Daniel and Jorge Explain the UniverseDaniel and Jorge talk about whether two planets could share an orbit around a star, or whether there would be too much gravitational squabbling.
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Hey, Orge, did you share a room with siblings when you were a kid?
I did.
I shared a room with my brother oooh. And was that like cozy and fun for the two of you? Or did you get on each other's nerves?
It was pretty fun. We get along pretty good. How about you.
I shared a room with two brothers and there was not a whole lot of coziness there. We got on each other's nerves a lot.
Yikes. Sounds intense.
Yeah, I remember after one argument my mom telling us if we couldn't get along, we'd have to live outside. Intense.
WHOA, sounds intense? But is that why you like camping so much? Because you did end up outside in the tent. That's that where your love of the stars came from. This is your origin story. It's all because of your brother, your nemesis or nemesi.
Is how I extracted something beautiful from a difficult story.
Sounds like a hallmark move. We know how you overcame sibling Riberry.
Depends on whether it ends well or not.
I guess it only ends when one of you dies. Hi am Jorhemrick cartoonis and the author of Oliver's Great Big Universe.
Hi.
I'm Daniel. I'm a particle physicist and a professor at u C Irvine, and I've been totally eclipsed by my younger brother.
Oh boy, is he a more shiny star or something.
He's taller than me. He's a professor at a fancier university than I am. What else is there?
But does he have a podcast?
There you go. I don't know if he wants a podcast, but yeah, that's a good point. I'm going to bring that up at Thanksgiving.
Thanks You know, it's super hard to do a podcast, or at least to start one. There's a high bar for putting things on the internet. They don't just let anyone post things on the internet, you know.
I guess we'll find out if he listens to the podcast, if he responds to this.
Oh boy one. In case you're listening, Hello, Daniel's brother. It sounds like we should have you on a podcast and break it down Oprah style.
He's been on the podcast once before when we talked about artificial intelligence, because he's a machine learning professor.
Oh I thought you were going to say he's a machine that too. But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of our Heart Radio.
In which we try not to eclipse your knowledge of the universe. We try to shine a light on it. We think everybody should be cozy with the physics of the universe, snuggling up to a deep understanding of what's going on out there, how it all works, how it started, and what story it tells us about where we live and where we will live in the future.
That's right. We tried to be your big brother in terms of telling you how the universe works and what we've learned from our mistakes and our discoveries so that you can navigate this fantastic universe with a little bit more ease and a little bit more confidence.
I'm not sure big brother has the connotations we're looking for. You know, it's a little bit overwhelminging, isn't it?
M not necessarily? Isn't there like a big Brother program where you mentor kids. That's a positive thing too.
Yeah, there you go. We're not telling you what to think of what not to think. We're just teaching you about the universe.
That's right, And there is a lot to know about the universe and to discover and to find out because it is pretty interesting, pretty complex, pretty vast, and also microscopic in the way that it reveals its secrets.
And one surefire way to figure out how the universe works is to ask basic questions about it. To look up at the night sky and ask why does this do this, and why does this not that. How can we see this happening and not some other thing happening. What are the rules of the universe that allow us to have huge planets like Jupiter but not other situations like planets shaped like a pretzel.
Yeah, because I guess you can look around and wonder like, are we a result of the rules of the universe or are we like an exception to the rules of the universe.
Well, we definitely a results of the rules of the universe, if you believe the universe has rules and it follows them. But there might be weird outcomes and there might be standard outcomes like it. It could be that our solar system is very unusual as an example of things in the galaxy, or it could be that we're totally vanilla.
Yeah, because this humans here in planet Earth. We only really have one view of one planet that we can study up close so far, and so it makes you wonder if other planets are the same, or if humans could have lived in other planets, could things have been different in our history?
That's true, and there are other planets nearby that we can study, though not quite as up close, to try to get more insight into what a planet can be like, where the various possibility how planets can turn.
Out, and do they get along as siblings? Or have any siblings been kicked out of the Solar System and are living in intents by the astory build That's right.
We talked recently about those really fun theories that there was once an ice giant in the outer Solar System that was ejected during a period of chaos.
WHOA, so you think it just couldn't get along with the other planets.
I'm wondering who the parent is in this analogy, Like, is it really the Sun's fault that it didn't work out?
Yeah?
And how responsible is it to kick your children outside and live outside? Yeah? Yeah, somebody should call it you universal child services.
But one thing we do notice about our Solar system is that the planets all seem to have their own orbits. Jupiter's got an orbit, or Earth has an orbit, Mars has an orbit. It's sort of like everybody's got their own room.
They all have their own lane.
You mean, yeah, exactly.
Like there's no other Earth on the other side of our orbit, right, or didn't scientists think for a while or maybe there was?
It sounds like a pretty cool science fiction story, right, second Earth on the other side of the Sun?
Planet X. Wasn't it called planet X?
I think planet X is an idea for a tenth or now ninth planet that might be out there tugging on distant planets, but wouldn't necessarily share an orbit with ours.
But it's technically is it possible that there's like another Earth on the other side of the Sun. Wech just have never seen it because it's on the other side of the Sun.
We wouldn't be able to see it directly from Earth. But of course we've had probes go all over the Solar System. We would have seen it, and we would have noticed its gravitational effects on Mars and Venus and other stuff in the Solar System.
So I guess each planet does have its own orbit its ow lane.
In our Solar system, each planet mostly does have its own lane. But of course the question is why is that and is that typical?
So today on the podcast, we'll be tackling the question why don't planet share an orbit?
Is it because they couldn't get along?
Yeah? And are we the little the younger sibling or are we the the more accomplished sibling.
I think the younger sibling is often motivated to eclipse the older siblings.
Boy, it sounds like a lot to attack there in your family.
I'm nothing but proud of my younger brother.
Let's stick to physics maybe, but yeah, it's an interesting question. I guess. The implied fact is that no planets in our Solar system share an orbit, meaning they're not going around the Sun at the same distance.
Yeah, although you could quibble about that in the case of Jupiter. There is some stuff in Jupiter's orbit, not another planet, but it does share its orbit with some other chunks.
M And I guess the other implied fact is that all the orbits don't overlap. I guess right, because most of the orbits are pretty much circular, right, They're not like ellipses that kind of overlap with each other.
You mean like if they intersect, Yeah, like.
They intersect, or if you look at them from the top, they're maybe like the two ellipsoids are overlapped.
Yeah, I think two ellipsoids that overlap and have different periods would eventually collide. So I don't think you could have that situation for very long.
But it's true that most of the orbits are pretty much circular, right.
They're all a little bit eccentric. None of them are exactly circular, kind.
Of like you and your brothers. They're all a little eccentric as well. I think a little is kind of generous there. We're all quite eccentric, actually, professors or something.
Exactly how does that happen? But in the case of the Solar system, none of the orbits are perfectly circular. They're all at least a little bit eccentric. But yeah, none of them are overlapping. They basically all have their own lanes.
So the question is why don't two planets have the same orbit? So, as usually, we were wondering how many people out there had thought about this question.
Thanks very much to our group of volunteers. We'd love if you joined them. Right to me two questions at Daniel and Jorge dot com and I'll send the questions on over.
So think about it for a second. Why do you think no two planets share an orbit. Here's what people had to say.
I think planets don't share orbits because the center of gravity would have to be absolutely stationary like and shake itself out of sync.
But I think they actually do. It's just very unlikely because it'd be a major coincidence.
So I'm under the impression that planet's by definition or celestial objects orbiting stars that have cleared their orbital path of competing debris like asteroids, and that as a planet forms, it's gravity gets stronger and it sucks up other objects in that orbital path. And that's one of the reasons that objects like series are not considered true planets, because they share their orbit with a number of other large asteroids and other competing debris.
Planets don't share orbits because of the way closters get together. If there is a clumble matter very close to another glombal matter, they will make a planet. And it is that are foot up are they will make another planet?
All right? Some interesting answers. Some people think it's maybe unlikely because they would be a big coincidence.
Or that gravity just doesn't work that way, that it can't pull things together into two planets. Oom.
Interesting, But I guess, you know, thinking about it, like the asteroid built has a whole bunch of mini planets and they're all kind of in the same orbit.
Mm hmm, yeah, absolutely.
I guess we'll dig into that. So first of all, Daniel, let's maybe start with the basics. What is a planet or what do you define as a planet?
So a planet famously has a fairly legalistic definition of what it is. And remember this is just astronomers giving names to things, like there's stuff out there in the universe. The rocks don't care whether you call them a rock or an asteroid, or a planet or a planet. It is well, they're still out there and doing the physics. This is just humans applying like our framework of categorization to the stuff, and different humans on a different planet or aliens whatever it might come up with a totally different categorization. It's mostly historical. It doesn't really reflect deep underlying physics. But it's language and we should all agree what the meanings of words are. And so In the case of a planet, there are three requirements. One that it's mostly spherical. This is basically, isn't big enough because any thing that's big enough is going to be mostly a spear due to gravity. Requirement Number two is that it orbits the Sun and not some other object like a planet, otherwise the Moon would be a candidate for a planet. And then the last one is that it's cleared its path of planetismals and other big objects. Basically that of all the stuff in its lane, it's gathered up all the bits into one object.
WHOA It sounds like by definition, then a planet cannot share an orbit because a planet, in your definition, is something that is by itself in an orbit exactly.
So technically speaking, like the legal answer is no, because then it wouldn't be a planet. But really we're asking, like, could you have two jupiters on opposite sides of the Sun orbiting in the same orbit. That would be really fascinating. Now astronomers from this committee would say, well, technically those aren't planets, and we say like, well, we don't really care. It's fascinating that they're doing this thing, whether or not you call them planets.
I mean if there was another Earth orbiting in our orbit, just on the other side of the Sun, we wouldn't be like, that's not a planet. We would still call it a planet, and you'd just be forced to change the definition of a planet.
Yeah. Maybe, or we'd have t shirts saying like Earth isn't a planet, or Earth is a planet, or I'm on Team Earth or whatever. It'd be a whole ridiculous battle really just over names. You know, where do you draw the dotted line?
But I guess maybe the question is like, why is that part of the definition? Why is it important for astronomers it a planet quote unquote has its own orbit.
It's a good question, and I don't think the answer is just scientific. I think it's historical. You know, we try to sometimes reverse engineer these definitions to reflect the decisions we've been making historically. We've been calling these things planets, and we sort of want to have a definition that aligns with the decisions we've already made, even if they aren't the decisions we would have made if we knew better at the time. You know, early on when we started talking about planets, we didn't understand everything that was out there in the Solar System, all the smaller rocks that were invisible to us. We didn't realize that there was like a full spectrum from tiny rocks to planetisimals, to dwarf planets to real planets.
But I guess maybe the idea that this is part of the definition maybe tells me that there are things out there that are spherical that orbit the Sun but that don't have cleared its path of planetesimals, that are maybe as big as some of the things we do call planets. Is that true?
Well, basically that's Pluto, right. Pluto got rejected as a planet because it was judged to have not sufficiently cleared the neighborhood around its orbit of other stuff. Basically, if you look in Pluto's orbit, you find other pretty big rocks that are roughly the size of Pluto. Pluto is just one of an example of lots of we call them dwarf planets that far out in the Solar System. They have not gathered together sufficiently to say there's one really dominant object and nothing else out there.
Wait wait, wait, wait wait, this was the rule that they kicked Pluto out because of you mean, it's not because of its size. I always thought it was because of its size.
It's sort of indirectly because of its size. I mean, if everything else out there in Pluto's orbit was just dust and tiny greens, then they would have said, Okay, it's a planet. But because there are chunks of rock out there that are sort of too big to ignore, then you say Pluto hasn't really cleared its orbit. I mean, really, technically speaking, nothing has cleared its orbit, because you know, there's dust in all sorts of other rocks in Earth's orbit certainly, right, It's not like it's perfectly empty space in Earth's orbit, So you can ask, like, well, what does it mean to clear its orbit? Even that definition gets a little bit fuzzy, but that's the reason why Pluto was demoted.
WHOA, I'm a little bit outraged. I feel like this is such a weird technicality to have kick Pluto for.
Well, this is the problem with creating categories, right, You're always going to find something on the edge of the category, and then you're going to have to try to crisply define the category, and then you discover the category is just artificial. You're just really drawing dotted lines between a smooth spectrum of stuff. Like I remember when we talked about space centaurs, the things that are so kind of comets and kind of asteroids, And what you discover is really the two things are part of a larger community. And you've drawn a dotted line between commets and asteroids, and it doesn't really make sense to distinguish between them. This is just the problem of humans creating categories that don't really work.
But there was sort of like a voter, there was a lot of discussion about Pluto. So what is it about not having anything in its path that is so important to scientists to kick you know, change change science books, basically for everybody. You know, why is that important whether or not it has little chunks of rocks in its path or not.
I think it's just sociological, Like we look at Earth and Jupiter and Mars and Venus and we say this is one kind of thing, and Pluto is sort of a different kind of thing because it's different in this way. Is just one example of many big chunks of rock out there in the Pluto orbit. So I think there is a distinction to be made between these two different categories of objects. Then again, it's sort of fuzzy.
Meaning like the fact that it has chunks in its path is maybe the only thing that sits it apart from the like, none of the other things that we call planets have things in its path, and so they're using this as an excuse to kick Pluto out exactly.
And when we talk about the history of the formation of the Solar System, you see that it's sort of like one step, you know, the Solar System. You have the planetary disk, and it gathers into rocks and those become planetesimals, and then in some cases they form big planets. In other cases they just stay a big spread of planet isimals, they don't really form planets. So in that sense, planets really are sort of like the next step in the formation of an object in the Solar System, and you've got to respect that or reflect it, or at least, you know, describe it.
You know.
I guess it would make sense if there were a lot of Pluto like planets that look like Pluto or the size of Pluto, but didn't clear its path, and then maybe I could see like, Okay, if you let Pluto in, you have to let all these other things in. But is that the case, or is Pluto just like a lone border case.
Well, Pluto is maybe the thing closest to the edge, But there are a lot of things out there, you know. There aren't a lot of these dwarf planets that you might call planets. You know, Series, for example, is a dwarf planet, but it's part of the asteroid belt, and there's lots of other stuff in the asteroid belt. So you can't say that Series really is a planet because it didn't gather together all the other bits. So I think that is true that once you open the door to Pluto, you have to let in lots of other folks. I don't know why that's the problem. Like, let's have a solar system with hundreds of planets. That sounds awesome to be big party.
Yeah, more siblings to fight with.
But there is something of a distinction here between objects that are basically alone in their orbit and objects that are not. And I do think it's really interesting to wonder, like, could you have two planet like objects orbiting the Sun in the same orbit. Why don't we see that in our solar system? Is that something you might see in other solar systems, et cetera.
I see you want to sort of pivot to the question of, like, what would happen if two planets did share an orbit or two large objects like the Earth share it an orbit?
Could that situation be stable? Why don't we see it? Could that form at all? I think those are really interesting sort of physics questions instead of like category legalistic questions.
That sounds like something siblings would argue about for no good reason. All right, well, let's dig into these interesting questions. But first, let's take a quick break.
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All right, we're asking the question here today why don't planets share an orbit? And we just went through a defined print to find out that astronomers call a planet something that doesn't share an orbit, So technically planets cannot share an orbit. But now we're going to pivot a little bit and ask the question, well, what would happen if something like the Earth share it an orbit with another big thing like the Earth? Well, first of all, it would get kicked out of the planet category, right.
Yeah, that's right. If you built an earth sized death star and put it in orbit on the other side of the Sun, then Earth would no longer be a planet.
Would you say, maybe the Moon is still something that we haven't cleared.
Well, but the Moon is orbiting the Earth, right, so mm hmmm, Oh, I see what you mean. Does the Moon disqualify the Earth because we haven't absorbed it into the Earth. Yeah, these are complicated legal questions.
That sounds like we need a lawyer. Is the brother a lawyer, also an accomplished lawyer.
Probably he's arguing in front of the Astronomy Supreme Court right now.
Yeah, now he just broken a machine.
To do it for him.
But yeah, so, I guess we're asking more of a physics question, not a technical definition question, like could you have two giant Earth or to Jupiter is kind of circling each other, maybe opposite sides of the Sun or maybe not. I don't know. That's kind of the question now, right, Yeah, I.
Think that's the interesting physics question, you know, is that possible? Is there some law physics that prevents that from happening, either to stop it from forming or to break it up if it did somehow randomly land that way.
Because I guess the question is have we seen that. We haven't seen that in our Solar system, or have we maybe like Pluto has it and anti Pluto out there.
So we don't have examples of Earth sized objects with other Earth sized objects in their orbits, but we do have in our solar system some sort of edge cases. Like Jupiter has a whole collection of asteroids that are not in the asteroid belt per se, the one you think about between Mars and Jupiter, but are actually in its orbit. There's two different groups of asteroids that are orbiting with Jupiter.
Wait wait, wait, Like on the opposite side of Jupiter from the Sun there's a whole bunch of debris.
So not on the opposite side of Jupiter thirty degrees trailing and thirty degrees ahead of it, because there are several stable points around Jupiter where you can balance the gravity of Jupiter and the gravity of the Sun, so they cancel out. These are called the lagrange points. And in the fourth and fifth lagrange points in front of Jupiter and behind Jupiter in its orbit, there's a whole collection of asteroids that are sort of hanging out there. WHOA, well, does that mean Jupiter is not a planet? I know exactly right, Like, has Jupiter really cleared its orbit? Well, it's so much bigger than these rocks that I guess you can say so. But like, if I were Pluto's lawyers, I would have raised that.
Example, right right. Why didn't anybody raise that example? Was nobody advocating for Pluto or four science books in general and not confusing the entire human population.
I'm not going to make the mistake of trying to defend any of these definitions because to me, they're all just arbitrary, and they all break down under some scenario. And this is an example, you know, and Earth even has one of these. Earth has an object like this that's in its orbit. That orbit's in one of its lagrange points. It's a pretty big rock. Wait, what really? How big is it? It's not that big. It's like five kilometers in diameter. We've only known about it since like nineteen eighty six when it was first spotted. It's got this terrible name I won't even try to pronounce, but it's spelled c r ui th and e. Maybe you can pronounce it, maybe, yeah, but not on air apparently anyway. It's the sort of feature of gravity. You know, there are places you can be where you can be in a stable orbit, like the James Web Space Telescope is in one of these lagrange points relative to the Earth and the Sun.
Well, I think what you mean by stable orbit points meaning that it has the same location relative to the Earth and it goes around the Sun at the same rate, Like it takes the same amount of time to go around the Sun. That's what you mean by like being in a stable position.
Well, there's two different things there. One is being in the same orbit, and that means following the same path, taking the same amount of time to go around the Sun. The second thing is whether you're stable, and stability beings like if you get a little push, do you return back to the path you are on or does that cause like a cascade where it creates a bigger push, and then a bigger push, and then a bigger push, and then you fall away, sort of the way like a pencil balanced on its tip is unstable. The tiniest little push and it will fall over. But like a ball in the bottom of a glass is stable. You can give a little push, gravity restores it back to where it was. So some configurations are stable, meaning that gravity will push them back to that configuration if they get pushed by a little rock or hit by the solar wind or whatever. And if you have two objects like the Sun and the Earth, there are five points they are called the Grange points that are stable. Three of them are in your orbit, and two of them are not right.
Meaning three of them are in the same circle that the Earth traces around the Sun, and two of them are like one of them a little bit further from the circle on the other one closer to the Sun. And if you're at those positions going at the right speed, then you're going to trace a circle kind of along with the Earth exactly.
The simplest one to understand is that there's a point between the Earth and the Sun where the gravity cancels up because the Sun is pulling in one way and the Earth is pulling another way. And if you're just the right point, it's not halfway between the Earth and the Sun, because the Sun is obviously more massive and has more gravity, But there is a point there where the Earth and the Sun gravity balances out, and if you're right there, there's effectively no gravity and you could just move with the same period, though not the same velocity as the Earth and stay there.
Meaning like you'll go around the Sun in exactly a year, just like the Earth exactly. That's what it means to kind of share an orbit.
In that case, you would have a different radius, so you'd have the same period, but it wouldn't really be the same orbit. But there are three of these points that are actually in the same orbit, meaning you're following the same path. One of them is on the other side of the Sun from the Earth, as you said, and that's very intuitive, right, and because you're like exactly balancing the Earth, and so you could just be like constantly one hundred and eighty degrees out of phase with the Earth. And then there are two points thirty degrees behind and thirty degrees ahead of the Earth also have this special gravitational balance where the Sun and the Earth effectively neutralize each other's gravity.
And does it depend on how big you are or how heavy you are.
So the location of some of these points do depend on the mass ratios, but the other ones are just geometric. So Jupiter has them, Earth has them. All the planets have these lagrange points, and that's why, for example, Jupiter has this collection of asteroids behind it and ahead of it because they're hanging out at one of these lagrange points.
Okay, so I think what we're trying to say here is that each planet has an orbit, and each orbit sort of has these spots where you could maybe put in another planet and it would still be stable and both of them will keep going around the Sun, like it's possible.
It's possible. One tiny little wrinkle there is that some of these lagrange points are stable, some of them are unstable. There are all locations where the gravity balance is out and if you're exactly at the right spot, you can stay there forever. But some of them are unstable, meaning if you deviate a tiny little bit, it's like a pencil leaning over on the table. It's going to fall over, and you're not going to where some of them are stable, meaning that if you deviate a little bit, you'll come back. So the fourth and fifth La Grange point, so one ahead and behind you in the orbit, are actually stable locations. So yeah, and principle, you could have another Earth that's not on the other side of the Sun, but just like thirty degrees behind us in our orbit, and it could be there and be stable gravitationally.
But didn't you say it sort of depends on the mass, like and you can imagine if I put a James Web telescope, it'll be stable there, or maybe a small asteroid or five kilometer rock. But if I put a whole Earth, wouldn't it just destabilize the whole thing? Or wouldn't it be totally a different point.
If you put another Earth there, it would be stable, Like the situation is symmetric and they could both survive there. There are mass limitations, like if you put a star there that's much more massive than the Earth and things is like a limit of like twenty five times the mass of the Earth, then you break some of the assumptions. It doesn't work anymore. But you could put a pretty massive planet there and it would be stable.
Interesting. But then now the question is, why isn't there something there? Because I'm sure at the beginning of the Solar System they're asteroids everywhere, there were rocks everywhere. Why didn't those two points start to gather things? And why don't we have a little twin Earth.
The answer to that is that we don't really know. I mean, we think that in the formation of a Solar system, it's possible for it to happen. Remember the story we told earlier. We start from a big cloud of gas and dust and leftover bits from other Solar systems that have died, and that collapses most of it into the star, but like one percent of it stays in this protoplanetary disc, a bunch of heavy metals and gas and dust and ice that is moving too fast to fall into the stars. The star gobbles up mostly gas and leaves behind this protoplanetary disc, and then gravity does this thing. It starts from little seeding locations, there are little dots of heavy metal or little gravitational spots that have over densities. Starts to cluster together other stuff and you get the string of rocks. So now instead of having like a disc, you have a bunch of rings, rings of these rocks of various sizes, which then gather together and form stuff. And so one option is that you know, it forms into a planet. There's like one dominant thing that pulls out all together into a planet. Another option is that it just sort of stays a ring of rocks because of tidal forces from neighboring planets that prevent it from gathering together. But there's nothing that says in physics that you couldn't have two objects coalescing. You know that two things couldn't have pulled together sort of in each other's lagrange points and ended up being what looks like two planets, although you know the astronomers would have said neither of them are planets.
Well, I feel like maybe we skip a step there. So you're saying, like the early Solar System was a flat disc, and then the stuff that was orbiting the Sun or the new Sun kind of arranged itself into rings, sort of like maybe Saturn's rings, So you can mentione the Sun in the middle a bunch of rings, just like Saturn has rings of rocks, and then those rings eventually collapse into planets. Now, what's the process for that, Like what causes a ring of rock to suddenly collapse into a giant ball on one specific spot.
Well, that's gravity. Also, you know, you have one spot along the ring that's denser than another, and so it's going to pull on stuff nearby, stuff that's ahead of it, it's going to pull closer. The stuff that's behind it, it's going to pull closer. And then as it gathers more stuff, it's going to have more gravity. It's gonna be able to reach further along that ring, and it essentially grab that stuff together.
But I can see that for gathering the things around it. But what about the things that we're on, like on the opposite side of the ring.
I think the answer is that we aren't one hundred percent sure how that happens. We don't know. For example, if you can really form stuff on the other side of the Solar System that stays there, like for example, Jupiter's asteroids, did those form? There is an example of exactly what you're talking about, stuff forming along that same ring and not getting absorbed into the planet, basically resisting because it's in a lagrange point and it's gravity is totally balanced, or is that stuff that came later. Did Jupiter actually clear its own path use up all the stuff in the ring, and then and later on stuff fell into Jupiter's legrange points. We just don't know the answer to that. If stuff really can form stably at the same time as a planet, that's what you're asking, right.
Well, I think I'm asking like it. Once we had a ring of rocks, sort of like Saturn has rings. Is it the case that each ring could only clasp into one planet because of the math. Basically, like the math doesn't let you make more than one big planet because anything else would be unstable.
No, the math would let you imagine a perfectly symmetric ring, and then imagine like ten or twelve or fifty tiny points perfectly spaced around that ring that are a little bit heavier. Those would gather up all the stuff around them. You'd end up with like ten or twelve or fifty or whatever, identically sized objects in an orbit around the Sun. Physically and mathematically, it is nothing preventing that from happening. I think the reason why it's not likely to happen is that it requires a lot of symmetry and balance For that arrangement to get set up. It's much more likely that one of them is bigger and one of them is smaller, and then the bigger when eats the smaller one.
But I feel like told me that before that there's only like two stable spots around and stable orbit where other things can be. You know, if I formed four miniplanets along the Earth's orbit, that wouldn't like some of them be unstable.
It gets much more complicated as soon as you add more than two objects. But now you could have that stable arrangement. You could have like a thousand planets in a stable orbit around the Sun as long as they're perfectly equally spaced in equal mass. That's also allowed. Earlier we were just talking about like two planets around the Sun. But if you allow them for more than two planets, there's a lot more configurations you could have, but they're also much more unstable, like they need to be perfectly balanced in mass. In fact, I found some guy on the web who's done a bunch of simulations of like crazy solar systems.
You know, we say, some guy on the web, Yeah, on the web, Wait what this is? Okay, mm hmm.
I found some guy in the web who posted a blog about simulations he had done about crazy solar systems where you have like sixty jupiters in orbit around the Sun and it's stable. Like if you arrange them perfectly, the sixty jupiters can stay in orbit around the Sun if you set them up perfectly.
I see, I see. But how do you know he's right? I mean, no offense. But if I told you that some guy in the Internet it's only something, would you assume it's true?
No, But this one makes sense. I mean, just from symmetry arguments, a ring could collapse and do a bunch of objects. As long as it does it symmetrically, then it's still stable. Every one of those objects is the same as any other objects, so it feels the same gravity from all the other ones. They all cancel out essentially.
But I wonder how true that is. I don't know. I mean not that I don't trust your guy on the internet, but you know, at some point, maybe, like if you have four things going around the Sun, maybe the angles and the way they interact with each other, and depending on how big they are, maybe it would make it unstable.
I think it's a pretty simple extension of the argument for why you don't feel gravity from anything that orbits the Sun at like the same radius as you or larger or for example, as you dig into the Earth, you're not feeling gravity from layers of the Earth that are larger than you because they all add up to cancel out. There are fewer that are closer to you and more that are further and they all balance out. It's basically the same argument. It's like gravitational shell argument. If you have a whole string of earths in orbit with you, all of their gravity will cancel out.
All right, Well, it sounds like we have a big mystery, which is that it sounds like it's possible for two planets to share an orbit definitions notwithstanding, but we don't really see that in our solar system or maybe in other solar systems out there in space. So let's dig a little bit more into that mystery. But first, let's take out a quick break.
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All right, we're asking the question why can't planets share what's going on? Why are they so selfish?
It's the parent's fault.
It's always the parent's fault. But yeah, the planets like to be loners. They don't like to share an orbit. And we sort of know that mostly from what we can see, right, because it sounds like theoretically it is possible to have like two Earth orbiting or maybe for Earth orbiting the same circle around the Sun. But we just don't see that, and so the question is like why not. Yeah, I think that mathematically it's possible. If you set up a solar system that way, you're like a solar system builder, and you bring into Jupiter's and you align them perfectly, you could build a solar system like that, which would be stable and last a long time. You know.
The other question is would a solar system like that form naturally? I think they're the answer is that it's most likely for things to collapse into one big planet, or for things to not really collapse at all, like Pluto or the asteroid belt because of tidal forces, or they're just two distant gravity. And for things to collapse into two really big or three big objects would require a sort of a perfect symmetry where they like divide up the mass budget into two or three portions and each of them gather it up themselves. I think that just requires a lot of balance for it to happen.
Like it basically a big coincidence you're saying, But I guess maybe paint the scenario for us. So we have initially around the young Sun sort of a ring of rocks. Let's say, like there's a ring of rocks around where Earth goes around the Sun in the Earth's orbit, and maybe there's a little bit more rocks in one spot, and so that once starts to gather more things, but there's a whole bunch of things all around the circle. Does that mean like the things on the other side of the orbit somehow speed up in order to catch up to the Earth or slow down. What causes those things to slow down or speed up? And if they did, wouldn't they change orbits?
No, you're right, something on the other side of the Earth like perfectly opposed to the Earth, feels no gravitational tug. Right, the Earth cannot pull that thing into itself because it's at a labraange point. Right, So something that's there will stay there. But that's a really tiny spot, the actual exact lagrange point. Anything slightly ahead of it or slightly behind it will feel a gravitational tug from the Earth, and eventually the Earth will pull it into itself.
But it has to go all the way around the circle, meaning like it speeds up in the orbit. But if it speeds up, wouldn't it change orbits?
Yeah, that's possible. Also, the Earth could tug it, not so that it ends up in the Earth, but that it falls into another orbit or falls into the Sun. Right. And remember that this is a very simplified way of thinking about the Solar System, just one planet and the Sun and one rock. In reality, there's lots of other things going on there, So lots of chaotic tugs from Jupiter and Mars as it passes by, so all sorts of stuff disrupting this pile of rocks, some of them falling out of this orbit and landing on Venus or landing on Mars, or some of them ending up on Earth. That's why Earth has rocks from all over the Solar System that are landing on it, not just things from our orbit.
Oh I see you're saying like maybe the picture I had of how planet's form is not quite correct, Like you know, maybe to go from the equivalent of Saturn's rings around the Sun to a planet, it doesn't mean like a ring of rocks is kind of collapsed into a planet, like Earth might be made out of the rocks from different rings.
Absolutely it is.
Other planets might be made from rocks from other rings.
Absolutely, yeah. There is is not just made from rocks from one specific range of radii. Definitely, stuff is contributed from the inner Solar System and from further out. Absolutely there's a big mix.
Oh I see. So like the history of the Solar System is that it pour into a disk, then rings. Then it was a free for all where all the siblings for fighting over who gets which toys and which rocks, and then and then whoever was left victorious, whoever it was bigger one exactly, yeah, and whoever it couldn't get along is sleeping outside in a tent.
And that's mostly the story. And to understand whether that's the story the Solar system. We look around to see if there's like weird stuff happening. You know, we'd love to have seen two planets in the same orbit so we could try to understand how that happened. There are some really fascinating and interesting counter examples to this that we see even in our own Solar system, like the moons of Saturn.
What do you mean, what's going on with the moons of Saturn.
So Saturn's got lots of moons, and there's two in particular that's swap orbits. So there's the moon Janus and then there's the moon Epimetheus, and these two swap orbits every four years, Like Janus is on the outside and Epimetheus is on the inside, which has a short orbit and it comes really close to Janus and tugs on it, and the gravitational interaction means that they end up swapping. So now Genus is on the inside and Epimetheus is on the outside.
Well, well, do they swap orbits or it's just that they both have really complicated orbits that overlap each other.
Well, there's two different radii and for four years, one of them has the inner one and the other one has the outer one and then they swap. So you can think about it two ways, like one is they have different lanes and they're changing lanes, or you could think about it in the more general sense, like each moon does four laps on the outer one and then four laps on the inner one, and they're perfectly syncd to never be on top of each other.
And the reason they change lanes is from the where they interact with each other, or just the way they interact with Saturn.
The way they interact with each other, like the one on the inner lane catches up to the one on the outer lane and tugs on it, pulling it in, and the gravitational interaction pushes the one from the inner lane.
Out well, meaning sort of like a younger sibling who's motivated to catch up to its older sibling and then or takes it for four years.
What else motivates younger siblings if not that? I mean, that's basically it.
Right.
Wait, are you trying to take credit for your brother's accomplishments? Is this what this is all about?
That's right?
Is this why this whole episode is just to take some of your brother's glory.
My entire life is just nothing but setting the stage to take credit from my younger brother. That's all right, right, yeah.
Okay, you're trying to overtake him and steal his orbit.
Yeah. The whole thing is a fifty year long con man.
The whole thing is a therapy session. I feel like, oh, in.
The end, you know, you're defined by the circumstances in which you're born, just like Saturn and Janus and Epimetheus doomed to do this dance forever. But it's really fascinating to see this like every four years. And it's a funny story because when these moons were discovered, they didn't realize there were actually two moons there basically in the same orbit. They saw one, and then somebody else was looking at that moon and realizing, huh, this doesn't look like the same moon that was previously reported, but it's in the right orbit. What's going on? And took people a while to figure out, actually the two moons here that are swapping orbits.
Whoa, And does the swapping happen like really fast or over years as well?
The swapping happens pretty fast, I mean it's every four years that happens. But the swap itself doesn't take years.
And could something like that happen elsewhere or is this like a super duper bizarre circumstance.
It's a pretty bizarre circumstance. You need exactly the right radii and exactly the right masses. We also don't know how long it can go on for. You know, it's pretty chaotic, and so it seems like something else passing nearby gives just the right tug and one of these things just plummets into Jupiter. But it's an example of how really weird, rare stuff can happen in the Solar System. And it might mean that even if two big planets forming in the same orbit seems unlikely because you need just the right balance, it might happen. It might happen in other Solar systems. It might happen more than we even expect.
Well, it sounds like it might happen, just from how random and chaotic the you know, young Solar system is, right, Like, you know, maybe you wouldn't get two plants from the same ring of rocks, but maybe, you know, you could get the Earth and then something like Mars drifts into our orbit or something, or we swap orbits with Mars, just like these two moons. That's totally possible, right.
It's totally possible, And some people think that something like that might have been a cause for the collision actually between the proto Moon and the proto Earth. Remember, the theory of the Moon's formation is that there were two proto planets that slammed into each other that formed the Earth the Moon. And it could have been like two planets that formed separately and then got too close to each other, or maybe they were in one of each other's lagarrange points for a while, but it wasn't stable and then it ended up colliding.
And it sounds like it could still happen maybe in our Solar system, Like you know those meteors that are orbiting with Jupiter ahead and behind it, those could kind of maybe cluster together into something that might look like a planet, couldn't they.
Yeah, eventually they might unionize and work together and form their own planet, and then they could protest Jupiter and they could get Jupiter demoted from being a planet or Pluto upgraded. You never know once you start an astronomical lawsuit what happens.
Right, it might be like a planetary conspiracy and they all get, you know, sued together.
Maybe we just get rid of this whole name for a planet, which is sad.
This stuff out there, man, let's just call them yeah rocks, big floating rocks, unless they're made out of gas.
Oh no, there's big planets the size of balls.
Just call them balls. If it's circular and it's southern space, just call them space balls.
I know. And they even require these things to be spherical, and none of them actually are sphericle, right, even the Earth is not technically spherical.
Oh boy, so it's a lawsuit waiting to happen. Well, how about out there beyond our Solar system? You know, now we can see with our telescopes really far away distant stars and distant planets. Have we seen two planets share in orbit out there in the wider universe?
We have not yet seen it, and we've looked. You know, we are pretty good at seeing planets around stars in other Solar systems in some scenarios. And we can tell because those planets like orbit the star and eclipse it, or they tug on the star, and we can tell about the change and the light from the star. And so it's looking for the wobble in the star and dips in its brightness that give us clues, and from those dips or from those wobbles, we can get a sensor like the period of the planet. And so far we've never seen like two wobbles or dips with the same period that would indicate like co orbiting planets. So so far we haven't seen anything like that.
I feel like maybe there's something else going on here, maybe something that physicists haven't thought about, because it seems possible, it seems sort of not impossible. And given so many planets and asteroids and things out there floating in space in our Solar system, and also all of the trillions of Solar systems we can see out there that we haven't seen one, it seems like there's something going on.
There's a conspiracy. Remember that we've looked at only several thousand stars to see if there are planets around them, and there's billions and billions of stars in the galaxy. That might mean that it's unlikely and not that common, but it doesn't mean it doesn't happen. In fact, there are astronomers that used a radio array in Chile to look at a planetary system that's still forming. There's a young star like three hundred and seventy light years from Earth, and it's not done making its planets, like they see exoplanets forming around it, but it's still got a really big disc of protoplanetary stuff. And when they studied this system, they found a big blob of dust in one of the lagrange points near that exoplanet. So that might be like a coplanet that's forming, or it could be that whatever this conspiracy is, you know, breaks that up before it.
Can happen, Like maybe it's not as stable as you think it is or something. That's what you're saying.
Right, Maybe it's technically possible, but it requires such ridiculous balance between all the factors that it's never practically achieved in the universe.
All right, well, it sounds like maybe the answer to the question of why don't you have two planets sharing in orbit? The answer is a like, we don't know. It seems possible, but we just haven't seen it yet.
Yeah, it seems possible. It might just require very very special set of circumstances that we just haven't had a chance to see yet. But as you look deeper into the universe, everything that's possible, everything that's weird. I think eventually you will see.
Like two brothers that get along statistically, it might happen.
Two brothers that are professors at exactly the same level of prestigious university.
There you go. Make it all depends on their intent. All right, Well, we hope you enjoyed that. Thanks for joining us. See you next time.
For more science and curiosity, come find us on social media where we answer questions and post videos. We're on Twitter, Discboard, Insta, and now TikTok. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.
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