evolve. I'm sure you know more about that than I do.

And you have to be able to hold on to that oxygen, right? If it all just drifted out into space, then you wouldn't be able to use it for more complex life. So right now we probably don't know, like, even for Earth-like life to evolve, does 50 to 150% of Earth's atmosphere work? Like, could life still have evolved if we had, you know, 50% the atmosphere we have now? We don't know that, I assume. We don't know anything about that. Yeah, like, we still have really no idea about how life started on Earth or what the crucial points were.

How many times it started on Earth, as we talked about earlier? this is the place where we're really in the dark. And so I don't want to be too negative about it because I think it is useful to try to think about these things and to take first attempts. And then the next generation of scientists coming along being like, okay, reasonable first step, but actually, and you know, that's the way progress is made. It's incremental. But I also really think we really haven't gotten anywhere on this question.

You know, because as you say, twice as much atmosphere might be fine. Or maybe it's even better if you have a denser atmosphere. just because no life evolved on Venus as far as we know doesn't mean that life can't evolve on Venus like planets it might be a little different it might enjoy the humidity and might like to land in Virginia but you know that's okay I'm open-minded I mean it would be so awesome if we could just find microbial life somewhere else in the solar system to give us like a handle on this kind of

stuff but all right so say that you are on a planet that's the right size and has the right atmosphere are there other criteria They argue in the book that there's an internal criteria, that having a planet with internal dynamics like lava or magma flow under the mantle, and having plate tectonics and volcanic activity is crucial for creating the conditions for life to evolve on Earth. And I think the argument is that like plate tectonics lead to like continental drift. And so you take populations and like separate them and then you can protect them. You get more diversity rather than like if you have a state. static planet and it's just like when one big brutish competition for life, you might have like one winner instead of like a bunch of different populations and different ecosystems with multiple

winners. That's the argument. I don't know how affected that argument is. It's more just like, hey, this seemed to have played an important role in the way that life evolved on Earth. Maybe it's also important everywhere. What do you think about that as a biologist? I mean, certainly plate tectonics has had interesting implications for biodiversity on Earth. And so you You know, you have gotten species that look very similar on different continents that do have, you know, shared evolutionary history, and now they look and act very different. And so that diversity is interesting. But, you know, we didn't, for example, see one winner on each of the different continents. You'd see lots of different solutions to problems.

And I would imagine tectonics would be helpful for increasing biodiversity, but not necessarily needed, especially if you have other things in the environment that are problems that need to be solved. that drive adaptations. Yeah, and people have tried to actually answer this question. And there are some recent studies that show that plate tectonics are sort of episodic, that you have like times when there's a lot of activity and times when it's stagnant. And some of the crucial developments in the evolution of life, as we know, it may come during those like stagnant periods when things are more chill. So again, like with everything on this topic, you could see it going both ways.

You could like convince yourself of A or not A, about equally well. But again, I think that you noted, it gives us interesting things that we should try to drill down and understand better and sort of focuses where we might want to do research in the future. Yeah. And one thing we've learned is that volcanic activity and internal motion in a planet might not be that rare. We thought for a long time, for example, Earth was the only place in the solar system that had volcanoes and had internal tectonics. But now we know that like volcanoes are everywhere in the solar system. They're on some of those moons that we talked about. We think that Pluto had volcanoes for a while. The moon used to have internal activity before it cooled, and so did Mars.

And so it might not even be as rare as people thought when this book was written. So maybe it is crucial for life to form, but that doesn't mean that life is rare because it might be that plate tectonics are everywhere. Yeah. Okay. Fair enough. One of the last bits they talk about is having a moon. You know, we have a big, fat moon in the sky, which is kind of cool and beautiful, even if you're in Virginia. But people speculate about the role of the moon in the evolution of life on Earth.

Having a big moon does seem a little bit unusual. Like in our solar system, there are not other planets that have just one big moon. And having a big moon like this makes for tidal effects. You know, the moon pulls on the ocean and you get high tide and low tide. It's really fascinating physics. And there's this theory I've heard before that having these tides helps life evolve because you get like the brackish conditions between the saltwater and, the freshwater, you get all the sloshing around which mixes up the amino acids and

imagining some like bubbling cauldron of prebiotic soup and you might get life evolving more easily if you're stirring that cauldron rather than just like letting it sit and be stagnant. But again, you're sort of talking yourself into this argument. What do you think about that? Is there no other way that stirring would happen? Like wouldn't you still get rivers going into the sea and do Coriolis effects influence the way water sort of moves around the earth?

It wouldn't be as extreme or as fun to surf on, but I think you could still get it to happen. How's the surfing in Virginia, by the way? I am not cool enough to answer that question. Well, we have beaches and they're beautiful. Okay. I believe you. I don't know the answer to that question. I don't know that anybody knows the answer to that question until we make the discovery you talked about,

which is like find microbes somewhere else and learn about the history there. But, you know, it's easy to point to these things which affected life on earth and say, oh, this must have been crucial and without it, life couldn't have evolved or would be so dramatically different, we wouldn't even recognize it. You know, another example is like the Earth's axial tilt. You know, we have enough axial tilt to give us variation, which leads to beautiful falls in Virginia, I'm sure. But it's boring in California.

And you don't get snow for Christmas, which makes it feel like not Christmas, which I know because I lived in California for a while. Hey, it snows in California and you can drive to the snow. It's optional. the best thing about the snow, you know, like stuck with it. All right, that's fair. I grew up in Ohio and I did feel stuck with it sometimes. Yeah, exactly.

And if you didn't have any axial tilt, that wouldn't give you any seasonal variations. And there's some suggestion that like having some variation is good for creating diversity and competition and all this kind of stuff. And again, these arguments are not from evidence. There's no like experiments or calculations that say this tilt is perfect. It's really just like retrospective is saying, we had it. this seems important and therefore it must have been crucial but I don't think the argument really holds up I mean so many of these arguments seem to be things that like maybe turned the diversity knob up or down and maybe speed up or slow down the process but human

intelligence only popped up once yeah and so I don't like do you need seven million species before you get intelligence or like I'm not entirely sure how this relates to intelligent life that you could communicate with I think it just creates diversity but Maybe it just creates a lot more beetles, but wasps, which is awesome. I want to be in the world with the most beetle species, but I don't necessarily know that all of these things are necessary for creating intelligent life. I don't think we really understand why we ended up the way we are. So if we find an alien planet and is covered in wasps and beetles, we can drop you on that planet? Yes.

If I can convince my kids. And you want credibility for your opinions about life in Virginia, huh? I think you just blew up your credibility there. Virginia's motto is Virginia is for lovers. And when we first moved here, we were amazed by how many different kinds of spiders there were. And I know I'm not selling Virginia very well. But we kept joking about getting shirts. Virginia is for spiders.

But anyway, there's lots of cool diversity here, is my point. Yeah. So, you know, in the end, a lot of these arguments, again, are like, here's how life evolved on Earth. This seemed to be an important factor, but we don't know, right? The takeaway message here is that we, just don't know. And by focusing on the particular outcome of life on earth, it makes the argument pretty weak. And you could equivalently argue like, hey, what are the chances that Daniel is here

and is on this podcast? You know, and you can even say, given that my parents existed and that they meet and all these things, what are the chances? Well, of course, the chances are tiny, right? But what are the chances my parents have some kid that maybe then grows up to be a physicist and starts some kind of podcast? Like, not that small. And that's That's really the question we're asking, right, is we know there's a huge variation of conditions out there in the universe, but what does that mean about how common life is? If we only ask, where are there humans that look like us? Then, of course, the answer is going to be nowhere or almost nowhere.

But that's not the question we want to answer, right? We just want to know if there's anybody else out there thinking about the universe. Right. And if they care how we smell. Exactly. And if they like nice weather or not, you know. and what they mean by nice weather anyway. That's right.

But fortunately, we are going to learn a lot about this in the coming years. It's fairly recently that we've even seen planets around other stars. That whole science is now like only 25 years old. And it's growing exponentially. You know, the number of planets that humans knew about went from like six for like thousands of years to like seven, eight, nine, and then back to eight. And now it's exploding. Now we're like we've seen five thousand planets around other stars.

stars. It's an incredible time to be human. There's like pre this moment and post this moment. And it really turns the page scientifically in terms of our knowledge of the universe. But we're still in that like exponential growth. We're building all sorts of crazy new space telescopes. Like there's the Hab X telescope, which is designed exactly just to do this. It's going to float in space and look at other stars. And it's got this amazing sunshade that's going to block out the light from the stars that we can just see the planet. It's going to be incredible to discover planets. So we're going to look back at the time when we only saw 5,000 planets as like the early days of this kind of science. And in 10 years, we're going to know so much more about the kind of planets that

are out there. We're going to be measuring what's in the atmosphere of those planets. We might detect oxygen. We might see biosignatures. We could see techno signatures. So it's an exciting moment because we don't know the answers to these crucial questions, but we might be about to find out. I hope so. I can imagine Habex giving us a lot of data to tinker with Drake's equation. I'm already super excited, but I will be really super duper excited when we find a second example of any life, intelligent or not, somewhere else. I feel like that will give us a lot of information that we really need to try to nail this stuff down. Absolutely. Even just two examples, you get to draw a second marble from the huge bag. If it's also an orange marble, you're like, wow, okay, maybe

they really are just orange marble. But it's something weird and crazy, which is, my personal preference. Then we're going to learn so much about life in the universe. And the frustrating thing to me and the exciting thing is that the answers are out there. There's either life out there right now sliming around and enjoying its various weather or there's not. You know, then facts are out there. We just need to like gather them. We just need to like scoop up the information. We need to be smart enough and devote enough energy and frankly money to just like purchasing this information from the universe. Yep. My bet is that the alien life is hiking in the equivalent of Shenandoah National Park

because there's nowhere more beautiful on this planet, but I guess we're just going to have to wait and see. Yeah, well, I guess we'll find out, is the universe for lovers, or is the universe for spiders? Oh, I hope it's for lovers. I really hope. There's a whole book about, turns out life somewhere else's

spiders and how we interact with them, but I can't remember the name of it now. Matt Inman, though, is the one who suggested that I read it. Oh, well, there's a great science fiction novel by Adrian Chikovsky called Children of Time, where spiders become intelligent, and it's fascinating. That's it. That's the one. It's a great book. Totally recommended. Well, anyway, thanks everyone for joining us on this tour of one of the most important, most ancient and least

answered questions in all of science, whether we are alone in the universe, what we can know, what we do know, and what we might know soon in the future. I thought it was going to be, is California or Virginia better? But no, probably you're right. That's probably a bigger question. And that's a question we actually already know the answer to. All right. Bye, everyone. Have a even those of you who live in Virginia. Thanks, everyone. Daniel and Kelly's Extraordinary Universe is produced by IHeart Radio. We would love to hear from you.

We really would. We want to know what questions you have about this extraordinary universe. We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you. We really mean it. We answer every message. Email us at questions at Daniel and Kelly.org.

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