How We Went From 5 Planets To 5 Trillion

How did we go from knowing a handful of planets in our own solar system to confirming thousands of worlds around other stars?

This episode traces the story of exoplanets, from ancient “wanderers” in the night sky to the modern methods astronomers use to detect worlds we usually can’t see directly. We talk Galileo, Jupiter’s moons, hot Jupiters, pulsar planets, lava worlds, water worlds, rogue planets, Planet 9, and why the universe keeps handing us planets we never expected.

Hosted by Dr. Dakotah Tyler.

Lead with curiosity.

Transcript

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Welcome to Dr. Star Kid. I'm your host, Dr. Dakota Tyler. Today we are going to talk about one of my favorite, if not my single favorite story in all of science, which is the story of how we went from knowing about a handful of planets that are nearby that humans could see from Earth, to today, where we've confirmed over 6,000 and potentially, potentially knowing that there are trillions of planets in our galaxy, quadrillions, sextillions in the universe, just unimaginable numbers. Some of those planets orbit dead stars, which sounds impossible. Some of them get so hot that the rock vaporizes and turns into a little quasi-atmosphere. Some of the planets have tails like comets. Some of them are just floating through space with no stars at all, just invisible, you know, just kind of meandering around the galaxy. And to understand this, you really have to start at the beginning to fully appreciate what we can do now when we talk about exoplanets, when we talk about in general, just what we know in astronomy. You kind of got to roll it back and think about the early days. I'm gonna tell this story from the perspective of early Western astronomy, which is a specific brand, right? It's like in the Mediterranean, it's the Greeks and the Romans, and they're you know, every culture had their own version of astronomy, their own mythology, their own relationship with the night sky. And it's not hard to imagine why that was the case. I mean, if you look at the stars each night, they don't really appear to change much. But if you look at them over the course of the year, they actually change a lot, and then it repeats every year. So, in a sense, looking at the night sky is like a built-in calendar. You know, the the the heavens, if you will, provide humans or you know, anybody who has the wherewithal to look up and make observations and keep track of what they're seeing a calendar. Now that could be super useful, right? Especially if you're tracking, say, the migration patterns of certain animals who are always gonna do the same thing around the same time of year. Or if you're trying to think about, you know, when is the best time to search for this type of fruit or this type of plant or vegetation, whatever. And it becomes it becomes so helpful that you can't help but think that, you know, the gods set this up, or you know, uh again, all different cultures, and this is called cultural astronomy, have a tight relationship historically with the night sky. Today we'll start from the the Western version because this is kind of the version that has propagated and you know is closely related to the progression that leads to like modern academic spaces in astronomy and astrophysics. I'll have another episode at some point where we talk about some of these other um some of the other mythologies and cultural astronomy perspectives. My favorite, which I'm already getting distracted a little bit, my favorite is the Polynesian Wayfinders. So these would be the peoples of Polynesia, um, in the Pacific Ocean, who were able to use the stars to navigate in the middle of the Pacific Ocean. I mean, I don't know if you've ever seen Hawaii on a map. It's like smack dab in the middle of the largest ocean on earth. You just imagine the precision, imagine the feel, the intuition that you have to have about the rotation of the earth, about how the stars move in the night sky, about how the ocean feels when you're getting closer to a shore or farther away. It's really remarkable, but that'll be on another episode. So, you know, the word planet comes from Latin, and it basically means wanderer or to wander, wander with an A. And the reason that they called these things wanderers is because the planets stand out with respect to all the other stars in the night sky. The stars, night tonight, don't really move much. I mean, the amount that they're moving is so small that you know the human eye can't really detect it. You need a hyper uh precise instrument to be able to see the difference in a given star tonight from yesterday. And of course, the reason that it will appear that that star has moved night tonight, because the earth is going around the sun. So the stars that you can see in the summer are not going to be the exact same stars in the exact same place that you can see in the winter because the earth is on opposite sides of the sun. So you just you literally can see different stars. Uh, very, very important. Also, only makes sense if the earth is going around the sun. Um people have known that forever, so uh I got but I guess flat earthers still don't. All right, so they notice that the planets, however, they seem to move a lot more than those background stars. Why is that? Night to night, a planet will be in a visibly different spot, not not super far, but it'll be it'll it'll have moved. And then every once in a while you'll notice that the planet, which was kind of creeping across the sky, will will slow down and then start to trace back and do a loop. This is called retrograde. It's when you know a planet literally changes the direction and then it keeps on going. And this is a bit of a mystery for a while. Like, what's what's going on there? Uh, at first, people didn't really know. Eventually, they tried to come up with this idea of something called an epicycle where they sort of thought that the earth was at the center, geocentrism, and everything orbits around the earth, which makes sense. It makes sense, it kind of looks like that, right? I mean, the sun comes up, goes down, the moon comes up, goes down, the stars, you know, they move over the course of the night. It makes sense that you would think that in the beginning, but there's some weird things, like this epicycle. Like, why is a planet out there doing this this loop? And again, they you know, they didn't know what what planets were. You know, they named them these wanderers, and so the planets move a lot more relative to the background stars. They also considered the sun and the moon to be planets, again, because by planet they meant this wanderer, this thing in the sky that's not like the other things in the sky, which are those distant stars, which are so far away that any motion will not appear to the naked human eye because it's so far away. You know, the idea, the concept here is called parallax. You know, you're going on driving on a highway, and any trees that are right outside your window, they look to be zipping by. But if you see mountains off in the distance, the mountains are not moving fast at all. Well, you're moving the same speed. What's the difference? The difference is they're farther away. So the amount that you have to move for them to shift in your field of view is going to be much greater. Same idea with stars. You know, the planets are moving nowhere near as fast as the stars, by the way. The stars do move very fast relative to our solar system, but again, they're so far away that it it looks like they're not moving at all. Kind of like an airplane, you see an airplane, it's not clear that it's going 400 miles per hour, but that's how fast they're going, you know, when they're at cruising altitude or something like that. New Mercury, Venus, Mars, Jupiter, Saturn. They knew about the sun and the moon, obviously. This is where we get our days as well, right? Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday. These are one day named after each one of the planets. Now, eventually, we come to know that the moon, the sun, and the planets are something very different altogether. And also that the earth is a planet, which they didn't know that. They called it Terra, meaning dirt. Literally, like there's the stars and planets as out there, astra, what they called them. Stars, planets, and then there's dirt, it's like the ground beneath us. But they didn't know that they were just on a planet, having this perspective from a planet, seeing other planets. But if they could, for example, stay on other planets, they would see the Earth, and that would also be considered a planet to them. They know about some of the planets, but not many. Then we invent these telescopes and telescopes, which are just like glass that's shaped in a way that can help focus light, essentially. And if you think about it, if you think about what your eye does, your eye, you know, has a little hole, and a bunch of light goes into your eye, and it kind of collects back in the back of your head, and your brain is like reconstructing these images of things in the world based on the incoming light. Well, if your eye was much bigger, you would be able to collect much more light, right? This would mean that you could see things better when it was dark, for example. You could see things that were dimmer better. And you you know that this is how your eyes work because at night, you know, your pupils will dilate, they'll open up, and after they adjust, you can see better in the dark. And then if somebody flips on a bright light, it hurts because you're getting way too much light. So it's pretty cool how our eyes evolve to be able to do something similar. But as astronomers, I mean, you just make a really big telescope, you can see something that's really far away and it's really faint. And the things that are farther away are fainter because light spreads out as it moves, and as it spreads out, it just becomes fainter. So if you you have a huge telescope, you can collect all that light and make it as though you're closer to it. That's kind of a way to think about it. All the light spreading out, but if you collect it all and put it back together, it's almost like you were much closer to the thing. Okay, we know now that we can see these objects a little bit better. And there's a number of important observations that were made, but one of them that tipped off, folks, that the earth can't be the center of the universe, geocentrism just doesn't work, is that Galileo he looks at Jupiter and he sees he sees these four little dots around Jupiter, and he's like, you know, he doesn't know exactly what those are, but it would make sense if you know Jupiter was a huge planet that those were little moons, and the moons appeared to be orbiting around Jupiter, but that it didn't make sense with the geocentric world, right? The Catholic Church, the the the ethos, the mythos of the Catholic Church and its relationship with the universe is this is a special planet, it's at the center of everything, God made it at the center of the universe, everything orbits it because we are the important things in this universe. Everything else orbits around us. That does not leave room for the sun being at the center of the universe, or the galaxy, the center of the galaxy being at the center of the universe. So there's this there's this inherent mismatch with what the way that the universe actually is and the narrative that people that the people who had a lot of power at the time want it to be perceived. Okay, so that's a whole other thing, and um it would actually be really cool if I could find an historian of the period to dig into that, because I don't really want to want to get lost in that right now. But alright. You see these little moons that are orbiting Jupiter, and this is very this is very important because you know if moon if if something can orbit Jupiter, then that means that everything isn't orbiting the earth, everything doesn't have to orbit the earth. Things can orbit Jupiter. I mean, why can't things orbit anything else? Things should be able to orbit the sun, they should be able to orbit Saturn. Of course, this is the case. So we make some more observations. A guy like Isaac Newton invents calculus and comes up with this idea for gravity that works really well and makes a lot of sense and can explain a lot of what we see, not everything that we see, but a lot of what we see in astronomy. And when we start to understand the way that planets orbit, um, you know, contributions from lots of people, uh, like Kepler, who discovers that planets don't orbit in perfect circles, but they orbit in ellipses. Again, this is a big deal because at the time they're thinking this is a perfect universe with Earth at the center, and everything is perfect spheres and perfect circles and perfect orbits. And no, there's no perfect orbit, there's no circular orbit. None of them are perfect circles, they are all slightly elliptical, right? This, like, this is a very cool theme, is that in perfection, in the can in the human constructed, uh, the human-constructed hubris universe, it's all it's all perfectly designed, right? Um, but in reality, that's not the case. The planets aren't perfect spheres. They don't orbit in perfect circles. It's not true, they just don't. I think it's very interesting. And I think that there's a deeper, there's like a deeper lesson to draw from about inherent imperfection and that being not a flaw, like that's not a stain. Like nothing in the universe was was perfect. A bit of a side sidetrack, but the way that matter was distributed in the early universe was not perfect, it was slightly imperfect, and that slight imperfection allowed for gravity to collapse things and things to jumble up here and there and eventually get stars and planets. If the universe started out perfectly evenly distributed, which is what you may expect if you think that you know everything's perfect, then nothing ever would have happened. You'd never get it, have gotten stars, you'd never have gotten planets. So imperfection, it's baked in and it's necessary for interesting things to happen like this. Okay, convinced that the sun is actually at the center of the solar system and all the planets orbit around it, the Earth also being a planet, the third one to orbit around the sun. And you see that, you know, we we we we see Mercury and we see Venus and Mercury's orbit, it doesn't, it doesn't exactly obey what Newton thinks that the orbits should do according to his equations. That's kind of weird. There's no answer for that yet. It won't come until Einstein's theory of general relativity, which I don't want to get into, but it's about gravity being the curving of space and time, both space and time, leading to what we call gravity. But Newton's equations are good enough to uh pick up on inconsistencies in the outer solar system. So we see Jupiter and we see Saturn, but Saturn doesn't exactly move the way that we think that it should. It moves as though there's another invisible object out there that's like tugging on it gravitationally, one way or the other. And so the way that you explain this, if we understand gravity, we should be able to make a prediction. Hey, there must be a planet out there that's causing Saturn to act this way. That's any good theory is predictive. If you have a theory that cannot predict the future, it's not a very good theory. You don't understand things very well. Sure enough, look with telescopes as they get better and they find Uranus. There is a planet that's tugging on Saturn. We you you saw that it belonged there, you predicted that you'd find it, and you find it. And then they observed Uranus, so now our uh planet count is up to seven. They observe Uranus, and they see Uranus doing the same thing. It seems like there's something beyond it that is affecting it in a way that cannot be described by just the inner planets and the sun in an orbit going around. It seems like there's another planet outside of Uranus. Telescopes get better, more sensitive. You predict that you find a planet and you find Neptune. There is another planet out there. Okay, it's so crazy. Then we make observations in Neptune, and they see the same thing. They see the same thing. Neptune is not exactly behaving like it seems like it should, if the only gravitational influences are coming from the inner solar system relative to it. So Uranus, the other planets, the sun. And they predict again, there must be another planet out there outside the orbit of Neptune. What do we find one day? We find Pluto. Now, this is a mistaken identity. Pluto ultimately, and this is the reason it's not a planet. Well, one of the reasons, it's not massive enough, and so it actually can't have the gravitational effect on Neptune that we that was observed initially. So Pluto is not not that massive, has kind of a weird orbit that's tilted. Um, we would say this is an obliquity, it's like a tilt with respect to the rest of the solar system. Uh, most of the planets in the are in the same plane, and then there are other massive objects that are kind of in its orbit, and I think it crosses Neptune's orbit at one point as well. So, dwarf planet, and not exactly what was responsible for the weirdness of Neptune. But then, okay, so then what was it? What caused Neptune to not orbit the way that we that we thought that it should? Is there another mystery planet out there that we just haven't found yet? And this is, you know, this is an active area of research. It's called Planet 9, and it would be a five, six Earth-mass planet, so like five or six times the mass of the Earth, orbiting at a very, very far, very far orbital distance, something like uh like a 20,000-year orbit to get around. So very, very far. If I had to guess, you know, I've seen the evidence is not conclusive, but it is suggestive. I wouldn't be surprised if there was a planet that was out there that was very massive, that's just so far away that no sunlight is reflecting off of it. So it's really hard for us to find. You know, space gets bigger the further away from us that you get, right? Because it's like this expanding sphere. We find out that there are all these planets. We suspect, now that we know over time, that all the other stars are stars. You know, it seems like physics is the same everywhere. Um I'm sure there's nothing particularly strange about our solar system that led to the formation of planets that wouldn't happen somewhere else, right? Like there's these other stars should also have planets. Now, the issue is finding them. How do you find a star that has planets? How do you find the planets? We can see Jupiter, Venus, and the night sky, Mars, but why? Because we can see them as Night. We can see them when the sun is behind us. Importantly, you can't see them during the day, right? Any day, if you look up at noon, you know, Mercury is orbiting the sun, it's on the shortest orbit. Mercury is up in the sky. You can't see it. You can't see it because the sun's too bright. Well, for us, during the night, we can see whatever planets are on that side, you know, behind our planet with respect to the sun. But when you get far enough away from a planet and a star, no matter when you look at it, the planet is always going to be next to the star. So this makes it incredibly difficult to see. Any given star system that we know of that has planets, they're so close to each other compared to how far we are away, hundreds of trillions of miles, light years in all cases. You know, the closest star to us is about four light years away. A lot of the stars that we study that have planets are hundreds of light years away, maybe thousands of light years away, definitely dozens. So, you know, how how can you possibly do it? Like, how can you see a tiny planet that's reflecting a little bit of starlight when you have this just super bright ball of light shining? And this is where astronomers have always been and continue to be extremely clever. And I am biased, but you know what you have to do to get information about the universe, particularly planets orbiting stars, is be very clever. You have to understand physics of light, orbital mechanics, and you have to leverage what we know about instrumentation and physics and quantum mechanics to tease out information that can allow you to reasonably infer that there are, for example, planets. Seeing a planet around a star is analogous to being in LA, which is where I live, and trying to find a firefly that's flying around a stadium floodlight in New York, like the other side of the country, a a floodlight, a firefly, and then I'm way over here in LA trying to see it in New York. It's very difficult, but not impossible because those things do exist. The firefly is there and it's doing something, right? I may have to have a super, super, super precise, sensitive instrument. Yeah, maybe. But it's not, it's not impossible. Sounds impossible, but it's not impossible. And in fact, one of the methods that you could do is you could imagine, you could imagine that the Firefly, which is a little bug, right, sometimes will buzz right in front of the floodlight from your perspective and block out a tiny bit of light, a fraction of a percent. And if you are staring at that floodlight long enough, you could witness that. And if the Firefly were buzzing around in a circle, you would see periodically that a fraction of a percent of the light from that stadium light dipped. And you could infer, oh, you know, something may be flying around it. This is one of the ways that we find planets. It's called the transit method. Every once in a while, a planet that's orbiting will come in between you and the star, block out some of the light. You can infer that there's a planet of a certain size there. Another method called the radial velocity method, a planet causes a star to to wobble. Like intuitively, we know that stars pull on planets, which is why they orbit, and that's uh directionally, I think, are the right way to think about it, even though really it's like space-time curving. But it's not just that stars are causing the planets to orbit them, the planets also exert a force on the star. So the planet causes the star to do a little tiny orbit as well. And if you have a sensitive enough telescope, you can see the star moving back and forth. Again, you can infer the presence of a planet. But this was not the way that the first planets were found. The first planets were found orbiting a pulsar. A pulsar is a neutron star, it's a remnant of a very massive star that explodes. The most massive stars, when they run out of fuel, they collapse. The collapse happens so fast, it like compacts the core, they rebound and explode, and what's left behind is a neutron star. And some of these neutron stars have these pulse beams that shoot out from the poles, and it's kind of like a lighthouse whipping around. Now, if you look at a pulsar, the beam faces you maybe once a second, maybe 10 times a second, maybe once every two seconds. The rotation rate is so fine that you can you can bank on it. It's always gonna be the exact same. And they were looking at this pulsar, and the rotation rate appeared to be changing slightly. It's as though, it's as though sometimes the pulse was coming sooner, and sometimes it was coming later. Like, why is that? Why would the pulse arrive sooner sometimes and later sometimes? The way that you would explain that is that there are planets orbiting this thing, and the neutron star is doing that little wobble. And so the light that's being emitted from the neutron star is having a slight delay or um speeding up relative to what you would expect as it orbits around. So the question is, how did planets get around a dead star? A massive star that will become a neutron star is gonna be so massive that it's gonna burn through its fuel really quickly. And if it burns through its fuel really quickly and only lasts tens of millions of years, how does it even form planets? We haven't really talked about it on this show yet, but it can take tens of millions of years, even a hundred million years, to fully form a planet. Well, if the star blows up before planets are done forming, how are planets surviving around it? How are planets surviving the explosion? You would expect that they would get vaporized by the explosion. That's not the case. Apparently, there are planets around pole stars. Still a mystery how these planets are existing there, by the way. Did they survive the explosion? That seems that seems far-fetched. I mean, it's hard to believe that those really, really massive stars even form planets, but maybe they do. Could they survive the explosion? A supernova, one of the most energetic uh explosions in the universe, hard to believe. I think the the leading theory, the one that most astronomers suspect is right, is that the there's the second round of planet formation that happens in the explosion. After the planet has or after the star has gone supernova, there's like this second round of planet formation that happens around the neutron star. Very, very cool. Very cool. Nobody expected that, by the way. Now, I don't think that you could ever have life on a planet like that, but who knows? Then we continue looking, and in 1995, when I was five years old, astronomers found a planet that was orbiting a sun-like star called 51 Peg B. And this was huge. I mean, this was an amazing discovery, but there was something weird about this planet too. It was the size of Jupiter, but it orbited its planet in just a few days. Wait a second. How is a gas giant as big as Jupiter orbiting a planet in just a few days? You know, the idea behind our working theories at the time, behind why the distant planets were more massive, the gas giants, Jupiter, very massive, Saturn, very massive, Neptune, Uranus, very massive planets, all much more massive than the Earth. The reason that they can get so massive is because they form far away from the sun. If you're close to your star, ices will vaporize. So, like carbon dioxide, methane, water. If you freeze those things, you can pack a lot more material in a small volume. But if you're close to the star, that stuff can't freeze and it vaporizes. It's a gas. So the further away you get, you have more ices, you just have more material to work with. And so those planets can get far more massive. Then how do you have a Jupiter-sized planet that's orbiting a star in just a few days? That just doesn't make sense based on how we think planets form. Then we have to refine our understanding of planet formation and migration. And oh well, maybe what happens is maybe what happens is that these big huge gas giants, as they're like plowing through the disk of stuff that is in a young solar system, they kind of lose momentum and they and they start to slide towards the star. And for some of them, they can slide all the way in and become a hot Jupiter. Now it's a good thing that that didn't happen in our solar system because it would have slid right into where we are and either ate us up or threw us out of the solar system or something like that. So lucky us that that didn't happen. But we find these hot Jupiters. Huge surprise, didn't think we'd find them. Only thought we were gonna find planets that were like the ones in our solar system. But so far, we found planets orbiting pulsars and then these hot Jupiter planets, just unheard of. So at first, we started slowly finding more and more planets, always bit ones that were big and large and massive, because they're easier to find. In fact, we have something called a detection bias in astronomy. It's easiest for us to find large planets that are close to their stars and very massive because they have easier signals to spot. Let's revisit the analogy of the floodlight with the firefly. Well, let's imagine that instead of a firefly, we were trying to find a gnat, which I think is smaller than a firefly. You know, them little fruit gnats that you get inside. Well, if there's a gnat buzzing around the spotlight, it still leaves some signal, but it's even harder to find. And then compare that to what's a huge bug? I don't know my insects for real. Let's imagine it was a what's the biggest bird uh uh that can fly? I almost said ostrich that can't fly. An albatross. I think that's the largest flying bird. Now imagine there was an albatross flying around the spotlight. It would still be hard to see because I'm in LA and the spotlights in New York, but it would be easier to see. The idea is simple. Large planets, massive planets, they're easier to find because they leave more findable signals. I mean, it's literally that straightforward. Okay, so as we get better and better and better at detecting these things, we get better and better at finding smaller planets that are further away. So now everything that we find isn't a hot Jupiter. We send out the Kepler mission, it's looking at the same stars every night for years, trying to find those little dips. And we end up finding several thousand planets from Kepler. And a lot of the research that I did in my dissertation was using that Kepler data set and some of the newer ones as well. At this point, we know of I think 62 or 6,300 exoplanets, and I will just talk about the categories of planets that we know. So I already said the hot Jupiters, so we know those exist. They're the super, super common planets called super Earths and sub-Neptunes. These are the most common planets out there. Most stars that have a planet have a planet that's slightly larger than Earth or slightly smaller than Neptune, in between the sizes of Earth and Neptune. And it's very interesting because we don't that's the exact that's the exact type of planet that we don't have in our solar system. We don't have a super Earth or a sub Neptune. Why is that? That's an interesting question. And you know, it's interesting knowing what you find. It's interesting also knowing what you don't find. Our solar system seems to be special, at least in this way that there are humans on it that can build, you know, telescopes and computers and you know submarines or whatever. And does something about having a planet that is favorable for life, does it is it connected to not having a super Earth or or sub Neptune? I don't know. I don't I can't imagine a mechanism in which that's the case. I don't really know. But is that maybe a question that is worth asking? And maybe one that has, you know, the answer has some sort of information that's informative in one way or another. I don't know. It's possible. Don't really know. So super Earths, sub Neptunes, those can be very hot, they can be close to their star, they can be cold, they can be far away. Uh, lava worlds, this is an interesting type of planet. We know of many of these, like rocky planets that are so close to their star that the surface literally has liquid rock. So Mercury is the closest planet to the sun and it has an 88-day orbit, but Mercury really isn't close in terms of averages for most of the planets that we find. Most of those super Earths and sub-Neptunes that we find are on orbits that are like under 50 days, so much closer. And if you take a rocky planet and put it close enough to a star, the rock is gonna melt. If it's close enough, the rock is gonna melt. If it's close enough, it may also vaporize, right? It may not just be liquid, it may vaporize to dust, and you could get like these quasi rock atmospheres, vaporized rock atmospheres around these hot, super hot, rocky planets. It's very interesting. Literally, like a hell, a hell planet. It would literally be like a hell planet. One of the more interesting types of planets that we can't even honestly really confirm are called water worlds. So the idea of a water world is probably not what you're thinking. When you hear water world, you know, it's like, oh, we got these oceans on Earth. Maybe there's just bigger oceans. Kind of. But if you really think about it, 75% of the Earth's surface is covered with water, yes. But that's just the surface. If you were to remove all of the water from the surface of the earth, you actually wouldn't change the mass of the earth at all. I think it's like 0.02% of the mass of the earth. So, yeah, we have water on the surface, we have vast oceans, but most of the planet is basically rock. There's kind of like a thin film of water on the surface relative to the overall mass of the planet, like right, like relative to that whole volume, to all that mass. But there are these other planets that seem like they're significantly comprised of water. A water world may be a planet that instead of having 0.2.02% of its mass and water, it has like 10 or 20 or 30% of its mass and water. Like we're talking about a fifth of the planet is literally just water. And that's very interesting. Because, yeah, there's probably a vast ocean somewhere, but a planet like that wouldn't have land. Like there wouldn't be a shore, you know, there wouldn't there wouldn't be land on a planet that was 30% water, it wouldn't have land. It would it would have a thick atmosphere. Sometimes we call these Hycian or Hycian. Hycyan sounds more sophisticated. Uh planets that that are like water worlds that that have like these H2O and hydrogen atmospheres. You almost think of a a planet sort of like uh Uranus or Neptune, where there's lots and lots of gas on the outside, and as you go in, there's more and more water. And at some point, you would expect that that water is liquid, and there is maybe something like a classic ocean. We really don't understand enough about these planets, and there's you know, people who do this the theoretical work of modeling planet interiors, and it's I mean it's hard because nobody would have guessed that planets like this even existed, but they may. And the reason I say they may is because you know, we're limited in what we can determine about a planet. We can find out the size and we can find out the mass, you know, like if you were to put it on a scale, what's the mass? That's different than the size, right? The size of this football is however long it is from here to here, but the mass of it is different, right? It's like how much does it weigh on a scale? So those are two different things the mass and the size, two different things. But if you combine them, you can get the average density. So I can tell you the density of this thing, right? You know, there's mostly air on the inside, but there's like material on the outside, and it has some average density. And when we come up with the average density for these planets that we think may be water worlds, you get that it can't just be rock like the earth. The average density of the earth, I think, is around 5.5 grams per cubic centimeter, which is basically a rock. So if you took the average density of the earth, you would have no idea that there was water on it. You really wouldn't. Uh luckily there is, luckily for us. But the average density of a water world, it's clearly not rock. It's also clearly not gas. Somewhere in between. And there's many ways that you could replicate that, right? It could be a small, rocky planet with a thick, puffy atmosphere. It could be a big planet that's like got a lot of water in it, a water world. So it's hard to determine. I don't think we've conclusively determined a water world yet, but there are several planets that are interesting candidates. And I think something that's interesting is what type of life, if any, could survive, thrive, evolve in a planet that was significantly comprised of water. What would that life look like? Is it is it even reasonable to consider ocean life on the earth? And you may think, yeah, you know, there, you know, maybe there'll be things like whales or or something like that. But like, no, I don't think that's reasonable, right? What I mean, what is a whale? Cetaceans, they evolved from land animals. That's why they breathe air, right? They breathe air because they evolved from animals that had lungs. Animals have lungs because they live on land. But a water world doesn't have land, right? So you wouldn't you wouldn't expect to find an animal like that. You can't have a land animal on a water world, possibly, if there's no land, then what would those animals be like? And so you may say, well, it would probably just be like other ocean animals, right? Like maybe just not the dolphins and the whales and and that sort of thing. But I don't know. I don't know. Because we don't know how life started on Earth. We susp you know, there's a handful of abiogenesis, which abiogenesis is the term for like the origin of life. A handful of hypotheses, but we don't know which one is right. And so it may not be possible to even form life. If you don't have land on a planet, maybe that has nothing to do with it. Maybe you can, uh, maybe you could form a different type of life. So I don't know. I think that that's an interesting thing to think about. The type of life, if any, that could evolve on a water world. I think another cool thing is you can have planets that are orbiting multiple stars. That's very interesting. This is like the Star Wars uh Tatooine style planet that you know you're standing on the surface, you look up, and there's multiple stars. Now, I don't know if a planet like that, right? It's a it's a very different environment. If we doubled the amount of sunlight that we got, Earth would be cooked. It would be way too hot, right? So, you know, there's there's a lack of stability, I think, that comes with orbiting um two stars or being a planet that is dealing with multiple suns. But again, you know, it's like the universe is a big place, so you you really you really don't know for sure. At most, we can make an educated guess. So I think that those are interesting, and then some of the scariest planets. Scary, not in terms of danger, but in terms of like blown potential, I think, are rogue planets. So a rogue planet is an exoplanet that is not orbiting a star, it's just free-floating through space. And the estimates based on how planets form and get ejected from young systems is that there's more rogue planets, there's more free-float free-floating planets out in space than there are planets orbiting stars. That's crazy. Interesting hypothesis, this is speculative. This is speculative, so don't, you know, I'm saying, don't take this to the bank. Interesting hypothesis for planet nine is that planet nine, maybe, maybe, is a planet that formed closer to the sun and was a super Earth or sub Neptune, and for some reason got kicked out to the outer solar system, and didn't quite have enough energy to leave, so it kind of just stays on a super distant orbit. One possibility. Another possibility, captured rogue planet randomly. You this would almost never happen, but it must happen sometimes because the universe is so vast because the galaxy has so many stars, even you know, one in a billion, one in ten billion odds will happen sometimes. That a rogue planet randomly happened to wander too close to the sun, and now it's kind of trapped in this super slow moving orbit that's on a 20, 30,000 year orbit around the sun. Again, speculative, we don't really know. I think it's very interesting though, and honestly, I think it makes sense. I think it makes sense that there are probably multiple. This is again, this is my speculation, that there are probably multiple planets that are way, way beyond Neptune that we just haven't found yet. I don't know what that number would be, but it wouldn't surprise me if there were a planet 9, 10, 11. Wouldn't surprise me. Just because we know how often planets get kicked out of solar systems. Just would not surprise me if some of them didn't quite have enough energy to fully escape and are just on these super distant orbits. Very interesting. We don't know a lot about exoplanets because they're hard to find and it's hard to get more information about them besides if they have like a thick atmosphere. Sometimes we can read what's in the atmosphere. Pretty easy to get the size, pretty easy to get the mass in most cases. So that gives you the average density. But again, like I said, uh an astronomer that could get the average density, an alien astronomer could get the average density of the Earth and would not know that there was water on the surface. And that may be something that an alien astronomer would be interested in. That's certainly something that Earth astronomers are interested in. Does this little rocky planet have water? If it has water, maybe it has life. Don't really know. Very, very, very interesting though. We're just scratching the surface on what we are able to determine about planets, and we're like constantly finding surprises. So it's a very exciting time, and some of the new age instruments that are coming out are going to be able to find thousands and thousands of more planets. You know, there are some estimates that in the next 10 years or so, the number of planets that we know of may increase by like a factor of 10. So I think we're at 62, 6,300. You can go check on if you Google NASA Exoplanet Archive, you can see the live counter for the number of planets that we have confirmed. Last time I saw it, I think it was 6300. Uh maybe within the next decade, that number is 63,000. Very, very interesting. Very, very interesting. All right, that's it for this episode. Thanks for tuning in. Always, always, always lead with curiosity.