How many rogue planets and stars are there?

Daniel and Kelly answer a question from a listener about planets without stars and stars without galaxies. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:35Speaker 01

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• 1:12Daniel Whiteson?

  You often hear that space is vast and mostly empty. That's all true, but it gives you the impression that our cosmic home is alone in the universe. But we're not. We have a nice, bright star to keep us warm and push back the darkness, and other planets are there to help protect us from interstellar interlopers. And our sun is part of a massive swarm of stars in the Milky Way, home to hundreds of billions of planetary cousins. It's actually kind of a cozy cosmic neighborhood. But is that true for all planets and stars? Could there be planets existing outside of a solar system, floating in the dark of space on their own? Or could a star form outside a galaxy?

• 1:54Daniel Whiteson?

  Could either of these be hosts to life who would evolve and see a very different view of the universe? Today on the pod, we'll dive into how planets and stars form, and whether any or many exist out there on their own. Welcome to Daniel and Kelly's extraordinarily rogue universe.

• 2:24Kelly Weinersmith?

  Hi, I'm Kelly Wiener-Smith. I study parasites and space.

• 2:28Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist and I like to think of myself as a rogue scientist.

• 2:33Kelly Weinersmith?

  Oh, nice. I could think of myself as a rogue scientist. I mean, you're like in academia and you're funded by the Department of Defense or something, right? How are you a rogue scientist?

• 2:42Daniel Whiteson?

  Excuse me, I have no Department of Defense or Department of War funding at all.

• 2:46Kelly Weinersmith?

  Oh.

• 2:47Daniel Whiteson?

  None of my research can be used to kill people. Thank you very much.

• 2:49Kelly Weinersmith?

  Sorry, who is it that's given you like lifetime funding? Is that the Department of Energy?

• 2:54Daniel Whiteson?

  There is no lifetime funding guarantee, but I have been supported by the Department of Energy since I was 10 years old.

• 3:02Kelly Weinersmith?

  10? Now, Daniel, you're brilliant. I have no doubt that you could have started physics research at 10, but was the Department of Energy funding your parents, I'm guessing?

• 3:11Daniel Whiteson?

  Yeah, exactly.

• 3:12Kelly Weinersmith?

  Okay.

• 3:13Daniel Whiteson?

  They put food on the table since I was 10. Yeah.

• 3:15Kelly Weinersmith?

  Nice.

• 3:16Daniel Whiteson?

  But my research direction is sort of off the beaten path, within the larger confines of academia. So that's why I think of myself as a rogue scientist. And you must as well. You're a pretty unusual character in your community, aren't you?

• 3:28Kelly Weinersmith?

  Yes, I am. Although, like many scientists, I'm underpaid. So, uh,

• 3:34Kelly Weinersmith?

  that's all right. I love what I do. I don't care.

• 3:37Daniel Whiteson?

  And all of us rogues come together to make a cozy community of rogue scientists. Nobody is on their own. And that's the topic of today's episode, our cosmic community, our cosmic neighborhood. Where are we in the universe? And do we have neighbors in far-flung, distant, dark reaches of space?

• 3:54Kelly Weinersmith?

  That's right. We're not alone. And thankfully for us, we're kept company by the community of extraordinaries who share their questions with us.

• 4:02Daniel Whiteson?

  We adore the adordinaries.

• 4:04Kelly Weinersmith?

  Adordinaries?

• 4:07Kelly Weinersmith?

  I feel like we're stretching it a little bit.

• 4:11Daniel Whiteson?

  Well, we love hearing from all of you. We love hearing what you are wondering about. We love being inspired by your curiosity and we love when you write to us to ask us questions. And today we have an episode inspired by a question from a listener. Here's Steve's question.

• 4:27Speaker 04

  Hi, Daniel and Kelly. I was looking at the stars last night and I know galaxies are full of stars. But are there any stars not in a galaxy and floating around by themselves? In that same vein, is there a planet floating around without orbiting a sun? Thanks.

• 4:45Kelly Weinersmith?

  Oh, rogue planets and suns. I love it. What a great question. And please send us your questions at questions at danielandkelly.org. We answer every email we get and a subset of them will end up getting answered on air.

• 4:59Daniel Whiteson?

  This is such a great question because you can hear Steve wondering if we are weird in the universe. If our situation is typical, like are all planets and stars out there in galaxies like we are? Or are there other situations? And this is exactly the kind of thinking that breaks us out of the box and helps us understand the universe. Because in lots of situations, we are typical and in lots of situations, we are weird. So it's important to ask these questions and then go out and take measurements and demand that the universe give us answers.

• 5:27Kelly Weinersmith?

  Well, and so often when someone sends us a question, it's a question that a lot of other people haven't thought about either. And so we wanted to know, has our audience thought about rogue planets and rogue suns? So we asked them, we asked the extraordinaries, are there rogue planets and rogue suns?

• 5:45Speaker 05

  I would say pretty common. Space is just so big and vast that I just don't think it's really gonna affect us much.

• 5:52Speaker 06

  Like, there's so many rogue planets out there because the initial conditions of solar systems are super chaotic.

• 5:58Speaker 07

  I think rogue planets and rogue stars are more common than we may think.

• 6:01Speaker 08

  I don't think there's a discovery yet, as these would be so dim. Every so often, one of them is flung out with quite a large velocity. I think we've seen quite a few of those, or observed at least a handful of them.

• 6:14Speaker 09

  There are probably a lot of rogue planets and stars out there. Even just one instance of a galaxy collision would disrupt a significant number of solar systems. I would venture to guess there are hundreds of millions, if not billions. I don't think they are common at all. Otherwise, we would be hearing about them all the time.

• 6:32Speaker 10

  I suspect they are quite uncommon. From what I've read and what I've heard from credible podcasts, not AI slop, is that rogue planets are quite common. However, I've heard nothing about rogue stars. So the only rogue star I can think of is P. Diddy, because, girl, he went rogue.

• 6:52Speaker 09

  I guess there's probably a lot of them, maybe even more than the well-behaved kind.

• 6:56Speaker 11

  Yeah, for once, physics went with an awesome name. Rogue planets is just such a cool thing to call them. I think I heard that one in three exoplanets are rogue planets. So they must be incredibly common. And I also think I heard that there are quite a lot of rogue stars as well. So I think there are loads of rogue planets, quite a lot of rogue stars.

• 7:20Speaker 12

  I know Kelly doesn't like the names of many of the concepts in physics, but Rogue Star and Rogue Planets are great titles.

• 7:30Kelly Weinersmith?

  I totally agree. I am so proud of physicists for coming up with a good name for once. Rogue planets and rogues. Or Daniel, was that just a Daniel Flourish? Do you all actually refer to them? Okay, all right.

• 7:41Daniel Whiteson?

  I can't take credit for that one, but I agree. It's a good name. It's cool, and it's nappily describes what it really is.

• 7:47Kelly Weinersmith?

  That's right. Way to go, physicists. Keep in mind that this can go well.

• 7:54Daniel Whiteson?

  We just do that occasionally to raise your hopes and then we disappoint you over and over again.

• 7:58Kelly Weinersmith?

  Oh, my gosh, so much disappointment these days. All right, let's get started because I'm kind of dying to know. I also did not know the answer to this question before you sent me the outline and I didn't read the outline very well, Daniel, so I'm still not sure I know the answer.

• 8:11Daniel Whiteson?

  For dramatic effect, I'm unprepared. Nice.

• 8:14Kelly Weinersmith?

  That's right. So before we talk about rogue planets and stars, how about we just talk about normal stars and planets? How are normal stars and planets formed?

• 8:27Daniel Whiteson?

  Yeah, it's important to know where stars and planets come from so that we can figure out whether there are any stars and planets out there in the black of space on their own, like, could they form out there by themselves, et cetera? And the understanding of how we get stars and planets is incredible because it's one of the great triumphs of our understanding of the universe. We have now an explanation for how you go from like a hot dense soup in the early universe with a few over densities and a few under densities, a few spots with like more stuff and a few spots with less stuff, and evolve that forward in time to get the structure of the universe that we see today. Briefly, the spots with more stuff have more gravity and they attract more stuff together and then it clumps and that just keeps happening so that you go from a spread out soup of stuff

• 9:11Daniel Whiteson?

  where everything is very close to the same density to spots of incredible density like stars and other places with massive voids in them.

• 9:19Kelly Weinersmith?

  That sounds pretty intuitive. Are we like super confident that this is how stars form or is this like the current hypothesis and we feel pretty good about it?

• 9:28Daniel Whiteson?

  We're very confident in this story because there's so many elements to it that come together. For example, the way galaxies form is that these massive clouds of gas come together, become dense, and then form stars. But if you just run a simulation of the universe, starting from those over densities and letting the clock go 14 billion years, you actually don't see galaxy formation. Because there's not enough gravity just from gas and dust and all that stuff to form galaxies. You need help from dark matter. Dark matter is most of the matter in the universe, so it provides most of the gravity. And dark matter forms these wells into which gas flows and forms these halos where you get star formation. So dark matter shapes the whole structure of the universe.

• 10:12Daniel Whiteson?

  When you add dark matter into those simulations, then you do see galaxy formation and structure formation and galaxy clusters and all the crazy stuff that we see out there in the universe. Not our exact galaxies, of course. We can't model that because we don't know the initial conditions. But the typical distributions of galaxies and clusters and all that kind of stuff all comes out beautifully in these simulations. If you add the mixture of normal matter and dark matter that we have in our universe.

• 10:38Kelly Weinersmith?

  All right, two thoughts. First thought, if I had created that simulation and what popped out was like a really nice simulation of the universe. Like, I can't even imagine how cool that must have been to like, see that on your screen and be like, I did it. I understand the universe. Like, that sounds amazing. But remind me, dark matter, dark energy, one of them, maybe both? We don't understand very well?

• 11:01Daniel Whiteson?

  We don't understand either of them very well, but we understand dark matter a lot better. Dark matter, we know it's some kind of matter that's out there in the universe. It's invisible, it's intangible as far as we can tell, but it does provide gravity. And we know a lot about where it is and how it flows and its temperature. It's quite cold, but we don't know what it's made out of at a particle level. But this is just like one independent line of dark matter evidence. We need it to make the large scale structure of the universe and also to make galaxies spin. And also it explained wiggles and the early universe radiation and all sorts of independent lines of evidence. Dark energy, we know almost nothing about. We know the universe is expanding and that expansion is accelerating. We don't understand the mechanism, we call that dark energy.

• 11:45Daniel Whiteson?

  But we also need that to explain the structure of the universe because it helps power the expansion of the universe. Anyway, back to stars. Dark matter and these initial over densities give you these clumps of gas and dust and those cool. And then they form stars. In order to form stars, you need like some seed, like a gravitational over density, but you also need the gas there to be kind of cold. Because if it's too hot, if they're moving too fast, then gravity, which is pretty weak in the end, can't grab on to these tiny gas atoms and collapse them together. So once it cools down to like 10 or 20 Kelvin, then it collapses into these stars. These pockets of densities become protostars, which get heavier and heavier. And eventually, they get enough mass that there's enough pressure

• 12:30Daniel Whiteson?

  at the core to raise that temperature up to get fusion. And that's how stars form.

• 12:35Kelly Weinersmith?

  Okay, and I'm finding myself wondering, why didn't everything just get sucked into the sun? Because it's massive. And I guess it's because everything else is moving so fast, it's able to keep orbit instead of getting sucked in. Is that right?

• 12:45Daniel Whiteson?

  Yeah, it's a great question. You might also ask like, why don't we just get one mega sun?

• 12:49Kelly Weinersmith?

  Yeah.

• 12:50Daniel Whiteson?

  Right? Instead of lots and lots of suns. It's because you have pockets of density. That's why you get lots of stars instead of individual ones. So each one forms a seed. And the same thing happens in the solar system. We'll dig into this in a minute when we talk about planets. But some things are going too fast to fall in immediately. And there are little gravitational over densities on the outer edge of these gas blobs, which then rush together to grab some gas before it all gets sucked into the sun.

• 13:15Kelly Weinersmith?

  Okay.

• 13:16Daniel Whiteson?

  So yeah, absolutely. And so the crucial thing to understand here is that stars do not form out in the middle of space. They form in these huge clouds that eventually leads to galaxies. And one way we know this is that when we look at stars, we often find them in binary star systems. Stars, even in galaxies, are not made on their own. They're typically made with siblings, two, three, sometimes seven stars all birth together. And you find them still near each other out there. So stars are not made all on their lonesome. They're made in these vast nurseries, where huge clouds of gas are collapsing into stars.

• 13:49Kelly Weinersmith?

  And it's impossible for that whole cloud to collapse into one thing? Or just like incredibly improbable.

• 13:56Daniel Whiteson?

  It's improbable, though we think that in the early universe, in the first round of collapse of stars, some of those stars were really monsters, like 300 times the mass of our sun, because you didn't have any metals yet. It was all just hydrogen and tiny bits of helium, so things were sort of smoother. Later on, when you'd form metals, those metals were excellent seeds. So you ended up getting more smaller stars. But in the early universe, we think there were some really huge monster stars that were formed. But there's also a sort of an upper limit on the size of a star, around 300 times the mass of our sun, because bigger than that, the core gets so hot that fusion rips apart the star. Remember, stars are a balance between gravity that's collapsing it and fusion that's providing pressure to keep it from collapsing. If fusion gets too hot, and fusion increases very rapidly

• 14:41Daniel Whiteson?

  with temperature, then it blows the star apart. So that's why you didn't get super huge galaxies that are just like one star, which would be kind of awesome, but doesn't really work with physics.

• 14:52Kelly Weinersmith?

  Lonely. It would be lonely.

• 14:53Daniel Whiteson?

  Exactly.

• 14:54Kelly Weinersmith?

  All right. So now we've tackled stars. What about planets? How does a planet form?

• 14:57Daniel Whiteson?

  Yeah. So planets are sort of the leftover bits of that formation. You have this cloud of gas that's collapsing, but you have other pockets of density. You have like a little bit of metal from an earlier star or a chunk of rock that forms a seed. And the rest of the cloud is either molecules of hydrogen or like micron sized dust grains, which can come together and stick together with like very weak van der Waals forces. And this just accumulates and you get like another seed. So if your disk of gas is very, very smooth, you might not get any planets, but that's very unlikely, because typically there are little clumps of gravitational over density that will then form the seed of little sort of mini collapses. For the same reason that you don't get one mega

• 15:38Daniel Whiteson?

  star from a molecular cloud, you also don't just have a collapse into one object. Though the star does gobble most of the mass in the solar system because it forms first and it's big and it's massive. And so it gobbles like 99% of the material. But if you're far away from the star, you also can take advantage of ice. If you're far enough away that like the star's radiation is not instantly vaporizing all ice, then you have another solid material you can use to build your planet.

• 16:05Kelly Weinersmith?

  I mean, it doesn't make me feel great that planets are like star crumbs, but I'll I'll take it. Are there any solar systems we've seen that have only one planet? Like, what's the average number of planets in a solar system, Daniel? Surely you have that at your fingertips.

• 16:22Daniel Whiteson?

  We have seen a lot of planets out there, something more than like 5,000 or so by now. And in some solar systems, we've only seen one planet because it's like a big one and it's close to the star. So it's easier to spot because being close to the star means it's like a bigger eclipse of the star. So we can use the transit method or a massive planet means it's pulling on the star. So we can use the wiggle method. So we don't know necessarily, but we suspect that most stars have many planets based on other observations and then extrapolating and also from models. It's very unlikely to get a single individual planet, but there's still lots of uncertainty here. The theory I just described is called the quark accretion model. You start with like a little clump and you gather more stuff around it to make

• 17:06Daniel Whiteson?

  a planet. There are other theories. There's one called like the gravitational instability theory that like an entire planet can form from a gravitational collapse all at once rather than gradually.

• 17:16Kelly Weinersmith?

  Wow.

• 17:16Daniel Whiteson?

  And neither theory perfectly describes everything we see. We can look out now into space and see planets form because remember, looking into space is looking backwards in time. And sometimes we can spot a star in formation and you can see there's like the protoplanetary ring around it after only like, you know, half a million years or so and can help us test our theories of formation. And sometimes we see like huge planets and multiple planetary rings and all sorts of crazy stuff that we don't really understand. It's often like this that we see things happening more quickly in the universe than we expect. And so it updates our models. So we definitely don't have a perfect theory of planet formation yet.

• 17:55Kelly Weinersmith?

  Okay. So quick summary, the sun, we feel super confident in how that is formed. Planets, a little less confident.

• 18:03Daniel Whiteson?

  Yeah, that's true. But the bottom line is that in general, stars and planets are made in big clumps in huge clouds of dust and gas. They're not made out on their own. You don't just get like a planet randomly forming in the middle of space.

• 18:18Kelly Weinersmith?

  All right, done. Thanks for the question and we'll see you next week.

• 18:23Kelly Weinersmith?

  Oh, no, wait, there's more.

• 18:24Daniel Whiteson?

  Oh, there's so much more.

• 18:26Kelly Weinersmith?

  So much more. After the break, we'll give you more.

• 18:38Speaker 13

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• 19:06Speaker 14

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• 20:18Kelly Weinersmith?

  All right. So Daniel just finished telling us how the sun is formed and how planets are formed, and it sounds like you shouldn't get rogue stars and planets. But you know, this is physics and so nothing is as it seems. What happens to give us rogue stars and planets, Daniel?

• 20:35Daniel Whiteson?

  Well, what we just learned is where stars and planets are made. That doesn't limit stars and planets to stay there forever, right? Lots of people grow up and then move away from home. And so stars and planets might also be able to do that. But what this means is that to get rogue stars and planets, you have to somehow form them in galaxies and then eject them. We need some method to get them out of the galaxies if they're going to be out there in the middle of space. And so let's start with rogue stars. Stars, we're very confident, begin in galaxies. Although I'll add, we talked about the development of stars as if that's where all the gas and dust is. But there's an enormous amount of gas between galaxies as well. There are these huge filaments of gas connecting

• 21:19Daniel Whiteson?

  the individual gravitational wells. Something like half of all the baryons, the normal matter in the universe, is outside of galaxies. It's just very, very dilute and very, very hot. So it's not the place where stars will form. But it's not like it's really empty space. There's these incredible filaments of gas connecting all the galaxies.

• 21:37Kelly Weinersmith?

  I love that you connected these rogue stars to like humans moving away. And now we've got this other connection here. The universe is full of gas and so are humans. And I'm feeling celestial today.

• 21:49Daniel Whiteson?

  Yeah, exactly. Well, maybe people moved out because they were too full of gas and their families couldn't stand it.

• 21:55Kelly Weinersmith?

  Begone.

• 21:58Daniel Whiteson?

  So the crucial question to understanding how a star can leave the galaxy is essentially like escape velocity. How can it get out of the galaxy? Galaxies exist for a reason because they have a lot of gravity. They tend to hang on to their stars. That's where they're formed and there's so much mass there that like, for example, our sun is orbiting the center of the galaxy and it takes a few hundred million years to go all the way around. But it's unlikely to just wander out of the galaxy. It is bound there, right? And we think we understand the gravitational dynamics of galaxies. This is how we first discovered dark matter. We saw that stars are not being thrown into intergalactic space very often because there's something in the galaxy providing that gravity to hold. Together. All right. But that doesn't mean that it's impossible.

• 22:41Daniel Whiteson?

  It just means you need a lot of speed, sometimes up to thousands or millions of kilometers per hour.

• 22:46Kelly Weinersmith?

  Wow.

• 22:47Daniel Whiteson?

  And that can happen. We know also that galaxies form in the method we just described, but then they also combine, right? The method we talked about makes essentially a bunch of little baby galaxies. But if we look at galaxies today, they're big, they're huge, and they show evidence of collisions of mergers. So the Milky Way, for example, is a combination of a bunch of little galaxies which came together. And when galaxies come together, it's sometimes peaceful, but often it's chaotic. And what happens is some stars get tossed into space.

• 23:19Kelly Weinersmith?

  I have to admit, I'm a little biased towards a particular listener question, and that was my daughter's listener question, because I think she asked you if our sun was thrown out of another galaxy and ended up where it is today. And I think you said no. Is that right?

• 23:35Daniel Whiteson?

  Yeah, I don't remember any detail, unfortunately.

• 23:37Kelly Weinersmith?

  What?

• 23:38Daniel Whiteson?

  Because we answer so many questions and we're trying to be democratic about them in a nutshell. Any favoritism.

• 23:43Kelly Weinersmith?

  All right, all right.

• 23:43Daniel Whiteson?

  But I'm glad that you remember.

• 23:47Daniel Whiteson?

  So in that sense, it's true that our star has always been a member of the Milky Way, but it's also possible that it was a member of another galaxy which then formed and became the Milky Way, like joined with the Milky Way. Though our star is fairly young for the Milky Way, like the Milky Way is almost as old as the universe, and our star is only a few billion years old. So it's also possible that it formed in the Milky Way.

• 24:08Kelly Weinersmith?

  Wow. Okay, cool. So sometimes galaxies combine, and how exactly does that result in something getting kicked out so that it's no longer part of the club?

• 24:17Daniel Whiteson?

  Yeah, so you have these two galaxies, each of which is already spinning, right? And now they come in and they combine, they form a new center of mass. And some things naturally have the right velocity and the right distance and the right direction in order to be orbiting the new center of mass, but sometimes they don't. And the new center of mass has like a gravity to tug on that star and just eject it. And so it's not guaranteed that everything in the old galaxies finds a new stable orbit in the new galaxies.

• 24:46Kelly Weinersmith?

  It's a real drag when things change.

• 24:47Daniel Whiteson?

  It can, exactly. And remember that at the hearts of these galaxies are supermassive black holes, these enormously massive, very dense, compact objects we don't understand very well, that can provide a huge gravitational boost. And so if the new stars coming in get close to the supermassive black hole of the other galaxy, boom, they can very easily get ejected.

• 25:08Kelly Weinersmith?

  Wow.

• 25:08Daniel Whiteson?

  Or even without a merger, if a star wanders too close to the supermassive black hole in our galaxy, for example, then it can get a big kick and become what they call a hypervelocity star.

• 25:18Kelly Weinersmith?

  Whoa. How do we detect and measure this stuff?

• 25:21Daniel Whiteson?

  So of course, space telescopes, one of humanity's greatest inventions.

• 25:24Kelly Weinersmith?

  Give more money.

• 25:25Daniel Whiteson?

  We can look at these stars and we can measure their velocity by looking at the redshift. We know how a star should emit light at what frequencies because we know what they're made out of. And we can look at the spectrum and say, oh, look, there's hydrogen, there's helium, there's other stuff in the atmosphere of this star. But if we see those things shifted from the fingerprints where we expect to see them, then we know that the star has a velocity because of velocity from the star will give a Doppler shift will change the light, it will redshift all the light from that star. So a star moving away from us will be redder and a star moving towards us will be bluer. So by looking at the spectrum of the star, you can measure the velocity of the star. This is how we see that most of the universe is moving away from us by looking at the redshift

• 26:08Daniel Whiteson?

  of stars in other galaxies. We can also do that for individual stars within our galaxy. And we've been doing that for like 20 or 30 years now.

• 26:16Kelly Weinersmith?

  Whoa. So how often have we seen something being ejected?

• 26:20Daniel Whiteson?

  So we've seen a bunch of stars inside our galaxy that have crazy high velocities, like this one called S5HVS1, moving at almost 2,000 kilometers per second relative to the center of the galaxy. And if you look at its trajectory, it looks like it visited the galactic center, got boosted by something in there, and is now headed straight out of the galaxy.

• 26:40Kelly Weinersmith?

  Is it gonna stop?

• 26:41Daniel Whiteson?

  Unless it gets deflected by something else along the way, it's moving out. And we just hope that it writes us letters and updates us on its life.

• 26:48Kelly Weinersmith?

  But it's just gonna like, keep going and going and going and oh man, sorry.

• 26:52Daniel Whiteson?

  Yeah, because the galaxy is pretty sparse, especially once you leave the center. And so it's not easy to like, hit another star. I mean, there's gonna be gravitational deflections, but this thing is moving super fast.

• 27:03Kelly Weinersmith?

  Wow.

• 27:03Daniel Whiteson?

  Another way you can get a hypervelocity star is being kicked by a supernova. So for example, if you have a binary star system, two stars form together, burning near each other. If one of them becomes a supernova, which happens sometimes if it's like, not enough mass to become a supernova and it becomes a white dwarf, and then later it gathers a little bit of extra mass, sometimes from its partner, then it can suddenly go supernova. That's the type 1A supernova. And that boost can kick the other star out of the galaxy. And we've seen one of these things. It's called US708, a helium-rich sub-dwarf star, moving super fast, more than 1,000 kilometers per second, probably ejected by a type 1A supernova. It's essentially its partner. So that's like a star divorce.

• 27:47Kelly Weinersmith?

  Oh, ooh, and divorces are rarely clean and nice things, but sometimes they are. But so a supernova, that's just like a giant explosion, right? And so the explosion sent it flying?

• 27:58Daniel Whiteson?

  Yeah, exactly. It's a huge explosion. It releases an enormous amount of energy. These stars are briefly brighter than the rest of the galaxy combined.

• 28:06Kelly Weinersmith?

  Wow.

• 28:06Daniel Whiteson?

  It's really a mind-boggling amount of energy. And all that radiation pressure can push the other star and send it out of the community to its lonely divorced dad apartment.

• 28:16Kelly Weinersmith?

  It's not me, it's you.

• 28:22Daniel Whiteson?

  Exactly. So the Doppler technique is the way we measure these things. And so you can look around and try to measure the speeds, but sometimes we can't even measure these speeds very well. We can only sort of find a minimum.

• 28:33Kelly Weinersmith?

  Why? Why is it so hard to measure the speed? Is it because they're so far away and moving so fast? I mean, it's amazing we can measure any of this stuff at all, to be fair.

• 28:40Daniel Whiteson?

  Yeah, well, we can essentially only measure the speed away from us, right? And so if a star is moving at an angle, then it has an additional velocity that we're not capturing. So this is sort of a minimum speed.

• 28:52Kelly Weinersmith?

  Cool.

• 28:52Daniel Whiteson?

  We have theories for how this might happen, etc. But people have been looking for these stars. And some astronomers from Vanderbilt identified more than 670 stars at the edge of the Milky Way, sort of between us and Andromeda. And these are stars that have already been ejected. They're like living out there in the middle of space. These are not stars that we're projecting are going to leave the galaxy. They're just like out there floating in the middle between us and the next galaxy.

• 29:18Kelly Weinersmith?

  Wow, and they must all be incredibly far away.

• 29:21Daniel Whiteson?

  Yeah, exactly. These are faraway stars. They're hard to see, but the light that comes from them also tells us about their origin. These stars are red giant stars. They have a lot of metal in them. And it's hard to find metals in between galaxies. Metals tend to be at the core, where there's a lot of gravity. There's like more metals at the center of the gravity than there are in our neighborhood of the galaxy. And there's even less in the outskirts of the galaxy. So the fact that these stars have a lot of metal in them tells us that they're probably formed close to the center of a galaxy. These are definitely not formed out there in space. So this is like direct confirmation of this whole idea that stars are formed in galaxies and then sometimes ejected, especially from the core, out into space.

• 30:02Kelly Weinersmith?

  Man, it's gotta be hard to go from the center of the universe to getting thrown out on your own.

• 30:09Daniel Whiteson?

  Yeah, exactly.

• 30:10Kelly Weinersmith?

  So I was really having fun when we were talking about explosions. Are there any more explosion-based ways that stars can go rogue?

• 30:18Daniel Whiteson?

  Sometimes stars go rogue and then they explode. This is super awesome. It's an intergalactic supernova. So imagine the scenario we had before, where you have like a white dwarf and a partner star. And earlier, we talked about that white dwarf exploding and kicking its partner out of the galaxy, which seems unfair, but hey, it happens. It's a cold universe out there. But sometimes those two stars get kicked out together by something else. Maybe they together visited the core of the galaxy and get ejected. Now they're out there in the middle of space between galaxies and they're still doing their thing. And then one of them can go supernova by grabbing some of the material from the other one. And so they can create an explosion out there in the middle of space.

• 31:01Kelly Weinersmith?

  And that blows both of them like out farther or just one of them?

• 31:06Daniel Whiteson?

  So then the other one, if it survives, they don't always survive. But if it does, it'll get ejected in some new direction. So it's like already got thrown out by the galaxy and now its partner is rejecting it. And it's getting shot out in some new direction.

• 31:19Kelly Weinersmith?

  Yeah. How could you possibly know that things had taken such a complicated path without observing that directly? That's amazing.

• 31:26Daniel Whiteson?

  It's hard to explain how you might get a supernova otherwise in between galaxies where there just isn't the material to form these stars.

• 31:33Kelly Weinersmith?

  Wow. So how many of these are there, do we think?

• 31:37Daniel Whiteson?

  So it's a great question. We've seen a lot of them, like hundreds, thousands by now of these intergalactic stars, these rogue stars floating out there. And we're confident that they come from galaxies. And then we take this step that astronomers often do, which is try to estimate how many of them are there. Like, we're seeing a small number of them, but we can also estimate our ability to see them. Like, if we think we're seeing only 1% of the stuff that's out there because it's blocked by something or because of our detection capabilities, then we can count the number we see and extrapolate to the total population. That's always a bit of a dangerous extrapolation because you never really know what fraction of stuff are you seeing? Because you don't see the whole denominator, but you can use models and theories, et cetera. So this is very speculative and there's lots of different estimates.

• 32:22Daniel Whiteson?

  Some early estimates suggested that 10 to 20% of all stars in galactic clusters are not in a galaxy.

• 32:29Kelly Weinersmith?

  What?

• 32:30Daniel Whiteson?

  That they're between galaxies. Yeah, like one in five.

• 32:33Kelly Weinersmith?

  That's way higher than I would have guessed. I don't know why. That seemed like a rare thing.

• 32:38Daniel Whiteson?

  It seemed like it should be a rare thing. On the other hand, I think we're biased because of our experience. We tend to think of our star and our planets, our galaxy as the typical thing. But we've learned over and over in physics that our experience is not typical. Our kind of matter isn't typical. Our star isn't typical. And so this is just another example. And then I read a more recent study that suggested maybe up to 50% of all stars are not in galaxies. This is much more speculative and it came from understanding the extragalactic background light, which is just like an overall general glow of light that comes from outside the galaxy that we don't fully understand and could be Coming from tons and tons of these hypervelocity rogue stars, but it's very indirect and speculative.

• 33:23Daniel Whiteson?

  So the bottom line is it's a big number. Rogue stars are out there. They're not rare. We don't know what fraction of stars in the universe are rogue. It might be 10%, it might be 20%, it might be 50%. It's some huge fraction of the stuff that's out there in the universe. It's not unusual.

• 33:39Kelly Weinersmith?

  So from a sci fi perspective, whenever I hear about Dyson spheres, I think, what right do you have to take all of the light from the sun and deprive all the planets that rely on that light, you know, of the sun's light? But if 50% of the sun's out there, yeah, don't have any planets around them, we could just go and capture that energy. And what does that harm?

• 34:01Daniel Whiteson?

  Well, that's a great question, actually. If a star is ejected from the galaxy, does it keep its planets or not? Right? Because I think you're assuming that a star that undergoes that kind of gravitational perturbation is going to lose its planets. And I think that's probably true, but not necessarily, right? If you think about the whole solar system as an object, it basically gets the same gravity. And so like the forces on the star are the same as the forces on the planets. Depends in detail on how like how close it comes to that supermassive black hole. Does it feel tidal forces? Is that black hole pulling the whole solar system apart? Or is it pushing on the entire thing together? Does the star have enough gravity to hang on to its planets?

• 34:39Kelly Weinersmith?

  Okay, so you're saying that like, some of the things that we've just talked about, when the sun gets ejected, it brings its planets with it and it forms a solar system in a new place.

• 34:50Daniel Whiteson?

  I think that's possible. I think it's more likely that they lose it because of the crazy physics involved, but it's not guaranteed. So some of those rogue stars probably do have planets. So hold off on stealing all of their energy, please, Kelly.

• 35:02Kelly Weinersmith?

  Well, you know that I'm a skeptic in all things. So I don't suspect this will come up soon. But, you know, I would caution people to see if there are planets before you steal a sun's energy.

• 35:14Daniel Whiteson?

  Sounds like a good policy. All right, so let's take a break. And when we come back, we'll talk about rogue planets out there without a galaxy or even without a star.

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• 36:01Speaker 14

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• 36:48Speaker 14

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• 37:01Speaker 19

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• 37:48Kelly Weinersmith?

  All right, we're back and we're talking about rogue planets. And so just to clarify, you were telling us a moment ago that suns can get ejected and sometimes they take their planets with them.

• 37:57Daniel Whiteson?

  We've never seen that because it's hard to see exoplanets, especially for really distant stars and the hypervelocity stars we've seen. We've never identified an exoplanet on a hypervelocity star, but it is possible.

• 38:08Kelly Weinersmith?

  Okay, but when we were talking about rogue planets now, we're not talking about planets that got pulled with their star. We're talking about planets that got pushed away from their star and now they're all out on their own. Is that right?

• 38:19Daniel Whiteson?

  Yeah, exactly. So a rogue planet would be a planet inside the galaxy still, but without a star to call home, right? Because the galaxy is mostly empty. I mean, it looks pretty bright when you look up at the stars at night and you can see the Milky Way and there are lots and lots of stars out there. But most places in the galaxy are pretty far from any star. If you get like randomly dropped into a spot in the galaxy, the odds are you're not close to any star. That mostly you'd just see like distant stars around you, but you wouldn't consider yourself gravitationally bound or held by any star.

• 38:53Kelly Weinersmith?

  But there'd probably be gas.

• 38:54Daniel Whiteson?

  There'd probably be gas. There's always gas.

• 38:57Kelly Weinersmith?

  There's always gas. The one constant in life is gas.

• 38:59Daniel Whiteson?

  And radiation. And so think of these planets as like having the same relationship with the galaxy as our sun does, right? The sun orbits the center of the galaxy. You could also have a planet out there orbiting the center of the galaxy, not necessarily orbiting a star directly. So that's what we would call a rogue planet.

• 39:16Kelly Weinersmith?

  So how do you get rogue planets then?

• 39:18Daniel Whiteson?

  So it's sort of the same process we talked about for stars, but in miniature. Like if two solar systems come near each other, then the stars can tug on each other's planets and cause gravitational instability. Our orbit around the star is pretty stable, like a little perturbation, we'll go back to our orbit, but it's not that stable. You give it a big enough push, we get the wrong direction, we are flying out of this solar system. And so it's certainly possible that a visitor can come and perturb our solar system and we could end up losing a planet.

• 39:47Kelly Weinersmith?

  There's the existential dread I've come to expect from a White Sun episode. Glad we got there.

• 39:54Daniel Whiteson?

  And also, sometimes the planets form and it's pretty chaotic. Like, we think of the solar system as very stable and chill. Everybody's driving around the star, staying in their lane. But, you know, this is billions of years along. And earlier on, there was a lot of chaos. As the planets formed, they gravitationally interact with each other. And any system that has more than two objects in it is chaotic and hard to predict. We had like Saturn and Jupiter, and we think another major gas giant in our solar system. And then Jupiter and Saturn moved in towards the center of the solar system. And then they got tugged by the other planet. And Jupiter and Saturn came back out, so they didn't like fall into the sun. But the other planet got ejected.

• 40:36Daniel Whiteson?

  And so this can happen, that you have this complicated interaction between the planets. Or sometimes a neighboring star can perturb things. And you get a lot of planets actually thrown out of solar systems.

• 40:48Kelly Weinersmith?

  What, so this isn't just when solar systems are initially forming? This could, this could happen now?

• 40:53Daniel Whiteson?

  You should be worrying about it right now, Kelly. In fact, if you don't worry about it, it's more likely to happen.

• 40:58Kelly Weinersmith?

  Oh, my gosh, kissing bugs, planets getting ejected. It's a scary world.

• 41:03Daniel Whiteson?

  It mostly happens in the early days of a solar system, but it could happen. Yeah, if we have another star come and visit us really close, then absolutely you could scramble our solar system. And we've done a bunch of calculations and they suggest that like maybe 25 to 50% of planets that get formed get ejected from stars.

• 41:22Kelly Weinersmith?

  What?

• 41:22Daniel Whiteson?

  Yes.

• 41:23Kelly Weinersmith?

  Holy cow.

• 41:23Daniel Whiteson?

  Exactly, which kind of makes sense because it's not that easy to get everything arranged in the right angles and velocities and distances to stay in a stable orbit, especially when you've got other planets tugging on you.

• 41:33Kelly Weinersmith?

  And so do these planets tend to like get sucked into other solar systems eventually? Or they're just off completely by themselves?

• 41:41Daniel Whiteson?

  They can be sucked into other solar systems, but a lot of them are just off by themselves because the galaxy is mostly empty. But yeah, you could end up being like captured by another solar system. Or if you're a tiny planet, you can join the like huge crowd of frozen stuff at the outskirts of solar systems, right? So absolutely, this can happen. And for example, like those interstellar visitors like Atlas and Oumuamua.

• 42:03Kelly Weinersmith?

  I love the way you always say that right the first time, which is totally different than how I say it.

• 42:09Daniel Whiteson?

  These things are demonstrations of how that works, right? These things were formed around another star and then gravitationally somehow detached from that star, sent through space and now hitting our solar system. And it's unlikely that they're going to get captured in our solar system and like join our neighborhood. They're going to fly through and get slingshotted out in some other direction. But there's stuff out there that's formed by one solar system, ejected and now flying through space, maybe interacting with other stars. And we think that there are a lot of planets that are like this.

• 42:37Kelly Weinersmith?

  Is there any evidence that any of the planets in our solar system were thrown out of another one and ended up in ours?

• 42:42Daniel Whiteson?

  You're wondering if any planets are adopted?

• 42:45Kelly Weinersmith?

  That's right. We love adoption.

• 42:47Daniel Whiteson?

  No, there's no evidence for that. And all the models suggest that the planets we have were formed here, but that there were some planets formed here that we've lost.

• 42:55Kelly Weinersmith?

  Okay.

• 42:56Daniel Whiteson?

  Yeah.

• 42:56Kelly Weinersmith?

  Are there any other ways?

• 42:57Daniel Whiteson?

  There are some other ways you might get a rogue planet. For example, if you have a star that's forming, but it's not enough mass to really have fusion, so it becomes like a sub-brown dwarf star. That's a star that isn't hot enough at its core to ignite real fusion. Sometimes you can get like a weaker version of fusion going, but sometimes they're in the middle, they're sort of like a super Jupiter, like a version Jupiter that's really big, bigger than Jupiter, smaller than what you need to get fusion going. A lot to argument about whether this should be called a rogue planet, because it's not really like a planet formed around a star and then ejected, or like a failed star. So, you know, astronomers argue about names.

• 43:36Kelly Weinersmith?

  Yeah, and those names carry baggage. Failed star.

• 43:41Daniel Whiteson?

  Would you rather be a rogue or a failure? Yeah, I'd rather be a rogue.

• 43:45Kelly Weinersmith?

  Rogue, obviously. Yeah, that's way cooler. Rogue implies you chose this path.

• 43:50Daniel Whiteson?

  Right, exactly. I was never trying to be a star anyway, okay?

• 43:54Kelly Weinersmith?

  That's right.

• 43:56Daniel Whiteson?

  And these are things that we can see out there in the galaxy. They're not nearly as far away as the hypervelocity intergalactic stars that we talked about, but they're harder to see because they're not bright, right? Stars emit light, so you can see distant stars pretty easily. Planets don't emit visible light, but they do emit in the infrared. Remember that everything that's out there in the universe that has a temperature, which is every Except for dark matter glows at some frequency and that frequency depends on the temperature. So the hotter you are, the higher the frequency, the colder you are, the lower the frequency. So Earth glows, for example, in infrared. If you take night vision goggles, you can look down at the ground, you can see light being emitted from the surface. Just the way you can see light emitted from another person and it's a different temperature. And

• 44:41Daniel Whiteson?

  that's how you can see like people at night, or that's how those thermometers work that look at your forehead and deduce your temperature without touching you. They're measuring the frequency of infrared light and inverting that process to deduce what your temperature must be. So we can directly see planets using infrared telescopes because they glow in that particular wavelength. We can't use this technique to see exoplanets because they're usually drowned out by their star. That is possible, but you can directly image rogue planets using like the WISE infrared telescope to see them in the IR, which is super cool.

• 45:14Kelly Weinersmith?

  That is super cool. Is that the only way we see them?

• 45:16Daniel Whiteson?

  No, the much more common way is to see microlensing. Like you're looking at a star out in space and they shouldn't twinkle, right? Out in space, stars don't twinkle. That's an atmospheric effect we have on the ground. In out in space, they shouldn't twinkle. But if something passes between you and the star, like a little mini eclipse, they call this microlensing, then you can deduce if there was something there. And so you can look out into space and count how often like stars twinkle, essentially. And there's a project here called OGL, Optical Gravitational Lensing Experiment. Awesome acronym, guys. And another one, MOA, Microlensing Observations in Astrophysics. And these things measure how often this happens. And then they do that same inversion step. They're like, well, we see 74 of these. How

• 45:59Daniel Whiteson?

  likely are we to see one if it's out there? You know, what are the chances that the star and the planet arranges themselves in exactly the right way? And then they estimate how many there are out there in the universe.

• 46:10Kelly Weinersmith?

  So if if you detect microlensing, something has got in the way of your image, you've got a little eclipse going on, how do you know that that's an exoplanet and not like a big comet or something?

• 46:21Daniel Whiteson?

  Yeah, it's a great question. Essentially, we could only see this for bigger stuff. So like, what's the difference between a big comet and a planet? Basically, just the size, right? A comet is just a chunk of rock and ice anyway that's been ejected from a solar system. Like, would you call Oumuamua or Atlas a comet or a planet? We just call them a comet because they're not as big as a planet. But we can only really see stuff that's like Jupiter-sized or a little bit smaller, because otherwise it's too small to really eclipse the star.

• 46:50Kelly Weinersmith?

  This is really cool.

• 46:51Daniel Whiteson?

  But one time, they saw a Jupiter-sized, free-floating planet, and they think they saw a moon around that planet.

• 46:58Kelly Weinersmith?

  What?

• 46:59Daniel Whiteson?

  So this is like an exo-moon around a rogue planet.

• 47:02Kelly Weinersmith?

  Oh, that's really cool.

• 47:03Daniel Whiteson?

  Yeah.

• 47:04Kelly Weinersmith?

  I want to visit.

• 47:06Daniel Whiteson?

  And this is due to like the pattern of the eclipse as the lensing event happens, right? It doesn't look like just a sphere. It looks like a sphere plus another sphere in order to model the pattern of the light. Super cool.

• 47:18Kelly Weinersmith?

  That's awesome.

• 47:18Daniel Whiteson?

  And we should see lots more of these. The Nancy Grace Roman Space Telescope will measure the deflection of background star's position to determine rogue planet masses. So we have like a lot of observations of rogue planets coming up in the future. But again, we think that rogue planets are not rare. We think that probably the Milky Way has billions of rogue planets. Not like seven, not like 42, not 65,000, but billions of rogue planets.

• 47:47Kelly Weinersmith?

  Wow, okay. And so, all right, so now we've got billions of data points, as I imagine them, out in space. Are they easier to study or harder to study than planets that are part of a solar system? Because I imagine they're not being like drowned out by their sun. On the other hand, maybe some light from a sun is helpful to view them? Like, how?

• 48:07Daniel Whiteson?

  Yeah.

• 48:07Kelly Weinersmith?

  Tell me about studying these guys.

• 48:09Daniel Whiteson?

  Yeah, you basically nailed it. It's easier and harder. So it's easier in some ways because they're not drowned out by their star, but it's harder because that star can illuminate them. Like, some studies we do on exoplanets require the light from that star. We can, for example, measure that light going through that planet's atmosphere and then use that to detect what's in the atmosphere of that star by which light's been absorbed and which light has not been absorbed. So that's a cool study you can do on an exoplanet around another star that you couldn't do on an exorogue planet. But exorogue planets, you could see their infrared light directly, which means you can do things like measure the surface temperature of that planet, you know, and understand what light it's emitting and all sorts of cool stuff. So, yeah, you can do different kinds of science.

• 48:50Kelly Weinersmith?

  Amazing. So what other kinds of science can we do now that we figured out that there are rogue planets and rogue stars?

• 48:56Daniel Whiteson?

  Well, it really helps inform our model of solar system formation and galactic formation, right? To understand how this happens because our models should predict this. Then we should go out and check it and see that we see the number of planets and stars that we are expecting. Remember, there's still a lot of uncertainty because a lot of the steps here do have direct observation to support them. But some of them have some extrapolation that relies on models, right? Like, what fraction of these things should we see? And so, you know, over the next few years, we'll get better and better and these estimates will get firmer and firmer. But I think we can be confident in the bottom line that there are a lot of them.

• 49:31Kelly Weinersmith?

  By the time this episode comes out, everybody should have their physical copy of Do Aliens Speak Physics? In their hands and they would have had at least a month to read it. And so I'm sure that you are all dreaming about aliens as much as I have been since reading the book. And so you're probably wondering, could life evolve on rogue planets? And what would it be like and could they communicate with us? So, Daniel, is there life on rogue planets?

• 49:56Daniel Whiteson?

  I certainly hope so because it would provide the kind of alternative aliens that I like thinking about in that book. You know, ways that aliens might have evolved that are very different from ours. That would lead them to explore and understand the universe very differently. Like, imagine evolving on a planet like that. You have no seasons, you have no days, right? Because all these things require a star. The surface is going to be extraordinarily cold. The atmosphere might be snow. It just might freeze and fall to the surface. The whole, like, top mile of an ocean could be frozen. So you're more likely to evolve, you know, underneath a very frozen ocean. If you're kept warm by like the internal geothermal of that planet. You know, on Earth, something like 99.7% of our energy comes from the sun.

• 50:42Daniel Whiteson?

  And so it'd be very difficult to evolve in that scenario. But if you do, you'd have a unique perspective on the universe for sure.

• 50:49Kelly Weinersmith?

  It is really good to live on Earth, which I think is the message that we come to at the end of almost every episode where we talk about space. It is pretty solid to be right here.

• 51:00Daniel Whiteson?

  It is. I love that we get to live on Earth. It's a pretty cushy place to evolve. But I hope that aliens are out there and they had a very weird experience because talking to scientists from that planet could give us a very different view on how the universe works. You know, if you evolve on a rogue planet, maybe you're not interested in stars and you haven't focused on solar systems. You have a very different way of seeing the universe and the galaxy. Maybe you don't break it up into the same units that we do. And so you've come up with alternative explanations or gone down different paths, which of course could just inform our joint understanding of the universe.

• 51:32Kelly Weinersmith?

  Well, I hope that one day we meet aliens from a rogue planet.

• 51:37Daniel Whiteson?

  And that they're not rogues. They're very friendly.

• 51:39Kelly Weinersmith?

  That's right. That's right. But Daniel will meet them anyway, even if they're gonna eat him, as long as he gets to ask them about physics first.

• 51:46Daniel Whiteson?

  That's right. Maybe they'll be part of the extraordinaries.

• 51:49Kelly Weinersmith?

  I hope so. How could they not be?

• 51:52Daniel Whiteson?

  All right. So thank you very much for going on this trip with us out into intergalactic space and into interstellar space to imagine where stars and planets could form. If you have questions about the universe, please don't be shy. Write to us. We love tackling your questions on the pod.

• 52:06Kelly Weinersmith?

  And thanks to Steve for his question. Let's go to Steve and see what he thought of the episode.

• 52:11Speaker 04

  Yikes. One in five planets or stars are rogue? That is way higher odds than I would have given. What I thought was a random idea is estimated to be quite common. Who would have thunk it? As you have pointed out, this is yet another example of our human bias when thinking about the universe. Thank you for the answer to my question. Now with this new knowledge, I'm hoping to see a hypervelocity star whiz by Earth sometime soon. Cheers.

• 52:47Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 52:52Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 52:58Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 53:05Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 53:11Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms, you can find us at D and K Universe.

• 53:21Daniel Whiteson?

  Don't be shy. Write to us.

• 53:25Speaker 20

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• 53:52Speaker 19

  Hey, this is Mario Lopez from all Mario Lopez Daily Podcast. Buy smart, drive for less, and trade for more. That's Toyota affordability. And it starts with a great deal on an all-wheel drive Camry, Corolla Cross, or a Corolla Sedan with the latest features in tech. Or test drive a spacious Grand Highlander or adventure-ready RAV4. And with so many hybrid models available, you can enjoy longer trips without all the stops. Well, Toyota's legendary resale value means you could get more at traded. So get a great deal today and great value tomorrow. Shop Toyota.com for deals and details. Toyota, let's go places.

• 54:29Speaker 17

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