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Sean Carroll: 00:00:00 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 fact 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 I believe right now even though 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, school children and older folks said, “Look, we know what the planets are. There are nine of them, and Pluto is one of them.” They said, “You 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.

Sean Carroll: 00:01:37 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 gonna let into the club.

Sean Carroll: 00:02:23 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.

Sean Carroll: 00:02:52 The IAU, 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, it’s today 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 @Plutokiller 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.

Sean Carroll: 00:03:45 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, Konstantine Batygin at CALTECH claim that there is evidence in the motion of known Kuiper Belt 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 gonna figure out why we think that 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 go. Mike Brown, welcome to the Mindscape Podcast.

Mike Brown: 00:04:40 Thanks for having me.

Sean Carroll: 00:04:41 So you’re technically an astronomer, but you’re in a Planetary Sciences Department here at CALTECH. What do you tell people you are when you meet them on an airplane?

Mike Brown: 00:04:50 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. I’m an astronomer who looks at planets in our solar system.

Sean Carroll: 00:05:03 Yeah, you don’t look at stars or galaxies like a real astronomer.

Mike Brown: 00:05:05 You know, they get in the way, and I have to somehow figure out ways to ignore them. But I look at the real planets.

Sean Carroll: 00:05:10 How did you get interested in that? What was … What captured your imagination?

Mike Brown: 00:05:16 It is not what I thought I was gonna 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. And-

Sean Carroll: 00:05:31 You’ve made a terrible mistake.

Mike Brown: 00:05:32 Yeah, so these most distant galaxies at the time were … The goal was to find things at a redshift 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 Hy Spinrad at Berkeley. And so I went to Berkeley to work with Hy Spinrad. Hy Spinrad had a hobby of looking 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 liked to look at comets is because he couldn’t tell the difference between galaxies and comets because they look the same.

Sean Carroll: 00:06:17 They look the same. And back in the day we didn’t even know. Right? Nebulae.

Mike Brown: 00:06:19 Yeah. Back in the day. It wasn’t that long ago. So we did actually … but he would take spectre 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’s 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, so 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.

Sean Carroll: 00:07:02 Right. Like exoplanets are cool but planets that are in actual our solar system.

Mike Brown: 00:07:08 So at the time, there were no known exoplanets so even the fact that exoplanets have now made nearby things because exoplanets are closer than distant galaxies, so now the ranking is a little different. But at the time, if you studied planets, you’re a loser. He forced his students to study comets for one summer before they could look at galaxies cause he just wanted them to get some work done for him.

Sean Carroll: 00:07:35 Hazing ritual.

Mike Brown: 00:07:35 Yeah. So I did. Mostly I wanted to work on comets, but I worked … I mean, galaxies. But I worked on this comet stuff and a moderately bright comet came by at the time and we went up to the telescope, Leuc 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, we were 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.

Sean Carroll: 00:08:10 With your eyes.

Mike Brown: 00:08:11 There in the sky. And that was it. I 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-

Sean Carroll: 00:08:26 Your heart beat a little faster?

Mike Brown: 00:08:27 It really did. For my thesis, my Ph.D. thesis, I studied Jupiter and its moons, and I would be there with 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 was just … I love that visceral feeling that what I’m studying is actually real as opposed to this very distant smudge.

Sean Carroll: 00:08:47 Those are also real by the way. Just so our listeners know.

Mike Brown: 00:08:49 I’m not convinced.

Sean Carroll: 00:08:51 But they are less visceral. They are further away.

Mike Brown: 00:08:53 You can’t … so 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.

Sean Carroll: 00:09:03 And you mentioned spectra, one of the things … I was an undergraduate astronomy major. I don’t know if you knew that. I have no degrees in Physics. My Ph.D and Bachelor’s Degree are both in Astronomy.

Mike Brown: 00:09:14 All right.

Sean Carroll: 00:09:14 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 … we take a picture of a comet. What does that tell us? How do we get information about it?

Mike Brown: 00:09:35 Yeah. So for the studies that we were doing in particular, it’s the spectrum. You take a … the comet’s 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 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 the very … we had a very, very, very fine spectrograph it’s called. Elaborate prism where we could really break up the light into an 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.

Sean Carroll: 00:10:33 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 of less close to the sun and moving by?

Mike Brown: 00:10:40 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, they heat up, their surfaces evaporate, they get all that stuff in the atmosphere and that’s when you can really study their details.

Sean Carroll: 00:11:02 And they’re dirty snowballs roughly speaking. That’s what I remember from my astronomy classes.

Mike Brown: 00:11:05 They’re dirty snowballs and then the interesting question is what is the dirt and what is the snow.

Sean Carroll: 00:11:10 Right.

Mike Brown: 00:11:10 Cause, yes, it’s mostly water in the snow and it’s mostly … 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-

Sean Carroll: 00:11:20 And it’s not really snow. There was never a snowfall on the comet. It’s ice.

Mike Brown: 00:11:24 It’s ice.

Sean Carroll: 00:11:24 Yeah, it’s an ice ball. So you moved on though from comets to-

Mike Brown: 00:11:28 I did.

Sean Carroll: 00:11:29 Planet-like things.

Mike Brown: 00:11:30 Well, so then as my Ph.D. I studied the volcanic emissions from Jupiter’s moon Io as they exploded off the surface of Io 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. And the same thing … it’s 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’s that … it was fantastic.

Sean Carroll: 00:12:03 Looking at all the chemicals that Io … I always said E-O, is it really I-O?

Mike Brown: 00:12:06 I can say both in the same sentence.

Sean Carroll: 00:12:08 You’re a professional. Okay. So, yeah, so the volcano, this huge volcano, right on Io?

Mike Brown: 00:12:14 There are a bunch of them.

Sean Carroll: 00:12:14 A bunch of them. Spewing stuff out int to the atmosphere. The whole neighborhood of Jupiter is like a mess with magnetic fields and radiation and a whole bunch of things.

Mike Brown: 00:12:23 And junk from Io. Actually junk from Io is one of the main components of the magnetosphere that’s going on there.

Sean Carroll: 00:12:29 Which would make it tough to go visit Jupiter and hang around in a spaceship.

Mike Brown: 00:12:32 This is actually why when spacecraft go to Jupiter, they usually spend most of their time pretty far away. And 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’ll get smacked basically by stuff that came from volcanoes at Io.

Sean Carroll: 00:12:58 Huh. Okay, so you’re still though studying things that we know to have existed. So Galileo discovered Io.

Mike Brown: 00:13:03 Yes, yes.

Sean Carroll: 00:13:04 That was a long time ago.

Mike Brown: 00:13:05 That is true.

Sean Carroll: 00:13:06 What made you move on to the further reaches of the solar system?

Mike Brown: 00:13:08 So while I was a graduate student at Berkeley, the very first object beyond Neptune since Pluto was discovered. First new Kuiper Belt objects. 1992 QB1 is its license plate number. And at the time I remember hearing abut it at the time from the discoverer. Actually the day before it went public, Jane Luu told me about it. She was in an office right down the hall from me. And I thought, “Oh that’s interesting. But who cares?”

Sean Carroll: 00:13:40 It’s just a rock.

Mike Brown: 00:13:41 Yeah, an 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.”

Mike Brown: 00:14:14 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 Mauna Kea 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.

Sean Carroll: 00:14:58 So let’s set the stage here what the solar system looks like. I mean, 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?

Mike Brown: 00:15:11 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.

Sean Carroll: 00:15:46 That’s not what the movies have taught me.

Mike Brown: 00:15:47 No it’s nothing quite like that.

Sean Carroll: 00:15:49 Very dangerous place as far as-

Mike Brown: 00:15:51 Yes. You could go through there a million times and miss every asteroid unless you were trying hard.

Sean Carroll: 00:15:58 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. Apparently, astronomers don’t believe that the Asteroid Belt used to be a planet.

Mike Brown: 00:16:10 No, but they did believe that at first. So the first asteroid was discovered January 1st, 1801. It was Ceres, and then in quick succession three more were discovered. Imagine how strange this was. There were … 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 object that we knew about in the sky that we didn’t know about before found with the telescope and kind of blew people’s minds.

Sean Carroll: 00:16:43 Well, sorry, Galileo and the moons. Right?

Mike Brown: 00:16:46 Yeah, okay. So there were moons and things that were being found. That’s actually … Saturn’s moons. Herschel discovered them I can’t think of the year that Herschel discovered them, but you’re absolutely right.

Sean Carroll: 00:16:54 Uranus is out there all by itself.

Mike Brown: 00:16:56 All by itself. And so it actually led people to search for a new planet systematically. And 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.

Mike Brown: 00:17:17 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 from the first one, but the Celestial Police found many of … the next three. So they found four in quick succession. Four, maybe four years, three years, four years, and they all had very similar orbits. 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, it must have been a planet that exploded.” Seems plausible.

Sean Carroll: 00:17:58 Or it was exploded.

Mike Brown: 00:18:00 Yeah. Something happened.

Sean Carroll: 00:18:01 It was blown up, yeah.

Mike Brown: 00:18:03 And nothing else was found until, I actually don’t know when the fifth asteroid was found, but it was 1840’s I think it was.

Sean Carroll: 00:18:11 How many do we know about now?

Mike Brown: 00:18:12 Oh, 300,000 maybe. Down to the size of a desk or something. 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. Whenever there’s anything going on in the inner solar system, it’s probably Jupiter’s fault.

Sean Carroll: 00:18:38 Right. Exactly. But I think that there’s a lesson that we’ll come back to you I think. We think of the solar system as big, the planets are relatively tiny and we sort of treat them independently I think in the mind of a non astronomer.

Mike Brown: 00:18:51 Yeah.

Sean Carroll: 00:18:52 But the lessons are over these millions and billions of year time scales, 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.

Mike Brown: 00:19:12 Yeah. If it hadn’t been for Jupiter, all these 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. In the end, they’re never able to form a planet.

Sean Carroll: 00:19:38 Or the Jovians didn’t want the competition so they destroyed the planet in its early days.

Mike Brown: 00:19:42 That is actually possible.

Sean Carroll: 00:19:43 Jupiter shouldn’t be as close minded. The establishment is hiding some things here. I’ve read them. So good.

Mike Brown: 00:19:48 So that’s the inner solar system.

Sean Carroll: 00:19:49 That’s the inner solar system. And then we get to Jupiter.

Mike Brown: 00:19:51 So Jupiter. So then there’s the realm of the giant planets. So Jupiter and Saturn are the two really big giant planets. It’s funny people have a very poor understanding of the sizes of planets because mostly they see them on kids’ lunch boxes where they’re all more or less the same size.

Sean Carroll: 00:20:10 And pretty close together.

Mike Brown: 00:20:11 Mercury’s a little smaller than Jupiter, but not that much smaller. So Jupiter and Saturn are huge.

Sean Carroll: 00:20:17 Whopping big planets.

Mike Brown: 00:20:18 And so these are … The distances we measure everything in is astronomical units. One being the distance from the Earth to the sun is one astronomical unit. Jupiter’s at five, so it’s five times further from the sun and then the giant planets are nicely arranged. Jupiter’s a five, Saturn’s a 10, Uranus is at 20, it’d be nice if Neptune were at 40, but no, it’s at 30. 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. They’re big but they’re not-

Sean Carroll: 00:21:05 I didn’t think that they were that small.

Mike Brown: 00:21:06 Yeah, they’re really kind of small. Jupiter, Saturn, huge. Jupiter is 315 … I think is the number times more massive than the Earth. Neptune is about 17 times more massive than the Earth.

Sean Carroll: 00:21:20 And they’re all gas giants? Does that mean they’re all gas or there’s little rocky cores? Right?

Mike Brown: 00:21:25 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.

Sean Carroll: 00:21:43 I mean, the great red spot has been on Jupiter for hundreds of years. Isn’t it … do people think that maybe it’s the reflection of that there’s some feature on the surface of the core-

Mike Brown: 00:21:52 No.

Sean Carroll: 00:21:53 Or it’s purely atmosphere?

Mike Brown: 00:21:54 Yeah. Purely atmosphere. The core is tiny. The core, so I said that 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 insignificant chunk going on. So that’s Jupiter and Saturn is very much like a slightly smaller version of Jupiter.

Mike Brown: 00:22:20 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 can 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. Some people say that they’re liquid on the interior. They’re not really liquid int eh interior but they’re very different from Jupiter and Saturn.

Sean Carroll: 00:23:02 We’ve never landed on any of these planets?

Mike Brown: 00:23:04 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-

Sean Carroll: 00:23:15 The metaphorical surface. It didn’t actually reach the core.

Mike Brown: 00:23:17 Yeah. 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 1 or 2 … I forget which one went. One of them diverted so it could do a fly by of the rings of Saturn and to do that, it had to basically go up out of the solar system and never go by anymore. But one of them went by Uranus and Neptune. That’s it. We know very little about these planets.

Sean Carroll: 00:24:03 Which means there’s a lot of room for young astronomers to grow up and study these more.

Mike Brown: 00:24:06 Yeah, they’ll find out.

Sean Carroll: 00:24:08 There’s a lot that we don’t know. Right. Okay, and 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? Like 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.

Mike Brown: 00:24:30 Yeah. It’s a weird … I remember before we understood Pluto’s place in 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 it was there, how it got there. It didn’t really make any sense. But everyone was like, “Well, I guess.”

Sean Carroll: 00:24:47 It was found kind of by accident. Like they were kind of looking for it-

Mike Brown: 00:24:49 Yeah, so well, this is how the whole problem with Pluto and planet hood 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 planet out there is because he looked at the orbits of Uranus and Neptune and they appeared to be being tugged by something.

Mike Brown: 00:25:31 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.” Leverrier predicted a planet based on perturbations and dude got super famous. I’m gonna do the same thing.

Sean Carroll: 00:25:48 I can do that.

Mike Brown: 00:25:49 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-

Mike Brown: 00:26:01 … 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 know, 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. 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, “Eh-

Sean Carroll: 00:26:33 A lot of stuff.

Mike Brown: 00:26:34 … I don’t see this giant planet that I’m looking for.”

Mike Brown: 00:26:38 So, he eventually passed away, but 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 Tombaugh took a couple pictures and realized that he didn’t know what a planet looked like … because he just … it looks like a star.

Sean Carroll: 00:27:01 Right. It’s a spot on your photographic plate.

Mike Brown: 00:27:03 Percival Lowell 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 Tombaugh said, “I don’t know.” And so, he realized that you take a picture one night, and then you take a picture the next night, and all the stars and all the galaxies are in the same place, planets move.

Mike Brown: 00:27:19 So, we did that. 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.

Sean Carroll: 00:27:29 It was a little dot moving.

Mike Brown: 00:27:30 And you know this is … 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, of the … when they announced the discovery, Ninth Planet Discovered in the Solar System, blah, blah, blah … and 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.

Mike Brown: 00:28:18 So, because it was a prediction, people … and so, people thought at first, that Pluto was as big as Jupiter. That is only wrong by a factor of 250,000. But, it … you know, if you think it’s as big as Jupiter … if it were as big as Jupiter, there 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 … 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.

Sean Carroll: 00:28:57 And then there’s this thing called the Kuiper belt, which has a kind of interesting history of its own, right? It wasn’t discovered by Kuiper.

Mike Brown: 00:29:05 It was vaguely predicted by Kuiper.

Sean Carroll: 00:29:08 Yeah.

Mike Brown: 00:29:08 So, it was named after Kuiper. I think this is a long standing and good tradition in astronomy that … 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 it 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 ice balls out beyond Pluto, they said at the time … out beyond Pluto. It’s the source … one of the sources of comets that come into the inner solar system.

Mike Brown: 00:29:48 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, so I’m actually very happy 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 … should be … the discoverers named it, and I think that’s … I respect the naming of the discoverers.

Sean Carroll: 00:30:16 Absolutely. And it’s different than the Oort cloud, which we’ve heard about in terms of where comets come from also. So, why do we need an Oort cloud and a Kuiper belt?

Mike Brown: 00:30:23 ‘Cause there are tow flavors of comet. One flavor of comet comes in, basically in the disc 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 Oort cloud … and cloud because it’s this uniform, very distinct cloud around the sun … and the Kuiper belt … belt because it’s a taurus of material out beyond Neptune. Just like the asteroid belt is mostly stuff that’s in the plane of the solar system, the Kuiper belt is the same way.

Sean Carroll: 00:31:07 And are these really two clearly distinct populations? Would they kind of blend into each other?

Mike Brown: 00:31:12 Well, 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 Oort cloud. I used to think I did, and now I know less than I used to.

Sean Carroll: 00:31:29 Science.

Mike Brown: 00:31:30 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 the … 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.

Mike Brown: 00:31:55 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 shock 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.

Sean Carroll: 00:32:22 And some of the moons and things of the planets we know about in the solar system might have been captured from the Kuiper belt? Is that a reasonable hypothesis?

Mike Brown: 00:32:31 So the moons … yeah, the moons that you generally know of, like the big ones, like the Galilean satellites, Titan 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 that … the regular satellites are in the plane of the planet. They rotate the same direction as the rotation of the planet. So, they’re all part of that initial disc. But, they all have … you know, it almost sounds like the Oort 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 … 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.

Sean Carroll: 00:33:22 Yeah. Okay. All right, good. So, I think that more or less finishes our tour. We’re able to catch up where Jane Luu, I think it was, had discovered a new … Kuiper belt object.

Mike Brown: 00:33:30 This new … yeah.

Sean Carroll: 00:33:32 Now, what year was that?

Mike Brown: 00:33:33 1992. That’s why it gets that license plate number of 1992 QB1.

Sean Carroll: 00:33:39 Okay. Is that in a similar orbit to Pluto? Or similar size?

Mike Brown: 00:33:42 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 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 the same orbit, I mean it is … it comes inside the orbit of Neptune. It’s tilted by 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 …

Sean Carroll: 00:34:31 Right.

Mike Brown: 00:34:31 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 in a residence, it’s called. So, Pluto and all of these other objects that are called Plutinos-

Sean Carroll: 00:34:53 Plutinos.

Mike Brown: 00:34:53 Yeah. That, I think is a good name too.

Mike Brown: 00:34:57 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 … every time they cross the orbit of Neptune, Neptune is on the other side-

Sean Carroll: 00:35:24 Neptune’s somewhere else.

Mike Brown: 00:35:24 … of the sun. So, actually Pluto comes closer to Uranus, than it ever does to Neptune, even though it crosses Neptune.

Sean Carroll: 00:35:31 That’s another example of this sort of gentle, but crucially important dynamical influence that the planets have on each other-

Mike Brown: 00:35:36 Right.

Sean Carroll: 00:35:36 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.

Mike Brown: 00:35:43 Yeah.

Sean Carroll: 00:35:44 And … good, so but this is … this discovery of more Kuiper belt objects. This is somewhat your fault … or in Caltech’s fault anyway, for giving you telescopes.

Mike Brown: 00:35:52 Yeah, or something. So … yeah, so this is … when I started here, there were … boy, I probably, when I started as a young naïve assistant professor, there might have been a hundred known Kuiper belt objects.

Sean Carroll: 00:36:06 Okay.

Mike Brown: 00:36:06 … which is a … going from one in 1992 to 100, was a lot of work for a lot of people. And I realize this is going to be big. 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 … 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 Tombaugh. But, there should be other things … very much like-

Sean Carroll: 00:36:51 Pluto-esque.

Mike Brown: 00:36:52 … like Pluto. 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.

Sean Carroll: 00:37:16 Ah, okay.

Mike Brown: 00:37:17 Finding a whale is a lot harder.

Sean Carroll: 00:37:19 I caught that analogy. That’s a good one.

Mike Brown: 00:37:20 You have to go sail all around. So, we didn’t have a good way of … we didn’t have a big net. Astronomers, at the time, were really good. They were just developing these electronic detectors, the CCD that everybody now has in their cameras, were pretty new then. And they’re pretty small. If you remember your first digital camera, it 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, ’cause you could-

Sean Carroll: 00:37:53 And this is late ’90s …

Mike Brown: 00:37:54 Late ’90s. So, if you wanted to cover large swaths of the sky, you couldn’t, with these digital detectors. So, I actually … I did, one of the very last projects I think, with photographic plates, that I … I’m probably the youngest astronomer to ever do a project using photographic 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. It’s the 48 inch Schmidt telescope had been built, basically to take wide field pictures of the sky, to help the 200 inch know where to look.

Sean Carroll: 00:38:35 Right.

Mike Brown: 00:38:36 … 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, ’cause you were about to go to the telescope, and you’d need to make sure you were looking at the right thing. They had these … Polaroid cameras designed specifically to … you mounted 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.

Sean Carroll: 00:39:19 Their internets and their Twitters, yeah.

Mike Brown: 00:39:20 Yeah. But, it was pretty fun back then.

Mike Brown: 00:39:24 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. It makes me very upset.

Sean Carroll: 00:39:36 Yeah.

Mike Brown: 00:39:38 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 difference is 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.

Sean Carroll: 00:40:09 Okay.

Mike Brown: 00:40:09 So, it was a painfully-

Sean Carroll: 00:40:10 This is why you’re the last one to have done this.

Mike Brown: 00:40:13 … painfully inefficient. But, it means I could cover big areas of the sky. So, I spent a couple years doing that, and … and just sort of caught the bug. I really was pretty convinced that we were going to find something big.

Sean Carroll: 00:40:25 You were just papering the sky. You were just looking all over the place.

Mike Brown: 00:40:28 We didn’t have enough time to paper the whole sky, and so we had to pick where to paper. And so, we did a swath right along the Ecliptic, the plane of the planets. You know, if we look up in the sky, and you see where the moon is and where Mars is, and where Jupiter … that makes this line across the sky … 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. It was pretty exciting ’cause we found not a thing.

Sean Carroll: 00:40:55 Pretty exciting.

Mike Brown: 00:40:55 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.

Mike Brown: 00:41:07 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.

Sean Carroll: 00:41:23 Okay.

Mike Brown: 00:41:24 But, we didn’t know that at the time. It turns out the bright Kuiper belt objects are preferentially above and below the Ecliptic, not on the Ecliptic. Who knew?

Mike Brown: 00:41:32 So, we didn’t find anything, but I just … it just really reinforced to me that this is something that you just …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 it … this was right about the time when the digital detectors were getting better, and bigger. They weren’t great, but you could kind of string a bunch of them together. It’s kind of like taking 100 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 cluegy 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 … 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 Kuiper belt objects. I’ll say, moderately big to me, is a 500 kilometer object in the sky … which is kind of cool. You’re sitting there looking, and you’re in your office, looking 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.

Sean Carroll: 00:42:50 And literally billions of miles away.

Mike Brown: 00:42:53 It’s pretty cool. Yeah. It’s pretty fun.

Sean Carroll: 00:42:53 Hanging out in the middle of nowhere.

Mike Brown: 00:42:55 I still get a 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’d be one that you would … you know, you’re just looking through the data, and you’d be like, “Oh, oh, oh” and you know, I would do quick calculations of how big it was, how far away, what was going on. And it was pretty cool, ’cause we find … you know, it started out … we found one that was half the size of Pluto. We thought that was pretty good.

Sean Carroll: 00:43:25 That’s pretty good.

Mike Brown: 00:43:26 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 of 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.

Sean Carroll: 00:43:44 Okay. Well …

Mike Brown: 00:43:45 Other than Pluto.

Sean Carroll: 00:43:46 Other than Pluto, yeah okay.

Mike Brown: 00:43:46 I guess the largest … oh, that’s when we found … I used to say that phrase. What was I talking about? I was talking about Eris … when we found Eris, which is later. And it’s not actually the largest. So, I was wrong.

Mike Brown: 00:43:57 So, at the time, 1845, it was like the … it was the largest object found since Pluto. How ’bout that?

Sean Carroll: 00:44:02 That’s pretty good.

Mike Brown: 00:44:02 That was good. And then we found … as time went on, we’d find one that was slightly bigger, like three quarters the size of Pluto. They just kept stacking up, and 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 … I remember seeing it on my screen, when I first saw it, and it was moving very slowly across the screen. And it was really bright. And my reaction was, “What did I do-

Sean Carroll: 00:44:32 We’re all going to die.

Mike Brown: 00:44:33 “What did I do wrong this time?” Because you know, 90% of your best discoveries are mistakes.

Sean Carroll: 00:44:38 Oh yeah. Yeah, yeah.

Mike Brown: 00:44:39 Maybe 95. Maybe 99% … I mean, I make a lot of really exciting discoveries … and most of them are wrong.

Sean Carroll: 00:44:45 I’ve had some really great theoretical ideas too.

Mike Brown: 00:44:47 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, “What if it 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, “Oh that actually is right. That was right too. Oh. That’s right too!” And I was like-

Sean Carroll: 00:45:21 It’s real.

Mike Brown: 00:45:22 “Oh.” So, I called up my wife, and I said, “I just found a planet,” because it was obviously-

Sean Carroll: 00:45:28 Back in those days …

Mike Brown: 00:45:29 … as big as Pluto.

Sean Carroll: 00:45:30 Yeah.

Mike Brown: 00:45:31 And Pluto’s a planet.

Sean Carroll: 00:45:33 Right.

Mike Brown: 00:45:33 It’s pretty clear that this thing was a planet.

Sean Carroll: 00:45:36 Eris.

Mike Brown: 00:45:36 Eris.

Sean Carroll: 00:45:37 You didn’t name it that.

Mike Brown: 00:45:38 I did. At the time, I did not name it that, ’cause it didn’t … we had code names for all the things that we found at the time … that we would talk about, just ’cause we needed … the first thing that it was called was the name that the computer gave it, which was … oh, 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 Xena, which I had been reserving for …

Sean Carroll: 00:46:07 Something …

Mike Brown: 00:46:09 … something bigger than …

Sean Carroll: 00:46:09 Some warrior princess like thing.

Mike Brown: 00:46:10 Something that was … so, you know … with the idea being that … people would always talk about Planet X. I wanted an X. I thought it would be nice to have an X. I wanted to have it … a good mythological name, and so okay, so it’s TV mythology.

Sean Carroll: 00:46:24 Still …

Mike Brown: 00:46:24 But, Pluto was named after a cartoon dog, so that seemed okay to us … not actually true, but … it’s mostly true.

Mike Brown: 00:46:31 And then, there were not enough female planet names and so I thought, good, if you wanted a mythological X, female name …

Sean Carroll: 00:46:41 Choices are limited. You picked a good one.

Mike Brown: 00:46:42 You can’t do better. You can not do better. It was an awesome … we found a satellite. There was an obvious name for this satellite. Satellite was Gabrielle. 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 Xena, and that story got out and everybody now knows-

Sean Carroll: 00:47:09 That’s the lead, right there.

Mike Brown: 00:47:10 For many years, it was mostly known as Xena, or the other official license plate was 2003 UB313. That was the international astronomical union’s license plate number.

Sean Carroll: 00:47:23 But, clearly the stuffed shirts at the international astronomical union are not going to let you get away with Xena as a long-term name for an important celestial object.

Mike Brown: 00:47:31 Yeah. Probably not.

Sean Carroll: 00:47:32 Right.

Mike Brown: 00:47:32 Although …

Sean Carroll: 00:47:33 So, now it’s Eris.

Mike Brown: 00:47:34 So, now it’s Eris, which is … I have to say, a fantastic name.

Sean Carroll: 00:47:37 It’s a good name.

Mike Brown: 00:47:39 So, we didn’t get to name it … they held off on allowing us to name it until they could decide what it was.

Sean Carroll: 00:47:48 Right.

Mike Brown: 00:47:48 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 to be a planet, and so I didn’t actually believe that Eris 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 could happen is, you can say no it’s not, and then Pluto doesn’t get to be a planet either. So, I you know … it’s win-win for me.

Mike Brown: 00:48:12 So, we called it the 10th planet. There were other people who were very adamant that it should be classified as a planet. And other people were like, “No way, that’s ridiculous.” And we just got to hand out and watch the arguments go. But, it meant that we couldn’t name it, because if it was a planet … well-

Sean Carroll: 00:48:27 Different standards.

Mike Brown: 00:48:28 … nobody knows how you name planet.

Sean Carroll: 00:48:28 Yeah.

Mike Brown: 00:48:29 But, if it’s just a regular Kuiper belt object, there’s ways to name it. So …

Sean Carroll: 00:48:33 So look, when Pluto was officially relabeled as a dwarf planet, not a planet, billions of hearts were broken.

Mike Brown: 00:48:39 Yeah.

Sean Carroll: 00:48:40 People got very upset. You’re considered to be a bad person, because of all this.

Mike Brown: 00:48:44 True.

Sean Carroll: 00:48:44 And yet, you kind of revel in that. You don’t back down. You lean into it, as they say today.

Mike Brown: 00:48:48 Yeah. Yeah.

Sean Carroll: 00:48:49 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-Plutonian objects or something like that”?

Mike Brown: 00:49:01 You will often hear arguments from astronomers who were 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.

Sean Carroll: 00:49:23 Right.

Mike Brown: 00:49:24 And classification is what we do as scientists to try to understand phenomena … in whatever field we’re studying. Bad classification lead 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 borrow in the ground, and all these things. And you know, you would study them in different ways, and that’ll be good. Or, you could be a scientist who studies birds, and you could say, “I’m going to study all the ones that have blue on their heads.” And you know, that’s a classification, and it’s a perfectly valid classification, it’s just not … doesn’t mean very much.

Sean Carroll: 00:50:13 Doesn’t latch on to anything real out there in the world.

Mike Brown: 00:50:15 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 here?” It doesn’t … you can’t ask any questions about it, because planet … the word is … the classification there is sort of meaningless.

Mike Brown: 00:50:46 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.

Sean Carroll: 00:50:55 The moon.

Mike Brown: 00:50:56 The moon … many moons. 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 there round things? That seems like the obvious question. Well, I just told you. It’s gravity.

Sean Carroll: 00:51:18 Yeah. Gravity.

Mike Brown: 00:51:18 We know the answer to that actually. If, instead, you classify the solar system … and 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 Oort 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-

Mike Brown: 00:52:00 … get to be the way that it is. By classifying it correctly you are led to that question. 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 planets … It doesn’t help people understand what the solar system is like. I would like people to understand what the solar system is like.

Sean Carroll: 00:52:23 Before we forget, what is the definition of a planet?

Mike Brown: 00:52:25 Not going to say.

Sean Carroll: 00:52:27 Nobody knows.

Mike Brown: 00:52:28 I refuse.

Sean Carroll: 00:52:28 It’s better off that no one knows.

Mike Brown: 00:52:29 It is better off that no one knows. Because the definition … There are people who are like, “It’s a stupid definition!” Okay, yes, it is a stupid definition. The fact that there is a definition is stupid. In astronomy, can you think of anything else, a phenomenon in astronomy, an object type in astronomy in the sky for which there is a definition that somebody has to check?

Sean Carroll: 00:52:55 Right.

Mike Brown: 00:52:55 In fact, planet-

Sean Carroll: 00:52:57 They argue about it.

Mike Brown: 00:52:57 Planet has a three part definition that you have to fulfill all three parts. The lawyers can argue about whether you fulfill-

Sean Carroll: 00:53:03 Lay it on us. Tell us the three parts. We need to know.

Mike Brown: 00:53:05 The IAU says you have to be …

Sean Carroll: 00:53:08 Round?

Mike Brown: 00:53:09 That’s the last one. You have to be in orbit around the Sun. People get confused by this one, too. The definition of a planet in the solar system is that you have to be in orbit around the sun.

Sean Carroll: 00:53:19 I thought part of the motivation for going through these arguments is because we thought we would be discovering planets elsewhere and we better have a definition.

Mike Brown: 00:53:25 No. The only motivation for this argument is to deal with Pluto.

Sean Carroll: 00:53:29 Okay.

Mike Brown: 00:53:29 Literally. There is no other reason for this definition of a planet than Pluto had to be dealt with, one way or the other.

Mike Brown: 00:53:37 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 … It’s phrased terribly, but I understand what they’re trying to say. It has to clear its orbit.

Sean Carroll: 00:53:53 Clear its orbit of other stuff.

Mike Brown: 00:53:55 Of other stuff.

Sean Carroll: 00:53:56 Right.

Mike Brown: 00:53:59 Instantly, the amateur astronomical lawyers say, “So Neptune is not a planet because Pluto crosses …”

Sean Carroll: 00:54:05 I was just going to say that, yeah.

Mike Brown: 00:54:07 Yes. It’s not because Neptune is not a planet. It’s because the definition is both poorly worded and a bad idea to have to begin with. What they’re trying to say is that the planets are the gravitationally dominant things out there. It’s super easy to make a calculation of something that you would call gravitation 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 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 kicking. Nobody kicks planets around. That’s a pretty good definition.

Sean Carroll: 00:54:56 by this definition, we have the eight planets and Pluto is just one of the various dwarf planets in the Kuiper Belt. 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.

Mike Brown: 00:55:12 Hundreds actually.

Sean Carroll: 00:55:13 Hundreds.

Mike Brown: 00:55:13 Hundreds.

Sean Carroll: 00:55:13 You’d be the discoverer of many, many planets.

Mike Brown: 00:55:15 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 I used your definition, I would be the biggest planet discoverer in human history? Do you know that?” The answer is no, they don’t know.

Sean Carroll: 00:55:30 No, they don’t know that.

Mike Brown: 00:55:30 They’re like, “You just do want to be.” Someone asked me back in that first year when Xena was still being generally called the tenth planet … I was doing an interview, and somebody said, “What does it feel like to have discovered the tenth planet?” I stopped and I thought about it. I said, “You know how it feels? It feels fraudulent.”

Sean Carroll: 00:55:51 Imposter syndrome.

Mike Brown: 00:55:53 No. I wouldn’t even say it’s imposter syndrome. I’d say it actually feels fraudulent. Herschel 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. Le Verrier did calculations on how the orbits were going, realized there was something else out there, had someone point a telescope, and boom, there was Neptune. Those are significant things in our solar system. 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 in exactly the same place. They don’t define the solar system in the same way.

Mike Brown: 00:56:41 It really did feel fraudulent to pretend that this was a major part of the solar system. You really had to pretend if you wanted to call it the tenth planet.

Sean Carroll: 00:56:51 The good news is now we’re basically done, right? We have the Kuiper Belt. We have eight planets. There’s no more planets ever to be found in the solar system.

Mike Brown: 00:56:57 Yeah, we’re not done.

Sean Carroll: 00:57:00 You’re saying there could be other planets?

Mike Brown: 00:57:02 Here’s what I’m saying. There is at least one other planet. I’m not saying could be.

Sean Carroll: 00:57:07 Bold. Bold move.

Mike Brown: 00:57:07 I’m not saying 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.

Sean Carroll: 00:57:33 It’s a long distinguished lineage, yes.

Mike Brown: 00:57:35 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, Konstantin Batygin. He and I came up with this idea a couple years ago. We were very reluctant. We started doing the calculations, we started seeing what we were seeing, and we were like, “God, it really makes sense that it’s a planet.” We do not want to be that 547th person saying, “We predict a planet and we’re right, and everybody else is wrong.” Here’s the interesting thing, is that we predict a planet and we’re right, and everybody else is wrong.

Sean Carroll: 00:58:10 Let’s just pause for a moment before getting to the evidence that you’re right. I want to just say again or highlight this way that science works. Not only do we look for evidence and so forth, but we have prior beliefs. Part of those prior beliefs are colored by history and what has happened through history.

Mike Brown: 00:58:28 Absolutely.

Sean Carroll: 00:58:30 Rather than you and Konstantin 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 I’s are dotted and the T’s are crossed.”

Mike Brown: 00:58:43 We also knew that there were no other planets. I was in 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 earlier observations of planetary positions were not exactly right. He didn’t have the precise mass of Uranus and Neptune. We didn’t have those until Voyager flew by Uranus and Neptune. At the final Neptune flyby, we got the precise mass of Neptune and redid all those calculations of where the planets are. They’re where they’re supposed to be. There is no Planet X.

Sean Carroll: 00:59:29 We’re done with perturbations of Planet X.

Mike Brown: 00:59:30 I knew that in graduate school. That paper came out. We all knew there were no new planets to be found and to think otherwise was …

Sean Carroll: 00:59:38 Heresy.

Mike Brown: 00:59:39 Yeah. Of course, there are no new planets. This led, when Konstantin and I first started looking at these phenomena, it was to prove that there wasn’t a planet.

Sean Carroll: 00:59:50 Right. What kind of phenomena do you look at?

Mike Brown: 00:59:52 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 … Most of the Kuiper Belt objects are in these kind of either kind of circularish orbits, a little outside of Neptune, or maybe a mildly elongated like Pluto. Some of them actually are on hugely elongated orbits and go out ten, 20 times further than the orbit of Neptune and then come back. 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.

Sean Carroll: 01:00:45 That’s where the aliens put them when they left their …

Mike Brown: 01:00:47 Even if the aliens …

Sean Carroll: 01:00:48 … listening stations.

Mike Brown: 01:00:48 Even if the aliens put them there, they would very quickly untangle themselves. Each of the objects in the whole solar system, but in particular these distant ones, their orbits change …

Sean Carroll: 01:00:59 They’d be scrambled by the planet …

Mike Brown: 01:01:01 … over time.

Sean Carroll: 01:01:01 … basically.

Mike Brown: 01:01:02 They precess, is what it’s called. The direction of their orbits changes over a couple tens to hundreds of millions time period. All these objects are like hands on a clock that are moving at different speeds. We happen to look up and they’re all aligned. It could be you just happen to look up when they’re all aligned. Or there’s something else going on.

Mike Brown: 01:01:25 We sort of split in two. My job was to decide whether or not it could be that they just happened to be all aligned and it was coincidence. Konstantin started doing calculations to-

Sean Carroll: 01:01:35 Math.

Mike Brown: 01:01:35 He started doing math. He actually started out by writing equations on his board. He’s like, “Well, what do you think about this?” I’m like …

Sean Carroll: 01:01:42 Looks good. I like the Greek letters.

Mike Brown: 01:01:44 Those are my favorite. So we really were trying to figure out what could have done this. We knew a planet could do that. You see that and you’re like, “Oh my God, must be a planet.” That’s kind of the last 150 years…

Sean Carroll: 01:01:59 Because it would be similar to how these trans Neptunian objects are sort of … There’s certainly places they could fit in without being disturbed by Neptune?

Mike Brown: 01:02:06 That’s exactly right.

Sean Carroll: 01:02:07 You’re saying it’s similar to that.

Mike Brown: 01:02:07 That’s exactly right. We didn’t know the details, but we knew, yeah, we get that there must be some gravitational perturbation that it’ll work. We know that. Of course that works, but 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. We tried really hard to make it not be a planet. There’s nothing else. There is no other way to make those objects line up the way that they do.

Sean Carroll: 01:02:35 Has the story gotten more convincing over time?

Mike Brown: 01:02:39 The convincing aspect of it … This has been two and a half years now our paper has been out.

Sean Carroll: 01:02:44 Planet Nine.

Mike Brown: 01:02:45 Planet Nine, as we called it at the time.

Sean Carroll: 01:02:47 Do you secretly in the back of your mind have a name you want to give it?

Mike Brown: 01:02:50 No, no. I am suspicious, Suspicious also, but 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.

Sean Carroll: 01:03:00 Okay.

Mike Brown: 01:03:01 I honestly do not have a name, which is pretty amazing to have blocked that part of my brain so strongly.

Sean Carroll: 01:03:06 Should we come up with a name?

Mike Brown: 01:03:06 No.

Sean Carroll: 01:03:06 Should we invent something?

Mike Brown: 01:03:06 No, no, no, no, no.

Sean Carroll: 01:03:09 All right. Go on.

Mike Brown: 01:03:12 Planet Nine. We did have a nickname. We have nicknames for everything. The nickname was Phatty at the time.

Sean Carroll: 01:03:19 Okay.

Mike Brown: 01:03:19 Spelled with a PH. It was because that was … Elsa, when I had a daughter, she had a nickname before she was born. I seem to make a habit of that. We had one picked out in case it was a boy. It was going to be Phatty. An old family name of mine is Jehosaphat. There were many Jehosaphat. We always talked about naming our names Jehosaphat, but we would call him Phatty. We would make sure that he was a jazz saxophonist because he’d be Phatty Brown. Doesn’t that sound … It has a pretty good name.

Sean Carroll: 01:03:50 Yeah, totally plays.

Mike Brown: 01:03:51 That was what we called Planet Nine at first. Eventually, Planet Nine was such a good name that we just kept with Planet Nine. Two and a half years ago-

Sean Carroll: 01:03:58 It also is a little poke to people who think that Pluto is still …

Mike Brown: 01:04:00 That was totally unintentional, I promise.

Mike Brown: 01:04:05 Two and a half years. One of the things that always happens in astronomy, I’m sure in physics, everything else, is that theorists are really good at explaining anything.

Sean Carroll: 01:04:17 Yeah, sure.

Mike Brown: 01:04:17 Yeah. If you …

Sean Carroll: 01:04:19 Twelve different ways.

Mike Brown: 01:04:19 Yeah. 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. Theorists are good. That’s their job. We knew that as soon as we published both the observations, that these things are lined up, and our hypothesis of Planet Nine very quickly there would be papers coming up with alternative explanations, alterative physics. We were curious what they were going to be because we couldn’t come up with any. In two and a half years there are zero. That baffles me that no one has come up with another way to make that alignment.

Sean Carroll: 01:04:58 I mean, dark matter, cosmic strings, aliens, black holes. I can come up with half a dozen.

Mike Brown: 01:05:02 Yeah, but write that paper.

Sean Carroll: 01:05:04 Okay.

Mike Brown: 01:05:04 It has to actually work.

Sean Carroll: 01:05:04 There’s a higher level of … You want it to work.

Mike Brown: 01:05:06 Yeah. No, I always get the explanations of like, “What if it’s …” Real physics really working, I’ve seen no explanation.

Mike Brown: 01:05:17 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. There are many ways that these observations could have been biased. 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 looked.

Sean Carroll: 01:05:45 Exactly.

Mike Brown: 01:05:45 For example. That would be an extreme version of the bias. We worried about that. It turns out to be very difficult to do that calculation right. It has taken us, in fact, two and a half years to do that calculation right. I just finished the paper on the final calculation this morning. Literally.

Sean Carroll: 01:06:06 Mazel tov.

Mike Brown: 01:06:08 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 a .1 percent, .14 percent, I think.

Sean Carroll: 01:06:19 .1 percent.

Mike Brown: 01:06:19 It means it’s possible.

Sean Carroll: 01:06:22 Those things happen, but not likely.

Mike Brown: 01:06:24 It’s a small number.

Sean Carroll: 01:06:25 Yeah, yeah.

Mike Brown: 01:06:26 Here’s what I say in the paper. I say, “Look, you don’t have to believe in Planet Nine. I’ll punch you, but you don’t have to believe in Planet Nine.”

Sean Carroll: 01:06:34 You don’t have to, yeah.

Mike Brown: 01:06:34 “But the effect is real. If you don’t believe in Planet Nine, there needs to be another explanation for it.”

Sean Carroll: 01:06:41 Why don’t you go look for it?

Mike Brown: 01:06:45 We are looking for it?

Sean Carroll: 01:06:46 Okay.

Mike Brown: 01:06:47 The two and a half years has bee both understanding those biases, because understanding the biases is critical to then using the observations to predict where it is. 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.

Sean Carroll: 01:07:08 Do you know where it should be?

Mike Brown: 01:07:10 I know the path that its orbit traces out in the sky. 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 phat.

Sean Carroll: 01:07:21 When it should be, right?

Mike Brown: 01:07:22 Yeah. The bad news is we’re not as good as Le Verrier. Le Verrier said, “There’s a planet. It’s right there.” They literally looked one night and found it. It was right where Le Verrier 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.

Sean Carroll: 01:07:53 Okay. Does it perturb Neptune?

Mike Brown: 01:07:55 It does not perturb Neptune. It’s so far out.

Sean Carroll: 01:07:58 Circular orbit far away.

Mike Brown: 01:07:59 It’s not a circular orbit. It’s an eccentric orbit.

Sean Carroll: 01:08:01 I’m sorry, eccentric orbit.

Mike Brown: 01:08:02 Its average distance away is about 500 AU. 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 Kuiper Belt objects. The only thing it affects are these very distant ones that go out into that realm. They go out into where it is and get affected by it.

Sean Carroll: 01:08:22 How big is Planet Nine supposed to be?

Mike Brown: 01:08:23 Now we know it’s right around seven times the mass of the Earth. It might be six, it might be five, it might be eight, but it’s not that much different from that.

Sean Carroll: 01:08:32 Okay.

Mike Brown: 01:08:32 Seven times the mass of the Earth is … Again, Neptune is about 17. It’s smaller than Neptune, bigger than the Earth. Probably it’s like Neptune in that it’s a mostly … It’s an ice giant. It’s mostly a liquidy core surrounded by gas.

Sean Carroll: 01:08:54 It’s way bigger than the Death Star, for example.

Mike Brown: 01:08:56 It could swallow the Death Star quite easily.

Sean Carroll: 01:08:58 Because you must be thinking, why is there such a thing out there, why is it in the wrong orbit, why is it so far away.

Mike Brown: 01:09:06 We think we know the … We have ideas. The idea that there’s an object out there is agnostic as to how it got there. 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 we figured out that it was something like seven Earth masses, ten Earth masses, and on this eccentric orbit, Konstantin and I both, the same light bulb went off in our heads at the same time. It’s like, oh, I know where it came from. Yep, me too. The answer is ten 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. Konstantin and I had actually 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 started out with five giant planets, in the regular giant planet region. 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.

Mike Brown: 01:10:13 We were interested in how that affects the outer solar system and everything else. We didn’t really … Once it got tossed out, we never worried about it again.

Sean Carroll: 01:10:19 The solar system is full in some sense planet wise.

Mike Brown: 01:10:22 Planet wise, yeah. You can’t stick new ones inside where the old ones are. If you try, they are most likely to get ejected. There’s no reason why there should have only been four cores formed. There should have actually been many, many cores formed. Probably there were cores being tossed out all the time. 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. We think that happens when the Sun is formed in a giant cluster of other stars. We think we know how that … Our hypothesis is that’s how it happens.

Sean Carroll: 01:11:03 Sure.

Mike Brown: 01:11:03 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 when you suggest that to theorists working on the solar system, they’re like, “Oh, yeah. Sure.”

Sean Carroll: 01:11:19 Yeah, totally happened.

Mike Brown: 01:11:20 Sure.

Sean Carroll: 01:11:21 Once it’s out there, does Planet Nine sometimes perturb Kuiper Belt objects and turn them into comets to come into-

Mike Brown: 01:11:26 Absolutely. 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 perturbs 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. They’re plunging into the solar systema and back out again, and then it drives them into the Sun and past Neptune.

Mike Brown: 01:11:55 The reason, the transition that Konstantin 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. We went out and realized that those things exist and that nobody had an explanation for them. They would be found and people would just say, “This is so weird. I don’t know why these things are coming in.” Our hypothesis now explains the alignment, some of the other detail dynamics. These are other objects …

Sean Carroll: 01:12:35 Fitting together, yeah.

Mike Brown: 01:12:36 At that moment, literally the moment we did that, I think both of just kind of look at each other like, “Oh. Oh, there’s actually … Oh. There’s a … Oh, there’s a planet out there.” It went from cute idea to, holy cow, there’s a planet. Let’s go find it.

Sean Carroll: 01:12:53 You’re optimistic about finding it? By finding it we mean literally taking a picture.

Mike Brown: 01:12:57 Yes. In the end, it’s a hypothesis that I am convinced is true. No one else need believe it until we go see it.

Sean Carroll: 01:13:07 Are people basically optimistic about it? Or are people scoffing?

Mike Brown: 01:13:11 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 Cal Tech in late spring of all the people around the globe who are in search of it and exchanged ideas on where we thought it was and who was searching and how they were finding it. There are people who are like … There are the general skeptics, like I think most scientists should be, who’ve probably not looked very hard at the evidence. Until you look at it really carefully your default is always going to, “Come on, really? Planet?”

Sean Carroll: 01:13:47 It should be. Right.

Mike Brown: 01:13:47 That’s right. That’s the way to be. Then there are the no way, it’s impossible, I’m going to prove you wrong. They try.

Sean Carroll: 01:13:56 Yeah, knock yourself out.

Mike Brown: 01:13:57 Yeah. They haven’t succeeded yet.

Sean Carroll: 01:14:00 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. Is that basically what happens?

Mike Brown: 01:14:09 Yes. I started in 1978 with photographic plates and now we’re continuing it now looking for something even more distant and a lot fainter.

Sean Carroll: 01:14:17 Do you really have to fly to Hawaii? Don’t they have robots now?

Mike Brown: 01:14:21 It depends on the telescope actually. The telescope that we’re using, the SUBARU telescope, on top of Mauna Kea. It’s the Japanese national telescope. They require you to be there. They don’t need to require you to be there, but they do.

Sean Carroll: 01:14:36 It’s a philosophical dilemma of necessity and requirement that I’m not quite qualified to adjudicate.

Mike Brown: 01:14:41 There’s no actual reason for us to be there. It would work just fine with us on video link somewhere else. Sometimes they actually do do that. They do make us come up there, which I don’t mind at all. It’s a spectacular place to go.

Sean Carroll: 01:14:56 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?

Mike Brown: 01:15:04 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 away, we can’t see things that far away unless they’re planets basically.

Sean Carroll: 01:15:27 Right, okay.

Mike Brown: 01:15:27 If we see something moving at the predicted speed … We won’t see it that night at the telescope. We bring all the data home and have it cranked through a computer. 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. I will be 98% certain at that point.

Sean Carroll: 01:15:48 Very quickly.

Mike Brown: 01:15:49 Really.

Sean Carroll: 01:15:49 Okay.

Mike Brown: 01:15:50 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. 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.”

Sean Carroll: 01:16:11 Take a picture. Not going to tell you why, but you know …

Mike Brown: 01:16:11 I’ll tell them. It depends on who it is. I have enough friends who would do it.

Sean Carroll: 01:16:15 Right.

Mike Brown: 01:16:18 If it’s where we predict, then it is 100% no questions, it is there. We know it. Then the fun starts. Finding it is fun.

Sean Carroll: 01:16:27 Yeah, but learning …

Mike Brown: 01:16:28 It’s actually studying it and learning about this new giant planet that … We only have four. We get a new one. Pretty cool. We have our day zero things that we want to do that we’ll start doing immediately.

Sean Carroll: 01:16:42 You killed Pluto, for which we can’t really forgive you, but it will compensate somewhat if you find another replacement planet. We’re rooting for you there.

Mike Brown: 01:16:50 This was all suggested by my daughter about four years ago. She said, “Daddy, do you know how to get people to stop hating you?” I was like, ” Gosh, no, I don’t.”

Sean Carroll: 01:17:01 Nobody knows.

Mike Brown: 01:17:03 “How should I do that?”

Sean Carroll: 01:17:04 Why is my four year old daughter thinking about these things?

Mike Brown: 01:17:06 She said, “You should go find a new planet and then people wouldn’t hate you anymore.”

Mike Brown: 01:17:11 I laughed and said, “Ha ha, but there’s no new planets.” Now I realize.

Sean Carroll: 01:17:16 The delusion of age.

Mike Brown: 01:17:17 She knew what she was talking about.

Sean Carroll: 01:17:18 All right. We are rooting for you. Mike Brown, thanks so much for coming on the podcast.

Mike Brown: 01:17:22 It was fun.