The centuries-long hunt for the largest hidden object in the solar system

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Connoisseur wearing bowler hat looks through spyglass in the dessert. (Ferrantraite/Getty Images)
Connoisseur wearing bowler hat looks through spyglass in the dessert. (Ferrantraite/Getty Images)

Every school kid learns that there are exactly eight planets in our solar system. But what if we told you there might be a ninth? A world that may be six times the size of Earth and take 12,000 years to orbit the Sun. The only thing is, while some scientists are convinced Planet Nine exists, no one has seen it. Yet.

In this episode, science journalist and WBUR producer Dean Russell (Endless Thread) traces the lives of two astronomers, separated by a century, bound by their thirst for finding that missing planet. This Last Seen story of obsession reveals the unexpected reward when one astronomer gets it wrong — and the fallout when another gets it, seemingly, right.

Show notes: 

Special thanks to Konstantin Batygin, Mike Brown, Samantha Lawler, Renu Malhotra, Richard Pogge, Nina Sankovich, Govert Schilling, Kevin Schindler, CalTech, and the Lowell Observatory for all of their time and wisdom.

Full Transcript:

This content was originally created for audio. The transcript has been edited from our original script for clarity. Heads up that some elements (i.e. music, sound effects, tone) are harder to translate to text. 

Nora Saks: Hey Dean, do you remember those mnemonic devices I think we all were taught to remember the names of planets as a kid? 

Dean Russell: So, mine was - my very excellent mother just served us nine pizzas. 

Nora: So can you give me the planets then, in that list? 

Dean: All right. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and pizzas. 

Nora: Pizzas. Exactly. Whatever happened to pizzas, I do mean Pluto?

Dean: Pizzas got demoted. We lost an entire planet - this little underdog named Pluto - all because astronomers got together to decide what the word “planet” actually means.

[International Astronomical Union Meeting:

Speaker: I would like to open the second session and the closing ceremony]

Nora: Well, what are we hearing? What happened?

Dean: It’s August 2006 and there’s this big conference happening in Prague. Hundreds of astronomers are gathering as part of this annual I-A-U meeting. International Astronomical Union.

Now typically, these things are, like, intellectually impenetrable and incredibly dull. Like, imagine a zoom meeting with 400 other people. But this year, in 2006, they’re voting on something big. The definition of the word planet. And that’s all because of this one guy.

Mike Brown: So it was early morning because this was midday in Prague, so I, I don't know, it was 4:00 or 5:00 in the morning. I got here in the dark.

Dean: Mike Brown is this astronomer at CalTech. He wasn’t at the conference, but he was live-streaming the vote from California.

Mike Brown: And all the film crews were here ready to go because nobody knew what was going to happen. Nobody even knew at the time what the final resolution to be voted on was going to be.

Nora: Woah, woah, why were all those film crews following him?

Dean: Yeah, Mike is surrounded by film crews, because this is kind of his fault. And he has a lot riding on this vote. Two different definitions are being debated. And those definitions will determine whether or not Pluto is considered a planet.

So Mike, wondering how this whole thing will go down, homes in on the vote’s moderator.

[International Astronomical Union Meeting:

Woman: We have planets, the eight that are named. We have dwarf planets.]

Mike: She had her props. She had some big inflatable planets thing and then she had a big Pluto doll for Pluto.

Dean: If Pluto gets the boot, it would instead be called a dwarf planet, which is different. Overnight, the number of planets in our solar system drops from nine to eight. Millions of textbooks will be suddenly out of date, research funding will likely be harder to get if you’re studying Pluto and, maybe most importantly, kids everywhere that have this inexplicable affinity for the solar system’s tiniest planet - like me - will have to accept that their favorite runt was really just a fraud.

Mike: I did not believe that astronomers would have the guts to demote Pluto. I assumed that we had Pluto forever.  

Nora: It kind of sounds like Mike wanted Pluto to be demoted. Why?

Dean: To Mike, Pluto was miscategorized. First of all, it’s tiny — about the size of Russia. Secondly, its orbit is way out of whack with the rest of the solar system. And then finally, and this ended up being one of the main points, compared to the planets, Pluto is a lot more like its immediate neighbors. These icy planetoids that make up the Kuiper Belt.

Nora: So what did all those other astronomers decide at that really tense and really nerdy meeting you were talking about?

Dean: This thing unfolds in a way that you wouldn't really expect a science conference to unfold. It gets very unruly. People are shouting and the moderator finally gets up and says, okay, everyone who doesn't want Pluto to be a planet. Raise your yellow card now, and it's a sea of yellow. 

Mike: And when you saw all those hands. The journalists in the room are all just kind of, you know, you could hear that intake of breath.

Dean: Pluto is dead.

[APPLAUSE from the International Astronomical Union Meeting]

Nora: Okay, so there’s one thing I don’t really understand. How is this Mike’s doing? 

Dean: Mike Brown was among all of these scientists debating whether Pluto is a planet. But the entire debate is happening because Mike Brown, one of these scientists, had been trying to find a new planet. And instead of making our solar system bigger, Mike's research shrunk it.

What he uncovered made everyone question what it means to be a planet.

And as I started to look into this, I realized that the story of Pluto and Mike and the planet Mike was searching for goes way, way back. It’s a lot bigger than that. Because the planet Mike was hunting, in one way or another, astronomers have been hunting for it for nearly 200 years. It’s literally one of the largest unsolved mysteries in human history. And Mike still believes he’s going to find it. He’s obsessed.

Mike: I think it's a little Ahab like. I am going to I'm going to go find this thing. And it may kill me. (LAUGHS)


Nora: Welcome to Last Seen, a show about people, places, and things that have gone missing and whether or not they can, or even should, be found. From WBUR - Boston’s NPR station. I’m Nora Saks.

Today, producer Dean Russell brings us the story of two astronomers, separated by a century, who spend their lives obsessed and searching for something that may not even exist. The largest hidden object in the solar system.

This is Episode 3 - The Lost World.


Dean: The commotion around Pluto and the search for a missing planet started a century before Mike was even born. In 1858, with a comet.

Kevin Schindler: His earliest recollection was when he was three years old and a comet called Donati's comet was visible from outside the window at the family home in Brookline. 

Nora: This is Kevin Schindler, a historian at the Lowell Observatory in Arizona. I called him to ask not about Mike Brown, but his planet-obsessed predecessor, Percival Lowell. Percival Lowell, who is, without question, one of the most famous planet hunters ever.

Kevin: He was brilliant. He was an articulate and vivacious speaker by all accounts. And it really captured the imagination of the general public.

Dean: When Percival was a kid, he was obsessed with the stars. His mother gave him a 2 and a half inch telescope, and he would spend hours comparing what he saw to the astronomy textbooks he collected. He was kind of a nerd.

But he came from a serious family of American aristocrats. They owned cotton mills, which made a lot of money in the early half of the 19th century in part because of slave labor.

His father was one of the wealthiest men in Boston — who made extreme success a prerequisite for his children.

Kevin: He had one brother, Abbott, who was president of Harvard for 24 years. He had one sister who married a relative of Theodore Roosevelt. He had another sister, Amy, who won the Pulitzer Prize.

Dean: Wow. 

Kevin: So that's a pretty heady family. 

Dean: Percival’s father didn’t see him as an astronomer. He wanted him to take over the family business. So, in 1876, after Percival went to Harvard,

Kevin: Because that's what you do.

Dean: He started running a handful of mills. He lasted six years. Couldn’t stand the life of a proper Bostonian. So, he quit. He traveled to Japan and Korea, writing travel logs about what, to him, seemed like other worlds. Then, something happened that would turn his attention back to his childhood obsession: the night sky.

Kevin: Mars made a really close approach to Earth or in opposition. And then this Italian astronomer also detected these apparent linear features that he called Canali. 

Dean: In 1877, Giovanni Schiaparelli discovered strange straight lines on the surface of Mars. These Canali, he called them. And as he was getting ready to retire, Percival decided to pick up where Schiaparelli left off.

Kevin: Percival Lowell thought, you know, these canali are too straight to be natural. They must be have been made by some sort of intelligent life. And in fact, Lowell intentionally modified the term canali to canals, which implies artificial.

Dean: He still wanted to impress his family. He thought there was no better way to do that than by proving the existence of aliens.

Kevin: He was a brilliant mathematician. He was self-taught in astronomy. But, you know, Percival wasn't a trained scientist.

Dean: He never trained in an observatory. He didn’t spend his twenties performing rigorous research. He was a dilettante. And he bought his way into astronomy. Literally. He financed his own observatory in Arizona. Commissioned a 32-foot telescope. After donating $10,000 to MIT, he pressured its president to give him a job as an astronomy professor. And it worked. These things gave him the credibility he needed to do and say what he wanted. Without evidence.

Kevin: Science in the ideal, perfect scenario, you collect data. And somewhere along the way, you come up with hypotheses and so on. But, you know, I think with Percival Lowell, it is probably backwards.

Dean: Historians have uncovered some of Lowell’s old journal entries. Where he basically says that you need to make things true. Bold theory first, proof later.

Kevin: You know, it's just it's not the normal course of science.

Dean: So he came along with sketchy data, telling the public that he had seen Martian-made canals and he drew pictures of what he saw as proof. And the public loved him. For them, he was this original Carl Sagan.

But to astronomers, his peers, Percival was a threat to their credibility and the credibility of the field.

Kevin: The scientific community pretty much, the consensus was, this isn't right. 

Dean: Percival was wrong about intelligent aliens living on Mars. Because those canals? They were artifacts from the telescope viewfinder. Percival never accepted this but he did notice that he’d become this laughing stock, by some accounts. So he put Mars on the back burner.

Kevin: When he started seeing that he was losing steam with that, he started picking up the search for another planet and I think it was, again, that desire to make a big discovery. 

Dean: He thought the only way to recover his legacy was to pursue a new idea, to find a planet, what he called Planet X. It was perfect really. Because the astronomers who discover planets are remembered forever.


Dean: Mike Brown’s entrée into the planetary search party started in the 1970s in Alabama.

Mike: Huntsville was this amazing place because basically everybody in town was there in some sense because of rockets, it was the Apollo program. The Saturn 5s were being built there and it was just a normal part of growing up. You grow up and everybody works on going to the moon. And you’re sitting in your room one morning and the whole place shakes.

Dean: Mike’s dad built rockets. But Mike didn’t want to do that.

Mike: The first thing I remember about a planet really is the Viking lander on Mars in 1976, 

[News Broadcast: Viking 1 and Viking 2 were readied for their separate journeys to Mars.]

Mike: and it was the first color photograph that ever appeared on my hometown newspaper front page. Just this, this full spread of this, you know, kind of these days we would look at it and think it looks pretty bleak. It looks a little bit like Death Valley, just rubble strewn everywhere. And that was, I think, the moment that suddenly planets caught my attention instead of just the moon.

Dean: Why do you think that was? 

Mike: Because it was real. I've always had a hard time being excited about sort of abstract stuff or even just like little dots in the sky that I didn't - I was not some, you know, a kid who went off and memorized all the constellations or could even point out where the planets were. But once things became real, I became enthralled.

Dean: Mike went to school for astronomy. He dug into the solar system. But in the early 90s, there was this feeling that we’d pretty much learned all that we could about the solar system. At least all of the big stuff. Astronomers just didn’t talk about the possibility of another planet, it was like talking about Atlantis. You’d become a joke.

Mike: And then in 1992, suddenly this tiny little object beyond Neptune in the vicinity of Pluto was found. And by the end of 1992, I can't remember the number, but there were probably two or three. By the end of the next year, there might have been a dozen. 

Dean: Telescopes and technology improved. Astronomers started conducting comprehensive searches of the night sky and began discovering what they called “Trans-Neptunian Objects.” Icy planetoids past Neptune, that made up a donut-shaped band of debris scientists named the Kuiper Belt. The solar system, it seemed, still had some secrets left. And Mike, he had a hunch.

Mike: I thought there was a pretty good chance that somewhere just beyond the Kuiper Belt, there would be something like a planet. 

And I didn't know what that meant. I didn't know what I meant by that at the time, but I meant something maybe the size of the Earth and maybe 20 percent further away than Neptune is what I kind of thought there might be a good chance that there would be out there.

Dean: In 1999, Mike was 34 years old. He was sitting in a big, domed observatory on a mountaintop near San Diego. It was night and fog rolled in, meaning the telescope was out of commission. He and a friend had nothing to do but talk and wait.

Mike’s dad had just died, so the friend was trying to keep things light. She asked him if he was excited about any upcoming work. And Mike just kind of blurted it out. “I think there’s another planet past Pluto.”

He had no evidence. But they made a bet on it.

Mike: Yeah, you sound crazy. You sound like you're going to go find a planet great. But you know, that's what friends are for. I wouldn't I wouldn't have made a bet with the public about this. But, but it's OK to make a bet with a friend.

Dean: Yeah, you wouldn't have pulled a Percival Lowell or anything like that? Told the world that you were gonna find something?

Mike: Yeah, yeah. No, no. Well, not until now, I guess. (LAUGHS)

Dean: His mission was now clear. But his method wasn’t. How do you find a planet? The answer, in a minute.


Dean: After about a year of searching, by 1907 Percival Lowell had only discovered one thing: an asteroid. But his search for a ninth planet was sporadic. He had a history of nervous breakdowns. He stopped and started. He kept the hunt a secret for almost a decade. And, eventually, he reluctantly accepted that to find a planet, he needed to do the boring work that scientists do. He needed clear, replicable evidence of a ninth planet. But the amount of space to scan with a telescope was unimaginably large. He had to use something called orbital dynamics.

Renu Malhotra. So most people think, of course, you know, we learn that planets move around the sun in elliptical orbits. But that's not the whole story. That's the tip of the iceberg.

Dean: Renu Malhotra studies this stuff.

Renu: I am a researcher, a professor at the University of Arizona, I study Orbital Dynamics and Planetary Dynamics.

Dean: Every piece of matter floating out there in space, all the rocks and planets and stars, they have mass and thus gravity. This means as they float around, they’re all pushing and pulling.

Renu: Not only the gravity between the sun and a planet individually but the gravity between every planet with every other planet.

Dean: Think about it like this: every time a bigger planet passes a nearby smaller planet, the gravity from the big guy gives the little guy a kick. That can cause the little guy to shake or veer slightly off course. There’s a word for that: aberrance. But,

Renu: It gets really complicated. 

Dean: Like, what happens when one or two planets are aberrant for no clear reason? Percival thought a mysterious third object with a lot of gravity was pulling on Uranus and maybe Neptune. He thought,

Renu: that there must be another large planet beyond.

Dean: And Percival thought he could reverse-engineer this, so to speak, and predict the location of the unknown giant. Again, Kevin Schindler.

Kevin: They started doing some calculations, you know, based on the movement of these bodies. Where is this unknown body? How big is it to cause these movements?

Dean: Percival was working with a team of human computers. Mathematicians.

Kevin: And as the computers were getting closer to what Percival Lowell thought was an accurate final answer, he would have his observers focus in that area of the sky. 

Dean: Eventually, the computers figured it out, they thought. Percival put together a memoir of all their work using orbital dynamics. It predicted that Planet X would be around seven times the mass of Earth, hanging out somewhere in the constellation Gemini.

After years of keeping this stuff under wraps, he decided to go public. He planned this big talk in Cambridge, Mass. January 1915. He was anxious because he knew his scientific peers would be judging him. He had calculations, but no picture. And like a person who can’t stop checking their email, Percival couldn’t stop checking in with his staff right up until the event. Like, did you get the picture yet? What about now?

Kevin: He would write telegrams to the staff with messages such as, don't be nervous about, you know, writing me of the discovery of the new planet. I mean, you know, if it's late and late at night, don't worry about waking me up with a telegram, you know, that sort of thing.

Dean: It’s not clear what happened at this talk. But he didn’t make a huge splash. Newspapers at the time reported on a gallery Percival set up at the Boston Public Library, showing images of the planets. The reporters rehashed the Mars canal debate. But, in all of the archives that I could find, they didn’t say anything about Planet X.

And the next year, Percival died.

Kevin: and that was kind of the end of the search for a while. 

Dean: How did he die?

Kevin: He had a stroke. And he was only 61 and he was, you know, Percival was, I would say a, you know, high strung probably, and in some ways an intense personality. After he passed away, his brother, president of Harvard, Abbott Lawrence Lowell, he later wrote a biography about Percival Lowell. And he said that, Percival’s greatest disappointment in life was not finding Planet X.

Dean: In 1930, 14 years after Percival’s death, another astronomer used his calculations like a treasure map and he found a planet. People thought it had to be Percival’s Planet X. But it was too small. Because, after all that number crunching, it turned out his data were flawed. Uranus and Neptune weren’t aberrant. The fact that the calculations led to anything was just a fluke.

What that astronomer found was Pluto. Percival is considered one of the discoverers.


Dean: So why did Pluto get demoted? It was January 2005. At this point, Mike Brown had discovered a handful of interesting Trans-Neptunian Objects, all a fraction of the size of Pluto, all hanging out in the Kuiper Belt.

But in 2005, the computer program he used to scan the sky wasn’t working like he wanted it. So, he rewrote it. As soon as he reran it, he struck gold.

Mike: It was brighter than anything else I'd seen, and it was moving more slowly than anything else I'd ever seen. And, you know, moving slowly means it's far away. Being bright means it's big. Moving slowly and being bright means it's really, really big.

Dean: It was about the same size as Pluto. Maybe bigger. And it was living in the same general area as Pluto.

Mike: And I just sat there and stared for a minute, and then I picked up the phone and called my wife and said, I think I just found a planet.

Dean: What he found, he nicknamed Xena. The X in Xena, a nod to Percival’s Planet X. Mike was excited to have discovered a tenth planet. But he also started to worry. If Xena was labeled a planet, what will happen when we find the next Pluto-sized planet? Mike was pretty sure there were more, potentially hundreds in the Kuiper Belt just like Pluto.

So Mike thought, maybe these tiny rocks shouldn’t be called planets at all. There are too many. And, universally speaking, they’re not that significant. Which is a pretty weird thought because Mike had finally discovered a planet. And now he was choosing principles over ambition. In the end, Mike didn’t have to choose. Because at that tense live-streamed conference in 2006, the I-A-U chose for him. Words were parsed. Definitions made. And, on the same day that Pluto got the boot from the planet club, so did Xena. They were un-planeted.

[International Astronomical Union Meeting:

Speaker: Then it’s clear Resolution 5B is not passed. (APPLAUSE)]

Mike: I don't feel remorse because I guess I don't feel like anything bad has happened to Pluto. I think Pluto has helped us to really understand how the outer solar system put itself together, and we are now classifying it correctly. I mean, the real thing to think about is it was never a planet, we just misunderstood it at first and misclassified it.

Dean: After the meeting, the solar system went back down to eight planets. And Mike stopped searching. He said he wanted to try something new for a change.

But that didn’t last long.

[Clip from NPR: An unseen planet about 10 times more massive than Earth is lurking in the outer reaches of our solar system.]

Dean: In 2012, Mike and a few other astronomers started to notice aberrations in the solar system. Some of the dwarf planets they had previously found are moving in strange orbits that can’t be explained. They’re floating way out there, past the Kuiper Belt, at the edge of the solar system. And their orbits all appear clustered and oddly angled and really elongated. It looks like something is tugging on them. But what?

Mike and a colleague, Konstantin Batygin, build a simulation of the solar system. They start plugging in ideas. Solutions. Maybe it was an ancient star that drifted by and warped their orbits. Maybe it's galactic tides. Maybe their data are wrong. Finally, they plug in their last idea.

Mike: And as soon as that happened, we were like, Oh. Oh, oh, you mean there's oh, there's a planet out there like, oh, my God, there's actually a planet out there. 

Dean: It was a missing planet. That’s what they said. And after they published their idea, the public went crazy. But some of their peers were skeptical.

Renu: Something has perturbed the outer solar system. Is it a large planet? I don't think that evidence is that strong.

Dean: Renu Malhotra was actually really into the idea to begin with. But as time passed, Renu, like others, started to notice problems with the data. Without getting too technical, Mike is basing his theory on a dozen or so objects. They may look clustered because of observational biases.

Renu: It's a statistics of small numbers

Dean: I voiced this criticism from Renu and others to Mike.

Mike: She's wrong. Haha. I mean, we have we didn't brush past any of that. We spent forever figuring this out correctly. And no one else, as far as I can tell, has done the entire data set like we have. It’s not it's not that we're saying that, you know, we are the thirty-fifth and thirty-six people to say that there's a planet out there and and everybody before us was wrong and you should believe us instead. It's just actually true. 

Dean: The technology and computing power today is leagues better than the days of Percival Lowell. And Mike has a solid track record. He’s discovered dozens of minor planets. So, their situations — Percival and Mike’s — they are different. But at the simplest level, they’re also not. Both Percival and Mike saw something odd about the solar system and came to the same conclusion.

Mike, however, pushes back on this idea.

Mike: There is no Planet X. Planet X does not exist.

Dean: He and Konstantin won’t call their predicted planet, Planet X. Instead, they call it Planet Nine. They don’t want to liken themselves to Percival. But honestly, it’s hard not to.

Since announcing their hypothesis in 2016, the duo have been refining their calculations. Today, they think their planet is about six times the size of Earth, floating somewhere near the constellation Gemini. Sound familiar?

Dean: In a lot of ways you are very, very different than Percival Lowell obviously, but you know, it can't be helped to compare part of the story to Percival Lowell and I'm curious about how you feel about that.

Mike: It's funny. I don't, I don't think about, I guess I don't really - Yeah. So Percival Lowell was obsessed with finding Planet X. Could we be the same way? I, you know, I just don't. I don't see it. I just don’t see the parallels because I know why they were wrong. They were wrong because they weren't careful about their data. It led them astray and I just don't think that's going to happen.

Dean: What if you're wrong?

Mike: I’m you know, I'm all in. I am still - then I will be the person who was a total idiot and thought there was a planet when there wasn't just like the 200 idiots before me who did it. But I'm, I don't, I just don't see that happening. Although none of them did, either, so, (LAUGHS)

Dean: Lurking behind all of this is a question. A great question about obsession without a clear answer. Finding a planet brings fame and book deals, but Mike already has those things. He says he searches because he knows it’s there. End of story. And if it’s not there, so be it.

And as I put this story together, I’ve thought a lot about why we search, even when our chances are slim. If it’s a missing person or a missing cat or a missing feeling, or a missing planet, many of us search — become obsessed — because it’s hard to imagine the alternative.

There’s something very human about that optimism. Deeply flawed, maybe. Not always as skeptical as the great halls of science might demand. But very human.

We have to believe in something, in making the un-right, right, in achieving a whole-form universe however you define “universe.”

Mike: This is part of our essential nature as humans, is to explore and to understand what's around us. And there is a huge part of our solar system that we knew nothing about just a few short years ago that is itching to be explored. And I think that is compelling to anyone who you talk to. That's why this idea of a planet resonates with the public so much. It's this new exploration. And I I feel as if, you know, if we if we stop exploring, we might as well stop being human. And this is this is just what we keep doing.

Dean: I think back to something Percival Lowell wrote when he first announced the result of his calculations of Planet X. I imagine him writing in the Arizona daylight after a sleepless night of searching, something he often did. And it's funny because he does something he didn’t often do, he admitted that he could be wrong.

But to that idea, he essentially says, “so what?” The work he and his staff have done, right or wrong, is meaningful. A thorough analysis of the problem is just as valuable as the solution.

“For that, too, means advance.”


Nora: Coming up next week, a story about a pair of bold leaders and their hope for a new African nation - that disappeared from our collective memory.

Brenton Zola: Not only could the future have been different, but it was on the path of being different. And we basically thwarted that path.

You can find all of our stories and show notes on our website and follow us on Twitter at @LastSeenPodcast.

You can pitch us your story ideas about people, places, and things, that have gone missing. We’re interested in pitches from contributors or just folks who want us to tackle the story. Drop us a line at

Headshot of Dean Russell

Dean Russell Producer, WBUR Podcasts
Dean Russell is a producer for WBUR Podcasts.


Headshot of Nora Saks

Nora Saks Producer
Nora Saks was a producer with WBUR's podcast team. 



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