Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 11 | Mike Brown on Killing Pluto and Replacing It with Planet 9

Episode Date: August 27, 2018

Few events in recent astronomical history have had the worldwide emotional resonance as the 2006 announcement that Pluto was no longer considered a planet, at least as far as the International Astronomical Union was concerned. The decision was a long time coming, but no person deserves more credit/blame for forcing the astronomical community's hand than Caltech astronomer Michael Brown. He and his team discovered a number of objects in the outer Solar System -- Eris, Haumea, Sedna, and others -- any of which was just as deserving of planetary status as Pluto. Rather than letting the planetary family proliferate without bound, astronomers decided that none of these objects dominated the orbits in which they moved, so none of them should be planets. Now Brown and his colleague Konstantin Batygin have found indirect evidence that there is another real planet far beyond Pluto's orbit -- which they have dubbed Planet Nine just to remind you that there are currently only eight. [smart_track_player url="http://traffic.libsyn.com/seancarroll/mike-brown.mp3" social_gplus="false" social_linkedin="true" social_email="true" hashtag="mindscapepodcast" ] Mike Brown received his Ph.D. in Astronomy from U.C. Berkeley in 1994, and is currently the Richard and Barbara Rosenberg Professor of Planetary Astronomy at Caltech. He shared the Kavli Prize in Astrophysics in 2012 for his discovery of major new objects in the outer Solar System, and in 2007 won Caltech's annual Feynman Teaching Prize. Home page Wikipedia page Blog Twitter How I Killed Pluto and Why It Had It Coming Online course, The Science of the Solar System Download Episode

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Visit your nearby Lowe's on West Pico Boulevard in Los Angeles. Hello everyone and welcome to the Mindscape podcast. I'm your host, Sean Carroll. You may have heard the one sign of an open, rationally minded thinker is the ability to change one's mind, to have an opinion and change it to something else in the face of new arguments or evidence. Personally, I don't even remember usually when I change my mind about things. One of the ways my brain works, and I don't think I'm unusual in this, is that I convince myself that I always believed that thing that I believe right now. even if I did change my mind. But there's one example where I remember very vividly in science that I changed my mind from a strong belief one way to another, and that's in whether or not Pluto is a planet. You may have heard, you may remember where you were when you first learned the news back in 2006.

The International Astronomical Union got together and decided that we would no longer classify Pluto as a planet, but instead as a dwarf planet. This caused outrage across the globe, as people, schoolchildren, and older folks said, look, we know what the planets are. There are nine of them, and Pluto is one of them. This issue even made it into an episode of Rick and Morty, the animated feature. But there, on Rick and Morty, the reason why Pluto had gotten demoted is because it was getting smaller,

that the plutonians were mining their own central core so that Pluto itself was shrinking over time. That's not why in the real world Pluto got demoted. It's not that Pluto changed, it's that our understanding of the solar system changed. Pluto didn't get smaller, the rest of the solar system got bigger. Out there beyond the orbit of Neptune, there's something called the Kuiper Belt, which is a collection of a large number of objects, many, many, many objects. Some of them get pretty big. It's only fairly recently that we discovered that there are a number of objects in the Kuiper

belt that are comparable in size. to Pluto. So basically the choice for astronomers was either to expand the solar system, to include all of these new discoveries as new planets, or to demote Pluto from the ranks. Of course, there's another option, which is the one that I originally believed, which is you could just grandfather Pluto in, right? You could say the planets are the nine planets that we know about, and Pluto discovered in 1930 is the last one we're going to let into the club. We've known for a long time that Pluto is small. Pluto is smaller than Earth's moon. But the important thing upon reflection is that Pluto is not even the most important object

in its orbit around the sun. Pluto's orbit crosses that of Neptune. It's at an angle. It's sort of not that important, dynamically speaking, in the solar system. So rationally, there is really no reason to keep Pluto as a planet and exclude the other ones. The IAUs, the IAUs, the International Astronomical Union eventually decided to just invent a new category, call them all dwarf planets. If anyone is responsible for this change in attitude towards the status of Pluto is today's guest, Dr. Michael Brown, an astronomer and colleague of mine at Caltech. Mike was the one who led the team that discovered these other large Kuiper Belt objects

that are now joining Pluto in the Dwarf Planet Club. He's received a lot of scorn for being the person who demoted Pluto, but he owns it rather than denies it. Mike's Twitter handle is Pluto killer, and his book is called How I Killed Pluto and Why It Had It Coming. That's actually the book that I read that finally changed my mind. As a scientist, it's important to be rational, to try to understand things, to categorize them properly, and when you face up to the evidence, Pluto does not belong in the Planet Club. These days, Mike is trying to make up for what he did to the solar system by finding a new planet, he and our colleague Constantine Batigen at Caltech, claim that there is evidence in the

motion of known Khyberbel objects for a new planet out there far beyond the orbit of Neptune, which they have dubbed Planet 9, just to remind us that Pluto is not one of the planets we already have. You can make up your own mind. The astronomers have made up theirs. Today we're going to figure out why we think the Pluto doesn't belong and what that tells us about the future of understanding what's going on in our solar system. So let's let's see. go. Mike Brown, welcome to the Mindscape Podcast. Thanks for having me.

So you're technically an astronomer, but you're in a planetary sciences department here at Caltech. Like, what do you tell people you are when you meet them on an airplane? So I will say, if someone says, what do you do, I say I'm an astronomer. But my official title is Professor of Planetary Astronomy, so I get to kind of have it both ways. But I'm an astronomer who looks at planets in our solar system. Yeah, you don't look at stars or galaxies like a real astronomer.

You know, they get in a way, and I have to somehow figure out ways to ignore them, but I look at the real planets. How did you get interested in that? What kept for your imagination there? It is not what I thought I was going to do when I went into graduate school to be an astronomer. I went to Berkeley for graduate school, and I went to Berkeley because I wanted to work on the most distant galaxies known to man. You've made a terrible mistake. Yeah, so these most distant galaxies at the time were the goal was to find things at a red shift of three, which is funny to people who know these days that people are finding things. I don't even know how much further away these days. But at the time, that was the big quest, find things at redshift of three. One of the people doing this, the best in the world was High Spin Red at Berkeley. And so I went to Berkeley to work with High Spin Red. High Spin Red. High Spin Red had a hobby of looking at at comets in addition to looking at galaxies. And he just liked, I'm not sure why. Actually, his wife claims that the reason he'd like to look at comets is because he couldn't tell the difference

between galaxies and comets. They're just so smudge. They look the same. And back in the day, we didn't even know, right? Nebulae. Yeah. Well, so back in the day, it wasn't that long ago, so we did actually know. But he would take spectra of comets, study the composition of comets. and he always tried to get his grad students interested in studying comets in addition to galaxies. And he could never get anybody interested because at the time, and it's still sort of this way. At the time in astronomy, there is a rank order of who is the coolest and who is the least cool. And the coolest kids are the ones who study the very most distant things. That's what I wanted to do.

So if you study very distant galaxies, super cool. If you study nearby galaxies, you're probably okay. If you study stars in our galaxies, you're kind of a loser. And if you study planets or anything in our solar system, like, why are you even there? Exoplanets are cool, but like planets that are in actual our solar system? Yeah, so at the time, there were no known exoplanets. So even the fact that exoplanets have now have made nearby things because their exoplanets are closer than distant galaxies. So now the ranking is a little different.

But at the time, if you study planets, you're a loser. So he forced his students to study comets for one summer before they could look at galaxies because he just wanted them to get some work done for me. Paising ritual. Yeah, so I did. Mostly I wanted to work on comets, but I worked, I mean, galaxies, but it worked on this comet stuff. And then a moderately bright comet came by at the time, and we went up to the telescope,

Lick Observatory, to study it. And I remember this moment forever. We were looking at comet Austin. I can't remember what year. Comet Austin was. Looking at Comet Austin through the telescope, getting a spectrum, it's coming out. We're seeing the composition. And I walk out into the dome of the telescope.

And I can sight up the barrel of the telescope. And I see the comet. There in the sky. Yeah. And that was it. It was like, oh, my God, this is not an abstract thing like a distant galaxy with coordinates that we're measuring. This is that thing in the sky. And I was stuck since then.

I have always... Your heartbeat a little faster. It really did. So for my thesis, my PhD thesis, I studied Jupiter and its moons, and I would be there the telescope, and I would get out my binoculars and stand outside and look at the moons and see the same thing I was seeing. And it's just, I love that visceral feeling that what I'm studying is actually real as opposed to this very distant smudge.

Those are also real, by the way. Just so our listeners now. I'm not convinced. I'm not convinced. But they are less visceral. They are further away. So, yeah, you can't see them. Almost everything I've studied. These days, I study some pretty faint things.

But at the time, they were all bright enough that you could take out your binoculars and see them. And that was pretty cool. And you mentioned spectra. I mean, one of the things that, I was an undergraduate astronomy major. I don't know if you knew that. I did know that. I have no degrees in physics. All my PhD and bachelor's agree are both in astronomy.

All right. I've forgotten it all by now. But one of the things that absolutely was drilled into me was the ability. of astronomers to take an incredibly tiny amount of data and spin an incredibly elaborate story about what they were looking at. So why don't you say a little bit about like, so we take a picture of a comet? What does that tell us? How do we get information about it? Yeah, so it's the, for the studies that we were doing in particular, it's the, it's the spectrum. You take a, the comets up in the sky, you collect the light from the comet, you stick it into a big

elaborate prism, and you split it up into all its colors. And all of the, you, you chemicals in the comet, in the atmosphere of the comet, in that coma that makes a comet a comet, each one of them has a different fingerprint of basically colors that it emits. And so we were simply trying to see what all the chemicals were in that coma by looking at their very, we had a very, very, very fine spectrograph. It's called, you know, elaborate prism where we could really break up the light into incredible number of colors and we could really in very detail, see not just what the chemicals were, but they slightly change their characteristics based on their temperatures and their velocities. And so we could map out all these things just by looking at that

one little spot of light. Because these comets are moving around the sun and you're catching them not when they're at their furthest away, but when they're more or less close to the sun and move by? Yeah, so they're, because we needed it to be pretty bright to be able to break up the light into all those components. So the comets that people tend to study in detail are ones that are pretty much the same distance away from the sun as the earth is. That's when they really start to get the heat up. Their surfaces evaporate. They get all that stuff in the atmosphere, and that's when you can really study their details. And they're dirty snowballs, roughly speaking. That's what I remember from astronomy. They are dirty snowballs. And then the interesting question is, what is the dirt and what is

the snow? Right. Because it's, yes, it's mostly water in the snow. And it's mostly, well, we don't know what it's mostly on the dirty part. But it's that studying the other parts of the comet are what's really been. And it's not really snow. There was never a snowfall on the comet. It's ice. Ice. Yeah. It's an ice ball up there.

So you moved on, though, from comets to planet-like things. Well, so I then as my PhD, I studied the volcanic emissions from Jupiter's moon, I.O. Okay. As they exploded off the surface of I.O. And then they would go into orbit around Jupiter. And then the magnetic field of Jupiter would grab a hold of them and start spinning it around. And so I was studying this elaborate dance of all these objects that you could do in the same thing. It was actually the same instrument that broke up the light into very small components that allowed me to see, here's this and here's that, and here's where it's going, and here was that.

So it was just, it was fantastic. So you're looking at all the chemicals that I-O. I always said EO. Is it really? I can say both in the same sentence. You're a professional. Okay. So, yeah, so the volcanoes, this huge volcano, right, on I.O.

They're a bunch of them. A bunch of them. And they're just spewing stuff out into the atmosphere. And the whole neighborhood of Jupiter. is like a mess with magnetic fields and radiation and a whole bunch of things going on. And junk from Io. Actually, junk from I. From I.O. is one of the main components of the magnetosphere that's going on there.

Which would make it tough to, like, go visit Jupiter and hang around in a spaceship. This is actually why when spacecraft go to Jupiter, they usually spend most of their time pretty far away. In fact, the Juno spacecraft that's there right now, it's on this very, very, very elongated orbit. It comes in really close, but then it goes off super far away. And it does that because it cannot spend that much time very close because it will get smacked basically by stuff that came from volcanoes and I.O. Okay, so you're still, though, studying things that we know to have existed. So Galileo discovered I.

Yes. That was a long time ago. That is true. What made you move on to the further reaches of the solar system? So while I was a graduate student at Berkeley, the very first object beyond Neptune since Pluto was discovered, the first new Khyber belt objects. 1992 QB1 is its license plate number.

And at the time, I remember hearing about it at the time, from the discover, actually the day before I went public, Jane Liu told me about it. She was in an office right down the hall for me. And I thought, oh, that's interesting, but, you know, who cares? It's a rock. Yeah, ice ball. Big deal. And very quickly, it became apparent that this region beyond Neptune

was full of stuff and that it was, it's in a sense the most pristine region of the solar system. It's not pristine, but it's the most pristine. It's like these things are cold. They've been in deep freeze since the beginning of the solar system, and you can study more and more about how the solar system formed by finding these very distant things. And so I thought this is an interesting thing to think about. But the big change was when I arrived at Caltech. and looking around at Caltech as an astronomer at Caltech,

I suddenly had access to telescopes that I had never had access to before. The big telescope at Palomar Observatory, the relatively new at the time, Keck telescopes out on Monacaea, are big telescopes that are really good at seeing faint objects. I made a deliberate decision to stop studying these relatively bright planetary objects because I had unique ability to study these faint ones, and I sort of changed path entirely and decided to start studying the outer parts of the solar system. So let's set the stage here what the solar system looks like. You have your planets, you have your asteroids. You mentioned

the word Kuiper Belt. What is the scale of all these things, and where does the planetary system end and stuff like that? Yeah, so, you know, the inner part of the solar system, most people would say the inner part of the solar system is everything inside the orbit of Jupiter. So we've got Mercury, Venus, and Earth, and Mars all in a line there. And then between Mars and Jupiter is this big region of relatively small, rocky asteroids, the asteroid belt. And the Asteroid Belt is not the Star Wars version where you have to dodge while you're flying through it. Every time a spacecraft goes to the outer solar system, they try desperately to fly near an asteroid so they can take a picture of an asteroid. And it's hard.

It's hard to find them unless you try really hard. That's not what the movies are taught. Oh, it's nothing quite like that. It's a very dangerous place as far as I can do. Yeah, so you could go through there a million times and miss every asteroid unless you were trying hard. When I was a kid, I was absolutely convinced that it used to be a planet that got destroyed somehow,

and I still kind of cling to that belief. But apparently astronomers don't believe that the asteroid belt used to be a planet. No, but they did believe that at first. So the first asteroid was discovered January 1st, 1801. It was a series, and then in quick succession, three more were discovered. And, you know, imagine how strange this was. You could look up in the sky.

The most recent discovery had been Uranus, and Uranus was the first thing discovered with a telescope, the first, you know, object that we knew about in the sky that we didn't know about before, found with a telescope and kind of blew people's minds. Well, sorry, Galileo in the moons, right? Yeah, okay. So there were moons of things that were being found. That's actually true. In Saturn's moons, Herschel discovered them.

I can't think of the year that Herschel discovered them. But your absolutely. But Uranus is out there all by itself. All by itself. And so it actually led people to search for a new planet systematically. And, you know, at the time there was this sort of numerology, the Bode's law that suggested there should be a planet right about where it turns out the asteroid belt is. Because it's easy to make up numbers that make you.

think something might be true. And so people started looking. They called themselves the celestial police. And they started scanning the skies. Actually, the celestial police did not find the first asteroid. There was an accidental discovery of the first one, but the celestial police found many of the, well, the next three. So they found three, they found four in quick succession, four, maybe four years, three years, four years. And they all had very similar orbit. So people were looking for a planet at about this location and they find four small things. And that was the not unreasonable at the time assumption. Like, well, must have been a planet that exploded.

Seems plausible. Or was exploded? Yeah. Something happened. And nothing else was found until I actually don't know when the fifth asteroid was found, but it was 1840s, I think it was. How many do we know about now? Oh, 300,000 maybe, you know, down to the same.

size of a desk or something. You know, there's tiny things out there that are found. And now we know that there was no planet that formed there. In fact, it's the opposite. It's that a planet would have formed there, but Jupiter messed with everything. You know, whenever there's anything going on in the inner solar system, it's probably Jupiter's fault. Right.

Yeah, exactly. But I think that there's a lesson that we'll come back to, I think. You know, we think of the solar system is big. the planets are relatively tiny and we sort of treat them independently, I think, in the mind of a non-astronomer. But the lessons are, over these millions and billions of year timescales, there's a lot of influence on what's going on in different parts of the solar system from the planets that are there.

And this fact that there's a whole bunch of things in more or less similar orbits between Mars and Jupiter has to do with gravity and dynamics and how Jupiter does things. Yeah. So if it hadn't been for Jupiter, all these other things. objects would have been able to coagulate together to form a planet. But instead, Jupiter is so close by every time an object gets close, but not even really that close. To Jupiter, it gets a little tug, and its orbit is kind of perturbed and shaken up. So basically, as these things are trying to coagulate, Jupiter comes by and shakes them. They try to coagulate, Jupiter shakes them. And in the end,

they're never able to form a planet. Or the Jovians didn't win the competitions. They destroyed the planet in its early days. It is actually possible. The astronomers shouldn't be as close-minded. The establishment is hiding some things here. I've read them. So good. So that's the inner solar system. That's the inner solar system. And then we get to Jupiter.

So Jupiter. So then there's the realm of the giant planets. So Jupiter and Saturn are the two really big giant planets. You know, it's funny. People have a very poor understanding of the sizes of planets because mostly they see them like on kids' lunch boxes where they're all more or less the same size, you know. And pretty close together. Mercury is a little smaller than Jupiter, but not that much smaller. So Jupiter and Saturn are huge.

Wap and big planets. And so these are, you know, so the distances we measure everything in is astronomical units. One being the distance from the Earth to the sun is one astronomical unit. Jupiter is at five, so it's five times further from the sun. And then the giant planets are nicely arranged. Jupiter's at five, Saturn's at 10, Uranus is at 20. It'd be nice of Neptune. We're at 40, but no, it's at 30.

But, you know, okay, close enough. So those are pretty easy to remember. So Neptune is the end of this realm of the giant planets. But Uranus and Neptune are actually not nearly as large as Jupiter and Saturn are. They are, I think, Uranus and Neptune are maybe three or four times the physical size of the Earth. You know, they're big, but they're not. I didn't think that they were that small. Yeah, they're really kind of small.

Jupiter and Saturn, huge. Jupiter is 315, I think, is. the number of times more massive than the Earth. Neptune is about 17 times more massive than the Earth. And they're all gas giants? So does that mean they're all gas, or there's little rocky cores? So the big ones, well,

so this is actually one of the prime reasons that the Juno spacecraft is at Jupiter right now in orbit around Jupiter is trying to answer that exact question. We think probably that they all have rocky cores. But we don't know for sure, and we're trying to find the answer to that with

these sorts of spacecraft. I mean, the great red spot has been on Jupiter for hundreds of years. Yeah. Isn't it, do people think that maybe it's the reflection of that there's some feature on the surface of the core? No. Purely atmospheric. Yeah, purely atmospheric.

The core is tiny. The core, so I said that the Jupiter weighs about 315 times more than Earth. The core might be 15 Earth masses of that. So it's really a very tiny fraction, a critical fraction that actually leads to the formation of Jupiter itself. But in terms of what's going on with Jupiter, it's actually a pretty interesting. insignificant chunk going on. So that's Jupiter and Saturn is very much like a slightly smaller version of Jupiter.

Uranus and Neptune are very, very different. Although we think of them all as gas giants. Many astronomers call Uranus and Neptune ice giants. So it's a better description of Uranus and Neptune. Let me step back. A good description of Jupiter and Saturn is mostly gas with a little bit of core. Uranus and Neptune are mostly core with a little bit of gas. It's not like you could stand on their surface,

and it's not like they have a solid surface in the core. It's this weird metastable liquid that I understand the physics of, but I don't really even understand what it means when I say it. So some people say they're liquid in the interior. It's not really liquid in the interior, but they're very different from Jupiter. We've never landed on any of these planets. Well, we've sent probes into a probe into Jupiter. It scratched the tiniest bit of the surface before it was imploded.

due to the pressure of the metaphorical surface. It didn't actually reach the core. It went in a tiny, tiny bit. People would love to send probes into the other ones. In particular, Uranus and Neptune would be fascinating because they're so different and we know so little about them and because planets like those seem to be very common throughout the galaxy. And so it would be very interesting to learn about what those planets are more like.

So Uranus and Neptune have been flown by, once, by Voyager, one or two. I forget which one went. One of them diverted so it could do a flyby of the rings of Saturn and to do that, it had to go, had to basically go up out of the solar system and never go by anymore. But one of them went by Uranus and Neptune, and that's it. We know very little about these planets. Which means there's a lot of room for young astronomers to grow up and study these things more, right? Not that we don't know, right? Yeah. Okay. And then it was always true about Pluto that it was a little weird.

If you saw a picture of the solar system that was a little bit more accurate, the orbit was way more eccentric, right? All these planets have circular orbits. Pluto's a very strong ellipse. It's tilted compared to everything else. And it's not even its own orbit. It crosses inside Neptune occasionally.

Yeah, it's so weird. So I remember before we understood Pluto's place and the rest of the solar system, it really was just considered sort of this oddball at the edge of the solar system. No one really knew why I was there. how it got there. It didn't really make any sense,

but everyone was like, well, I guess. And it was found kind of by accident? Like, they were looking for it. Yeah, so well, so this is how the whole problem with Pluto and planethood started is that people were looking for planet X. Now, when you say Planet X, people just think that means anything out there that you don't know about. But Planet X was an actual thing. There was a prediction of a specific planet,

from Percival Lowell. Percival Lowell had letters for all of his predictions, and X just happened to be the one that he was predicting. And the reason he thought that there was a planet out there is because he looked at the orbits of Uranus and Neptune, and they appeared to be being tugged by something. And of course, this is how Neptune itself was found. And so the day that Neptune was found in 1845,

astronomers everywhere were like, oh wow, Laverier predicted a planet based on perturbations, and dude got super famous. I'm going to do the same thing. And so, I mean, literally, from that day, people have been saying, I predict a planet, I predict a planet, I predict a planet. Every single one of them has been wrong until very recently. But Lowell had predicted that there was this planet and set out to find it. And he actually, one of the very first times he looked for it.

He sent a team up to Mount Wilson right here above us here in Pasadena. You can look out the window and see the telescopes. The big telescopes didn't exist yet. This was something like 1916. There was a small station up there. And he said, go look in this location. And they took a big photographic plate of the sky right there and brought it back down. And he looked at it and was like, I don't see this giant planet that I'm looking for.

So he eventually passed away. but he had founded Lowell Observatory with one of the goals was to find his planet X that he predicted. And that's why Clyde Tombaugh was hired off the farm to come take pictures of the sky looking for this planet. Clyde Tombao took a couple pictures and realized that he didn't know what a planet looked like. Because he just looks like a star. Right. The spot on your photographic plane. Percival Loew thought that it was going to be big.

So he thought he would know what it looked like. It would be a big spot instead of a small one. Clyde Tombao said, you know, I don't know. And so he realized that you take a picture one night and you take a picture the next night and all the stars and all the galaxies are in the same place. Planets move. So we did that. So we took photographic plates, looked for things that moved.

And very quickly and very close to the predicted location of planet X, he found Pluto. There's a little dot moving. And, you know, I think about this a lot because this is one of the ways that science can get it wrong at first. and then eventually correct itself. There was a prediction of a giant planet at this location. Something was found at this location. Therefore, it must be that thing. So if you go to the New York Times headlines when they announced the discovery,

9th planet discovered in the solar system, blah, blah, blah, right below the headline, New Planet, 4 billion miles from the sun. I think that's the right number. I can't do miles well, but that's 4 billion miles from the sun. possibly as large as Jupiter and meets predictions. And that's the meets predictions

is where science can get itself in trouble. So because there was a prediction, so people thought at first that Pluto was as big as Jupiter. That is only wrong by a factor of 250,000. But if you think it's as big of Jupiter, if it were as big as Jupiter, it would be no question that it's a planet

in its own right. it took a long time for that mass of Pluto to slowly work its way down to the realization now that it's like it's a tiny fraction. I mean, it's smaller than our moon and it's a tiny fraction of the mass of the moon because it's just a little ice ball. So it's in the grand scheme of things in the solar system, it's pretty small. And then there's this thing called the Kuiper Belt, which has a kind of interesting history of its own, right?

Like it wasn't discovered by Kuiper. It was vaguely predicted by Kuiper. So it was named after Kuiper. I think this is a longstanding and good tradition in astronomy. It was basically named by those people who found 1992 QB1, which was, I would say, the second Kuiper Belt object after Pluto being the first, but we didn't know at the time. And they said possible Kuiper Belt object.

And they named it after Kuiper because Kuiper had written a paper that suggested that possibly there is this belt of iceballs out beyond Pluto, they said at the time, out beyond Pluto, that's the source, one of the sources of comets that come into the inner solar system. The Kuiper paper was really nearly a throwaway. It was not a very detailed calculation or really much of a prediction. But it was, you know, Kuiper is a large figure in planetary astronomy. me. So I'm actually very happy that it's, that it was named for him. There are people who argue, oh, that was the inappropriate name. It should be called the blah, blah, blah, blah, blah.

But it's not, it should be. It's the discovers named it. And I think that's, I respect the naming of the discovers. Absolutely. And it's different than the ord cloud, which we've heard about in terms of where comets come from also. So why do we need an orc cloud and a Kuiperbell? Because there are two flavors of comet. One flavor of comet comes in, basically, in the disk of the solar system. And we see a lot of comets that are basically on the same types of orbits as all the planets are tilted by a little bit, but not very much. And then we see a second set of comets that come from everywhere, all directions equally with no preference. Those come from the Orte Cloud and cloud because it's this uniform, very distant cloud around the sun. And the

Kuiper belt, belt because it's a torus of material. out beyond Neptune, just like the asteroid belt is mostly stuff that's in the plane of solar system, the Kuiper Belt is the same way. And are these really two clearly distinct populations? Do they kind of blend into each other? So, this is an

active question that we would like to know the answer to. They probably come from the same original source, but we don't know very much about the transition from the Kuiper belt to the org cloud. I used to think I did, and now I know less than I used to. Science. Yeah.

It's true. So the Kuiper Belt is as we're continuing our tour of the solar system. So Neptune is the edge of what we know of as the realm of the giant planets. And then out beyond Neptune there is this belt of icy material that is completely analogous to the asteroid belt. It's not the same as the asteroid belt. It's icy instead of rocky. But the reasons for it, the way it behaves are exactly the same. The reason there is a Kuiper belt instead of a planet beyond Neptune is because Neptune messed with stuff. Neptune did the same thing to the Kuiper belt

that Jupiter did to the asteroid belt. There would have been a planet beyond Neptune had Neptune not formed fast and then shook up everything out there and didn't let it form into a planet. So there's no planet out there. There's just a belt of debris, basically, that never got a chance to form a planet.

And some of the moons and things of the planets that we know about in the solar system might have been captured from the Kuiper belt? So the moon, Reasonable hypothesis? Yeah. So the moons that you generally know of, like the big ones, and like the galleon satellites tighten around Saturn,

those all formed in place. So those are all part of the planetary system. But all of the giant planets have what's called irregular satellites. Regular meaning the regular satellites are in the plane of the planet. They rotate in the same direction as the rotation of the planet. So they're all part of that initial disk. But they all have, you know, it almost sounds like the or. cloud that I was talking about. They all have these clouds of small moons around them that are just

going in all kinds of crazy directions. And those are absolutely captured from the Kuiper Belt or from the region where the Kuiper Belt got started to begin with. So some of them were probably, they were probably captured early on before there was even a Kuiper Belt, but it's the same stuff. It's those same icy things that are out there. Okay. All right, good. So I think that more or less finishes our tour and we're able to catch up where Jane Liu, I think it was, had discovered. a new, this new, yeah.

This new, yeah. And then what year was that? 1992. That's why it gets that license plate number of 1992 QB1.

Okay. And is that a similar orbit to Pluto or similar size? It's about 200 kilometers across. So it's small

compared to Pluto which is about 2,400 kilometers across. And it does not have the same sort of orbit as Pluto. It's a little

further away, further out. It's a little bit more circular than Pluto is. What we now know is that that there are several different classes of objects in the Kuiper Belt.

There are many, many, many, many objects with orbits just like Pluto. And an orbit just like Pluto, I don't mean it's exactly in the same orbit. I mean it is, it comes inside the orbit of Neptune. It's tilted by, you know, anywhere from zero to 30 degrees. It's elongated like Pluto is elongated. If I drew a diagram of all those objects and put Pluto's orbit in there too, you could not distinguish. So there's one flavor that's like that.

A little bit further out, there's a flavor of ones that are like this 1992 QB1. They're a little bit more circular, a little bit more, they don't cross the orbit of Neptune. To cross the orbit of Neptune and to live, you have to be on a very special orbit on a resonance it's called. So Pluto and all of these other objects that are called Plutinos. Plutinos. Yeah, that I think is a good name too. all of those objects Neptune they go around two times

the sun they go around the sun two times precisely for every three times Neptune goes around the sun so they're locked into this very precise dance that they are through complicated gravitational mechanism they're forced to be locked into that they can't escape it but by being locked into that

they never come close to Neptune so they're there every time they cross the orbit of Neptune Neptune is on the other side of Sun So actually Pluto comes closer to Uranus than it ever does to Neptune, even though it crosses Neptune. And it's another example of this sort of gentle but crucially important dynamical influence that the planets have on each other. And random objects in the solar system can't be in any old orbit. There's certain orbits that are happy for a planet to be in. And good.

But this is this discovery of more Kuiper Belt objects. This is somewhat your fault or in Caltech's fault anyway for giving you telescope. Yeah, something. So yeah, so this is a. when I started here, there were, boy, I probably when I started as a young, naive assistant professor, there might have been a hundred known Kuiperbald objects. Okay. Which is, you know, going from one in 1992 to 100 was a lot of work for a lot of people. And I, and I realize this is, this is going to be big. I want to, I want to get into this. And so I started doing

a couple projects, studying the known objects. And what I really got excited about was the realization that it was very clear. It was obvious to people who had looked carefully at it, that there would be some large Kuiper belt objects out there. They might, like large, I mean, Pluto size, bigger than Pluto size, we didn't know, but that Pluto was not an outlier. It just happened to be the one that was found first by Clyde Tombao. But there should be other things very much like Pluto-esque. The hard thing is, that finding objects, the small number of large objects

is a lot harder to find than the large number of small objects. The analogy I used to always make when I was doing this is that if you, you know, if you go out into the ocean and you get a big net and you scoop it, you'll find a ton of small fish. But you're probably not going to get a whale. Finding a whale is a lot harder. I got that analogy.

That's a good one. You have to go sail all around. So we didn't have a good way of, you know, we didn't have a big net. Astronomers at the time were really good, and they were just developing these electronic detectors, the CCD that everybody now has and their cameras were pretty new then. And they're pretty small. If you remember, your first digital camera might have been one of those like 380 by 500 pixels that was, you know, tiny compared to what we have now. Astronomers were the same way. We could only look at a tiny area of the sky, but these things were so good.

This is late 90s? It's the late 90s. So if you wanted to cover large swaths of the sky, you couldn't with these digital detectors. So I actually did one of the very last projects, I think, with photographic plates. I'm probably the youngest astronomer to ever do a project using photograph plates. That might actually be a true statement. I'm not 100% sure that's true. but I used this old telescope at Palomar Observatory,

which had been built at the same time as the big 200-inch telescope. The 48-inch Schmidt telescope had been built, basically to take wide-field pictures of the sky to help the 200-inch nowhere to look, what was interesting in the sky. And there are these famous, at the time, these famous sky pictures that you could go to any astronomical library, and they would all have drawers that you would pull out

and get the Palomar prints for that point in the sky, and you could see what was in the sky. Now you can get them all online, and it's less fun. But at the time, if you were going to look at something, you would go to the library, you'd pull out the print. You would take a picture of it because you were about to go to the telescope and you needed to make sure you were looking at the right thing. They had these, you know, Polaroid cameras designed specifically to,

you mount in it here, and you could take a picture, and you'd walk back with your picture of where you were looking in the sky, your finding chart, we called them. Now, you know, kids these days. they just look on their computers. Their internet and their Twitter's, yeah. Yeah. But it was pretty fun back then.

So it was built to do that sort of thing. And it was, nobody was doing things with photographic plates anymore. So it was spending a lot of time just sitting around doing nothing. I don't like it when telescopes sit around and do nothing. Makes me very upset. So I realized that I could use it to take pictures of vast areas of the sky. The disadvantage is photographic plates are not, very sensitive compared to digital detectors. And so in a, the, the differences in about a half an

hour of exposing a photographic plate on the sky, I can see things about the same faintness as I can now from the same telescope that's still being used, but with digital detectors in about 30 seconds. Okay. So, so it was a painfully, painfully inefficient, but it means I could cover big areas of sky. So I spent a couple years doing that and I and just sort of caught the bug. I really was pretty convinced that we were going to find something big. So you were just papering the sky. You were just looking all over the place. We didn't have enough time to paper the whole sky. And so we had to pick where to paper. And so we went, we did a swath right along the, the ecliptic, the plane of the planets. And we know if we look up in the sky and you see where the moon is and where Mars is and where

Jupiter is, they make this line across the sky. That's the thing. That's the thing. That's the line across the sky that we looked at. And we looked a little above and a little below and went along there and spent three years. And it was pretty exciting because we found not a thing. Pretty exciting. Zero. Literally zero. There were no objects in the sky bright enough that we could find them in that part of the survey that we did.

Turns out, had we, so I told you, we looked at the ecliptic and we looked a little bit above and a little below, had we gone 50% more below or 50% more above, we would have found things back in 1998. Okay. But we didn't know that at the time. It turns out the bright Khyber belt objects are preferentially above and below the ecliptic, not on the ecliptic. Who knew?

So we didn't find anything, but it just really reinforced to me that this is something that we just needed to do. Somebody needs to go out there and cover the whole sky. photographic plates were a little bit of a pain, but this was right about the time when the digital detectors were getting better and bigger. They weren't great, but you could start, you could kind of string a bunch of them together.

It's kind of like, you know, taking a hundred of those 580 by 300 cameras and mounting them all on a big board and pointing them at the sky. I mean, it was as sort of clugee as that. But in the end, we could cover inefficiently still, but we could cover vast parts of the sky.

And so it took about six more years, seven more years to cover the whole sky, but we covered the whole sky to much better than we could do with the photographic plates. And slowly as we were covering the whole sky, we would find, we found a lot of moderately big Khyberbald objects. I'll say moderately big to me is a 500 kilometer object in the sky, which is kind of cool. You know, you're sitting there looking and you're in your office, at the images from the night before and suddenly you see this thing that's a big chunk of ice that no human has ever seen before.

And literally billions of miles away. Yeah. That's pretty fun. Hang it out in the middle of nowhere. I still get at charge every time I find one of these new ones. But every once in a while and actually more as we got further and further off of the ecliptic, there would be one that you would, you know, you're just looking through the data and you'd be like, oh, oh, oh.

and I would do quick calculations of how big it was, how far away, what was going on. And it was pretty cool because we would find, you know, it started out, we found one that was half the size of Pluto. We thought that was pretty good. That's pretty good. We didn't know how big it was at the time. We thought maybe it was going to be bigger than Pluto, but we learned later it was about half the size Pluto.

It was the biggest new thing that had been found at the time. It was the largest new object that had been found since, I think it still is true. largest object found in the solar system since 1845. Okay. Well, other than Pluto. Other than Pluto. I guess the largest. Oh, that's when we found error.

I was found like, I used to say that phrase. What was I talking about? I was talking about ERIS when we found errors, which is later. And it's not actually the largest. So I was wrong. But so at the time, 1845, it was like the largest objects found since Pluto. Okay. That's pretty good.

That was good. And then we found, you know, as time went on, we'd find one that was slightly bigger, like three quarters the size of Pluto. They just, they just kept stacking. up and it just by chance we found the smaller ones first and the bigger ones next and then one of the last ones we found was in fact eris eris was this one that uh i remember seeing it on my screen when i first saw it and it's it's it was moving very slowly across the screen and it was really bright and my reaction was we're all going to die what did i do wrong this time um because

you know 90% of your best discoveries are mistakes yeah yeah maybe you're 95, maybe 99%. I mean, I make a lot of really exciting discoveries, and most of them are wrong. I've had some really great theoretical ideas, too. Yeah. So same thing. So I was like, what did I do? It's moving slowly. Our typical sequence was we would take an image every hour and a half. We would take three images over the course of three hours and see how fast the things were moving. So I thought, you know, what if I just screwed up and I accidentally took them every 10 minutes? And I see this thing not moving very much. It's because it's actually. an asteroid really close by, moving fast.

That's why it's bright. What did I do wrong? I went and checked everything. I'm like, well, that actually is right. Oh, that was right too. Oh, that's right too. And I'm like, it's real. Oh.

So I called up my wife and I said, I just found a planet. Because it was obviously as big as Pluto. And if Pluto's a planet, it's pretty clear that this thing was a planet. Aris. You didn't name it that. At the time, at the time, I did. At the time, I did not name it that because it didn't. We had code names for all the things that we found at the time that we would talk about just because we needed, you know,

the first thing that it was called was the name that the computer gave it, which was, I can't believe I can't even remember this one. Anyway, some string of letters and numbers, most of which used to mean a lot to me and now I can't remember it. But we gave it code name Zena, which I had been reserving for something. bigger than the... Or your princess-like thing, yes. Something that was... So, you know, with the idea being that people had always talked about planet X, I wanted an X.

I thought, you know, it'd be nice to have an X. I wanted to have it, you know, good mythological name. And so, okay, so it's TV mythology. But Pluto was named after a, you know, a cartoon dog, so that seems okay, too. It's not actually true, but it's mostly true. And then there were, you know, there were not enough female planet names. And so I thought, good. If you wanted a X,

Mythological X female name. Choices are limited. You can't do better. You cannot do better. It was an awesome one. We found a satellite. There was an obvious name for the satellite. Satellite was Gabriel.

So that's what we called it for the first. The time while we were still studying it before we were working on our papers to announce it to the world. By the time it got announced, it still didn't have a name. So I told, I believe I told one reporter that we called it Zina. and that story got out and everybody now knows. That's the lead right there. For many years, it was mostly known as Zena. Or the other official license plate was 2003,

UB313. That was the International Astronomical Union's license plate number. But clearly the stuff shirts at the International Astronomical Union are not going to let you get away with Zena as a long-term name for an important celestial object. Yeah, probably not. Although... So now it's ERIS.

So now it's ERIS, which is, I have to say, a fantastic name. It's a good name. So we didn't get to name it. They held off on allowing us to name it until they could decide what it was. Right. There were some people who were pushing very hard that it be called a planet. When we announced it, we said 10th planet.

Because I figured, even at the time, I did not think Pluto deserved be a planet. And so I didn't actually believe the heiress deserved to be a planet. But I thought, look, if you guys are going to call Pluto a planet, I'm going to call this a planet. And the worst that can happen is you can say, no, it's not. And then Pluto doesn't get to be a planet either. So it's win-win for me. So we call it the 10th planet. There were other people who were very adamant that it should be classified as a planet.

And other people who are like, no way, that's ridiculous. And we just got to hang out and watch the arguments go. But it meant that we couldn't name it because if it was a planet, well, nobody knows how you name planets. But if it's just a regular Kuiperbald object, there's ways to name it. So look, when Pluto was officially relabeled, as a dwarf planet, not a planet. Billions of hearts were broken. People got very upset.

You're considered to be a bad person because of all this, and yet you kind of revel in that. You don't back down. You lean into it, as they say today. Yeah, yeah. What is the best sales pitch for saying, no, we shouldn't call these things planets, or at least we shouldn't call Pluto a planet? Why can't we just let Pluto be a planet

and call these other things post-Putonian objects or something like that? So you will often hear arguments from astronomers who are tired of talking about this, that it doesn't matter. Pluto is Pluto no matter what you call it, blah, blah, blah. It's semantics, it doesn't matter. And I get what they're trying to say, but I actually disagree completely with the statement that this is just semantics. It doesn't matter.

It's not semantics. The word planet, the word that you use is semantics, but it's classification. And classification is what we do as scientists to try to understand phenomena in whatever field we're studying. And bad classification leads to a lack of understanding of what you're going on. You could be somebody who studies birds and you decide to classify them all and you might classify them as, you know, sea birds and birds that live here and birds that burrow in the ground and all these things. And, you know, you would study them in different ways and that would be good.

Or you could be a scientist who studies birds and you could say, I'm going to study them all the ones that have blue on their heads. And, you know, that's a classification, and it's a perfectly valid classification. It just not, doesn't mean very much. It doesn't latch on to anything real out there in the world. It doesn't lead you to ask any important questions. And so when you look at the solar system

and you think about a classification in the solar system, the classification should lead you to the important questions. So if you had the eight planets plus Pluto as a planet, the main question you would ask about the solar system is, what the heck is Pluto doing there? It doesn't, it doesn't, you can't ask any questions about it because planet the word is, the classification is sort of meaningless. There are other people who suggested that you should have all round things should be planets, which would include Pluto and Eris and 200 other objects in the Kuiper Belt. The moon? The moon, many moons. And so there are many things.

And it's true, they are different because the being round means you have enough gravity that you have pulled yourself into a sphere, which is very different. And so the question you would ask yourself about the difference between round things and not round things is why are the round things? That seems like the obvious question. Well, I just told you. It's gravity. We know the answer to that, actually.

If instead you classify the solar system, and if you were to say there are four terrestrial planets, rocky planets, Mercury, Venus, Earth, Mars. There are four giant planets, or maybe two giant planets and two ice giants, Jupiter, Saturn, Uranus, Neptune. Between Mars and Jupiter, between the terrestrials and the giant planets, there's an asteroid belt. Beyond Neptune, there's a Kuiper belt. Even further out, there's an Orch cloud. That leads to profound questions. And the profound question, the obvious question is why. And that why is the main question that we as planetary scientists are trying to answer. How did the solar system get to be the way that it is?

And by classifying it correctly, you are led to that question. And by not, we as scientists would still ask the right questions, but I still feel like it's a public disservice to pretend that we're going to call all these other things planet. It doesn't help people understand what the solar system is like. And I would like people to understand what the solar system is like. And before we forget, what is the definition of a planet? going to say it. I refuse. Nobody knows. It's better off than no one knows. It is better off than no one knows.

Because the definition, I mean, there are people like, ah, it's a stupid definition. And like, okay, yes, it is a stupid definition. The fact that there is a definition is stupid. In astronomy, I mean, can you think of anything else of a phenomenon in astronomy, an object type in astronomy in the sky for which there is a definition that somebody has to check? It's this. In fact, planet has a three-part definition that you have to fulfill all three parts, and the lawyers can argue about whether you fulfill them. Let it on us. Tell us the three parts.

So the IAU says you have to be... Round. So that's the last one. You have to be an orbit around the sun. So this says people get confused by this one, too. The definition of a planet in the solar system is that you have to be an orbit around the sun. I mean, I thought part of the motivation for going through these arguments was that we thought we would be discovering planets elsewhere. The only motivation for this argument is to deal with Pluto.

Okay. I mean, literally, that if there is no other reason for this definition of a planet, then Pluto had to be dealt with one way or the other. So the definition is, in orbit around the sun, round, big enough to be round, and then the third part kicker, which is where all the arguments come about, and it's phrased terribly, but it's, I understand what they're trying to say, it has to clear its orbit. clear its orbit of other stuff.

Of other stuff. Right. So the, you know, instantly the amateur astronomical lawyers say, well, so Neptune's not a planet because Pluto crosses planet. I was just going to say that. And it's, you know, yes, it's because it's because the, it's not because Neptune's not a planet. It's because the definition is both poorly worded and a bad idea to have to begin with. So what they're, I mean, what they're trying to say is that the planets are the gravitationally dominant things out there. And it's super easy to make a calculation of something that you would call

gravitational dominance and see that the eight planets are incredibly different from everything else in the solar system. They are big dominant bodies that kick around everybody else. You know, you could say the argument is the planets, all the things that are not planets are sort of flitting in and out of the orbits of all the planets, getting kicked around by the planets. And the planets are the ones doing the kick in and nobody kicks plants. around. Right. That's a pretty good definition.

And so by this definition, we have the eight planets. Right. And Pluto is just one of the various dwarf planets in the Kuiper Belt. And as you sort of alluded to, alternatively, the only sensible alternative that didn't sort of just make Pluto a thing all by itself would be to have dozens of planets. Hundreds. Hundreds. And you'd be the discoverer of many, many planets.

Yeah, this is what I always find funny when people like, you just hate Pluto, so you don't want them to be a planet. I'm like, dude, do you know that if, if you're, you know, if you're, you're not, you if I used your definition, I would be the biggest planet discoverer in human history. Do you know that? And the answer is no. They don't know that. They're like, you just don't want to be.

I'm like, someone asked me back in that first year when Zina was still being generally called the 10th planet, I was doing an interview and somebody said, you know, what does it feel like to have discovered the 10th planet? I stopped and I thought about it. And I said, you know how it feels? It feels fraudulent. You know Imposter syndrome No, I wouldn't even say it's imposter syndrome I'd say it actually feels fraudulent

So Herschel You know opened up his brand new fancy telescope pointed to the sky and found Uranus, this thing that's 17 times more massive than the Earth a big chunk of the solar system the Vary A did calculations

on how the orbits were going realize there was something else out there had someone pointed telescope and boom there was Neptune those are those are significant things in our solar system and if you removed any of them our solar system would be a different place if you removed eris or Pluto

or any of these other objects the solar system is exactly the same place these are you know they don't define the solar system in the same way so it just it really did feel fraudulent to pretend that this was a major part of the solar system and you really had to pretend if you wanted to call it the 10th planet. But the good news is now we're basically done, right? We have the Kuiper Bell, we have eight planets, and there's no more planets ever to be

found in the solar system. Yeah, so we're not done. So you're saying there could be other planets? So here's what I'm saying. There is at least one other planet. I'm not saying could be. I'm not saying it might be. I am as close to 100% convinced as you can be in this business that we have found gravitational evidence for a ninth planet. If you recall, I am about the 575th person to say this since 1845. It's a long distinguished lineage, yes. It's not distinguished. It is really scary to say this. It was scary for me and my colleague who came up with this idea, Constantine Batigen. He and I came over this idea a couple years ago. And, we were very reluctant. You know, we started doing the calculations. We started seeing what we're seeing

and we're like, God, it really, it really kind of makes sense that as a planet. Like, we do not want to be that 547th person saying, we predict a planet and we're right and everybody else was wrong. But here's the interesting thing is that we predict a planet and we're right and everybody else is wrong. Well, let's just pause for a moment before getting to the evidence that you're right. I mean, I want to just say again or highlight this. way that science works. I mean, not only do we look for evidence and so forth, but we have prior beliefs, right? And part of those prior beliefs are colored by history and what has happened to history. And so

rather than you and Constantine just running out and saying, hey, maybe there's a planet, you say, look, we all know that this has been claimed before. We should be extremely cautious and really make sure the eyes are dotted and the T's are crossed. We also knew that there were no other planets. I was graduate school, when the final nail was put in the coffin of planet X, I should have mentioned this when we were talking about the discovery of Pluto and the perturbations and all this stuff, is in the end, the reason that Percival Lowell thought there was a planet X is because some of the early observations of planetary positions were not exactly right, and he didn't have the precise

mass of Uranus and Neptune. And we didn't have those until Voyager flew by Uranus and Neptune. So at the final Neptune flyby, we got the precise mass of Neptune and redid all those calculations of where the planets are and there were there supposed to be. So there is no planet ice. We're done with perturbations. And I knew that in graduate school. That paper came out. And we all knew there were no new planets to be found and to think otherwise was. Heresy.

Yeah. And so of course there are no new planets. And so this led when Constantine and I first started. looking at these phenomena, it was to prove that there wasn't a planet. Right. So what kind of phenomena do you look at? So it's one specific thing that we eventually found, it took us a while. It's that if you look at the most distant objects in the Kuiper Belt. So most of the Kuiper Belt objects are in these kind of either kind of circular-ish orbits

a little outside of Neptune or maybe a mildly elongated like Pluto. But some of them actually are on hugely elongated orbits. orbits and go out 10, 20 times further than the orbit of Neptune and then come back. So they're on these big ellipses like this. If you look at the ones that go the furthest, we realized something unexpected, which is the ones that go the furthest are preferentially lined up in a particular way. The direction that they're going when they go out the furthest is a specific direction. There's no reason that should be so.

It's where the aliens put them when they left their listening stations. Even if the aliens put them there, they would very quickly untangle themselves. So each of the objects in the whole solar system, but in particular these distant ones, their orbits change over time. And they precess, is what it's called. The direction of their orbits changes over a couple tens to hundreds of millions time period. And so all these objects are like hands on a clock that are moving at different speeds. And we happen to look up and they're all aligned.

And so it could be you just happen to look up when they're all aligned or there's something else going on. Right. And so we sort of split in two. My job was to decide whether or not it could be that they just happen to be all aligned. And it was coincidence and Constantine started doing calculations. He started doing math. He actually started out by like writing equations on his board.

And he was like, well, what do you think about this? I'm like, uh, uh, uh, you like the Greek letters. Yeah, those are my favorite. And so we really were trying to figure out what could have done this. We knew a planet could do it. You see that and you're like, oh my God, it must be a planet. That's kind of the last 150 years. Because it would be similar to how these trans-Neptunian objects are sort of,

there's certain places they could fit in without being disturbed by Neptune. Exactly right. You're saying a similar thing. It's exactly right. And we didn't know the details, but we kind of knew like, yeah, we get that there must be some gravitational perturbation that it'll make them, it'll work. We know that. Let's, like, of course that works, but that's, that's a crazy thing to jump to initially. So let's figure out what's really going on because it's not a planet.

And we tried really hard to make it not be a planet. And there's nothing else. There is no other way to make those objects line up the way that they do. Has the story gotten more convincing over time? So the convincing aspect of it. So this has been two and a half years now. Our paper has been out. Planet Nine. Planet Nine, as we called at the time. Do you secretly in the back of your mind

have a name you want to get? No, no. I'm suspicious, suspicious also, but I'm superstitious, but definitely suspicious too. I'm superstitious enough to feel like if you really start to think about a name, you will not find it. Okay. And so I really, I honestly do not have a name, which is pretty amazing to have blocked that part of my brain so strongly. Should we come up for the name? Should you invent a lot? I'll do this. No, no, no, no, no, no. All right, go on. So Planet 9, you know, we did have a nickname. Like we have nicknames for everything. The nickname was fatty at the time, spelled with a pH.

And it was because that was going to, that was the, also, when I had a daughter, she had a nickname before she was born. I seemed to make a habit of that. And we had one picked out in case it was a boy. It was going to be fatty because that's my, an old family name of mine is Jehosephat. There were many Jehosephazepats. And we always talked about naming our son. Jehoshaphat, but we would call him fatty. Because I just thought, and we would make sure that he was like a jazz saxophonist

because he'd be like fatty brown. Doesn't that say that's a pretty good name? Yeah, totally plays. So that was what we called Planet Nine at first. But eventually Planet Nine was such a good name that we just kept with Planet Nine. So two and a half years ago. It also is a little poke to people who think that Pluto is to a place. That was totally unintentional, I promised.

Two and a half years. So one of the things that always happens in astronomy, sure in physics, everything else, is that theorists are really good at explaining anything. Oh, yeah. Yeah. So if you...

Twelve different ways. Yeah. And if you said, oh, no, I'm sorry. I didn't mean that. I actually found that instead like, oh, okay, then I can explain that too. So, I mean, theorists are good. That's their job.

So we knew that as soon as we published both the observations, that these things are lined up. And our hypothesis of Planet 9, very quickly, there would be papers coming up with alternative explanations, alternative physics. And we were curious what they were going to be because we couldn't come up with any. And in two and a half years, there are zero.

And that baffles me that no one has come up with another way to make that alignment. I mean, dark matter, cosmic strings, aliens, black holes. I can come up with a half a dozen. Yeah, but write that paper. Oh, okay. So it has to actually work. It has to actually work. Yeah.

So, no, I always get the explanations. is like, what if it's, yeah. But real physics really working, I've seen no explanation. So the one potential explanation and the one part that we still worried about is the idea that it was just a random coincidence. And not only a random coincidence, but a random coincidence can be helped along by some sort of bias in your observations. And there are many ways that these observations could have been biased.

So you can imagine that if you're saying that all of your objects are lined up in one particular way, well, what if that's the only place you look? Yeah, exactly. For example. I mean, that would be an extreme version of the bias. But we worried about that. And it turns out to be very difficult to do that calculation right. And it has taken us, in fact, two and a half years to do that calculation right.

And I just finished the paper on the final calculation this morning, literally. Mazel to. And the answer is, The probability that it's just due to chance, taking into account all the biases of all the observations and everything else is 0.1%, 0.14%, I think. So it means it's possible. Those things happen, but it's a small number. So here's what I say in the paper. I say, look, you don't have to believe in Planet 9.

I'll punch you, but you don't have to believe in town. But the effect is real. And if you don't believe in Planet Nine, there needs to be another explanation for it. Right. And so why don't you go look for it? So we are looking for it. Okay. So I've spent the two and a half years has been both understanding those biases,

because understanding the biases is critical to then using the observations to predict where it is. So as of right now, now that I understand the biases, I have a very tight prediction of the orbit of planet nine in the sky. Do you know where it should be? So I know the path that its orbit traces out in the sky, and that is not the same as where it should be. That is where it should be except that I don't know where along the path. Not when it should be, right?

Yeah. So the bad news is we're not as good as Laverier. Laverier said, there's a planet. It's right there. And they literally looked one night and found it. It was right where Laverier said it was. he got a little lucky. He had a couple things going for him. One is he could make some assumptions like it's a circular orbit in the plane of the solar system. Ours is definitely not a circular orbit and it's actually not in the plane of the solar system. It's tilted by 20, 30 degrees.

Okay. And so does it perturb Neptune? Does not perturb Neptune? It's so far out. It's a circular orbit. It's not a circular orbit. It's an eccentric orbit. And it's average distance away. It's about 500 A.U. Remember, Neptune was 30, so it's nearly 20 times further away than Neptune. It has no effect on Neptune or even the relatively nearby Khyber Belt objects. The only thing in effects are these very distant ones that go out into that realm. And so they go out into where it is and get affected by. And how big is planet nine supposed to be?

So now we know it's right around seven times the mass of the Earth. Might be six, maybe five, might be eight, but it's not that much different from that. So seven times mass of the Earth is, again, Neptune, is about 17. So it's smaller than Neptune, bigger than the Earth. Probably it's like Neptune, and then it's a mostly,

it's an ice giant. It's mostly a core, a sort of liquid e-core, surrounded by gas. It's way bigger than the Death Star, for example. It could swallow the Death Star quite easily. Because, I mean, you must be thinking,

why is there such a thing out there? Why is it in the wrong orbit? Yeah. It's so far away. So we think we know that we have ideas. So the idea that there's an object out there is agnostic as to how it got there. So our evidence for its existence is solid. Then we just get to make up stories on how it could have gotten there.

Interestingly, as soon as we figured out that it was something like seven Earth masses, 10 Earth masses, and on this eccentric orbit, Constantine and I both just the same light bulb went off on our heads at the same time. It's like, oh, I know where it came from. Like, yep, me too. And the answer is 10 Earth masses is a special mass in the solar system. Ten Earth masses is, as we've talked about, the mass of the cores of the giant planets. Constitina and I actually had written this paper six or seven or eight years ago now about what would happen in the solar system if instead of four giant planets, you start out with five giant planets in the regular giant planet region. And the answer is, nearly all the time, one of those giant planets gets destabilized, gets a little too close to Jupiter and gets tossed out.

And we were interested in how that affects the outer solar system and everything else. And we didn't really, once it got tossed out, we never worried about it again. So the solar system is full in some sense, planet-wise. Planet-wise, yeah. So you can't stick new ones inside where the old ones are. And if you try, they are most likely to get ejected. There's no reason why there should have only been four cores form. There should have been actually many, many cores form.

So probably there were cores being tossed out all the time, and we never really thought about what happens when they get tossed out. The idea that one gets tossed out and then gets, there's still a little bit of a waving of a magic wand that has to happen because it has to then get stabilized in the outer part of the solar system and not come back in and not go back out. And so we think that happens when the sun is formed in a giant cluster of other stars. So we think we know how that happening.

Well, you know, our hypothesis is that that's how it happens. But it all kind of makes sense. It fits perfectly. It doesn't mean it's true. But it's the idea that there was a core that got ejected and recaptured is so uncontroversial that, you know, when you suggest that to theorists working on the solar system, like, oh, yeah, sure. Yeah, that's what they do. Yeah, sure. And once it's out there, does Planet 9 sometimes perturb Khyber Belt objects and turn them into comets to come in time?

Absolutely. And it does something really interesting to them. It does that by twisting. their orbits. Instead of just perturbing them and throwing them in, it slowly perturts them. So things that used to be in more or less the same plane as the solar system get their orbits twisted by about 90 degrees. And so they're plunging into the solar system and back out again. And then it drives them into the sun in past Neptune. And the reason, the transition that Constantine and I took from thinking this was a cute theory that could explain things.

It's easy to come up with theories that explain things, but you don't believe them most of the time. When we started believing it is when we realized that we were predicting these orbits twisting and coming into the solar system, and then we went out and realized that those things exist and that nobody else, nobody had an explanation for them. They would be found and people would just say, it's so weird, I don't know why these things are coming in. And now, so our hypothesis now explains, you know, the alignment, some of the other detailed dynamics, these other objects were like at that moment, I mean, literally the moment we did that. I think both of us just kind of looked each other and like, oh, oh, there's actually, oh, there's a, oh, there's a planet out there.

And it went from just, you know, cute idea to, holy cow, there's a planet. Let's go find it. you're optimistic about finding it? By finding it, we mean literally taking a picture. Yes. I mean, in the end, it's a hypothesis that I am convinced is true. No one else need believe it until we go see it. Are people basically optimistic about it, or are people's coffee?

Some of each. There's a whole group of people who are desperately trying to find it because they're convinced. We had a workshop here at Caltech in the late spring of all the people around the globe who are in search of it and exchanged ideas and where we thought it was and who was searching and how they were finding it. But there are people who are like, so there are the general skeptics like I think most scientists should be who've probably not looked very hard at the evidence, you know, until you look at it really carefully, your default is always going to be, come on, really, plan it. And that's right. That's the way to be. And then there are the no way it's

impossible. I'm going to prove you wrong. And they try. Knock yourself out. Yeah. They haven't succeeded yet. But meanwhile, you're flying to Hawaii going up on top of a mountain where there's big telescopes and taking snapshots and hoping to see a dot moving. Yeah. It's what I started in 1978 with photographic plates. And now we're continuing it now looking for something even more distant and a lot fainter. Do you really have to fly to Hawaii? I mean, don't they have robots? So it depends on the telescope, actually. So the telescope that we're using, the Subaru telescope on top of Monique is the Japanese National Telescope. And they require you to be there. They don't need to require you

to be there, but they do. As a philosophical dilemma of necessity and requirement that I'm not quite qualified to adjudicate. It would be, it would, there's no actual reason for us to be there. It would, it would work just fine with us on video link somewhere else. And sometimes they actually do do that, but they do make us come up there, which I don't mind at all. It's a spectacular place to go. When you see a moving dot, how quickly will you know and with what level of certainty that it actually is the planet you've been looking for? So I think that if we found a moving dot in the Subaru survey, the things that you want to know are how fast is it moving, because that tells you how far away it is, and how bright it is, because that tells you about how big it is. Basically,

if we find anything that is five or six or 700 AU.A.U.A., we can't see things that far away unless they're planets, basically. So if we see something moving at the predicted speed, and, you know, we won't see it that night at the telescope. We bring all the data home and have it crank through computer. But basically, I'll be sitting in my office. I'll be looking through candidates, and one will come up, and it'll be consistent with everything. And I will be 98% certain at that point. Very quickly. But what we'll do, one is I'll assume it's real and I'll think about what I'm doing about it.

But the other thing that we'll do is we'll very quickly predict where it should be, this will probably be a week or two later it took us. We'll predict where it should be that night based on what we did. And we will find some astronomer somewhere in the world at some telescope and say, go take a picture right here and tell me what's there. Depends on who it is. I have enough friends who would do it. And if it's where we predict, then it is 100% no questions.

It is there. We know it. And then the fun starts because finding it is fun. Yeah. But it's actually studying it and learning about this new giant planet that, you know, we only have four. We get a new one. Yeah. Pretty cool.

So we have our day zero things that we want to do that we'll start doing immediately. Well, you know, you killed Pluto for which we can't really forgive you. but it will compensate somewhat if you find another replacement plant. So we're rooting for you there. It was all suggested by my daughter about four years ago. She said, she said, Daddy, do you know how to get people to stop hating you? I was like, gosh, no, I don't. Nobody knows.

How should I do that? Why is my four-year-old daughter thinking about these things? She said, you should go find a new planet, and then people wouldn't hate you anymore. And I laughed and said, ha-ha, but there's no new planets. and now I realize she knew what she was talking about. All right. Well, we're rooting for you.

Mike Brown, thanks so much for coming on the podcast. It was fun.