It is the mission of children on beaches around the world: to dig through the centre of the planet and come out the other side. But such an endeavour is far from simple. Earth isn’t just sand and rocks all the way through – it holds a sea of molten iron, and the temperature and pressure near the middle would be enough to melt any ambitious digger, along with any tools they might use to make their hole.
In the second episode of the Dead Planets Society podcast, our intrepid hosts Leah Crane and Chelsea Whyte dig into the question of what might happen if we were to bore a hole through a planet. Gas giants are probably a no-go, because the temperatures and pressures below their clouds are too intense for any material humans have ever made to stay intact, let alone for actual humans to survive.
For an indestructible vessel, though, the journey would be interesting, with strange gravitational effects and phases of matter we have never seen before. Maybe on a smaller world, like Pluto, you wouldn’t need an indestructible vessel – in fact, Pluto’s surface is so cold that a person’s body heat would be enough to start a borehole. Planetary scientists Konstantin Batygin and Baptiste Journaux join our hosts this week to talk about the logistics of drilling through an entire world, and what would happen if we could actually pull that off.
Dead Planets Society is a podcast that takes outlandish ideas about how to tinker with the cosmos – from unifying the asteroid belt to destroying the sun – and subjects them to the laws of physics to see how they fare.
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Chelsea Whyte: Now I want to skateboard with intention through Mars and do a sick flip on the way out.
Konstantin Batygin: There you go. There you go. Yes.
Chelsea Whyte: The X Games goes galactic.
Leah Crane: Welcome to the Galactic X Games, also known as Dead Planets Society.
Chelsea Whyte: This is a podcast where we imagine what would happen if we were given cosmic powers to rearrange the universe. I’m Chelsea Whyte, senior news editor at New Scientist.
Leah Crane: And I’m Leah Crane, physics and space reporter at New Scientist.
Chelsea Whyte: And today, we’re talking about destroying a planet. But only mostly destroying it. And we’re not discriminating, any planet will do.
Leah Crane: And we don’t necessarily want to wreck it entirely. We just want to bore a hole straight through the middle.
Chelsea Whyte: Yes. So big, small, doesn’t matter. Rocky, gas giant, who cares? Let’s go get one of these suckers.
Leah Crane: Yeah. We’ve got to figure out which planets it would be possible to drill through.
Chelsea Whyte: And that’s probably not going to be Earth, right? For a lot of reasons.
Leah Crane: Yeah, almost definitely not Earth. But we’ll get into that in a little bit when we talk about what it would be like to drill this big ol’ tunnel and how we could get it to stay open. But the planets are all different and this is really complicated, so we got some expert help.
Chelsea Whyte: Right. So we spoke with Baptiste Journaux from the University of Washington, and we’ll bring him in a little bit later.
Leah Crane: Yes but right now, we’ve got some information from Konstantin Batygin from Caltech who talked a bit about what the best planet to drill through might be.
Konstantin Batygin: You’d have the best chance of actually drilling a hole through Mars. Because, like, if you think about the Earth, right, eventually you’ll reach the liquid iron core and then you’re going to have to worry about the fact that it’s liquid, so it’s hard to drill a hole through liquid.
Leah Crane: Okay. So we want to pick the smallest one without a magnetic field.
Konstantin Batygin: Yes.
Leah Crane: Because no magnetic field means no moving liquid metal in the middle.
Konstantin Batygin: That’s right. So Mercury’s magnetic field is much more complicated, so we’ll see. But I think Mars is a good bet for this.
Chelsea Whyte: I mean, I’m on board, I’ve always wanted to shoot, Mars but it seems like we might have a hell of a time trying to get through the rock.
Leah Crane: Yes, that’s why Baptiste said we might want to aim for something a little bit smaller.
Baptiste Journaux: Digging a hole through a planet is incredibly hard, or near impossible if you think really, you know, about the physics of it. So literally the smaller the better, you know, as you might expect.
Chelsea Whyte: Is the smaller the better simply because there’s less distance to go? Or less gravity? Or all of it?
Baptiste Journaux: Actually, none of the above.
Chelsea Whyte: Oh.
Baptiste Journaux: The main problem is temperature. Because as soon as you start to go below the surface of a planet, there’s going to be remnant heat from the formation of that planet. Very quickly, you’re going to rise to temperatures that are way above the melting temperature of metals so you would just literally melt, like, the boring bits that you use. So that’s the main issue.
Chelsea Whyte: Okay. So our machinery would melt.
Baptiste Journaux: Yes. I mean, before it would melt it would probably act like play dough, in a way. You would start to dig in but eventually you would get, like, so hot that even metals would start to become soft and they will just like, yes, become like very, almost gooey.
Chelsea Whyte: Okay.
Leah Crane: Okay, so if we’re using anything metal to drill this hole it’s going to become gumby and then melt.
Baptiste Journaux: Yes. I mean, just to get things, like, if you actually look at real things that happened, we actually tried to dig a hole, the deepest possible hole in Siberia.
Leah Crane: The Kola Superdeep Borehole?
Baptiste Journaux: Yes, that’s right. The Kola Superdeep Borehole. And they went all the way down to roughly twelve kilometres. So twelve kilometres, that might seem a lot but it’s so small compared to the entire thickness of the Earth – that’s closer to 6,300 kilometres. So we didn’t even pass the crust. We were still inside the crust, we didn’t even punch through the first very thin layer of the Earth, we didn’t even enter the mantle, because the crust is roughly 30 kilometres in that area. And they had to stop mostly because of temperature because, like, the drill bits would just get destroyed.
Leah Crane: I wonder, I mean, I guess you have the same problem, but as much as smaller is better seems like the obvious choice, it also seems like gas is easier to get through than rocks. Would a gas planet be easier for a little bit, and then much worse, or…
Baptiste Journaux: Pretty much, I mean, the problem with gas is that it doesn’t stay in place so if you dig a hole then the gas that are next to it are just going to replace the gas that you just removed. But if you are to imagine that you would be able to apply a force field that just keeps the gas from going in-
Leah Crane: Yeah, or we just leave a tunnel behind us.
Baptiste Journaux: Here you go. We have this magic power and we can just keep whatever we remove from the hole from being replaced by the gas that is next to it, very quickly you’re going to run into the exact same type of problems, which is mostly temperature. Because on planets like Jupiter, Saturn, Uranus and Neptune, even though the surface is really cold and, you know, you have the cloud deck and then you get to a higher pressure, the main problem is that the temperature is rising very quickly so very fast you’re going to past the melting point of lead or aluminium, all the other metals-
Chelsea Whyte: Humans. Yes. (Laughter).
Baptiste Journaux: And humans. And humans. That’s actually one of the things I tell in my class is that, what happens if you just drop someone in Jupiter? First they would probably suffocate because, you know, you can’t really breathe the atmosphere. But after this, while you fall, yeah, you’re going to literally get cooked and eventually you will dissolve in what we call metallic hydrogen. So it’s, like, hydrogen that is so compressed that it becomes metallic and it’s so hot that it can dissolve pretty much everything. And so you would just, like, dissolve things in the planet before you’d even reach even halfway through the planet you will get totally dissolved before that.
Chelsea Whyte: You become the gas planet.
Baptiste Journaux: Yes. So gas and ice planets they are, kind of, it’s not a realistic description for what most of the volume is. Okay, there is gas at the exterior, but very quickly you become fluid because you pass this point, the thermodynamic point that we call the critical point where you cannot make the distinction between gas and liquids because you’re too high pressures and too high temperatures. And most of, Jupiter and Saturn for example, are mostly in this, like, super high pressure, super high temperature fluid state so they’re more like fluid planets rather than gas planets. So the temperature we’re talking about, I mean, very quickly you get into the thousands of kelvin but at the centre you can get to, yeah, tens of thousands of kelvin. I think it’s around, like, 30,000 kelvin or something like that.
Leah Crane: So even if we were able to dig through and leave a, sort of, slide behind us, of openness, the tunnel would be a really unpleasant place to hang out.
Baptiste Journaux: Oh, absolutely. Absolutely. It would be a terrible place. Actually if you have a tunnel, very quickly you will reach a place with this type of temperature and they would actually glow because, you know, anything that is hot emits a blackbody radiation. But because it’s hotter than the surface of the sun it would shine brighter than the surface of the sun so you would have, like, a hole that is emitting a bunch of light probably.
Leah Crane: Ooh.
Chelsea Whyte: Okay, but would the light come out either end?
Baptiste Journaux: Possibly, yes.
Leah Crane: It’s sounding more fun now.
Baptiste Journaux: You would have, like, a very, very expensive torchlight.
Leah Crane: So it would be blindingly bright.
Baptiste Journaux: Yes, very impractical.
Leah Crane: Thousands of degrees.
Baptiste Journaux: Yeah. I mean, at 30,000 kelvin which is the temperature of the centre, yeah, most of the light coming from it would probably be in the ultraviolet but you will still have a lot of light coming from the visible spectrum so it will be very, very bright. So you have this extra bright spot coming from the tunnel, probably.
Chelsea Whyte: So you’d be blind and cooked. But let’s say I jumped in-
Baptiste Journaux: Yes, and dissolved.
Chelsea Whyte: And dissolved.
Leah Crane: You’d be soup.
Chelsea Whyte: If I wasn’t soup and I jumped in, would I also get stuck in that bad, awful middle place? Like, would the gravity, sort of, pull me in? Even if I got going pretty fast and overshot it wouldn’t it, kind of, yank me back and I would end up stuck?
Baptiste Journaux: So let’s take the idea of, like, we have a hole through a planet and you’re not cooked, you’re not burned or whatever, but you drop from the same altitude as the surface and just fall through the entire planet. So every planet is different and the evolution of the gravitational pull with distance to the centre can either increase or decrease when you get closer. So for example, on Earth, the gravitational attraction is pretty much the same until you reach, like, the core of the Earth. And then it starts to decrease. For planets like Jupiter or Saturn, the gravity actually increases as you go down because you get closer to the high density areas of the planet. So if you have, like, super high density areas it will actually attract you more. So if you were to just fall through that thing, what’s going to happen is you’re going to happen is you’re going to accelerate and the more you fall, you know, the more acceleration you get and so you arrive at the centre with an incredible speed.
Chelsea Whyte: So Konstantin had thoughts about this too. I asked him if I would go through all the way through and, sort of, pop out the other side and land on the surface or if I would get caught in the middle by gravity and fling back and forth forever.
Konstantin Batygin: At the centre, there’s zero gravitational acceleration because there’s no mass interior to you. But what would happen is you would fall in, you’d accelerate, you’d reach maximum speed as you go through the centre, and you’d come out the other side. I mean, it’s just like half pipe, right? Like, if you’re going down a half pipe on a skateboard, you’re going fastest at the bottom where it’s flat. Right? And then you come up to the other side of the half pipe and you’re not going very fast at all which is why you can do whatever you guys like to do on the half pipe.
Leah Crane: And if I’m not jumping through with intention then I’m just going to end up, sort of, wobbling back and forth, just like I would if I didn’t drop into the half pipe with intention.
Konstantin Batygin: Right.
Chelsea Whyte: Okay but now I want to skateboard with intention through Mars and do a sick flip on the way out.
Konstantin Batygin: There you go. There you go. Yes.
Chelsea Whyte: The X Games goes galactic.
Leah Crane: I love it. This would be the worst slide ever.
Chelsea Whyte: Yeah. It would be very unpleasant.
Baptiste Journaux: I mean, that would be really fun for the first five minutes. Maybe.
Leah Crane: That’s longer than I expected.
Baptiste Journaux: Yes. After that it becomes very unpleasant but it’s going to be very unpleasant for a very short amount of time, so.
Leah Crane: Right. And then you’re dead.
Baptiste Journaux: It’s not going to be a very long torture. You’ll be cooked very quickly. I mean, the temperature in Earth for example, in the crust, increases by 30 Celsius per kilometre so, you know, after two or three kilometres you will already be above the boiling part of water so you’ll literally boil out and cook out after the first three kilometres, so. And that’s really close to the surface.
Chelsea Whyte: I think even just the first kilometre sounds like enough for me. That’s a lot of heat.
Leah Crane: Okay, so let’s say we’re not jumping in because of, we don’t want to die.
Chelsea Whyte: Fair enough.
Leah Crane: Then we don’t have to keep the tunnel open so it seems like a gas giant might be an easier target, because I can imagine myself burrowing through gas more easily than the liquid iron core of a planet.
Konstantin Batygin: I mean, you’d be burrowing through metallic hydrogen so it would be not too different after all. Right? Like, the moment you go down, I think it was 0.82 Jupiter radii or 0.92 but if you started going inside Jupiter, pretty quickly you reach a situation where hydrogen becomes a metal. And the interior pressure, of course in Jupiter, is larger than inside the Earth at, sort of, at the tens of megabars level.
Chelsea Whyte: Just to interject here, a megabar is a unit of pressure that’s about a million times the atmospheric pressure at sea level on Earth.
Leah Crane: Every once in a while you get a reminder that a gas giant is maybe a bit of a misnomer.
Konstantin Batygin: Yes, I mean, it’s made out of hydrogen but hydrogen goes metallic under high pressure.
Chelsea Whyte: But what if you didn’t go straight through the centre? What if you, like, did a glancing blow? Sort of through the upper parts of Jupiter? I’m having a hard time picturing punching a hole through gas, in general, but would it be possible to keep something open?
Konstantin Batygin: I mean, it’s like being in an aeroplane. Right? And also Galileo had a probe that, sort of, did this. Galileo, not the person, but Galileo the spacecraft dropped in a probe into Jupiter and, you know, that’s how we know some of the abundances in the atmosphere. So yes, it’s a lot like being in an aeroplane.
Leah Crane: Yeah, I feel like the glancing blow is really, like, if we were to do a glancing blow through the centre of Earth, that’s just, like, a water line. Those exist, we’ve got tunnels. You’ve been on a train? That’s a glancing blow through Earth.
Chelsea Whyte: Yeah, yeah.
Konstantin Batygin: I think from now on we should rename all tunnels to glancing blows through the Earth.
Chelsea Whyte: Yes, correct.
Konstantin Batygin: It’s like, imagine you’re driving, right? And whatever your Siri or your Google Maps is like, ‘And now, execute a glancing blow to the Earth for point one miles.’
Leah Crane: Yeah. It’s like, “I’ll be there in fifteen minutes, I’m just travelling through the centre of the Earth.”
Chelsea Whyte: I like it.
Leah Crane: ‘Like, the centre?’ ‘No, just a little bit below the surface.’
Konstantin Batygin: Yes. I like it. I like it, this is good.
Leah Crane: So, my other thought if we’re not maintaining this bore hole is that I could just burrow through something icy like Pluto, like, inside of a heated drill bit or something.
Baptiste Journaux: Probably. Yeah, on Pluto-
Chelsea Whyte: But could a person live inside something hot enough to burrow through Pluto but not too hot to cook you?
Baptiste Journaux: So the main advantage of Pluto is that it is so cold, the surface is around 30 kelvin, you know, even a human at the surface, by just the body heat that we produce, would actually sink through.
Chelsea Whyte: You, yourself are the drill bit.
Baptiste Journaux: Yes.
Leah Crane: Yeah.
Baptiste Journaux: Yes, you, yourself are the drill bit. Until you actually emit enough heat that your body temperature starts to cool down and then you just, like, freeze in place. I mean, that would be a very terrible way to die actually, like, drop someone on the surface of Pluto and-
Leah Crane: Just watch them melt.
Baptiste Journaux: See them, like, slowly sink. Yes, like, slowly sink through the surface and eventually disappear and being re-covered by nitrogen ice for example.
Leah Crane: Be just buried alive inside Pluto.
Baptiste Journaux: Yes because on Pluto we have different types of ice because it’s so cold that, you know, we’ve all heard that liquid nitrogen is really cold and we probably have seen liquid nitrogen, solid nitrogen is even colder and so if you were to put just a human- even in a spacesuit, the temperature of it will be enough to sublimate the nitrogen so you would just, like, literally sublimate yourself through until a certain depth and then, yes, you will get cool enough and you would probably get stuck there.
Chelsea Whyte: But Pluto is an interesting test case because we were talking about how other planets would get too hot, do we think Pluto would get very hot at its centre as well?
Baptiste Journaux: I mean, the temperature eventually will get too hot, that’s guaranteed. But it’s like, at what depth? That’s the main question I have. So yes, probably the first 300 kilometres would be okay, you know, at 300 kilometres we could be close to room temperature.
Chelsea Whyte: Oh.
Leah Crane: We can build a little house 300 kilometres under the surface of Pluto.
Baptiste Journaux: I mean, you would still be at a super high pressure so it would be better for, like, deep sea fishes. They would be very comfortable there.
Chelsea Whyte: Oh, okay.
Leah Crane: Okay.
Baptiste Journaux: It would be temperate for them.
Chelsea Whyte: So we just need a whale on this. Yes.
Baptiste Journaux: Yes, like, a sperm whale would be very happy there probably.
Chelsea Whyte: Okay, heat up the sperm whale, send him to Pluto.
Baptiste Journaux: Yes, exactly. It’s super cheap. Small rockets.
Chelsea Whyte: Yes, just a tiny project.
Baptiste Journaux: Yes. Yes, like, the sperm whale space programme.
Leah Crane: You could just build a really big catapult. Big trebuchet. Chuck a whale to Pluto.
Chelsea Whyte: In, like, a little water capsule that’s warm. See how far we can get it into the planet.
Baptiste Journaux: I mean, there’s not that many left so many we should leave the sperm whales alone.
Chelsea Whyte: Yes, I mean, we should be nice to them. But I think that would be, like, the most historic sperm whale. They would go down in sperm whale history.
Leah Crane: Yes, they could repopulate.
Baptiste Journaux: Yes. I guess, yes. But, like, so when you go through Pluto you get to a possible ocean and at the bottom of the ocean it’s probably going to be around room temperature, but after you go below this you’ll probably hit, kind of, a rocky core probably, and this rocky core, actually the temperature will rise much faster. So once you get to the rocky core then it actually starts to become too high to be comfortable.
Leah Crane: You know how, like, fishing lakes are repopulated with fish? They basically have, like, the big cannon that they shoot salmon out of. It feels like we could do that in this situation.
Chelsea Whyte: With large whales?
Leah Crane: Just shoot a bunch of fish. If they’re not living at the bottom then they don’t even necessarily need to be whales, right? If they’re in that ocean, at the top of it.
Baptiste Journaux: Yes. I mean, the greater problem there is that you’re going to have to convince NASA that it’s a good idea in terms of what we call planetary protection. Have you heard of that?
Leah Crane: Mm. To turn Pluto into a big fish tank.
Chelsea Whyte: Yes, I don’t think they’re going to go for it.
Baptiste Journaux: It’s a little far. You know, it took us nine years with one of the fastest spacecrafts ever made, with New Horizons. That was launched in 2006 and it arrived in 2015, so it took us nine years and it was too fast to actually stop, so I’m not a huge believer in inter-planetary fishing.
Leah Crane: I think they’d all be dead by the time they got there. We’d have to create a salmon inter-generational spacecraft.
Chelsea Whyte: An inter-generational fish spaceship? What are you talking about? That sounds great.
Leah Crane: Go along then. You can be the fish queen.
Chelsea Whyte: My lifelong dream.
Leah Crane: You might just be a glorified aquarium technician.
Chelsea Whyte: Yeah okay, less good.
Leah Crane: And that’s our show, folks. Thank you to Konstantin and Baptiste for joining us today and, as always, a special thanks to our listeners.
Chelsea Whyte: And finally, if you have any cosmic object you want us to figure out how to destroy, let us know and it could be featured in a later episode of the podcast. Our email is email@example.com.
Leah Crane: And if you enjoy our podcast, you might also enjoy my free monthly space newsletter, Launchpad. Check it out at newscientist.com/launchpad.
Chelsea Whyte: And if you just want to chat about this episode, or wrecking the cosmos in general, you can find us in Twitter @chelswhyte or @DownHereOnEarth.
Leah Crane: Thanks for joining us.
Chelsea Whyte: Bye.
Baptiste Journaux: First, we don’t know if there is an ocean so these poor salmons are going to get thrown onto a frozen surface, you’re going to end up with a bunch of frozen salmon. And we know how to do that, you know, it’s already something we know how to do.