Ep. 705: Water Worlds – Looking for Life Beyond Earth

Wherever we find liquid water on Earth, we find life, so it makes sense to search for water across the Universe, and hopefully we can find evidence of life. But what about worlds which are completely covered in water, oceans hundreds of kilometers deep. Can there be too much water?


(This is an automatically generated transcript)

Fraser Cain [00:01:50] Astronomy Cast. Episode 705 Waterworld. Looking for life beyond Earth. Welcome to Astronomy Cast, a weekly facts 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, I’m the publisher of Universe Today. With me, as always, is Doctor Pamela K, a senior scientist for the Planetary Science Institute and the director of Cosmic Quest. Hey, Padma, how are you doing? 

Pamela Gay [00:02:13] I am doing well. I just want to alert all of our listeners. We’re going to take a completely random week in January off. Yes, because next week I’m going to be in Orlando for the ship Podcast Conference and Sonic Bloom Awards. Astronomy Cast is nominated for a People’s Choice Award. All of the voting occurred while we were on hiatus, so Lord knows how that’s going to work. Yeah, yeah. But, yeah, I will I will be away next week, and I’m actually going to take a couple of days off. My colleague in crime, Annie Wilson, she and I are going to reprise our is the save rocket travels through Disney and Universal. 

Fraser Cain [00:02:57] That sounds great. Wherever we find liquid water on Earth, we find life. So it makes sense to search for water across the universe. And hopefully we can find evidence of life. But what about worlds which are completely covered in water? Oceans hundreds of kilometers deep? Can there be too much water? So this idea of water worlds. I mean, we learned everything we needed to know from that Kevin Costner movie, right? 

Pamela Gay [00:03:21] No. Now, one of the things that that I was thinking about preparing for this show is just how magnificently wrong in pretty much every way that movie appears to have been, except from a climate change perspective. And really, if you want one thing to be wrong, you want it to be the climate change part that’s wrong. 

Fraser Cain [00:03:44] So yeah, but climate change causes sea levels to rise by a couple of hundred feet in the worst case scenario, right? 

Pamela Gay [00:03:52] No. That’s true. It won’t eat the entire planet. It will still be soil. 

Fraser Cain [00:03:55] The coastlines will be roughly where they were, especially in places that are very mountainous. So, you know, won’t be hard where you’re trying to find evidence of violence. No, no, but this idea of water worlds, these we know they exist here in the solar system, and we’re starting to we assume that they’re out there. So. So give us sort of like an idea. What is the quintessential water world here in the solar system? 

Pamela Gay [00:04:22] So within, our system, we’re looking at the more moon like definition of world, where we have things like Europa and Solidus and potentially things I personally would never have expected 20 years ago, like Ganymede, these these are worlds that have what appeared to be solid surfaces. And from all of our data and the occasional observation of geysers. It looks like there are massive liquid oceans beneath these seemingly solid surfaces. 

Fraser Cain [00:04:59] Yeah. I mean, we’re looking at like, you see Ganymede, we’ve got Pluto. Sharon said. Yeah. Eris. Homer. Triton. Like probably there are dozens, if not maybe a hundred objects in the solar system that could be sort of in that classification of a water world. And so when we think about the kinds of worlds that are out there, they are the, the vast majority of the kinds of habitable environments. But, of course, you know, we have to sort of change our definition of what, of what habitable is. So, so let’s imagine then that we are starting to look for exoplanets. Yeah. What are some kinds of water world exoplanets that we might be able to find out there? 

Pamela Gay [00:05:49] Well, there has been for a while now theoretical models that there should be these super Earth sub Neptune mass objects with rocky cores, massive complete oceans, and hydrogen atmospheres. I have apparently been mispronouncing these worlds since the first time I read about them, because the word looks like it should be pronounced piscean, but it’s actually a portmanteau, which is where you smash two words together to get a new word like portmanteau. I don’t do French. I’m sorry. 

Fraser Cain [00:06:30] Apartment okay. Apartment. Okay, I oh, you do know. 

Pamela Gay [00:06:38] Sorry. But anyways, ignoring my inability to pronounce words. Podcast is an example where where, we’re broadcasting these pods of information essentially. 

Fraser Cain [00:06:55] To iPods, to Apple iPods. 

Pamela Gay [00:06:57] Yeah, yeah. 

Fraser Cain [00:06:58] Is the the historical. What’s an iPod, grandpa? 

Pamela Gay [00:07:02] I know, I know. But, it’s a combination of the words hydrogen and ocean. And when you combine hydrogen and ocean, you do not get high. Seein which I’m sure each of us will say at least. 

Fraser Cain [00:07:18] What I’m going to say. From here on out, I’m not. I’m not changing. 

Pamela Gay [00:07:22] All right. Yeah. The correct pronunciation for the people who came up with the word is apparently Haitian. 

Fraser Cain [00:07:28] Yep. Not doing it. 

Pamela Gay [00:07:30] We’re moving on. I can’t. All right. All right, that’s fine. So these, as I said, these are super Earth sub Neptune sized objects. Mass objects with rocky core, massive oceans and hydrogen atmosphere. And we have started finding them. 

Fraser Cain [00:07:46] Right. And I guess so. I’m sort of imagining a world like we’ve got a planet, like maybe Earth or a little bigger than Earth. It’s got ocean and it’s got this thick atmosphere that is majority hydrogen. And so is it like a warm ice giant. I mean, I think about a one in the interior of, say, Neptune. Right. You’ve got an atmosphere and then it’s got a lot of hydrogen and it’s got various ices and water and stuff inside of it. It’s just close to the sun. 

Pamela Gay [00:08:23] So. So you can’t make assumptions on temperature since these things can be moved around inside their solar system. It comes down to do they have enough gravity to hold on to the hydrogen. And do they have enough heat in their cores to maintain a liquid ocean at some point. And the fraction of liquid ocean to icy surface is going to vary with age of the system, distance from the sun kind of star, all that sort of craziness that makes making any assumptions about anything in our universe very difficult. 

Fraser Cain [00:09:04] Well, and the thing that is really exciting about these worlds is that, hydrogen is a really good insulator. Yeah, it’s a very potent greenhouse gas. And so you get a much wider habitable zone in a star system for where you can maintain a liquid water underneath that atmosphere. So here Venus is on the very inside of the habitable zone. Mars is on the very outside of the habitable zone. But you can have a habitable zone that is from Venus all the way out to the asteroid belt and still have liquid water on the surface of your planet. And so now what is a habitable zone is now has to be redefined. And that’s going to require, you know, if we can find some of these high C in worlds out there, Asian worlds out there. See, I did it. Then. Then and you find them, say, out of the orbit of Mars, beyond the over to Mars, the asteroid belt on the way to Jupiter. Ish. Suddenly the rules have changed. 

Pamela Gay [00:10:11] And and what I’m loving is the propensity for liquid water to exist beneath shells means the volume of space available for life has gone up significantly. There’s been some really cool work looking at Pluto and modeling the energy transport necessary to go from the heart on one side to the chaotic terrain on the other side. And, just what kind of density profile would allow that to happen? A a researcher, a Denton, has, has done all of this, and it’s amazing work that she’s done. And if you run the math on if from here to here is liquid ocean inside of Pluto. Calculate the volume and then compare that volume to the volume produced. If you take everything within plus or -two kilometers of sea level here on Earth, you end up with a greater volume available for for life in Pluto than on Earth. 

Fraser Cain [00:11:26] Wow. All right, we’re gonna talk about this some more, but it’s time for another break. 

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Fraser Cain [00:13:25] And we’re back. So I actually interviewed the guy who came up with the name and was sort of pioneering these, these worlds, and we were talking about just how wide ranging these. Haitian. Ask so hard. These Haitian worlds are. And. And, you know, as I said, you know, the habitable zone is much wider that even the like rogue planets are back on the table because you could have, like, say, a brown dwarf star with one of these worlds orbiting around it. And if you get this tidal interactions, then it’s enough heat that it could keep the surface of this planet liquid, even though it is doesn’t have a star per se. 

Pamela Gay [00:14:12] In the fullness of time, it will cool off. But you’ve got a lot of times. 

Fraser Cain [00:14:17] Yeah, they got a lock. They got you got to stop. You know, you got to have multiple, multiple planets orbiting this brown dwarf so that you’ve got those resonances in there interacting with each other. Yeah. But yeah, eventually and eventually it will lock. But but again, like, suddenly what you thought was like the only place for us to look for life is in these terrestrial planets orbiting around sun like stars will. Now everything is as we note. So, so that’s like one type of of Waterworld. But let’s talk about like a more traditional one that we’re thinking about, like one that is more like Earth doesn’t have that thick hydrogen atmosphere is within the temperate zone. What is you know, what sort of what are our options there. 

Pamela Gay [00:15:02] I mean technically that makes us the quintessential example. We are a world that we have a high density core surrounded by liquid water and a splash of land and. As we look around the the nearby star systems trying to find Earthlike worlds, we keep finding things with different densities for their given size. And it seems like if you can imagine it, the universe will create it. So we can expect there to be, through collisions and other natural interactions, as well as from formation, a suite of worlds with the entire distribution from tiny core, lots of water, nitrogen atmosphere to massive core oceans and crust with more crust than we have. Wow. And and it’s going to be a continuum. And this is what I’m loving. We’re finding just looking around our solar system, more and more evidence for massive collisions happening during planetary formation as, instruments like, the Atacama Large Millimeter and submillimeter Array and Meerkat look at planetary forming disks are starting to find evidence for, increased, collisions in other solar systems while they form. And every collision offers a new opportunity for weird planets to exist. 

Fraser Cain [00:16:44] Yeah. And so I’m sort of imagining, like, like, let’s take Earth as the standard planet, which of course, is is ridiculous to say. But, you know, we have maybe, whatever, ten, approximately ten kilometers deep at the deepest part of the ocean. I don’t know what the average depth is. A couple of kilometers across the Earth is. Yeah, 70 something percent water. But that’s just, you know, Europa has more water than Earth does. And so, yes, you could imagine a world that maybe has no land that it has. Yeah. That oceans that are not that are maybe 100km deep or more. 

Pamela Gay [00:17:23] Yes. And and the one thing that we do seem to be seeing consistently is due to the distribution of materials out there and how gravity works, you are going to end up with dust grains colliding to form, planetesimals colliding to form bigger and bigger cores. And it is these cores that are able to attract in the volatile elements. So the gases and ultimately water in the early solar system, and then it appears to be through collisional processes that water gets delivered later. So you’re imagining solar systems that built up this core have sufficient gravity to hold on to their atmosphere, and then got the bejesus pounded out of them by things carrying water to them, or they’re orbiting a star that didn’t blast them dry early in their solar system, because they either migrated from farther out inwards towards their star, or there’s a mechanism we just don’t understand yet. And I mean both. Yeah, seems to always be the universe’s answer. 

Fraser Cain [00:18:35] Well, and it’s funny. Yeah, I think both sounds great because we are it’s funny. Like we’re posting stories on Universe Today, I would say every couple of weeks, which are some variation of where did Earth get its water from? And it is always the Earth is too close to the sun, it’s within the frost line and so water shouldn’t be stable. It should be blasted away by the radiation and sun. Just look at the moon and you hear that sounds of water. So where did that water come from? Like, did it form in place or was it delivered later by comets, as you said. And and they are finding that and there and in fact, astronomers have found with James Webb evidence of water in the dust grains of newly forming planetary systems. And so it really both sounds exactly right, that you had this water present in the early material that formed the planet, and if the planet had enough gravity, then it could hold on to the water, it could keep it safe inside its interior, and then at the same time, it could have that stuff delivered. And then if some mechanism that stopped it from, from, you know, that that allowed it to keep all this water out on the surface, then then who knows where you’re going to get. And so I think you’re 100% right that in the end it’s going to end up with all of it. C all of the above. Right. 

Pamela Gay [00:19:51] Well, and what I’m loving is we actually don’t know how common or uncommon it is for life to form. So everything has its own probability. There. There is the the probability. I’m going to somehow manage to make it through an entire week without getting coffee on my counter. While making coffee in the morning, the probability of that happening is close to zero. There is the probability of getting through an entire. Higher week, without going to bed before midnight, the probability of that happening is close to 100%. There’s so many things that are in between. And if life has even a 30% chance of forming under a water nutrient temperature gradient, kind of. This has the potential to support life environment. We will find life in our solar system on one of these icy worlds, in fossil relics, on a worlds like Mars. Potentially even living on the surface and really small, not very intelligent formats on Titan. And and so within our solar system, we have the potential to figure out what is the probability of life finding a way. And as we look out at other worlds, we’re already starting to see with Kepler K2 18 b, a world that has a biosignature that doesn’t have to be a biosignature, but could be a biosignature. And this is an ocean world. And if we find out. Maybe it’s being argued. 

Fraser Cain [00:21:39] Yeah. Yeah. 

Pamela Gay [00:21:40] There’s dimethyl sulfide. It’s made by algae here. 

Fraser Cain [00:21:43] Maybe. 

Pamela Gay [00:21:45] All right. I looked at. Go ahead, tell us more. I simply looked at the spectra and went, yes, that looks like a spectra. Okay. 

Fraser Cain [00:21:53] Yeah, I know what you’re talking about. And there’s been quite a lot of, of people talking about sort of. And there was this original hydrogen world and there was the discovery of a potential. But but like just the amount of segments are just too low. So yeah. So the the presence of the biosignature is very weak evidence. And it’s a big claim. It’s a big, bold claim to say, we’ve found a we found a chemical in the atmosphere of a plane that can only be formed by life there for life. 

Pamela Gay [00:22:22] That is I would I wouldn’t say the second half. So so what I’m comfortable saying is molecules are messy, molecules are hard to detect. There is a spectra from the James Webb Space Telescope that one way to interpret the spectra is the overlapping patterns of carbon dioxide, methane and dimethyl sulfide overlapping, making it really hard to figure out how much any of these things, what proportions they exist in. And it is a non-unique signature. I’m not willing to say dimethyl sulfide. Sulfide, that gas, that gas is only made by life. I mean, the universe is more creative than we are. But it’s an intriguing spectrum that needs replicated. We just don’t have a bigger telescope to replicate it with. 

Fraser Cain [00:23:17] Well, more time with with Webb to pull it off. But but yeah, like, I’ve been obsessing about Exoplanetary atmosphere observations for the last couple of months, and I’ve probably done 4 or 5 interviews in the last in this time with people who specialize in this. And. And and there I mean, they’re really good at what they do. And the message keeps coming back loud and clear, which is that we are not ready to recognize the signal, a biosignature that the scientists haven’t even agreed on. What is a potential biosignature. They argue about Venus as a potential biosignature. We’re so far away from that, and so weirdly, I can’t believe this is coming from me, but I think it is premature for people to be saying that a biosignature it’s getting found on this planet. We actually did another article in Universe Today about how it’s actually really hard to tell the difference between a planet where one half of it is covered in lava and one half of it is covered in water, that in fact they give off a very similar signal, produce a very similar chemical trace in the atmosphere. Because if all you’re doing is measuring the chemicals, you don’t know whether it’s farting bacteria or farting volcanoes. And so true. Yeah. And so we’re just so there was one to the last thing I really wanted to talk about, which was just this idea. Like, if you do have an ocean that is hundreds of kilometers deep. Is that a problem for life? 

Pamela Gay [00:24:52] I was giving this a good hard think the other day because at the bottom of the ocean you’re dealing with extremes in temperature, you’re dealing with extremes in pressure, and there is not the same. Kinds of life found at the bottom of the ocean that we find at more moderate depths, because they do have to find a way to not go squish. 

Fraser Cain [00:25:24] Yeah. 

Pamela Gay [00:25:27] But. Just as we’ve talked so often about the possibility for, balloon based studies, Venus, where you’re working at reasonable pressures and what you have at the surface of the world, you can imagine there being the kinds of thermodynamic mixing that allow there to be a depth that is amenable for life. But the question then becomes, how can it form? We don’t fully understand formation of life on Earth by any means. There are folks arguing that it started in clays, that it started in ice, that it started in hydrothermal springs. It’s possible it did all three. There is debating arguments over whether or not life emerged once or multiple times. It’s looking like, we can’t say how many times singular cellular life emerged. But multicellular life tried to spring forth in a many tentacled version, died off, spring forth with more solid bodies later. So when we can’t even answer with our own question, how did life form? How many times did life initialize, the zero to life versus the single cellular to multicellular level? I don’t think I’m comfortable saying what limits we need to place on other planets. 

Fraser Cain [00:26:59] Yeah. So there so there’s a couple of, like, interesting papers on on this one is that. Yeah. When you have that much water, as you said, you know, reaching intense pressures and. Yeah, and, and temperatures that in fact, you’re starting to approach the kinds of ice that you find inside worlds like Uranus and Neptune, that you’re actually the stuff is moving away from just liquid water and starting to turn into more exotic forms of, of water, like high pressure water. 

Pamela Gay [00:27:27] We did an entire episode. 

Fraser Cain [00:27:28] Yeah, yeah, yeah. And so you get to a certain point and now this stuff is no longer just water. It is now slushy. 

Pamela Gay [00:27:36] Chaotic environments of molecules that aren’t crystalline. Exactly. 

Fraser Cain [00:27:40] That’s the first thing. And then the second thing is, is that there might not be any mechanism for nutrients to get from the mantle, from the from the crust up. Yeah. The up the water column up to a place where it could then be able to provide for life. And so you’ve got the bottom of the ocean is uninhabitable because it’s just not it’s not liquid water. The topic would be habitable, but there’s no source of nutrients that’s able to come up for life to be able to work with. And then if. 

Pamela Gay [00:28:07] There’s an icy saw, if there’s an icy, surface, the nutrients from. So. So, yeah. Yeah. Raining down on nutrients from space. There’s some cool papers on that. Totally. 

Fraser Cain [00:28:18] Yeah. So you get, like a slight trickle of I mean, here on Earth, we get a little bit of 100 tons of dust a day. Yeah. And so that would be landing on. And so you can imagine the life really quickly scooping that stuff out and trying to incorporate it. Yeah. And but any time anything dies and sinks to the bottom of the ocean, then those nutrients lose ones. Yeah, yeah, yeah. So it’s like it just upends our idea. And we have these naive assumptions about what these places are going to be like. And yet nature tends to make it more complicated, as you said earlier on. And, and so it’s it feels like the vast majority of the worlds out there that we’re going to find are going to be some variation of water worlds, right? Maybe by a factor of 101. But how habitable are they? That’s still the question. 

Pamela Gay [00:29:09] And and I’m I hate this. Reaching the point in our lives where we have to start asking, will they answer these questions while I still care? Well, you still care. It’s like. It’s like, hurry up, humans. 

Fraser Cain [00:29:25] Do you mean like, while you’re still alive? Or do you expect there’s going to be time when you won’t care anymore? 

Pamela Gay [00:29:30] Well, I mean, there’s always multiple ways to approach the end of life. Asymptotically or directly. Yeah. So. Yeah. 

Fraser Cain [00:29:40] Yeah. 

Pamela Gay [00:29:40] That’s a darker topic for a different day. 

Fraser Cain [00:29:43] Yeah. But yeah, it’s a it’s an exciting time and it’s such a time of flux, like, like I can imagine a more exciting time than where we’re at now because we have the tools. We have a mission. The Europa Clipper is on its way to Europa soon. We’ve got the the Juice mission. We’ve got the Titan Dragonfly that’s going to be flying at Titan. We’ve got telescopes like James Webb and the upcoming Habitable Worlds Observatory. There’s the ground based telescopes like the Extremely Large Telescope. 

Pamela Gay [00:30:11] Like we have got to name things better. 

Fraser Cain [00:30:13] The pieces are all in place to finally be able to answer these questions. But but everyone’s gonna have to be patient while this work gets done and water worlds are on the list. 

Pamela Gay [00:30:27] And please name telescopes better. I know we’re writing stories last week. There is now the Large telescope array being built, the Extremely Large telescope being built. There is already a very large telescope, a very long bearing array. Yes. We need to do a better. 

Fraser Cain [00:30:44] No, this is purely the European Southern Observatory and that’s their style. So they named it the Very Large Telescope. And then they named it the Extremely Large Telescope. And the other telescope. It’s going to be called the Overwhelmingly Large Telescope. 

Pamela Gay [00:30:58] I approve. I love it one. 

Fraser Cain [00:31:00] Yeah. 

Pamela Gay [00:31:00] And that one went away. We need to bring the owl back. 

Fraser Cain [00:31:03] Yes. So, no, I think more power. Keep going. But. But with more ridiculously large names for their telescopes. I love it, every part of it. 

Pamela Gay [00:31:13] Yes, yes. 

Fraser Cain [00:31:14] Thank you, Pamela. 

Pamela Gay [00:31:15] And thank you, Fraser. And and thank you so much to to all of our patrons out there. I’m wearing my Patreon hoodie because it’s really warm and comfy. And thank you. This week I would like to thank in particular Joel Stein, Richard Drum, les Howard, Gordon doers, Adam and these Brown, Alexis Wanderer and 101 Kim Baron Felix good Astro sets William Andreas gold, Roland Vermeer, Dharma, Jeff Collins, Simon Parton, Stuart. Mills, Jeremy. Cohen, Kellyanne and David Parker, Harold. Burden, Hagen, Alex. Cohen, Claudia mastroianni. Conception, Fiona. Go and Esau, Matt Rucker, MH, w 1961 Supersymmetric Call. Mark Steven Resnick shares some, Abraham Cottrell, Paul L Hayden, Andrew Stevenson, Alex Rain, Benjamin Carrier, Bart Flaherty, the Lonely Sand Person, Daniel Loosely, and Jim Schooler. Thank you all so very much. 

Fraser Cain [00:32:16] Thanks, everyone, and we’ll see you next week. No. In two. 

Pamela Gay [00:32:19] Weeks. No. In two weeks. Yes. Bye bye. Astronomy cast is a joint product of the Universe Today and the Planetary Science Institute. Astronomy cast is released under a Creative Commons Attribution license. So love it, share it, and remix it. But please credit it to our hosts, Fraser Cain and Doctor Pamela Gay. You can get more information on today’s show topic on our website. Astronomy. Cars.com. This episode was brought to you. Thanks to our generous patrons on Patreon. If you want to help keep the show going, please consider joining our community at Patreon.com Slash Astronomy Cast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We are so grateful to all of you who have joined our Patreon community already. Anyways, keep looking up. This has been Astronomy Cast. 

Show Notes

Hycean planet (Wikipedia entry)

Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b (NASA Webb Telescope Team)

Hycean planets might be habitable ocean worlds (Paul Scott Anderson, Earth Sky)

‘Hycean’ exoplanets may not be able to support life after all (Paul Sutter, Space.com)

‘Mini-Neptunes’ beyond solar system may soon yield signs of life (Nicola Davis, The Guardian)

Evidence for the volatile-rich composition of a 1.5-Earth-radius planet (Piaulet, C., Benneke, B., Almenara, J.M. et al, Nature Astronomy 7, 206–222 (2023))

Habitability and Biosignatures of Hycean Worlds (Nikku Madhusudhan et al, 2021, The Astrophysical Journal, 918 1)

The Runaway Greenhouse on Sub-Neptune Waterworlds (Raymond T. Pierrehumbert, 2023, The Astrophysical Journal, 944 20)

Validation and atmospheric exploration of the sub-Neptune TOI-2136b around a nearby M3 dwarf (K. Kawauchi, 2022, Astronomy and Astrophysics, 666)