Daniel and Kelly’s Extraordinary Universe - What is the densest planet?

Episode Date: December 1, 2022

Daniel and Jorge wonder about which planets would float if you dunked them in a cosmic tub.See omnystudio.com/listener for privacy information.

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He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want or gone. Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Why are TSA rules so confusing? You got a hood of you. I take it all. I'm Mani. I'm Noah. This is Devin.

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Hey, Jorge, are you ready to dig into another controversial topic? Uh-oh. Did you take some classified physics documents? to your house or something? Oh, they don't let me see any of that stuff. But this is much more important than that. I'm wondering where you land on the debate about bread. Do you prefer it light and fluffy or dark and heavy?

Ooh, I didn't even know there was a debate about bread. Mostly I just eat it. I mean, if people argue about like chocolate versus vanilla ice cream, you know they're going to argue about French baguettes versus German rye. Well, it kind of depends on how hungry you are. If you're hungry, you'll eat any bread. But isn't that more of a cultural topic than a physics topic? It's a cultural topic, but it's also a physics topic because it's all about density. Do you like bread that floats or sinks when you drop it in the pool? It sounds

like you've done this experiment. It's a fundamental question about every object in your life. Will it float? Have you tried this on your children as well? Yes, and I can confirm that my children do float. And are they dense? Only the best way possible. Well, in terms of bread, I'd rather you don't throw it into a pool. I'm not sure I like my breath soggy. I'm sure there's someone out there who disagrees with you. That'll be a very dense debate. I'm sure I'll rise to the occasion. Hi, I'm Jorge McCartunist and the co-author of Frequently Asked Questions about the Universe. I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I really miss Dave Letterman's bit called Will It Float? Oh, is that the one where he makes things float or is that the one where he throw things out of the roof?

I love that he did so many physics experiments, but one of my favorites was Will It Float, where he just brings out a tub of water and random stuff and he asks the audience, do you think it's going to float? And then, of course, he throws it in the pool and they find out. It's pure physics. Right, right, yeah. What kinds of things would he test? Well, you know, we had bowling balls and chainsaws, but also, you know, loaves of bread, pumpkins, all sorts of things sort of right on the edge that divided people and tested their intuition. I feel like it's also kind of an engineering question, like how watertight something is. Because something could be light and float, but eventually you'll sink. Yeah, that's true, like a paper boat will float initially until it gets waterlogged. Yeah, it's all about engineering at the end.

Truly the most important discipline. But anyways, welcome to our podcast, Daniel and Horhe Explain the Universe, a production of iHeartRadio. In which we float various ideas about how the universe works, how it all comes together, what is made out of at its smallest bits, and how those bits to and fro and zip back and forth and buzz together to make the universe that we are familiar with, the one that we experience, the one we are so curious to understand. We dig deep into all the questions about the basic nature of reality and how everything. behaves and try to explain all of it to you. That's right, because it is a pretty dense universe, pack full of interesting and amazing facts and phenomena to discover and to explore. And we like to make it all light and fluffy for you here on the podcast and dipable, I guess.

You know, I was just Googling Will It Float and discovered that Letterman once released an Xbox version of Will It Float? What? Like, will it Xbox float or sync? Or can you play it inside the Xbox? That is a good question, whether an Xbox will float or sink. probably it'll sink. But no, he had a version of it, which was a game you can play on the Xbox.

Like you predict whether various things are going to float or sink. And then they dump them in a virtual tub. I feel like that's a little bit of an overkill for the Xbox's abilities here. I think maybe he was just playing a joke about merchandising and commercialization of stuff. How would your son feel if you threw his Xbox into a tub of water? He would probably jump in afterwards. And then I would learn whether it floated and whether he floated. At the same time, why did you throw both in?

You only need to throw one in and then he jumps in after it, so. As long as it's not still connected to the plug in the wall, it's not a bad exercise. But as much as we like to joke about whether things float, it's part of just wondering how things work and what's inside of them. There's a long history of dunking things in water to understand them all the way back to measuring the volume of things by immersing them in a bathtub. Yeah, and as you know, whether something floats or doesn't float, it has everything to do with its density. If something is denser than water, then it's going to sink. If it's less dense than water, it's going to float.

And the universe has all kinds of densities in its existence. That's right. And people have all kinds of densities in their bodies and stuff around us has all sorts of densities, which is why people have been building boats out of wood and not out of stone for thousands of years. You can make a stone boat, right? You could make a stone boat if it had a big air pocket in it, right? That's how those ships are made of steel. Steel, obviously, much denser than water.

but a modern steel ship has a huge pocket of air inside it and like a big steel balloon essentially. Yeah. And actually, if you study civil engineering, a big ride of passage in a lot of schools is to make a concrete boat. Is that right? So that's something people really do? Yeah. That's awesome. Wow. I wonder what the densest material you can make a boat out of is. Can you make a neutron star boat? Nuclear pasta boat. You could use sheets of nuclear lasagna, I suppose. Yeah. I guess it would collapse the entire. planet but you know at least he would test that theory you'd win that engineering competition along

the way and for those of you out there confused about why things that are denser than water can float the key is not the density of the container necessarily but the overall average density of the object so if you make a big balloon out of neutron star material as long as there's enough air inside of it the average density is less than the density of water and then it can float yeah and so the universe has all kinds of dense things we've talked on this podcast about neutron star which are some of the densest things in the universe. And we've also talked about the opposite, which is the emptiest spots in the universe as well.

The extremes are really fun places to think about because they show us what the edge cases are, what the rules are, what it's possible to achieve. And they also illuminate how various forces and various factors come together to achieve such densities or such emptinesses. So understanding how things can get really, really dense can give us some understanding about how things work.

Yeah, there are all kinds of forces in the universe. A lot of them are pushing things together like gravity or the strong nuclear force and a lot of things are also pushing things apart like the electromagnetic force. Density plays an important role in the heart of our sun allowing us to warm our toes on that incredible cosmic fire even though it's millions and millions of miles away.

It's density that creates the conditions necessary for fusion at the heart of our star. Yeah, and it plays a big role in our solar system. I guess especially here on Earth, right? If Earth was less dense than it was, it would probably, what, collapse or float away? Well, I think what's interesting is to ask how dense is Earth and why is it that dense? And then to look around to the other planets and wonder like, hmm, are they more dense? Are they less dense?

What's going on? It's part of understanding how our solar system came to be the way that it is and understanding whether it's unusual. Like is our solar system weird compared to other solar systems? Are other planets out there more massive, more dense? Are they fluffier than a French baguette? Are they denser than German rye? So today on the podcast, we'll be asking the question.

What is the densest planet? Now, my first question to you, Jorge, is do you think this is something to be proud of or something to be embarrassed about? Which one? What do you mean? Like asking questions or eating bread? If we're going to label one planet as densest, do you think that's something the denizens of that planet should be proud of? embarrassed by. Yes, I was going to ask you that you thought this was an appropriate question to ask.

I guess it depends. First of all, if there are people in other planets, that's amazing. But also, I guess if you're talking about their planet, then they wouldn't necessarily be offended. Yeah, dense has some interesting connotations, right? No person would want to be called dense because it implies that you're not very smart. Unless you're dense with awesomeness. Jam packed with wonderfulness. Dance with talent.

On the other hand, if you're talking about a planet, maybe it means that you're filled with diamonds and gold and all sorts of valuable heavy metals, right? A huge blob of platinum is pretty dense, and I'd rather take that than a blob of water. To drink? You mean to drink or to flirt it? No, when they're like assigning mineral rights in the solar system. You know, I'll take the denser asteroid, please, thank you very much. Because then you can do what with it? You can't live in it if there's no water in it.

You know, you can't live in it if there's no water, but there's plenty of water out there in the solar system. But, you know, some of those asteroids are just like huge blobs of platinum or other rare earth metals that we need here on Earth to make batteries for all of our fancy gizmos. Yeah, so there's a huge range of density in our solar system. I guess you can go from planets that are made out entirely out of gas, which is gas. And then there are planets that are made out of rock and metals, which are some of the densest things around. And the range of densities tells us something about how they, the solar system was formed and what's under our feet and the incredible balance between gravity and the other forces.

Well, as usually we're wondering, how many people have thought about the question, which is the densest planet? And so Daniel went out there into the internet to find out. So thanks very much to everybody who participates. If you've been listening to the podcast and you would enjoy speculating about the next topics for future episodes, please don't be shy. Write to me to questions at danielandhorpe.com and we'll set you right up. Think about it for a second. Which do you think is the densest planet?

Here's what people had to say. I think it would either be Earth or Mars, but I'm pretty sure there's more water on Earth than on Mars. So just based on that, I think Mars is the densest planet in the solar system. I'm going to say, just to throw it out there, the core of Jupiter or Mercury. Right now I'm thinking about all this. crazy exoplanets discovered recently, but I can only refer to our solar system. And from our solar system, the densest planet is Earth. Yes, it's Earth.

I'm going to go out on a limb and say that the densest planet is actually Jupiter. Because even though it is the most massive planet in our solar system, I'm guessing that there's Probably a part of it that's super dense to keep it all together. In our solar system, I'm going to guess one of the gas strands, the one with the most gravity. Yeah, let's go with that. All right. Some dense answers here. Not a lot of fluff here.

Yeah, a lot of variety in the thinking and in the answers. I love it. I like the people who argue that maybe we didn't specify enough that we meant planted in our solar system. Because, like, are we asking what's the densest planet in the universe or just in our little local neighborhood? Yeah, well, the physicist in me wants to know what's the highest density possible in a planet anywhere in the universe. And also, what is the most dense planet in our solar system? I want to know answers to all those questions. You're dense with curiosity.

But I guess maybe it depends on what you mean by a planet. Like, what's the definition of a planet? Like, can a neutron star be a planet? Like, if you lived on the surface of a neutron star, it would technically be your planet. I'm not sure. Maybe it would be your star. You would be the planet. But let's not get drawn into a 40-minute rabbit hole about the definition of a planet. That's a whole briar patch we don't want to get thrown into. Well, what's the one-sentence answer from a physicist? Well, the official definition of a planet is a body that mostly orbits the sun and has cleared its own path in the solar system. A-sun, right? Not just our sun.

Yeah, that's right. A-sun. Or maybe multiple suns, right? Because there are systems out there with binary stars at their hearts. Well, we have how many planets in our solar system? Eight planets? We have officially eight planets in our solar system and a bunch of dwarf planets also. One of them is the densest, and I guess one of them is the lightest planet. Or what's the opposite of dense? Fluffiest. Is that the official physics word?

I don't know what the SI unit is for fluff, but it's the opposite of dense. It's maybe measured in podcast episodes. It's measured in puns per podcast episode. Or chuckles. If we have too many chuckles, it's not a very dense episode. All right, well, let's dig into this, Daniel, and let's maybe start with just like an average planet. What are planets made out of in our solar system? So planets come in quite a variety of stuff, right?

The things that planets are made out of depend on where they were when the planetary system was formed. Because it all starts from one big blob of ingredients, but the process of planetary formation and stellar formation pulls different ingredients in different places. So where you are in the solar system, as things are forming, determines what you sort of get made out of. So it all comes down to understanding that process of how you go from a huge cloud of gas and dust and bits of rock left over from other solar systems that have now died and turn that into a new solar system. Right. And maybe just to take us back, we start out in space, right? And there's usually a huge cloud of stuff that came from the Big Bang or maybe the remains of other suns that blew up. And slowly gravity brings all of that stuff together, which is what forms the sun and the planets, right?

That's exactly right. And mostly it's hydrogen. You know, we've been burning stuff in the hearts of stars for billions of years, turning the hydrogen from the Big Bang into helium and neon and carbon and iron and heavier stuff. But we haven't made that much progress. The universe is still like 90 something percent hydrogen. It's really just mostly hydrogen to first approximation. And so even though I'm not made of hydrogen and you're not made of hydrogen, hydrogen is the most common thing out there in the universe. So if you're imagining this cloud that formed the solar system, it's mostly hydrogen with like a sprinkle of other stuff, like the iron and the magnesium, all that stuff,

is like the spice on top of the meal. Right. Although something interesting to think about is how much hydrogen we actually are, right? I mean, we're mostly made out of water, which is H2O, right? And even our molecules are, you know, hydrocarbons are called hydrocarbon because they have hydrogen in it. Yeah, that's true. And there's different ways of thinking about it also.

hydrogen by number or hydrogen by mass because even if you have a lot of hydrogen, it's not very heavy. And so the universe, for example, is like 92% hydrogen by number of molecules, but it's only 70-something percent hydrogen by mass because the other stuff is so much more massive. Even though there's less of it, it counts for a bigger proportion of the mass of the universe. So like in our bodies, maybe hydrogen is the most popular atom, but maybe not the most in terms of weight. Exactly. And so. And if you eat a lot of bread, then, yeah, you're mostly carbs. And for example, the sun today is like 95% hydrogen, but the Earth is much less.

The Earth has a huge fraction of oxygen and magnesium and iron. And so the Earth and the Sun and Jupiter, they're not all like representative samples of the stuff that started the solar system. It differentiated itself. It separated. The process of that gravitational collapse led things to accumulate in different ways, which led to different. sort of scoops of each material in different places. Right. Because you were saying it sort of depends on where you are in the initial kind of cloud that eventually became the solar system because I think what happened was that most

of the hydrogen that cloud kind of rushed to the center and that's where the sun formed, right? Yeah, exactly. There's sort of three zones to think about. There's the center which forms the star and you're right. That gathers most of the gas, at least in the inner solar system. Then there's the inner part of it, which is who you say doesn't have much gas. for two reasons. One is that the sun steals a lot of it. And the other is that the sun blows away some of the gas. Once the sun gets going and starts emitting photons and solar wind, it will strip the inner planets of any gas they had. Remember that the earth, when it was first formed, it had a primordial atmosphere of hydrogen and helium. But most of that was lost because of the

incredible radiation from the sun. So the inner planets don't really keep much gas. That's why they're mostly rock. They're iron and silica and that kind of stuff. because the gas either falls into the sun or gets blown out into the outer solar system by the radiation from the sun right and i guess where did the rocks and the rocky planets come from they came from the hearts of other stars right those rocks are made of heavier elements and remember astronomers think of everything that's not hydrogen or helium as a metal and so these metals came from fusion inside the hearts of other now dead stars they started as hydrogen from the big bang or maybe a tiny little bit of helium they were fused into half heavier stuff and lived for millions or billions of years inside another star, which then died. And when it died, it blew out a lot of that stuff out into the galaxy. And that's where these raw materials come from. They float out there in these big clouds until eventually they collapse back again, triggered maybe by a shockwave from a nearby supernova or just by a gravitational over

density that gradually pulls this stuff back together. Right. So initially we had this cloud of hydrogen and some helium and a little bit, tiny little bit of some rocky and metal and solid stuff. And then I think what happened was that the gas, because it's lighter, kind of rushed to the middle, right, leaving some of the heavier stuff just around the sun. That's where the rocky planets came from. Yeah, so the processes you have the stars forming.

And at the same time, you get what's called a protoplanetary disk. This disk of materials, sort of like the star has a ring system. All the material, which will eventually form the planets, is now flattened into a disk. Gravity is doing its job of pulling it together, but it's hard to pull it all. into the sun because if it's spinning around it has a lot of angular momentum and so it sort of stays in orbit rather than collapsing. So gravity collapses it into that disk and places where you have

enough density where you have like heavy stuff like iron and magnesium and rock, that's able to compete with the gravity of the sun and pull some more stuff together. So it's sort of like a race. You know, who can get enough stuff to survive? If you don't pull enough stuff in to get massive enough, then you just get drawn into somebody else's gravity well. And so in our solar system, we had a huge planet Jupiter start to form, and it must have started from a gravitational over density, and then it got bigger and bigger and bigger because the more massive it is, the more gravity it has, the more it pulls stuff in. And so each of the planets sort of start from a spot in that protoplanetary disk where you

already had a little bit of extra blob of stuff. And the mass of the planets tells you something about the initial size of that over density. The bigger it is, the faster it's able to gather mass, and the bigger the planet ends up being. Kind of like a, maybe like a game of musical chairs, kind of like everyone's trying to grab as much stuff as possible before somebody else grabs it, right? Yeah, sort of like gravitational hungry, hungry hippos, right? Everybody just grabbing more and more. And the bigger you are, the more hippos you get and the easier it is to grab stuff. And in the inner solar system, you end up with planets of rock and silica and iron because those are the dense materials that can hold themselves together and resist being pulled into the sun.

And then the gas, of course, is blown out. Further out in the solar system, the sun's radiation is weaker and the sun's gravity is weaker. And so gas giants can form. Jupiter and Saturn have huge contributions from helium and from hydrogen because they were far enough away from the sun and grew fast enough that they were able to compete with the sun's gravity. And their own gravity could protect their gas from the sun's radiation, which is also weaker that far out in the solar system. And voila, that's how we got the planets. And some of them are denser than others. and some of them is going to win the title of densest planet.

And I guess another one is going to win the fluffiest planet, which is the one where I want to go. And so let's get into those details. But first, let's take a quick break. Ah, come on. Why is this taking so long? This thing is ancient. Still using yesterday's tech, upgrade to the ThinkPad X-1 Carbon,

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the Houston Lab that takes on the most hopeless cases to finally solve the unsolvable. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. All right, we're talking about the densest planet in the solar system. Daniel, where would you put your money? I guess you know the answer. I guess I know the answer.

But I do like thinking about it in terms of floating. Like imagine some giant cosmic tub of water on some, you know, cosmic version of the David Letterman show. And you're putting the planets in it. Like, will Saturn float? Or if you drop Mercury into a huge, type of water will it sink? That's just sort of a fun experiment to do. Yeah, let's do it. How much would it cost? I don't know. That's an engineering problem. It's called David Letterman. He did it pretty well, right, with this show? He's sitting on some money. Yeah, this would be like a great way out of

retirement for him. He's got some of that Netflix money now. But it is an interesting question, which is the densest planet. And maybe, you know, I feel like as we learn about the planets, we learn about their size. Like, you sort of learn that Jupiter is the biggest one and that Mercury's smallest one, right? But you maybe never think about the density of them. Yeah, we do focus on the size because that's what we can see. You look through your telescope, you can see Jupiter is big. You can see Saturn is huge with these big rings. So we know something about the size. And it's fun to think about, right? It's fun to try to wrap your mind around the incredible variety of sizes. Right. Like Jupiter is enormous compared to the Earth. So it's really fun to just like try to

imagine filling Jupiter with Earths. And then doing the same thing for the Sun, like filling the sun with Jupiter's and imagining how many Earths would fit inside the sun, which is like a million. And would Earth float in Jupiter? If you like liquefied Jupiter and put it in a tub. I don't know. Or if you like through Earth at Jupiter, would it float? Would Earth float in Jupiter? Actually, it would not, right?

Earth is much denser than Jupiter, it turns out. So Earth would sink like a stone in the ocean of Jupiter. But I guess how far, right? It also depends on your definition of floating, kind of. Like, are we technically floating around the sun? I don't think there's any buoyancy, right? It's only gravity. I think in order to be floating, you need to have some sort of forces of buoyancy,

you know, the displacement. And I think that's happening in our solar system. You need to stuff from the thing that you're floating on to be pushing you up. That's what you're saying. Yeah, exactly, to be balancing gravity. All right, well, we're trying to get to the answer of this question. What is the densest planet? And we've talked about how planets are foreign,

because what you're made out of sort of depends on where you are in the solar system when things started to form. Yeah, there's sort of four different categories of stuff that planets can be made out of. There's like very heavy metals like iron. You're going to find that the inner planets are mostly made out of stuff like that. And then there's rock and that's also what we found

mostly in the inner planets. And then there's gas, which of course, the sun and Jupiter and Saturn have huge servings of. And then there's water. There's a lot of water in the solar system. And there's this very interesting point where you're far enough away from the sun. It's called the frost line or the snow line

past which water is ice instead of vapor, and beyond that line, water is a solid. And so if you have a bunch of water, it's frozen and it sort of contributes to the mass of the planet, and it helps planets grow faster. So planets like Uranus and Neptune, these are ice giants because they have huge amounts of ice in them. Even without accumulating a lot of hydrogen and helium, they still were able to grow really large. So the proportion of what you get in the solar system as you form really does depend on where you are. Right. Although if you put water out in space, even near Earth, it's going to

freeze too, right? It's a really good question and it's sort of tricky. If it's close enough to the sun, then it will vaporize, right? Because it's very low pressure out there in space. And the phase of a material depends not just on the temperature, but also on the pressure. And so water will vaporize in space unless you're really far out. So you mean like it's really the difference between vapor and solid. Like you'll jump straight from a vapor to at some point, if you put some water out there, it will turn into an ice cube. Exactly, which is why I think if you go out into space, parts of you will boil instantly, right? Where boiling doesn't mean you're getting really hot. It just means it's sublimating. It's going directly to gas. And so you can boil at a

very low temperature if the pressure is very, very low. Like you know how if you're at high altitude, you can boil your water at lower temperatures. It's like harder to cook pasta at the top of Mount Everest. than it is in Death Valley because the water boils at a lower temperature because of the lower pressure. Out in deep space, water boils at very, very low temperature and so it turns into vapor even if it's very cold. So you were saying that the inner planets are mostly met out of rocks because of where they are

and the outer planets are mostly met out of gas because of where they were. But that doesn't mean that there aren't rocks and metals out there in the outer solar system. There still are, right? There's still a lot of metal and Earth, maybe the same amount as there is in the inner. the inner solar system. It's just that the inner planets don't have gas because it was all blown away.

Exactly. And there is definitely metal and rock out there in the outer solar system. We know Pluto, for example, is a lump of rock. And we suspect that at the core of Jupiter and Saturn, there is metal. There is rock. Absolutely. It's just that they also have a lot of gas that accumulated all this hydrogen and this helium as well. Because, right, because the sun didn't get to suck up that gas before Jupiter could take it all by for itself. Exactly. And Jupiter probably did begin as just metallic or rocky blob and then it gathered all that gas up with it. And the same thing for the ice giants. They also definitely have some rock and some metals inside of them. They just also gathered a bunch of water as well. So that helps you understand the mass. The density is this combination of size and mass, right? And so you might imagine that the densest things would then be in the inner solar system because that's like where you have almost only the denser materials. Right. But

that's kind of interesting to think about like if you took jupiter or any of those giant planets out there and you strip them of all their gas or you got rid of all the gas in the solar system like all the planets would maybe be kind of the same kind of right small dense rocky tiny balls that's right if you like gas blasted jupiter so you blew away all of its helium and its hydrogen then you would be left with a core which we think we don't know we're not sure because of course we've never probed it the way we probe the earth we think that probably there is a rocky core there well i read that jup Jupiter's core is actually more fluffy. It's not like a rocky core.

It's more like a kind of like a fuzzy rocky core. Yeah, well, we're not exactly sure. There's definitely some rock and some ice. There's also really interesting other chemical effects like surrounding the rock and the ice. There is hydrogen, but it's not hydrogen in the form that you're familiar with it. It's called metallic hydrogen. Hydrogen that's under such intense pressure and temperature that has formed this really strange phase. It's like liquid hydrogen.

But I think the point is that, you know, if it wasn't for gas, the gas that the outer planets have and maybe some of the water, then all the planets would be sort of the same. It would be small and rocky and we wouldn't be talking about maybe the densest one. But because those outer planets have a lot of gas, then their density changes. Yeah, and there's a really interesting connection between the mass of a planet and its density. Because remember, there's more than just one thing going on. It's not just like how much stuff do you have and what element is it, right? You might think it's not just how much iron do you have or versus how much gas do you have,

but how massive are you? Because the larger the planet gets, the stronger its gravity and the stronger its ability to compress itself to make it more dense. So it's not like as you add mass to a planet, it just gets bigger and stays the same density. As you add mass to a planet, it actually also gets more dense. So its size doesn't grow as quickly as its mass. Right. But maybe it also depends on what you put into it, what you're feeding it, right?

Like if you're feeding it a gas, then that's going to be maybe a little harder to compress than a metal. Actually, the gas is easier to compress the metal, right? It has to do with how compressible these materials are. And things like iron and rock are harder to compress than things like hydrogen and helium. And so if you look, for example, at planets made out of ice or silicon or iron, then there tends to be a bit more of a spread between their mass and their density. It's not as closely connected. But if you look at giant planets and not just in.

in our solar system, but in other solar systems, you see a much tighter connection between the mass and the density. As you add more mass, the density increases pretty quickly for gas giants. It's a not as tight relationship for the lower mass planets made of metals and rock. But in general, the planets closer to the sun are made mostly out of rocky stuff. For example, let's talk about mercury. Yeah, so Mercury, you might imagine, which should be the densest thing, right? It's closest to the sun. It's had all the low density stuff below. off of it. And in fact, it is a really crazy planet. It's got like a metallic core that's 85% of

its interior. So this thing is mostly metal. It's really just like a huge scoop of heavy metals surrounded by a thin layer of silica, thin layer of rock. So to compare, Earth's core is like 55% of its interior is this metal. Whereas Mercury, it's 85%. So Mercury is really a very dense blob of stuff out there in the universe. Yeah, it's really into heavy metal. But I think here you mean actual like metal metals, right? Like for Earth, it's mostly iron, right? Like our core is mostly made out of molten iron and as opposed to rocks. Yeah, that's exactly right. We're talking about iron and there's also some nickel in there. And you know, there's a lot of uncertainty here. We know a lot about the Earth's core because we can do things like study earthquakes. And as those

earthquakes reverberate around the surface, they bounce across those layers in the Earth's core and they tell us something about the density. It's like ringing the Earth like a bell and then studying those sound waves. We can't do that as well on Mercury because we don't have Mercury quake sensors. We do have those kind of sensors on Mars now and on the moon so we can take direct measurements. But the other ones we're guessing a little bit more. They come from planetary models and from our understanding of the mass and the radius of these things. All right.

So Mercury is basically like a solid ball of metal, right? Like a BB almost, like a little ball of metal. And so you would think maybe it's the densest planet in the solar system. You would think so. And it's pretty dense. It comes out about 5.4 grams per cubic centimeter. And so, you know, to orient yourself, water is one gram per cubic centimeter. So mercury would definitely not float.

It's more than five times denser than water. All right. So it's not mercury, the densest planet. I guess maybe a question is what exactly makes something dense or not? Well, density here is really connected to size. as you add more stuff to the planet, it doesn't just get bigger, right? If you put a whole other load of mass on Mercury, if you, like, double Mercury's mass, it wouldn't double its volume and then have the same density because the gravity would get stronger.

And so because the gravity gets stronger, you would squeeze those atoms more tightly. So the way to increase Mercury's density would be to increase its mass. But Mercury is a pretty small planet. It's not nearly the mass of the Earth or a Venus. for example. Oh, I see what you're saying. Like if maybe I added more stuff to Mercury, even if it was light stuff like gas, then it would increase the gravity and maybe squeeze that metal core even more and make it more dense. Exactly, because the metal core of Mercury right now is denser than like if you just took that metal and had it floating out into space. If it was very cold and there was no

pressure on it, it would be much less dense. You take that same amount of metal and you put it inside Mercury's core, it's going to get squeezed down. It's going to be more dense than it other wise would. So you add a bunch more stuff to Mercury, it's going to increase its density. But maybe only up to a point, right? Like this, is there a maximum size for a rocky planet? Actually, it's really interesting. We think that there's no maximum density to a planet, but there is a maximum size to a rocky planet. As a rocky planet gets larger and larger, it also gets denser and denser because you're adding more stuff and then increases the gravity and that makes it denser. And there's sort of this asymptotic size. You can't really make a rocky planet bigger than about

10,000 kilometers in radius. When you get to that radius, it's so dense inside that if you add more stuff, it doesn't actually increase the size at all. It just increases the density. Like the gravity on that planet pulls that new blob of stuff in and it just makes it more dense. So for rocky planets, there's no maximum density unless you think about black hole levels, but there is probably a maximum size. Well, I think what you mean is when you're adding more rock to it, right? Like if you make you have a rocky planet and you add more rocks to it, it's not going to increase in size. It's just going to get denser. But if you add other kinds of stuff to it, it is going to increase in size, right?

We're talking about rocky planets having a maximum size. So if you can only make it out of rocks and metal, then there is a maximum size. But you see sort of the same kind of effect happening in our solar system with Jupiter and Saturn, for example. Jupiter is only a little bit bigger than Saturn in terms of size. Like its radius is 70,000 kilometers, whereas Saturn. is about 60,000, but Jupiter is more than three times more massive than Saturn, right? And that's because as you add gas to Saturn, for example, try to turn it into Jupiter, it doesn't just grow in volume.

That gas gets compressed. And gas is easier to compress than iron and metals. And so that's why adding a lot more mass to Jupiter also wouldn't make it much bigger. It would make it denser faster than it would make it bigger. Right. But it does get bigger, though, right? It does get bigger, absolutely. So you can have really, really huge gas.

planets. They can get much bigger than rocky planets. Absolutely. Right. But I guess so you're saying like Jupiter is maybe like maybe started off like a mercury, but then you added a bunch of gas. And if you had added rocks, it wouldn't have grown in size. But because you're adding gas, it does increase in size, although it also increases in density because that gas gets compressed. Exactly. And there's also a maximum size to a gas planet. Because if you add enough gas to Jupiter or what happens, it turns into a star. The pressure in the interior ignites fusion. And then fusion creates a lot of outward pressure.

So then as you add more mass, the density goes down. Like bigger stars are less dense than smaller stars. Once you cross that threshold from gas giant into star, then as you add mass, the density actually goes down because the fusion increases and it fluffs up the star. Interesting. And so I guess maybe in the case of Jupiter, I would think that maybe that's a good candidate for being the densest object then.

Even though it's made out of a lot of gas, there's a lot of gas that's there and a lot of solid stuff in the middle. Maybe it's compressing all that gas enough to make it super dense. It's a good idea. And one of our listeners might be right that the core of Jupiter is probably very, very dense. But Jupiter has so much hydrogen and hydrogen is just not very dense that is actually not very dense at all. Jupiter is like 1.33 grams per centimeter cube. Jupiter would not float, but it almost would. Right.

I think that's what I meant earlier is that, you know, gas is sort of easy to compress, but ultimately, if something like Jupiter, the gas doesn't compress to something denser than rock. Yeah, exactly. The gas is very low mass, right? So it doesn't contribute as much gravity as well. And Saturn is even less dense than Jupiter. Saturn is actually 0.7 grams per cubic meter, which means Saturn would float. If Letterman could build a huge tub of water,

Saturn would actually float in it. Well, I guess maybe a question is, why doesn't gas compress as much as rocks? It just doesn't have as much mass, right? It's not inherently as dense. You don't have as many protons inside there. So the compression is just due to gravity. Why can I squeeze them closer together than I can with the metal? It's not about squeezing the atoms closer together.

It's about the nucleus of the atom having more mass already. There's all those extra protons in the nucleus of iron and in silicon and in carbon and in oxygen that makes it more massive and therefore having more gravity. But couldn't I then squeeze the hydrogen atoms closer together? The reason it's hard to compress hydrogen, of course, is because the electrons repel each other. And if you squeeze even further, then the protons in the nuclei will reject each other as well, will repel each other. So the atoms don't like to be compressed beyond a certain point.

But if you have the same sort of number density of hydrogen and number density of iron, like you have a million iron atoms in a cubic meter versus a million hydrogen, atoms. You're going to get a lot more gravity from the iron atoms. You're going to get a lot more density. All right. Well, Mercury apparently is not the densest planet, even though it's the smallest and the rockiest and Jupiter is the largest planet, but it's also not the densest. And so let's get into which planet is the densest planet in our solar system. But first, let's take another quick break. Ah, come on. Why is this taking so long? This thing is

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To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. Hello, it's Honey German. And my podcast, Grasasas Come Again, is back. This season, we're going even deeper into the world of music and entertainment with raw and honest conversations with some of your favorite Latin artists and celebrities.

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Listen to the new season of Grasasas Come Again as part of Michael Toura Podcast Network on the IHartRadio app, Apple Podcasts, or wherever you get your podcast. All right, we are giving out an award for the densest planet in the solar system. Daniel, what is our planet going to win for being the densest planet? It's going to win a huge blue ribbon, and we're going to give it a nice pat of butter after we toasted for lunch. I thought you were going to get it a bouquet of bread. or a nice day at the spa where they can float in a tub all day.

Or not float, I guess, or sink into a tub. They're going to have to float in like a tub of molten lead or something like that. Well, yeah, I mean, that could be relaxing, I guess, if not totally poisonous. Yeah, you do you, man. Treat yourself. All right, well, the densest planet apparently is not Mercury, which is the smallest and rockiest one that is mostly made out of metal. And it's not Jupiter, which is the biggest planet and most massive planet in our solar system. Daniel, which one is the densest planet?

It's not Pluto, is it? It's not Pluto, no. It's actually the Earth. Earth is the densest planet in the solar system. We win this one. What? Surprise plot twist. It was us all along.

It was us all along. The answer was the planet we loved along the way. But not by much, right? The Earth is like 5.5 grams per cubic centimeter. And Mercury is like 5.43, which is pretty close. Yeah. And the reason the Earth wins is just because it's more. massive like whatever happened in the very early solar system earth started forming before mercury or it

formed from a larger initial blob either one and so he was able to like hungry hungry hippo its way to just more stuff and so because we have more stuff we don't just get bigger again we get more compressed right more stuff means more gravity which means more gravitational pressure which means that the stuff we're made out of is squeezed more than the stuff mercury is made out of just because there's more of it and so that's why Earth is able to eke out above Mercury, like you would naively expect, and if all the planets had the same mass, you would expect that the ones closer to the sun would be denser for the reasons we've discussed several times. And as you went out, there would be less and less dense. But there's another factor there, which is the total mass. More mass means more density for these planets.

But we're also bigger than Mercury, right? Like we're both larger in size and also denser. Exactly. We're larger. We're more massive and we're only a little bit denser. than Mercury, we're like just barely eke out. It's like 5.51 versus 5.43 grams per cubic centimeter. So neither of us would float in water. Right. But again, it's not just because we have more mass, because Jupiter definitely has thousands of times more mass than Earth,

but it's not as dense as Earth because gas doesn't get compressed as much. Yeah, Jupiter's around 300 times the mass of the Earth, but you're right, it's much less dense. So the reason that we're denser is because we are made out of rocks and metals instead of gas and we also have a bigger scoop of stuff than Mercury. So we're denser than Jupiter because we're made of denser stuff and we're denser than Mercury because we're made of more dense stuff. All right. So the Earth is about 5.5 grams per cubic centimeters and Mercury is 5.4 grams per cubic centimeters. What were some of the other close runner ups? Venus, which is right between us, is 5.2 grams

per cubic centimeter. So it's like right around there. You know, these three interplanets are all very similar in mass super close right like if you were to round to the you know nearest single digit they're about the same yeah and except for the earth it sort of follows the trend you expect you know earth is the most massive but then it's mercury then venus then mars which is almost four grams per cubic centimeter so the earth is sort of an outlier there right it's sort of like a weird one other than that it follows the rule of the inner planets being the densest and then it falling as you get further from the sun so the earth just sort of like happened to be a big boy or big girl or big thing.

It's just having to get more stuff, right? Because Mars is further out, but it's smaller. Yeah, Mars is further out. And it's also smaller. And so it can't really compete. Right. And then at some point in the solar system, the planets become gas planets, right, which is where Jupiter and Saturn come in. And those guys are about roughly the density of water. You know, Jupiter is just a little bit more dense than water at 1.3 grams per centimeter cubed. And Saturn is a little bit less dense than water at 0.7 grams per centimeters cubed. So it's interesting because then if you had a cup of water, it would sink in Jupiter, technically, but it would float. No, it would sink in Saturn, but it would float in Jupiter.

Yeah, exactly. If you spill your beverage in Jupiter, don't worry. You can just, you know, bend down with your straw and slurp it up. It's going to be floating over the surface of Jupiter. Right. Well, technically also maybe in Saturn, right? Because we're talking about average densities. Yeah, exactly.

And it's a little bit fuzzy there, right? Like, because the edge of Jupiter is fuzzy, it's easy to say where we think the earth ends, right, at the surface, but it's harder to say, like, where to draw the line for Jupiter because it doesn't have a hard surface the way Earth does. So they've sort of arbitrarily defined some drop off in the density as the edge of Jupiter, but that changes the number. You know, if you push further out to include Jupiter's full atmosphere and exosphere, then the density would drop even further. The density at the core of these planets is much higher than the density near the edges. Right. Like you said, Jupiter has metallic hydrogen at its core, and Saturn, I think it rains diamonds too, right? I think they're pretty intense inside of these fluffy planets. Yeah, and that's one reason that it's hard to study. We have dropped probes into some of these things, but they don't last very long because the pressure gets very intense pretty quickly.

Right. So then after these gassy planets, then you have the icy planets, right, Uranus and Neptune. And those are actually denser than the gassy planets. Yeah, Uranus is about the same as Jupiter. but Neptune is even denser. It's 1.6 grams per centimeters cubed. And that's just because it has more water, more ice. It's out past the frost line. And so it can get a little bit more solid. It's a little bit more like the rocky planets than like the gas planets.

I mean, it's still more like a gas planet, but sort of in the rockier direction because it has more ice in it. Right. It kind of seems to depend on what your most of your mass comes from, right? Like if most of your mass comes from rocks, then you're going to be more dense. If it mostly comes from gas, you're going to be the least dense, and if it mostly comes from water, then you're sort of in the middle.

Yeah, and it's a really cool way to sort of indicate what you're made out of, which tells you where you were formed in the solar system and something about the whole history of the solar system's formation. There's so much information wrapped up in these few numbers, you know, the mass, the radius, the density. It tells you a lot about the history of each planet. All right, so Earth, we are the densest planet in our solar system. I guess that's a good distinction, right? Like, dense is good. It means more is happening.

We're not to be taken lightly, that's for sure. Yeah, it's a pretty heavy topic. But, you know, on cosmic scales, these are not very impressive densities. You were talking earlier about neutron stars. Remember that if water is one gram per cubic centimeter, a neutron star is 10 to the 11 kilograms per cubic centimeter. It's just like way off the scale, you know, orders and orders of magnitude. Remember that a teaspoon of neutron star material is like 700,000 Eiffel towers all squeezed into a tiny spot. So the universe is capable of creating stuff at much, much higher densities than we see in our solar system.

Right. I guess it depends on how much mass you get to accumulate into a small spot. So basically anything would float in a neutron star, even 600,000 Eiffel towers. I don't know if you would call it floating. Neutron stars actually have a crust, right? So you could build Eiffel Towers on the surface of a neutron star if you made them strong enough because nothing can be higher than about a millimeter above the surface of a neutron star

because the gravity is so intense that it just gets flattened. So yeah, if you spill your drink on a neutron star, it's going to be a very thin puddle on the surface. Right, yeah. Well, well, you'd be a puddle, but you'd still be floating. Well, those are the planets in our solar system. What about out there into the cosmos? One of our listeners here was asking whether we meant the solar system

or the exoplanets out there in other parts of the galaxy and other galaxies. Yeah, one of the really fun things about modern astronomy is that we are now able to use our telescopes to study planets around other stars, which gives us this amazing window into the question of whether our solar system is weird or typical. You know, in the end, it's sort of a statistics question. We're like one example out of many, many stars, and we want to know, are we usual? Are we weird? You know, how could we be different?

is the fact that there's life on this solar system mean that our solar system has to be different or if our solar system is normal and usual, does that mean there's life everywhere in the solar system? It's a really big and fun question. And it's actually not that hard to think about the densities of these planets because to measure the density,

you only need to know their mass and their size. Right, because it's an interesting question because it could be that maybe other solar systems out there in the universe are totally different than ours, right? It could be that ours is like, you know, a weird one where we form planets, but it could be maybe that in other solar systems,

maybe you don't even form planets, or you form like one giant planet, or you only form two planets or something like that, right? Yeah, or they're all gas giants. Maybe rocky planets are super rare in the universe, or maybe they're all rocky planets, right? Until we started looking at other solar systems, we didn't know the answer to this.

Now we actually know that there are a lot of gas giants out there. We call them hot Jupiters. A lot of them are really big gas giants close to their sun. We also have identified a lot of rocky planets. So we know that there's rocky planets out there and there are gassy planets out there. And there's a huge variety also in the density of these planets. Right. It's interesting because like you say, these things are really hard to see.

Like we barely even seem like or have photos of one planet out there beyond the solar system. We kind of have to backtrack what these planets look like or how dense they are from what we can see of how their stars wiggle or how much light they block from the star when they. pass in front of it, right? It's kind of a tricky problem. It's a very tricky problem and it's amazing what we can figure out. You know, you want to know what is this planet made out of? Well, you can't go visit it. You can't land a probe on it. You can't even really measure the light that comes from it very well. So how do you figure out what it's made out of? Well, its density is a huge clue, right? If you were an astronomer in a far away solar system studying hours and you could measure the density of Earth and the density of Jupiter, that would tell you, oh, one of those is probably a gas

planet and one of those is a rocky planet because one of them is much denser than the others. So just getting a measure of the density of planets and other solar systems tells you immediately what kind of things they might be made out of. And you're right, it's very tricky, but we can measure the mass of those planets and the radius of those planets. The mass comes from understanding the orbital dynamics, like how long does it take to go around the star? How far away from the star is it? We can just solve the Newton's equations, you know, use Kepler's laws to understand what is the forces of gravity? How fast is it going? And therefore, how massive is it? Just by understanding its orbit, we can tell what the mass of a planet is. Like Jupiter, if Jupiter was denser

or less dense, it would still have the same trajectory around the sun. Exactly. Its mass and its radius determine the orbital velocity. Or like the Earth, if the Earth was fluffier or more hardcore, a year would still be a year on Earth. Yeah, exactly. Because gravity, yeah, exactly because when you're dealing with Newtonian gravity, you can always like replace an object with a point particle of the same mass and you get in effect the same gravity as long as you're on the outside of the object. So replace the earth with a particle, the mass of the earth, and it would move the same way the earth does, right? And so by observing the motion of those exoplanets, we can tell what their mass is, regardless of what their size is, right? It's an independent thing. But then to figure out

what their density is we do need to know what is the size of that planet. And that we can do by watching them eclipse their sun. As you were saying, one way that we can detect those planets are there is that they pass in front of their star. And so they block the light from that star a tiny little bit. But our telescopes are sensitive enough to see that. It's called the transit method. So as that distant planet passes in front of the star, it decreases the light and decreases the light more if it's a bigger planet and less if it's a smaller planet. So by seeing how much it decreases the light, we can measure the radius of that planet, the size of it, separately from its mass. Right. And once we know their mass and their size, then you can tell their density. And that maybe tells you like, hey, this is a rocky planet or an icy planet or a gas planet. Exactly. Or it's like, what? This is a really weird planet. What's going on? This planet seems to be super fluffy or super dense. This was made out of bread. That's so weird. It's a baguette planet. Maybe the French have been colonizing before we even knew it. But, you know, it's definitely going to be testing our assumptions. We have these ideas, these models for how big a

rocky planet can be. I'm sure we're going to find one that breaks that rule if we look far enough and that's going to tell us something we didn't understand about how planets form. So it's very exciting to look for the extremes of these planets. All right. Well, that means density is an important thing to know about a planet because it tells you a lot about what it's made out of and where in the solar system, wherever that solar system might be, it came from. Yeah. And so we have been looking and there are a few really fun candidates. One of them is called Kepler 131C, which just means that the Kepler telescope discovered it is the 131st that it spotted. And this is very uncertain, but this is the planet out there with the highest estimated density. It's more than eight times

the mass of the Earth, but it's actually smaller than the Earth in radius. And so the current estimate of this thing's density is 77 grams per cubic centimeter. Remember, the Earth is like five grams per cubic centimeter. So this thing is like 15 times the density of the Earth if these numbers are correct. Right, but is that weird or is that pretty much what you'd expect if the earth was, you know, if you gave it eight times the mass and rocks, would the earth also be that size? Yeah, if you took a bunch of earths and you squeeze them together, you would get something very dense, right? But we think that it's possible to get larger than the earth. Remember, the upper limit for the radius of a rocky planet is like 10,000 kilometers. And the earth is

6,000 kilometers. So it's possible to get bigger than the earth. If you plopped a bunch of Earths together, you would expect them to be larger than the Earth by a little bit. So for this to be that dense, it has to also be made of denser stuff than the Earth. So maybe it's just like a huge blob of lead or like has a lot of magnesium or osbium in it. It's not a crazy number, but it's definitely out there on the extreme edge. Whoa, like a whole planet gist of a single metal or something. I know this fluffy rock that makes this less dense. Maybe it's like an alien engineering project, you know, some alien university.

Like, make a planet out of concrete or make a planet out of osmium or something. Yeah, and see if it floats. And see if it floats. Maybe there's a David Letterman in that planet with a very big budget for his or her show. Exactly. So I hope somebody got an A for that project. And then there's another interesting planet that we found out there, right? So number two on the exoplanet density top 10 is 55 Cancree E.

This one is 60% larger than the Earth. So it's a bigger radius. also about eight times the mass of the Earth. So about twice the density of the Earth, which gives it the density of about lead. You know, that's the average density, which means that probably near the surface, it's less dense. And in the core, it's much, much more dense. And it's really fun to think about, like, how these planets came to be. Was there a huge blob of metal that formed a planet? Or are there other processes that we don't understand that contribute to planetary formation? You know, we're also able to image protoplanetary disks. Like, we look far

enough back in time, which means looking at things far away, you can see planets form it. We can see stars with disks around them, not just planets. Those are actually easier to spot than planets because the disks are much bigger. Well, this one's interesting because it's also eight times the mass of the Earth, but it's only twice the density, as opposed to like 10 times the density. Yeah, and so it might be made out of less dense stuff. You know, it might have a lot of water in it. We just don't know. Interesting. And we can use some of these facts sometimes to figure out which are maybe habitable planets, right? Like, we don't want to land in a Jupiter or a Saturn because that would be probably not livable for us. And we don't want to live in a maybe like a mercury, right?

We want to live in a planet. Maybe that's similar to Earth in density. Exactly. If you're planning an interstellar road trip, then you probably want to target a rocky planet. It's more likely to have water on the surface of it, for example, to be in what we call the habitable zone. If you're just wondering about what's possible for planets. If you're a scientist and you want to visit the craziest planets out there, then yeah, you want to find stuff with really low density or really high density to help, like, inform your models of the universe. But it's amazing how much you can learn about a planet just by understanding its density.

Right. But then using information that we know from our solar system to kind of extrapolate and say, hey, that one's probably icy or rocky or gassy. Yeah, exactly. And our knowledge of how these elements work and our models for, planetary formation, which come, of course, from what we learned in our solar system. So science is good. But, you know, I'm sure we're wrong about a lot of how this works.

And if we ever do get to visit these solar systems in detail, we will find planets to make us go, what? How is that even possible? We were totally wrong. All right. Well, that's kind of a pretty good lesson, I think, as you say, of why it's important even to study our backyard or why it's important to have curiosity about these things. Because, you know, the more you learn about how solar systems form and how planets form

and what determines density, the more you can learn about the rest of the universe with limited information, right? And it tells you something about our own history, which is fascinating in its own right. Something happened early in our solar system to make Earth bigger and more massive than Venus and Mercury, and that's why it wins the crown today. All right. Well, that was a pretty dense episode. I guess you can make it denser by playing it twice as fast or increasing the playback speed.

You have control over the density of your podcast experience. You know, we often talk about people playing the podcast at higher speeds. I wonder if there are people out there who play us at like half speed. Mmm, to make us more fluffy. Yeah, like spread it out like butter on toast, you know? Right, right. But that would also spread out your chuckles, so it'd be more sinister. Be more like, hohoho, ha, ha, ha.

I try to make my chuckles pure and innocent. Well, we hope you enjoyed that. Thanks for joining us. See you next time. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeart Radio. For more podcasts from IHeart Radio, visit the IHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.

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