Our series on Universe weirdness marches on. This week we take a look at the habitable zone, and how things aren’t as simple as we thought.
In this episode we mentioned donations. Click to learn more!
- Habitable Zones of Different Stars (old NASA article)
- What is the Habitable Zone? (Guide to Space 2015)
- The Habitable Zone (Penn State Astronomy 801 Class notes 2018)
- Circumstellar habitable zone (Wikipedia)
- Why just being in the habitable zone doesn’t make exoplanets livable (Science News 2019)
- What is Tidal Locking? (Guide to Space 2015)
- Tidal locking (Wikipedia)
- Methane on Titan (Wikipedia)
- The Mystery of Methane on Mars and Titan (Scientific American 2009)
- Methane-Filled Lakes on Titan are “Surprisingly Deep” (Universe Today 2019)
- Are there geysers on Europa? (EarthSky 2018)
- Saturn Moon Enceladus Blasts Rings with Geysers in Gorgeous Cassini Photo (Space.com 2018)
- On Icy Pluto, Volcanoes May Spout Liquid Water (Space.com 2019)
- Pluto Was Supposed to Be Fully Frozen – But It Looks Like It Has Liquid Oceans (Science Alert 2019)
- The Habitable Epoch of the Early Universe (press release)
- Life on Mars (Hypotheses) (Wikipedia)
Transcriptions provided by GMR Transcription Services
Astronomy Cast, Episode 543
Weird Issues: The Habitable Zone
Fraser: Welcome to Astronomy Cast, our weekly fact-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of CosmoQuest. Hey Pamela. How you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Great. What’s new?
Pamela: It suddenly became fall. It went from in the 90s Fahrenheit to in the 50s in the two weeks that I was away, and I feel like I missed a season in there, and I would’ve enjoyed it.
Fraser: Yeah. All the leaves turned to brown and yellow, and then we had a big storm come in and it just threw them all off the tree. So, normally you get like a couple of weeks of really nice leaves, but we’ve just had storm after storm, and so every leaf that’s on the trees is all just down already. So, here comes the raking and the wheelbarrowing to clean off the lawns.
Pamela: Adulting is hard.
Fraser: It really is. All right. Our series on universe weirdness marches on. This week, we take a look at the habitable zone and how things aren’t as simple as we once thought. All right, Pamela. Sort of the same game we’ve been playing every week. You head back 10 years and ask 10 years ago Pamela, what is the habitable zone?
Pamela: I would have said that you take a sun-like star and assume that those are the only kinds of stars that can earth-like planets, and then you find that area the correct distance from the star that water is allowed to exist as a water, and it isn’t too hot that it evaporates it, and it isn’t too cold that it freezes it, and in that region of the solar system where planets can have liquid water, there, and only there.
Fraser: Right. There’s where you might find life.
Pamela: Exactly. It might be on a moon, but if it’s on a moon, we’re talking Endor.
Fraser: Whoa, whoa, whoa. Earth-sized world.
Pamela: We’re talking Endor. Endor could be Earth-sized.
Fraser: Okay, fine. Endor orbiting around a gas giant.
Fraser: Okay, great. All right. And so that was the – that narrowed the search criteria down to sun-like stars, Earth-sized worlds, fine. They can orbit a gas giant within a very specific region where liquid water can be present on the surface of a planet. That’s it. Exoplanet hunters – go. How’s that changed?
Pamela: We kinda took all of it and threw it out a window.
Fraser: We sure did. So, then when astronomers think about the habitable zone today, what are all the factors? I mean, it’s really like habitability places. I mean, is there even a better term than habitable zones?
Pamela: Well, the ridiculous thing is it’s hard to kill off language. So, the phrase “habitable zone” continues to get used even when it makes no sense. So, we’ve had two different things happen. The first thing that we’ve had happen is the realization that you can have red dwarf planets that have around them this set of radii where liquid water is allowed to exist, but if you put a planet in that region, it is going to tidally lock itself to its star so that only one side is ever facing that star. And that particular world is likely to have a particular hellstorm of winds with a single cellular catastrophe of wind blowing from one side of the world over to the other side of the world. Life is highly unlikely. So, yeah.
Fraser: Well, that’s only part of it, though, right? Being snuggled up close to a flaring, red dwarf star is also a bad day.
Pamela: Right. Well, we think they only flare for a few billion years and in the fullness of time, maybe a world that has been completely irradiated, embroiled, and burnt by that misbehaving red dwarf, maybe things can happen that make it wet and happy again. But even if it survives, and even if water comes back, when it’s stable later, it’s still gonna be tidally locked. Nothing is taking that tidal locking away.
Fraser: So, and normally, I take everything you say and I agree and go along, but I actually had a chance to do a video on this subject, and I talked to an exoplanet researcher from McGill University, and sort of this is his specialty. And he was saying that actually, tidally locked planets are looking now like they’re not as bad as people thought as long as they have deep oceans.
Fraser: And so, what you’re gonna get is you’re gonna get deep oceans that are gonna very efficiently move the warm, move this overly hot, sunbaked temperature through the air and especially through the water, and circulate it with currents to the far side of the ocean. And so, the front side of the planet will be more like a jungle, and the back side of the planet will be more like Antarctica.
Pamela: And so, the thing I wanna point out with this is to get those kinds of deep oceans, you have to have a massive amount of asteroid strikes and comet strikes after the red dwarf is done with its –
Fraser: Oh, yeah. For sure. But now, like before, again if we were talking three years ago, say tidally locked, that’s it – def nil for the planet. You might have this tiny little region around the edges, but actually half of the planet is jungle-like and the other half is arctic.
Pamela: In the special case of a massive heavy bombardment occurring midway through the life of the solar system, yeah.
Fraser: Yeah. You take an Earth-sized world, you put it beside a red dwarf star, and you’re gonna get – that is likely Earth but the amount of water that the Earth has – you’re gonna get half the planet jungle, half the planet arctic. And so, actually you could have life all the way around on that front side. That’s all. Just adding that to the tool box.
Pamela: That is cool. I totally didn’t know that. Yeah. We keep trying to use computers to find exceptions and it turns out they keep existing. And this is the whole problem with habitable zones is these exceptions keep being found to exist. And this includes our own solar system. And one of the things that we’ve been saying over and over in recent years on this show is “and now, it looks like – and this moon might be able to have life.” And I’m not sure if we started with Titan or Europa, but those are the two worlds where all of this got started.
With Titan, it was initially noticed that the methane appeared to be out of balance and there were other chemical signatures that were out of balance with equilibrium. Now, the methane out of balance with equilibrium simply means it’s being produced somewhere. Sunlight breaks down methane. The fact that we continue to see methane on Titan means something continues to produce methane on Titan. But the other chemistry that was noticed means that there’s something going on that is driving ongoing chemical reactions on the surface of the world that the easiest explanation is respiration, but that’s very uncomfortable. That’s very, very uncomfortable. So, most people are like we’re just gonna assume there’s chemistry we don’t understand going on.
Pamela: So, from Titan where we see liquid methane filling the role that water forms here on Earth, we can jump over to Enceladus where we don’t even have to replace the water. You have the water via means that we hadn’t expected in the past. I’m so sorry. I had an itch in my throat that just was killing me. So, as we look at Europa, we’re finding that this world is tidally stretched and compacted as it gets, well, thrown around by the moons of Jupiter and Jupiter itself, and this constant contracting and compressing has the effect of heating its core.
And that heat is driving a liquid ocean that may be only a couple of kilometers between an icy surface shell that protects that water from radiation and all of the other brutalities of space.
Fraser: And so, when you think about the places like Enceladus, it’s possible that there is, as you said, there is something similar on Europa. There’s probably something similar on Ganymede and Callisto, probably Triton, possibly Pluto and Sharon. There’s Eris. I mean, it’s possible that there are hundreds of worlds, depending on how big the solar system is and what’s out there, that could have some amount of liquid water under their surface, under some kind of icy shell.
And so, instead of saying “oh, the habitable zone is this place,” the habitable zones are that place where the liquid water can exist on the surface, or any world that has a lot of ice. So, once you get outside the frost line, there’s a whole bunch more habitable zones.
Pamela: And even this story has been radically evolving over the past few years. We settled into the idea of well, we see geysers at Enceladus. We think we have evidence of geysers at Europa – that’s now much more confirmed. So, we’re willing to believe liquid water is there and well, it’s a harder argument for Enceladus, we can explain this through tidal features of squishing and contracting. Okay, fine. But, we still expected pluto to be completely dead.
Pamela: And then, New Horizons got there, and New Horizons was like “huh, huh, okay. This world is nothing like we expected.” And it appears to have – who-knows-what’s driving it – geology that is causing recurring – well, not plate tectonics but ice tectonics – to be a feature on its surface.
Fraser: It’s such a neat idea. And when we think about say the future of the Earth, and as the sun heats up and as it boils off the oceans in a billion years from now, and even as the sun dies, these worlds are going to be around again for billions and billions, trillions of years, and could be still warm inside from the decaying radioactive elements and whatever tidal forces are going on for a long time after the solar system – the sun has died – the Earth on the surface. So, in fact, these places could be the last places that we find life in the universe as well.
Pamela: And this gets to the other side of how we understand habitability. Initially, when we thought of habitability, we didn’t just assume that you had to have liquid water, but we also assumed you had to have sunlight. So, when we were children, we were taught that life requires sunlight, air, nutrients, and it didn’t have to be air like you and I breathe. It could be oxygen that is in water that fish are respirating out of the water. But then, as we began to explore the Marianas Trench and the volcanic features that exist in the deep ocean, we found vast, teeming life in all kinds of absurd diversity around these volcanic vents, indicating that life totally didn’t need sunlight.
That was just not a requirement, so let’s get rid of that and instead say you need a thermal gradient of some sort. And now, as we explore more and more with mines, with ice samples, with even looking at the surroundings of radiation spills, we aren’t finding environments where life isn’t willing to exist.
Fraser: Right, yeah. Life, as they say, finds a way. But there are – we talked about the red dwarfstars. So, let’s examine some of the other kinds of stars. As you go up the spectrum, is there like a sweet spot for the size of your star? The red dwarf stars – they’re gonna last for 10 trillion years, but they may very well flare away all of the heavy metals and make the planets unlivable before they settle down. A star like our sun is really only gonna provide us on the surface say two billion years of animals walking around, total time. Is there like a sweet spot in between?
Pamela: We’re still figuring these things out. This is one of the things that we’re hoping to be able to solve with spacecraft-like tests. The problem is we don’t know what is the frequency that different size stars actually have planets. The Kepler Mission looked out at a cone of space. It looked at a single field on the sky. It looked at it for a long period of time. And when you look at a single field on the sky, you’re sampling a small volume nearby and a progressively larger and larger volume at greater and greater distances. Well, at greater distances, you’re looking at brighter stars.
And so, it didn’t give us a complete sample of this is how frequently red dwarf stars, this is how frequently the mid temperature stars, this is how frequently sun-like stars. We need to understand what is the likelihood that a star of any given size is going to have planets. And then it’s not quite the Drake equation, but it’s going to be a modification to that Drake equation where you take the size of the star – in this case, the initial mass function – what is the ratio of stars at a different size one to the other, and then you convolve it with what is the likelihood of stars of each of those sizes having planets?
And that new convolution is going to be what tells us how many planets are out there. And that, in turn, allows us to say “okay, these stars last for this amount of time. Let’s convolve it again and take into account that the fact that well, red dwarfs have a terrible childhood and bigger stars only live a short period of time, and get at that duration that a civilization could exist.
Fraser: Yeah. I can imagine like if you have a star that’s maybe a little smaller than the sun, like I don’t know what the classification is that’s below what the sun is –
Pamela: K stars.
Fraser: A K star, yeah. So, you’ve got a K star. They’re gonna last double the length or 70 billion years and they’re gonna be less intense, but maybe they still will be fairly flarey in the beginning and then settle down, and maybe there’ll be other reasons. And then, what about stars that are bigger and hotter than the sun, like –
Pamela: And this is where it comes down to trying to figure out how fast can life possibly evolve. We are now finding, thanks to stromatolites in western Australia that life already had appeared on Earth in the form of bacterial mats 3.5 billion years ago. So, that’s a billion and a half years into our solar system’s existence. We were finally a solid object when these stromatolites were getting formed. Now, while there are stars that don’t live to see a billion years, could it be that these significantly shorter-lived stars, these stars that are only 3-4 billion years in age, have the potential to quick-start life and up because they evolve so quickly, in turn evolving life quickly?
We don’t know these things, and these are more questions to ask. We used to think that massive stars couldn’t have planets, and well KELT-9b is out there going nope. We’ve got them. KELT-9b is this fabulous little fricasseed world that its surface temperature on the side facing it’s very, very hot star – well that surface temperature matches the surface temperature of our sun. It’s being baked.
Fraser: Right. Not that world. We don’t wanna go to that world.
Pamela: No. That world bad. Bad world, bad.
Fraser: Well, this idea that life could’ve – we don’t know how long life takes to form. And yet, here in the solar system here on Earth, life formed it feels like literally the moment that it could.
Fraser: Like as soon as there was one spot that had cooled down and wasn’t on fire anymore, then life found a way. And there’s this great idea – and this was proposed by Avi Loeb – I don’t know if you’ve heard this idea before that in the early universe at around 8 million years after the big bang, the average temperature of the entire universe was around say 20 degrees Celsius. So, it was room temperature – the entire universe.
Pamela: I hadn’t thought of that, but it makes perfect sense.
Fraser: Yeah, yeah. And so, all water in the universe would have been liquid, everywhere.
Fraser: And so, you could imagine this moment early on where life could’ve gotten going, and then a few million years later, everything cooled down to the point that it all froze. And it’s such a neat idea. I don’t know if there’s any way to test it or anything, but still, it’s a super cool idea.
Pamela: Well, just that idea that you could have had, for this brief epoch, a great, flourishing. I mean that would have been the moment for panspermia to take off.
Fraser: Right. Exactly. Yeah, yeah.
Pamela: Oh, there’s so much science fiction that I know, which I had the ability to write.
Fraser: I know. It’s super science fiction. I know. It’s great. I’ll send you the paper. It’s such a great idea. It’s called The Habitable Epoch or The Habitable Era, or something like that, of The Universe.
Pamela: Imagine if they just got to trilobites at that period and put trilobites in the entire universe and –
Fraser: Swimming across the universe, yeah.
Pamela: It’s a Star Trek discovery waiting to happen.
Fraser: Yeah, it really is. So then, even just this idea of – like before, the habitable zone was really all we considered when we thought about is a planet habitable or not. And now, I think, even that idea is more complicated, right? Like we now have a lot more factors that go into whether we think a planet is going to be habitable or not.
Pamela: And so now, what we’re looking at is things like we are pretty sure the surface of Mars is not habitable because the surface is getting constantly hit with high energy radiation, and this isn’t the kind of radiation that bacteria likes to eat. This is the kind of radiation that blasts apart DNA in an unrelenting manner. So, here if you wanna have life on Mars, you need to get beneath the surface. And the next topic we’re gonna take on is actually astrobiology and how we look for life’s signatures, and the quandaries that we’ve had looking for life on Mars.
Pamela: Now, as we consider where is there a habitable place, we need to consider do you have protection from ionizing radiation? Well, under the ice of Enceladus, under the ice of Europa, under the ice of Ganymede, of Ceres, yes – you have protection from that radiation. So, then the next question is do you have nutrients? And we’re finding organic molecules just about everywhere. And there’s a new release coming out that is explaining that you can get complex organic molecules, these polycyclic hydrocarbons, just by taking normal carbon hydrogen atoms of [inaudible] [00:22:29] and blasting them with galactic radiation.
So, the existence of carbon and hydrogen together in the presence of that deadly radiation can create the organic molecules we need for life. It’s kinda twisted.
Fraser: Yeah. And so, the thing with all of that, right, is I think 10 years ago, again we would have said if you wanna have a world to be habitable, it’s gotta be in the habitable zone. It’s gotta have large amounts of water, and it’s gotta have rock and other elements that are required. But now, it almost feels like it’s gone more general again. Like because there are findings – you said you’re finding the raw materials in a different shape at Enceladus than you do on Earth. You’ve got protection from radiation from space. It’s our atmosphere and the magnetosphere, but on Enceladus, it’s ice. You’ve got water on Earth that’s liquid on the surface, and on Enceladus it’s under a big, thick ice shell.
You’ve got the food for life on Earth – photosynthesis, plants. On Enceladus, maybe it’s hydrogen gas dissolves in the ocean. And so, I think that it’s almost like astronomers got too specific about what they wanted, sort of the way they thought that should be constructed. Now, it’s almost like –
Pamela: And here, I think we have to blame the biologists.
Fraser: Maybe. But then, you have to kinda go take one step back now and just kind of go back to first principles about this, right? Solvent energy source. Right? Raw materials.
Pamela: Yeah. And it’s gonna come down to what is the probability of a given environment having life. Now, the key question that we still haven’t answered at all is how hard is it to create life. Now, we know that if there is a place you can have life on Earth, it has life on Earth. We know that if there’s a place you think there isn’t life on Earth, there’s probably life on Earth. We’ve kinda got life literally coming out our ears because even we are covered in microbes that aren’t of our own making.
Fraser: Literally coming out of our ears, yeah.
Pamela: Yeah. It’s kinda gross to think about. Again, not a wet scientist. I deal with stars.
Fraser: And super death volcanoes.
Pamela: It’s true.
Fraser: You’re a hobbyist on that. You’re just an amateur.
Pamela: Yeah. But as we look out, we don’t know if we are unique. We don’t know if life often evolves at the single cellular level. We don’t know if it very easily goes from single cell to multi-celled bacterial mats. There are hints on Mars of seeing features that look like stromatolites. We don’t know if the first squishy, flagellan-wielding life that we found here on Earth had a chance to well, [inaudible] [00:25:31] its way through the waters of Europa. We need to answer these questions.
Fraser: Yeah. But it’s like – it’s kind of exciting. Like on the one hand, the possibilities have totally opened up. And then on the other hand, the possibilities have totally opened up.
Pamela: We know nothing.
Fraser: And so, we’ve got too many places to look now. But it’s really exciting. And so, next week, we’re gonna talk about how do we look for life and how this has actually gotten a lot more complicated than we ever thought. And it got weird.
Pamela: It got weird, yes. That is an accurate description.
Fraser: Yeah. Do you have any names for us this week?
Pamela: I do. So, as a reminder, we are made possible by all of you out there listening right now. It is your donations, through Patreon in particular and also through PayPal, that really support us. And this week, we would like to thank Tim Garish, Frederick Shorga, Gregory Joiner, Thomas Tupeman, Eric Franiger, William Andrews, Dwayne Isaac, Shannon Humbard, David Gates, Ryan James, Keslina Penflienco, Rachel Frye, Darcy Daniels, Kristin Brooks Dean, Dan Litman, Martin Dawson, Jason Semanski, and Russell Peto. Thank you. We are here thanks to your support.
Fraser: Thanks, Pamela. We’ll see you next week.
Pamela: See you next week.
Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can email us at firstname.lastname@example.org, tweet us @astronomycast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific, or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Searle, and the show was edited by Susie Murph.
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Duration: 28 minutes