Could we tweak the solar system to make Pluto a planet again?

Pluto’s back, baby! Seventeen years after it was demoted from planet to dwarf planet, it is time for Pluto to retake its former title…at least according to our hosts, Chelsea Whyte and Leah Crane.

In this episode of Dead Planets Society, Chelsea and Leah get into the nitty-gritty of what it would take to officially make Pluto a planet – not by changing the rules laid out by the International Astronomical Union, but by changing the solar system. They are joined in their quest by Kathryn Volk at the University of Arizona and Konstantin Batygin at the California Institute of Technology.

Of the requirements to officially be a planet, the one Pluto misses out on is the ability to clear its orbital path of debris: the distant little world is just not big enough to sweep away all the other rocks in its orbit. So what if it were bigger? It would take a lot of mass to make that happen, and there would be consequences to super-sizing it.

Maybe it would be easier to simply drag Pluto to a better orbit, somewhere that is already mostly empty – as long as that orbit is far enough from the sun that Pluto doesn’t just evaporate. There is always the option to drop a small black hole into Pluto or shrink the solar system around it. Those may be tougher to accomplish, but our hosts are up to the challenge.

Dead Planets Society is a podcast that takes outlandish ideas about how to tinker with the cosmos – from snapping the moon in half to causing a gravitational wave apocalypse – and subjects them to the laws of physics to see how they fare.

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Transcript

Kat Volk: It would be visually stunning because it would make, you know, the best, most awesome, largest comet of all time to stick Pluto in the inner solar system.

Leah Crane: You heard that right. Pluto’s coming back to stay. Welcome to Dead Planets Society.

Chelsea Whyte: This is a podcast where we imagine what it might be like 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 peporter at New Scientist. And if there’s one space thing that Chelsea is mad about-

Chelsea Whyte: It’s Pluto. Pluto was robbed. As a child of the ’80s, I loved Pluto, and I was truly horrified when it was demoted from being a planet to, well, not one.

Leah Crane: As some background, in 2006, the International Astronomical Union, the IAU, changed the designation of Pluto from planet to dwarf planet.

Chelsea Whyte: So, like, what does that entail? What are the specific requirements to be considered a planet?

Leah Crane: There’s a couple. It has to orbit a star.

Chelsea Whyte: Check.

Leah Crane: It has to be massive enough that its own gravity has pulled it into a spherical shape.

Chelsea Whyte: Check.

Leah Crane: And it needs to be big enough that it’s cleared its orbit of all the other large objects.

Chelsea Whyte: So, Pluto got booted because it wasn’t tidy enough?

Leah Crane: Yeah, it didn’t do its chores and it got kicked out.

Chelsea Whyte: Rude.

Leah Crane: I know you have a lot of feelings about it, and there are a few people who do, but I think most scientists don’t really care that much either way.

Konstantin Batygin: I have, like, no personal dog in that fight at all.

Kat Volk: Back in high school, which was a very long time ago, I built a website explaining why Pluto wasn’t a planet. I don’t feel nearly as strongly about it now as I did then.

Leah Crane: That was Kat Volk at the University of Arizona, and Konstantin Batygin at Caltech, both planetary scientists and our experts for today’s chat.

Chelsea Whyte: I mean, okay, they may be experts, but they’re wrong. We should make Pluto a planet again.

Leah Crane: Alright. The argument is that we can have seven planets, just the big ones, or we could have thousands, because there’s tons of stuff out there that’s big enough to be a sphere but hasn’t cleared its orbit.

Chelsea Whyte: Okay. Well, number two, bring it on. I want thousands of planets to play around with. But maybe let’s start with just this one.

Leah Crane: Okay.

Chelsea Whyte: Okay. Could we do it? You know, what would it take to get Pluto to count as a planet again, and could we make it any bigger? Could we move it to another spot? What do we do?

Leah Crane: Yeah, I agree, enough quibbling with definitions. Let’s just actually make it a planet. So, we asked Kat the best way to do that.

Kat Volk: Oh, it’s actually a bit of a tricky question. I mean, with your magic powers the best way would be to increase its mass perhaps. I looked up, because thankfully someone has actually answered this question in the literature of quantitatively how massive does something have to be as a function of its orbit to clear its orbit, for some reasonable definition of clear, according to the IAU. And to get Pluto to clear its orbit at its current position, you don’t need to add too much mass to it. You only need to get it up to, you know, something like a tenth of an Earth mass, which isn’t ridiculously large. So, you know, Mars-ish sized.

Chelsea Whyte: What’s the gap there between where Pluto is now and how much we’d have to add?

Kat Volk: So, right now it is about .002 Earth masses, so quite small compared to the earth. So, we need to make it quite a bit bigger than it is right now.

Leah Crane: So, we’re not really adding mass to Pluto so much as putting a whole planet where Pluto is now?

Kat Volk: Yeah, pretty much. Yeah, it’s not just, like, a simple doubling or, you know, a couple big objects that you could add to it. And in fact, I also looked up what are the current best estimates for, like, the mass of the whole Kuiper belt.

Leah Crane: For those of you who may not know, the Kuiper belt is a ring of icy objects that starts just past the orbit of Neptune and includes Pluto and a bunch of other dwarf planets. The Kuiper belt consists of two parts, the classical belt, which is made of objects in circular orbits in about the same plane as the planets, and the scattered disc, which is made of more distant, randomly oriented objects.

Chelsea Whyte: If we have to just add a bunch of mass, well, in my mind, there’s a lot of mass out there in the Kuiper belt, right? Like, we should be able to gather it all up in some kind of net and smush it all onto Pluto. But is there very much mass out there?

Kat Volk: Yes, so, it turns out right now there’s not all that much mass out there. So, the classical Kuiper belt we think probably never had more than about a tenth of an Earth mass of material in it. And, kind of, the estimates I’ve seen for its current mass is maybe 5 per cent the mass of the Earth. So, you’d be pretty close if you took a net and gathered up the entire classical belt and smushed it into Pluto. You’d get close to getting Pluto to be big enough. So, if you were to get a magic net to capture up all the mass, it wouldn’t be so ridiculous to sweep up the whole classical belt, because at least they’re pretty localised in space. But it’d be kind of marginal whether there’s enough mass if you collected them all together to add them to Pluto to make Pluto clear its orbit. If you include the scattered disc population, which are things that extend out much further from the sun, you could get to the total amount of mass because there’s probably another tenth of an Earth mass or something in the scattered disc. But it’d be a much harder net to gather up.

Chelsea Whyte: Yeah. Like, I wonder if there’d be a better way to do it. I don’t know why this came to mind, but I’m thinking of putting, like, a snow plough just in the way and letting everything pile up over a long period of time, I suppose.

Kat Volk: You’d need a really big snow plough.

Chelsea Whyte: A really big snow plough and a lot of time. If we did, you know, gather some Kuiper belt objects somehow and throw them at Pluto, first of all, would they stick? Or would we need to put them in orbit around Pluto and just let it, sort of, gather them itself?

Kat Volk: You would want them to collide with Pluto, but very, very gently. You know, when you have two bodies collide, if you want the final remnant to be bigger than the initial target, so what we call a merger in planetary simulations, you need the collision to happen at less than the escape speed of the body you’re trying to add to. So, right now Pluto’s escape velocity is, like, 1.2 kilometres per second. So, it is actually currently being hit by things from the Kuiper belt, that’s why we see craters on the surface. But most of those things are hitting at larger than the escape speed, so you couldn’t just try to, like, maybe shake up the classical belt and encourage things to hit Pluto. You would have to, kind of, somehow gather them up and get them to very gently collide with Pluto because their natural speed distribution hitting Pluto right now does not favour mergers. Also, there aren’t enough really big things, so even if you do “merge” a 1-kilometre body into Pluto, it doesn’t solve our problem.

Leah Crane: Right. And we don’t want them to be, like, hurtling in there and chipping Pluto away, because that would be counterproductive.

Kat Volk: Yes. So, you’d have to very gently collide them, and you’d probably have to do it over a long period of time because you’d have to worry about heating up the ices into, like, gas form. If you did too much at once, you might evaporate too much of Pluto, so it might take a while. Or maybe you could put some sort of balloon around the whole thing to make sure all those escaping volatiles are contained. So, I would expect, if we were adding mass to it and colliding things onto it, the orbit would change over time. And whether or not that new orbit would be stable, it might just be luck of the draw.

Chelsea Whyte: So, if it’s not stable, does Pluto just fling out of the solar system at some point maybe?

Kat Volk: Probably. So, if it’s not stable, it will have a close gravitational encounter with Neptune at some point. And then you kind of have maybe a one-third chance of being scattered inward by Neptune and a two-thirds chance of being scattered outward by Neptune.

Chelsea Whyte: Are you saying we could make Pluto a bullet that would just zip through the solar system?

Kat Volk: Yeah, it certainly could be ejected. Most likely it’d be ejected outward into the scattering population.

Chelsea Whyte: Right, but if we’re lucky it could go in and get Jupiter, and then we don’t have to take care of that guy.

Leah Crane: Chelsea, do you hate Jupiter more than you love Pluto?

Chelsea Whyte: Yes, absolutely, I do.

Kat Volk: Well, I don’t think Jupiter would even care if we hit it with Pluto. I’m not sure it would notice.

Leah Crane: Because it’s big and awesome and it doesn’t care, you can’t mess it up.

Chelsea Whyte: But regardless of consequences, we still want to make Pluto a planet. Maybe there’s an easier way to give Pluto a lot of mass. So, we asked Konstantin if he had any ideas.

Konstantin Batygin: So, here’s the way you make it, kind of, massive, is you draw a primordial black hole inside of Pluto so that it both consumes it, right, but the primordial black hole then is five Earth masses, or ten Earth masses, or whatever.

Leah Crane: Can a black hole technically be a planet by the IAU’s definition?

Konstantin Batygin: No, certainly not by the IAU’s definition. But I think the IAU’s definition is only there to explain what is not a planet, right? Well, actually, let’s think about it. The IAU’s definition is that it has to deform itself into a spherical shape under its own gravity, which, you know, certainly the Schwarzschild radius is a radius, so it’s a sphere, and no-hair theorem tells us that it really is, kind of, a sphere.

Leah Crane: Black holes seem like they’re way better at that than any other planet.

Konstantin Batygin: That’s right, they’ve done a good job at gravitationally deforming themselves, right. But you can imagine a massive enough one that clears out its orbit and it orbits the sun, it’s a planet.

Chelsea Whyte: Oh, this I love.

Leah Crane: I feel like this is just going to lead to a whole new argument about whether a black hole could be a planet, which isn’t really solving our problem of doing away with the quibbling in making Pluto unarguably a planet.

Chelsea Whyte: Yeah, I mean, maybe we just need to think bigger. Maybe the target isn’t Pluto itself, but rearranging the solar system around Pluto to bring it back into the club.

Konstantin Batygin: All you’ve got to do really is decrease the mass of the sun by a lot, like, decrease it to some negligible fraction. Because then, as you decrease the mass of the sun, the mass ratio of Pluto, whatever you want, to the sun becomes a huge number. Then things, even not very massive things start acting like planets.

Leah Crane: I was just going to say, if we decrease it suddenly, say, by chopping it in half and blooping the other half away from the solar system, I feel like wouldn’t Pluto just fly into interstellar space?

Konstantin Batygin: Okay, so this is a great question. If you were to snap your fingers and decrease the mass of the sun by 90 per cent or whatever you want, indeed what would happen is Pluto has some momentum, it’ll keep going and it’ll leave the solar system. But if you do a different exercise of slowly decreasing the mass of the sun such that the number of orbital revolutions of Pluto, or whatever objects you’re considering, is very large compared to the characteristic, you know, timescale over which you would decrease the mass of the central body. As one goes down, the other expands, but you still stay bound. So, you could, you know, expand the solar system that way by a factor of 100, which I think is a good idea. But yeah, you could totally do it.

Chelsea Whyte: Wait, so am I understanding- you’re saying that if we very slowly decrease the sun’s mass, like, I don’t know, we give it an illness or something, make the sun sick and it starts to lose mass and then it starts to shrink, are you saying that the rest of the planets would stay where they are and continue in orbit and not change?

Konstantin Batygin: Yes, they’d keep orbiting around the star and their orbits will slowly expand in concert with the decreasing mass of the central body. So, it’s like the architecture of the solar system will actually not really change, but the, kind of, size that it occupies will. Right now the interactions between small objects like Pluto, et cetera, are small because the mass of Pluto divided by the mass of the sun is a very, very small number. But as you start to crank that up, right, you’ll trigger instabilities, the solar system will enter a transient period of, kind of, orbital chaos, which I love talking about that, it’s just the best thing ever.

Leah Crane: So it seems like not only would this be good for Pluto potentially, while also having orbital chaos, so being bad for everything, but it would make my least favourite planet Mercury maybe a little bit better because it wouldn’t be so hot. Like, maybe we end up with a habitable Mercury.

Chelsea Whyte: Well, yes, and an uninhabitable Earth, and, like, who knows what happens to Jupiter.

Konstantin Batygin: Well, so, you know, what’s weird is beyond the realm of speculation, all of this is actually going to happen in five billion years, right. So, the sun will go through the red giant branch, it will consume Mercury, Venus, and Earth, hopefully Mars as well. After that, right, as the sun turns into a white dwarf, it will lose about half of its mass, and the solar system will expand, the orbits will expand by about a factor of two, which is pretty cool. Unfortunately, there’s no way to make Mercury habitable, and I think that’s a good thing. I think we should just have the sun eat it just as soon as possible.

Leah Crane: Instead of messing with the whole solar system, it might be easier to just physically move Pluto to somewhere where it can clear its orbit more easily.

Chelsea Whyte: Yeah, okay. I love that, yes.

Kat Volk: Because then you only have to move one object. So, I guess, like, a tugboat or something to grab onto Pluto and drag it into the inner solar system.

Chelsea Whyte: Oh, I like that. Where would we put it?

Kat Volk: So, you’d need to put it about between Venus and Earth to get it to clear its orbit on, kind of, the age of the solar system. So, you’d have to move it in quite a ways, but not quite all the way to Mercury.

Leah Crane: Is that just because there’s less stuff in that area, or it’s moving faster?

Kat Volk: It’s more because it’s moving faster, I think. So, we tend to think of clearing up your orbit, if you think about things going around the sun, you know, every orbit is an opportunity to encounter material that is near you. And out in the outer solar system, that takes a very long time because the orbits are very, very long. So, if you’re in the inner solar system, you’re going around faster, the stuff you’re trying to clear is going around faster, you have more opportunities and you can kind of kick stuff around. So, that’s why the terrestrial planets, which are much, much smaller than the giant planets, still can, kind of, clear their orbits of debris at smaller masses.

Leah Crane: Based on that logic, what I’m thinking is I want it to become a planet as fast as possible because I have no patience ever. So, it seems like the best thing to do is to lasso Pluto and drag it into the inner solar system and chuck it right over by Mercury because I want it to be going really fast, I want it to become a planet very quickly. So, I guess my concern is that that might mess up the inner solar system, like, a lot. But it’s so small that maybe it wouldn’t?

Kat Volk: Yes, you know, it’s kind of marginal. But we also have to remember the inner solar system in and of itself, we’re not quite sure if it’s stable for the age of the sun. So, even without messing up things by adding another planet in the inner solar system, it’s possible that- it’s usually Venus and Mercury that become unstable in these long-term numerical simulations of the inner solar system. So, we’re already kind of dancing on the edge of stability in the inner solar system. I mean, we personally have nothing to worry about, in the simulations this happens about ten billion years into the solar system’s history, so, you know, we’ll all be dead, it’ll be fine.

Chelsea Whyte: Then I say chuck an extra dance partner in and see how it goes. Like, that’s great.

Kat Volk: Yes, it would be visually stunning because it would make, you know, the best, most awesome, largest comet of all time to stick Pluto in the inner solar system.

Leah Crane: Let’s do it.

Chelsea Whyte: Yes.

Leah Crane: I think we should do it. I’ve written about Pluto a lot, and a lot of the things that I write about are, like, ‘You know, these glaciers are liquids and the water ice is the bedrock.’ And there’s a lot of ice on Pluto. If we put it over by Mercury, would it melt away?

Kat Volk: Yes, you’d get some pretty vigorous sublimation of that ice. So, even right now where Pluto is, it has a tenuous nitrogen atmosphere, and there are still, I think, debates over how that atmosphere changes as Pluto’s distance from the sun changes. So if you brought that into the inner solar system, you’d get more than a tenuous atmosphere. You’d get very vigorous sublimation of those surface ices, which is why it would make an unbelievably bright comet if you brought something that large. Think of the comets that we see in the sky, you know, we see them in the night sky typically. There have been a few comets in history that have been bright enough to see during the daytime. Pluto would be pretty impressive, probably even during the daytime, because it would be very big.

Leah Crane: So, you were saying that this would create an atmosphere around Pluto. My very obvious and very stupid follow-up question is, are we making Pluto not only a planet, but potentially a habitable planet?

Kat Volk: You might need an atmospheric scientist for that. I’m going to guess not because it would probably be such vigorous sublimation that it would be, you know, more like this giant comet where you are losing material to space and creating this tail.

Chelsea Whyte: That’s what I was going to ask. Would it run out pretty quickly in terms of planet lifetimes?

Kat Volk: That is an excellent question. So, I mean, certainly, you know, quickly compared to the age of the solar system, but not quickly compared to people timescales. I think our best estimate is that when you have, like, a normal comet that’s a couple of kilometres across at most, that it can sublimate for hundreds to maybe thousands of years before it depletes the surface ice. I do not have a back of the envelope estimate for how long it would take Pluto to fade. I mean, that nitrogen glacier is pretty big, so I bet it would take a while to completely sublimate. And then you’ve got all those water ice mountains, because remember, you know, the rocks that we see on Pluto are actually water ice, it just behaves like a rock because it’s so cold. So, all of that would get warmer in the inner solar system and would be available to sublimate. So, I bet Pluto could be active for quite a while.

Leah Crane: So, it would be really awesome for a while, and then we would just have a second, smaller, worse Mercury?

Kat Volk: I don’t know. I think it’d be a better Mercury because at least you’d have Charon along for the ride, so you’d have binary objects – a little more interesting than Mercury.

Leah Crane: Now I’m that we should just, like, billiards it and send Pluto in, kick Mercury into the sun, who needs that, and then we’ve got a pre-cleared orbit.

Kat Volk: You know, I guess actually we don’t know for sure there’s nothing inside Mercury’s orbit. It’s really hard to observe asteroids that close to the sun. There’s been much debate over whether you could have, they call them vulcans I think, vulcanoids. So, I guess we don’t know for sure there’s nothing that it would need to clear, but there doesn’t appear to be, like, a whole asteroid belt.

Chelsea Whyte: So, we’ve found a way to make Pluto a planet, but it’s going to take a lot of time.

Leah Crane: And for all of that time, we’re going to have seven planets instead of nine. So, Chelsea, are you happy now?

Chelsea Whyte: I mean, actually, kind of, yes.

Leah Crane: Well, put Mercury on notice because I’m going to yeet it into the sun.

Chelsea Whyte: Thanks to Kat Volk and Konstantin Batygin for joining us today.

Leah Crane: And thanks to you for listening to Dead Planets Society. If you like this podcast, you might also enjoy my monthly space newsletter at New Scientist called Launchpad. You can check that out at newscientist.com/launchpad. And finally, if you have any cosmic object you want us to figure out how to destroy, let us know and we might feature it in a later episode of the podcast. Our email is deadplanets@newscientist.com. Or if you just want to chat about this episode or breaking the cosmos more generally, you can find us on Twitter at @chelswhyte and @DownHereOnEarth. See you next time.

Chelsea Whyte: Bye.

Konstantin Batygin: Pluto has roughly the same surface area as Russia, and one could make the case that Russia is a planet, but that would be about as good of a case as Pluto being a planet.