We’ve always assumed that habitable planets would need to be like Earth; a terrestrial planet orbiting a sunlike star. But now astronomers have been discovering planets in the habitable zone around very much non-sunlike stars. What strange places could be habitable?
Kepler and K2 (NASA)
Red Dwarf (Swinburne University)
Proxima Centauri, closest star to our sun (EarthSky)
What is Tidal Locking? (Universe Today)
Tidally locked exoplanets may be more common than previously thought (University of Washington)
VIDEO: The Search for Alien Earths – How Coronagraphs Find Hidden Planets (NASA JPL)
The real Tatooine: Could there be life on other circumbinary planets? (University of Warwick)
Planetary bodies observed for first time in habitable zone of dead star (University College London)
Final moments of planetary remnants seen for first time (University of Warwick)
Ocean Worlds (NASA)
What Astronomers Are Still Discovering About the Big Bang Theory (Smithsonian Magazine)
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 633: Weirdly Habitable Worlds. Welcome to Astronomy Cast, your 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, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela, how you doing?
Dr. Gay: I am coming to the conclusion that we should no longer ask that question. And I think I mentioned this in the last show.
Fraser: Oh yeah, you did.
Dr. Gay: We just need to ask, are you okay, and the answer is yeah, I think I’m okay. Are you okay?
Fraser: Are you okay? Yeah. I’m okay, yeah. So, then, if I say – because I can’t not ask how you’re doing, but you can say, I’m okay.
Dr. Gay: I’m okay. I can say that. It’s true.
Fraser: Exactly. Well, we’ve always assumed that habitable planets would need to be like Earth, a terrestrial planet orbiting a sun-like star, but now, astronomers have been discovering planets in the habitable zone around very much non-sun-like stars. What strange places could be habitable? So, give us the textbook definition of a habitable planet.
Dr. Gay: The textbook definition is a planet that exists in the region orbiting a star where it gets enough energy that you would expect water at its surface to be liquid, if there is significant gravity and pressure. So, Earth, Venus, Mars all have the potential to be habitable to a degree. Mars didn’t stay that way. Venus overshot. And this brings us to the Goldilocks phenomena of our own planet Earth.
Fraser: And so, if Venus had a better atmosphere and maybe plate tectonics, it would have liquid water on its surface. If Mars had a thicker atmosphere, maybe a protective magnetosphere, it would be – and maybe five times the mass, it could have liquid water on its surface. And so, obviously, there’s lots of reasons why you can discount it further, but just as a baseline, just distance from the star creates this zone. And so, that is the habitable zone. And obviously, there’s all kinds of reasons why things could be uninhabitable from there, but then, if that’s the textbook definition, what was sort of the assumed planetary system where you would find planets that could be habitable?
Dr. Gay: So, beyond just the thermodynamics of the issue, which you summed up really nicely, there was also this assumption that you needed to have a sun-like star, something that was long-lived, that gave off a similar spectrum of light, that didn’t go through the terrible first few billion years of nasty flares. And so, that brought us to the conclusion that you needed to have a sun-like star. You needed to have a terrestrial planet orbiting in the habitable zone that was roughly the rough size, spin rotation, magnetosphere. And so, we started out looking for planets around sun-like stars because that’s kind of where we placed the limits.
And then, of course, you also have to have the correct composition. So, the solar system has to have the stuff to actually make planets.
Fraser: But it turned out, finding planets around sun-like stars was tricky.
Dr. Gay: Right. So, sun-like stars are fairly big, and because they’re fairly big, we don’t see the wobble as easily with more distant Earth-like planets, as we would with a much smaller planet that has a much closer and habitable zone, and a much friendlier ratio of sizes.
Fraser: And I guess, the other issue is that because the habitable zone is so far away from the star, any planets that are passing in front of the star do on a very long, slow timescale, like once a year. And so, it’s gonna take you many years to confirm the discovery of a planet in a habitable zone around a sun-like star.
Dr. Gay: And it was really hoped that Kepler would give us that opportunity, but as happens far, far too often, the reaction wheels just didn’t let that be a thing.
Fraser: Yeah, poor Kepler. Poor, never Kepler. And so, weirdly, right now, we have still not found a Earth-size world orbiting within the habitable zone of a sun-like star, but we have found a lot of other planets in the habitable zones of their stars. They’re just not very sun-like stars.
Dr. Gay: No. And every time we think we have placed a limit on this kind of star cannot have planets, unless you’re talking about that metallicity constraint again, the composition, every other restriction we’ve put in, we’ve been proven wrong. It turns out, planets just planet pretty much everywhere.
Fraser: So, let’s start with red dwarfs.
Dr. Gay: So, the nearest planet to our solar system that is not in our solar system are the planets orbiting Proxima Centauri. This is the nearest star to the planet Earth, and we have found a variety of planets, including a Earth-sized one in the star’s habitable zone. And so, it turns out, these itty-bitty little stars that we didn’t think could support planets because how were they gonna hold onto enough mass while they were forming, are actually forming Earth-sized worlds. Now, the thing is, because these are much fainter, much smaller stars, we’re looking at when they have things in their habitable zone.
We’re looking at order of 10-day orbital periods. So, a year and a day are not going to be anything like what we’re used to, but it’s the thermodynamics, the temperature regime that defines habitable or not.
Fraser: So, even a red dwarf has a teeny-tiny little habitable zone, and we’ve found planets orbiting within this – and I guess the surprise, as you said, was that they are Earth-sized worlds. You would expect teeny-tiny planets around teeny-tiny stars, but actually, we’ve got Earth-sized worlds around these small stars.
Dr. Gay: And the most famous of these systems is the TRAPPIST system, where TRAPPIST-1 has a small entourage of planets that are many of them, potentially habitable, that are again, sizes that we can imagine finding evolution on of some sort. These are rocky worlds, again, just orbiting a itty little tiny star.
Fraser: What are the downsides of these planets? I mean, they definitely are in the range where they’re warm enough to have liquid water on the surface but being around this red dwarf star comes with a few downsides.
Dr. Gay: Yes. So, the No. 1 downside is, if you are on the inside of that habitable zone, the Venus side of the habitable zone for these little star systems, you are also probably tidally locked to your star like the moon is tidally locked to the earth. This means one side of your world is always facing the star, and that side gets really, really hot, while the other side, not so much. And you have massive windstorms. Now, there are models out there saying that you can have habitable conditions on these planets for some definitions of life. Building large buildings probably is not gonna be a thing, but it’s possible.
Fraser: I mean, I talked to a scientist about this, and there’s sort of this misnomer that the frontside would be completely baked, and the backside would be frozen solid, and that the only place that you could live would be on this thin little ring. But the reality is, is that the whole frontside would be warm, but because of those winds, as you mentioned, it’s circulating that cold air and regulating the temperature. And think of it more of like a jungle, that the whole frontside would be quite hot, but not boiling water hot. And so, in theory, one whole hemisphere of the planet would be habitable at the same time. And there would be no day-night cycle.
It would just be always day or always night, but nobody lives where it’s always night, so it’s always day.
Dr. Gay: Well, and even creepier than that, the sun always stays in the same place in the sky. And so, as you move around the planet, you see the star moving through your sky, and it’s not just that north-south adjustment we can see if we go on a road trip from say Florida to Toronto. It is, you can actually road trip to sunset, and that’s just a fascinating idea.
Fraser: And the stars themselves are a little temperamental, a lot temperamental.
Dr. Gay: When they’re young, they do go through a terrible two billion years or so. Early on, red dwarf stars, as they’re settling into their fully convective beings, these are stars where the entire atmosphere of the star is circulating like a lava lamp. The fuel from the outer layers of the star is getting brought into the core, and it’s just cycling. And early on, while it’s settling into figuring out how to be a stable star, there are some X-ray outbursts, which have the ability to do bad things to early worlds. And so, the question becomes – we know that in our solar system, there was a much shorter period of badness that occurred.
And afterwards, water was somehow brought into the inner solar system again. Planets were able to settle out and evolve into what we see now, after those first couple billion years that included the great heavy bombardment. Well, what happens in a solar system where maybe not your biggest enemy is the rocks falling out of the sky but the sun flaring at you and that the sun keeps flaring at you through the end of that period of bombardment? Can you still develop an atmosphere? Can you still get your water back? These are questions we don’t know.
Fraser: And it’s important to understand this. These flares can be hundreds of thousands of times more powerful than anything that the sun puts out. You’re close to your star because that’s where it’s warm enough, and so you’re just getting hit by these shotgun blasts of radiation and stellar material that will strip your atmosphere away, blast away your ozone layer, radiate the surface. So, it does sound like a rough period, but who knows? Who knows? If the magnetosphere gets locked in early enough, then maybe you can be protected. Maybe life can get going on the shaded part of the planet, and then move back over to the frontside of the planet once the star has settled down. So, they’re pretty fascinating.
Well, we’re gonna talk about this some more in a second, but it’s time for a break.
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Fraser: And we’re back. Let’s go to the other end of the spectrum. Let’s look at giant stars, stars vastly bigger than the sun. Can they have planets too?
Dr. Gay: They can. My favorite example of an absolutely not habitable planet is KELT-9b. This is a very hot Jupiter that is getting its atmosphere removed. Its atmosphere at its surface is hotter than the surface of our sun. And it’s getting externally heated by the giant star that it is orbiting. Now, while KELT-9b is having a really rough time of it, you can move out further, and further, and further from that star until you find that zone that is cool enough, with a long enough orbit that you can actually start to get Earth-like conditions with a very different duration of year.
Fraser: And I guess, it’s the exact opposite problem. The planets around red dwarves have been so easy to find because they zip around, they tug at the star, they block the light from the star on a regular basis. But if you had a planet orbiting around a giant star, now your habitable zone is like the distance to Jupiter. The planet is taking 10 years to go around. It’s making almost no impact, as it crosses the surface of the star, and makes almost no impact in terms of gravity. So, these are gonna be really tricky to find, if they’re out there.
Dr. Gay: They are going to be extremely tricky to find. They fall into the category of things that maybe easiest to find through direct imaging, as we start to develop coronagraphs, but even then, we’re looking at needing massive systems of the kind that it stretches what we’re currently building and probably needs to wait a couple more generations of scopes. But the fact that we’re finding close in planets on these kinds of stars tells us all kinds of planets should be possible, and this then gets us to the point of okay, so we’re five billion years, six billion years into the age of our solar system. These stars don’t live that long.
Fraser: So, you better hurry.
Dr. Gay: So, there is a good chance that you’d hit the point of little, tiny microbes going hi, I’m here, I’m a microbe, except they are single-celled.
Dr. Gay: So, they can’t think. Yeah, and just as they pop into existence, the star pops out of existence. And then, what happens?
Fraser: So, let’s talk about a double star. Let’s talk about a binary star. Can we have a habitable zone around there?
Dr. Gay: Yes. It’s a bit more complicated because there are so many different kinds of orbits in binary systems. So, if you have two stars close in together being orbited by planets further out that treat those two stars essentially as one gravitational center, you can have an outer habitable zone. If you have a planet that is orbiting one of two stars in a binary system where the other star is extremely far out and is just out there being overly bright in your sky but not really producing heat, also okay. You can then get into the complicated geometries of what happens if you are on a highly elliptical orbit, trying to handle these two stars simultaneously.
It’s all going to depend on the geometry of the system, but if you have those magical geometries of – and by magical, I mean statistically exactly right – of either the two stars being close together and not with their orbiting each other, varying in brightness enough that it would take you out of the habitable zone. That will work. And if the two stars are separated enough, and you’re going around only one of them, that can also work.
Fraser: So, I’m sort of envisioning, you’ve got these two stars that are orbiting around each other, and then the distance of the earth away from them. You’ve got this big habitable zone band. And you could have a planet orbiting in that, going around both of them.
Dr. Gay: Yes.
Fraser: And liquid water on its surface, and everything is stable for a very long period of time because I guess from that distance, those two stars act as if one. And so, if you were on the surface of one of these worlds, and you were sort of looking at your day, what would it be like?
Dr. Gay: It would be essentially that Tatooine, multiple star system, where you see if the stars are far enough apart in the sky to allow you to separate them. You see these two stars that sometimes are super close together, other times might be further apart depending on the orientation of their orbit to you. You’d see them setting and rising together in the sky. And I love this idea that if they’re going around each other, and you’re going around all of them, you basically see over time, the two stars passing super close, and then further away in the sky. That just would be cool.
Dr. Gay: It’d just be cool.
Fraser: And there was a discovery, fairly recent. I don’t know if you’ve been tracking this story, Kepler-16b, which they knew about, but they’ve been able to piece together that it is going around these two stars, circumbinary. But it’s not in a habitable zone, and it’s the size of Saturn, so I don’t think a habitable zone planet has been discovered yet.
Dr. Gay: Yet.
Fraser: But theoretically, there’s no reason to think why – and I guess you could scale that up. I mean, you could have a system with eight stars and have planets, as long as things are stable.
Dr. Gay: And it gets harder and harder to get stable as you get more and more stars piled in, but it’s still cool.
Fraser: What about dead stars?
Dr. Gay: What dwarves actually have planets in habitable zones, or at least one does, and that’s kind of cool.
Fraser: How’s that even possible?
Dr. Gay: This is one of those things we’re still trying to figure out. So, we know that planets in solar systems like ours will eventually get consumed, the innermost ones, as the star expands out. We also know that all the objects that are left behind will get disrupted, both by the gravity and the ultraviolet light of the youngest stage of life once that white dwarf forms. So, you end up with debris disks around white dwarfs, and these debris disks can turn around and apparently form new planets. Or alternatively, it’s possible that white dwarfs are just thieves and steal gravitationally, planets from other systems.
We’re still figuring that part out. We just know that we’re starting to find examples.
Fraser: Yeah, I mean, they discovered – I mean, it’s kind of horrific, but they discovered a planet orbiting around a white dwarf, as it crashed into the white dwarf.
Dr. Gay: Right, that –
Dr. Gay: – is not the best way to find a planet, but it’s possible.
Fraser: We found a planet, and it’s gone. But I guess the point being, that white dwarf will put out radiation because it’s essentially the exposed core of the dead star, and it’s cooling down to the background temperature of the universe.
Dr. Gay: Slowly.
Fraser: And it’s gonna take a long time.
Dr. Gay: And so, worlds that end up around a white dwarf through either a new generation of planet formation or through thievery, have the chance to evolve and evolve over time. And we’re finding on our own planet Earth, it is a bit easier to deal with problems of your star cooling than your star heating to a point. And so, you can imagine having a long time for a civilization to slowly evolve. And the question is, does that happen? What is left behind? And how does it deal with all the early ultraviolet light? That is a concern, but we don’t know the limit to life.
Fraser: And so, we’ve discounted the big Jupiters, and the Saturns, and stuff, but if that planet is orbiting within the habitable zone of its star, it could have moons that are habitable.
Dr. Gay: Exactly.
Fraser: So, even if it’s a bad planet, wrong kinda planet, it might have the rights kinds of moons.
Dr. Gay: Yes. And that starts to raise the potential of, imagine the icy worlds out orbiting Jupiter and Saturn someday being heated up and becoming ocean worlds. Now, they don’t have the gravity to really allow liquid water to be maintained at their surface. There isn’t the kinds of magnetospheres you would like, but there are exceptions like Titan. What happens when Titan gets just a little bit warmer? Does it bump it so far out of that triple point, or do we start to get new and interesting physics?
Fraser: Now, as we just wrap up today’s show, I’m gonna blow your mind. There is a really interesting paper written by Avi Loeb and team, that a few a million years after the Big Bang, the average temperature of the entire universe was room temperature.
Dr. Gay: Fair.
Fraser: So, 20-ish degrees Celsius, and everywhere, every single part of the entire universe. And so, the entire universe was in the habitable zone. And –
Dr. Gay: But we didn’t have planets yet.
Fraser: Who knows? Obviously, you didn’t have a lot of time, didn’t have a lot ofdirt, but it’s such a weird idea to think that you had this, you had –
Dr. Gay: Room temperature hydrogen, helium.
Fraser: – room temperature and some trace other elements everywhere across the entire universe. So, who knows what kinds of shenanigans life could have gotten up to in that time period? Who knows?
Dr. Gay: With four elements.
Fraser: Four elements, yeah, okay. I can hear the sarcasm in your voice. Well, I think it’s awesome, so…
Dr. Gay: It is awesome.
Fraser: Well, thank you, Pamela.
Dr. Gay: And thank you, Fraser. And thank you so much, everyone out there. Now, we stuck one extra show in this month. So, it turns out, we’ve already read the names for all of our patrons this month. And I just wanna say, we’re so grateful to all of you. Now, if you wanna help us end up with more names to spill into more weeks, please consider joining our Patreon at patreon.com/astronomycast. Thank you.
Fraser: Yeah, if you wanna hear Pamela mangle your name, now is your chance.
Dr. Gay: It’s true.
Fraser: Yeah, so join our Patreon. Thank you, everyone, and we’ll see you next week.
Dr. Gay: Thank you, bye-bye.
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