Why do we think the solar system might have another planet? Why do we think it doesn’t? How do test for something that’s too far away to see? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
How do you know something is there even if you can't see it? It's a tough problem, isn't it? There there are many things that I've discussed in this show that we know even though we've never seen it. We've never seen the inside of a supernova as it went off in our backyard, and let's hope we never do. And yet we kinda sorta know how the inside of a supernova works.
We've never seen a planet forming in real time, and yet we kinda sorta know how it works. We can't see dark matter. It's in the name, and yet we have a pretty decent idea of how it works. But there is something special about seeing, about direct observation, about holding it in your hands and showing it to the class that you wanna be able to point to and say, yep. Right there, it exists.
But what if that's not an option? What if we can't directly observe? Science is all about inference, which is a big fancy word, and that means educated guessing. We take the evidence, and then we build a case for that evidence. And if the evidence is strong enough for what we're claiming, we're gonna go ahead and believe it for now.
But if new evidence comes to light or the evidence weakens, we're gonna discard what we believe. It's as simple as that. And what I'm talking about today when it comes to inference and knowing something that we can't really see is about our solar system and the planets of the solar system. How many are there? Where are they?
What are they like? And I'm not talking about the whole dwarf planet thing, the whole categorization brouhaha that's been going on for years now. Pick whatever definition you want. It's a pretty straightforward but frustrating question to answer. Once you've defined a planet, how many planets are there?
Why is it hard to answer? Because planets are, a, small compared to the distances we're talking about, and b, very far away. And that combination of being small and far away makes them hard to spot even with the most powerful telescopes. Put this in perspective. We've only known of the existence of Neptune for less than two hundred years.
Less than two hundred years. We figured out steam engines before we discovered Neptune. Why did it take so long to find Neptune? Because it's small and far away and hard hard to spot. And Neptune wasn't discovered through a telescope, but a pen.
There was a French astronomer, Alexis Bouvent, in 1821. He was looking at the orbit of Uranus, and there was something odd about the orbit of Uranus. We knew all the stuff in the solar system. We we understood physics and Newtonian gravitation and all that. And when you put it all together, you should be able to 100% accurately predict the motion of all the planets in the solar system.
And we could, except for Neptune, if something wasn't adding up. Another mathematician, Urbain Le Verrier, looked at this and said, you know what? Okay. You know what would fix the orbit of Uranus? A brand new plan.
If we stuck a planet out there with a very certain kind of orbit, it would explain why we can't explain the orbit of Uranus. And so this new planet, if it exists, if we really are getting everything right, should be found roughly over here, and you can imagine me just pointing in a general direction in the sky. German astronomer, Johann Gottfried Gell, found it. Boom. Neptune.
Making a claim for a new planet to exist like Neptune is a big deal because it assumes you know everything there is to know about the solar system. You know all the players, and you've accurately measured them and their motions, and you can do it all, and you know everything there is to know about physics. Because what if you're getting a little bit of physics wrong? Then your predictions don't mean squat, and there's no Neptune. There's just a whole bunch of empty space.
You get one little thing wrong, one little bit of physics wrong, one little bit of solar system census wrong, head counts wrong, then your claim is just trash. And throughout history, there have been claims of new planets here and there pretty much all the dang time. Why? Because we don't fully understand the solar system. We haven't measured everything there is to measure, and everything is very small and very far away.
And most of these claims of new planets end up being wrong. The winners, like Neptune, are the exception to the rule. Most of the time, it's more like the story of Vulcan where everyone's like, hey. We don't understand the orbit of Mercury. Maybe there's a new planet inside the orbit of Mercury that is explaining its strange orbit.
We understand physics and but we're missing something about the solar system. There were even many sightings of this planet, Vulcan, from astronomers around the world. That kinda died down once Einstein developed general relativity and said, maybe we don't understand there is everything to understand about physics. In general relativity, he was able to explain the orbit of Mercury, and Vulcan went bye bye. There's also the story of Pluto because same as what happened to Uranus, we weren't fully understanding the orbit of Neptune, and we predicted the existence of a new planet.
We actually did find something. We found Pluto, but that was actually a lucky break because then we went back to the original calculations, and it turns out we were getting the math wrong. We actually weren't understanding as much as we should have the orbits of Uranus and Neptune. So that was actually a bad prediction that accidentally got right. Some claims of new planets are more often wrong than right.
And today, we're examining the latest claim, something called planet nine. Now this itself, even the name is controversial because some people you know, Pluto is no longer a planet. It's a dwarf planet. So, technically, there are eight planets in our solar system. So this would be the ninth planet in our solar system.
But some people say Pluto ought to be a planet. So it's already number nine. So, really, this is planet 10. So even the name is up for arduous debate, but it is a claim for a real doggone planet, something big. And most of the questions I got about Planet Nine came to me a few years ago when it first hit the mainstream, and I almost did an episode way back when, but I didn't.
I wanted to give it a few years to see where the evidence went, to see what would be revealed. Maybe we'd spot it. Maybe the evidence would get stronger. Maybe it'd get weaker. I just wanted to see.
It it it was a little bit too fresh back then. And so here we are a few years later from the initial claims, and, the short version is this. As of early twenty twenty, we don't know if Planet Nine exists. We don't know. And so like I did with the string theory series, although not nearly as long, we'll wrap this up in a single episode, I want to put Planet Nine on trial.
I opened the show about evidence and inference and the stories of Neptune and Vulcan so you could get a taste of how these arguments are going to go. Because we don't have a picture of planet nine, otherwise, I would just be talking about planet nine. We just have vague mathematical arguments going back and forth, and that's the kind of language we need to get used to. We can't point to planet nine. We can't see planet nine.
We have to infer the existence of planet nine, which means we have to make an educated guess as to whether planet nine exists. So here's the scene. We've got eight major planets of the solar system. We've got an asteroid belt. And since 1930, we've had Pluto, and Pluto's weird.
It's way weird. It's got a funky orbit. It's really elliptical in its orbit. Sometimes it's even closer than Neptune. Most of the time, it's not.
It's tilted with with respect to the other planets. Just weird. Okay? It's really tiny. It has a big moon.
And in general, it just doesn't fit with the rest of the planets. But it was still, for a while, considered a planet, and for decades, that was it. That was our solar system. That was the portrait of our home. You can make a poster of it and stick it on your wall and call it a day.
But through those decades, astronomers thought that there ought to be more things in the solar system. They were starting to understand how solar systems form. We were starting to do computer simulations, and these simulations were suggesting that there ought to be more than these planets in AstroVault. There should be some junk left over in the outer reaches of the solar system. But they couldn't find anything out there for decades because very small, very far away.
And that all changed in 1992 with the discovery of the first Kuiper Belt object. And the Kuiper Belt is a region outside the orbit of Neptune that contains all the leftover bits of the formation of the solar system. It's like the asteroid belt, only more so and colder. And that is where most of the action of our trial will take place. The first Kuiper Belt object to be identified.
And as you can imagine, a member of the Kuiper Belt is called the Kuiper Belt object. They don't get special names. Like, the asteroids were named first, and then we got asteroid belt. Now we have they're not gonna be Kuiper's in Kuiper Belt. They're gonna be Kuiper Belt Object because, honestly, who the heck knows?
Anyway, the first Kuiper Belt Object or KBO was one five seven six o Albion. It's just a hundred miles across. It's about 40 times further away from the sun than the Earth is. That gives it in the astronomical nomenclature an AU of astronomical unit, which is the distance between the Earth and the sun. We say that one five seven six o Albion is 40 AU away from the sun, 40 times the distance from the Earth to the sun.
So forty forty times further away from the sun. I spent way too long explaining AU, but now you get the gist. Since then, since 1992, thousands of KBOs have been discovered and named and categorized and subcategorized and sub subcategorized because that's what astronomers like to do, and for the most part, it's no big deal. It's okay. Kuiper Belt is a colder, bigger, farther away from the sun asteroid belt.
Whatever. Then 02/2003 happened. Sedna happened. And if you thought Pluto was weird, then Sedna will just blow your mind. The closest to Sedna gets to the sun is seventy six AU.
The farthest this gets from the sun is nine hundred thirty seven AU. That's 30 times the distance of Neptune. Neptune is already 30 times the distance from the sun that the Earth is. It has an orbit over lasting over eleven thousand years. So one year in Sedna takes eleven thousand Earth years, and it's big.
It's not as big as Pluto, but it's in the same class of, you know, relatively large objects for this kind of distance from the sun. It has a really funky, really extreme orbit. I mean, it's far from the sun, but it's also wickedly eccentric. And it's relatively massive, like I said, for this part of the solar system. It's a big chunk of rock doing something weird.
Why? How? We don't know. We need to explain this mystery. How did Sedna get to be so big and so far away?
Because something that big shouldn't have formed that far away. There's just not enough stuff out there for it to form natively at that distance. So it had to form closer into the sun, somehow get out too far away from the sun, and also get this really tweaked orbit. What's going on? Could something else be out there making it weird?
Sedna was our first clue that something funky might be going on in the solar system, especially the outer solar system. But definitive statements are hard to come by because planets in general are small and far away, and members of the Kuiper Belt are very, very small and very, very far away. So it's really hard to tell what's going on. And, also, everything out there is moving super slow. Like, all of recorded human history is contained in less than one orbit of Sedna.
So it's really, really hard to measure orbits out here. Okay? It's like trying to study the mating habits of polar bears while living in Ecuador. I don't know if that metaphor makes any sense or not, but either way, you should contribute to Patreon for more nonsensical metaphors. That's patreon.com/pmsuder.
It is your contributions that keep all this outreach work going, and I truly appreciate it. It also helps me spend time coming up with horrible metaphors. So, anyway, Sedna, KBOs. And here's another random bit of jargon that we need to use. I know it's a lot of jargon here, but we need we need to talk about it.
Because if you're gonna read about planet nine, you're gonna read this word, t n o. That is trans Neptunian object. And there's also e t n o. E a t n o is just anything past the orbit of Neptune. If you got an orbit bigger than Neptune, you are a trans Neptunian object.
I I I don't know why either. Okay? We're just gonna go with it. And there's also extreme TNOs, which are so far away from the orbit of Neptune that they don't even care that Neptune exists. Like, if you're close to Neptune and then, like, if we're to magically pluck away Neptune, you would notice.
You would care. You're like, oh my gosh. Where's Neptune? My orbit's changed. But if you're an extreme TNO and we magically plucked away Neptune, you'd be like, whatever.
Who's Neptune? All Koiper Belt objects are trans Neptunian objects, but not all trans Neptunian objects are Koiper Belt objects. Just there you go. Jargon done. We know of thousands of TNOs, KBOs, whatever you wanna call them O's.
And for the most part, they're very boring. The Cena is a massive exception. But for the most part, these objects are just on their orbits, and they're not really doing anything interesting. But starting about a decade ago, we we began to spot a few weird ones. I won't I and here, I'm deliberately avoiding saying the names of astronomers who are involved in this because what's at stake here is the existence of a new planet.
And there are a couple teams each claiming to be first in discovering this new planet. And so if it's found, the Nobel Prize Committee has a lot of work to do, but it's not important for us of who's doing the discovery and why and who's the privacy and who is first and blah blah blah. That's for the history books. It's not for us. What we care about is the evidence.
So starting about a decade ago, we started to see a few weird ones out here in the Kuiper Belt. And the first weird ones came in the form of a half dozen extreme TNOs. And what made them weird? Their orbits. And what's weird about their orbits?
Well, a couple things. One, their orbits are elliptical, which isn't that special, but they're all rather largely elliptical. Okay. And their ellipses, their orbits are kind of clustered together. And the visual I like to have in my head is imagine a bunch of flower petals on a flower, And each one of these flower petals is an elliptical orbit, so of of each one of these objects.
And so, you know, there's a point where they get to the center of the flower and they get close to the sun. They swing back out. Now they're far from the center of the flower. They're far from the sun. They swing back in.
And on a flower, you expect to see petals in all sorts of different directions. You've got all these ellipses pointing in different directions. But these six extreme TNOs were like picking up a flower and seeing petals all clustered on one side. They're all really close to each other in space. And when they approached the sun, they all tended to approach the closest to the sun at relatively the same location.
Not at the same time, so it's not like they crashed into each other, but the place. Like, if one of these weird e TNOs dropped a ball at its closest point to the sun, then another e TNO could pick up the ball. That's how close these orbits aligned, which is odd. And all of these e 10 TNOs had a similar tilt in their orbit, which is strange. And they all had very, very, very fine aphelion, which is the word astronomers choose to mean farthest distance from the sun.
In their orbits, they were all very, very, very high, like, exceedingly high. Like, when they're far away from the sun, they really mean it. And what's weird about these six e TNOs and their orbits is that we shouldn't see it by pure random chance. It was a very low probability that if we were to just observe six random e t and o's, that they'd have these particular orbits. As an example, imagine, you wanna do a survey of a classroom.
Like, you wanna know you're gonna walk up to the teacher and say, I wanna know what your students are like. Alright? I wanna know how tall they are. I wanna know their hair color. I just want you know, get some facts of your students.
So, teacher, I haven't met your students. I haven't stepped foot inside the room. Give me six random students. I'm just gonna take a sample. I don't have time to study every single student.
So just give me six of them, and I'll see where I go where we go. And the teacher says, okay, and brings out six students, and you measure their heights. Maybe you weigh them. You take their eye color, their hair color, and and you go about your day. And let's say those six kids that were randomly pulled out of the classroom, maybe they're all redheads with green eyes and freckles, and they're all about the same height.
I picked six random kids from this class. What are the chances that these six random kids look very, very similar? Something funny is going on here. By pure random chance, these kids should not look similar. There's something funky going on.
There must be something to explain why this entire class. Because remember, I'm using these six kids that I've observed to represent the entire class. Something funny is going on with the entire class because I shouldn't just pull six random kids and end up with very, very similar hair colors and heights. Something funny is going on. I shouldn't get this from pure random chance.
I need something to explain the weirdness that I'm seeing in this class. Maybe these six extreme trans Neptunian objects that we've observed have very similar orbits because not through random chance because there's a very low chance of that. Maybe it's because there's something else in the solar system shepherding them, shaping their orbits. Maybe there's a planet nine. And the gravity of this unknown planet nine is what is making these e t n o's be weird.
There's, like, a hidden thing in our solar system that's shaping things at these distances. Is that planet nine? Well, we can only guess about it using the properties of the orbits of the extreme trans Neptunian objects. We have to take everything we know about the solar system and everything we know about physics, plug it all in, and say, The only way to explain the data is the presence of a new planet. The first estimates that came out said that Planet Nine was massive, at least five times the mass of the Earth, but sitting at an incredibly far distance.
The nearest it gets to the sun is 400 AU, and the farthest it got from the sun is 800 AU. So it doesn't even pretend to come close to the sun. Why this kind of orbit? Why that kind of mass? Because that was required to explain the data we were getting from these six extreme trans Neptunian objects.
So let's find it. Right? Just like we we did a search for Neptune. We did a search for Vulcan. We did a search for Pluto.
Let's do a search for planet nine. Difficulty. We don't know where it is in its orbit. We can predict its orbit, but we don't know where the planet is right now. And, also, even though it's big, five times the mass of the Earth, it's small, and it's very far away.
Very far away. It's hard to spot. So while we're doing that, while we are scanning the skies, hoping we can catch a glimpse of planet nine, what are some other ways to bolster our case? Because that's our main piece of evidence. We've got these six weird extreme trans Neptunian objects.
These orbits are weird. They should not be collected together like this at a pure random chance something is shaping and shepherding them. Do we have any other pieces of evidence? Well, some evidence came in a couple years later in the form of some more extreme trans Neptunian objects with orbits that were completely perpendicular to the solar system. This was a prediction of planet nine.
That if planet nine is out there and it's shaping the orbits of the members of the extreme transneptunian objects in the Kuiper Belt, then it should send some of them scattering to completely perpendicular orbits. And lo and behold, here are some objects with perpendicular orbits. No one else is explaining it except for planet nine. And since the original findings, even more extreme trans Neptunian objects have been found, about, one to two dozen depending on how you count, Using the updated orbits of these e t n o's, we've updated the characteristics that, planet nine, its mass, and its orbit. So we've had to change where we look on the sky because, oh, oh, oh, no.
Our first guess was wrong. We've got more data. We're smarter now. But the basic story remains the same. There is something fishy going on in the outer solar system.
The only thing that easily explains all the available evidence is the existence of a new planet, period. So case closed. Right? It's just a matter of searching the sky, and it'll turn up one of these decades. Let's you know what?
Let's just go ahead and book that nonrefundable flight over to Stockholm for the prize ceremony, and I I should really start shopping for a good tux. I wonder what would look best on me. Something longer? But wait. The opposition has not presented their evidence.
The biggest counterargument there is to the existence of planet nine is that planet nine making that claim assumes we know everything there is to know when it comes to physics, and that it assumes we know everything there is to know about the solar system, but we could be getting something wrong. And the first thing that could be wrong is that these special extreme trans Neptunian objects may not be special after all. The planet niners, which is my own main up phrase for people who advocate for the existence of planet nine, the planet niners argue that they've picked a representative random fair sample. Like, the teacher went in and gave you six random students across the classroom so you had a fair sample. You don't get to see the whole classroom.
You don't have time for that. You only get six, and there's something weird about these six. But what if it's not representative? What is what if it's not random? What if it's not fair?
Later surveys that have gone on in the past couple years have suggested that these special e TNOs are not randomly selected at all. What if instead of picking random kids from the class, the teacher just gave you the first six? You know, this one, this one, this one, this one. What if all those six were siblings, like sextablets? There were six all related to each other.
And you then come out, and they've all got red hair and freckles and green eyes and about the same height, and you're like, wow. Something's going on with this class. No. Something was going on with your selection method. The teacher was just being lazy.
You haven't learned anything at all about the class because you had a bad sample. You didn't get a random fair representative sample of kids from the class. You got the six siblings. That does that just teaches you about the siblings. It doesn't teach you about the class.
There could very well be all sorts of e t and o's out there with all sorts of orbits, and we just happened, because of the way our surveys are constructed, to catch the ones in these weird orbits. It could be that as we measure more and more and stare and do more and more surveys, we'll find that the eTNOs that we're using to claim the existence of planet nine aren't special at all. We just happen to see those first and that there's more out there in perfectly normal orbits, and so there's nothing to explain. Nothing special is going on. And later surveys have suggested this that, you know what?
Those extreme trans Neptunian objects, those weird ones that were sticking the claim on planet nine may not be weird at all. They were just the first, and we were good at finding those kinds in those orbits. As we get better, we see all the others. We get to see the rest of the class. And as for those perpendicular objects and the follow-up e t n o's, you know, the bolstering the case ones, well, we were looking for those, weren't we?
So, of course, we found them. We always see what we wanna see, and we don't look for the things that, you know, might contradict our hypothesis. Very natural human inclination. Not gonna blame anyone for doing it. This is debatable in both directions about how good the statistics really are.
So we can take it one step further. Let's assume that the e t and o's really are weird. They are really representative. We're not getting statistics wrong and that they demand some explanation. Well, there are other options in a planet nine.
There could be an extra disk of material beyond the Kuiper Belt, the leftovers of the leftovers from the formation of the solar system, and that might be enough to tweak some orbits. There could just be extra gravitational interactions among the members of the Kuiper Belt that we haven't accounted for yet because everything's slow and hard to map out, and we're not so good at predicting orbits at these distances. Plus, there are some arguments against the existence of planet. Like, oh, you want a planet nine? Well, how do you get a planet nine?
How do you get something five times the mass of the Earth at that kind of extreme distance? There isn't enough stuff out there to form a planet, so I had to come from somewhere. Was it an ejected core from our own solar system? Is it a captured rogue planet? Well, if they are, how do you form a planet nine?
What are the chances of that scenario compared to the chance of random orbits of the extreme trans Neptunian objects? If you're saying, woah. These are these orbits are weird. It's a very low probability that we would that they would just happen to be clustered this way. Well, what are the chances that you would form a planet five times the mass of the Earth and send it into that orbit?
You need to compare it because maybe maybe it's more likely for the extreme trans Neptunian objects to just be randomly aligned that way than to have a massive planet out there. You can also say, look. If Planet Nine exists and is shaping the orbits of the extreme trans Neptunian objects, that's fine. But wouldn't it also affect other things like Pluto and Sedna? It's hard to have the rest of the outer solar system behave when you have a five Earth mass monster out there.
Yes. You can explain the e TNOs, but what about the regular TNOs? What about the rest of the KBOs and all the other o's? There's also the fact that we've been looking for planet nine for a few years, and we haven't found SQUAD. And yet, if the extreme trans Neptunian objects truly are weird, then planet nine is indeed the simplest explanation for the evidence that we have.
All the other proposed mechanisms that I mentioned have their weaknesses and can explain all the data. And Planet nine hypothesis is flawed. It's not perfect. Definitely has some weaknesses, but it is the least flawed hypothesis for the orbits of these extreme trans Neptunian objects. It's it's the front runner.
So as of now, the whole crux of the argument comes down to whether the weird extreme trans Neptunian objects that we've observed truly are weird. If what we're seeing is weird is due to observational bias, then the existence for plan nine goes out the window. If we just not have run deep enough surveys and gone long enough to capture more friends out there, more classmates, and eventually it might evaporate? Or is this a real thing that we're really seeing and there needs to be something new in the solar system? That's what it comes down to.
In other words, do we truly know all we need to know when it comes to the outer solar system? And the answer right now is we don't know. Welcome to science. Case closed, except it's not. Thank you to Clyde v via email, Scott m on email, Matthew a on email, the Manly Astronaut on YouTube, Scott Manty, Michael h, and Eric c on Facebook for the questions that led to to today's episode.
Of course, thank you to all my loyal Patreon contributors, my space cadets. I love all of you. But I love the top contributors more. I won't lie. Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nudrude, Chris c, Robert m, Nate h, Andrew f, Chris l, John Cameron L, Nalia, Aaron s, Kirk Seed, and doctor Johnny Fever for your contributions that made this show possible and all the shows possible.
That's patreon.com/pmsutter. Don't forget. You can buy a book. Go to pmsutter.com/book for information on how to die in space. It it's a hoot.
Alright? It's I think you'll enjoy it. I really think you do. You can send questions to a hashtag ask a spaceman. Hit me up on social media.
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I can't thank you enough, and I will see you next time for more complete knowledge of time and space.