Why The Former Planet Pluto Got Demoted In 2006 the former ninth planet in the solar system, Pluto, got demoted to a mere Kuiper Belt object. The man who was in large part responsible for that demotion, Caltech planetary scientist Mike Brown, discusses the status of Pluto and why it doesn't qualify as a planet.

Why The Former Planet Pluto Got Demoted

Why The Former Planet Pluto Got Demoted

  • Download
  • <iframe src="https://www.npr.org/player/embed/133498152/133498131" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript

In 2006 the former ninth planet in the solar system, Pluto, got demoted to a mere Kuiper Belt object. The man who was in large part responsible for that demotion, Caltech planetary scientist Mike Brown, discusses the status of Pluto and why it doesn't qualify as a planet.


I'm Ira Flatow. This is SCIENCE FRIDAY.

Up next, I want to read you a couple of lines. See if they mean anything to you. Here they - here's the first one. My very excellent mother just served us nine pizzas. Did you have to learn that line in school, science class? How about this one? Martha visits every Monday and just stays until noon, period. Does that ring a bell? If not, you don't have to worry because both of those sayings are relics of the past. They're mnemonic devices for remembering the nine planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and, well, in those days, Pluto, which, as of 2006, as you know, is no longer classified as a planet. It's been demoted to a mere Kuiper Belt object.

But don't feel sorry for Pluto because my next guest says Pluto had it coming. Mike Brown is the author of "How I Killed Pluto and Why It Had It Coming." He's also professor of planetary astronomy at the California Institute of Technology, Caltech, out there in Pasadena. Welcome back to SCIENCE FRIDAY.

Professor MIKE BROWN (California Institute of Technology): Oh, thank you. I'm delighted to be here.

FLATOW: Why did Pluto have it coming?

Prof. BROWN: You know, it really was that astronomers made a mistake back in 1930. And so, in 2006, when Pluto got demoted, it was really fixing the solar system back to the way it probably should have been, as opposed to doing something mean to Pluto. So it's really not that we were against Pluto or had any big complaints about it. But it just never should have been classified with those eight big things.

FLATOW: And so what was your role? "How I Killed Pluto" is your book. What was your role?

Prof. BROWN: Well, I have to admit that my original role and - as I start out the book, my original role is that I was searching for a 10th planet, because I knew, being a very reasonable person, I knew that if I found something bigger than Pluto, clearly it had to be a planet. And so I wanted to go find one so I could be known as the person who found the 10th planet.

But by the time I got there, by the time I found the object that we now know to be more massive than Pluto, it was really clear that there were so many other things out there, that it really made much more sense to step back and call those eight big ones planets. And all these other things that I'd been finding are just parts of the Kuiper Belt, cosmic debris.

FLATOW: And what was that planetoid thingy?

Prof. BROWN: Yeah, the one that's more massive than Pluto...


Prof. BROWN: ...is - it now goes by the name of Eris, E-R-I-S, the Greek goddess of discord and strife.

FLATOW: Mm-hmm. And it's still believed to be more massive because, you know, sometimes these things go back and forth on how massive they are.

Prof. BROWN: They actually go back and forth on the size. In fact, Pluto, when Pluto was discovered, it was thought to be an extremely large planet. And this is one of the reasons why it was called a planet back in 1930. And over time, it has been getting smaller and smaller and smaller, and we now know its size pretty well.

And Eris was the same way. When we first discovered Eris, we didn't know how big it was. And we thought, wow, maybe this is going to be a real planet the size of Mercury or the size of Mars. And measurements have refined that size down. And it's now - as close as anyone can tell, Pluto and Eris are more or less the same size. Whichever one is bigger is probably bigger by 10 kilometers out of 2,300 kilometers. It's ridiculous how close they are together. But the one thing we know extremely well is that Eris is more massive. We can measure the mass very precisely.

FLATOW: Well, why do Pluto and Eris not fall into the planet category?

Prof. BROWN: They - if you look at the eight planets, the eight planets are the massive objects that dominate the solar system. They're all in essentially circular orbits in a single disc around the sun. And they have - nobody kicks them around.

Everything else in the solar system - the - Pluto and Eris and the many other thousands of Kuiper Belt objects out in those similar regions past Neptune, all of these objects have tilted orbits, have elliptical orbits. They get kicked around by the planets. They slip in and out of all these other planets. They're simply different sorts of objects.

Now, what we assign the word planet to maybe doesn't matter as much. But recognizing that the eight large things in the - that dominate the solar system are different from Pluto and Eris and all the rest of them is an incredibly important thing to understand about the solar system.

FLATOW: Mm-hmm. And do you think that if you or others look long enough, you'll find another one?

Prof. BROWN: Well, that's always my hope. Although I have to admit, I think the probability is pretty low. To be really classified as a planet with the other planets these days, you would have to be massive. You would have to be, essentially, one of these things in a circular orbit...

FLATOW: Right.

Prof. BROWN: ...dominating your part of the solar system. And if something like that existed, we would probably have found signatures of it in the outer solar system by now. So I certainly end the book speculating about what else might be out there that we might find. And I had a bet that I started with - that I was going to find a planet one of these days, and I still I still have - I officially won the bet because the bet was only to find something bigger than Pluto, and yet those bottles of champagne that I won are still sitting on my shelf, waiting in the hopes that someday there's something else out there.

But I think if I'm going to have to find a planet, I'm going to have to do what everybody else is doing these days and go look around other stars.

FLATOW: Now, you originally had named it Xena.

Prof. BROWN: I did.

FLATOW: And then they had to rename it. Why? Did you get in trouble for that?

Prof. BROWN: The name Xena was originally just a nickname because nobody knew what to classify it as, as soon as we discovered it. Nobody allowed us to name it because depending on what you are, you get different categories of names.

And so we had a nickname. We had chosen this nickname earlier, so that when we talked amongst ourselves about it before we had announced it, we knew what we were talking about.


Prof. BROWN: And Xena seemed like a perfectly good name for a 10th planet. It's got the X in it. It's got the, you know, sort of the mythological sound to it. So that nickname - because it didn't get a real name for a long time, that nickname sort of got to be the name that a lot of people knew it by for that first year.

FLATOW: Let's talk about some really fascinating news from the Kepler Mission this week. A paper published in Nature that had a couple of things. First there was the scientists having discovered six transiting planets around a sun-like star they're calling Kepler 2. Tell us what is significant about that.

Prof. BROWN: It's actually Kepler-11.

FLATOW: Kepler-11. I'm reading Roman numerals. I'm thinking there's 11. Absolutely right.

Prof. BROWN: Yeah.

FLATOW: Yeah. So what is so fascinating about that discovery? So you have six, you know, little planets around a star.

Prof. BROWN: So it's - I have to say that it's rare, in my scientific life, where I open up a paper and read it and I just have to then pick my jaw up off the floor, and I really had to this time.

First off, the Kepler Mission is finding planets by looking at these very, very small dips in the light as the planets pass in front of them. So the fact that all of these planets are so perfectly aligned that all six of them get little dips like that, is just - it's just amazing that they're there.

But the scale of that planetary system is just astounding. The five interior planets are all sort of between two and 10 times the mass of the Earth, so they're big. There's nothing that's quite the size of the Earth out of there.

But they're all packed in incredibly tightly down inside where the orbit of Mercury is in our planetary system. And then just outside of them is something that's maybe the mass of Jupiter, or perhaps even very larger, but it's still packed in tight like that.

It just - it really challenges everything we think about a planetary system. And in fact, it really challenges everything we think about what to call a planet, although I still think that the ideas that we have developed here in the solar system for what planets are, are going to transfer very naturally. But boy, that's a crazy planetary system.

FLATOW: Are they bigger than the Earth, these six planets?

Prof. BROWN: All of the ones that are found there are bigger than the Earth. The smallest one is about twice the radius of the Earth, and you can measure that very precisely because you can just see how much sunlight they block.

FLATOW: And so you have six of them packed inside what would be equivalent to inside Mercury's orbit.

Prof. BROWN: Yeah. It's...

FLATOW: That's - that is.

Prof. BROWN: It's crazy. So when we first look at this, I thought that this can't be right. There's no way these can even be stable in there without tossing each other all over the place, but in fact they are.

They're all just like our own solar system planets. They appear to be on essentially circular orbits in essentially the same disc again, showing that these ideas that I talk about in the book about what a planet is are really universal and extend throughout the galaxy.

FLATOW: We're talking with Mike Brown, author of "How I Killed Pluto and Why It Had It Coming."

Let's go to Caroline in Louisville, Kentucky. Hi, Caroline

CAROLINE (Caller): Hi. I go to St. Francis School(ph), and I did a science project on Eris. So I just wanted to ask, when did they find Eris's moon?

Prof. BROWN: Eris's moon is now called Dysnomia. Dysnomia is the demon spirit of lawlessness, and it's the daughter of Eris. Of course, when Eris was Xena, the moon had to be called Gabrielle because it's the only thing that made sense.

So we discovered that one from the Keck Observatory in Hawaii, and that was discovered in October of 2005. So shortly after the discovery, we got a chance to use that big telescope out in Hawaii. And sure enough, right next to Eris, there was this tiny little dot, Dysnomia.

FLATOW: Caroline, why did you choose to study Eris?

CAROLINE: Well, he said that you had to choose, like, a planet or something. And I said I wanted to choose something else, like a dwarf planet. And so I chose Eris because I thought it was really just cool.

FLATOW: Do you talk about planets and dwarf planets and things like that a lot?

CAROLINE: Yeah, kind of - maybe a little. I'm kind of obsessed with them.

FLATOW: Yeah. Which is your favorite planet?

CAROLINE: I like Jupiter a lot (unintelligible)...

FLATOW: Because?

CAROLINE: Because, like (unintelligible) the giant spot printed on Jupiter. So - and its stripe too.

FLATOW: Yeah. Mike, does that spot - does it change in size at all?

Prof. BROWN: It does. It gets bigger and smaller, it changes in color over the past hundred of years that we've watched it. But I have to say, I agree with Caroline there, that Jupiter is just - how could it not be your favorite planet? It's got, it's got it's got some of everything. It's got storms. And it's got moons that have liquid - water, oceans. It's got all kinds of stuff. It's a great place to be fascinated by.

FLATOW: All right. Caroline, thanks for calling. Good luck to you.


FLATOW: Good luck with your report.

CAROLINE: Thank you. Goodbye.

FLATOW: Bye-bye. 1-800-989-8255 is our number. Let's see if we've got some time for a couple of more. Yeah, we do. Doug(ph) in (unintelligible) Ohio. Hi.

DOUG (Caller): Yeah.


DOUG: Hi. Thank you for having my call.

FLATOW: Mm-hmm.

DOUG: I remember growing up and being in high school, learning that Pluto had a moon, Charon or Charon - I can't pronounce right.

Prof. BROWN: Neither can astronomers.

(Soundbite of laughter)

DOUG: Yeah. I'm curious. Since it's not a planet anymore - I totally forgot about this since last summer when they, you know, demoted Pluto - what happens to Charon now?

Prof. BROWN: Well, you know, it's a - having a moon really doesn't change anything about being a planet or not a planet. And of the 1,500 objects that we know now past Neptune, something like a third of them have moons also. So having a moon doesn't make you a planet. And in fact, you know, as everybody knows, Mercury and Venus don't have moons and they're still planets...

DOUG: Right.

Prof. BROWN: ...at least for now. But - so it's now just a moon of a Kuiper Belt object or moon of a dwarf planet. And so having a moon - and in fact, since we've learned about that one moon, there are actually now two other moons known around Pluto too. So Pluto is actually packed full of moons out there.

DOUG: (Unintelligible)

Prof. BROWN: And having those moons is a tremendous scientific boon, because it's only way that we can learn how much these dwarf planets weigh and figure out what's going on inside of them. So it's very exciting when we find these moons around them.

FLATOW: Thanks for calling, Doug.

DOUG: Right. Thank you.

FLATOW: And this is SCIENCE FRIDAY from NPR. I'm Ira Flatow, talking with Mike Brown, author of "How I Killed Pluto and Why It Had It Coming." What are the other names of the moons on Pluto?

Prof. BROWN: There's Pluto and Charon. And then the other two names are Nix and Hydra, also characters associated with Pluto in the underworld in Greek mythology.

FLATOW: Now, I know that because Pluto is no longer part of the planetary system, we need a new mnemonic device for remembering the planets. And you have a favorite one.

Prof. BROWN: I do. This one was given to me. And when - I was on a different radio program just the day that Pluto had been demoted, and somebody called in, and they were trying to come up with all sorts of different ones. But somebody called in with this one, that very first day, which I still think is the best one, which is: mean, very evil men just shortened up nature.

FLATOW: Mean, very evil men just shortened up nature.

Prof. BROWN: I think that works pretty well.

FLATOW: Is it true?

Prof. BROWN: You know, it was not mean, very evil men, and there were a lot of women involved too. It was - it doesn't fit the mnemonic right, but I would instead characterize it as many rational people set nature straight. But then we'd have to rearrange the planets.

(Soundbite of laughter)

FLATOW: And in the short time we have, you talk about 1,500 exo-planets out there. That number has just jumped recently, right?

Prof. BROWN: It's - well, so I talked about 1,500 objects in the Kuiper belt and yet that number is about the same number - it used to be much more than the number of exo-planets we used and now it's about the same. So yeah, the discoveries of planets around other stars, particularly from the Kepler Mission, is just going crazy. It's hard not to feel like this is the most exciting thing going on in all of astronomy right now, all these new discoveries. And they will just keep pouring in over the next couple of years.

FLATOW: You know - and I'm surprised how much airplay that those discoveries got this week.

Prof. BROWN: It's because - lot of it is because some of those are very close to being Earth-sized and they're the first close to being Earth-sized things that have been found in the inhabitable zone. So they're these things that are potentially at the right temperatures and conditions for liquid water, for conditions that might really support life. It doesn't mean that they do, or that there's life on any of them. But for the very time in human history, you can point to a star and say that's star has planet around it where life really could exist in a way that we understand it.

FLATOW: They call it the Goldilocks...

Prof. BROWN: That's exactly right. You know, it's not too hot, not too cold, and just perfect where life could be.

FLATOW: And how can we not end on that thought? Well, what would you - one last - because we have just less than a minute - what would you like to do? What kind of discovery do you want to see next?

Prof. BROWN: You know, what I'm very excited about in our own solar system is whatever else is out past the region where Pluto and Eris and the Kuiper belt are, the next phase of the solar system. We found one object out there, Sedna. We discovered that one about - almost 10 years ago now. And we know that there are many, many more out there. And if we can find them, they're going to hold these ancient clues to the very birth of our sun. And that's going to be an incredible scientific boon that we are we are desperate to find out more about.

FLATOW: All right, and we'll be in there with you on the chase, Mike.

Prof. BROWN: I can't wait.

FLATOW: Thank you very for taking time to be with us today.

Prof. BROWN: It's been my pleasure.

FLATOW: Mike Brown is the author of "How I Killed Pluto and Why It Had It Coming." He's also professor of planetary astronomy at Caltech in Pasadena.

Copyright © 2011 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.